Counterweight jack mechanisms for cranes and the like
United States Patent 1191 Leigh i 1 COUNTERWEIGHT JACK MECHANISMS FOR CRANES AND THE LIKE [75] Inventor: Theodore M. Leigh, Cedar Rapids,  
 Iowa  
 [73] Assignee: FMC Corporation, San Jose, Calif.  
 [22] Filed: May 16, 1973 [21] Appl. No.: 360,726  
 Primary ExaminerRobert B. Reeves Assistant ExaminerHadd Lane Attorney, Agent, or Firm-J. F. Verhoeven; C. E. Tripp in] 3,874,515 [451 Apr. 1, 1975 [57] ABSTRACT Resilient, adjustable jack mechanisms work in cooperation with front and/or rear end counterweights on the carrier frame of a mobile crane to improve the cranes stability. At least one jack mechanism is attached to the carrier frame adjacent each counterweight. Resilient means using preloaded springs or a precharged hydropneumatic accumulator in the jack mechanism are disclosed. When a load is lifted over the front end of the crane, the front of the carrier frame deflects downwardly and the resilient means in the front jack mechanism transfers the weight of the front counterweight directly to ground while the rear of the carrier frame deflects upwardly and the resilient means in the rear jack mechanism reacts to lift the ground contacting float clear of the ground so the rear counterweight is then effective to counterbalance some of the lifting moment. When a load is lifted over the rear end, the opposite actions take place so the cranes stability is improved in both modes of operation.  
 14 Claims, 13 Drawing Figures PATENIEU H975 3,874.515  
 saw 1 0F 4 Tll3 l COUNTERWEIGHT JACK MECHANISMS FOR CRANES AND THE LIKE BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates to mobile material handling machines. such as carrier mounted cranes. and more particularly to resilient jack mechanisms to work cooperatively with front end or rear end counterweights attached to the carrier frame to improve the stability of the machine.  
 2. DESCRIPTION OF THE PRIOR ART With mobile. full revolving. carrier mounted cranes it is often necessary to increase the stability of the machine by adding counterweight to the forward end of the carrier. such as by a front bumper counterweight. to permit erection of long booms or tower-boom combinations over the rear end. This is true even when the entire machine is blocked up on conventional outriggers to increase the stability of the machine over that provided when the machine is supported on its rubber tires as the primary ground-engaging vehicle support means. Placement of the counterweight forward has the desirable effect of moving the center of gravity of the complete machine toward the front end of the carrier. thus improving the stability of the machine when the boom is over the rear end ofthe machine. However. this forward counterweight then reduces the stability when the machine operates with the boom over the forward end. Furthermore. the extra weight of the front counterweight can impose added loadings on the front outriggers and thus limit machine capacities during operation in the forward work area quadrants in order that the machine&#39;s structural capacity is not exceeded. In some cases. the addition of a counterweight attached to the rear of the carrier frame will improve the stability of the machine. particularly when the boom is over the forward end of the machine. This added weight would then impose additional load on the rear outriggers. however. possibly limiting some of the advantage of the rear counterweight.  
  One known method to overcome these disadvantages is to place the added counterweight on the ground heneath the front bumper and to connect the counterweight to the carrier by vertically arranged. taut chains or other flexible members. With this arrangement the normal structural deflections. which occur when raising long booms over the rear. will cause the front end of the carrier frame to raise sufficiently to lift and transfer a portion. or all. ofthe weight of the added counterweight to the forward end of the frame and thus provide the stability sought. When the crane upper section is rotated to place the boom over the forward end. the structural deflections depress that end and allow the entire weight of the added counterweight to be transferred to the ground. thus avoiding the loss in stability and the lindesirable structural loadings. However. this method is inconvenient as the added counterweight must be handled. usually by lifting it with the crane itself from the counterweight&#39;s travel position on the carrier and placing the counterweight beneath the carriers front bumper and then adjusting the chains to the proper length and connecting the chains to the counterweight and to the carrier.  
  A second known method to avoid loss of stability over the forward end of the carrier when using a front bumper counterweight is to block up or jack under the front bumper to move the tipping fulcrum forward to the front bumper. This method imposes severe bending moments in the carrier frame. To build the carrier frame to adequately withstand these high bending moments requires a substantial increase in frame weight and cost. Any increase in frame weight adversely affects the roadability ofthe mobile crane and usually increases the running gear. drive line components and engine with cumulative increases in cost and reduction in roadability. This second method is usually impractical as the normal requirement is to make the carrier frame as light as possible to attain the best roadability with the lowest cost.  
  Known patented prior art breaks down into several groups ofdevices. One such general group might be referred to as permanently attached automobile or \ehiclc jacks with or without built-in shock absorbers. These are exemplified by U.S. Pat. Nos. 1.193.570 to H. LOHMILLER: 1.281.275 to L. H. BERRY; 1.544.611 to l. M. THOMPSON et a1: 1.701.232 to W. FISCH: 2.837.312 to H. .l. TROCHE; 2.978.115 to R. PRESTEL&#39;. 3.362.683 to E. K. HANSEN; 3.401.915 to T. B. DALTON. and 3.442.531 to B. J. RUTLEDGE. A second general group might be referred to as adjust able props and load holding mechanisms. These are exemplified as US. Pat. Nos. 2.695.764 to K. GREBE; 3.133.720 to l. T. BOOP: 3.136.528 to R. W. EATOUGH; 3.321.182 to W. D. ELENBURG: 3.333.808 to P. L. DUBOFF and 3.426.995 to F. S. ALLINOUANT. A third general group might be referred to as crane or derrick outriggers and stabilizers. These devices provide auxiliary ground supports movahly connected to the vehicle to increase the stability of the crane or derrick when the machine is supported on its rubber tires. This group is exemplified by U.S. Pat. Nos. 1.877.373 to COHEN-VENEZIAN; 1.887.553 to O. C. HAMRE: 2.082.017 to G. A. Mc- CLAIN; 2.370.661 to H. B. HAYES; 2.978.115 R. PRESTEL: 3.035.713 to L. R. ISERMAN; 3.037.643 to C. P. HERMANN: 3.109.542 to G. C. NOLL et a1. 3.253.716 to R. R. STRATTON: and 3.310.181 to W. D. SYMMANK. A fourth general group might be referred to as special crane counterweighting arrangements and this group is exemplified by U.S. Pat. Nos. 1.877.373 to C. COHEN-VENEZIAN; 2.370.661 to H. B. HAYES; 2.978.] 15 to R. PRESTEL: and 3.051.325 to W. L. RISTO. Special means for erecting crane booms is disclosed in U.S. Pat. Nos. 2.370.661 to H. B. HAYES and 3.109.542 to (1C. NOLL et a1.  
 SUMMARY OF THE INVENTION In the present disclosure one or more jack mechanisms with a vertically extend-able shaft and a ground engaging float are provided adjacent a front bumper counterweight on the front or forward end of a wheel mounted mobile material handling machine. such as a carrier mounted crane. Each jack has a housing attached to the carrier frame. The housing encloses a spring system and the shaft mounting. The lower end of the shaft and the float project below the housing and are extendable from a retracted traveling position to a ground engaging operative position. The spring system of the jacks is preloaded to a total value approximately equal to the weight of the front bumper counterweight. Each jack mechanism can be quickly adjusted to its 0perative position with its float in contact with the ground. and then the shaft is locked at that particular extension. When the machine is to be moved. the float can be quickly retracted to a non-operative position for traveling without causing interference. The shaft of the jack mechanism may be extended by manually rotating a nut on a threaded postion of the shaft. by the rotation of the nut by a hydraulic motor and gear train. or by a hydraulic power cylinder having the shaft formed as an extension of the piston rod. in the ease of a hydraulic power cylinder the resilient action of the preloaded springs may be replaced by providing a precharged hydropneumatic accumulator in the hydraulic circuit to accomplish the same function as the springs.  
  A counterweight may also be attached to the rear of the carrier frame when desired since this rear counterweight may improve the lifting capacity and/or improve the stability of the machine. particularly when handling a load over the forward end of the machine. One or more jack mechanisms of the same type as described above would then be added to the rear of the carrier frame adjacent the rear counterweight.  
  When a lift is made with the boom over the rear end of a machine having a front counterweight and having a front jack mechanism in its operative position, the bending moments of the lifted load applied to the carrier frame result in structural deflections which cause the front end of the carrier frame to rise. Since the pre load on the spring of the present jack mechanism at least balances out the weight of the front counterweight, the shaft in the jack mechanism does not move relative to its housing. As the front end of the carrier frame deflects upwardly. the jack mechanism lifts with the carrier frame and the float on the lower end of the shaft rises clear of the ground. Since the float is then free of contact with the ground, the full effect of the front counterweight will be applied to counteract some portion ofthe lifted load and thus improve the stability of the machine.  
  If the machine also has a rear counterweight, and has a rear jack mechanism in its operative position, the rear end of the machine will deflect downwardly while the lifted load is over the rear end. Since the spring system of the rear jack mechanism is also preloaded to at least the equivalent of the weight of the rear counterweight, the counterweight is rendered ineffective by the preloaded spring. With the downward deflection of the frame and the float already in contact with the ground, the weight of the rear counterweight is transferred directly to the ground through the rear jack mechanism. Without the rear jack mechanism, the rear counterweight would add moment to the lifting moment of the load on the boom over the rear end and thereby reduce the stability of the machine. Therefore. this rear jack mechanism negates the effect of the rear counterweight, with the boom over the rear end. so the stability of the machine is further improvedv When the boom of the above machine is rotated over the front end of the carrier frame, the resulting actions ofthe counterweights and their associated jack mechanisms are reversed. The front jack mechanism then transfers the weight of the front counterweight directly to the ground so the front counterweight would be ineffective. The float of the rear jack mechanism lifts free of the ground so the full effect of the rear counterweight would be applied to counteract some portion of the lifted load. Thus. the jack mechanisms of the present disclosure will function to improve the stability of the machine with either position of the boom relative to the ends of the carrier frame.  
  It is therefore an object of the present invention to provide a resilient counterweight jack mechanism which will improve the stability of a mobile material handling machine with a rotatable load handling device, such as a boom.  
  It is a further object of the present invention to provide a counterweighting system for a mobile material handling machine that provides additional stability when required but does not reduce stability under conditions when use of normal dead weight systems would adversely affect such stability.  
  It is another object to provide a spring loaded jack mechanism for a mobile material handling machine having a counterweight that provides additional stability without also imposing excessive bending moments in the machine structure.  
  It is another object to provide a jack mechanism for a mobile material handling machine which selectively and automatically transfers the weight of a counterweight from the machine to the ground supporting surface or permits the counterweight to be effective as determined by the deflection of the machine structure to which the jack is attached.  
  It is another object to provide a jack mechanism to improve the stability of a mobile material handling machine which permits quick application and adjustment with minimum effort and which causes minimum change in roadability of the mobile machine.  
  Is is yet another object of the present invention to provide hydraulic means for operating a jack mechanism with an extendable shaft and a ground engaging member for a mobile crane having a counterweight, said jack mechanism having a spring system with a preloading equivalent at least to the weight of the counterweight and said jack mechanism in cooperation with the counterweight improves the stability of the crane.  
  A further object of the present invention is to provide hydraulic means for operating a jack mechanism with an extendable shaft and a ground engaging member for a mobile crane having a counterweight. said hydraulic means including a precharged hydropneumatic accumulator which provides a preloading equivalent at least to the weight of the counterweight and said jack mechanism acting with the counterweight to improve the stability of the crane.  
 DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a material handling crane employing the jack mechanism of the present invention in cooperation with a counterweight on the carrier frame.  
  FIG. 2 is a side elevation of a forward portion of the crane of FIG. 1 showing the jack mechanism of the present invention in more detail.  
  FIG. 3 is a front elevation of the crane of FIG. 1 showing the jack mechanisms of the present invention.  
  FIG. 4 is a sectional view of the jack mechanism of the present invention taken along lines 44 of FIG. 2  
  FIG. 5 is a cross-sectional view taken on the line 55 of FIG. 4.  
  FIG. 5A is a partial elevational view of an alternate embodiment of the footplate shown in FIG. 5.  
  FIG. 6 is a cross-sectional view. similar to FIG. 5, of a second embodiment of the jack mechanism of the present invention.  
  FIG. 7 is a cross-sectional view. similar to FIG. 5. of a third embodiment of the jack mechanism of the present invention.  
  FIG. 7A is a cross-sectional view taken on the line 7A7A of FIG. 7.  
  FIG. 8 is a schematic hydraulic circuit diagram for the second embodiment shown in FIG. 6.  
  FIG. 9 is a schematic hydraulic circuit diagram for the third embodiment shown in FIG. 7  
  FIG. 10 is a cross-sectional view. similar to FIG. 5. of a fourth embodiment of the jack mechanism of the present invention.  
  FIG. 11 is a schematic hydraulic circuit diagram for the fourth embodiment shown in FIG. 10.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS There is shown in FIG. 1 a carrier mounted mobile crane I5 having a pivoted boom I6 attached to a rotatable uppepr section I8 and having a carrier frame 20. A pair of forward outrigger beams 22 with ground engaging footplates 24 are attached to the carrier frame slightly to the rear of the forward wheels 26 and are extendable laterally on each side of the carrier frame. A  
 similar pair of outrigger beams 28 with ground engaging footplates 30 are also provided slightly to the rear of the rear wheels 32. The upper section contains a crane operator&#39;s cab 36. and the conventional hoisting machinery and power plant 38. Jack mechanisms 40 and 40 of the present invention are illustrated in FIG. I at both the front and rear ends respectively of the machine.  
  In FIGS. 2 and 3 a jack mechanism 40 of the present invention is shown mounted on either side of the front of the carrier frame 2&#34; with a bumper counterweight 42 suspended from the carrier frame between these jacks. While two jack mechanisms 40 will hereinafter be described as indicated in FIG. I. one centrally located jack mechanism might equally well be used and in this case the single jack would be centrally attached to the front cross member of the carrier frame 20 in back of the counterweight. The present jack mechanism 40 might also be attached to the counterweight 42 itself if the counterweight is rigidly connected to the carrier frame 20.  
  Under some conditions a counterweight may also be added to the rear of the carrier frame. This will improve the stability of the machine with the boom over the forward end and therefore provide for handling increased loads. When a rear counterweight is used, the addition of a jack mechanism 40&#39; of the present invention to the rear of the carrier frame 20, adjacent the rear counterweight (not shown) will reduce the load on the rear outrigger beams 28 and provide similar advantages to the jack mechanisms 40 used with the front counterweight as will be described more fully hereinaf ter. Since the rear jack mechanism 40&#39; is similar to the front jack mechanism 40, only the detailed description of the front jack mechanism will be given.  
  As shown more clearly in FIGS. 4 and 5, the jack mechanism 40 comprises a housing 44, a vertical shaft 46, a pair of nut members 48, a retainer plate 50, a spring seat 52, a compression spring 54 and a float or footplate 56. The housing 44 is securely attached to the end of the carrier frame 20 and has a top plate 58 with a central opening 60 having an inner extending vertical flange 62. The lower end of the housing has an external horizontal flange 64 with a plurality of spaced holes 66. The vertical shaft 46. which has upper and lower threaded portions 68 and 69. is mounted in housing 44 and extends below the housing. Float or footplate 56 is attached to the lower most end 70 of shaft 46. One of the nut members 48 is mounted on the upper threaded portion 68 of the shaft and the other nut member is mounted on the lower threaded portion 69 of the shaft. The nuts are provided to adjust the axial position of shaft 46 prior to the application of the load to the jack. Each nut member 48 has a plurality of spokes 72 for hand spinning of this nut in extending or retracting shaft 46 with its footplate 56. A circular boss 74 is located on the bottom of nut member 48 and is coaxial with the axis of the internal thread in the nut member. The outer diameter of boss 74 is slightly smaller than. and concentric with. the central opening 60 in the housing top plate into which opening the boss projects so as to guide the shaft 46. As shown in FIG. 4. the nut member 48 is provided with a horizontally projecting hub portion 76 which has a radial slot 78 to form a split hub locking device. A thumb screw 80 passes through a clearance hole 82 in one side of the split hub 76 and engages a threaded hole 84 in the other side of the split hubv When the nut member 48 is finally positioned on shaft 46. thumb screw 80 is tightened to firmly clamp nut member 48 to the shaft 46 to act as a safety device to prevent the nut member from rotation on shaft 46.  
  Compression spring 54 fits in housing 44 around shaft 46 against housing top plate 58 with annular flange 62 acting as an upper spring guide. Spring seat 52 forms a lower seat for the spring and this spring seat is retained by retainer plate 50 which is fastened to lower housing flange 64 with bolts 86 and nuts 87. The second nut member 48. which is a duplicate of the upper nut member 48 previously described locks the shaft against the spring seat 52 when finally positioned. Spring seat 52 is circular in shape. has a central bore 88 slightly larger than the diameter of shaft 46, a vertical sleeve section 90 as a spring guide. and a horizontal circular flange section 92 slightly smaller than the inside of housing 44 so it will move freely within the housing as the spring is compressed. The lower end of spring seat 52 also has a cylindrical projection 94 which extends into a mating coaxial circular opening 95 in the retainer plate 50. The boss 74 on the lower nut member matingly fits into the central bore 88 of spring seat 52 so the lower end of shaft 46 is guided by the combined cooperation of spring seat projection 94 in opening 96 in the retainer plate and the boss 74 on the nut member in the central bore 88 of the spring seat 52. Spring 54 is compressed to a preselected preloaded condition when the retainer plate 50 is drawn up tightly against the lower housing flange 64 by the nuts 87 on the bolts 86 which pass through holes 66 in the flange 64 and mating holes 93 in the retainer plate 50.  
  The characteristics of spring 54 are determined on the basis of the number of jack mechanisms 40 being used (usually one or two) and the weight of the front bumper counterweight 42 for the particular duty of the crane involved. The combined spring system for the number of jacks being used is then selected so that under a preloaded condition the spring reaction will at least equal the weight of the counterweight being used. The spring rate, or pounds of force per inch of deflection of the spring, is also selected so that upon deflection of the springs under loads greater than the preloading. the spring reaction from the housing to the crane carrier frame will not impose a bending moment on the frame greater than the frame can readily withstand. Normally this upward force can be two or three times the oppositely acting weight of the counterweight by itself without exceeding limiting bending stresses in the carrier frame so additional load over and above the weight of the counterweight can be balanced out by the springs when desired.  
  As further shown in FIG. 5. the lowermost end 70 of shaft 46 is provided with a rigid float or footplatc 56. This footplate comprises a dish shaped bottom plate 96 with a huh 97 and a plurality of stiffening ribs 98 welded as a unit to the end of the shaft.  
  An alternate embodiment to the float or footplate 56 is shown in FIG. SA. In this construction float 100 is removable front the end of shaft 46 and is also selfaligning to readily adjust to an uneven or a non-level ground surface. The lower end of shaft 46 is then provided with a grooved section 102 and the extreme end of the shaft 104 is made convexly hemi-spherical in shape. Float 100 comprises a dish shaped bottom plate H16. a central vertical hub portion 108 and a plurality of ribs H0. The upper portion of hub [08 is provided with a horizontal aperture 112 having retaining flanges I14. slidably fitting into groove 102 in the end of the shaft. and a concave hemi-spherical bottom portion [[6 matingly receiving the hcmi-spherical end 104 of shaft 46. The plate 106. hub 108 and and the ribs are welded together to form the unitary assembly 100. When float 100 is assembled on the end of shaft 46 by fitting aperture 112 over the grooved section [02 in the end of the shaft. the float will be held in place by the retaining flange 114 but the float will be free to swivel and universally adjust itself to the prevailing ground conditions to obtain firm support for the jack mechanism.  
  As already noted. the present disclosure provides a jack mechanism 40 or 40&#39; for use with a counterweight. which jack mechanism includes resilient means in the form of a preloaded compression spring system. The amount of the preloading of spring 54 is selected so that the upward reaction ofthe spring system is at least equal to the weight of the counterweight with which it is used and so that deflection of the spring beyond the preloading causes a controlled upward reaction on the carrier frame whose bending moment imposes bending stresses which do not exceed the limiting bending stresses in the carrier frame. When the loading of the carrier frame is such that the end of the carrier frame at the particular jack mechanism involved is deflected downwardly. such as with the boom located over a for ward counterweight and jack mechanism. there are two beneficial results of the present jack mechanism which act to improve the stability of the machine. The first result is that the weight of the counterweight associated with the jack mechanism is removed from the carrier frame and transferred through the jack mechanism directly to the ground thus negating the effect of the counterweight. The second result is that by deflecting the spring beyond its preloaded value. the downward movement of the carrier frame is partially restrained or decreased by the upward reaction of the spring acting through the jack housing into the carrier frame to which it is rigidly attached. Since the spring is selected so that the carrier frame cannot be overstressed this upward restraining force imposes only a controlled. permissible bending moment on the carrier frame. Thus. by taking full advantage of the capability of the carrier frame in selecting the spring rate of the spring. additional upward spring reaction may be provided to counte ract more of the downward reaction of the lifted load over the jack mechanism and thereby obtain even greater stability. or increased lifting capacity. than when the spring system is selected merely to balance out the weight of the counterweight under preloading.  
  It is to be noted that the resilient means of the jack mechanism 40 or 40&#39; works automatically to improve the stability of the machine Once the float of the present jack mechanism is placed in its operative position, the jack mechanism functions without further manual control to permit the counterweight to be effective as a counterbalancing force when the end of the carrier frame at the jack mechanism deflects upwardly under a lifted load and to negate the effect of the counterweight when the end of the carrier frame at the jack mechanism deflects downwardly under a lifted load. Both conditions result in improving the stability of the machine.  
  The present disclosure provides an advantageous system of a counterweight and at least one resilient jack mechanism mounted on the carrier frame cooperatively working to improve the stability of the machine. A single system of a counterweight and a jack mechanism may be used on the front end of the carrier frame or on the rear end of the carrier frame. A dual system of a counterweight and a jack mechanism may also be used on both ends of the carrier frame simultaneously.  
  With reference to a single system of a counterweight and a jack mechanism mounted on the front end of the carrier frame, the resulting operation of the machine will now be described. When a heavy lift is made with the boom 16 over the rear of the machine 15, the bending moments applied to the carrier frame 20 will cause an upward deflection of the front end of the carrier frame and lift the attached extended jack mechanism 40 with it. The jacks float S6 loses contact with the ground supporting surface since the preloaded spring 54 prevents movement of shaft 46 relative to its housing 44 until the value of the preloading is exceeded which would be more than the weight of the front counterweight. With the float 56 clear of the ground, the full effect of the front counterweight hanging on the front of the carrier frame 20 with a moment arm nearly equal to the length of the carrier frame will be applied to counteract some of the moment of the load being lifted. This is due to the fact that the counterweight and the jack mechanism are located on the front end of the carrier frame and are therefore on the opposite side from the lifted load of the fulcrum or pivot point. ln this case the pivot point is usually the footplates 30 of the rear outrigger beam 28 or the rear wheels 32 if outriggers are not used. The stability of the machine using the front counterweight when the load is over the rear is therefore improved.  
  When the rotatable upper section l8 of the crane is swung around to operate the boom over the forward end of the machine, the bending moment conditions are reversed and the use of a front counterweight, without the presently disclosed jack mechanism 40, would tend to reduce the stability of the machine. The front counterweight would then be on the same side of the pivot point. usually the footplates 24 of the forward outrigger beams 22 or the forward wheels 26 if outriggers are not used. as the lifted load which is over the front end. The front counterweight effect would then add to the effect of the load being lifted and thus reduce the stability of the machine. However. with the front jack mechanism 40 in its operative position. the front end of the carrier frame will deflect downwardly and the shaft 46 with its float 56, already in contact with the ground. will not move. The weight of the front counterweight is therefore transferred directly through the front jack mechanism 40 to the ground. The front counterweight would then be ineffective and would not add its adverse effect to the moments. With the effect of the front counterweight ineffective. the stability of the machine with a front jack mechanism will be improved. If the load on the spring transferred from the carrier frame through the housing 44 exceeds the prc loading. the spring compresses and imposes an upward reaction force on the carrier frame tending to counteract the effect of the load being lifted. thus further improving the stability.  
  Again with reference to a single system of a counter weight and a jack mechanism, the operation ofthe machine when the counterweight system is mounted on the rear of the carrier frame will now be described.  
 When a heavy lift is made with the boom l6 over the rear end of the machine 15. the bending moments applied to the carrier frame will cause a downward deflection of the rear end of the carrier frame 20. Since the float 56 of the rear jack mechanism is already in contact with the ground in its operative position. and since the preloading of spring 54 prevents movement of the shaft 46 relative to its housing 44 under the weight of the counterweight only. the weight of the rear counterweight is therefore transferred directly through the rear jack mechanism 40 to the ground. Since the rear counterweight would be on the same side of the pivot point as the lifted load which is over the rear end. and would normally add to the effect of the lifted load. the use of the rear jack mechanism 40&#39; has negated the effect ofthe rear counterweight by transferring it directly to the ground. so the stability of the machine with a rear counterweight and jack mechanism has been improved.  
  Again when the rotatable upper section [8 of the crane is swung around l80 to operate the boom over the forward end of the machine. the bending moments are reversed. With the lift over the forward end and the counterweight and jack mechanism 40&#39; on the rear end. the rear end of the carrier frame 20 will deflect upwardly and lift the jack mechanism 40&#39; with it. The jacks float S6 loses contact with the ground since the preloaded spring 54 prevents movement of shaft 46 relative to its housing 44 under just the weight of the rear counterweight. With the float 56 clear of the ground. the full effect of the rear counterweight will be applied to counteract some of the moment of the lifted load. This is due to the fact that the jack mechanism 40&#39; and the counterweight are located on the rear end of the carrier frame and are therefore on the opposite side from the lifted load of the pivot point. The stability of the machine using the rear counterweight system with its jack mechanism 40&#39; when the load is over the front is therefore improved.  
  With reference to a dual system of a counter weight and its associated jack mechanism on each end of the carrier frame simultaneously the resulting operation will now be described. Each counterweight and its associated jack mechanism will work individually as de scribed above in the single systems depending upon the location of the boom with respect to the ends of the carrier frame. Thus. when the boom 16 is oycr the rear of machine 15 and both jack mechanisms 40 and 40&#39; are in their operative positions with their floats 56 in contact with the ground. the bending moments of the load being lifted over the rear will cause the front end of the carrier frame 20 to deflect upwardly lifting jack mechanism 40 with it. and will cause the rear end ofthe carrier frame to deflect downwardly. These deflections acting through the respective jack mechanisms at each end will result in the front end counterweight being free to act effectively as a counterweight to counteract some of the moment of the lifted load. and will result in the weight of the rear counterweight being trans fcrrcd directly to the ground. thus negating the effect of the weight of the rear counterweight. When the boom 16 is over the forward end of the carrier frame and both jack mechanisms 40 and 40&#39; are in their operative positions with their floats 56 in contact with the ground. the bending moments of the load being lifted over the forward end will cause the rear end of carrier frame to deflect upwardly lifting jack mechanism 40&#39; with it. and will cause the front end of the carrier frame to deflect downwardly. These deflections acting through the respective jack mechanism at each end will result in the rear end counterweight being free to act effectively as a counterweight to counteract some of the moment of the lifted load and will result in the weight of the front counterweight being transferred directly to the ground. thus negating the effect of the weight of the front counterweight.  
  A second embodiment of the jack mechanism of the present invention is shown in FIG. 6. ln this embodiment the jack mechanism comprises a housing I32. a cylinder 134. a ram and piston assembly [36. a compression spring 138 and a retainer plate 140. The housing 132 is vertically attached to the carrier frame 20. as by welding. and includes an upper cover plate 142. This cover plate is securely attached to the housing and has a central opening I44. An external horizon tal flange 146 with a plurality of bolt holes 148 is securely attached to the bottom of housing 132. The cylinder 134 includes a top head I50 with a central shaft opening [52. The cylinder 134 further includes a bottom head I54 with a central shaft opening 156 and a flange I58 extending horizontally beyond the outer sur&#39; face of the cylinder. Top cylinder head 150 has a shaft seal [62 retained in its central opening 152 and bottom cylinder head 154 also has a shaft seal I64 retained in its central opening 156. The ram and piston assembly [36 comprises a ram shaft 166 with an integral piston 168 located a finite distance from the upper end of the ram shaft. seals 162 and 164 maintaining fluid tightness with ram shaft 166. The piston 168 further contains piston ring seals 170. The ram shaft M6 is provided with two fluid ports, 172 and 173, at its upper end. a first longitudinal fluid passageway 174 extending from fluid port 172 to below the piston 168. with a transverse connecting passageway 175 from the passageway 174 to the interior of the cylinder below the piston and a second longitudinal fluid passageway 176 extending from fluid port 173 to above the piston with a transverse connecting passageway 177 from the passageway 176 to the interior of the cylinder above the pistonv The lower end of the ram shaft 166 extends below housing 132 and has a float or footplate 178. Float 178 may take either form shown in FIGS. and 5A as previously described under the first embodiment. The upper end of ram shaft 166 passes through cylinder head 150. Spring 138 is concentrically located within housing 132 and surrounds cylinder 134. The bottom of spring 138 seats on flange 158 of the cylinder 134 and the top of the spring seats against cover plate 142. The cylinder 134 and spring 138 are retained in the housing 132 by the retainer plate 140 which has a central opening 180 and a plurality of bolt holes 182 matching the bolt holes 148 in bottom flange 146 of housing 132. When tightened nuts 186 on bolts 188. which pass through bolt holes 148 and 182 draw up retainer plate 140 into contact with flange 146 to preload spring 138 to a value at least equal to the weight of the counterweight with which it is used. or its proportional part of the weight if multiple jack mechanism are used with one counterweight for the same reason previously described in the first embodiment. The top of cylinder 134 extends through opening 144 in the upper cover plate 142 of housing 132. The cylinder 134 is free to move vertically in housing 132 as spring 138 is compressed beyond its preloading.  
  Ram and piston assembly 136 with cylinder 134 form a hydraulic power cylinder when supplied with fluid under pressure. A typical schematic hydraulic circuit for this embodiment of the invention is shown in FIG. 8. Oil is supplied from a hydraulic system 184 which comprises a fluid pump (PF) 186. a system relief valve 188. a sump or tank 190. a 3-position 4-port manually operated spring centered control valve 192 having its pump port P&#34; connected to a tank port and its cylinder ports &#34;A&#34; and &#34;B&#34; blocked. in the centered position a pressure compensated speed control valve 194, and a hydraulically operated spring centered 3- position Z-port pilot valve 195. A flexible conduit 196 connects valve 195 to port 173 on the ram shaft 166 and a flexible conduit 197 connects port 172 on the ram shaft with cylinder port 13&#34; on control valve 192. A pilot pressure line 198 is connected from the pres sure line at port A&#34; of valve 192 to the pilot operator on the top of valve 195 and a second pilot line 199 is connected from the pressure line at port &#34;B&#34; on valve 192 to the pilot operator on the bottom of valve 195. Pilot valve 195 is placed on or closely adjacent to the jack mechanism 130 since it acts as a safety device or check valve to prevent the piston and ram shaft 136 from moving under load when at rest. This is accomplished by the blocked center position of valve 195 which prevents movement of oil from the top of piston 168.  
  To extend the float 178 to its operative position in contact with the ground, the control valve 192 is positioned to direct fluid pressure through valve port which pressure is transmitted through speed control valve 194 and through pilot valve 195 to port 173 on the ram shaft and on to the top of piston 168. The return oil from under piston 168 is discharged through port 172 in the ram shaft to valve port 8&#34; of control valve 192 and on to tank 190. When float 178 is firmly in contact with the ground, the control valve 192 will be returned to its centered position and valve ports A&#34; and 8&#34; will be blocked and pilot valve 195 will also return to its blocked center position. The system relief valve 188 will be adjusted so that when the float is firmly in contact with the ground. the spring 138 is not compressed beyond its preloading. This will prevent an excessive uplift force being applied against the carrier frame 20 by the application of too much hydraulic pressure on top of piston 168.  
  To retract float 178 to its traveling position. control valve 192 is raised to direct fluid pressure to valve port The pressure is transmitted directly to port 172 in the ram shaft 166 and on to the bottom of the piston 168. The return oil from the top of piston 168 passes through port 173 in the ram shalt through pilot valve 195, through speed control valve 194 to valve port A&#34; of control valve 192 and on to the tank 190. When the float 178 reaches its uppermost position, control valve 192 is again centered and valve ports A&#34; and &#34;B&#34; will be blocked. Pilot valve 195 will also return to its blocked center position and thus prevent movement of the ram shaft 166.  
  Once positioned in its operative position. jack mechanism will act in the same manner as jack mechanism 40 of the first embodiment. When the lifted load is over the counterweight and jack mechanism system. that end of the carrier frame will deflect downwardly. The float 178 is already in Contact with the ground and the preloading of the spring system 138 is at least equal to the weight of the counterweight. The weight of the counterweight is therefore transferred directly through the jack mechanism 130 to the ground. and the counterweight would then be ineffective. Therefore the counterweight does not add its adverse effect to the bending moments and the stability of the machine is improved. 1f the load on the spring 138 exceeds the preloading. the spring compresses and imposes an upward reaction force on the carrier frame thus further improving the stability. When the lifted load is over the opposite end of the carrier frame from the counterweight and jack system. the end of the carrier frame at the counterweight will deflect upwardly lifting the jack mechanism with it. The full effect of the counterweight will then be applied to counteract some of the moment of the load being lifted and thus improve the stability of the machine.  
  A third embodiment of the present disclosure covering a fluid motor driven screw jack mechanism 200 is shown in FIGS. 7 and 7A. This jack mechanism comprises a housing 202. ajack screw shaft 204.21 compression spring 206. a lower spring guide 208, a lower retainer plate 210, an upper guide bearing 212, a split retaining collar 214. a gear train 216. and a hydraulic motor 218. Housing 202 has a body portion 220 which is square in cross section (FIG 7A) and is securely attached to the carrier frame 20, as by welding with its longitudinal axis vertical. Housing 202 further has an upper housing head 222 with a central opening 224 and with an inner extending annular vertical flange 226. An external horizontal flange 228 with a plurality of bolt holes 230 is attached, as by welding. to the lower end of the body portion 220 of the housing. Jack screw shaft 204 has an upper end 232 extending through the housing head 222. The upper end of the shaft is splined, and the shaft has a main central section 234 which is threaded. The lowermost end 236 of shaft 204, which extends below housing 202, is fitted with a float or footplate 238. The float 238 may take either form as previously described under the first embodiment and as shown in FIGS. 5 and SA. The lower spring guide 208 has a main body portion 240 which is square in outline (FlG. 7A) and has a cylindrical hub 242 whose bore 243 is threaded to mate with the threaded portion 234 of shaft 204 and act as a nut. The square outline of spring guide 208. which fits in the square body portion 220 of the housing 202. prevents rotation of this spring guide as shaft 204 is rotated. but permits the spring guide to move vertically in the housing 202 under the compression of spring 206. Hub 240 has an upper portion 244 projecting above and a lower portion 245 pmjecting below the main square body portion 240 of spring guide 208 to form a lower seat and guide for spring 206. The upper end of spring 206 seats against housing head 222 and is guided by the vertical annular flange 226. The spring 206 and the lower spring guide 208 are retained in housing 202 by the lower retainer plate 210 which also has a central opening 248 to receive the lower hub projection 245 of spring guide 208. Retainer plate 210 further has a plurality of bolt holes 250. matching holes 230 in housing flange 228. to receive bolts 252. In the assembly of spring 206 in housing 202. the spring guide 208 and retainer plate 210 compress spring 206 to the desired preload value by the tightening of nuts 253 on bolts 252 which draws retainer plate 210 into contact with the lower housing flange 228. The preload value of spring 206 is at least equal to the weight of the counterweight with which it is used. or. if multiple jack mechanisms are used with one counterweight. the preloading of the spring system of the combined multiple jack mechanims is at least equal to the weight of the counterweight.  
  The upper end of jack screw shaft 204 is guided by upper bearing 212. Bearing 212. which is made of bronle. is a cylindrical bearing bushing held in annular vertical flange 226 of the housing head 222. Bearing bushing 212 will have an integral upper flange 254 and a lower flange 256 defined by an annular plate secured to the bottom of bearing bushing 212 by countersunk screws 258. Bearing 212 further has a central opening 259. concentric with the axis of jack screw shaft 204. which is splincd to slidingly mate with the splincd upper section 232 of the shaft 204. Thus. the shaft 204 can move vertically within bearing 212 on the splined surfaces while bearing 212 rotates with the shaft and the outer cylindrical surface of the bearing rotates on the stationary insidc cylindrical surface of the annular flange 226. The bearing flanges 254 and 256 retain the bearing 212 vertically and absorb any resulting thrust.  
  The gear train 216 (FIG. 7) comprises two sets of gears forming a double gear reduction between fluid motor 218 and jack screw shaft 204. Gear 260 which is mounted on the upper end of jack screw shaft 204 has a central bore 261 which is splined to slidingly mate with the splincd upper section 232 of shaft 204. Gear 260 further has a lower hub section 262 with groove 263 in the periphery of the hub. Gear 260 is retained in a fixed vertical position by split retaining collar 214. Collar 214 is provided with a circular flange 264. which projects into groove 263. to retain the gear against the top of bearing 212. Collar 214 is fastened to the top of housing head 222 with countersunk screws 265.  
  A bracket 266 is secured to the carrier frame 20. as by welding. and supports stationary idler shaft 268 and the fluid motor 218 in spaced relation as determined by the centers of the gears in the gear train. Pinion 270 is integrally made with an intermediate gear 272 above. the two gears being vertically spaced apart by a short hub section 273. Pinion 270 and intermediate gear 272 are free to rotate on idler shaft 268 and are retained by a collar 274 on the top of the idler shaft. Pinion 270 is positioned on shaft 268 to mesh with gear 260. Fluid motor 218 is mounted on bracket 266 with its output shaft 276 extending vertically upward. A pinion 278 is mounted on the output shaft 276 and is positioned to mesh with intermediate gear 272. Thus the fluid motor. when supplied with hydraulic fluid under pressure from a hydraulic system 280 and conduits 282 and 283. rotates pinion 278 and the remaining gears in gear train 216 to rotate jack screw shaft 204. The threaded sec tion 234 of jack screw 204 turns in lower spring guide and nut 208. which is restrained from turning by its square outline fitting in square housing body 220 FIG. 7A). so the jack screw moves up or down depending upon the direction of its rotation.  
  A typical hydraulic circuit diagram for operating the hydraulic system 280 is shown in FIG. 9. Oil under pressure is supplied by a fluid pump (PF) 284 from a tank or sump 286. A system relief valve 288 is connected to the pressurized line from the pump and the pressurized fluid passes through a pressure compensated specd control valve 290 to a manually operated spring centered 3-position 4-port directional control valve 292. In centered position. valve 292 has its pump port P&#34; to open to the tank port and its cylinder ports &#34;A&#34; and B&#34; blocked. A second relief valve 294 is connected in conduit 282 between directional control valve 292 and the fluid motor 218 to limit the force which can be applied in extending jack screw shaft 204 and its float 238 when the float engages the ground sup porting surface. The return fluid passes through conduit 283 front the fluid motor 218 to the directional control valve 292. and then to tank 286.  
  In operation of this third embodiment (FIG. 7) of the presently disclosed jack mechanism to extend jack screw shaft 204 into its operative position with float 238 in contact with the ground supporting surface. the directional control valve 292 is moved to its first position to direct oil under pressure into conduit 282 connected to fluid motor 218. The fluid motor 218 then rotates gear train 216 to extend jack screw shaft 204 until its float 238 firmly engages the ground supporting surface. When the footplate 238 bears against the ground surface. the setting of the second relief valve 294, which is adjusted to some value lower than the system relief valve 288, prevents compression of spring 206 and thus prevents excessive uplift forces being applied against the end of carrier frame 20. To retract jack screw shaft 204 to its traveling position. it is merely necessary to position directional control valve 292 to direct fluid into conduit 283, in lieu of conduit 282. so fluid motor 218 operates in the reverse direction to drive the jack screw shaft 204 in the upward direction.  
  Once positioned in its operative position, jack mechanism 200 will act in the same manner as jack mechanism 40 of the first embodiment. When the lifted load is over the counterweight and jack mechanism system. that end of the carrier frame will deflect downwardly. Since the float 238 is already in contact with the ground and the preloading of the spring system 206 is at least equal to the weight of the counterweight. the weight of the counterweight is transferred directly through the jack mechanism 200 to the ground. The counterweight would then be ineffective so the counterweight does not add its adverse effect to the bending moments. thus impro ing the stability of the machine.  
 If the load on the spring system 206 exceeds the preloading. the spring compresses and imposes an upward reaction force on the carrier frame thus further improv ing the stability. When the lifted load is o\ er the oppo site end of the carrier frame from the counterweight and jack system. the end of the carrier frame at the counterweight will deflect upwardly. lifting the jack mechanism with it. Since the preloaded spring prevents mmcment of the shaft 204 until the value of the preloading is exceeded. the float 238 loses contact ith the ground. The full effect of the counterweight will then be applied to counteract some of the moment of the load being lifted and thus improve the stability of the machine.  
  A fourth embodiment of the present disclosure covering a hydraulic cylinder actuated jack mechanism 300 having an accumulator in the hydraulic circuit is shown in H65. and H. The jack mechanism 300 comprises a cylinder or housing 302. a piston and piston rod assembly 304. and a hydraulic system 306. Housing 302. which is in the form of a hydraulic cylinder. is securely attached to the carrier frame 20 so that the piston and piston rod assembly 304 having a portion located internally of the housing moves vertically in the housing. Housing 302 has a top cylinder head 308 integral with the upper body portion ofthe housing and having a first fluid port 310. The lower body portion of the housing has an external horiyorttal flange 3l2 with a plurality of spaced holes 314. and a second fluid port 316. A bottom cylinder head 318 with a plu rality ofspaced holes 320. matching holes 314 in llange 3l2. is bolted to housing flange 312 with bolts 322 and nuts 323. The bottom cylinder head 318 further had a central shaft opening 324 and a shaft seal 326. The piston and piston rod assembly 304 includes a piston rod 328 which projects below the housing 302. a piston 330 integral with. or securely attached to the upper end of piston rod 328. said piston having sealing rings 332. and a float or footpiate 334 attached to the lowermost end of piston rod 328. The piston rod 328 passes through central shaft opening 324 and fluid tightness is maintained by the shaft seal 326. Float 334 may take either form shown in FIGS. 5 and 5A as previously de scribed under the first embodiment.  
  Piston and piston rod assembly 304 with cylinder or housing 302 form a reciprocahle hydraulic power cylinder when supplied with fluid under pressure from the hydraulic system 306. A typical schematic hydraulic circuit for this embodiment is shown in FIG. H. The hydraulic system 306 comprises a fluid pump (P F) 336, a system relief valve 338. a sump or tank 340. a 3-position 4-port spring centered manually operated direction control valve 342. The valve has a pump port &#34;P which is connected to tank port T in the center position. and has cylinder ports A&#34; and B blocked in the center position. The hydraulic system includes a pilot operated Z-position pressure controlled valve 344. a check valve 345. and a hydropneumatic accumulator 346. A flexible conduit 348 connects valve port A&#34; of directional valve 342 to port 3l0 of cylinder 302 and supplies fluid to the top of the piston. A second fluid conduit 349 connects port 3l6 on cylinder 302 to valve port 8&#34; of directional valve 342 and permits the fluid from below the piston to return to tank when the valve is in its first position. The accumulator 346 is charged with a gas under an initial pressure about equal to the system relief valve pressure se ing. The pilot operated pressure controlled valve 344. which is adjustable. would be set for a pressure slightly above that of the system relief valve pressure setting so that flow from the accumulator 346 to the cylinder 302 can only occur when the pressure in the accumulator is higher than the sy stem pressure. Fluid can pass from the cylinder port 3l0 through check alve 345 into the accumulator 346. by-passing pressure controlled valve 344. but cannot return in the opposite direction through the check valve. The flow of fluid out of the accumulator. when permitted by the pressure controlled valve 344. is prevented from returning to tank by the blocked valve port of directional control valve 342 in its centered position, but is free to go to the cylinder 302. Return flow from under the piston 330 through port 316 is also we \ented by the blocked valve port 8&#34; of directional control valve 342 in its centered position.  
  The prechargcd accumulator 346 in this jack mechanism 300 acts as the resilient means comparable to the preloaded compression spring 54 of the jack mechanism 40 or 40 of the first embodiment. When the lifted load is such that the end of the carrier frame 20 at the jack mechanism 300 and its associated counterweight is deflected downwardly. the attached cylinder or housing 302 is carried downwardly. with the frame. Since the float 334 in its extended operative position is already in contact with the ground surface. the float and the piston 330 do not move while the cylinder 302 moves so a decreased volume in the cylinder portion above the piston results. The fluid displaced from above the piston due to this change in volume in the cylinder flows through check valve 345 into accumulator 346 causing the initial charge of gas to compress. increasing the hydraulic pressure then in the accumulator above that of the system relief valve. This increase in hydraulic pressure acts on the upper cylinder head 308 of the cylinder 302 and thus tends to provide an upward reaction force on the carrier frame counteracting some of the downward force from the lifting momerit. With the proper selection of the volume of the accumulator in relation to the displaced volume of fluid from the cylinder, and with the proper initial charge pressure of the gas in the accumulator so the upward reaction of force of the cylinder on the carrier frame is at least equal to the weight of the counterweight involved with one jack mechanism or its proportional part if multiple jack mechanisms are used with the counterweight involved. the weight of the counterweight can be balanced out. The effect of the counterweight is then transferred directly through the jack mechanism to the ground surface, thus negating the increased load on the carrier frame due to the counterweight. lf the initial charge of the gas in the accumulator is selected to more than compensate for the dead weight of the counterweight. this higher-than-system pressure from the accumulator acting on the cylinder head would then create a net upward reaction force on the carrier frame tending to counteract some of the downward forces due to the lifting moments in addition to cancelling out the effect of the counterweight. Thus. the pre-charged hydropneumatic accumulator provides the resilient means to counteract at least the weight of the counterweight involved or some pre-selected load greater than the weight of the counterweight. It is to be noted that while the preloaded spring 54 of the first embodiment will deflect in a straight-line relationship of force versus deflection, the pressure-volume relationship of the pre-charged gas in the accumulator will follow a curve which can be made to approximate the straight line relationship within the expected range of normal operation of this jack mechanism.  
  To extend the float 334 to its operative position in contact with the ground. the directional control valve 342 is positioned to direct fluid under pressure through valve port which fluid then passes through fluid conduit 348 to cylinder port 310 and on to the top of piston 330. The initial operation applies system pres sure to the accumulator 346. thus approximately neutralizing the initial pressure of the gas by fluid flowing through check valve 345 as pressure controlled valve 344 would be blocked. The fluid from under piston 330 returns through cylinder port 316 and fluid conduit 34) to valve port &#34;B&#34; of directional control valve 342 and then to tank 340. The system relief valve 338 will be adjusted so that an excessive uplift force cannot be ap plied against the carrier frame 20 by applying too much hydraulic pressure on top of piston 330 and thus to float 334 while the latter is in contact with the ground.  
  To retract float 334 to its traveling position. direc tional control valve 342 is positioned to direct fluid under pressure to valve port 8&#34; while fluid then passes through fluid conduit 34) to cylinder port 316 and on to the bottom of piston 330. The return fluid from the top ofpiston 330 passes through cylinder port 310 and fluid conduit 348 to valve port A&#34; of directional valve 342 and on to tank 340. When the float 334 reaches its uppermost position directional valve 342 is again centered and valve ports &#34;A&#34; and &#34;B&#34; will be blocked. Since the pressure in the accumulator 346 will he at system pressure. the pressure controlled valve will also be in its blocked position so fluid cannot be discharged from the accumulator to the tank 340.  
  Once positioned in its operative position jack mechanism 300 will act similarly to jack mechanism 40 of the first embodiment. When the lifted load is over the counterweight and jack mechanism system. that end of the carrier frame 20 will deflect downwardly. Since the float 334 is already in contact with the ground and the pre-chargcd accumulator is pressurized to system pressure. the downward movement of the frame will carry cylinder 302 with it. thus decreasing the volume in the cylinder above piston 330. The fluid displaced will pass into the accumulator 346 through check valve 345 and so compress the initial gas charge in the accumulator above that of the system pressure. The resulting in&#39; creased pressure in the accumulator. acting on the cylinder head 308, would create an upward reaction force on the carrier frame at least equal to the weight of the counterweight ifa single jack mechanism is used. With multiple jack mechanisms used with one counterweight. the total of the upward reaction forces of the combined jack mechanisms would at least equal the weight of the counterweight. If the initial pressure of the gas in the accumulator had been selected just so the resulting hydraulic pressure balances the weight of the counterweight with the addition ofthis displaced fluid. no additional upward reaction force results on the carrier frame. The weight of the counterweight is transferred directly through the jack mechanism to the ground so the counterweight does not add its adverse effect to the bending moments thus improving the stability of the machine. However. if the initial charge pressure of the gas in the accumulator had been selected for some reaction value greater than the weight of the counterweight. but still within the permissible bending capacity ofthe carrier frame. then the balance of upward reaction force. after cancelling out the effect of the counterweight. provides a net upward reaction force which counteracts some of the downward forces due to the lifting moments. This condition further improves the stability of the machine.  
  When the lifted load is over the opposite end of the carrier frame from the counterweight and jack mechanism system. the end of the carrier frame at the counterweight will deflect upwardly. Since the valve ports A&#34; and 8&#34; ofdirectional control valve 342 are both blocked in the valves centered position. the fluid under piston 330 is trapped so it will not move relative to its cylinder 302 even with system pressure supplied to the top of the piston by the accumulator. Thus. the piston. piston rod and float remain fixed relative to the cylin der and as the cylinder rises with the end of the carrier frame under the upward deflection. the float will be lifted from the ground surface, When the float is free of the ground surface. the weight of the counterweight is no longer supported by the jack mechanism 300. so the counterweight can act in a normal manner. as with out the presentjack mechanism. to assist in counterbalancing some of the lifted load moment applied to the carrier frame. This. then. acts to improve the stability ofthe machine with the load over the end of the carrier frame opposite from the counterweight and jack mech anism system.  
  Although the best modes contemplated for carrying out the present invention have been shown and described. it will be apparent that modifications and vari ations may be made without departing from what is regarded to be the subject matter of the invention as defined by the attached claims.  
 What is claimed is:  
  l. A resilient jack mechanism for use in cooperation with an associated counterweight mounted on a carrier frame of a mobile material handling machine to improve the stability of the machine. the combination which comprises:  
 a. a housing attached to one end of the carrier frame of the machine adjacent the counterweight. said housing having a top plate with a central opening. having an open bottom and having an external bottom flange;  
 b. a vertical shaft centrally mounted in said housing. said shaft having an upper end projecting through said central opening in said top plate and &#39;having a lower end projecting below said housing. said shaft having a screw thread on its upper end and having a screw thread on at least a portion of its lower end.  
 c. a ground engaging float connected to the lower most end of said shaft. said shaft with said float movable between a retracted traveling position and an extended operating position with said float firmly in contact with the ground;  
 d. a first nut member engaging the screw thread on the upper end of said shaft. said nut member supporting said shaft on said housing top plate and having a lower projection guiding said shaft in said central opening;  
 e. a spring seat slidingly fitting into the interior of said housing. said spring seat having a hub with a bore larger than said shaft;  
 f. a retainer plate removably secured to said housing flange. said retainer plate having a central opening into which the lower portion of the hub on said spring seat extends and centrally positions said spring seat:  
 g. a compression spring mounted in said housing around said shaft and between said housing top plate and said spring seat. said spring being initially compressed to a preselected preloading when assembled in said housing and retained by said retainer plate. said preloading providing an upward spring reaction force at least equal to the weight of the counterweight with which the jack mechanism is associated; and  
 h. a second nut member engaging the screw thread on the lower end of said shaft. said second nut member having an upper projection extending into the bore of said spring seat and guiding said shaft. said shaft being vertically movable by the selective rotation of one of the two nut members and the loosening of the other nut member. both of said nut members locking said shaft after it is located in a selected position.  
  2. The resilient jack mechanism of claim 1 wherein said first and second nut members each include locking means to prevent movement of said shaft under load.  
  3. The resilient jack mechanism of claim 2 wherein the locking means for each of said nut members comprises a horizontally extending hub portion which is vertically split therethrough in a radial direction into the internally threaded bore. and a horizontal screw which passes through a clearance hole in one section of the split hub perpendicularly to the radial split and en gages a threaded hole in the other section of the split hub. whereby when each nut member is finally positioned on said shaft and against said housing or said spring guide. said horizontal screw is tightened thereby distorting said nut member sufficiently to clamp said nut member to the shaft and to prevent movement of said shaft.  
  4. A resilient jack mechanism for use in cooperation with an associated counterweight mounted on a carrier frame of a mobile material handling machine to improve the stability of the machine. the combination which comprises:  
 a. a housing attached to one end of the carrier frame of the machine adjacent the counterweight, said housing having a top plate with a central opening, having an open bottom and having an external bottom flange;  
 b. a vertical fluid cylinder mounted to move within said housing. said cylinder having a top head with a central shaft opening. a bottom head with a central shaft opening coaxial with the central shaft opening in the top head. and a bottom horizontal flange external to the outer surface of the cylinder;  
 c. a vertical ram shaft having a piston thereon mounted in said cylinder. portions of said shaft extending through said central openings in the top and bottom heads of said cylinders;  
 d. a ground engaging float connected to the lowermost end of said shaft. said shaft and said float movable between a retracted traveling position and an extended operating position with said float firmly in contact with the ground when fluid under pressure is supplied to the interior of said cylinder above or below said piston depending upon the desired direction of travel:  
 e. a retainer plate removably secured to said housing flange. said retainer plate having a central opening coaxial with said shaft to permit passage of said movable shaft:  
 f. a compression spring mounted in said housing around said cylinder and between the housing top plate and the bottom flange of said cylinder. said spring being initially compressed to a preselected pre-loading when assembled in said housing and retained by said retainer plate. said preloading providing an upward spring reaction force at least equal to the weight of counterweight with which the jack mechanism is associated: and  
 g. a hydraulic system for providing fluid under pressure to said cylinder and for controlling the movement of said shaft and piston. said hydraulic system including valve means to trap the fluid in said cylinder to prevent undesired movement of said shaft when located in either of the traveling or the operating positions.  
  5. A resilient jack mechanism for use in cooperation with an associated counterweight mounted on a carrier frame of a mobile material handling machine to improve the stability of the machine. the combination which comprises:  
 a. a housing attached to one end of the carrier frame of the machine adjacent the counterweight. said housing having a polygonal cross-section and having a top plate with a central circular opening. and open bottom and a bottom horizontal external flange;  
 b. an upper guide bearing rotatably mounted in the central circular opening in the top plate of said housing. said bearing having a vertically splined bore coaxial with the axis of the central circular opening in the top plate;  
 c. a vertical shaft mounted in said housing on said guide bearing, said shaft having a vertically splined upper portion and a threaded lower section. said shaft movable vertically on said splined section while rotating in said upper guide housing;  
 d. a ground engaging float connected to the lowermost end of said shaft. said shaft and said float movable between a retracted traveling position and an extended operating position with said float firmly in contact with the ground;  
 c. a lower spring guide having a main body portion of the same polygonal shape as the cross-section of said housing. said spring guide having a hub which has a central bore internally threaded to matingly engage the threaded section of said shaft and which hub has a projection below said body portion, said spring guide movable vertically in said housing, but restrained from rotating by said polygonal shaped body portion matingly engaging the polygonal shape of said housing;  
 . a retainer plate removably secured to said housing flange. said retainer plate having a central opening into which the projection of the hub of said spring guide extends and positions said shaft coaxially with said upper guide bearing;  
 g. a compression spring mounted in said housing around said shaft and between said housing top plate and said lower spring guide. said spring being initially compressed to a preselected preloading when assembled in said housing and retained by said retainer plate. said preloading providing an upward spring reaction force at least equal to the weight of the counterweight with which the jack mechanism is associated:  
 h. a fluid motor with a vertical output shaft mounted on the carrier frame;  
 i. a gear train arranged to drivingly connect the output shaft of said fluid motor and the upper splined end of said vertical shaft mounted in said housing, said fluid motor thereby rotating said vertical shaft and since the mating threaded lower guide member is restrained from rotating. this vertical shaft is moved vertically up or down depending upon the rotation of said fluid motor; and  
 j. a hydraulic system for providing fluid under pressure to said fluid motor and for controlling the movement of said vertical shaft and said float, said hydraulic system including a relief valve adjacent said fluid motor to limit the pressure applied to the fluid motor so as to prevent excessive forces being transmitted from the jack mechanism to the carrier frame and including valve means to prevent rotation of said fluid motor when said shaft is located in either of the traveling or the operating positions.  
 6. A resilient jack mechanism for use in cooperation with an associated counterweight mounted on a carrier frame of a mobile material handling machine to improve the stability of the machine, the combination which comprises:  
 a. a fluid cylinder attached to one end of the carrier frame of the machine adjacent the counterweight, said cylinder having an upper and a lower fluid port;  
 b. a piston and piston rod shaft assembly having a piston within said fluid cylinder and a piston rod extending from the piston to below the bottom of said cylinder;  
 c. a ground engaging float connected to the lowermost end of said piston rod, said piston rod and said float being movable vertically between a retracted traveling position and an extended operating position with said float lirmly in contact with the ground when fluid under pressure is supplied to the interior of said cylinder; and  
 d. a hydraulic system for providing fluid under pressure to said cylinder and for controlling the movement oi said piston and piston rod assembly, said hydraulic system including a hydropneumatie accumulator connected into a conduit delivering pressurized fluid to the top of said piston in said cylinder. the accumulator having an initial charge of gas at a pressure approximately equal to the operating pressure of the hydraulic system and having a volume related to the cross-sectional area of said cylinder and the amount ofdeflection of the carrier frame under a load at least equal to the weight of the counterweight, whereby as the end of the carrier frame deflects downwardly under load with said float in its operative position the fluid displaced l&#39;rom the top of the cylinder will flow into the accumulator compressing the gas further and increasing the fluid pressure in said cylinder, which increased pressure then provides an upward reaction force through said cylinder to the end of the carrier frame at least equal to the weight of the counterweight. thus resiliently counteracting at least the downward force due to the weight of the counterweight.  
  7. The resilient jack mechanism of claim 6 wherein said hydraulic system further includes a power driven fluid pump connected to a tank having a supply of fluid, a spring centered directional control valve located between the pump and the cylinder and having its valve ports to the cylinder blocked and its pump port connected to the tank port when the valve is in its centered position; a system relief valve located between the pump and the directional control valve. said relief valve being adjusted to a pressure setting which will prevent an excessive up-lift force being applied to the carrier frame when said float is in its operative position; flexible fluid conduits connecting said directional control valve and the upper and lower fluid ports of said cylinder; said accumulator being connected to the conduit from said directional control valve leading to the upper fluid port of said cylinder; at check valve located in the conduit to said accumulator preventing flow from the accumulator; a pressure controlled valve located in a conduit by-passing said check valve, said pressure controlled valve permitting i&#39;low to the upper fluid port of said cylinder from the accumulator only when the pressure in the accumulator exceeds the pressure setting of the system relief valve, and fluid conduits interconnecting the pump. the tank. the relief valve and the directional control valve so as to complete the hydraulic systerm 8. A counterweight and resilient jack mechanism system for mounting on a carrier frame of a mobile material handling machine to improve the stability of the machine, the combination which comprises:  
 a. a counterweight mounted on one end of the carrier frame of the machine; and  
 b. at least one jack mechanism adjacent said counterweight, said jack mechanism comprising a housing attached to the carrier frame adjacent said countcrweight. said housing forming a hydraulic cylinder having an upper and a lower fluid port, a vertically movable shaft mounted in 9. in a mobile material handling machine having a carrier frame, a boom rotatably mounted on said frame and selectively positionable to lift a load over one end of the machine or over the opposite end thereof, and primary ground-engaging vehicle support means receiving the weight of the machine and the load lifted thereby.  
 the improvement to improve the stability of said machine under load comprising:  
 a counterweight attached to one end of said carrier frame,  
 a preloaded jack for said counterweight, said jack being mounted on said frame adjacent said one end and outwardly spaced from said ground-engaging vehicle support means substantially the same distance as said counterweight,  
 said jack including a substantially vertical downwardly extending shaft having a ground-engageable foot plate,  
 means for vertically moving said shaft between a retracted elevated position for machine transport and an extended position with said foot plate proximate the ground for load-handling operations,  
 resilient means disposed between said jack shaft and said frame, said resilient means exerting a downward force on said shaft substantially equivalent to the weight of said counterweight, thereby to preload said jack,  
 means limiting downward movement of said shaft under said resilient urging, and,  
 means mounting said shaft for limited upward movement relative to said frame against the preload downward force of said resilient means,  
 whereby 1 when said boom is positioned over said frame opposite end remote from said counterweight and a load is lifted thereover, deflecting motion of said frame on said ground-engaging support means toward said load elevates said counterweight at said one end and said jack foot plate thereat limited against further downward movement so that said counterweight aids in counterbalancing the load, and,  
 (2) when said boom is positioned over said frame one end adjacent said counterweight and a load is lifted thereover, deflecting motion of said frame toward said load forces said ground-proximate shaft foot plate firmly against the ground with said resilient preload substantially counteracting said counter&#39; weight so that said counterweight does not contribute to the effective unbalance load weight on said primary ground-engaging support means and the stability of the machine is thus not diminished by the counterweight presence.  
 to. The machine of claim 9 wherein the connection of said foot to the lowermost end of the shaft is selfaligning so as to automatically adjust to the ground surface with which it is in contact.  
 II. The machine of claim 9 wherein said one end of said carrier frame to which said counterweight and we loaded jack are attached is the forward end of the carrier frame of the machine to primarily improve the stability of the machine when the load handled is over the forward end of the machine.  
  12. The machine of claim 9 wherein said one end of said carrier frame to which said counterweight and preloaded jack are attached is the rear end of the carrier frame of the machine to primarily improve the stability of the machine when the load handled is over the rear end of the machine.  
  13. The machine of claim 9 wherein said one end to which said counterweight and jack are attached is the forward end of the carrier frame of the machine, and,  
 a second counterweight and a second preloaded jack therefor are attached to said opposite end of the carrier frame of the machine whereby the combined actions of the two counterweights and preloaded jacks improve the stability of the machine when the load is handled over either end of the machine.  
  14. The machine of claim 9 wherein said resilient means comprises a compression spring cooperatively associated between said jack shaft and said frame and wherein said spring is preloaded to a value at least equal to the weight of said counterweight whereby the shaft is prevented from upward movement relative to said frame until the load applied thereto exceeds the value of the preloading of the spring.