US3150781A

US3150781A - High capacity draft gear

Info

Publication number: US3150781A
Application number: US78367A
Authority: US
Inventors: Richard J Housman; Eldred H Natschke
Original assignee: Cardwell Westinghouse Co
Current assignee: Cardwell Westinghouse Co
Priority date: 1960-12-27
Filing date: 1960-12-27
Publication date: 1964-09-29
Anticipated expiration: 1981-09-29

Description

Sept. 29, 1964 R. J. HoUsMAN ETAL 3,150,731

` HIGH CAPACITY DRAFT GEAR Filed Dec. 27; 1960 4 sheets-sheet 1 777mm, @Lawn/w77@ alli rif. VA I Sept. 29, 1964 R. J. HousMAN E'rALA HIGH CAPACITY DRAFT GEAR Filed Dec. 27, 1960 4 Sheets-Sheet 2 m, S w www m.\ Syn. Way@ 14H V\ a@ @m @A |A All- JAHWMAWM E@ -T |.W l |.\.||.|.||.l||| M W9 Humm. mw m A-m$ Q NW1 n. Nm.. Niv uw www ANA- @u Nw WN A. I A

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United States Patent 3,150,781 HIGH CAPACITY DRAFT GEAR Richard J. Housman, Dolton, and Eldred H. Natschke, Bourbonnais, Ill., assgnors to Cardwell Westinghouse Company, a corporation of Delaware Filed Dec. 27, 1960, Ser. No. '78,367 Claims. (Cl. 213-23) This invention relates to high capacity draft gears foi application in the draft pockets of railroad car frames and has for its principal object the provision of a draft gear arrangement having a positive resistance type of spring cushioning unit that is operable to cushion draft loads which are relatively light and a hydraulic fluid throttling type cushioning unit that is operable in conjunction with the first unit to jointly resist buff loads which are relatively stronger, with the rst unit operating during initial buff travel to limit shock effects during set up of the hydraulic unit.

There have been many arrangements devised for utilizing hydraulic cushioning units in draft gear applications. For such applications, hydraulic units have advantageous features which are highly desirable, such as, offering high capacity and offering increasing resistance and absorption as the speed of load application increases. However, hydraulic units have certain disadvantages which make them undesirable for draft gear application. Some amount of initial cushioned travel must be provided during which the hydraulic unit may set up and condition itself for yieldably resisting closure.

An object of the invention, therefore, is to provide an arrangement wherein a positive resistance type of spring cushioning unit acts in series with a hydraulic unit to handle the light loads and the initial shock effects of the heavy loads to which the hydraulic unit may be subjected.

Still another problem that is inherent in the use of hydraulic units for draft gear applications relates to the possibility of the development of destructive internal pressures when extremely high energy impact loads are encountered.

A further object of the invention is the provision of an arrangement wherein the maximum loading on the hydraulic unit is positively limited to an acceptable range and wherein a spring cushioning unit acts to resist loads in excess of the maximum capacity of the hydraulic unit.

Another difficulty with hydraulic units arises from the f act that they normally will undergo thier full closure travel if they are subjected to a light but steady force. Under a constant force or pull, the hydraulic oil bleeds through the flow restricting orifice. Draft loads of this nature are commonly encountered during train operation and objectionable train run-in and run-out can result when hydraulic units are applied for handling draft loads. It may also be noted that where a hydraulic gear which has a relatively slow recovery characteristic is permitted to operate, even partially during draft, a situation can occur wherein a sudden reversal of forces from draft to buff would leave the hydraulic unit partially closed and hence incapable of providing its full closure travel for resisting the buff loading. Accordingly, another object of the present invention is to provide a draft gear arrangement incorporating a hydraulic unit in a fashion such that the hydraulic unit is precluded from operating under draft loading.

Brieliy, in a preferred embodiment of the invention, the high capacity draft gear arrangement is constructed for mounting in a 36 draft pocket of a railway car frame Vto act between the usual front and rear stops of the pocket and provide a total buff travel on the order of 4% with a nominal capacity of 130,000 foot pounds, and a total draft travel on the order of 11/2. The arrangement includes a housing structure having an intermediate abutment wall separating oppositely opening front and rear chambers with a hydraulic cushioning unit mounted in the front chamber and with a rubber spring unit mounted in the rear chamber to bias the housing against the front stops for disabling the hydraulic unit from reacting during draft loading. The rubber spring unit which is operable during both buff and draft may have a travel of 11/2 and the hydraulic cushioning unit which is operable only during buff may have a travel of 3%.

The force-closure characteristics of the two units are correlated so that under buff loading, the rubber unit initially undergoes limited closure travel while the hydraulic unit is setting up for providing a substantially constant reaction force throughout its entire closure travel. A positive stop arrangement is provided to relieve the hydraulic unit of excessive internal pressure at the end of its travel whereupon the rubber unit undergoes further closure to handle those impacts that exceed the capacityof the hydraulic unit.

Other objects and advantages will become apparent during the course of the following description.

In the accompanying drawings forming a part of this specification and in which like numerals are employed to designate like parts throughout vthe same;

FIG. 1 is a vertical sectional view through a railway car frame and illustrating the draft gear of this invention mounted in place therein with parts of the draft gear being broken away and sectioned to facilitate the disclosure;

FlG. 2 is a horizontal sectional view illustrating the draft gear in mounted position within the draft pocket of the railway car frame;

FIG. 3 is a transverse cross-sectional view through the gear and is taken on the line 3-3 of FIG. 2;

FIG. 4 is an enlarged fragmentary sectional view as taken at the rear end of the gear and illustrates a followerlock piny arrangement;

FIG. 5 is a fragmentary horizontal sectional View illustrating an alternative rear follower arrangement;

FIG. 6 is a fragmentary horizontal sectional view illustrating an alternative fro-nt follower arrangement;

FIG. 7 is an enlarged lengthwise sectional view through the hydraulic unit to illustrate its details of construction;

FIG. 8 is a side elevational view of the hydraulic unit, with parts thereof broken away and sectioned, and illustrates the unit in a closed position;

FIG. 9 is a fragmentary transverse cross-sectional view through the hydraulic unit and is taken on the line 9-9 of FIG. 7 to illustrate a piston guide rib arrangement;

PIG. l0 is a fragmentary transverse cross-sectional view through the hydraulic unit and is taken on the line 1010 of FIG. 8 to illustrate the orifice passage configuration;

FIG. l1 is a diagrammatic lengthwise sectional view illustrating an alternative compression chamber construction for use in the hydraulic unit of the draft gear;

FIG. 12 is a force-closure diagram illustrating the high and low speed buff closure characteristics of the combined action of the rubber and hydraulic units; and

FIG. 13 is a capacity-closure diagram illustrating the high and low speed bui closure characteristics of the combined action of the rubber and hydraulic units.

For purposes of disclosure, in FIGS. 1 and 2 the draft gear of this invention is designated generally at 15 and is shown mounted in a draft gear pocket provided within a railway oar frame having the usual center sills 20 equipped with front and rear draft gear lugs or stops 21 and 22, respectively, with a vertical yoke 23 shown encircling the gear and connected to a coupler shank 24 by a draft key 25. The draft gear has a housing structure of a length substantially less than the distance between the front and rear stops to accommodate its lengthwise shifting movement within the pocket. The housing structure includes 4an intermediate abutment wall 26 that separates a forwardly opening chamber 27 of generally circular cross-sectional configuration and a rearwardly opening rear chamber 28 of generally rectangular crosssectional configuration.

A self-restoring cushioning unit 29 of the hydraulic fluid throttling type is located in the front chamber for cushioning inward or butf movement of the coupler shank. A separate self-restoring type of cushioning unit 30 of the positive resistance spring type is shown mounted in the rear chamber and equipped with a rear follower 31 to react through the rear follower 31 and the rear lugs 22 to bias the housing structure towards the front lugs 21 when the gear is in its normal release position.

Under buff loads, the coupler shank 24 moves inwardly to transmit forces serially through the hydraulic unit 29 and the spring unit 30 with the housing structure moving rearwardly through the pocket a distance determined by the travel of the spring unit 30. Preferably, the cushioning characteristics of the spring unit are selected to prevent the housing structure from going solid against the rear stops.

Under draft loads the yoke 23 moves forwardly against the rear follower 31 to transmit draft forces into the spring cushioning unit 30 which reacts directly against the front stops 21 through -intermediate portions of the housing structure.

The housing structure, illustrated in FIGS. l-3 for the purpose of disclosure, is comprised of an assembly of elements and includes a hollow box-shaped casting 33 generally of rectangular cross-sectional configuration for housing a rubber mat type of spring unit and closed at one end to form the intermediate abutment wall 26, a cylinder 34 for the hydraulic unit and composed of a length of seamless tubing of adequate wall thickness to withstand the internal fluid pressures developed in the hydraulic unit, an end cap 35 seated in the front end of the cylinder, `a set of four mounting bolts 36 external of and eX- tending lengthwise alongside the cylinder to secure it in tightly sealed relation between the end cap 35 and the casting 33, and a pair of thrust bars 37 interposed between opposing external surfaces on the rectangular casting and the end cap to transmit forces therebetween and thereby relieve the seamless tubing of excessive buckling loads.

As is best shown in FIGS. l and 2 the rear face of the intermediate abutment wall 26 is formed with a set of recesses to receive the nuts 36N of the mounting bolts in counter-sunk relation.

As is best shown in FIGS. 7 and 8, the seamless tubing that forms the cylinder 35 of the hydraulic unit is snugly seated on a circular embossment 26C provided on the front face of the casting wall 26 and it is sealed in place by a peripheral weld line 38 at its rear end and an O-ring 39 at its front end. The other ends of the mounting bolts 36 are threaded into suitably tapped sockets provided in a rearwardly directed face 35F of the end cap. At the rear end of the casting horizontally extend- 'ing recesses 33k are formed along the rear edges of the top and bottom walls to accommodate full clearance travel for the rear end of the yoke 23.

It will be apparent that the

elements

33, 34, and 37 may be cast integrally to provide a one-piece housing structure having an intermediate abutment wall separating front and rear chambers thereof.

The end cap is generally rectangularly shaped to provide the abutment face 35F for the thrust bars and mounting bolts and includes a circular inner end 35E for tight fitting engagement in the front of the cylinder 34 and it also includes forwardly projecting lateral arms 35A for abutment against the front stops 21 to disable the hydraulic unit from any response during draft load- Cil 4 ing. The end cap is also arranged to provide forwardly facing internal abutment shoulders 35S for solid engagement with a front follower 40 which may, as illustrated herein, be formed integrally with the piston 41 of the hydraulic unit, there being adequate clearance for free movement of this front follower in the space between the lateral arms of the end casting.

The spring unit 30 for the rear chamber preferably consists of an aligned stack of rubber mats ntercalated with metal spacer plates, a preferred construction of this type being shown in copending application, Serial No. 6,305, filed February 2, 1960, in the name of Bertil E. Peterson.

For use in the draft gear application of this disclosure, however, a locking arrangement such as is shown in FIG. 4 may be employed wherein the outermost corner regions of the rear follower 31 are cut out and aligned holes 33H are provided in the adjacent edges of the side walls of the casting. When the gear is initially assembled the rubber spring unit is compressed until these holes come into registry with the cut out regions of the corners and lock pins P are inserted to hold the parts under initial compression. It will be apparent that other locking arrangements known to the art may also be employed as desired.

While the characteristics of a iubber spring unit of the foregoing type are preferred for use in combination with a fluid throttling type of hydraulic unit, it will be apparent to those skilled in this art that coil springs might be employed in place of the rubber or in conjunction With the rubber, and it is also contemplated that coil springs or rubber might be employed in combination with a friction clutch meachanism within the rear chamber. All such types of cushioning units are defined herein under the generic phrase a positive resistance type of spring cushioning unit, and it will be apparent that each of these is also self-restoring in that it returns to its initial configuration upon removal of the load.

The details of a preferred form of hydraulic cushioning unit, which is illustrated herein for purposes of disclosure, are best shown in FIG. 7 wherein the piston 41 is shown with its integral front follower portions 40 spaced 3% inches from the abutment shoulder 35S provided internally and facing forwardly on the end cap 35. These portions engage in solid abutment, as illustrated in FIG. 8, to limit the maximum travel of the hydraulic unit to 3%" and, in addition, to positively limit the internal fluid pressures at full closure of the hydraulic unit. The positive stop arrangement prevents the hydraulic unit from going solid on the oil, a condition which, under the severe impacts that must be taken into account in draft gear practice, could readily lead to rupture of the cylinder 34.

The piston 41 has an enlarged inner end flange 41E within the cylinder 34 and the piston works against a Idouble coil helical type compression spring 42 housed within the cylinder to restore the hydraulic unit after release of the applied loads.

In the normal release position of the hydraulic unit the enlarged inner end of the piston is urged forwardly by the action of the compression spring 42. The circular outer end of the piston is of a diameter to nest in fluid tight sealing relation within the outer end of the circular bore of the end cap and an annular rolling diaphragm type seal 43 is provided in the end cap and clamp rings 43C mount it in sealed relation against both the piston 41 and the inner end 35E of the end cap for preventing leakage from the cylinder of hydraulic fluid and for preventing entrance to the cylinder of dust and dirt. A seal scraper or wiper ring 44 is clamped to the end cap 35 in recessed relation to the abutment shoulder 35S thereof to exclude dust, dirt, or other foreign matter from entering between the slideably engaging surfaces of the piston and end cap.

During inward movement lof the piston the hydraulic iluid is displaced from the rear of the cylinder to occupy an annular space created at the front side of the enlarged inner lend ilange of the piston. It will be apparent that this annular space increases in volume at a subtantially lower rate than the decrease in volume at the rear of the chamber due to the fact that the entrance of the piston into the cylinder progressively subtracts from the initial Volume of the cylinder chamber. To accommodate the loss of volume within the cylinder the piston 41 is provided with a rearwardly opening axial pocket that registers with a pocket provided in a hat-shaped orifice cap 48 that is anchored across the inner end of the piston. These internal pockets in the piston and orifice cap cooperably define an accumulator chamber 49 for receiving the excess hydraulic fluid displaced by the piston during its inward stroke.

The accumulator chamber 49 is provided with a rolling diaphragm 50 of rubber or other suitable material trained about a sleeve piston 51 and urged rearwardly towards the orice cap by a helical coil type compression spring 52 one end of which seats against an internal face 41F on the piston and the other end of which seats against a ball bearing type thrust bearing assembly 53 that is tted within the sleeve piston 51. A vent passage 54 is shown extending axially and then transversely to open through the follower portions 40 of the piston. These vent pass-ages laccommodate the required free ow of air out of and into the accumulator chamber during closure and release movements, respectively, of the assembly of the diaphragm 50 and sleeve piston 51. The compression spring 52 within the pocket determines the resistance to closure characteristic of the diaphragm.

It is preferred that the gears characteristic be independent of an entrapped air pocket since the pressures involved make-it substantially impossible from a practical standpoint to assure that the entrapped quantity of air will not undergo some significant leakage over a period of time.

The hat shaped oriiice cap 48 is formed with uniformly peripherally spaced full-length oriiice passages 45 which are preferably of uniform cross sectional configuration throughout their length and of the shape shown in FIG. 10. A set of three such passages are provided to act as ow restricting orifices through which the hydraulic uid is throttled during closure of the gear. Essentially it is desired that the uid pressure existing in the cylinder will be substantially constant throughout a given closure stroke of the gear. For high speed closures the fluid pres sure will be substantially greater than for low speed closures. From the standpoint of the gear design as well as from the standpoint of the structural members of the car framing, a constant uid pressure relationship within the hydraulic gear is important, as it assures a maximum utilization of the energy absorbing characteristics and a maximum utilization of the closure travel and thereby protects against rupture of the parts which lare subjected to load under buff impacts.

In order to prepare the hydraulic unit for subsequent cushioning action it is desirable that the piston be restored as swiftly as possible. For this purpose a set of six substantially uniformly peripherally spaced fluid return passages 46 (see FIG. 7) are provided in the orifice cap 48 and are normally held closed by suitable ball check assemblies 47 which respond to internal fluid pressure at the inner end of the cylinder to become tightly seated and block fluid ow during the cushioning stroke of the piston. These ball check assemblies 47 unseat to accommodate a relatively rapid return stroke under the action of the double coil compression spring 42. An annular mounting ring 47R retains the ball check assemblies 47 in their mounted position and the return passages open rear- Wardly through under cut regions of the `orifice cap to accommodate the desired free How around the edge of the ring 47R.

In the arrangement illustrated the orice cap 48 has a central flow thnottling orifice passageway 48P for restricting the iiow of hydraulic uid into the accumulator chamber during gear closure. This cap has a set of return ow channels 48C controlled by one-Way ball check assemblies 53 to assure rapid return of tluid from the accumulator chamber to the cylinder during the release stroke of the gear.

During the closure stroke the hydraulic iiuid in the main cylinder of the hydraulic unit is throttled through the set of three orifice passages 45 in the enlarged inner end of the cylinder and through the axial orifice passage 48P in the orifice cap to generate energy absorption in the hydraulic unit. The amount of absorption varies with the speed and energy of the impact as these factors determine the pressure within the main cylinder and hence the flow through the throttling orifices. The pressure within the main cylinder chamber also depends upon the design of the compression spring 52 of the accumulator chamber and the size of the throttling oriiices.

Upon release of the load, Huid is returned to the main cylinder through the various return` flow passages 46 and 43C under the control of the ball check assemblies 47 and 53, respectively, and through the throttling passages of both the orice cap and the inner end of the piston.

To lock the piston 41 against rotation within the cylinder sleeve 34, the keying arrangement, as is illustrated in FIGS. 7 and 9, is employed. An elongated key K is fixed to extend lengthwise within the cylinder 34 and it engages in a lengthwise keyway 48K along the edge of the cap 48.

The rolling diaphragm 50 and the spring 52 assembly shown in the accumulater chamber 49 for the arrangement of FIG. 7 may be replaced with an aligned stack of metal type bellows element 55 such as is illustrated in FIG. 1l. Each bellows element is resiliently collapsible under pressure to create increased volume for receiving the excess hydraulic liuid and upon the release of the load each bellows element restores automatically to its original shape to return the hydraulic fluid to the main reservoir. Where bellows are used the vent passage 54 is utilized to permit of bleeding all of the air from the accumulator chamber during the initial assembly of the gear. The vent passage is then sealed off by removable plugs (not shown) to make the closure characteristics of the gear dependent upon the resiliently collapsible nature of the bellows and not upon entrapped air pockets.

The gear performance under buli loading is depicted in FIGS. l2 and 13. The curve 56 in FIG. l2 illustrates the force closure characteristic for high Velocity buff impacts and the curve 57 illustrates the force closure characteristic for low velocity butt impacts.

As is apparent from a consideration of curve S6, during the first inch or more of closure travel the rubber cushioning unit acts to resist closure and during this time the hydraulic unit is becoming set up and conditioned to permit a continuous and smooth iiuid throttling flow through the orifices of the piston and orifice cap. The hydraulic unit then undergoes its full closure travel at a reaction force of 400,000 pounds and at the end of this travel when the front follower portions 40 abut the shoulders 35S on the end cap (see FIG. 8) the cushioning characteristics of the hydraulic unit are fully exhausted. The solid stops prevent buff impacts from going solid on the oil of the hydraulic unit and this is important for avoiding internal pressures of rupturing magnitude. The iinal 1/2 inch of closure travel is then resisted bythe rubber unit which is preferably arranged so that the housing structure is not actually permitted to go solid.

The low velocity impact curve 57 illustrates that the rubber gear again operates first but due to the slower closure rate and lower capacity impact involved, the hydraulic unit comes into play after less than one inch of initial travel. Thereafter the hydraulic unit undergoes closure at a constant reaction force value of 200,000

pounds which for the illustrated condition is sufficient to handle the full energy of the impact.

The capacity-closure characteristics are shown in the curves 58 and 59 of FIG. 13 to illustrate conditions for high velocity and low velocity impacts, respectively.

In FIG. an alternative rear follower arrangement is illustrated wherein a rear follower 31 is shown as having corner portions 31P overlapping the end face of the side walls of the casting 33 for engaging in solid abutment therewith. Where such an arrangement is utilized, the casting will be shorter by the amount of the thickness of the corner portions 31P in order to maintain the same length closure travel as is provided in the preferred embodiment.

While in the preferred embodiment, the piston 41 and its integral follower portions 40 are locked in the cylinder at all times by the inner end construction of the end cap the invention contemplates other end cap structures that need not include such a locking feature. For this purpose, the front follower 40 is of sufiicient lateral extent to overlap with the front lugs 21 and create solid abutment therewith in establishing the release position of the gear in the pocket.

It should be understood that the description of the preferred form of the invention is for the purpose of complying with Section 112, Title 35, of the U.S. Code and that the claims should be construed as broadly as prior art will permit.

We claim:

1. A draft gear for mounting in the draft pocket of a railway car frame, said pocket having front and rear stops, said gear including housing structure of a length substantially less than the distance between said stops disposed in said pocket for movement lengthwise therein, said structure having an intermediate abutment wall and endwise opening front and rear chambers on opposite sides of said abutment wall, a first self-restoring cushioning unit of a hydraulic fluid throttling type mounted in said front chamber to react against said abutment wall and a second self-restoring cushioning unit of a positive resistance type mounted in said rear chamber under predetermined compression to react between said abutment wall and said rear stops and normally bias said housing structure against said front stops for conning draft loads to said second cushioning unit.

2. The draft gear of claim 1 wherein said first cushioning unit incorporates means providing such unit with a substantially constant reaction force characteristic throughout its closure travel and said second cushioning unit incorporates means providing such unit with a reaction force characteristic that rises progressively throughout its closure travel from a value substantially beneath that of said first cushioning unit to a value substantially above it. Y

3. In a draft gear for mounting in the draft pocket of a railway car frame, the combination of a first self-restoring cushioning unit of a hydraulic iiuid throttling type, a second self-restoring cushioning unit of a positive resistance type, means for transmitting loads serially through said first and second units to close the same simultaneously during application of buff forces to said gear and means for loading only said second unit during application of draft forces to said gear.

4. In a draft gear for mounting in the draft pocket of a railway car frame, the combination with a first self-restoring cushioning unit of a hydraulic fluid throttling type, said first unit incorporating means providing such unit with a substantially constant reaction force characteristic throughout its closure travel, a second self-restoring cushioning unit of a positive resistance type, said second unit incorporating means providing such unit a reaction force characteristic that rises progressively throughout its closure travel from an initial value that is substantially beneath the constant reaction force of said first unit to an ultimate value that substantially exceeds the reaction force of said first unit, means for transmitting loads serially through said first and second units to close the same simultaneously during application of buff forces to said gear with said second unit undergoing initial closure travel during which said first unit sets up for resisting subsequent closure travel, and means for loading only said second unit during application of draft forces to said gear.

5. In a draft gear rigging that includes a draft gear mounted in the draft pocket of a railway car, said pocket having front and rear stops and said rigging having a coupler yoke assembly encircling the gear, said gear comprising a housing unit of a length appreciably less than the distance between said stop disposed in lengthwise movable relation in said pocket, said housing unit having endwise opening front and rear chambers separated by fixed intermediate abutment structure, a first self-restoring cushioning unit of the positive resistance type under predetermined compression in said rear chamber and engaging said abutment structure to react between said front and rear stops through said housing unit and through rear follower means respectively, to bias said housing against said front stops for confining draft loads to said first cushioning unit, a second self-restoring cushioning unit of the hydraulic fluid throttling type mounted in said front chamber and engaging said abutment structure to transmit forces serially through said second and first units under buff loading applied through said coupler yoke assembly, and means for applying draft forces from said coupler yoke assembly forwardly through said first cushioning unit and said housing unit, said second cushioning unit incorporating means providing such unit with a substantially constant reaction force characteristic throughout its closure travel and said first cushioning unit incorporating means providing such unit with a reaction force characteristic that rises progressively throughout its travel closure from a value substantially beneath that of said second cushioning unit to a value substantially above it, such that during application of buff loads said first unit undergoes initial closure travel during which said second unit sets up for resisting subsequent closure travel.

References Cited in the file of this patent UNITED STATES PATENTS 2,208,338 Munro et al. July 16, 1940 2,816,670 Edwards et al. Dec. 17, 1957 2,825,472 Peterson Mar. 4, 1958 2,915,198 Spencer Dec. 1, 1959 FOREIGN PATENTS 303,141 Great Britain July 18, 1929 384,642 Great Britain Mar. 2, 1931 695,109 Great Britain Aug. 5, 1953

Claims

1. A DRAFT GEAR FOR MOUNTING IN THE DRAFT POCKET OF A RAILWAY CAR FRAME, SAID POCKET HAVING FRONT AND REAR STOPS, SAID GEAR INCLUDING HOUSING STRUCTURE OF A LENGTH SUBSTANTIALLY LESS THAN THE DISTANCE BETWEEN SAID STOPS DISPOSED IN SAID POCKET FOR MOVEMENT LENGTHWISE THEREIN, SAID STRUCTURE HAVING AN INTERMEDIATE ABUTMENT WALL AND ENDWISE OPENING FRONT AND REAR CHAMBERS ON OPPOSITE SIDES OF SAID ABUTMENT WALL, A FIRST SELF-RESTORING CUSHIONING UNIT OF A HYDRAULIC FLUID THROTTLING TYPE MOUNTED IN SAID FRONT CHAMBER TO REACT AGAINST SAID ABUTMENT WALL AND A SECOND SELF-RESTORING CUSHIONING UNIT OF A POSITIVE RESISTANCE TYPE MOUNTED IN SAID REAR CHAMBER UNDER PREDETERMINED COMPRESSION TO REACT BETWEEN SAID ABUTMENT WALL AND SAID REAR STOPS AND NORMALLY BIAS SAID HOUSING STRUCTURE AGAINST SAID FRONT STOPS FOR CONFINING DRAFT LOADS TO SAID SECOND CUSHIONING UNIT.