US2179765A - Walking tractor - Google Patents

Description

@v. M. 1939. H. F. STOCK 2.179.765

WALKING TRACTOR Filed May 24, 1938 4 Sheets-Sheet 1 his A T TURN; Y.

Nwv. 14, 11939. H. F. STOCK v WALKING TRACTOR Filed May 24, 1938 4 Sheets-Sheet 2 IN VE N TUE- HARE Y E 5 TUE'K hi5 ATTUl-F'NE Y Nnv. 14, 1939. H F, sTogK 2,179,765

WALKING TRACTOR Filed May 24, 1938 4 Sheets-Sheet 5 IN VE N THE.

- HARRY I STUD? I p% his ATTDIFNE"F- Patented Nov. 14, 1939 UNITED STATES 18 Claims.

'This invention relates to a walking tractor and is designed primarily for propelling heavy machines, such as excavating machines. It is often necessary for excavating machines, particularly 5 drag line excavators, to work in and to be propelled over ground which is so soft or swampy that the traction devices, such as crawling traction mechanism, will sink into the ground to an extent that will render the operation of the propelling mechanism very diflicult and often impossible. It has been proposed heretofore to provide such a machine with a large base to support the same in working position and with narrow platforms or treads at the sides of the 5 machine, operating mechanism being provided to advance the treads and to then lift the forward portion of the base, and the machine as a whole, and mpve the same forwardly with respect to the treads, the rear portion of the base dragging my on the ground during the forward movement thereof. With such an arrangement the base will sink into the ground a substantial distance and for the most satisfactory operation the forward edge of the base should be elevated in a substantially straight line far enough to clear F the upper edge of the depression, in which the base has been resting, before forward movement is imparted thereto. Heretofore the operating devices for such propelling mechanisms, so far w as I am informed, have been of such a character that the base begins its forward movement simultaneously with its upward movement and as a result it must displace the front wall'of the recess and the ground in front thereof as the g base moves forwardly and upwardly to its highest position. Further, if the surface of the ground slopes upwardly in front of the base, and even if the base rests substantially on the surface of the ground, it will in its forward movement engage the ground and be required to displace substantial portions thereof as it advances. This displacement of earth greatly increases the power required to operate the propelling mechanism, as well as retarding that operation, and is otherwise objectionable.

One object of the present invention is to provide a propelling mechanism of'this general type in which the forward edge of the base will be moved a substantial distance in a substantially vertical line before any forward movement is imparted thereto, thus avoiding the necessity of displacing the ground in front of the base, or greatly reducing the amount of ground which is so displaced. A further object of the invention is to provide PATENT OFFICE WALKING TRACTOR Harry F. Stock, Marion, Ohio, assignor to The Marion Steam Shovel Company, Ohio, a corporation of Ohio (1927) Application May 24, 1938, Serial No. 209,769

Marion,

a propelling mechanism of this kind which will be simple in construction and of a strong durable character.

A further object of the invention is to provide such a propelling mechanism which will operate 5 continuously to advance the machine by rapidly recurring steps.

A further object of the invention is to provide a propelling mechanism of such a character that a slight yielding under excessive stresses will not 1 cause breakage or binding.

Other objects of the invention will appear as the mechanism is described in detail.

In the accompanying drawings Fig. 1 is a plan view of an excavating machine embodying my invention, with the cab structure in section and the boom removed; Fig. 2 is a side elevation of such an excavating machine, partly broken away; Fig. 3 is a detail view of the driving mechanism which operates the propelling mechanism; Fig. 4 is an end elevation of the machine; Fig. 5 is a transverse sectional view taken through one of-the treads and its operating mechanism; Fig. 6 is a section taken on the line 6-6 of Fig.

5; Fig. 7 is a section taken on the line 1-1 of Fig. 5; Fig. 8 is a side elevation of one of the tread operating mechanisms showing the tread in its advanced position with the base resting on the ground; Fig. 9 is a similar view with the forward edge of the base in its elevated position; and Fig. 10 is a similar view showing the base in its fully advanced position and again resting upon the ground.

In these drawings I have illustrated one embodiment of my invention and have shown the same as applied to a drag line excavator of a known construction but it will be understood that the invention may take various forms and may be applied to machines of various kinds.

The drag line excavator here shown comprises a platform I! which supports the usual boom i2, the power plant and the mechanism for operating excavating element or scoop, which is not here shown but which is supported by a cable from the point of the boom. The power plant 5 and the operating mechanism for the scoop form no part of the present invention but, as here shown, the power plant consists of a Diesel engine l3 connected by a belt IS with a shaft l6 on a frame I1 carried by the platform. Secured 5() Mounted on the shafts 23 and 25 are v cable drums 26 and 21 on which are wound the cables for controlling the scoop. The usual clutch and brake mechanisms are provided for controlling the operations of the drum but are not here shown. A pinion 28 secured to the shaft 23 meshes with a gear 29 secured to a shaft 30 which drives the propelling mechanism as will be hereinafter described. Mounted on the platform at each side thereof are side frames 3| of a strong rigid character which, in the present construction, form parts of the cab which encloses the mechanism on the platform. At each side the platform is provided with lateral extensions 32 of relatively narrow fore and aft width which project substantial distances beyond the body of the platform. Mounted on each lateral extension 32 is an upright frame member 33 the upper end of which is connected by a transverse member 34 with the adjacent side frame 3|. A diagonal frame member 35 connects the lower end of the upright frame member 33 with the inner end of the transverse frame member 34 and thus provides a very strong rigid frame structure which is of approximately triangular shape. Preferably the upper portion of the upright frame member 33 is offset outwardly so that it projects a short distance outward beyond the lower end of that frame member.

The platform is supported normally by a base 36 which is preferably circular in form and is here shown as of. a diameter slightly less than the distance between the outer ends of the frame extensions 32, the diameter being in the present instance approximately thirty feet. The platform is so connected with the base that the latter will move upwardly and forwardly with the platform. In the construction here shown the platform is rotatably mounted on the base and the connecting device is of such a character as to permit of the rotation of the base while holding the platform and base against relative vertical movements. The connecting device may be of any suitable construction and as here shown comprises a stud or pintle 3! rigidly secured to the base and journaled in a bearing 31a rigidly secured to the platform and provided with a bushing 31b which is movably supported in a curved seat in the fixed part of said hearing so that the stud 31 may tilt slightly with relation to said hearing. The stud extends above the bearing and has secured thereto a flanged member 31d forming a socket which receives a stud 31c rigidly secured to the platform and having a head projecting beneath the flanged top of the socket to cause the stud 31 and base to move upwardly with the platform. The platform is maintained in substantial parallelism with the base by means of rollers 38 arranged between and traveling on annular tracks 39 and 40 mounted respectively on the platform and the base. If desired, the base may be provided with a circumferential flange 36a and brackets 36b may be secured to the forward portions of the platform and provided with lugs 360 which extend beneath the flange and are adapted to engage the same and thus limit the tilting movement of the platform with relation to the base.

Rigidly secured to the base is a ring gear 4| with which meshes a pinion 42 on a vertical shaft 43 which extends upwardly through the platform and is connected with the usual driving mechanism, not here shown, to rotate the platform on the base.

At each side of the platform there is mounted a long and relatively narrow platform or tread 45, the treads being spaced apart a distance slightly greater than the diameter of the base 36, and, in the present instance, having their inner edges extending beneath the outer ends of the respective platform extensions 32. These treads are connected with the platform by suitable operating mechanism which will support the treads in elevated positions during the operation of the machine and will move the treads downwardly and forwardly into engagement with the ground in advance of the base and will then lift the platform and base with relation to the treads and move the same forwardly to an advanced position, this mechanism being in the present instance of such a character that the initial upward movement of the forward edge of the base will be in a substantially straight line and this forward edge of the base will have a relatively short forward movement as it completes its movement to its maximum elevation, and will thereafter be moved gradually forwardly and downwardly into engagement with the ground. It may be noted here that a drag line excavator is usually propelled from one position to another in a direction away from the point of excavation, so that with relation to the propelling movement the boom end of the machine constitutes the rear end thereof and the forward movement of the machine will be to the right, as shown in Fig. 2. Preferably the operating mechanism is connected with the platform in such relation to the center of gravity thereof that the forward portion only of the base will be elevated and the rear portion of the base will drag over the ground as the base is moved forwardly.

The operating mechanism for accomplishing this result may take various forms but, in the construction here shown, each operating mechanism comprises a lever 46 which is pivotally mounted at its lower end on the tread 45 near the center of the latter and is pivotally mounted at its upper end on a crank 41 mounted on the platform, the crank in the present instance being provided with a wrist pin 48 which is journaled in a bearing 49 at the upper end of the lever 46. It isalso preferable that the tread should have movement about both a transverse axis and a longitudinal axis in order that it may adjust itself with relation to any irregularities in the surface of the ground on which it rests. For this purpose the tread is provided with bearings 50 arranged in alinement lengthwise of the tread and in these bearings are journaled trunnions of a second bearing member 52, which is provided with ears 53 arranged to receive between them the lower end of the lever 46, which is connected with the second bearing member of a pivot pin 54 which extends through the ears and through the lever.

The cranks 41 may be actuated in any suitable manner but preferably they are connected with the respective ends ofa transverse shaft, such as the shaft 30, which is driven from the engine on the platform. Due to the possibility of some yielding or relative movement of the parts of the frame mechanism under excessive strains it is desirable that this shaft should be capable of a slight flexing movement to avoid binding or injury to the mechanism under such conditions. It is therefore here shown as comprising three parts, a main or intermediate portion 55 and end portions 56, the end portions being connected with the intermediate portion for a limited movement about transverse axes with relation thereto. In the construction illustrated the side frames 3| are provided with bearings 51 in each of which is rotatably mounted a sleeve 58 adapted to receive the adjacent ends of the intermediate portion 55 and one end portion of the shaft, the ends of the two portions of the shafts being keyed or otherwise secured in the sleeve for rotation therewith but being capable of a slight movement with relation to the sleeve about a transverse axis. Each crank 4'! is provided with a trunnion member 59 which is rigidly connected with the adjacent end portion 56 of the shaft, as by means of opposed flanges 60 bolted one to the other. The trunnion 59 is journaled in a bearing 6| in the adjacent upright frame member 33 and this hearing is so shaped, as by curving the same about a transverse axis, as to permit of a slight tilting of the trunnion in the bearing.

Means are provided for so controlling the positions of the levers 46 with relation to the cranks during the movement of the latter as to cause the desired 'movements to be imparted to the treads and to the platform and base. In the construction illustrated each lever 46 forms one arm of a bell crank, the other arm 62 of which extends lengthwise of the platform. Preferably this arm 62 extends forwardly from the upper end of the lever 46 at approximately a right angle thereto. The arm 62 is braced against the lever 46 by a bar 63 rigidly connected with the forward end of the arm 62 and with the lower portion of the lever 96, and in the present instance formed integral with the arm and lever, to rigidly brace the same one against the other. Pivotally connected with the forward end of the arm 62 is a bar or link 64 which extends upwardly therefrom and is pivotally connected at its upper end on the upright frame member 33 at a point spaced a substantial distance above the crank, as shown at 65. As has been stated, the upper portion of this frame member 33 is offset outwardly and the link M is connected withthis ofiset part so that the link will lie in a substantially vertical plane.

When the platform is supported on the base 36, in working position, the cranks 4'! will normally extend upwardly from the shaft 30 and will thus support the treads 45 in their elevated positions, in which positions they will not interfere with the rotation of the platform. When it is desiredto propel the machine the shaft 39 is connected with the engine and rotated thereby to cause the cranks to rotate in a clockwise direction, as shown in Fig. 2, thus moving the cranks from their upwardly extending positions to forwardly extending substantially horizontal positions, as shown in Fig. 8, and moving the treads downwardly into engagement with the ground. During this movement of the cranks the downward movement of the treads and levers 46 with relation to the platform will cause the links 64 to swing the levers forwardly as they move downwardly andthus move the treads to advanced positions with relation to the base. The treads being now in contact with the ground they can have no further downward movement and as a result the continued rotation of the shaft, in the same direction, causes the cranks to fulcrum on the upper ends of the levers and to move the shaft upwardly with relation to the treads, this upward movement of the shaft serving of course to elevate the forward edge of the base and the platform. As the shaft moves upwardly with relation to the treads the upward movement of the platform will cause the lower ends of the links 64, which are connected with the arm 62 of the bell crank, to move rearwardly and upwardly, from position a to position b in Fig. 9, and this rearward movement of the bell cranks shifts the upper ends of the levers,

on which the cranks are fulcrumed, rearwardly and toward the vertical plane of the axis of the shaft, as shown in Fig. 9. This rearward movement of the fulcrums maintains the same in such relation to the shaft 30 that the latter will move in a substantially straight line during a substantial portion of its upward movement. Due to this straight line movement of the shaft the forward edge of the base will also move upwardly in a substantially straight line from its initial posi tion 0, which is the position shown in Fig. 8, until it has reached the position shown in broken lines at d in Fig. 9. The continued rotation of the shaft will then cause this forward edge of the base to move upwardly and forwardly through a short are e, to its maximum elevationQas shown in full lines in Fig. 9. It will be apparent therefore that the base will complete its upward movement during a relatively short forward movement thereof, this forward movement being in the present instance approximately two-sevenths of the entire distance which the base is to travel. This straight line movement of the forward edge of the base may be sufficient to move the edge of the base above the forward edge of the depression in which it has been resting but even if the depression is of a depth equal to the thickness of the base, the latter will be elevated in a straight line to such a height that only a relatively small portion of the ground in advance of the depression will have to be displaced by the base. While a substantially straight line movement of the forward edge of the base during its initial upward movement will enable the edge of the base to clear all or asubstantial part of the ground in front thereof I prefer that the line of upward movement shall curve slightly to the rear during this initial movement of the base, as shown at f in Fig. 9. As shown in the drawings the action of the links 64 is such as to move the axes on which the cranks fulcrum slightly to the rear of the vertical plane of the shaft axis, thus causing the forward edge of the base to move rearwardly out of contact with the ground in front of it and also providing a vent to relieve the suction beneath the base, thereby further reducing the resistance to the upward movement of the base.

During the movement of the base to its maximum elevation the cranks will have moved into vertical positions extending upwardly from their fulcrum points on the levers. The continued rotation of the shaft will cause the same to move forwardly and downwardly but the first portion of this movement will be through a shallow are so that the forward edge of the base will have very little downward movement and as the shaft continues its downward movement the forward edge of the base will move forwardly and gradually downwardly into contact with the ground, the links 64 so controlling the positions of the levers that the base will be lowered gently onto the ground. The parts then assume the positions shown in Fig. 10 in which the cranks extend rearwardly from the axis of the shaft but inasmuch as the base is now resting on the ground the cranks will again fulcrum on the platform and the treads will be again restored to their elevated positions,.as shown in Fig. 2. The con tinued rotation of the shaft'will again move the treads and base through this same cycle of operations and will cause the machine to be advanced by rapidly recurring steps as long as the shaft continues to rotate.

For the purpose of controlling the tilting movements of the treads about their pivotal connections with the levers 46 the treads are provided in front of and to the rear of the transverse axis 54 of the arms with stops 68 and 69. Rigidly secured to the levers near the lower ends thereof and in front thereof are stops 10 arranged to cooperate with the stops 68, and secured to the platform extensions 32 are stops H arranged to cooperate with the stops 69. The arrangement of the several stops is such that when the treads are in their elevated positions the stops 68 and 10 and 69 and II will respectively be either in contact one with the other or close together so that the treads can have little or no movement about their pivotal connections with the levers. As the treads are moved downwardly with relation to the platform these stops will be separated, as shown for example in Fig. 8, so that the treads are free to adjust themselves in accordance with their movement and in accordance with the irregularities in the ground with which they engage.

While I have shown and described one embodiment of my invention I wish it to be understood that I do not desire to be limited to the details thereof as various modifications may occur to a person skilled in the art.

Having now fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a propelling mechanism for a machine which comprises a platform and a supporting base. connected with said platform for upward and forward movements therewith, treads at the respective sides of said base, operable devices connecting the respective treads with said platform, means carried by said platform for operating said devices to move the forward edge of said base upwardly and to move the base as a whole forwardly, and means connected with said operable devices and with a relatively stationary part of said machine for positively controlling the movement of said operable devices to cause the forward edge of said base to move upwardly substantially in a vertical line, to then move upwardly and forwardly to its maximum elevation and to then move forwardly and gradually downward to its supporting position.

2. In a propelling mechanism for a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, treads at the respective sides of said base, cranks rotatably mounted on said platform above the respective treads, members connecting said cranks with the respective treads, means for rotating said cranks, said cranks acting through said connecting members to move said treads into engagement with the ground and to then move said platform and said base upwardly and forwardly with relation to said treads, and means connected with said connecting members and with a relatively stationary part of said machine for controlling the movement of said connecting members to cause the forward edge of said base to move in a sub stantially vertical line during the initial upward movement thereof, to then move upwardly and forwardly quickly to its maximum elevation and to then move slowly downwardly and forwardly into engagement with the ground.

3. In a propelling mechanism for a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, treads at the respective sides of said base, cranks rotatably mounted on said platform at the respective sides thereof, means for rotating said cranks, levers pivotally connected at their upper ends with said cranks and pivotally connected at their lower ends with said treads, and means for controlling the positions of said levers with relation to said cranks during the movement of the latter to cause said cranks to move said treads into engagement with the ground and to then fulcrum on said levers to move the forward edge of said base upwardly in a substantially vertical line, then upwardly and forwardly to its maximum elevation and then downwardly and forwardly into engagement with the ground, and to then cause said cranks to elevate said treads with relation to said base.

4. In a propelling mechanism for a machine which comprises a platform, and a supporting base connected with said platform for upward and forward movements therewith, treads at the respective sides of said base, cranks rotatably mounted on said platform at the respective sides thereof, means'for rotating said cranks, levers pivotally connected at their upper ends with said cranks and pivotally connected at their lower ends with said treads, arms rigidly secured to said levers, and means acting on said arms to control the positions of said levers with relation to said cranks during the movement of the latter.

5. In a machine comprising a platform and a supporting base connected with said platform for upward and forward movements therewith, propelling devices at the respective sides of said machine, each propelling device including a tread, a. member pivotally mounted on said tread and extending upwardly therefrom, a crank rotatably mounted on said platform, pivotally connected with the upper end of said member and adapted to fulcrum alternately on said platform and on said member, and means controlled by the movement of said crank while fulcrumed on said member to move said member about its pivotal connection with said tread, and means for actuating said cranks.

6. In a machine comprising a platform and a supporting base connected with said platform for .upward and forward movements therewith, a

shaft rotatably mounted on said platform, a crank connected with said shaft at each side of said platform, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected with the adjacent crank, means for rotating said shaft to cause said cranks to move alternately about the axis of said shaft and about the axes of their pivotal connections with said levers to move said platform and said treads with relation one to the other, and means controlled by the relative movement of said platform and said treads to move said levers about their pivotal connections with said treads.

7. In a machine comprising a platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft rotatably mounted on said platform, a crank connected with said shaft at each side of said platform, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected with the adjacent crank, means for rotating said shaft to cause said cranks to move alternately about the axis of said shaft and about the axes of their pivotal connections with said levers to move said platform and said treads with relation one to the other, and means operating while said cranks are fulcrumed on said levers to move those parts of said levers on which said cranks are fulcrumed toward the vertical plane of the axis of said shaft.

8. In a machine comprising a platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft rotatably mounted on said platform, a crank connected with said shaft at each side of said platform, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected at its upper end with the adjacent crank, means for rotating said shaft to move said cranks from upwardly extending positions to forwardly extending positions, thereby moving said treads into engagement with the ground, and toy then cause said cranks to fulcrum on said levers and move said shaft upwardly, and means acting on said levers during the initial upward movement of said shaft to move the upper ends of said levers toward the vertical plane of the axis of said shaft.

9. In a machine comprising a platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft rotatably mounted on said platform, a crank connected with said shaft at each side of .said platform, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected at its upper end with the adjacent crank, means for rotating said shaft to move said cranks from upwardly extending positions to forwardly extending positions, thereby moving said treads into engagement with the ground, and to then cause said cranks to fulcrum on said levers and move said shaft upwardly, and controlling members connecting said levers with said platform and acting on said levers during the initial upward movement of said shaft to move the upper ends of said levers toward the vertical plane of the axis of said shaft.

10. In a propelling mechanism for a machine which comprises a platform and a supporting 7 base connected with said platform for upward and forward movements therewith, cranks rotatably mounted on said platform at the respective sides thereof, means for rotating said cranks, bell cranks pivotally mounted on the respective cranks and each having an arm extending downwardly from'its axis and pivotally connected with the adjacent tread and having a second arm extending from the first mentioned arm adjacent said axis lengthwise of said platform, and means for movably connecting the second arm of said bell crank lever with a fixed part on said platform to control the position of said bell crank with relation to its crank as the latter moves from one position to another.

11. In a propelling mechanism for a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, cranks rotatably mounted on said platform at the respective sides thereof, means for rotating said cranks,

bell cranks pivotally mounted on the re"pective cranks and each having an arm extending downwardly from its axis and pivotally connected with the adjacent tread and having a second arm extending from the first mentioned arm adjacent said axis lengthwise of said platform, and links,

respective sides of said base, cranks rotatably mounted on said platform at the respective sides v thereof, means for rotating said,cranks, a lever "pivotally mounted at its upper end on each crank with said arms and with said frame members at points spaced from said cranks.

13. In a propelling mechanism for a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, upright frame members rigidly supported on said platform at the respective sides thereof and each having a bearing, a shaft mounted on said platform and having end portions journaled in said respective bearings, a crank secured to each end portion of said shaft, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected with the adjacent crank, said lever having a part extending lengthwise of said platform, a bar pivotally connected I at one end with said partof said lever and pivotally connected at its other end with the adjacent frame member above said crank, and means for rotating said shaft.

14. In a propelling mechanism for-a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, bearings at the respective sides of said platform, a transverse shaft journaled on said platform and having end portions journaled in the respective bearings, said end portions being connected with the intermediate portion of said shaft for limited movement transverse thereto and said bearings being arranged to permit the end portions of said shaft to have movement about a transverse axis, cranks secured to said end portions of said shaft, treads at the respective sides of said platform, a lever connecting each tread with the adjacent crank, and means for rotating said shaft.

15. In a propelling mechanism for a machine which comprises a' platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft on said platform, cranks secured to said shaft, treads at the respective sides of said platform, a lever pivotally connected with each tread and with the adjacent crank, means for'rotating said shaft to cause said cranks to raise and lower said treads with relation to said platform and to raise and lower said platform with relation to said treads, and cooperating stops arranged to prevent substantial movement of said treads about their pivotal connections with said levers when said treads are in their elevated positions and to permit such movement of said treads when they are in their lower positions.

16. In a propelling mechanism for a machine which comprises a platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft on said platform, cranks secured to said shaft, treads at the respective sides of said platform, a lever pivotally connected with each tread and with the adjacent crank, means for rotating said shaft to cause said cranks to raise and lower said treads with relation to said platform and to raise and lower said platform with relation to said treads, stops carried by said treads on opposite sides of their pivotal connections with said levers, and other stops carried by said levers and said platform respectively and so arranged that they will be in operative positions with relation to the respective first mentioned stops when said treads are in their elevated positions and will be in inoperative positions with relation to said first mentioned stops when said treads are in their lower positions.

17. In a propelling mechanism for a machine which comprises a platform and a. supporting base connected with said platform for upward and forward movements therewith, treads at the respective sides of said base, operable devices connecting the respective treads with said platform, means carried by said platform for operating said devices to move the forward edge of said base upwardly and to move the base as a whole forwardly, and means connected with said operable devices and with arelatively stationary part of said machine for controlling the movement of said operable devices to cause the forward edge of said base to move rearwardly during the initial portion of the upward movement thereof.

18, In a machine comprising a platform and a supporting base connected with said platform for upward and forward movements therewith, a shaft rotatably mounted on said platform, a crank connected with said shaft at each side of said platform, a tread at each side of said platform, a lever pivotally mounted on each tread and pivotally connected at its upper end with the adjacent crank, means for rotating said shaft to move said cranks from upwardly extending positions to forwardly extending positions, thereby moving said treads into engagement with the ground, and to then cause said cranks to fulcrum on said levers and move said shaft upwardly, and means acting on said levers during the initial portion of the upward movement of said shaft to move the axes about which said cranks fulcrum on said levers slightly to the rear of the vertical plane of the axis of said shaft.

HARRY F. STOCK.

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