Impact tool with hydraulic cocking mechanism
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to an impact device for delivering blows to a working tool, and in particular to an improved hydraulic cocking mechanism for an impact tool. 
     2. Description of the Prior Art 
     Many types of impact tools for mining and breaking up concrete and the like are known. These devices have essentially an energy storage means such as a coil spring or gas chamber, a hammer, and a working tool. The energy storage means when compressed, causes the hammer to accelerate to the working tool to deliver a blow. 
     Various mechanisms are used to automatically recock the hammer to repeat the cycle and to release the hammer for the blow. Some are mechanical, using a rotating cam and cam follower. Others are hydraulic. The hydraulic types usually have a piston attached to the hammer for urging it upward to compress the energy storage device. Some have external control valves to alternately supply fluid to the piston chamber to cycle the hammer. Others have internal valves for automatically cycling the fluid to the piston chamber. 
     Some of the internal valve types have a solid piston, such as shown in U.S. Pat. No. 2,559,478. However, this requires that the piston push the chamber fluid out an exhaust port on the downstoke. Appreciable drag may result. Others of the internal valve type avoid this by having a piston made up of two separate components. For example, in U.S. Pat. No. 3,687,008, the shaft has an annular stop fixed to it that is smaller in diameter than the compression chamber. On the upstroke, the annular stop contacts a sleeve, which combines with the annular stop to define a piston. At the top of the stroke, the annular stop and sleeve separate with the annular stop and shaft going to impact. Since the annular stop is smaller in diameter than the chamber, it does not have to push all of the fluid below it out an exhaust port on the downstroke. At the bottom of the stroke, the sleeve is reseated with the annular stop. On the upstroke, a resistance or bias has to be applied to the sleeve, otherwise pressure below it would push it off of its seat with the annular stop. Also, some pressure from above has to be applied after the downstroke begins in order to push the sleeve back into seating contact with the annular stop. The device of U.S. Pat. No. 3,687,008 accomplishes this by placing a port at the top of the compression chamber with a fairly large back pressure. 
     U.S. Pat. No. 3,866,690 avoids having to bias the sliding sleeve into contact with the shaft annular stop by placing the sleeve below the annular stop. On the upstroke, the sleeve pushes the annular stop upward. At the top of the stroke, the sleeve must be returned to starting position first, then the annular stop is forced to impact by the gas spring. 
     U.S. Pat. No. 3,792,738 avoids having to push the piston chamber fluid out on the downstroke by using a piston with a central passage through it. A cylindrical valve above it is biased into contact with the piston passage to seal the passage on the upstroke. At the top of the stroke, the valve and piston separate opening the passage. This reduces the hydraulic pressure below the piston to allow the spring to push it downward. On the downstroke, fluid in the piston chamber passes through the central passage. The valve is biased and returned by the same hydraulic input pressures that drive the piston upward. The valve&#39;s pressure area is smaller than the piston, however, to provide a net upward force for cocking. 
     While these proposals may be suitable, improvements to an internally-valved hydraulic cocking mechanism are desired. In particular, means to absorb sealing surface shock as the annular stop and sleeve reseat is desirable. 
     SUMMARY OF THE INVENTION 
     It is the general object of this invention to provide an improved impact tool. 
     It is the further object of this invention to provide an improved hydraulic cocking mechanism for an impact tool. 
     It is the further object of this invention to provide an improved hydraulic cocking mechanism for an impact tool with internal valves for automatic cycling. 
     In accordance with these objects, a hydraulic cocking mechanism for an impact tool is provided that includes a housing having compression and bias chambers. A shaft with a fixed annular stop is carried in the compression chamber. A compression sleeve is carried in the compresssion chamber. It is located above the annular stop and has a lower seat for contacting the annular stop to define a piston. A bias member is reciprocally carried in the bias chamber. The bias member extends to the compression sleeve to maintain it in contact with the annular stop on the upstroke. Input fluid pressure is applied both below the piston and to the bias chamber. The areas of the piston and bias member are selected to provide a net upward force. Ports allow the compression sleeve and annular stop to separate at the top of the stroke and to reseat after impact. The annular stop includes an annular member connected with the shaft and a metal ring secured to the annular member to buffer the shock on reseating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical sectional view of an impact tool having a hydraulic cocking mechanism constructed in accordance with this invention. 
     FIG. 2 is an enlarged vertical sectional view of the hydraulic cocking mechanism of FIG. 1, as shown in the position beginning immediately prior to the upstroke. 
     FIG. 3 is a view similar to FIG. 2, with the mechanism shown at the top stroke position. 
     FIG. 4 is a view similar to FIG. 2, with the mechanism shown in the downstroke. 
     FIG. 5 is a view similar to FIG. 2, with the mechanism shown in a position after impact but before the compression sleeve has completed its downstroke. 
     FIG. 6 is a partial enlarged view of the mechanism as shown in its position in FIG. 2. 
     FIG. 7 is a partial enlarged view of the mechanism as shown in its position in either FIG. 2 or FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the impact tool includes an enclosure 11 within which a striker or hammer 13 is reciprocally carried. A working tool 15 is reciprocally carried within an anvil 17 at the bottom of the enclosure 11. Anvil 17 is mounted in the enclosure by a buffer spring 19. Anvil 17 and buffer spring 19 absorb blows from the hammer if the working tool 15 is not in contact with the workpiece or if it breaks through the workpiece. A coil spring 21 is compressed between the top of the hammer 13 and the top 23 of enclosure 11. The shaft 25 is connected to the hammer 13 and extends upward through the top 23. Coil spring 21 serves as energy storage means to accelerate the hammer 13 to deliver a blow to the working tool 15 when the coil spring is compressed and the hammer released. 
     The hydraulic cocking means assembly 27 is bolted to the top 23 of enclosure 11. It includes a housing 29 mounted to the top of enclosure 11 and extending axially upward. A shaft 31 is reciprocally carried in housing 29. Shaft 31 which is connected to shaft 25 by pin 33, is urged downward by the energy storage means. The terms &#34;downward&#34; and &#34;upward&#34; are used herein with reference to the stroking movement of the hammer. The tool is operated in many orientations other than truly vertical. 
     In FIGS. 2 through 7, minor construction details have been eliminated to more clearly illustrate the hydraulic cocking mechanism. Referring to FIGS. 2 through 5, the housing 29 includes a bias or upper chamber 35 and a compression or lower chamber 37. Both chambers are cylindrical. The compression chamber 37 is larger in diameter than the bias chamber 35 and has an upper annular end or latching surface 37a. The bias chamber extends upwardly from the latching surface 37a. 
     The bias chamber 35 has a housing upper inlet port 39 and a housing lower inlet port 41. Ports 39 and 41 are spaced apart vertically and are joined by two vertical passages 43 (only one shown) in housing 29. Port 39 is connected to a pump (not shown) which supplies a direct flow of hydraulic fluid. The compression chamber 37 has an exhaust or outlet port 45. Port 45 leads without restriction or regulation to the return of the pump. Port 45 is located on the same vertical level as the latching surface 37a. 
     Shaft 31 extends through the compression chamber 37 and bias chamber 35. Shaft 31 has an upper portion 31a and a lower portion 31b. Lower portion 31b is larger in diameter than the upper portion 31a. Shaft 31 has an internal, longitudinal or axial passage 47. A row of upper inlet ports 49 in shaft upper portion 31a extends from passage 47 into the bias chamber 35. A row of lower outlet ports 51 in shaft portion 31b extend from passage 47 into compression chamber 37. An annular band or member 53 is formed rigidly on the shaft at the intersection of the upper portion 31a with the lower portion 31b. Annular member 53 is located in the compression chamber 37 and is lesser in diameter than the compression chamber. As shown in FIG. 6, annular member 53 is solid and has an upper tapered surface 53a and a lower tapered surface 53b. The upper surface 53a increases in diameter as it proceeds downwardly to an intersection with a lower tapered surface 53b. The lower tapered surface decreases in diameter as it proceeds downwardly. The annular member 53 also has an upper annular surface 53c that is perpendicular to the axis of the tool. 
     A modified Belleville seal or ring 55 is secured to shaft 31b below annular member 53 by a retaining ring 57. Ring 55 is a frusto-conical, metal ring facing upwardly so that its upper surface is in flush, mating contact with the lower tapered surface 53b. The diameter of ring 55 is larger than the diameter of the annular member 53, but smaller than the diameter of the compression chamber 37. As shown in FIG. 6, the Belleville ring 55 is modified in that its thickness increases from the inner diameter outward, with the outer thickness slightly more than twice the thickness at the inner edge. 
     A compression or lower sleeve 59 is reciprocally carried in compression chamber 37. Compression sleeve 59 is cylindrical, with a periphery in sliding and sealing contact with the compression chamber wall. The compression sleeve 59 is axially movable with respect to shaft 31. Resilient bands or seals 61 are secured in the outer wall of sleeve 59. Compression sleeve 59 has an upper latching surface or shoulder 59a adapted to contact the latching surface 37a of the compression chamber. As shown in FIG. 6, compression sleeve 59 has a lower tapered surface 59b. A lower seat 59c joins the tapered surface 59b on its periphery. Lower seat 59c is perpendicular to the axis of the tool. The compression sleeve 59 has an inner annular surface 59d perpendicular to the axis of the tool. Lower seat 59c is adapted to contact the ring 55 before the surface 59d contacts the annular member surface 53c. Downward deflection of the ring occurs on impact, allowing surface 59d to bump or strike surfaces 53c. After initial impact, and on the upstroke, surfaces 53c and 59d separate, leaving a clearance. Tapered surfaces 59b and 53a do not contact each other at any time. The ring 55 serves not only as a seal but as resilient means to absorb some of the shock of contact when the compression sleeve 59 reseats. The deflection of ring 55 reduces damage to the sealing surfaces. Annular member 53 and ring 55 define an annular stop which combines with the compression sleeve 59 on the upstroke to form a piston. 
     Compression sleeve 59 is cup shaped, with a large diameter central opening 63. Twelve longitudinal or reseat passages 64 extend from the bottom of openings 63 to the lower tapered surface 59b, completing a passage from top to bottom of the compression sleeve. 
     A bias or upper sleeve 65 is reciprocally carried on upper shaft portion 31a in sliding and sealing contact. Bias sleeve 65 is cylindrical with an upper portion 65a in sliding and sealing contact with the bias chamber 35 wall. Bias sleeve 65 has a relieved or lower portion 65b that is of lesser diameter than upper portion 65a. This lower portion fits within the central opening 63 of the compression sleeve 59, bearing against the bottom of opening 63 adjacent reseat passages 64. The bias sleeve portion 65b is of lesser diameter than the central opening 63, communicating fluid above the compression sleeve with the reseat passages 64. Bias sleeve 65 moves in unison with compression sleeve 59 at all times, and could be constructed integrally with it if so desired. The upper edge 65c of the bias sleeve defines a pressure area to be acted on by hydraulic fluid pressure in bias chamber 35. This allows the bias sleeve to serve as bias means to urge the compression sleeve 59 into contact with the ring 55. 
     As shown in FIG. 7, a radiused recess 67 is formed on the inner upper edge of the bias sleeve 65. Shaft upper ports 49 are located in a reduced cylindrical portion 69. The length of bias sleeve 65 and the diameter of the portion 69 are selected to provide a clearance, or a seat orifice indicated as 71, to allow fluid to pass from the bias chamber into longitudinal passage 47. The pressure drop across the seat orifice 71 assists in urging sleeve 59 into contact with ring 55. 
     In brief summary of the operation, FIG. 2 shows the components at the moment the compression sleeve 59 contacts the Belleville ring 55 after impact. Hydraulic pressure at port 39 acts on upper edge 65c of the bias sleeve, urging the compression sleeve 59 into seating contact with the ring 55. Once seated, the combinationn of the ring 55 and compression sleeve 59 defines a piston in compression chamber 37. At the same time, fluid pressure from port 39 forces fluid through orifice 71 and out shaft lower ports 51 into compression chamber 37. Due to the difference in pressure areas of the bias sleeve upper edge 65c and the piston, the net force is upward, compressing power spring 21. 
     At the top of the strke, as shown in FIG. 3, the compression sleeve 59 contacts the upper annular end or latch surface 37a of the compression chamber and closes the exhaust port 45. At this time, the relieved area 65b of the bias sleeve opens housing lower port 41 to the compression chamber above the piston. This reduces the difference in pressure above and below the piston, allowing the power spring 21 to push the hammer 13 to impact. Fluid in the compression chamber passes around the periphery of ring 55 as the shaft moves downward. 
     The compression sleeve 59 remains momentarily latched to the latch surface 37a, after the shaft 31 starts downward as shown in FIG. 4. However, downward movement of the shaft 31 with different diameters tends to create a suction in chamber 37, causing the compression sleeve 59 to break loose before hanmer impact and move downward. This closes lower housing port 41, as shown in FIG. 5. Then, pressure at upper housing port 39 urges the bias sleeve 65 downward, causing the compression sleeve 59 to sealingly contact the ring 55 after hammer impact. At the time of sealing, the upper edge 65c of the bias sleeve exposes the upper shaft ports 49 to fluid pressure through orifice 71. This causes the cycle to repeat, these cycles occurring approximately ten per second. 
     In order to determine the various parameters to achieve the cycling, the steps of the cycle will be considered in more detail, assuming that: 
     D 1  =diameter of lower shaft portion 31b 
     D 2  =outer diameter of ring 55 
     D 3  =outer diameter of compression sleeve 59 
     D 4  =diameter of upper shaft porion 31a 
     D 5  =outer diameter of bias sleeve upper portion 65a 
     D 6  =&#34;average&#34; or &#34;mean&#34; diameter of the latching surface 59a 
     P in  =hydraulic fluid pressure at port 39 
     P lift  =hydraulic fluid pressure below the annular member 53, when seated with compression sleeve 59 
     P latch  =pressure in central opening 63 
     F=power spring load at the top of the stroke 
     P in  =input flow at port 39 
     Q separation  =flow through port 41 at the top of stroke 
     Q reseat  =flow through passages 64 in compression sleeve 59 while it moves downward 
     A reseat  =area of passages 64 
     Initially, the various diameters, pressure and flow rates are selected so that to start the cocking from a dead stop, the input fluid Q in  will force bias sleeve 65 and compression sleeve 59 to seat on the ring 55. The size of the seat orifice 71 is selected to cause a pressure drop so that P in  is greater than P lift . These pressures and the pressure areas they act on keep the compression sleeve 59 seated on the ring 55 during cocking. As pressure P lift  builds up with input fluid passing through shaft passage 47 and out the lower ports 51, the downward force on the bias sleeve 65 is greater than the upward force caused by P lift  acting on the pressure area defined by the compression chamber diameter less the ring 55 diameter, i.e.: 
     
         .sup.P lift&lt;P.sub.in 
    
     
         (D.sub.3.sup.2 -D.sub.2.sup.2)&gt;(D.sub.5.sup.2 -D.sub.4.sup.2) 
    
     The closing of the compression sleeve 59 on ring 55 defines a piston in compression chamber 37 and starts the cocking stroke. The lower end now has a larger pressure or hydraulic area than the upper end, and the shaft 31 and sleeves 59, 65 move upward as follows: 
     
         P.sub.lift (D.sub.3.sup.2 -D.sub.1.sup.2)&gt;P.sub.in (D.sub.5.sup.2 -D.sub.4.sup.2) 
    
     The power spring 21 is compressed until the compression sleeve latch surface 59a strikes the compression chamber latch surface 37a as shown in FIG. 3. A metal to metal face seal is formed between the latch surfaces. The momentum in the moving components causes deflection of ring 55 upon contact of compression sleeve 59 with latch surface 37a. The pressure P latch , acting on the latch surface 59a builds up due to part of the input flow Q separation . The deflection of ring 55 adds to the pressure P latch . As the pressure P latch  increases, the hydraulic lifting power of the actuating mechanism diminishes to a point where it can no longer support the power spring load, i.e.: 
     
         P.sub.lift π/4 (D.sub.2.sup.2 -D.sub.1.sup.2)-P.sub.latch π/4 (D.sub.2.sup.2 31 D.sub.4.sup.2)&lt;F 
    
     The power spring drives the shaft and hammer to impact as shown in FIGS. 4 and 5. The impact stroke of the hammer 13 is slightly impeded by a drag force created by fluid flowing around the ring 55. Keeping the area D 3  -D 2  as large as possible minimizes this source of blow energy loss. 
     The compression sleeve 59 remains latched to the latch surface 37a, as shown in FIG. 4, for a short period of time after separation because: 
     
         (P.sub.in -P.sub.latch) (D.sub.5.sup.2 -D.sub.4.sup.2)&lt;P.sub.latch (D.sub.3.sup.2 -D.sub.6.sup.2) 
    
     As the shaft lower portion 3lb moves out of the compression chamber 37, the volume it occupied at the top of the stroke cannot be completely filled since the upper portion 31a is of lesser diameter. Since the exhaust port 45 is closed, a vacuum will be drawn in the chamber 37 unless additional fluid is supplied to fill the void being created. Fluid is being added to the chamber through relieved portion 65b, however this flow, Q separation , is selected to be insufficient to fill the void being created by selecting the sizes of vertical passages 43. The flow into chamber 37 from passages 43 is selected as follows: 
     
         Q.sub.separation &lt;C (D.sub.1.sup.2 -D.sub.4.sup.2) V, 
    
     where C is a constant, and V shaft impact velocity. 
     This vacuum delatching provides a point at which P latch  can no longer keep the compression sleeve 59 latched before impact. The compression sleeve moves downward, as shown in FIG. 5. The lower housing inlet port 41 is closed by the bias sleeve 65 and the input flow Q in  drives the sleeves 59 and 65 to seat on ring 55. The vacuum delatching serves as means to move the bias sleeve downward after the shaft has commenced its downstroke to close the lower housing inlet port. 
     The reseat passages 64 in compression sleeve 59 form a passage through which fluid below the compression sleeve must pass for the lower seat 59c to reseat. This flow, Q reseat , is determined by: ##EQU1## 
     P lift , prior to starting another cycle, should be raised to 200--600 psi. This reduces the shock of supplying high pressure input fluid to an empty chamber 37 when the upper shaft ports 49 are open to port 39. This is accomplished by adjusting the size of the passages 64, i.e.: 
     
         (Q.sub.reseat/ 24 A.sub.reseat).sup.2 
    
     should be in the 200--600 psi range 
     Upon recontact of compression sleeve 59 with ring 55, the ring 55 will initially deflect. The diameter of ring 55 is selected so that ring 55 will not contact the wall of compression chamber 37 under maximum deflection. 
     In summary, the spacing of the shaft ports, the housing ports, the bias sleeve, and the passages through the compression sleeve, serve as means for supplying hydraulic fluid pressure to the bias and compression chamber to move the shaft upward, then reducing the difference between the pressures above and below the piston to allow the energy storage means to accelerate the piston downward, for separating the sleeve from the annular stop during the downstroke, and for reseating the sleeve on the annular stop after impact for recocking. 
     One impact tool constructed in accordance with this invention uses a spring that requires a force of 7,000 pounds to compress it at the top of the stroke. The tool delivers 250-600 blows per minute. The flow, Q in , is 12 to 28 gallons per minute for 21-22 blows per gallon. The input pressure, P in , averages 1500 to 1600 pounds per square inch. The following dimensions are used: 
     D 1  =2.250 inches 
     D 2  =3.521 inches 
     D 3  =3.750 inches 
     D 4  =2.000 inches 
     D 5  =2.500 inches 
     D 6  =2.687 inches 
     Passages 43 are 0.045 to 0.055 inches in diameter 
     Passages 64 are 3/16 inches in diameter 
     The clearance on the upstroke between annular member surface 53c and compression sleeve surface 59d is in the range from 0.015 inch to 0.040 inch. The ring 55 is preferably beryllium copper of thickness 0.033 inch at the inner diameter and 0.078 inch at the outer diameter. It should be apparent that an invention having significant advantages have been provided. The hydraulic cocking mechanism provides stroking for high energy impact tools. It performs cocking and releasing of the hammer automatically, without external valves, controls, or internal springs other than the energy storage means. The separate sleeve and annular stop define a piston on the upstroke that separates for impact, thus the piston does not have to push fluid out an exhaust port on the downstroke. The sleeve is located above the annular stop, and does not have to be returned to its original position prior to allowing the shaft to move to impact. By using input fluid to bias the sleeve into contact with the shaft annular stop, mechanical springs or restricted exhaust ports are not required. The exhaust valve is closed on impact avoiding surges of fluid out the exhaust port. The Belleville ring reduces the sealing surface shock of the compression sleeve and annular stop colliding. The flexibility of the ring allows relative movement of the compression sleeve and the annular stop during reseating, and also at the top of the stroke. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit thereof. For example, energy storage means such as a gas spring could be used rather than a coil spring.