CA1105895A - Pallet truck - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A pallet truck for moving articles is disclosed having a load-bearing chassis which can be elevated or lowered.
A hydraulic lifting mechanism is provided for lifting the chassis. The lifting mechanism comprises at least one piston which is fully immersed in the hydraulic fluid employed and a pump with at least one pump piston for actuating the lifting piston. Valve control means are provided in the steering head base which also carries the steering handle. The steering handle is used to actuate the pump piston. The valve control means can be manipulated by the operator for the pallet truck so as to select lowering or raising of the chassis. A neutral position in which the pump is disconnected from the lifting mechanism is also provided. A second valve control means permits the pump circuit to adapt to various loads. The truck in accordance with the present invention is of particularly durable construction and can easily be controlled by the operator.

Description

58~5 ~ Al.L,~' r~ ) (' K
l3~lC-gi~o~ of,_ti~ r'~
1'11is invelltion rc~]ates to -improvemellts ln palle-t trucks and more par-ticlllcll:ly Lo improvements in or relating to raising and lowering of thc l,oa(1-bearing platform or platforms of such pallet trucks.
I~allet t:rucks are well known in the material handling field. They usl,lally comprise a load-su;)porting cantilevered platform or chassis mov~able upon a plllra]ity oE supporting rollers or wheels. The chassis can be raised by means of a jack or lifting mecllanism which is actuated by a steering handle to raise the item or items carried on the chassis.
I,owering of the chassis is carried out by releasing the hydraulic fluid from ~I-le ]ifting cylinder associated with the lifting mechanism. rl'hese pallet trucks ar-e normal]y rated at a capacity of about 5 metric tonnes and, thus, are useful in moving pallets, containers, cra-tes and the like from one location to another where heavy-duty equipment, such as fork-lifts, cannot economically be ernployed.
Review of the Prior Art _ Pallet trucks as briefly described in the foregoing are known and have been described in the patent literature.
U.S. Patent 3,567,240 issued March 2, 1971, to Brassington discloses a dual lif-ting system for pallet trucks.
In accordance wi,th the proposal by Brassin~ton, lowering of -the chassis of a pallet truck is carried out at a ~aster ra-te than that used for raising o-E the chas.sis.
According to the patent of Brassington, there is provided a liEtinc~ nlechanisrn with an inner ]ift piston ~oined to an outer lift piston having a larger diameter than the inner lift piston, each piston being associated with a corresponding piston chamber. ~t a predetermined pressure, ~ check valve associated with the inner lift piston permits entry of ~58~S
)Iy(~ f~ ln~ c~t~ ,n ~ c,~Jf~ ron~ ? illller li~t chamber to ~llc~ outl~r ]i.ft l~alnb(~ . I'he t)roposal by Brassin(lton incluck~.s .:ip~on me.llls to cnsure that the outer lif-t chamber i.s sur.~.ic:i.ently (:il].e(l Wi.t}l hydrau].ic [luid. Membrane means are also provi(led ~:o !~revent leakaqe of a;iy hydraulic fluid from the outer pistorl chamber.
U.S. Patent 3,603,922 issued September 28, 1971, to Best et a]. disclos(?s a ~oot opei-a-ted pllmp to foree hydraulic fluid from a resc~rvoi.r .i.~ o a E)i.ston charnber, thereby raising the frame or chascs;.s o~ thc truck. .~ manually operable release mechanism is disposed tlear the steer:i.lly handle of the truek.
This mechanism incluc~es an ohli.(luely clisposed wedge mernber attached to the lowermost end of an actuating rod. The wedge member serves to actuate, via an intermedia-te finger, the plunger for the fluid control valve. Movement of the plunger of the valve in this manner causes opening of the valve, there-by draining the hydraulic fluid from the piston chamber, so that the chassis of the truck will be ].owerecl.
U.S. Patent 3,940,333 issued Yebruary 24, 1976, to Bryntse discloses a lifting mechanism for a pallet truck in which a pivotally mounted control lever may be moved -from a neutral locked positi.on, in which the chassis is at res-t, to one of two further posi-tions to cause lowering or raising of the chassis, respectively. With the control. lever in the neutral position the steering handle can he usecl for pulling : and steering of the pa].].et: truek.
U.S. Patent 3,959,970 issued June 1, 1976, to Bos et a]. is concerned with a hydraulic lift meehanism or jack for lifting vehieles therewith and, particularly, with the provision of a jack llavirlg a base plate on wllieh there are disposed a jaeh cy].i.nder and a pump cyli.nder, both equippecl with respective pi.stotls, and in which theJ^e are provided communication passages and control valvcs. The advantagc of S~S

~ ol~os~ sl~ si(les i~ elilnirlation of a separate va~e I~OU'~ n(l in tl-~c~ l)rc)vis i C~rl of a base plate made of substantially llni rorm mal~ri~l. (hickness.
~ .S. I'atellt ~ 82,767 i.ssued Ser~tember 28, 1976 to Larsson et a] cliscloses a hand operated pallet iruck having a release arrangelllellt for act-latincl an outlet valve of a lifting piston cham~er, ~o low~r the chassis of the pallet -truck. The primary advantaqe o-r the truck in accordallce with the ~arsson reference resides in the pro~ision of a pallet truck wherein the axis of the stc~crirlc3 wlleel intersects the vertical axis about which the whec~l ancl the jack assembLy rotate.
I].S. Patellt 4~olo~r~o9 issued March 8, 1977, to Boutroy et al discloses a pump piston arranged to face a jack piston, both pistons beincl arranged in a common, double-walled cylinder and havinc-l disposed between them a fixed, transverse partition. The clevice proposed by Boutroy is particularly useful for adjusting the height of components in machinery, for moving parts of seats, for office chairs ancl the like.
Pallet trucks are also disclosed in the following 20U.S. Patents:-Issued

2,424,417 Quayle July 22, 1947 2,439,781 Quayle April 14, 1948 2,681,202 Germond June 15, 1954 2,940,767 Quayle June ]4, 1960

3,136,128 Suter June 9, 1964 3,183,989 Trusock et al May 18, 1965 3,188,107 Quayle June 8, 1965 3,202,233 Do]phin et al AucJust 24, 1965 303,249,170 Quayle May 3, 1966 3,260,534 Knic3hts ~l~ly l2, l966 3,261,617 l~ecker et al Julv 19, 1966 8~5 3,28("9~5 T'dera Novemher 22, 1966 3,455,21]. ~ cJ~lay ~July l.rj, :l,969 Summary of ~he l'resent Invent.ion It. i.s all obje~ct: of the [)resent imvelltion to provide a pal],et truck whereill lowerinc~ alld raisin(3 of t~,le chassi.s of the truck are readily control,led.
It is a].so an object of the present invention to provide a fluid actuated piston and piston chamber arrangement which is especially suitab1e :Eor controllinc~ the rai.s:i.ng and lowering of a chassis of a pal,Let truck.
In accordance with the present i,nven-tion, there is provided a pallet truck comprising a chassis moveable between a-t least one load bearing position and at leas-t one load releasing position; a flui~i actuatecl pi.s-ton and piston cllamber arrangement disposed adjacent one end of said chassis whereby said piston operatively engages said chassis for movement thereof between the said positions; pump means for circulati.ng fluid flow to said piston; passage means for communicating said piston chamber with said pump means; and valve means disposed in said passage means for controlling the fluid flow between said pump means and said piston, said valve means selectively initiating and terminating fluid flow to move said chassis be-tween the said positions.' Also in accordance with the present invention, there is provided a fluid actua-ted piston and piston chamber arrangement comprising a base member having an aperture passing therethrough;
sealing means mounted in said aperture; a hydr.aulic fluid reservoir housing above said base member and carried thereby; a piston chamber housing extending downwardly from said base member;
a piston mounted within said piston chamber housing and extending upwardly therefrom through said sealing mearls and through sai.d r 8~5 reselvoir hc)llsincl to ~)roiec~ p~Jardly therefroirl, said piston slidably ~nqaqillg the sea],iny means; va]ve means mounted in said base member, purlll) mearls connected to sa;d valve means, first passage rlecllls ext,end;llg ~rom ttle resc~rvoir throuc3h the base member to the valve mealls, secol-cl passaye means extending from the piston chamber to the valve mea]ls, said pump means and valve means being operable to pump fluid from the reservoir through the first passage means, the valve means and the second passage means to the piston chamber to raise the pis-ton relative -to -the base member;
said valve means beinc3 operable to permit fluid -to return to the reservoir from the piston chamber througll the second passage means, the valve means and the first passage means and thereby permit lowering of the piston relative to the base member, said piston having a lower end portion shaped to provide by-pass means permitting fluid to pass from the pis-ton chamber to the reservoir past said seal means when said lower end por-tion of said piston traverses said sealing means.
Description of _he Drawings Specific embodiments of the inven-tion will now be described in more detail with reference to the accompanying drawings, wherein:-Figure 1 is a perspective view of a pallet truck inaccordance with the present invention;
Yigure 2 is a side elevation of the pallet truck shown in Figure l;
Figure 3 is a perspective view of the components of the lifting mechanism arranged on the steering head base of the pallet truck;
Figure 4 is a perspective, exploded view of the components arran(3ed on the steering head base, S 8 ~

~ r) j-~, " (licl~r.lml~lati.~ s~r~ o:[ ~h~
I)~clratll:ic circ~ fc-~ l.iEti~ mec~larli.sm o~ the pa.Llet truck;
I'igure h is a det~ )E t.he select:incJ hanclle o:E the tr~lck i.I~ a Fi.I~ O~;it.i~ cl l;`igure 7 i.~. a detai.] o~ the selec-ti.ng han(Ile in a third pos:i.tion.
Specific Desc__ptioIl ~ pal.let~. truck i.ncorporcll;i.ncJ t he Eeatures of the present invention -is (lenel~al].y il]ust1-atc(3 in Figures 1 and 2.
The truck has a ma;.n chassis 10 compr;sinc~ -two :Eork extensions 12 joined hy a connecting arch 14 with a forwardly directed boss portion 16.
The :Eork extensions 12 of chassis 1.0 are supported by wheels or rollers 1.3 which are associatecl wi-th linkaye sys-tem 20, not shown in det-ail.. ~'he :Eorwarcl pa:rt of -the linkage system is pivotall.y joined to the un(le]sside of transverse thrust plate 22. ~ steering head base 24 is di.sposed in -thrust plate 22~ Steering heacd base 24 has a lowermost rod extension 26 (Flgure 4) on which are mounted the Eorward two steering rollers or wheels 28. An upwardly extending clevis 30 is provided on the upper surface of steering head base 24 for mounting therebetween a steeriny hand]e 32.
The lower end of steeriny hand.le 32 (E'igure 3) serves to actuate a pump assembly generall.Y identi:Ei.ed by nuIlleral 33 for lifting mechanism, comprised o.f a ].ifting piston assembly generally designated by numeral 34; with a Elow control means comprised of a flow control valve assembly 35 and check valve assembly 36 arranged between the pump and pis-ton assemblies, all to he described i.n-detail.
The llitiIlg ,ci.ston assemhly 34 of the liftinc3 mechanism is capal)l.e of rai.sing, and can ~e controll.ed, by way of the flow cont.rol valve assembly 35, to lowe:r -the chassis 10.

~l~he rais~cll~osit;(,ll Or' (,l~cl~si~ ) is itld;cat,(~d in d~lsh lines in Fi(lure ~, ~vhile lhe lc,\~e~ (l pc).~ iol~ oF the chassis is shown in solid lilleS.
, In respor~s~ lo ,-~tuat:ioll Or Ille f]ow control valve assembly 35, the chassic; 1,0 cat) he p],aced in a neutral position or condition. Dur:illq tllis cond;l-ion, the chassis may be in the raised or lowcre(l yc-sit,ion or :in a position between these two ex-tremes. I~n ~-his conc~ition, ~he pUItl~ assemhly 33 is efective]y disconrlectecl from l,he ]iF~incJ pi~.ton assembly 34 ancd actuation o[ I-lle stecrinc3 hand]e ~,2 wil] not apply pumpiny pressure in the liJ-ting mec}lanism. rl'l-lus, during this COnditiOIl, the steering hal~l~l]e 32 is used to steer and move the truck from one location to another.
For the p~]l-pOsc oc this spc~cification, the Eorward end of the palle-t truck is that end havinc3 thc steering head base 24 associate(l therewith.
Thus, thrust plate 22 has ~ vertica] bore, not shown, through which the rod extension 26 of steer:ing head base 24 passes, a collar q0 and an antifriction beari,ng 42 arranged be-tween the respective facing walls of the steering head base 24 and the thrust plate 22. Beariny 42 is adapted to receive radial and axial loacls. Collar 40 and bearing 42 are retained by a snap ring 44 i,n a suitable groove, as is known in the art.
Near its lowermost end, the rocl exterlsj,on 26 has a transverse bore A6 in which a wheel shart 48 is journalled by a needle bearing 50. Wlleels 28, of WhiCil only one is shown in Figure 4, are secure(1 hy washers 52 an(l pins 54; and bolt 56 fixes the wheel assembly in bore 46.
Steerillq handle 32, Eormed of s~luare cross-section tubing has at its upper end a handle grip 58. Steering handle 32 is pivo-tally moull-ted with its lower encl in clevis 30 by way of an inverted U-silape(l bracket 60, rollers 62 and support bolts 64. The support bolts 64 also mount an actua-ting roller 58~

66 which transfers the pumping motion of the steering handle, i.e., movement of the handle 32 back and forth through a small arc as indicated in ~igure 2, to pump piston 68. Pump piston 68 has a head portion 69 and is received with its rod end in the pump cylinder chamber 70 of steering head base 24, with a wiper ring 72 and a seal ring 74 being provided for the pump piston 68.
While the steering handle 32 can move the pump piston 68 into the pump cylinder chamber 70, wnen handle 32 is moved in pumping action, tra-vel of the piston 68 is reversed by the action of coil spring 76 which is disposed between the upper face of the steering head base 24 and the pumping piston head 69.
A longitudinal roller 78 is supported on pin 79, mounted in the arms of clevis 30, for guiding parts of the actuating mechanism of the flow control valve assembly 35.
The actuating mechanism for the flow control valve assembly 35 firstly comprises a selecting handle 80 extending within the handle grip 58 of steering handle 32. Selecting handle 80 is securèd to or forms part of a guide end 82 slidingly disposed in the upper end of steering handle 32. A
finger tip lever 84 is rotatably secured on pin 86 which, in turn, is secured in guide end 82. Finger tip lever 84 has a circular, f irst cam section, eccentrically disposed with respect to the pin 86 and has a second cam section, somewhat paraboloid in shape.
As can best be seen with reference to Figures 6 and 7 the position of selecting handle 80 can be varied, depending on the position of finger tip lever 84. As shown in solid lines in Figure 6, the handle is in its lowermost position since the finger tip lever 84 rests with its side on the upper end of steering handle 32. For the sake of brevity, this position of the selecting handle 80, and of the finger tip lever 84, is ~ 1~58Q~;
re~erred to as position 1. During this position, the pump assembly 33 of the lifting mechanism is effectively connected to the lifting piston assembly 34 so that the chassis 10 can be raised in response to actuation of the steering handle 32 in pumping motion.
On repositioning of the finger tip lever 84, in clockwise direction, to the position shown in solid lines in Figu.re 7, the selecting handle 80 assumes the position indicated likewise in Solid lines. This position is referred to as position 2. During this position, the pump assembly 33 of the lifting mechanism is effectively disconnected from the lifting piston assembly 34 so that the chassis 10 will remain in the prevailing position, and handle 32 can be used to steer and/or pull the truck without inadvertent use of the pump assembly 33.
The selecting handle 80 can be moved further in an upward direction by the operator, as indicated in dotted lines, this movement only being limited by the handle grip 58 (Figure 7).
This action raises the finger tip lever 84 to position 3 and the flow control valve assembly 35 is thereby actuated so as to release the working pressure from the lifting piston assembly 34, thereby permitting lowering of the chassis, due to gravity.
Movement of the selecting handle 80 into one of the positions described is transmitted by rod 88 (Figures 8 and 4), a short chain section 90 and connecting pin 92 to a bell crank type operating lever 94, pivotally mounted on pin 96 in a trunnion 98 forming part of, or secured to, the side wall of steering head base 24. Pin 92 engages the bifurcated end 93 of operating lever 94 by way of a nut 100.
The vertically disposed arm of operating leve.r 94 interacts with control piston 102 (Figures 4 and 4) forming part . of the flow control valve assembly 35. Piston 102 has a sealing ring 104 associated therewith and a needle end 103 for receiving thereon spring 106.

_ g _ ~ 5~
The piston 102 is moved in bore 108 in steering head base 24, in response to the positions assumed by the selecting handle 80 transmitted by the linkage system compos~d of rod 88, chain section 90 and connecting pin 92 and by operating lever 94.
Thus, when the selecting handle 80 i5 in position 1, the vertically disposed end of lever 94 will just touch, but not displace, the control piston 102. When selecting handle 80 is moved to position 2, the operating lever 94 moves control piston - 10 102 into the bore 108 in base 24. When the selecting handle 80 is brought to the position 3, piston 102 is stilI further moved into bore 108.
In order to best describe the various passages and other components of the hydraulic circuit, the description will now proceed with reference to Figure 5. This shows, schematically, on the right hand side of the page, the pump assembly 33, previously described with respect to Figure 4, and the flow control valve assembly 35 described thus far is shown alongside of the pump assembly 33.

Bore 108 narrows into control passage 110, which passage leads into ball chamber 112 having disposed therein ball 114.
Chamber 112 communicates with a ball valve bore 116 receiving therein a composite ball valve member 118 having longitudinal peripheral passages 119 and being sealed by seal ring 120. Fluid flow through the longitudinal central passage 122 and transverse bore 124 is controlled by a needle 126, ball 128 and spring 130.
The ball valve bore 116 is closed by means of seal 132 and plug bolt 134.
Disposed alongside the flow control valve assembly 35 is check valve assembly 36 which is comprised of an adjustment screw 136 threadingly insertable in bore 138, the screw 136 being covered exteriorly by a cap 140 and washer 142. Screw 136 serves to compress a spring 144 thereby biasing a ball 146.

58~5 Tl~ ifi:in(~ o~ mb:ly 34 wi.ll t~e ck~cscribed next.
Il- compris~s l:i~tillcJ pistol) 150 c~ispos~d with its lower end in a bore l52 rorm~ a lowc~r cy];n(ler chamber :in rod extension 26 of steerin(J hc-.~a(ll:,.lc;c 2~. 'I'hc~ upt:)cr cn(:l o~ ~he p;.ston rod ].50 extends in uppc~. cy]incler (llambc~r 1.54 f-~xtcn(lil-~y above the upper face of the stceri ng hea(l bclse 24.
~s can hest be seell in Fi(~urc 5, a seal. 156 is arranc3ed i.n a slli.lal~l.e c~roove in Ihe ~alls oE bore 158 in steerillg hcad ba.~-;(? 24, a~ roximately ck~linratirlcJ the transition between lo~er cy]:inclei~ chamber 152 and ur~)er cylinder chamber 154. ~ reservoil^ nllt :I.60 with an associated sea]. ring 162 and wiper ring 164 closes the upper cy].incler chamber 154.
The o]lowing communications passages are provided between the main componen'cs of the hy(3raulic ci.rcuit shown in Figure 5. Passac~c ]70 -joins the upper cylirl(ler chamber 154 with bore ].38 o~ chec]c valve assembly 36. Uore 1.38, in turn, is connected to bore 108 by passage 172. q'he ball charnber 112 is connected to pump cylinder 70 by passage 1.74 and passage 176 leads from ball valve bore 116 to lower cyli.nder chamber 152 near seal 156. Passage 178 connects the check valve assembly 36 with the passage 176.
~t its lowermost end liftincJ rod 150 has in i-ts circumference a planar depression ].80. This cleE)ression allows hydraulic fluid to pass :From lower cy]inder chamber 152 to upper cylinder chamber 154 when the piston is disp]aced in upwardly direction until the depression 180 is bridged by the seal 156.
Lifti.ng rod 150 is Eurnished at i-ts upper end with a ball-receiving scat 182 for ball 184. I,all. 184 i.s receivecl in a suitable depression in a corresponclincJ ball. seat, not shown, located in the boss portioll 16, and serves as the friction-reducing joint hCtWeC'I~ boss portion 16 ancl steering head base 24.

~1~58~5 Operation of the pallet truck in accordance with the present invention is as follows:-In order to raise the chassis 10 of the truck, the finger tip lever 84 is brouyht to position 1, shown in solid lines in Figure 6, if required. This wili ensure that the operating lever 94 merely touches the control piston 102. When the handle is moved to actuate the pump piston 68 in a pump stroke, the hydraulic fluid, usually oil, is moved through passage 174 towards ball chamber 112 and ball 114 closes control passage 110; ball 128 is unseated and hydraulic fluid passes through transverse bore 124 into passage 176 and from thence into lower cylinder chamber 152, thereby raising the cylinder piston 150. Pumping of hydraulic fluid can thus be continued until the chassis 10 is raised to the desired level.
The chassis can be lowered by moving ~he selecting handle 3Q to position 3. This actlon causes fullest inward - movement of the control piston 102 into bore 108 by the inter-vention of operating lever 94. The ball 114 is moved by the needle extension 103 of control piston 102 to depress needle 126, which will unseat ball 128 in the central passage 122 of composite ball valve 118. The hydraulic fluid is now free to return from lower cylinder chamber 152 through passage 176, ball chamber 112, control passage 110, bore 108 and passage 172, bore 138 and passage 170 into cylinder chamber 154. The piston 150 will thus be lowered in response to the load carried on the chassis, due to gravity.
The chassis will rémain at rest in a prevailing position when the finger tip lever 84 is brought to position 2 (Figure 7) even though the handle 32 is moved in pumping motion. In this position operating lever 94 moves the needle end 103 of control piston 102 so as to keep the ball 114 unseated in ball chamber 112 and hydraulic fluid is merely moving back and forth through passages 170 and 172 and 174 r ~ 5~3~

wil.-l~o~lt In(~vi.llg Ll~c~ pi~;tOIl 15() ~nl i:l a clif:l~erent posi tion is selectecl.
~ d-just~ ml: c-f ~lle .screw 13(; wil] permit reciulation of t}le E)rC~ic;~ c~ r~ Or ~ Lr~ . '['1-~ ,, as t~ spri~ ]44 is compres~ed, by tllrll-ing t~le c,crew 136 s() as to ~;énetrate deeper into bore ]38, lhc ra~;rlcl o~ thc~ unit is increased. Turning of the screw in the opposi.t e (lireCtiOIl wi.1.1 re~luce the pr~ssure rating o~ the uni~.
'I'he check va]ve assemblv is operative when the pressure in ~I-Ie systelll is excessive, i.e., the load to be carried on the cl~as.sis ]0 is too great. This has -the following effect:- hyc1raulic ~luicl is moved throuc~?l the passage 176 and thence through pa~sage 178, since the pressure in the system will maintain ba]l l28 in closing posit:ion in -t}le composite ball valve member 118. 'I'he hyclraulic f I Ui cl wil] thus unseat bal] 146 and retul^ll to cylincler chambel- 154 thereby permitting lowerinc3 of the pislon rod ]50.
SprincJ 106 is biasincl the conlro] piston 102 so that the system is ready for use, i.e., ready for ra:isin~ the chassis by moving the handle 32 in pumping mo-tion.
The upper limi-t of -travel of piston 150 is conveni-ently controlled by -the planar depression 160 near the lower-most end of piston 150. Thus, when reachinc3 -the seal 156, with this depression, the hyclraulic fluicl is ~ree l-o pass from tlle cylinder chamber ]52 to chamber 154 without a physical re-- s-traint beinc~ placcc~ o~ the piston 150 thus a]so adding to the longevity of the system.
It wil] be apparent from the fore~going -that the pallet truck in accordance with the pre~sent inventioll has a number of irnportantt ac~vantac~es.

For one, t-he liftillc~ piston 15() ~it-h its associated seal 156 in the liFLin(l system is completely immersed in the hydraulic fluid, normally oil. This is in contraclistinction to p:rior ~Irt proposa:J.!-~ wl~e-rei.n oi is a(3mi.t.ted to the pis-ton from a separat-c oi.l reservoir~ t.hereby givinc3 rise to moisture accumulati~ ancl :Ic~(l:in(l l-o dalllclclc l~y rno;.st:~lre, i.c., corrosion, of the ~)ist:on all(l detel~:ic-rati.c)rl o[ I-hc~ assoc.iclted sea]. or seals when the pistcll ;.s e.Yp0se(l I:o sucll conditions (:Ibrillc3 l.ong periods of rest.
~ urthermorc, ~hc pall.et truck in accordance with -the present invention Call easi].y be o~)erat-cd by the manipulation of a selectincJ hanclle -tha~. c~S~l-en(]s subst~rl~.ia].]y coextensivel.y within the main s~-ec:rinq yrip. Easy col-l-trol of position selection is affol^(le(l by finyer ti.p l.ever 84.
Furthermore, the pressure rating of the system is reaclily adjusted to ensure that the palle-t truck can accomodate different weights ancl ~rc.ssurcs.
~ elec-tion of the neutral posj.ti.on or concli-tion of the truck is useul. in e].iminating undue pre.ssure in the system leading to possi}.~]e }~reakclge of componen-ts thcrein.

- ].4 -

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluid actuated piston and piston chamber arrangement, comprising:
a base member having an aperture passing therethrough;
sealing means mounted in said aperture;
a hydraulic fluid reservoir housing above said base member and carried thereby;
a piston chamber housing extending downwardly from said base member;
a piston mounted within said piston chamber housing and extending upwardly therefrom through said sealing means and through said reservoir housing to project upwardly therefrom, said piston slidably engaging the sealing means;
valve means mounted in said base member, pump means connected to said valve means, first passage means extending from the reservoir through the base member to the valve means, second passage means extending from the piston chamber to the valve means, said pump means and valve means being operable to pump fluid from the reservoir through the first passage means, the valve means and the second passage means to the piston chamber to raise the piston relative to the base member;
said valve means being operable to permit fluid to return to the reservoir from the piston chamber through the second passage means, the valve means and the first passage means and thereby permit lowering of the piston relative to the base member, said piston having a lower end portion shaped to provide by-pass means permitting fluid to pass from the piston chamber to the reservoir past said seal means when said lower end portion of said piston traverses said sealing means.

2. A piston and piston chamber arrangement according to claim 1 wherein said valve means comprises a valve having a spring-biased closure member movable to an open position by pump pressure to permit fluid to pass therethrough to said second passage means, and also including manually-operable means for moving said closure member to an open position to permit fluid to pass from the second passage means to the first passage means, said closure member moving to the open position in a plane substantially perpendicular to the direction of movement of the piston and substantially perpendicular to the shortest imaginary line between the valve and the piston.

3. A piston and piston chamber arrangement according to claim 2 wherein the valve means also comprises a second valve, said second valve having a closure member responsive to a predetermined pressure in the second passage means to move to an open position to permit fluid to flow from the second passage means to the first passage means, the closure member of the second valve moving to the open position in a direction parallel to and in the same plane as the direction of movement of the closure member of the first-mentioned valve.

4. A piston and piston chamber arrangement according to claim 1 wherein said pump means comprises a pump piston housing mounted on the base member and a pump piston slidably mounted therein and projecting upwardly therefrom.