US3542212A - Double acting hydraulic cushioning device - Google Patents

Description

United States Patent [72] inventor David W. Daugherty,Jr 3,164,263 1/1965 Novikov et a1 213/43X Downers Grove, Illinois 3,175,699 3/1965 Price et a1........ 213/43 [21] AppLNo; 718,551 3,220,561 11/1965 Blake 213/8 [22] Filed Aprll3, 1968 3,275,164 9/1966 Peterson... 213/43 [45] Patented Nov. 24,1970 3,368,698 2/1968 Cardwe1l.. 213/43 [73] Assignee Cardwell Westinghouse Company 3,412,870 11/1968 Rollins 213/8 8 corporatmn of Delaware Primary Examiner-Drayton E. Hoffman Aztorney- Mann, Brown, McWilliams & Bradway [54] DOUBLE ACTING HYDRAULIC CUSHIONING DEVICE 5 Claimsg Drawing Figs ABSTRACT: A two-way hydraulic unit particularly adapted for use as a cushioning device when mounted on the end of a [52] US. Cl 213/43; railmad cat in which a ram is mounted in a closed cylinder 188/88; 267/64 fixed to the underside of the car, a slidable box element im- [51] Int. Cl. B6lg 9/02, poses b h buff and draft on the ram mounted in the cylinder 8 9/02 8 9/06 8 9/12 and a metering pin mounted in the ram controls the flow of [50] Field otSearch 213/8, 43; hydraulic fluid from the high pressure side of the cyunder to 267/64 65; 188/88- 88503 965 the low pressure chamber during both buff and draft. An ex- 56 R f Cit d pansible storage chamber is provided into which fluid can flow l e erences e through a double acting check valve from the low pressure UNITED STATES PATENTS side only of the cylinder. The unit is provided with springs and 1,282,712 10/1918 Barrell 188/88.503X spring retaining canisters and bolts at the four corners to 3,150,783 9/1964 Campbell et a] 213/43 return the unit to its neutral position.

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17 V//////////7///// L' I T- m 33 :25 175 mm 1 J9 V/////////////Ms Lk\\ J 1 Q\\\ 39 107 p as 1 IIIIIIIIIIIIIIIIIIIII I, 7 ti P at'ent ed Nov. 24, 1970 Sheet MW m MN AWN x 2% jw% Puhid Na. 24, 1970 Sheet Patented Nov. 24, 1970 Sheet "fa M DOUBLE ACTING HYDRAULIC CUSHIONING DEVICE BRIEF SUMMARY OF THE INVENTION Hydraulic units suitable for mounting on the end of railroad cars to absorb impacts are well known. Several of such units are shown and described in U.S. Pats. Nos. 3,150,782 and 3,368,698. These units are one-way shock absorbers designed to act as such only when the unit is being compressed, and do not absorb energy when the unit is being expanded or returned to neutral position. The present invention is a two-way unit adapted to absorb shock both when the unit is being compressed and when being expanded. In order to enable the present invention to act as ajtwo-way hydraulic unit, as compared to those of the above-mentioned patents, the hydraulic cylinder is closed at the rear end instead of leaving it open so that fluid can flow directly therefrom into the expansible boot attached to the rear end of the cylinder, thereby creating two separate chambers within the cylinder, one on either side of the ram or piston. The piston or ram head is equipped with a two-way check valve so that fluidcannot flow from the high pressure side of the cylinder into the boot. The metering pin is constructed in such way that it will open due to high pressure fluid in either of the two cylinder chambers. A slidable box keyed to the coupler is adapted to exert both impact and draft or pull on the ram. Suitable spring guides and spring elements are provided to return the unit to starting or neutral position.

FIGS. 1 and 1A represent in two parts a vertical cross section of the ram and cylinder structure forming part of the invention;

FIGS. 2 and 2A represent in two parts a horizontal cross section through the center of the hydraulic unit, illustrating the invention with certain parts shown in plan view;

FIGS. 3 and 3A represent in two parts a vertical longitudinal cross section through the center of thehydraulic unit, illustrating the invention, with certain parts shown in elevational view;

FIG. 4 is a cross-sectional view, half of which is taken along the line 4-4 of FIG. 2 and theother half of which is taken along the line 4-4 of FIG. 2A,

DETAILED DESCRIPTION OF-THE INVENTION Referring to FIGS. 1-5, the numeral 1 indicates the hydraulic cylinder mounted in the center sill 3. Cylinder 1 is formed with flanges 5 and 7 in order to mount it solidly in' the sill by means of lugs 9 extending inwardly from the sill. Instead of or in addition to the lugs 9, the cylinder may be held stationary by horizontal keys passing through slots in the center sill and between flanges 5 and 7.

Slidably mounted in cylinder 1 is ram 11 formed with piston forming head 13. Ram 11 is formed with chamber 15 having an orifice ring 17 set in the outer end of chamber. 15. A retaining ring 19 holds the orifice ring 17 in place. A metering pin 21 is slidably mounted in the chamber 15 with the surfaces 23 adapted to slide against surface 25. The outer surface of the metering pin 21 is formed with a shoulder 27. The wall portion of the metering pin on the end thereof beyond the surface 23 (Le. on the left in FIG. 1) is thinner than that of the wall portion 29 on the other side of bearing surface 23, also shown on the right in FIG. 1. The ram stem 31 is bored at 33 in order to. provide a recess for the coil spring 35 which abuts at its inner end against the shim 37 and at its outer end against the inner end of the metering pin.'Coil spring 35 serves to hold the metering pin in closed position until it is subjected to sufficient fluid pressure to bias it to open position. A diagonal passageway 38 connects chamber 15 to the cylinder chamber on the left side of the ram head. A drive pin 39 is inserted in the ring 17 across the diameter thereofin order to act as a stop for the metering pin. Pin 21 and orifice ring 17 thus provide a pressure sensitive orifice arrangement.

The end 41 of cylinder 1 is closed by an annular closure member 43 held in place by retaining ring 45 threaded to the inside end of the cylinder. Sealing rings 47 are provided between the inner wall of the cylinder 1 and the outer wall of closure element 43 to prevent leakage between the two walls.

' A bronze or other suitable bearing 49 is mounted between the closure element 43 and the ram stem in order to provide a suitable surface for the ram to slide back and forth. One end of a rubber or other flexible and/or an expansible boot 51 is fastened to closure element 43 by bead ring 53. The other end of the boot is fastened to the ram stem by means of band ring 55.

A longitudinal bore 56 extends from the face of the ram head 13 to the outlet 57. The outer end of the bore 56 at the ram head is closed by a threaded pipe plug 59. A cross bore 61 connects the area 63 of chamber 15, adjacent the surface 23, with the bore 56. A threaded pipe plug 65 closes the outer end of bore 61. An oil deflector ring 67 is mounted on the ram stern opposite outlet 57 by means of drive pins 69 in order to prevent hydraulic fluid from impinging against the boot wall. The ram head is equipped with a piston ring 70 in line with a radially extending bore 71 to seal the area between the ram head and piston wall. Tapered plug 72 closes end of bore 71 and seals it by virtue of the pressure of fluid between the plug 72 and the piston ring.

Also bored in ram head 13 and intersecting bore 71 at right angles thereto is bore 73, which is connected to bore 56 through bore 71. Bore 73 communicates with the cylinder chamber on both sides of the ram head. Loosely mounted in bore 73 is valve stem 75 having at each end a valve closure element or head 77 and 78 adapted to seat in valve seats 79 and 80, respectively. The valve seats are frustroconical in contour and the valve elements have a rounded or arcuate surface adjacent the valve seats. The rounded or arcuate surface of the valve closure elements enables the valve to close tightly even when the stem 75 is somewhat deflected.

At the opposite side of the ram head there is a second valve structure indicated generally by the numeral 81. This valve comprises an orifice 83 closed by an orifice pin 85 mounted on stem 87 and held in closed position against the orifice ring 89 by means of coil spring 91. Passageways 93 are provided through the ram head in order to permit hydraulic fluid to flow from one side to the other side of the cylinder when valve structure 81 opens. The purpose of valve 81 is to prevent damage to the unit in the event of an impact creating pressure above the design pressure of the unit. Valve 81 is set to open only when the pressure applied to the ram exceeds a predetermined amount and will open to an extent commensurate with the pressure of the fluid.

' This is to be distinguished from the functioning of pin 21 and the orifice it controls, which constitute the primary means for controlling the pressure within the cylinder high pressure chamber during normal cushioning operations of the cushioning unit in buff and draft. A threaded screw plug 09 closes opening 101 in the wall of cylinder 1 through which the cylinder is filled with hydraulic fluid. Base plate 103 is threadedly mounted at its center on the end 105 of the ram stern. A passageway 107 connects the area between sealing rings 47 with the boot chamber.

Referring now more particularly to FIGS. 2, 2A, 3, 3A, 4 and 6 a spring plate 110 having a rectangular contour and a circular center opening fits over the cylinder 1 and is adapted to abut against flange 7. A sliding box 112 of rectangular contour is adapted to fit in and slide in the sill 3. Box 112 is formed at the inner end thereof with slots 114 adjacent the top and bottom thereof through which keys 115 extend and serve to attach base plate 103 to the box.

Box 112 is keyed to a coupler 117 by means of key 118 passing through a slot 119 in the center of the box and extending through a slot 120 in the coupler. Reinforcing plates 121 are welded to the inner wall of box 112 at the end of slot 119.

A follower plate 123 corresponding in cross section to the interior cross section of the box is welded to the interior wall thereof on the side of key 118 opposite from which the coupler extends out of the box. Plate 123 is adapted to abut against base plate 103 attached to the end of the ram. A spring plate 125 similar to spring plate 110 abuts against blocks 127. Blocks 127 are welded to the inside of the sliding box 112 abutting against movable spring plate 125 at the corners thereof in order to provide fastening means for the bolts about to be described and to form an abutting surface for the keys 115 on draft or pull.

Mounted between the spring plates 110 and 125 at the corners thereof, are four pairs or sets of bolts and spring retaining canisters and bolt guides. Two bolts 129 are diagonally positioned within the sill 3 and are fastened at one end to blocks 127 by being threaded into tapped holes 131 in the blocks.

Collars 132 are fastened to plate 125, concentric with bolts 129 to act as spring guides. The opposite end of bolts 129 have enlarged heads 133 welded or otherwise fastened thereto and extend into spring retaining canisters and bolt guides 135. The end of the canisters into which the bolts 129 extend have stop elements 137 welded or otherwise fastened thereto in order to prevent the bolt head from being withdrawn from the canister and to act as an abutting surface for the head 133. The other end of canisters 135 is threadedly fastened at 138 to spring plate 110. The canisters and bolts are of sufficient length to permit maximumtravel for which the unit is designed.

In the other two diagonally opposite corners are bolts 141. One end 143, of bolts 141, is threadedly fastened to the cylinder 1 by being screwed into tapped holes in the flange 7 and cylinder wall. Collars 144 are fastened to spring plate 110 concentric with the bolts to act as spring guides. The other end of bolts 141 extends into spring retaining canisters and bolt guides 145. The opposite ends 147 of canisters 145 are threadedly fastened to spring plate 125 by screwing the threaded ends of the canisters into tapped holes in the corners of the plate. Bolts 141 have enlarged heads 149 welded or otherwise fastened to the end of the bolts and the canisters 145 have stop elements 151 welded to the ends thereof in order to prevent the ends of the bolts 141 from beingwithdrawn from the canister and to act as an abutting surface for the heads 149. Coilsprings 153 surrounding the canisters and bolts are retained between plates 110 and 125. Coil springs serve to return the unit to its starting position after pressure on the unit is relieved.

Referring now to FIG. 5, there is shown a modified type of ram and metering device. Orifice ring 17' has a knife edge 155 defining the orifice opening. The head 157 ofmetering pin 21 closes the orifice opening. The orifice pin has an elongated stem 159 which extends into the recess 33 of ram stem 31. A spring collar 16]. is held in place in recess 33' by means of two drive pins 163 which fit into annular grooves in the outer wall of the collar 161 and through the wall of ram stem 31. A coil spring 165 is mounted under compression between the shoulder 167 at the end of stem 169 and the collar 161. Another coil spring 169 under compression is mounted between the collar 161 and the shoulder 171 formed at the juncture of the stem 159 metering pin head 157. The chamber is connected to the cylinder chamber by means of passageways 38'. The area of the face of head 157 is greater than that of the surface of the end 172 of the pin, thereby enabling the hydraulic pressure to overcome the action of the respective springs 165 and open the orifice in either direction.

The device shown in FIGS. 1 to 4 operates in the following manner: I

The unit is mounted in the sill at the end of a car and the sliding box is keyed to the car coupler. Upon impact of the plate 1231against base plate 103 attached to the end of the ram, the ram moves to the right in cylinder 1 building up pressure in the fluid on the right side of the ram. By reason of the fact that the cross-sectional area of the surface 173 of portion 21 of pg,7 the metering pin is greater than that of the surface 175 more pressure is exerted on surface 173 than on 175 and the pin will overcome the bias of coil spring 35 and move toward the left, opening the orifice in the orifice ring 17, thereby allowing fluid to pass into chamber 15 and thence through passageway 38 into the'low pressure or left side of the cylinder 1'. At the instant pressure is built up on the right side of the ram, the pin 75 moves to the left causing the valve closure element 77 to close, thereby preventing fluid from the high pressure side of the cylinder from flowing through the orifice 56 into the boot while allowing the fluid from the low pressure side of the cylinder to pass through the valve closure element 78 through orifice 56 into the boot. As the ram travels to the right in the cylinder, the force of impact is gradually diminished until the coil spring 35 is able to overcome the pressure of the hydraulic fluid and close the orifice. In the event of excessive impact the relief valve 81 opens and permits fluid to flow from the high pressure to the low pressure side of the cylinder, as distinguished from the pressure responsive variable orifice functioning provided by the pin 21 during normal operation of the cushioning unit. Valve 81 is set to open at design pressure, as for example impact forces above 500,000 pounds. In the event of leakage of fluid from recess 33 past surface 25 the fluid will pass through passageway 61 into passageway 56 and thereby prevent buildup of back pressure in area 63 against the metering pin. Although passageway 61 is provided, the unit will function without it since pressure in area 63 will be relieved by chamber 15.

During impacts in buff, spring retaining canisters 145 and bolts 129 move to the right together with the sliding box 112. The spring canisters 135 and bolts 141 remain stationary during impact or buff. After the force of the impact is dissipated the coil springs 153 return the unit to its starting position shown in FIGS. 2, 2A, 3 and 3A.

When the unit is subjected to pull or draft, sliding box 112 moves tothe left, pulling with it the ram by virtue of the fact that the plate 103, attached to the end of the ram, abuts against the keys 115 which in turn abut against blocks 127 welded or otherwise secured to the end of the box 112. Valve pin 75 moves to the right closing closure element 78. When the ram begins moving to the left, high pressure is created in the cylinder chamber to the left of the ram, forcing fluid through passage 38 into chamber 15 against shoulder 27 causing the metering pin to open and allowing fluid to flow through the orifice from chamber 15 into the low pressure side of the cylinder to the right of the ram. As the ram moves to the left bolts 129 also move to the left pulling canisters 135 along with them. The spring plate 110 which is screwed fast to the other end of the canisters 135 is similarly pulled to the left causing compression of springs 153. Since bolts 141 are fastened to cylinder 1, canisters 145 remain stationary together with spring plate which separates from box 112. When the force of the draft or pull is dissipated, the coil springs 153 surrounding canisters and bolts, having been compressed, return the unit to its starting position. The cylinder 1 remains stationary and as previously pointed out is held fast to the sill by means ofthe lugs 9.

The hydraulic unit and metering device shown in FIG. 5 function in the following manner:

Upon buff the ram 11' moves to the right causing metering pin 21' to open the orifice formed by the orifice ring 17'. Hydraulic fluid flow through the orifice into chamber 15' and thence through passageway 38' into the low pressure side of cylinder 1'. As the force is dissipated the coil spring 169 returns the metering pin to closed position. When draft or pull is exerted on the device, the ram 11 moves toward the left causing high pressure fluid in the left-hand side of the cylinder to flow through passageway 38' into chamber 15' causing metering pin 21 to open toward the right by virtue of the fact that the area of the shoulder 171 is greater than that of shoulder 167. The metering pin is guided by means ofthe snug fitting end 172 and the spring collar 161.

Although springs for retaining the metering pin in closed position and for returning the ram to normal position have been disclosed as being springs under compression, it is apparent that the springs can be under tension if attached at their ends to the bearing surfaces.

While the invention has been described as being particularly useful as a hydraulic unit for attachment to the end of railroad cars, it may be used in any environment where it is desired to provide cushioning effect against both impact and pull forces.

lclaim:

1. A double-acting hydraulic unit comprising;

a. a substantially closed cylinder;

b. a ram having a head reciprocably mounted in said cylinder in substantially leak-proof relationship therewith, said ram head dividing said cylinder into two chambers;

. a double-acting check valve in said ram head controlling a first passageway extending through said ram head and connecting said two chambers;

d. a second passageway connecting an intermediate point of said first passageway with an expansible hydraulic fluid storage chamber;

e. a cavity in said ram;

f. a third passageway connecting said cavity with one of said cylinder chambers;

g. an orifice at the outer end of said cavity, communicating with the other of said chambers;

h. a metering pin, subjected to fluid pressure in said two chambers, controlling said orifice;

i. resilient means for biasing said metering pin to neutra position;

j. and means to impose both buff and draft forces on said ram;

k. said double-acting valve being adapted to cut off communication between said one cylinder chamber and said expansible storage chamber and provide communication between said other cylinder chamber and said expansible storage chamber when the ram is subjected to draft; and to cut off communication between said other cylinder chamber and said expansible chamber andprovide communication between said one cylinder chamber and said expansible chamber when the ram is subjected to buff.

2. A double-acting'hydraulic unit in accordance with claim 1 in which said double-acting check valve comprise: a

stem;

n. two spaced valve seats cooperating with said closure elements; 1 o. and said second passageway connecting with an intermediate point of said first passageway between said two valve seats. 3. A double-acting hydraulic unit in accordance with claim 2 in which:

p. said stem is loosely mounted in said first passageway; and q. the surface of the valve closure element adapted to seat on the valve seats is arcuate. 4. A double-acting hydraulic unit in accordance with claim 3 in which: i

r. the valve seats are frustoconical. 5. A double-acting hydraulic unit in accordancewith claim 1 including:

s. a relief valve mounted in said ram head, said valve providing communication between the two cylinder chambers and being adapted to open only during buff when the pressures in the cylinder chamber cushioning buff forces exceed a predetermined maximum. 6. A double-acting hydraulic unit in accordance with claim 1 in which said means for imposing both buff and pull'forces on said ram comprise:

1. said ram including a stem extending to the outside of said cylinder; 1 m. a base plate mounted on the outside end of said ram stem; n. a slidable element slidable in the same direction as said ram and having a follower plate attached thereto, said follower plate being adapted to abut said base plate on the side opposite to that to which said stem is fastened;

o. a movable spring plate on the side of said base plate opposite to that which the follower plate abuts, said spring plate being independent of said slidable element;

p. a first set of oppositely disposed, parallel, elongated bolt guides and spring retaining canisters fastened at one end to said spring plate and having the other end free;

q. an elongated bolt extending into each such canister and slidable therein in a direction parallel to said ram, the end extending into the canister having an enlarged head adapted to abut against astop element adjacent the free end of the canister to prevent withdrawal of the enlarged head;

r. the other end of each said bolt being fastened to said cylinder;

s. a second set of oppositely disposed elongated spring retaining canisters and bolt guides, parallel with said firstmentioned canisters, fastened at one end tov a second movable spring plate and free at the other end;

t. a second set of elongated bolts, each having one end thereof extending into the free end of one of said second set of canisters and being slidable therein, said bolt end having an enlarged head adapted to abut against a stop element adjacent the free end of the canister in order to prevent withdrawal thereof, and the other end of each said bolt being fastened to said slidable element; and

u. spring means between said spring plates adapted to return said unit to starting position when not subjected to pull or draft.

7. A double-acting hydraulic unit comprising:

a. a cylinder;

b. a ram head snugly and slidably mounted in said cylinder;

c. a chamber in said ram communicating with the interior of said cylinder on both sides of said ram;

d. means for controlling passage of fluid from said chamber into and from the interior of said cylinder on both sides of said ram;

, e. means for preventing passage of fluid from oneside to the other side of said cylinder in either direction;

f. said ram including a stem extending to the outside of said j. a first set of oppositely disposed parallel, elongated bolt guides and spring retaining canisters fastened at one end 7 to said spring plate and having the other end free;

it. an elongated bolt extending into each such canister and slidable therein in a direction parallel to said ram, the end extending into the canister having an enlarged head adapted to abut against a stop element adjacent the free end of the canister to prevent withdrawal of the enlarged head;

l l. the other end of each said bolt being fastened to said cylinder;

m. a second set of oppositely disposed elongated spring retaining canisters and bolt guides, parallel with said firstmentioned canisters, fastened at one end to a second movable spring plate and free at the other end;

n. a second set of elongated bolts, each having one end thereof which extends into the free end of one of said second set of canisters and being slidable therein, said second set bolts end having an enlarged head adapted to abut against a stop element adjacent the free end of the canister, and the other end of said bolt being fastened to said slidable element; and

0. spring means between said spring plates adapted to return said unit to starting position when not subjected to draft or buff.

8. A double-acting hydraulic cushioning unit in accordance with claim 7 including:

p. a coupler attached to said sliding element.

9. A railroad car having a bottom center sill in which is mounted at the end thereof the double-acting hydraulic unit defined by claim 8, the cylinder of said unit being rigidly fastened to the center sill and said sliding element being slidably mounted in said center sill.

10. A double-acting hydraulic unit having:

a cylinder closed at both ends;

a slidable ram in said cylinder and including a ram head reciprocable in said cylinder and forming a variable volume chamber on either side thereof;

pressure response variable orifice means for metering flow of hydraulic fluid through said ram from the high pressure to the low pressure side of said cylinder, said means comprising a chamber formed in said ram adjacent said head thereof; a

an open ended hollow metering pin slidably mounted in said ram chamber and extending inwardly from the face of said ram head;

an orifice at the outer end of said ram chamber opening to the cylinder chamber that is formed in part by said ram face toward which one end of said pin is spring biased to normally close the orifice, with said pin one end being exposed thereto;

said pin being formed with a shoulder about its outside wall;

the cross-sectional area of said pin adjacent the orifice being greater than that of the other end of the pin whereby pressures developed in the cylinder chamber that is defined in part by said ram face on movement of the ram to contract same create differential forces acting on either end of said pin tending to move same against its spring bias to open said orifice; and

passageway through said ram connecting said ram chamber to the cylinder other chamber on the side pposite the face of said ram, said passageway connecting to said ram chamber at a locus which will subject said pin shoulder to the pressure of the fluid entering said ram chamber through said passageway from the other cylinder chamber whereby pressures developed in said other cylinder chamber on being contracted by said ram creates differential forces acting in opposite directions on said pin tending to move same'against its spring bias to open said orifice.

11. A double-acting hydraulic unit in accordance with claim 10 in which:

said shoulder is intermediate the ends of said metering pin;

said passageway opening into said ram chamber between said shoulder and said'orifice; and

said pin being adapted to open said orifice by movement in one direction only and to close it by movement in the opposite direction.

12. A double-acting hydraulic unit in accordance with claim 10 in which:

said shoulder is adjacent said one end of said pin and is can tered within said orifice;

said pin is equally biased in opposite directions by spring means acting on either end of same; and

said pin moving in either direction to open said orifice under the differential action of fluid pressures acting on said shoulder and said other end of said pin.

13. A hydraulic cushioning unit for railroad cars adapted to cushion buffand draft impacts, said unit comprising:

a cylinder;

a ram reciprocably mounted in said cylinder and having a head in said cylinder forming a variable volume chamber in said cylinder on either side of said head and a shank extending from one end of said cylinder;

a passageway extending through said ram head and between said cylinder chambers;

double-acting check valve 1 means mounted in said passa eway for controlling h draulic fluid flow theret rough and comprising a va ve member extending through said passageway and having a valve head at either end thereof each cooperating with a valve seat at the respective ends of said passageway;

pressure responsive variable orifice means in said ram head for metering the flow ofhydraulic fluid through said ram head from the high pressure side to the low pressure side of said cylinder when buff and draft impacts are occasioned;

said orifice means constituting the primary means for controlling the pressure on the high pressure side of the cylinder during normal operating conditions;

said unit including an expansible hydraulic fluid storage chamber about said ram adjacent said cylinder one end;

conduit means formed in said ram for connecting said expansible chamber with said passageway intermediate the ends of the latter; and

means for imposing buff and draft forces on said ram.

14. The cushioning unit set forth in claim 13 including:

relief valve means mounted in saidram head for limiting the pressure developed in said chambers to a predetermined amount and including means for bypassing hydraulic fluid through said head when said predetermined pressure has been reached.

15. The cushioning unit set forth in claim 13 wherein said means for imposing buff and draft impacts on said ram comprises:

a base plate mounted on the outwardly extending end of said ram shank;

a slidable element slidable in the same direction as said ram and having a follower plate attached thereto;

said follower plate being adapted to abut said base plate on the side opposite to that to which said ram shank is fastened;

a movable spring plate on theside of said base plate opposite to that which the follower plate abuts;

sai'd spring plate being independent of said slidable element;

a first set of oppositely disposed, parallel, elongated bolt guides and spring retaining canisters fastened at one end to said spring plate and having the other end free;

an elongated bolt extending into each such canister and slidable therein in a direction parallel to said ram, the end of the respective bolts extending into the canister having an enlarged head adjusted to abut against a stop element adjacent the free end of the canister to prevent withdrawal of the enlarged head;

the other end of each said bolt being fastened to said cylinder;

a second set of oppositely disposed elongated spring retaining canisters and bolt guides, parallel with said first-mentioned canisters, fastened at one end to a second movable spring plate and free at the other end;

a second set of elongated bolts, each having one end thereof extending into the free end of one of said second set of canisters and being slidable therein, said bolt ends of said second set of bolts having an enlarged head adapted to abut against a stop element adjacent the free end of the canister in order to prevent withdrawal thereof, and the other ends of each said bolt of said second set of bolts being fastened to said slidable element; and

spring means between said spring plates adapted to return said unit to starting position when not subjected to buffor draft.

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