US2820608A

US2820608A - Lifting device

Info

Publication number: US2820608A
Application number: US560638A
Authority: US
Inventors: Braselmann Ferdinand
Original assignee: Individual
Current assignee: Individual
Priority date: 1952-09-26
Filing date: 1956-01-23
Publication date: 1958-01-21
Anticipated expiration: 1975-01-21

Description

Jan. 21, 195s F@ BRASELMANN 2,820,608

. LIFTING'DEVICE Filed Jan. 23, 1956 United States Patent "O The present invention relates to a lifting device or jack.

More particularly, the present invention relates to a lifting device or jack by means of which a load may be raised or lowered in a step-wise manner.

There exist lifting devices capable of raising and lowering loads in a step-wise manner wherein the arrangement,

of the parts is such that a lever is continuously moved to and fro or oscillated so as to bring about step-wise raising or lowering. It has been found, however, that in existing lifting devices there exists danger that the load acts in such a manner as to overcome the normal step-wise operation of the device so that during raising and more fr equently during lowering the load will fall in one uninterrupted movement. This is undesirable, not only from the point of View of safety, but also inasmuch as rapid and unchecked descent of the load may very easily damage the same.

It is therefore an object of the present invention to provide a lifting device or jack which overcomes the abov disadvantages.

It is another object of the present invention to provide a lifting device or jack by means of which a load may be raised and lowered in a step-wise manner.

It is yet another object of the present invention to provide a lifting device or jack which under all circumstances prevents rapid and uncontrolled descent of the load supported by the lifting device.

The objects of the present invention also include the provision of a lifting device which is sturdy, which comprises a minimum number of parts, which may be built at very low cost, and which will give long periods of trouble free service.

With the above objects in view, the present invention mainly resides in a jack which includes an elongated shaft and a work member mounted on the shaft for sliding movement relative thereto in opposite directions. A first clamping member is arranged on the shaft and is movable relative thereto between an unclamping position wherein the first clamping member is freely slidable relative to the shaft and a clamping position wherein the first clamping member wedgingly engages the shaft in such a manner that sliding movement of the first clamping member in one direction is prevented. The first clamping member cooperates with the Work member in such a manner that when a force in the one direction is exerted on the work member, the same engages the first clamping member and tends to move the same in the one direction but also moves the same into and tends to maintain the same in its clamping position. A second clamping member is also arranged on the shaft and is movable relative thereto between an unclamping position wherein the second clamping member is freely slidable relative to the shaft and a clamping position wherein the second clamping member wedgingly engages the shaft in such a manner that sliding movement of the clamping member in the opposite direction is prevented. The second clamping member has. a tendency Patented Jan. 21, 1958 Fice normally to occupy its unclamping position. First moving means are adapted to be interposed between the first and second clamping members for first moving the second clamping member into its clamping position and for thereafter moving the first clamping member into its unclamping position against the effect of a force acting on the work member in the one direction and consequently acting on the rst clamping member to move the same into and to maintain the same in its clamping position. Thus, the force is permitted to move the work member and the first clamping member relative to the shaft in the one direction a distance which is just short of suliicient to move the first clamping member out of engagement with the first moving means whereupon the first clamping member is again moved into and maintained in its clamping position by the force acting on the Work member in the one direction, thereby preventing further movement of the work member relative to the shaft in the one direction.

A third clamping member is also arranged on the shaft and is movable relative thereto between an unclamping position wherein the third clamping member is freely slidable relative to the shaft and a clamping position wherein the third clamping member wedgingly engages the shaft in such a manner vthat sliding movement of the third clamping member in the one direction is prevented. The third clamping member has a tendency normally to occupy its unclamping position. Second moving means are adapted to be interposed between the third clamping member and the work member for first moving the third clamping member into its clamping position and for thereafter moving the work member relative to the shaft in the opposite direction against the effect of a force acting on the work member in the one direction.

This application is a continuation-impart application of my copending application Serial No. 255,406, filed November 8, 1951, now Patent No. 2,731,842.

The novel features which are considered as characterisn'c for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a side elevational view, partly in section, of a preferred embodiment of the present invention; and

Fig. 2 is a front elevational view of the embodiment illustrated in Fig. l. v

Referring now to the drawing, there is shown a preferred embodiment of a lifting device or jack according to the present invention. The jack includes a substantially vertical elongated shaft 5 which has a base portion 5a adapted to rest on the ground. If desired, a point 5b may be provided which is adapted to penetrate the surface upon which the jack rests so as to prevent sliding of the base portion 5a.

A bifurcated support member 4 is mounted on the shaft for upward and downward sliding movement relative thereto. The support member 4 has a pair of spaced substantially horizontal upper and

lower prongs

4a and 4b which are respectively formed with apertures 4a' and 4b therethrough. The apertures are aligned with each other and surround the shaft 5 in such a manner that the support member 4 is freely slidable relative to the shaft in opposite directions. The support member 4 additionally has a substantially vertical portion 4c which joins the upper and

lower prongs

4a and 4b. The portion 4c is formed with a recess or opening 4c which constitutes a receiving means the purpose of which will be described below.

A support element-6 is connected to thesupport mem- 'The clampingmemberlfisformedwith:anapertureila "therethrough which Ysurrounds"theshaftf'S rand '.which's so"shape`d thafthe'clamp'ing"member 1"is"movablerelatve to -theshaft between anunclam'ping position wher'eirrthe '-'clampingememberil is freely rslidablerielativeto 'the shaft and l a clamping" position-whereinitheclamping" member Y 1" wedginglyA engages "the shaft'in 'such"a"'m'anne1` that "downward slidingmovement ofthe' .clamping member 1 N'is prevented. TheI clamping member" 1 Lis "formed with fa" projecting-portion 1b which isadaptedfto'be` received jby Vthe receiving emeans' 4c'1 in *such "amanner `that" ythe supportmember 4-is"supported `bythe*clampingmember "1 at 1c. Ay coil'spring'7 is'interposed betweeneand abuts yagainst the lower pronglband the'clamping lmember ".1 for urging' the latter upwardly.

It `will be seen'that the clamping 'member' 1 `cooperates with the support member4V in such-a'mannerl'that when ar downward force-is 'exertedontheisupportmember, `was, forexamplq whenfthe' jack supports"aloadjthesupport -member -4 engages the clamping member 'lfand Vtends'to '-'move the' same downwardly. Additionally, the* support vmember `moves the clamping lmember f1 into -aidP-ten'ds,

ttomaintain thesame'in its clampingposit-i'on.

Y The"`clampingmemberl 2 isl formed with-amaperture 22b therethroughand is arranged" above 'theclamping mem- `4`bery 1 and between the latter'and theupper'prong""4a. TheY aperture '2b vsurroundsY the shaft-5 4and is so'shape'd 'that the'clamping Amember 2 is movable 1relativetothe 'shaft' betweenan unclampingposi'tion whereinthe 'clamping member 2 is freely slidable relativeto theshaftand V"a 'clamping position wherein the clamping-member 2 *wedgingly engages-thefshaft in-suchfa"mannerthat upward sliding movement `of 'the 'clampinge'member 2k -is prevented. -The clampngmember 2 has 'ae tendency fnormally to occupy. its unclampingpositionfbut a coilspring 10 may be` interposed between and abutting againsti'the Vclamping 'members VZand f3 for-i constantlyurging the v:former intoits unclampingjpo'sition. "I'he-"eect'offthe y'coilspring -10 upon they clampingA member'3`^w'il1 be2dis -cussed below.

The third clamping memberlis formedw'ith 4ani-aperf'ture af'there'through f arid isf :arranged below i the f-fupper prong i`4aand-'between the?` latter-and` vthe"'clampingfmem- 'ber`2. `Theaperture'Swsurronnds-theshaft' yand is shaped so that the clamping member 3 ismovableirelative tov` the shaft/between ani unclamping` position--whereini the iclampin'g 4member lis @freely Ysliiable relative `to the Vshaft "and-22211 vclamping#positionewherein` fthe i' clamping :member `3 1-wedgin'g1yf-' engagesthe shaft -infsuch- -amanner that idownward slidingmovementeoff'the clamping' member'- 3 is'Lprevented. TTherlampingf-meinber may'fhave-'a tend-V aency'* normally fto #occupy its -unclampingf position, *but may be constantly urged thereinto by the'coil-sprmg110 ewhich is -interposed'fbetweenthe'f'clamping Amembers 2 i fan'd 3.

. It will besseen'ffromethe -bove that'when'theiackis fsupportingca' loadffnot IAshown) which 'exerts -adownward force oni'the supporti member 4,A the y'clamping members v'.will: occupy' the f followingfrest positions -which vare `shown A-inFig 1: "'Ifhezclampingmemberllfwill,under the infuence ofr the ixiownwardifforce `acting on thevlsupport :member-At and'tran'smitteditofthe clamping member 1 aan 1'c; .occnpy its clampingi posit-ionfwhereinthe. 'clamping fmembenltis rwedged.@against1downward\,'=movement V.and secnnseqnentlwabler.:totsupport: thessupp ort member-a4, sr'sfliereas. the :.clampingrzmemherssz andzzioccnpytther respective unclamping positions. iii-Suitable :'movingsmeans are -provdedtfonrrasing .aand

lowering the support member 4, in a step-wise manner,

mromthe rest positionshown in`Fig."1.

The moving means for^lowering the support member 4 are constituted by a double-armed lever or cam 11 5 which is pivotally mounted on the clamping member 2 at-11a and which is adapted to engage a cam surface 1d which is formed on theiclamping member 1. When the cam 11 is pivoted in a clockwise direction, it is interposed between the clamping'.membersland 2. The arrangement of thehpartsfissuch that the clampingmemberfi.

is"v flrstmoved into' its clamping position against the action of the. spring;y 10; so; `thaty the clampingmember: 2. -is'wedged `against upward movement. Withngpward movementmf the clamping `rl=ren1ber^-^2 `vbein g prevented, the clamping 15 member 1 is thereuponmovedtoutfoffits clamping position into its unclamping position against the eect of the downward force acting on the support member 4 `and consequently acting on the clamping member 1 to main- -tain-fthesamein:its clamping-position. :aflnasmuchfas' the 20.1. c1amping.member.-;1 is :now no` longerrwedgedhagainst downwardsmovementg f the downward..forceaacting-:onthe support member-.4fisppermitted to movethersupportmembersls-.as .wellfasl the-clampingf-member 1 downwardly -,relativetotheshafteS. IIt-.will be.,nnderstood,1:however, n

-thatoncer this downward. force has.l movedl ythe:V clamping .memberr, `1 downwardly, a fdistanceewhich l: iscjust lsho'rt. ,of V sufiicient tamme--the clampingfmember 1' out-,of Iengagement.. with.- the f cam` 11, the.; downward forceeacting .onthesupport member#` will once.- againmove :the'clamping .member...1into,..and,maintain ittfinitszclamping .posi- V.tiolumtherebypreventing.further .downward .movementfof -thesupport.memberAzrelative tothe shafts. :,.Thiswi'll abe readilymnderstood .whenv it is keptfin .mindthatfthe ,.cam..11. y,or y.other movi-ng means interposed between .-:the

clamping members 1 and 2 is relied on to maintainathe clamping.. member 1 y,in .A its 1.unclamping posi-tion-;l during .downwardmovement yof the-support member4: andithe. .Y

vclamping member 1. But for the action of-.the camll,

the clamping Amemben 1` is. maintained.; in itszunclamping 40 position .by vthe `idownward force Y `acting on'.l the ;support member 4. In.practice, therefore, 'once .the camxshas been 4pivoted in` a y.clock-wise zdirection, ,fthe downward .movement of 'the supportamem-ber 4'.and thezclamping member 1 will .cease while Athere is:still;1somephysical contactl between the cam 11 .andthefzcam.surfacefldof the clamping .rnember.1.

".In l order to continue the downwardmovement; of .the

. support member. .4, the cam lllrspivotedam a .counter-` clockwise direction outtof ,engagement lwith `.the earn.k surface""1d. The clamping member 2 is `.thereuponrfree-to If l:desiredgthe,cam "11 may .beiformedwith .a sleeve Vportion 11b'having.a recess' 11c which. is..adapted.to.re

,ceive"`therewithin .theffreeend Aof alever. 12. .Thelever "maybe'removably received within then recess A11C.

'Additional fmoving means la1:e.provided .fonzstepfwise -raising iofthe"support member "4.` ".Thesemovingfmeans may .inchideatwarmedlever or Vcam '8 whichV is piv- 'otallyfmountedmnthe .upper prong 4a as indicated :at 8a. "'The: cam/f8! is=adaptedto Tengage a VcamA surface'r'b Awhich Amay fbe constituted `by`v the outenperipherhsmj- Ffacevofs air rollercewhichiisfrotatably "mounted-ofthe scopes clamping member 3 at 3d. When the cam 8 is pivoted in a clock-wise direction, it is interposed between the prong 4a and the clamping member 3 for first moving the latter into its clamping position against the action of the spring 10. Downward movement of the clamping member 3 is thus prevented so that during continued clock-wise rotation of the cam 8 the support member 4 is moved upwardly relative to the shaft 5 against the elect of the downward force acting on the support member. Inasmuch as the clamping members 1 and 2 are free to move upwardly, they will be so moved by the spring 7 which is interposed between the lower prong 4b and the clamping member 1. Thus, the clamping member 1 will at all times be maintained in engagement with the support member 4 at 1c, so that after the support member 4 has been raised a certain distance and the cam 8 is thereafter pivoted in a counterclock-wise direction, downward movement of the support member 4 will be prevented by the clamping member 1 which, as described above, is maintained in its clamping position by the effect of the downward force acting on the load. Thus, while in practice at least some physical contact is normally maintained between the cam 8 and the surface 3b of the roller 3c during counterclock-wise rotation of the cam 8, when the latter ceases to apply a substantial downward force on the roller 3c, the clamping member 3 will be moved upwardly into abutting engagement with the upper prong 4a and also into its unclamping position by the spring 10. The parts therefore, will once again assume the rest position illustrated in Fig. 1, and the above described elevating cycle may be repeated.

The cam 8 may be formed with a sleeve 8b having a recess 8c adapted to receive therewithin the free end of a lever 9. In practice, only one of the cams 8 and 11 will be actuated at any one time, so that the levers 9 and 12 may be constituted by the same element.

It will be seen from the above that when the jack supports a load, the same may be raised or lowered in a stepwise manner depending upon which of the moving means is actuated. Moreover, the arrangement of the parts is such that irrespective of whether the load is being raised or lowered, the same cannot undergo a continuous or other than step-wise descent.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of lifting devices differing from the types described above.

While the invention has been illustrated and described as embodied in a lifting device or jack for raising and lowering a load in a step-wise manner, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A jack comprising, in combination, an elongated shaftga Work member mounted on said shaft for sliding movement relative thereto in opposite directions; a first clamping member on said shaft and movable relative thereto between an unclamping position wherein said first clamping member is freely slidable relative to said shaft and a clamping position wherein said first clamping member wedgingly engages said shaft in such a manner that sliding movement of said first clamping memberin ope direction is prevented, said first clamping member.

i9 cooperating with said work member in such a manner that when a force in said one direction is exerted on said work member, the same engages said first clamping member and tends to move the same in said one direction but also moves the same into and tends to maintain the same in its clamping position; a second clamping member on said shaft and movable relative thereto between an unclamping position wherein said second clamping member is freely slidable relative to said shaft and a clamping position wherein said second clamping member wedgingly engages said shaft in such a manner that sliding movement of said second clamping member in the opposite direction is prevented, said second clamping member having a tendency normally to occupy its unclamping position; rst moving means adapted to be interposed between said first and second clamping members for first moving said second clamping member into its clamping position and for thereafter moving said first clamping member into its unclamping position against the effect of a force acting on said Work member in said one direction and consequently acting on said first clamping member to move the same into and to maintain the same in its clamping position, thereby permitting the force to move said Work member and said first clamping member relative to said shaft in said one direction a distance which is just short of suicient to move said first clamping member out of engagement with said first moving means whereupon said first clamping member is again moved into :and maintained in its clamping position by the force acting on said work member in said one direction, thereby preventing further movement of said work member rela. tive to said shaft in said one direction; a third clamping member on said shaft and movable relative thereto between an unclamping position wherein said third clamping member is freely slidable relative to said shaft and a clamping position wherein said third clamping member wedgingly engages said shaft in such a manner that sliding movement of said third clamping member in said one direction is prevented, said third clamping member having a tendency normally to occupy its unclamping position; and second moving means adapted to be interposed between said third clamping member and said work member for first moving said third clamping member into its clamping position and for thereafter moving saidv work member relative to said shaft in said opposite direction against the effect of a force acting on said work member in said one direction.

2. A jack comprising, in combination, a substantially vertical elongated shaft; a support member mounted on said shaft for upward and downward sliding movement relative thereto; a first clamping member on said shaft and movable relative thereto between an unclamping position wherein said first clamping member is freely slidable relative to said shaft and a clamping position wherein said' first clamping member wedgingly engages said shaft in such a manner that downward sliding movement of said first clamping member is prevented, said first clamping member cooperating with said support member in such a manner that when a downward force is exerted on said support member, the same engages said first clamping member` and tends to move the same downwardly but also moves the same into and tends to maintain the same in its clamping position; a second clamping member on said shaft and movable relative thereto between an unclamping position wherein said second clamping member is freely slidable relative to said shaft and a clamping position wherein said second clamping member wedgingly engages said shaft in such a manner that upward sliding movement of said second clamping member is prevented, said'second clamping member having a tendency normally to occupy its unclamping position; first moving meansV adapted to be interposed 4between said first and second clampingvmembers for first moving said second clamping member intoits clamping position and for thereafter- 7 rrlovi'ng, said rst lclarrlping member` `into its unclamping position against the effect- 'of" adownward force acting #on said support' member and consequently acting on saidrtrst clampingl memberto moveY the sameinto and'to maintain thelsame in its` clampingL position, thereby permitting -the downward force. to move said` support member and said rst clamping member downwardly relative to-said' shaft a distance which is justy short ofsuicient to move said i'stclampingmember out of" engagement with said rst movinglmean's whereupon said'irst clamping memberis again moved into and maintainedin itsl clamping position by the'l downward forceA lacting onsaid support member; thereby'preventing further downward movement of said support member relative to said shaft;V a third clamping member on said shaft and movablev relative -thereto betweenan-'unclampiug position wherein ksaid'third clamping member is freely slidable relative 'to said shaft and a clamping position wherein said third clamping member wedginglyfengagessaid shaftin such ay manner that-downward `sliding movement of said third clamping member is'prevented, said third clamping memberfhaving-y a tendencynormally to occupy lits-.unclampi'ng position; and secondt moving means adapted to beinterposed lbetween said'v third clamping member'and said support member for- .rst movingV said third clamping member into its clamping position and for thereafter moving Vsaid-support membenupwardly'relative to said'shaft against the effect offa'fdownward force acting: on saidsupport member.

. 3. A1 vjack comprising, in combination, a substantially verticalelongated shaft; a forked'support'member having a `pair :of spaced substantially horizontal upper and lower prongs: each of which is formed wit-h an aperture VVtherethrough, said apertures-being alignedwith eachother and surrounding said shaft Vin'suchfa manner; that said support member is slidableupwardly and downwardly -relative to said. shaft; arst clampingV member formed with an apert1n1e..therethroughi and arranged between said'prongs, saidav lastementioned` aperture surrounding said shaft and being. sofshapedthatfsaidiirst clamping-"member is movable relative, to. said shaft .between an. unclamping position 'l whereinasaid iirst clamping member is freely slidable relative to; saidshaft ancha zclamping; tpo'sition: wherein said rstl'amping member' wedgingly.v engages said shaft in such a manner. that .downward slidingf movement ofi said' mst-clamping'. memberisaprevented, said' rstl clamping member` cooperating: with'. said '.suppor-t; member in such a manner that when a downwardsforcezsveerted onfsaid.

support: member, the .same engages: :said first; clamping nlernher'and' tends-.temeva .the samezdownwardly. but also movesthe; samegintaand-.tendsio maintairr the same in itsizclarnpingposition; 'a second-.clamping member formed with 1an aperture atherethroughf andarrangedibetween1 said' first; slammingv member and; said upper prong,. :said lastmentioned aperture: surrounding said .shaft and being-so shaped:` thatzsaid'i'second, clamping member. is movable relative to; saidshaft@ between an unclamping position wherein; saidL .seoondclamping member. is. freely lslidahlel relat-ire to saidfshaftand a. clamping position wherein said second clamping ;memben wedgingly: engages. said? shaftk ill-:such a mannerfthat.upwardifsliding 'movement of said second :clampingfmem'bel:is'prevented, saidsecondelamp ing. member; having. a. tendency .normally `-toL oecupyits unclampng,` position;A first?. moving means adapted to-V bc` move-,the same into and tomaintain the same .in its clampngposition, thereby-permitting thef'downward'force tomovesaid' support member and.` said'y rst clamping memberrdownwardlyireiative'tofsaid'shaft a-distancewhich sjltslshort 'of sufficientito move said'tirst'clarnpingmenr- '8 ber outfoff engagementz with saidf' rst movingymeans vsfhereuporrsaidv yfirst clampingjmem'ber isfagain' moved into and maintained'iin' its clamping'positionby the down.- wardlforce actiugio'n` -sa'idv supportV member; therebyi pre.- ventingifurtherdownwardmovementfof saidsupport mem ber relative-.to said shaft; a third clampingmemberformed with@anraperturegtherethrough and arranged between'said secondi clampingj member and'- said upper .pron'g, `said -lastb mentio'ne'daperturey surrounding `said-shaft andbeingfso shaped .that .-saidrthirdh clamping memberisL movable relative to said shaftl'betweenf -anun'clamping position wherein 4. A jaelr,- comprisingalini combination, a-substantially vertical :elongated shaft; aforkedrsupportfmember having-- a pair1of:spaned:substantially'horizontalfupper andllowery prongs.- eachf onwhieh its-formed. with. an. aperture theref through; saidiapertureseheing alignedfwith each.other-.:and1l surroundihgrsaidzzsliaft. @such a manner .that said 'support 'z-membe'riisslidablefupwardly. and: downwardly rela tivefto;.;saidshaft;fantirstrclamping rmember formedwith an aperture therethrough. :and: arranged .between fsaida prongs,` said; iasthnentioned. lapel-.turesurrounding said shaft andeheinggsoi'shaped than said rstclamp'ing membei is moa/able; :relativa to. saidzzshaftrbetwee'nfan un.A clamping :pnsitionv wherein :said: first.. clamping member is freely,` slidable reiativeftos*saidafshaftrandfazclanrpingl position whereinsaidf tirstv clampingmember.- wedginglyI` engages rsaid. shaft-.fin sucha'manner thatfdownwardslid-T- ing movement-of saidffirst velamphi-g.memherisl prevented; said. first clampingfmernher -cooperang saidsuppolt? -member in such a manner thatiwhenaar'downwarcl: .forcelis exerted on said support memberthe,same engages-said irst clamping memherfand'tends; lto fmove-.the tsanre down.-

wardly. but also.:moves;-itlfrefsameiinto .andten'ds to :main` l tain the same in its.clampingposition;ja second1clamping, membery formed'. with; 'ani aperture. therethrough: and arranged.. betweenxsaid. .-tirstrcl'arnping; member. said upper prong, ,said ilastementirxnedaperturenurroiusndingsaidA shatandfheing so@shapedfthatrsaidfsecomkelamping member is movable relatiiveftoi said'zsh'aft between an unclamp.- ingl Iposition wherein said*v seconde clamping .members-is; freely slidable relative; to.: said .shaft and.. a1clamping po sitioniwherein; said; secondclampingmember wedgingly enga-ges,V said shaft. inil such a,v manner thatf upward slid.- ing movement of. said seeondolamping member. ris prevented; a third iclamping; member formed; with: an` aperture; therethrough and;Y arranged between said .second clamping; mem-ber and. said. upper. fprongiv said; 'last-.nrenLY tionedapertures `surrounding5 lsa-id-V shaft. and being so. shaped that saidthirdaclarnping. member isV movable rela-l tive tosaid. shaft .between anunclampingpostion wherein said third clamping member is 'freely lslidable relative to. said .shaft andA a clamping position whereinV saidA third clamping member wcdgingly engagessaid shaft' insuch a manner. that downward' sliding movement offsaid clampinggmember isprevented', saidthird clamping member `havinga tendency normally to Qccupyjitsunclamping position; Y spring means; for.. constantly yurging saidl secondl andthi'nd clamping members Vintoftheir respectiveeunclamp.-

Y ing positions; ii'rs't moving means adapted' to be inter-y posedv between. saidfirst` and second ctampingm'embers.

for'trst' movingisaid isecond clamping member'into' 'its clamping position against the action of said spring means and for thereafter moving said irst clamping member into its unclamping position against the eiect of a downward force acting on said support member and consequently acting on said first clamping member to move the same into and to maintain the same in its clamping position, thereby permitting the downward force to move Isaid support member and said first clamping member downwardly relative to said shaft a distance which is just short of suicient to move said iirst clamping member out of engagement with said irst moving means whereupon said rst clamping member is again moved into and maintained in its clamping position by the downward force acting on said support member, thereby preventing further downward movement of said support member relative to said shaft; and second moving means adapted to be interposed between said third clamping member and said support member for first moving said third clamping member into its clamping position against the action of said spring means and for thereafter moving said `support member upwardly relative to said shaft against the ciect of a downward force acting on said support member.

5. A jack as defined in claim 4 wherein said Ispring means are interposed between and abut against said second and third clamping members.

6. A jack as defined in claim 4, and additional spring means interposed between and abutting against said lower prong and said first clamping member for urging the latl 'ter into and maintaining the same in cooperation with said support member during upward movement thereof during actuation of said second moving means.

7. The combination defined in claim 6 wherein said rst moving means are first camming means including a first cam mounted on said second clamping member for pivotal movement relative thereto and a rst cam surface on said first clamping member and adapted to be engaged by said first cam, and wherein said second moving means are second camming means including a second cam mounted on said upper prong of said support member for pivotal movement relative thereto and a second cam surface on said third clamping member and adapted to be -engaged by Isaid second cam.

8. The combination defined in claim 6 wherein said support member has a substantially vertical portion joining said upper and lower prongs and formed `'with receiving means, and wherein said Jfirst clamping member is formed with projecting means adapted to be received by said receiving means in such a manner that said support member is supported by `said first clamping member.

References Cited in the le of this patent UNTED STATES PATENTS