Hydraulic bolster dampening device
United States Patent [191 Schwam Apr. 1, 1975 HYDRAULIC BOLSTER DAMPENING DEVICE Inventor: Stuart A. Schwam, Broomall, Pa.  
 Railroad Dynamics Inc., Newtown Square, Pa.  
 Oct. 26, 1973 Assignee:  
 Filed:  
 Appl. No.:  
 References Cited UNITED STATES PATENTS 11/1937 Sproul 267/4 6/1962 Funkhouser .1 267/8 R 12/1971 Wiebe /197 DH 11/1973 Wiebe 105/197 DH Primary Examiner-M. Henson Wood, .1 r. Assistant Examiner-Howard Beltran Attorney, Agent, or Firm-Louis V. Schiavo [57] ABSTRACT The truck of a railroad car or similar vehicle is provided with a hydraulic fluid single action piston and cylinder motion damping device located in each spring group, between the spring seat and the truck bolster. The cylinder is divided by the piston head into a variable volume compression chamber under the piston head and a variable volume compensating chamber over the piston head. The device is also provided with unvalved passages by means of which hydraulic fluid is by-passed around the piston head between the variable volume compression chamber under the piston head and the variable volume compensating chamber over the piston head, and with valved passages by means of which hydraulic fluid flows back and forth between the compression chamber and a reservoir. Within the compression chamber is a purge valve by means of which air is forced out of the cylinder back into the reservoir through the valved passages in response to retraction of the piston.  
 16 Claims, 5 Drawing Figures 20 abv is&#39;z us mman H915 #874,307 33am 2 Q5 3 TRUCK BOLSTER aeo SPRING GROUP PISTON RETRAGTED msrou :xreuoeo START or :xrsussou START OF coumasswu STROKE STROKE u-----P|STON smoke a p N E x F G L 4 FORGE OUTPUT (COMPRESSION) FIG. 3.  
 PATENTED APR 1975 3. 874, 307  
 \ BODY OF VEHICLE I&#39;IIIIIII/I/I 1 SPRING SE AT OF TRUCK SIDE FRAME FIG. 5  
  1 HYDRAULIC BOLSTER DAMPENING DEVICE BACKGROUND OF THE INVENTION:  
 1. Field of the Invention This invention relates generally to devices for reducing car body roll to a tolerable level at all speeds, the purpose being to eliminate wheel lift-off and toreduce pitch and vertical motion.  
 2. Description of the Prior Art In my now abandoned application for US. Pat., Ser. No. 300,109, filed Oct. 24, 1972, there is disclosed an improved motion damping device. The device is provided with valved passages at the bottom thereof through which hydraulic fluid flows back and forth between a compression chamber and a reservoir in response to actuation of the piston. The reservoir is only partially filled with hydraulic fluid; the space in the reservoir above the hydraulic fluid is filled with air. When the device is disposed with the right end thereof uppermost, the hydraulic fluid is at the bottom of the reservoir, interposed between the air in the reservoir and said passages. As a consequence, when the piston is extended, hydraulic fluid is readily drawn from the reservoir through said passages into the compression chamber. However, when the device is tilted from the vertical sufficiently or disposed with the wrong end thereof uppermost. the hydraulic fluid and the air in the reservoir exchange places. As a consequence, the air is interposed between the hydraulic fluid in the reservoir and said passages, and when the piston is extended, air, which is highly compressible, instead of hydraulic fluid, which is not compressible, is drawn into the compression chamber. Thus, there occurs a sharp loss of efficiency. and for this reason the device disclosed in my said abandoned application for US. Pat. is not entirely satisfactory for some purposes. It is limited to use in situations where the device will always remain generally in a vertical position.  
 SUMMARY OF THE INVENTION A principal object of the present invention is to pro- I vide an improved motion damping device for the truck with the wrong end thereof uppermost is automatically expelled from said cylinder and forced back into the reservoir when the device is positioned upright again.  
  Still another object of the present invention is to provide such a device wherein air in the compression and compensating chambers is expelled therefrom and forced back into the reservoir by a few compression strokes of the piston immediately after the device is returned to its upright position.  
  An additional object of the present invention is to provide such a device wherein air in the compression and compensating chambers is expelled therefrom and forced back into the reservoir through a purge valve within the compression chamber.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section through an hydraulic motion damping device constructed in accordance with the present invention, the piston being shown in its fully extended position (full lines) and in a partially retracted position (broken lines);  
  FIG. 2 is a plan view showing a spring group supporting one end of a truck bolster and the motion clamping device of the present invention in the midst of the springs;  
  FIG. 3 is a diagram wherein compression force output, i.e., resistance to retraction of the piston, is plotted against a full stroke of the piston to illustrate how the compression force output varies with each successive stroke during the purging operation.  
  FIG. 4 is a fragmentary side elevation of a railroad car constructed in accordance with the invention; and  
  FIG. 5 is an enlarged fragmentary section on line V-V in FIG. 4.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT The following description is directed to the specific form of the invention shown in the-drawings. It is not addressed to the scope of the invention, which may be practiced in various forms.  
  A motion damping device constructed in accordance with the present invention comprises an extensible and retractable assembly working in a stationary main body assembly 12 which mounts a compression coil spring 14.  
  The stationary main body assembly 12 includes a casing 16 having an upright cylindrical wall 18 with a top which is open and a bottom which is closed by a wall 20. The upper portion of the cylindrical wall is reduced in diameter, as at 22, to provide a circumferentially extending shoulder 24. The spring 14 embraces the reduced diameter portion of the cylindrical wall, its lower end is seated upon the shoulder 24, and it is of a length to extend upwardly a substantial distance beyond the casing 16.  
  Formed integrally with the bottom wall 20 is a centr ally disposed cylindrical boss 26 over which is fitted an annulus 28 seated directly upon the bottom wall 20. The inner periphery of the annulus is fitted with an O- ring 30 embracing the boss 26. The outer periphery of the annulus is provided with a recess affording a shoulder 32 extending about a reduced diameter portion 34 of the annulus.  
  Overlying the annulus is an annular member 36 provided with a series of circumferentially spaced openings 38. At its inner periphery the member 36 is provided with a short depending cylindrical rim 40, and at its outer periphery the member 36 is provided with a longer depending cylindrical rim 42. Thus, in transverse section, the member 36 is of inverted J-shape, as shown. The rim 42 is fitted over the reduced diameter portion 34 of the annulus 28, and interposed between the rim 42 and the shoulder 32 of the annulus is an O- ring 44 embracing the reduced diameter portion 34 of the annulus 28. Thus the rim is disposed a substantial distance above the top of the annulus 28.  
  Also overlying the annulus is an annular member 46 provided with a cylindrical body 48 fitted over the member 36, and with a radially outwardly extending flange 50 seated upon the shoulder 32 of the annulus. Thus, in transverse section, the member 46 is L-shaped, as shown.  
  Fitted over the cylindrical body 48 of the member 46 is the lower end portion of a cylindrical member 52, the lower terminal edge of which is seated upon the flange 50 of the member 46. lnterposed between the members 46 and 52 is an O-ring 54 embracing the cylindrical body 48. The member 52 is of a length to extend upwardly into the portion 22 of the casing 16 for mounting a member 56.  
  The member 56 is provided centrally with a bore 58. The outer periphery of the member 56 is provided with a recess in the undersurface thereof affording a shoulder 60 extending about a reduced diameter portion 62 of the member 56, and with a recess in the upper surface thereof affording a shoulder 64 extending about a reduced diameter portion 66 of the member 56. The reduced diameter portion 62 is fitted into the upper end portion of the cylindrical member 52, and interposed between the members 56 and 52 is an O-ring 68 embracing the reduced diameter portion 62 of the member 56.  
  Overlying the reduced diameter portion 66 of the member 56 and seated thereon is a ring member 70, the under-surface of which is provided with a circumferentially extending recess 72 for accommodating an O-ring 74.  
  Slidably fitted into the reduced diameter portion 22 of the cylindrical wall 18 is an annular member 76 provided with a central bore 78. The upper surface of the member 76 is recessed, as at 79, to accommodate a wiper ring 81, L-shaped in transverse section, as shown, and an O-ring 83. The undersurface of member 76 is counterbored, as at 80 and 82, to accommodate the ring member 70, O-ring 74 and annular member 56. The outer periphery of member 76 is provided with a circumferentially extending recess which affords a shoulder 84 and a reduced diameter portion 86. lnterposed between the member 76 and the reduced diameter portion 22 of the cylindrical wall 18 is an O-ring 88.  
  Threaded into the reduced diameter portion 22 of the cylindrical wall 18 is a ring member 90 nested in the recess which affords the shoulder 84. The bottom of the member is seated on the shoulder 84. Thus members 76, 56, 52, 46 and 28 are clamped in position between the ring member 90 and the bottom wall 20 of the casing 16.  
  The extensible and retractable assembly comprises a piston rod 92 which is slidably projected through the central bore 58 of the member 56, ring member 70, wiper ring 81 and the central bore 78 of the annular member 76. Threaded onto a reduced diameter lower end portion of the rod is a piston head 94 for working in the cylindrical member 52. lnterposed between the piston head 94 and the member 52 is an O-ring 96.  
  The boss 26 is provided with a central bore 98 and counterbores 100 and 102. The counterbore 100 accommodates a plate 104, and threaded into the counterbore 102 is a member 106 which secures the plate 104 in position. Overlying the member 106 is a collar 108 provided with a radially outwardly extending flange 110 seated upon the member 106. Extending about the collar 108 and seated upon the flange 110 thereof is the lower end portion of a compression coil spring 112, which extends upwardly therefrom to the piston rod 92. The piston rod is provided with a central bore 114 for receiving a substantial length of the spring 112, which urges the piston rod upwardly and the piston head into engagement with the member 56.  
  Underlying the plate 104 is a poppet valve 116 provided with a hollow cylindrical body 118 slidably nested in the bore 98. Extending across the upper end portion of the body 118, in spaced close relation to the plate 104, is a cross-axially extending wall 120 mounting a centrally disposed tapered pin 122. At the base of the pin are a series of circumferentially spaced openings 124 extending through the wall 120. Underlying the wall 120 is a compression coil spring 126 which urges the poppet valve 116 upwardly and the body 118 into engagement with the plate 104. Thus the pin 122 is projected freely through an orifice 128 provided in the plate 104. In a plane coincident with the undersurface of the plate 104, the orifice 128 is of a diameter sufficient to provide an annular space extending about the base of the pin 122.  
  The bore 98 in the boss 26 is permanently in comm unication with the annular space provided between the cylindrical member 52 and the cylindrical wall 18 of the casing 16 through a system of passages including one or more radially extending passages 132 formed in the boss 26, a passage 134 extending about the base of the boss 26 and provided by a suitable recess in the undersurface of the annulus 28 extending about the inner&#39; periphery thereof, and a plurality of radially extending passages 136 provided by suitably grooving the undersurface of the annulus 28.  
  The annular space provided between the cylindrical member 52 and the cylindrical wall 18 of the casing 16 may be placed in communication with the interior of the cylindrical member 52 through the several passages 136, and a plurality of openings 138 circumferentially spaced about the annulus 28 and extending therethrough, there being one opening 138 for each passage 136. Overlying the annulus 28 and covering the several openings 138 is a ring plate 140 serving as a check valve. The valve is biased downwardly by a compression coil spring 142, the upper end portion of which embraces the rim 40 of the annular member 36 and seats against the undersurface of the member 36.  
  The upper surface of the piston head 94 is provided with a circumferentially extending groove 144, and extending from the groove downwardly through the piston head 94 are a series of circumferentially equally spaced passages 146.  
  Referring particularly to FIG. 1, the interior of the cylindrical member 52 affords a working chamber, and the space within the cylindrical member 52, between the annulus 28 on the one hand and the piston rod 92 and piston head 94 on the other hand, affords a compression chamber 148. When the piston head moves downwardly against the influence of the return spring 112, for example, from the extended full line position thereof shown in FIG. 1 to the partially retracted broken line position thereof shown in FIG. 1, the annular space within the cylindrical member 52 and extending about the piston rod 92, between the piston head 94 and the member 56, affords a compensating chamber 150. The annular space within the casing 16 and extending about the cylindrical member 52, between the bottom wall 20 of the casing 16 and the member 76, af-  
  The compression chamber is provided with a purge valve consisting of a pair of tubular members 154 and 156 disposed upright within the compression coil spring 112 in axial alignment with the pin 122 of the poppet valve 116 and the orifice 128 in the plate 104. The lower end portion of member 156 extends freely through the collar 108 and threads into the member 106. The upper end portion of the member 154 threads into the upper end portion of the piston rod 92. When the piston head is fully extended, as shown, the lower end portion of the member 154 remains telescoped into the upper end portion of the member 156. The inside diameter of the member 156 is somewhat greater than the outside diameter of the member 154. The upper end portion of the member 154 is provided with a pair of diametrically opposite openings 158.  
  A motion damping device constructed in accordance with the present invention may be placed in the center of a truck spring group to replace the outer coil or the inner and outer spring combination at that location. No mechanical attachment to either the side frame or bolster of the truck is required, as a consequence of which the device may be applied as simply as a truck spring. Its presence in the spring group is not apparent. Such an arrangement is shown in FIG. 2, wherein the motion damping device, designated l012, is disposed in the midst of a spring group, generally designated 160. Referring particularly to FIGS. 4 and 5, both the motion damping device and the springs of the spring group are seated upon the truck side frame, designated 162, and when the vehicle is empty, the compression coil spring 14 and the springs of the spring group 160 conjointly act to support the truck bolster, designated 164, at a predetermined level which provides a substantial distance between the upper extremity of the piston rod 92 and the truck bolster. The compression coil or retention spring 14, being partially compressed, functions to hold the motion damping device in position.  
  In the operation of the motion damping device, i.e., when the vehicle is loaded, downward movement of the truck bolster is against the influence of retention spring 14 until the upper extremity of the spring is flush with the upper extremity of the piston rod 92. Thereafter, downward movement of the truck bolster is not only against the influence of the retention spring 14 but also against the influence of the coil compression spring 112 and the resistance to flow of hydraulic fluid through the annular space extending about the base of the pin 122, in the plane of the undersurface of the plate 104.  
 As the motion damping device is compressed, the pis- I ton head 94 moves downwardly, as a consequence of which the compensating chamber 150 forms and increases in volume while at the same time the compression chamber 148 decreases in volume. Hydraulic fluid is forced from the compression chamber into the compensating chamber via the several large unvalved passages 146 in the piston head 94, but the compensating chamber is incapable oftaking all of the hydraulic fluid displaced from the compression chamber. As a consequence, the ring plate or check valve 140 being closed, the major portion of the hydraulic fluid is forced into the reservoir 152 via the purge valve, the annular space at the base ofthe pin 122 defined by the pin projecting upwardly through the orifice 128 in the plate 104, the several openings 124 in the poppet valve 116, the bore 98 and the several passages 1 32, 134 and 136.  
  When the truck bolster moves upwardly, the compression coil spring 112 acts to urge the piston rod and piston head upwardly. As the piston head approaches its initial extended position, the compensating chamber decreases in volume, while at the same time the compression chamber increases in volume. Hydraulic fluid is forced from the compensating chamber into the compression chamber via the several large unvalved passages 146 in the piston head, but the compression chamber is capable of taking more hydraulic fluid than is displaced from the compensating chamber. As a con sequence, a vacuum is developed in the compression chamber, and the check valve is raised from its seat against the influence of the compression coil spring 142. Thereupon, fluid is drawn from the reservoir 152 sufficient to fill the void in the compression chamber, the flow path being via the several passages 136 and the several large openings 138 in the annulus 28.  
  With a hydraulic system of the nature described, there is obtained a force output which is solely a function of velocity input. Unlike a spring, which absorbs energy upon compression and returns it back into the system as it extends, the motion damping device of the present invention absorbs energy from the system upon compression and dissipates it in the form of heat. None of the energy is returned to the system.  
  The motion clamping device has an overall height greater than that of the spring group when the retention spring 14 is fully extended. Internally the dimensions are such that internal bottoming is avoided. Externally the dimensions are such that the truck springs will go solid first and function as a mechanical stop preventing the device from going solid.  
  In the absence of the purge valve 154156, the motion damping device described hereinabove can be used to best advantage only in situations where the device will always remain more or less disposed with the right end thereof uppermost, i.e., as shown in FIG. 1. In this position, the hydraulic fluid in the reservoir is disposed at the bottom thereof, interposed between the air in the reservoir and the valved passages controlling the hydraulic fluid flow path between the reservoir and the compression chamber. As a consequence, when the piston head 94 is extended, hydraulic fluid is readily drawn from the reservoir 152 through said valved passages into the compression chamber 148. However, when the device is tilted from the vertical a substantial angular distance or disposed with the wrong end thereof uppermost, as when it is installed right end up on a railroad car which is then temporarily turned bodily upside down to unload the same, the hydraulic fluid and the air in the reservoir exchange places. As a consequence, the air is interposed between the hydraulic fluid and said valved passages, and when the piston head 94 is extended as the car dumps its load, air, which is highly compressible, instead of hydraulic fluid, which is not compressible, is drawn into the compression chamber 148. Accordingly, when the car is turned right side up again and the piston head 94 is retracted, the piston must compress the air in the compression chamber 148 sufficiently to bring the pressure up to a predetermined level before the device can begin to function for the purpose intended. Thus a sharp loss in efficiency occurs. Use of the purge valve 154-156 provides the device with means whereby the compression and compensating chambers are automatically purged of air by a few compression strokes of the piston, pr0- gressively when the device is disposed generally in an upright position.  
  For example, when the device is disposed with the wrong end thereof uppermost, the hydraulic fluid and the air in the reservoir exchange places. Accordingly, the hydraulic fluid which normally underlies the poppet valve 116 and fills the several passages 132, 134, 136 and 138 is replaced by air, and when the piston is extended, the air is drawn past the ring plate check valve 140 into the compression chamber. The air drawn into the compression chamber lodges next to the annulus 28 and ring plate check valve 140.  
  When the device is turned upright again, i.e., again disposed with the right end thereof uppermost, the hydraulic fluid and the air in the reservoir again exchange places. Accordingly, the hydraulic fluid returns to the bottom of the reservoir, for example, to the level A, and the air returns to the space above the level A, designated B. At the same time, the hydraulic fluid and the air in the compression chamber exchange places. Accordingly, the hydraulic fluid returns to the bottom of the compression chamber, for example, to the level C, and the air rises to the space in the bore 114 above the level C, designated D.  
  The operation of purging the air from the compression chamber normally requires several compression strokes. When the first stroke begins, movement of the piston is against the influence of compression springs 14 and 112. Hydraulic fluid from the compression chamber passes into the compensating chamber through the several openings 146 in the piston head. However, initially, there is no resistance to movement of the piston due to internal fluid pressure.  
  As the piston retracts, the air in the space D is compressed, as a consequence of which the internal fluid pressure increases until it is sufficient to actuate the poppet valve 116 against the influence ofthe compression spring 126. Continued contraction of the piston increases the internal fluid pressure still further, causing the poppet valve 116 to open still farther. As a result, air, forced into the tubular member 154 through the openings 158 at the upper end thereof, in turn forces the hydraulic fluid in the tubular members 154 and 156, below the level C, through the poppet valve 116 and passages 132, 134 and 136, back into the reservoir. At the same time, some of the air from the space D is forced back into the reservoir via the said flow path.  
  On the return stroke, i.e., when the piston is extended again after being retracted as described hereinabove, the ring plate check valve 140 is raised from its seat against the influence of compression spring 142, and hydraulic fluid is drawn from the reservoir into the compression chamber through the passages 136 and 138. The quantity of hydraulic fluid thus drawn into the compression chamber is in excess of that forced back into the reservoir during the preceding compression stroke. Thus the level C of hydraulic fluid in the compression chamber rises and the space D becomes smaller. Thus one cycle of the purging operation is completed. Each succeeding cycled is similar, cycle with each more air is forced from the compression and compensating chambers back into the reservoir. At the same time, the level C of hydraulic fluid keeps risingand the air space D becomes smaller and smaller until it is eliminated altogether.  
  Referring to FIG. 3, the several plotted curves E through M illustrate what happens during the few successive cycles necessary to complete the purging operation. Compression force output (due to internal fluid prssure) is plotted against stroke of the piston. The curve E for the first compression stroke is typical. During the initial portion of the compression stroke, i.e., until the internal fluid pressure is sufficient to actuate the poppet valve 116, there is no compression force output, as indicated by the flat portion of the curve extending from N to 0. During the next following portion of the compression stroke, i.e., while air is being forced from the compression chamber back into the reservoir, the compression force output rises rapidly, as indicated by the portion of the curve extending from O to P. Thereafter, as the piston approaches the fully retracted position thereof, the compression force output falls back to zero, as indicated by the portion of the curve extending from P to Q. During the return or extension stroke of the piston, the compression force output remains zero, as indicated by the flat portion of the curve extending from Q to N. The compression force output curves for the compression strokes that follow, designated F through M, in that order, are generally similar to the curve E. However, since some air is expelled from the compression chamber and forced back into the reservoir by each compression stroke, the next following stroke reaches its maximum compression force output sooner, i.e., the distance from N to 0 becomes progressively shorter with each succeeding stroke until the point 0 finally coincides with the point N (see curve M), at which time the compresion and compensating chambers have been fully purged of air.  
  While in accordance with the provisions of the patent statutes, I have illustrated and described the best form or embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the structure described without departing from the spirit and scope of the invention set forth in the appended claims. For example, the tubular member 154, instead of fitting loosely into the tubular member 156, i.e., instead of the OD. of the member 154 being substantially less than the ID. of the member 156, may be provided with a sliding fit into the member 156 and an O-ring on the lower extremity thereof. In addition, if desired, a compression coil spring may be housed within the member 156 to react between the lower extremity of the member 154 and the plate 104 in aid of the compression coil spring 112. Furthermore, as noted hereinabove, the lower end portion of the tubular member 156 extends freely through the collar 108 and threads into the member 106, which in turn threads into the boss 26, and, as shown and described, the member 106 and collar 108 are separate members. However, they may be made integral, and the details thereof may follow the teaching of the corresponding part shown and described in my now abandoned application for US. Pat., Ser. No. 300,109, filed Oct. 24,1972, and/or my pending application for US. Pat., Ser. No. 388,785, filed Aug. 16, 1973.  
 What is claimed is:  
  l. [n a motion damping device, the combination comprising A. a casing,  
 B. a member within said casing defining a working chamber and conjointly with said casing defining a reservoir extending about said member,  
 C. a piston including l. a head operating in said working chamber and dividing the same into variable volume compression and compensating chambers, and  
 2. a rod extending upwardly from said head and through the top of said casing,  
 D. spring means yieldably biasing said piston head upwardly to extend said piston rod,  
 E. hydraulic fluid normally filling said compression and compensating chambers and partially filling said reservoir,  
 F. means valved for passage of said fluid from said compression chamber to said reservoir when said piston is retracted against the influence of said spring means and for passage of said fluid from said reservoir to said compression chamber when said piston is extended by said spring means,  
 G. purging means in said compression chamber operable in response to retraction of said piston for expelling air from said compression chamber and forcing it into said reservoir, and  
 H. a compression coil spring seated upon said casing and extending upwardly therefrom a substantial distance above the upper extremity of said piston rod.  
  2. The combination according to claim 1 wherein the pruging means comprises a first tubular member extending upwardly from the bottom of the compression chamber, a second tubular member depending from the top of said compression chamber and extending downwardly into the upper end of said first tubular member. and means providing for free passage of fluid from said compression chamber into said second tubular member at the upper extremity thereof.  
  3. The combination according to claim 2 wherein the compression coil spring extends about the tubular members and upwardly from the bottom of the compression chamber, through the piston head and into a hollow in the lower end portion of the piston rod, and the second tubular member is affixed by its upper extremity to the upper end portion of said piston rod and provided with an opening through the wall thereof close adjacent to the upper end portion of said piston rod for passage offluid from said compression chamber into the interior of said second tubular member.  
  4. The combination according to claim 3 wherein when the piston is retracted, the movements of the first tubular member into the hollow in the lower end portion of the piston rod and that of the second tubular member into said first tubular member are free and unobstructed at least for distances equal to the stroke of said piston.  
  5. The combination according to claim 4 wherein the passage for fluid from the compression chamber to the reservoir includes the interior of the purging means.  
  6. The combination according to claim 5 wherein an orifice plate for a valve at the bottom of the compression chamber and in the fluid flow path from the compression chamber to the reservoir is axially aligned with the tubular members.  
  7. The combination according to claim 6 wherein the tubular members are concentrically related, and the lower end portion of the second tubular member fits loosely into the upper end portion of the first tubular member, the inside diameter of said first tubular member substantially greater than the outside diameter of said second tubular member.  
 8. In a motion damping device, the combination comprising A. a casing provided with an upright cylindrical section, B. a cylindrical member within the upright cylindrical section of said casing, said member defining a working chamber and said member and easing conjointly defining a reservoir, C. a piston including 1. a head for operating in said working chamber and thereby dividing the same into a variable volume compression chamber under said piston head and a variable volume compensating chamber above said piston head, and  
 2. a rod extending upwardly from said piston head and slidably through the top of said casing, D. hydraulic fluid normally filling said compression and compensating chambers and partially filling said reservoir, E. hydraulic fluid passage means provided with a fixed area orifice plate for a valve in a flow path from said compression chamber to said reservoir when said piston is retracted, F. a compression coil spring extending upwardly from the bottom of said compression chamber, through the piston head and into a hollow in the lower end portion of the piston rod, said spring being operative to yieldably urge said piston upwardly to extend the same, G. means for purging air from said compression chamber when said piston is retracted including 1. a first tubular member axially aligned with the opening in said orifice plate and extending upwardly therefrom with said spring through said piston head into the hollow in the lower end portion of said piston rod, and  
 2. a second tubular member affixed by its upper extremity to the upper end! portion of said piston rod and provided with an opening through the wall thereof close adjacent the upper end portion of said piston rod for passage of fluid from said compression chamber into the interior of said second tubular member, said second tubular member being axially aligned with and loosely telescoped into the upper end portion of said first tubular member, the upper extremity of said first tubular member being set back from the nearest obstruction to movement thereof into the hollow in the lower end portion of said piston rod a distance at least as great as the stroke of said piston, and the lower extremity of said second tubular member being set back from the nearest obstruction to movement thereof into said first tubular member a distance at least as great as the stroke of said piston,  
 H. unvalved hydraulic fluid passage means providing for a restricted but continuous free flow of hydraulic fluid between said compression and compensating chambers in response to all movement of said piston in either direction,  
 I. means for converting said fixed area orifice to a variable area orifice and progressively increasing said area and the consequent flow of hydraulic fluid from said compression chamber to said reservoir in response to retraction of said piston and the consequent increase of hydraulic fluid pressure in said compression chamber as it decreases in volume,  
 J. valved hydraulic fluid passage means, the valve being operable for checking the flow of hydraulic fluid from said compression chamber to said reservoir through the associated passage means when the compression chamber is decreasing in volume, and being operable for opening and thereby permitting a free flow of hydraulic fluid from said reservoir to said compression chamber through the associated passage means when the compression chamber is increasing in volume, and  
 K. a compression coil spring seated upon said casing and extending upwardly therefrom a substantial distance above the upper extremity of said piston rod.  
 9. In a railroad vehicle, the combination comprising A. a vehicle body,  
 B. a pair of trucks respectively at opposite ends of said body, each including 1. a pair of wheeled frames,  
 2. a bolster, and  
 3. compression coil spring groups each carried by one of said frames and supporting the associated end of said bolster, and  
 C. unitary motion damping means each associated with one of said spring groups including 1. a casing seated upon the associated one of said frames, 2. a member within said casing defining a working chamber and conjointly with said casing defining a reservoir extending about said member, 3. a piston including a. a head operating in said working chamber and dividing the same into variable volume compression and compensating chambers, and  
 b. a rod extending upwardly from said head and through the top of said casing,  
 4. spring means yieldably biasing said piston head upwardly to extend said piston rod,  
 5. hydraulic fluid normally filling said compression and compensating chambers and partially filling said reservoir,  
 6. means valved for passage of said fluid from said compression chamber to said reservoir when said piston is retracted against the influence of said spring means and for passage of said fluid from said reservoir to said compression chamber when said piston is extended by said spring means,  
 7. purging means in said compression chamber operable in response to retraction of said piston for expelling air from said compression chamber and forcing it into said reservoir, and  
 8. a compression coil spring seated upon said casing and extending upwardly therefrom a substantial distance above the upper extremity of said piston rod and coacting with the associated spring group to support the overlying end of said bolster.  
  10. The combination according to claim 9 wherein the purging means comprises a first tubular member extending upwardly from the bottom of the compression chamber, a second tubular member depending from thetop of said compression chamber and extending downwardly into the upper end of said first tubular member, and means providing for free passage of fluid from said compression chamber into said second tubular member at the upper extremity thereof.  
  11. The combination according to claim 10 wherein the compression coil spring extends about the tubular members and upwardly from the bottom of the compression chamber, through the piston head and into a hollow in the lower end portion of the piston rod, and the second tubular member is affixed by its upper extremity to the upper end portion of said piston rod and provided with an opening through the wall thereof close adjacent to the upper end portion of said piston rod for passage of fluid from said compression chamber into the interior of said second tubular member.  
  12. The combination according to claim 11 wherein when the piston is retracted, the movements of the first tubular member into the hollow in the lower end portion of the piston rod and those of the second tubular member into said first tubular member are free and unobstructed at least for distances equal to the stroke of said piston.  
  13. The combination according to claim 12 wherein the passage for fluid from the compression chamber to the reservoir includes the interior of the purging means.  
  14. The combination according to claim 13 wherein an orifice plate for a valve at the bottom of the compression chamber and in the fluid flow path from the&#39; compression chamber to the reservoir is axially aligned with the tubular members.  
  15. The combination according to claim 14 wherein the tubular members are concentrically related, and the lower end portion of the second tubular member fits loosely into the upper end portion ofthe first tubular member, the inside diameter of said first tubular member being substantially greater than the outside diameter of said second tubular member.  
 16. In a railroad vehicle, the combination comprising A. a vehicle body,  
 B. a pair of trucks respectively at opposite ends of said body, each including 1. a pair of wheeled frames, 2. a bolster, and 3. compression coil spring groups each carried by one of said frames and supporting the associated end of said bolster, and C. unitary motion damping means each associated with one of said spring groups including 1. a casing provided with an upright cylindrical section and seated upon the associated one of said frames, 2. a cylindrical member within the upright cylindrical section of said casing, said member defining a working chamber and said member and casing conjointly defining a reservoir, 3. a piston including a. a head for operating in said working chamber and thereby dividing the same into a variable volume compression chamber under said piston head and a variable volume compensating chamber above said piston head, and  
 b. a rod extending upwardly from said piston head and slidably through the top of said casmg,  
 4. hydraulic fluid normally filling said compression and compensating chambers and partially filling said reservoir,  
 5. hydraulic fluid passage means provided with a fixed area orifice plate for a valve in a flow path from said compression chamber to said reservoir when said piston is retracted,  
 6. a compression coil spring extending upwardly 7, means for purging air from said compression chamber when said piston is retracted including a. a first tubular member axially aligned with the opening in said orifice plate and extending upwardly therefrom with said spring through said piston head into the hollow in the lower end portion of said piston rod, and  
 b. a second tubular member affixed by its upper extremity to the upper end portion of said piston rod and provided with an opening through the wall thereof close adjacent the upper end portion of said piston rod for passage of fluid from said compression chamber into the interior of said second tubular member, said second tubular member being axially aligned with and loosely telescoped into the upper end portion of said first tubular member, the upper extremity of said first tubular member being set back from the nearest obstruction to movement thereof into the hollow in the lower end portion of said piston rod a distance at least as great as the stroke of said piston, and the lower extremity of said second tubular member being set back from the nearest obstruction to movement thereof into said first tubular member a distance at least as great as the stroke of said piston,  
 8. unvalved hydraulic fluid passage emans providing for a restricted but continuous free flow of hydraulic fluid between said compression and compensating chambers in response to all movement of said piston in either direction,  
 9. means for converting said fixed area orifice to a variable area orifice and progressively increasing said area and the consequent flow of hydraulic fluid from said compression chamber to said reservoir in response to retraction of said piston and the consequent increase in hydraulic fluid pressure in said compression chamber as it decreases in volume,  
 10. valved hydraulic fluid passage means, the valve ll. a compression coil spring seated upon said casing and extending upwardly therefrom a substantial distance above the upper extremity of said piston rod and coacting with the associated spring group to support the overlying end of said UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,874,307 Dated APRIL 1, 1975 Inventor(s) STUART A. SCHWAM It is certified that error app ears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 COLUMN 7, line 61, after &#34;similar,&#34; change &#34;cycle&#34; to &#34;and-w comm 9, CLAIM 7, line 6, after &#34;ber&#34;and before &#34;substantially&#34; insert --b&#39;eing--.  
 COLUMN l4, CIAIM 16, line 3, change &#34;emans&#34; to --means-.  
 Signed and Scaledl this thirtieth ,D ay Of September I 9 75 [SEAL] A tresr:  
 RUTH C. MASON (&#39;ummissimzvr oj&#39;lurents and Trademarks