US2928560A

US2928560A - Air-charged hydraulic unloader

Info

Publication number: US2928560A
Application number: US707273A
Authority: US
Inventors: George R Wilkin
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-01-06
Filing date: 1958-01-06
Publication date: 1960-03-15
Anticipated expiration: 1977-03-15

Description

March l5, 1960 G. R. wlLKlN AIR-CHARGED HYDRAULIC UNLOADER 5 Sheets-Sheet 1 Filed Jan. 6, 1958 INVENTOR. GEORGE R. WILKIN BY BUCKHORN,CHEATHAM 8 BLORE Array/VE vs March 15, 1960 G. R. wlLKlN AIR-CHARGED HYDRAULIC uNLoADER 5 Sheets-Sheell 2 Filed Janf e, 1958 )llllllli illlllll INVHVTOR. GEORGE R. WILKIN BY BUCKHORNCHEVATHAM 8\ BLORE A WOR/VE YS March 15, 1960 G. R. wlLKlN AIR-CHARGED HYDRAULIC UNLOADER 5 Sheets-Sheet 3 Filed Jan. 6, 1958 R. m N m m W II. O Q 1 Wm R N R m Y G B l I @di m @E NE Sr; W wb @2. i a vb mm w3 NQ g @L w" .5 NS S QN@ m. n: 2 |5 N Il, 1| .Il l .||||||H i. FQ l @m11 JI @n l E.- IL 1 LI l\|uV||| f Il NQ S E zal Q w@ BUCKHORN,CHEATHAM 8. BLORE ATTORNEYS March l5, 1960 G. R. wlLKlN AIR-CHARGED HYDRAULIC uNLoADER Filed Jan. 6, 1958 INVENTOR. GEORGE R. wlLKlN BY BUCKHORN,CHEATHAM BLORE S Wm. D

March l5, 1960 G,RW|1 K|N 2,928,560

AIR-CHARGED HYDRAULIC UNLOADER v Filed Jan. 6, 1958 5 Sheets-Sheet 5 LO s, Q I i t L1. l Q 2 L- NLJ? t MEM FIG. I4

FIG. l5

INVENTOR. GEORGE R.WILKIN BY J BUCKHORN,CHEATHAM 8BLORE A TTOPNEYS f i States 2,928,560 AIR-CHARGED HYDRAULIC UNLOADER George R. Wilkin, Portland, Oreg. Application January 6, 1958, Serial No. 707,273 l Claims. (Cl. 214-75) apparatus with which the lifting cables are associated.

` The principal object of lthe present invention is to provide a traveling crane associated with rails which may be attached toanyilat-bed truck, which traveling crane is capable of following distortions of the truck bed as the truck traverses uneven surfaces'whereby the crane will remain in position regardless of the roughness of the road. In connection with this object of the invention the traveling crane comprises a pair of side frames each of which is supported upon four a'nged wheels, and which are connected together by a trunnion. device extending across the truck at a height above the bed whereby the opposite sides of the traveling crane may assume various angular positions with respect to each other. In connection with this object of the invention I have devised a sturdy traveling crane comprising a light weight, easily constructed frame assembly.

A further object of the present invention is to provide means whereby heavy loads may be unloaded from a truck by a single operator. It is quite common for a truck to be loaded with heavy articles, either on pallets or skids, or crated, or by themselves, in a terminal, manufacturing plant, or other locality where loading devices are available, such as fork trucks, overhead cranes, or the like. At least, the loading point for many commodities will have a number of men available to lift heavy objects onto the truck. However, when the truck arrives at its destination, there may be no equipment or men available to unload the truck, necessitating the dispatching of a work crew or lift trucks or other equipment to accomplish the unloading. The present invention surmounts this difficulty by providing means whereby a single operator, such as the truck driver, may unload heavy loads by himself.

A further object of the present invention is to provide means whereby an unloading device of the foregoing character may be regulated to unload loads of various weights.

A further object of the present invention is to provide means of the foregoing character whereby the hydraulic system Vfor operating the unloader may be charged with air at a controllable pressure, and whereby the air pressure may be regulated to compensate for temperature rises. The lat-ter object is particularly necessary where a load may start out in the cool of the morning and arrive at its destination when the temperature is at a maximum.

The foregoing objectsand advantages will be more particularly apparent from inspection of the following specification taken in connectionrwith the accompanying drawing wherein like numerals refer to like parts throughout.

In the drawings, Y

.fis- .l Side. elevation et a... ckprovided .with th;

apt-ent l' A present invention, palletized loads thereon being shown in full line in connection with a full line showing of the unloader in its traveling position, the view also showing tail rail mechanism to provide an unloading position and the unloader illustrated thereat in dash line;

Fig. 2 is a plan view of Fig. 1; Fig. 3 is a vertical section taken along line 3-3 of Fig. 1;

Fig. 4 is a vertical section on an enlarged scale through a portion of the mechanism taken substantially along line 4-4 of Fig. 5;

Fig. 5 is a vertical section taken substantially along, line 5-5 of Fig. 4;

Fig. 6 is a horizontal section taken substantially alongline 6-6 of Fig. 4;

Fig. 7 is a vertical section, on an enlarged scale, taken substantially along line 7--7 of Fig. l;

Fig. 8 is a horizontal section taken along line 8--8 of Fig. 7;

Fig. 9 is a horizontal section taken along line 9-9 of Fig. 7;

Fig. 10 is a view in end elevation, partly in section; through the pump and valve assembly of the present invention;

, Fig. 11 is a partial vertical section, on an enlarged scale, taken substantially along line 11-11 of Fig. 2;

Fig. 12 is a schematic view showing the arrangement of the lifting cables and the operating mechanism therefor;

Fig. 13 is a partial vertical section taken substantially along line 13-13 of Fig. 1l;

Fig. 14 is a partial vertical section taken substantially along line 14-14 of Fig. 1;

Fig. 15 is a rear view of a truck having the tail rail sections folded against the end of the truck for traveling;

Fig. 16 is a partial vertical section taken substantially along line 16-16 of Fig. 17, showing the tail -rail hinge and lock; and

Fig. 17 is a horizontal section taken substantially along line 17--17 of Fig. 16.

The invention is herein illustrated in combination With a flat bed truck,indicated generally at 10, having a at bed 11. At each side of the bed there is positioned al rail 12, each rail being rigidly fastened to the surface Y of the truck bed. By reason of the construction of' the traveling crane, as will presently appear, I am ableto utilize a very simple form of rail, comprising an elongated, iiat strip of sheet metal such as cold rolled steel in the order of in thickness, which may be readily attached to the surface of any at-bed truck by suitable fastening means such as screws (not shown). Each railA .is provided with longitudinally aligned cog openings 13.l

The truck is shown loaded with a plurality of palletized loads 15 mounted yupon pallets 16 of the type having provision for the insertion of the forks of fork lift trucks. Pivotally mounted at the rear end of the truck are a pair of tail rails .17, each of which is provided with a few cogwheel openings 18 in alignment with the cogwheel openings 13 when the tail rails are swung rearwardly, and a long, parallel row of tail rail cogwheel openings 19 which are offset outwardly from the cogwheel openings 13. Each tail rail is pivotally mounted at an outer f corner of the end board 20 of the bed of the truck, upon a pair of vertically aligned, upwardly extending hooks v21 which engage journals 22 at the inner end of thel tail rail. The end board is provided with a pair of arcuate slots 23 which engage arcuate lugs V24 on the journals' 20 when the tail rails are folded against the end board, there-.1 l. by preventing accidental loss of a tail rail while the truckis traveling. The tail railsmay be lifted slightly in order that the inner portion bearing the cogwheel openings 18 lmay rest uponthe rear endet/the rail 12 whenthe-tail-l liaizented Mar. 15, 1960 i rail is folded back against .the end board of the truck.` Eaehtail rail is provided with a pivotally mounted, 'adjustable, landing jack which is fixed upright beneath the end of the tailrail when 'the tail railis ,extended rearwardly, so as to support the same against ithegweight of a load thereon; t j

A traveling crane unloader` generally indicated by the i i e #3,928,550*

numeral is positionedY upon the flat bed'truck, the Y,

same being removable so that one crane may b e utilized with any number of trucksV provided with rails, orso .that the truck may be utilized without arcrane when desired. The crane comprises a pair of substantially

upright frame tubes

31 and 32, one at each side of fthe crane,- connected together near their'tops by a journal means 33, and a pair of

side tubes

34 and 35 extending longitudinally A` pair of transverse partitions 80 are fixed within the Vside tube' 34, thus providing a fluid tank 81 within which a supply 82 of hydraulic fluid is contained.` YA liquid supply line 83 extends downwardlyV into the tank 81 and is connected to the inlet of a valve housing 84 mounted upon the lower portion of the tube 31. A first ball check valve 85 is `seated inthe inlet cavity 86 of the valve Vhousing and ,isurged `downwardly' by a .spring 87 against j a valve seat inthe bottom thereof. A plunger 88 mounted forward. in alignment with the

tubes

31 and 32 respefV tively, being attached thereto and communicating there-` with'. 'The *sideV tubes lie horizontally andfiply close to the bed of the truck, in position to embrace a palletized load between them. The forward end of Aeach side tube is connectedwith

averticahfront leg tube

36 and 37 respectively. The frame construction thus-described is preferably made of` large pipe sections,` five illhes in diameter for example-.thus forming a7 rigid, sturdyV oonstructionof light weight.4 The side tubes 34 Y and 35 being 7close to the truck bed, there is no 'necessitylforY lateral bracing of the structure, as Y`would be the cast-'Vifr r the side tubes were above the palletized loads. K A transverse shaft 41-extends across the lower,v end off.'`

' the legA tube 36, and a transverse shaft 43 extends across the lower end of the leg tube 37, the shafts 41 and 43 projecting laterally beyond `each side of the leg tubes."`

Shaft 41 supports at its outer ends a flangedcogwheel 44 at the outside, and a llangedfwheel 45 atr'thejinside. Similarly, shaft 43 projects laterally and supports acogwheel .46 at the outside and a flanged wheel 47atzthe inside. The flanges of wheels 44V to 47, inclusive, L engage the edges ofi bed rails 12. The lower end of the tube. 31 supports a central cogwheel 48., -a flanged cog` wheel 49, and a llangedwheel 50, on a transverse shaft 51, Vand the lower end of .-tube 32 supports a central.-`

' cogwheel 52, a flangedcogwheel 53,.an d a flanged wheelV 54, on a transverse shaft 55.VV Thus, thetraveling crane n unloader is supported above the flat bed of the truck by fourzllangedwheels .45, 47, 50 and 54 engaging the inner i edges of rails 12, and by four flanged cogwheels 44, `46, 49 and 53 engaging the outer edges of rail 12. When the `traveling crane unloader is above the tail `rails 17 it is `,supported by the four

flanged fcogwheels

44, 46, 49 and 53 engaging `thercog openings V19 inthe tail rails. All of the cogwheels and flanged Wheels are fixed to their respective shafts.

A sprocket 60 fixed to shaft 51 drives a sprocket chain f i' 61 extending upwardly through tube 31 anda sprocket 62 at thetop. of the tube. Sprocket 62 is fixed to a cross shaft'63 which extends from tube Y31 into tube 32 and has a sprocket 63lixed thereto within thetube. Sprocket 63' `drives a sprocket chain ,64 which extends around a sprocket Y65 fixed to shaft 55, thereby transmitting any movement of shaft 51 vto coextensive movement of shaft whereby both sides ofthe unloadermove in'unieson.

The Outrend of shaft 51 is provided withia square solet 66 which maybe engagedbya square-lug 67 on af hand crank Y63 whereby the erane'may be' caused to move longitudinally of the truck bed and side rails.` A transverse gear shaft 7G mounted in tube 31 is provided with a square socket71rwhich may also be engaged by the lug,` 67. A gear 72 fixed to shaft `'70 drives a gear` 73 meshing V'therewith and fixed to a second transverse shaft 74. Shaft 74 is also fixed to a'gear 75 which mesheswith a gear 76 rxed to shaft 51. A` reduction gear drive is thereby provided which may he selectively` e usedto .drive the unloader crane longitudinally at a slowY sPtQd. Thisisdesirable when `the crane is ,supporting Il heavyiload, which might cause the mechanism to be Y ,immlvble `by direct drive'through theshaft 51. i

vertically beneath the'inlet 86 is normallyin raised position, the plunger having a ball lifterV extension 89 which engages the ball and lifts it from its seat so that fluid may flow into the valve housing. The plunger is retained by alimit screw 9,0 engaging a slot in tho plunger and permitting limited vertical movement thereof. The

-Pluugor is normally held in raised postiouby a sable .91

fixed lo its ,lower end, extending upwardly through tubo 31 andilaterally into ,tho iourualliug means 33. with ,its

other end fixed to the outer oud of a buffer spring 9,2 mounted upondpallitio 93 in the journallingnieans,

' When the Vbulier spring "is expanded the

plunger

8,8 is

lifted andthe first ball check valve 8,3A- held open; Theifirst ball vcheck yalye cavity 86 .communicates with` j a passage 95 within the valve body, which passage is constantly opefnto asiirst bypass passage 96 within the valve body, This pasgagenornially communieates with 4the .cylff 1 Yinnerispiace:97sof a manually ao'tuatod pump 9.8 rlountedA upon the valve body, thorolbeiug a` pump inlet .obs-.ok

valve 991m' prevent return flow of the fluid when the `Plump lis actuated. The puma comprises a piston lllll l operating within thooylind'or and pli/orally .connected to a. manual lever .101, whereby tbe piston may be regir roated. `Fluid is forced ,from the @cylinder space 97 into passagelz "in the valve body, having@ pump outlet check valve 103 therein. .Fluid isfored from the pas.- sage 102 into a discharge lino 104 Ywhich leads .to the lift: iug mechanism which will boflater described, fho first bypass passage, including portions 96 and 1,02J is .oon-

nected into the fluid supply line comprising the

tubes

33 and 104 between the Vfirst check valve 85 and a ,SQCOIld spring-pressedchecklvalve 105 comprising a ball and spring to'preventbackllow of uid into thepassage 9 5,

' with LS Ot'hol' .end connected illtO thesupply lino biyglld the second .check valve 105; 'The punlp is eonneotcd into ,the first bypass passage.

A second ,bypass passage extends froluthe first bypass;

passage lllzinto the passage 95, this second passage ,106

hdl/ing a manually controlled butterfly valve lllT-tlrerein,y

the stem o f which extends outwardly to a valve handle ball `check valve A85' iS `also closed, the fluidA .cannot be drawn from lthe tank 81. Likewise, under `this set of non?. ditions. fluid is prevented from returning through line 104 i bythe check valves 105 andlll, and the manual valve 107, blocking both bypass passages. However, when the manual valve 107 is opened fluid may freely llovvY through passages/,102V and 95, lifting Ythe ball check valve 85 if there is st illicient force applied through the fluid, dlld returning to the supply tank 81.

A pair of partitions 110 is provided in side tube 35.

thus providing a compressed airk Supply tank llLAu alr supply tube 11g2rleads therefrom up through vertical tube 32, then partially through the jouruallirig means 33, then into and down through the vertical tube 31,V then laterally abovevthe horizontal side; tube 34 and into the liquid Y supply tank 8 1. `Theconnection of tube' `112 to liquid supply i tank-81 comprises an elbow 1.1 3 supplied d'with a pneumatic tire valve 114 whereby the system may be charged with air at a controlled pressure. The

tubes

112 and 83 are protected by a shield 115 through which the valve stem projects so that the operator may bleed off air when desired, such as when the mechanisml is to be disassembled for shipment. The tube 112 is constantly open between tank 111 and tank 81 so that therliquid supply in tank 81 is constantly under the desired pressure.

The portion of journalling means 33 connected to ver-v tical tube 31 comprises a cylinder 118 within which a piston 120 reciprocates horizontally, crosswise of the truck. The cylinder mounts a guide block and seal 121 provided with an annular recess 122 from which extend longitudinal openings 123 so `that fluid supplied to the recess 122 through tube 104 may exert force against the piston 120. The piston rod 124 connected to piston 126 extends into a tube 119 having one end telescoped over the end of cylinder 11S and its other end connected to the vertical tube 32. The journalling means therefore comprises two telescoped tubular members which are free to rotate or move laterally apart with respect to each other. The piston rod 124 is connected to the cable system to be described, and when moved to the left in Fig. 7 will raise the lifting cables. The pressure in the system is so set that the cables are normally raised, the piston 120 normally being extended to the left and partially collapsing the buer spring 92. As a result the first check valve lifting plunger 88 is normally retracted so that the first check valve 85 normally prevents further ow of fluid against the piston. However, the plunger 88 may be manually raised, whereupon operation of the pump 93 by a few strokes of the handle 101 will further collapse the buffer spring 92 to a limited extent, for a purpose to be described. The air pressure charge in the system is such that the piston will compress the buffer spring 92 to a limited extent when a load is being supported, and manual actuation of the pump 98 is required to compress the buffer spring to its full extent.

The piston rod 124 supports a Vclevis 130 in which are mounted a first pair of pulleys 131 and a second pair of pulleys 132. A bracket 133 xed to a removable cover 134 for an opening in the wall of the tube 32 supports a third pair of pulleys 135 and a fourth pair of pulleys 136. A first lifting cable 137 is fixed to the bracket 133 and successively passes over one of the pulleys 131, the opposed one of the pulleys 135, and down through the upright tube 32. The lifting cable 137 then passes over one of a pair of pulleys 138 at the lower end of tube 32, a pulley 139 at the rear end of the side tube 35, and is attached to a lifting bar 140 at its rearward end. A second lifting cable 142 is attached to the bracket 133, and successively passes over the other pulley 131 and the other pulley 135. The cable then extends laterally through the enlarged portion of the crosshead tube 33 and through an opening in the welding ring 125 and alongside the upper surface of the remainder of the crosshead tube 33. It is then trained over one of a pair of pulleys 143 at the top of upright tube 31, downwardly through the tube and over one of a lower pair of pulleys 144, then over pulley 145 at the rear end of the side tube 34 and is attached to the rearward end of an opposed lifting bar 146. A third lifting cable 150 is attached to bracket 133, then passes successively over one of pulleys 132, one of the pulleys 136, the other pulley 138, and a pulley 151 at the front end of side tube 35, and is attached to the frontend of bar 140. A fourth lifting cable 152 is attached to the bracket 133, then passes over the second pulley 132, the second pulley 136, through the crosshead tube 33 and over the `second pulley 143, then downward through the vertical tube 31, over the second pulley 144, and over a pulley 153, and is attached to the front end of the lifting bar 146. Thus, extension of the piston rod 124 causes separation of the clevis 130 from the bracket 133 and simultaneous, coextensive raising of the lifting

cables

137, 142, 150 and 152, and vice versa.

The lifting bars and ,146 vare channel bars their flanges facing inwardly, and the cables are con. nected to the vertical surfaces thereof. Apair of pipes may be inserted crosswise beneath a pallet with their ends inserted into the channel bars asillustrated in Fig. l2. When a lifting force is exerted the channel bars are canted inwardly as shown in Fig. 13, thereby pinching the ends of the pipes to secure them firmly to the lifting bars. The pipes 160 are stowed during traveling of the truck in a receptacle formed by a pair of channel,- bars 161 welded inside the bed frame, the pipes being; insertable therein through an opening 162 in the'side skirt of the frame. 4 Y 1 The crane travels at a position such as illustrated in Figs. l and 2, with the lifting bars substantially at the level of the truck bed. When it is desired to unload` a load, a pair ofthe pipes are removed from their storagey places, inserted beneath a load, and their ends engaged by the lifting bars. The operator then manually opens theA rst check valve and operates the pump handle several times further to compress the buffer springs 92, which lifts the load a shortdistance from the truck bed. The operator may then apply the crank handle to the reducing gear train and rotate the crank handle to cause the crane to roll rearwardly, transporting the load olf the truck bed and onto the extended tail rails. When the loadvis in the clear space behind the truck bed, the operator canA open the manual valve 107 to the desired extentV to permit return of uid into the uid supply tank, the speed of return, and hence the speed of lowering of the load to the ground, being controlled by the extent of opening of the valve 107 and the weight of the load, as well as the,`

charge pressure of the system againstl which the load,

exerts force. When the-load rests upon the ground the" operator, by stepping upon the outer, upper edges of thelifting bars, disengages the lifting bars from the pipes'.

Fluid may now return to thespace 122 due to the air' pressure charge until the buffer spring 92 is .partiallycompressed, Vat which point the bars will remain at an elevation slightly above the truck bed. The unloaded crane may now be cranked at high speed by direct engagea ment of the crank with the wheel shaft back to a position to pick up another load.

The cylindrical sections 118 and V119 of the journalling' means 33 are only partially telescoped as illustrated-in Fig. 7. The weight of the lifting bars 146 is multiplied through the cable and pulley system so that sufficientforce is exerted to maintain the end of the larger tubular.,

section 119 against an abutment 170 fixed to the outer surface of the smaller tubular section 118. The abutment is so arranged as not to restrict relative rotation of

tubular sections

118 and 119. Therefore, when one edge of a truck bed becomes skewed with respect to theother edge during rough travel, one side portion of the frame may assume an angular position with respect toy the other side of the frame as permitted by the journalling-v means. It will also be observed that each side section of the frame rests upon four anged wheels and that the cables extend downwardly `within the quadrangles definedA by each set of four wheels. Therefore, any force exerted by load on the frame does not exert a lateral force against either side of the frame, with the result that the relatively' light tension on the cable system is all that is needed to.,

hold the two frame sections together. As a result, the traveling crane is much lighter than any construction heretofore devised for a similar purpose. The slight amount of skewing which may occur between the two side sections of the crane does tend to place a distorting` force on the cross-shaft 63; however, the movement ofone end of the cross-shaft 63 relative to the other is so slight that no binding occurs.Y The traveling crane thus is light, sturdy and remains on the rails regardless of rough treatment.

Having illustrated and described a preferred embodii ment of the present invention, it should be apparent 'to asas-,boo

-those skilledin the art that the same permits of modiiica- Y tion in detail and arrangement. 1I claim as my' invention all such modifications as come within thel true spirit a fluid conduit extending from saidlluid supply tanjk toV said cylinder, a first check valve in said conduit which t when opened permits fluid Vtofiow in to said cylinder wherebysaid piston is extended to raise the `lifting cables,` a buler spring in said cylinder engaged by said piston f when extended, said buffer spring being partially compressed thereby due to the force exerted by the comsaid conduit at one end between said'checkY valves and t at its other end between said second check valve and said pressed air against the fluid, a check valve opening plunger engaging said first check valve, means connecting said buffer vspring and said plunger to hold saidfiirst check lvalve open until said buffer spring is partially'compressed by said piston, a second checkvalve in saidV cylinder, and a manually `actuated bleed `valve in said second vby-pass which when opened permits the return of fluid tofsaid fluid supply tank, whereupon the load will descend to the ground at the unloading position, the air pressure in the system automatically `re-extending said piston when the load is released from the lifting cables whereby the lifting cables aretraised in readiness for engagementrwith a succeeding load on the truck bed.

3. An air-charged hydraulic system for operating the lifting cables of a load handling device comprising a cylinder, a piston in said cylinder connected to the lifting cables, a fluid supply tank containing a supply of fluid, a compressed air tank containing a supply of com pressed lair and openly connected to said fluid supply tank constantly to exert pressure on the fluid in said fluid supply tank, a fluid conduit extending from said uid supply tank to said cylinder, a first check valve in said conduit vwhich when opened permits iluid to flow into saidcylinder whereby lsaid piston is extended to raise the liftingcables, a buffer spring in said cylinder engaged by said piston when extended, said buffer spring being conduit between said first check valve and said cylinderY and arranged to prevent the return of fluid from said cylinder to said supply tank, a first by-pass connected to said conduit at one end between said check valves and at its otherend between saidsecond check valve and said cylinder, a manually actuated force pump in said first by-pass, said plunger being manually engageable whereby said first check valve may be manually reopened and said pump operated to forcerfurther extension of said piston in order to raise'a load slightly, a second by-Vv pass connected to said conduit at one end between said l check valves and at its otherend between said second ch'eckvalve and said cylinder, and =a manuallyactuated` bleed'lvalve Vin said second by-pass whichrwhen lopened permitsthe return of fluid tosa'id fluid supplytank, where-` upon the load will descend, the air pressure in the system automatically re-extending said piston when the vload is released from the liftingcables whereby the lifting cables are raised in readiness for engagement with asucceeding load. Y Y,

2. An air-charged hydraulic system for operating the lifting cables of a traveling crane truck unloader corn.l

prising a cylinder, a piston in said cylinder connected `to the lifting cables, a fluid supply tank containing ag supplyA of uid, a compressed air tank containing a sup# ply of compressed air and openly connected to said Vfluid supply tank constantly to exertv pressure on'rthe fluidin said uid supply tank, a fluid conduit extending from` saidfluld supply tank to said cylinder, a' first check valve `in saidV conduit which when opened permits fluid to fiowinto Vsaid cylinder whereby said-piston'is ex?.4 tended to raisev the lifting cables, a buterspring in said' cyIindenengaged by `said piston when'extended, said buffer springbeingrpartially compressed thereby due to tlie force exertedby the compressed air against the fluid,

a check valve opening plunger engaging said first check valve, mechanical means connecting said buffer spring and said plunger to hold said first check valve open until said buier spring is partially compressed by said piston, a second check valve `in said conduit between said first check valve and said cylinder and arranged to prevent the return of fluid from said cylinder to said supply, tank, a first by-pass connected` to saidconduit at onek end betweensaid second check valve and said-cylinder, a manually actuated force pump in .said first `'by-pass, said plunger being manually engageable whereby said first check valve may be manually reopened and said pump operated to force further extension of -said piston'` in orderto raise aV load slightly from a truck bed whereupon the load may be transported by the travelingcrane to anunloading position, a second by-passr tionnected` to partiallycompressed thereby due -to the force exerted by the compressed air against the' fluid, a check valve opening plunger engaging said first check valve, a cable connectingsaid' buffer spring and said plunger to hold said `frstrcheck valve open until said buffer spring is partially compressed'by said piston, a second check valve in said conduit betweensaid first check valve and said cylinder and arranged. to prevent the return-of fluid from said cylinder to said supply tank, a first by-pass connectedV to said conduit at one end between said check valves and at its other end between said second check valve and saidV cylinder, a manually actuated force pump in said rst by-pass, said plunger being manually engage- Vable whereby said'ffirst check valve may be manually reopened andsaidV pumploperated to force further exten- 'Y sion of said pistonl in order to raise a load` slightly from itsresting place, a Vsecondby-pass connected to said Vconduit at one end between saidcheck valvesand at its. :other end between said second' check valve and said cylinder, and amanuallyfactuated bleed valvein said second -by-pass whichwhen opened permits the returna of fluid to said fluid supply tank,`,whereupon` the load will descend, the air pressure in the system automatically re-extending said piston when the load is releasedfrom the lifting cables whereby the lifting cablesl are raisedl in readiness for engagement with a succeeding load.

4. The .structure set forth in claim 1 wherein` said compressed air vtankV is provided `with a releasable charging valve whereby the compressed air supply tank may be controllably charged with 4air under pressure.

5. The structure set forth in claim 2` wherein said unloader`comprisesa cable supporting structure including a pair of sheave supports closely adjacent the bed of the truck, one at 'each side, saidstructure being` open at one end whereby said structure may extend around three sides of a load on a truck.

References Cited in the file of this patent UNITED STATES PATENTS V640,242, Turner et al. Ian. 2,V 1900 983,426 Carlson" 7 Feb. 7, 1911 2,334,708 Joy Nov. 23, 1943 2,359,112 l Hymans Sept. 26, 1944 2,424,899 `Priester July 29, 1947 2,512,150 Geren June 20, 1950 2,605,914 Hala Aug. 5, 1952 2,606,001 Lepper Aug. 5, 1952 V2,614,833 Laugaudin Oct. 21, 1952 2,701,655 Crile L Feb. 8,1955 s 2,797,550. Stelzer July 2,1957` 2,812,868 Crile Nov. 12, 1957 2,836,315 i Fuhrmann et al. May 27, 1958