GB2190896A - Lift truck - Google Patents

Abstract

A lift truck comprises a steerable power section and a load-handling section (11), which is adjustable in height relative to the power section by means of a lifting cylinder (6) that is mounted in the power section. A load-carrier consisting of load-carrying fork arms extends at a fixed angle to the load-handling section and rollers (37) are disposed under the load carrier and are mounted on a swivel lever. The swivel levers are pivotally movable in height, in dependence on an elevation control device, which corresponds to the operation of the lifting cylinder (6), so that the load carrier is adjustable to a position in which it is parallel to the riding surface. A linkage is a selectively operable by an adjusting device (29), to vary the angular position of the load carrier relative to the riding surface from a position which is parallel to the riding surface. In one embodiment the adjusting device consists of a control cylinder. Such an adjusting device may be incorporated in rods, which serve to adjust the swivel levers and are pivoted to the latter. <IMAGE>

Description

SPECIFICATION Lift truck comprising a steerable power section and a vertically movable load-handling section This invention relates to a lift truck comprising a steerable power section and a vertically movable ioad-handling section, which is guided on the power section and which by means of a lifting cylinder in the power section is adjustable in height relative to the power section, wherein rollers which are movable in height and are particularly mounted on a swivel lever are disposed under a load carrier, which extends at a fixed angle to the load-carrying section and particularly consist of load-carrying fork arms, and the carriers for said rollers consist particularly of swivel levers and are movable in height in dependence on an elevation control means, which operates in correspondence with the operation ofthe lifting cylinder, whereby the load carrier can be adjusted in height while it is held parallel to the riding surface.

The load-handling section which is movable in height is preferably guided on the power section parallel to the latter.

The term lifttruck includes also a truck in which the load carrier consists of a plate, which at that end which is remote from the load-handling section is supported by means of rollers of the stated kind and arrangement.

Such lifttrucks are also called low-l ift trucks and may be operated by hand or mechanically and may optionally be provided with a driver's seat. Such trucks are often provided with handlebars, which at their end carry a control head with actuating levers for various functional means.

In a particularly preferred embodiment the swivel levers are pivotally movable by means of rods, each of which extends between a swivel lever and a pivoted link, which may be incorporated in a linkage provided between the power section and the load-handling section.

In the normal operation of the above-mentioned trucks the forks are inserted into a pallet or moved under a load and are then raised sufficiently to liftthe load from the ground.

Fork lift trucks which are provided with adjusting rods and are of the kind described first hereinbefore are known from German Utility Model 1769 144and German Utility Model 1787367.

Whereas in a fork lift truck disclosed in German Patent Publication 1,199,954the power section and the load-handling section are interconnected only at three points, the head of the piston rod must be provided with a ball-type joint for supporting the load-handling section because the other lowerjoints are constituted only by pivot points disposed adjacent to a hinge ofthe linkages.

A problem arising in such known trucks resides in thatthefork arms should be adjustable relative to the power section so that the orientation which is parallel to the riding surface can be changed on a level riding surface and the tips of the fork arms can be raised or lowered to facilitate the loading and unloading. Another known lifttruckis intended to handle loads which have been placed only on sheets of paper.

Belgian Patent Specification 571,395 discloses a lift truck having a lifting mast which is provided with the load carrier and can be adjusted relative to forward backing rollers carried by forwardly protruding arms. That adjustment is permitted because a pair of wheels are movable on said protruding arms and said wheels have a larger ground clearance than the backing rollers so that the forward tips ofthe protruding arms are raised and the carrier of the lifting frame is thus pivotally moved. That pivotal movement can be increased by a provision for an additional adjustment ofthe subassembly including the protruding arms relative to the steerable power section of the truck. In that embodiment the load carrier proper is guided over the protruding arms so that the adjustment is restricted. That truck involves a considerable structural expenditure.The object resides inthatthetruck, particularlywith a picked-up load, should be ableto travel upgrade oron uneven ground without a contact of the load with the ground and the load should safely be supported.

The same object is to be accomplished by the invention.

German Patent Specification 1,051,196 discloses a lift truck in which a guide mast is rigidly mounted on the frame of the truck and a load-carrying fork is guided on said mast and can be raised so that its forward edge rises obliquely from the riding surface or can be lowered so that the forward edge of the fork approaches the riding surface. Thatadjustmentwill ensure that a picked-up load will safely be held during a travel of the truck. Forthat purpose the known lift truck comprises controllable means for supporting the body of the truck on its rear axle, which is constrained by swivel levers, so that the entire body of the truck with the driver's seat and the guide mast can be pivotally moved.This is expensive and permits only a restricted adjustment although certain adjustments are permitted for a reliable and safe holding of pallets or other loads during travel.

The operation of that expensive truck body involves considerable inertia effects.

In particular, the p problem arises that in the handling of pal lets by means of such trucks of the kind described first hereinbefore the surface on which the truck begins to travel and the surface on which the pallet is supported do not extend in the same plane. For instance,the pallet may be horizontally supported whereas the truck approaches on an inclined plane, such as a ramp, or vice versa. In that case the fork arms should be capable of an adjustment by which the differential inclination ofthe load-supporting surface and the riding surface is compensated and the fork arms or the load carrier can be entirely moved under or into the pallet without striking against the latter.

In that respect the known embodiment described last is inadequate because the design ofthetruck does not permit of an adequate pivotal movement and the body of the truck is mounted on the rear axle for an adjustment in height.

It is an object of the invention to provide a truck which is ofthe kind described first hereinbefore and has a simple structure and in which the load carrier can be adjusted to an inclined position relative to the riding surface and loads can be picked up even when the riding surface and the surface on which the load is carried are at an angle to each other. Besides, the means for adjusting the load carrier to an inclined position relative to the riding surface should be simplified and the picking up and delivery of loads and a travel over uneven ground should be simplified.

This object is accomplished in accordance with the invention in that the linkages for connection to the rollers, which are adjustable in height, comprise a selectively operable adjusting device and the angular position of the load carrier relative to the riding surface can be selectively altered.

Owing to the selectiye adjustment ofthe roller carriers and the resulting selective adjustment ofthe angular position of the load carrier by a movement which is superposed on the movement in height of the load carrier when it has a parallel orientation, it is possible not onlyto effect a normal operation of such a lifttruck provided with a lifting cylinder, i.e.,to operate the lifting cylinder orto move the handlebar, e.g., in order to raise or lowerthe load-carrying fork while said fork is parallel to the riding surface, but also to operate the adjusting device so as to adjust the load carrier consisting, e.g., of load-carrying fork arms so that the load carrier extends at an angle to the riding surface whereas the power section and the load-handling section are moved in parallel.It must be borne in mind that the load-carrying fork arms are mechanically held at a fixed angle to the load-handling section and that in case of an angular adjustmentthe entire truck performs a pivotal movement aboutthe point of contact of at least one carrying wheel of the power section.Two features have been combined because there is always a parallel guidance between the power section and the load-handling section so that the guiding means may bevery simple, butthe angle between the load-carrying fork or of a corresponding load carrier, which is included, and the riding surface can be altered in thatthe forward edge of the load carrier or the tips ofthe load-carrying arms can be lowered to or raised from the riding surface. it is surprising that this can be accomplished in a truck of the type concerned without a need for an articulated joint between the power section and the load-handling section. Such an articulated joint would have disadvantages as regards its arrangement and as regards the loads applied between different adjusted angular positions.

A parallel guidance ofthe load-handling section and the power section on each other is desirable in itself because it permits an exact adjustment of the load-carrying fork arms or of a corresponding load carrier. In a certain raised position, i.e., in a certain position of the lifting cylinder, the at least one adjusting device incorporated in the linkages permits an alteration of the inclination of the load carrier and particularly ofthe load-carrying fork. This will be true even when there is no exact parallel guidance.

The aspect mentioned immediately before may be applied to an arrangement in which the rollers mounted on a swivel lever are pivotally movable by means of rods which extend between respective swivel levers and a pivoted link. In that case a particularly preferred embodiment may be so designed that the rods constitute links between the swivel levers and the other linkage members and can be adjusted in length bythe adjusting device.

It is surprising that even existing trucks can easily be altered to incorporate such an arrangement because such trucks having rods between the swivel levers and linkage members are conventional.

The meansforadjusting each linkage member may be provided in various forms. A mechanical device may comprise a turnbuckle nut having end portions formed with mutually oppositely handed screw threads in screwed engagement with rod portions, and an adjusting drive for each turnbuckle nut.

In a particularly preferred embodiment the adjusting device consists of a cylinder-piston unit. In a preferred arrangement the rods consist of piston rods, which extend into respective cylinders, which are pivoted to the linkage members. An adjustment ofthe piston rod can be effected by a controlled supply of pressure fluid to at least one end ofthe piston.

In the latter case, in which the pressure is applied to one end only, the return movement is effected under the load applied bythetruck.

The underlying problem is solved in a surprisingly simplemannerbythe provision of rods which are variable in length.

The term linkage includes a lever arrangement which is disposed between the load-handling section and the power section and by means of connecting members consisting particularly of rods is linked to the roller carriers consisting particularly of swivel levers.

In that connection a desirable embodiment of the invention of the kind stated first hereinbefore may comprise a mechanical adjusting device consisting of a rope drive or a system of levers. In accordance with the remarks made first hereinbefore such an adjusting device may suitably serve to actuate at least one cam or swivel lever which engages the linkages to alterthe setting imparted to them bythe lifting cylinder.

In connection with a linkage consisting of a lever arrangement the adjusting device preferably comprises at least one control cylinder, which is pivoted in the power section and has a piston rod that is pivoted to a lever in the at least one linkage.

This is a simple embodiment, which can be connected to the hydraulic system that is associated with the lifting cylinder.

In a particularly preferred em bodiment the lifting cylinder and the control cylinder or cylinders are included in a hydraulic control system. Inthatcase the setting of the elevation may be used to control also the inclination of the load carrier and/orto adjust certain dependencies.

In a preferred arrangement a bell-crank lever is mounted in the power section and by means of a coupling link that is pivoted at both ends is connected to a particularly arcuate swivel lever that is pivoted in the load-handling section and to a rod, and the control cylinders are movably mounted on the bell-crank lever. This results in a particularly simple design of the linkages because it is sufficient to provide a bell-crank lever, a coupling link connected thereto, and a swivel lever, in orderto permit a large number of settings.

Particularly that design provides for a dual constraint between the power section and the load-handling section by the coupling linkwhich connects the bell-crank lever and the swivel lever and buy a backing roller, which is movably mounted at the free end ofthe bell-crank lever and is engageable with a track in the load-handling section and can be moved along and lifted from the track.

This proves a double support, i.e., a multibar linkage of special design, because the association of the supporting hinges is altered during operation.

But that multibar linkage affords the advantage of providing for a constraint having lateral stability and fora high adaptability.

In a preferred arrangement, the track is parallel to the fork arms.

Inadesirableembodimentthecoupling linkand the control cylinder or its piston rod are pivoted on a pin disposed at the bend ofthe bell-crank lever, which is open toward the load-handling section.

Such linkage provides numerous possibilities for an adapting adjustment. But it is essential to minimize also the controlling force. For this reason the swivel levers suitably have unequal arms and the lever arm to which the rod is pivoted preferably has one-halfthe length of the lever arm that extends to the coupling link.

From that aspect another desirable embodiment comprisesacoupling linkwhich hasapproximately the same length as that lever arm of the bell-crank Ieverwhich is pivoted to said coupling link and the angled lever arm provided with the backing roller has approximately one-halfthat length.

A lift truck, which is particularly of the kind described hereinbefore and comprises laterally disposed, vertical track rails on the power section or the load-handling section and track rollers provided on the load-handling section or power section and running in said track rails, is desirably so designed that only one track roller is provided on each side in the lower portion ofthe truck and the constraint at the top at the resulting three-point constraining mechanism is provided by the connection between the piston rod of the lifting cylinder and the load-handling section. This arrangement results in a novel three-point support, because only lowertrack rollers, which are particularly coaxial, are guided in channel bars and the piston rod of the lifting cylinder is mounted so asto preventadeviation.This is a requirement for a structure which is stable and does not cant but owing to the provision ofthe control cylinder permits of numerous adjustments.

As regards the upper joint between the lifting cylinder and the load-handling section it is emphasized that that joint is non-displaceable and non-pivotal.

The lifting cylinder and the control cylinder are desirably connected by sliding control valves to a hydraulic pump and a connecting line comprising a check valve which closes to the lifting cylinder is provided between the lifting cylinder and the sliding control valves and associated with the control cylinders and is adapted to be selectively connected in a sense to retract or extend the control cylinder. In the particularly preferred embodiment an inclination-sensing switch is incorporated in the meansforcontrolling the operation of the lifting and/orcontrol cylinder.

The provision of such an inclination-sensing switch will promote the automation of the arrangement because a setting which corresponds to the normal mode of operation will be automatically obtained during normal operation.

In another, further embodiment, in which the foregoing features are incorporated, the sliding control valve forthe lifting cylinder is so adjusted that when said lifting cylinder is shut off the connecting line will be directly connected to the sliding control valve for the control cylinder. This provides for an interdependence of the lifting cylinder and the control cylinders.

Athird sliding control valve may be used to providefora returnflowfrom the pump.

In the preferred embodiment ofthe invention taking the above-mentioned aspects into account the lines leading to the working chambers of the control cylinders are connected thereto by adjustable throttle valves and by-pass check valves, and that of said check valves which is connected to the valve spaceforeffecting a retraction ofthecontrol cylinders opens to said valve space and the other check valve connected to the other valve space closes to that valve space. This results in a simple design.

In a preferred arrangement the sliding control valves are actuated by solenoids and two solenoids associated with the ports for supplying and discharging the hydraulic fluid are provided for each of the sliding control valves for controlling the supply and discharge of the hydraulicfluid to and from the control cylinders and when said sliding control valve is in its neutral position the ports of the control cylinder are connected to the drain. Such an arrangement is known for sliding control valves used for different purposes.But it is preferred in accordance with the invention that an actuating device consisting particularly of a switching drum is provided and in dependence on the desired mode of operation can be used to energize or de-energize the pump and simultaneously to switch selected solenoids for actuating sliding control valves. In one mode of operation onlythe sliding control valvefor the return flow from the pump and the sliding control valve for the control cylinder are operated for normal operation, in which the fork arms have a parallel orientation for the travel of the truck. Another mode of operation comprises the simultaneous use of the sliding control valveforthe lifting cylinderandfor the return flow from the pump.

The invention will now be described with reference to illustrative embodiments which are shown in the drawing, in which Figure lisa sectional side elevation showing a fork lifttruckwith parts broken away and serves to explain essential elements of the invention.

Figure2 is an elevation which is similarto Figure 1 but shows a different operating position corresponding to a so-called normal elevation.

Figure 3 is an elevation which is similarto Figure 1 but illustrates the use of the specially described means for effecting an adjustment to a setting which differs from the normal elevation.

Figure 4 is a view that corresponds to Figure 3 but illustrates a different setting ofthe load carrier relative to the riding surface, particularly on a ramp.

Figure 5is a sectional view taken on line V-V in Figure 1 viewed from the left in Figure 1.

Figure 6is a diagrammaticviewshowing a hydraulic control circujtforthe embodiment substantially in accordance with Figures 1 to 5.

Figure 7is a diagram illustrating an electric circuit used in conjunction with Figures 1 to 6.

Figure 8 is a diagrammatic side elevation showing in section a corresponding fork lifttruckwith parts broken away for illustrating elements which are essential fora modification.

Figure 9 is an elevation which corresponds to Figure 8 and shows a different operating position.

Figure 10 shows a fork lift truck in a position that corresponds to the operating position shown in Figure 8 but with the load carriers in a different position which is due to the design in accordance with the invention.

Figure 11 is a viewthat issimilarto Figure 8and shows a further operating position to illustrate a further setting which can be adopted.

Figure 12 is a diagram illustrating a hydraulic control circuitforexplainingthefunctional embodiment illustrated in Figures8to 11.

In Figures 1 to 12, like parts are designated with the same reference characters.

The fork lift truck comprises a power section 1 including a drive motor 2 for a steerable drive wheel 3. The power section 1 might be provided with a driver's seat and corresponding controls and power sourcesforthedrive. lntheillustratedembodimenta fork lift truck is described which has a handlebar 4 provided with a control head 5 with handles and control levers for initiating individual function sequences.

Alifting cylinder6 is mounted in the power section 1. Forinstance,the lifting cylinder6 is mounted on a support7 provided on a stiffening member 8 and may be laterally pivotally movable, e.g., about a longitudinallyextending pin 9, in case the riding surface is uneven in the transverse direction. This is a desirable feature.

The extensible piston rod 10 is fixedly connected to a load-handling section 11, which is movable in height. It is apparent that a pin 12 ofthe piston rod extends into a corresponding opening 13.Asa result,thepiston rod l0constitutesapartofthe load-handling section, which is movable in height, and the piston rod controls the movement in height.

The power section is provided on the outside with two vertical channel members 14, 15(Figure5), which are outwardly open and constitute vertical track rails, which extend only over about one-third of the height of the truck or of its power section 1. In dependence on the vertical guidance ofthe load-handling section the track rails may be even shorter. Two track rollers 16, 17 run in the respective track rails and are freely rotatably mounted, e.g., on the load-handling section where it overlaps the track rails on the outside. The track rollers 16, 17 atthe bottom and the mounting ofthe guide pin 12 at the top provide a non-pivoted three-point support, which is believed to be particularly desirable and essential for such trucks.

Reverting to Figure 1 a linkage 18 is mounted on the stiffening member 8. Such linkages are provided on both sides. The linkage is pivoted at the top about a pivot pin 19 for a movement that is transverse to the longitudinal direction of the truck and at that mounting comprises a bell-crank lever 20, which has two arms 21,22, which differ in length. The shorter arm 22 is remote from the pivot pin 19 and carries a backing roller 23, which is associated with a track24 provided on the load-handling section 11. That track 24isvirtuallyparalleltoa riding surface during normal operation and in any case is parallel to a load carrier 25 which consists of fork arms.

Another pivot pin 27 is movably mounted atthe bend 26 ofthe bell-crank lever 20 and is acted upon by the adjusting device either directly or by means of a cam or by a toggle joint and in the preferred embodiment is hinged to a piston rod 28 of a control cylinder, which will subsequently be designated 29.

Acoupling link30 is pivoted atone end on the pin 27 and at the other end is pivoted by a pin 31 to an arcuate swivel lever 32, which is pivoted on a pin 33, which is mounted on the load-handling section 11.

The swivel lever 32 has two arms, which differ in length, and its shorter arm is connected by a hinge or pivot pin 34to a rod 35, which extends underthe load carrier 25 and particularly under individual forkarms and is pivoted to a carrier 36 for rollers 37. The carrier 36 consists specifically of a swivel lever, which is pivoted on a pivot pin 38 mounted in a fork arm 39 of the load carrier. A coupling link40 is pivoted on the pivot pin 38 and at its other end is non-rotatably connected to the carrier consisting particularly ofthe swivel lever 36.

Itwill be understood that a toggle joint oran adjustable cam might be pivoted at one end in the power section just as the control cylinder 29. A rope drive or linkage mightdirectlybeconnectedtothe pivot pin 27, if desired.

For this reason a position as shown in Figure 2 can be assumed on which the swivel lever 36 provided with the roller 37 will be swung out as the piston rod 10 is extended whereas the control cylinder 29 remains in that functional position in which the piston rod is locked. The piston 29 is pivoted to the power section on a transverse pin 41.

A comparison between Figures 1 and 2 reveals that the means thus shown permit a normal operation.

The arcuate swivel lever 32 has such a curvature that the bell-crank lever 20 provided with the backing roller 23 can be moved within the arc.

The control cylinder 29 which supports the linkage 18 permits an overriding control during the function sequence.

Figures 1 and 2 illustrate a normal operation.

Figure 3 illustrates a modified mode of operation because the riding surface 42 is inclined relative to a surface 43 on which a pallet 44 is supported. In that embodiment the carriers 36 have been swung in so that the rollers 37 have been swung in relative to the load-supporting surface or the fork arm 39 whereas the drive wheel 3 provides for a tilted position ofthe entire truck relative to the riding surface 42. That tilted position is assumed because the lifting piston rod has been extended and the control cylinder 29 two of such control cylinders may be provided on opposite sides or a single control cylinder may be provided and may be connected by a crosspiece to the two lateral linkages 18 - has extended its piston rod 28 and has thus sv;ung outthe bend 26andthe pivot pin 27 from the position shown in Figures 1 and 2.The adjustment to that swu ng-out position is assisted in that the backing roller 23 is now rolling on thetrack24 so thatthebell-crank lever 20 is not perfectly free to turn about its pivot pin 19 but the angular position will be determined by the track 24.

In that embodimentthe coupling link30 ensures that the carrier 36 and particularly the associated swivel lever will remain in its swung-in position even underthe load applied by the rollers 37 on the carriers 36.

Itis apparentthat in an arrangement as shown in Figure3the load carrierand particularlytheforkarm 39 can move smoothly into the pallet 44.

That inserting movementwithouta resistance will also be permitted when the truck is moving on the inclined riding surface 42 to the pallet 44. But in that case the parallel guidance of the fork arms 39 our a corresponding load carrier will be compensated by the lifting cylinder 6, which retracts its piston rod 10, which carries the load-handling section 11.

Figures 3 and 4 illustrate embodiments which are comparable in that respect but in which the backing roller 23 assumes different positions on the track 24.

The embodiment shown in Figure 4 is desirable because the bell-crank lever 20 at its bend is just receiving the corner45 ofthe load-handling section 11. If the control cylinder 29 in the embodiment shown in Figure 4would retract its piston,the load-handling section 11 would be lowered and the backing roller 37 would be extended. These different effects are obtained in the special embodiment.

It is apparent from Figure that in addition to the outer guide means two rods 35, 35' are disposed in the lower portion and are embraced by arcuate swivel levers32, 32', 32", 32"'. Each swivel lever is connected to a coupling link 30, 30',which is pivoted at its other end at 27 or 27', where the piston rods of the control cylinders 29, 29' are pivoted too. The connection of the coupling linkto the mounting comprising the pivot pins 27, 27' provides a controlled connection to the bell-crank lever 20,20' ofthe embodiment described hereinbefore.

Figure 5 shows also a particularly back-lashfree connection 46togetherwith other advantages which have been mentioned.

The lifting cylinder6 andthecontrol cylinder29 are shown also in Figure 6. Whereas the lifting cylinder 6 has only one fluid port 47, the control cylinder 29 has two ports 48, 49, each of which is connected to an adjustable throttle valve 50,51 and a shunt-connected check valve 52,53. Via the supply lines 54,55, the cylinder chambers can be pressurized as desired. The illustrated throttle valves permit of an adaptation.

The two supply lines 54,55 are primarily controlled only by a sliding control valve 56 in dependence on a command for retracting or extending the control cylinder. This is effected by actuating solenoids 57, 58, which are controlled buy a command generator, which may consist, if desired, of a switching drum or of individual levers, which are coupled to each other.

The feed line to the pump is controlled by another sliding control valve 59, which precedes the lifting cylinder 6. A connecting line 60 is connected to the fluid port 47 by a check valve 61,which openstothe sliding control valve 59. In dependence on the position of the sliding control valve 59 the connecting line 60 supplies pressure fluid to one end or the other of the control cylinder 29. A backflow is prevented bythe check valves 52,53 even when the sliding control valve 59 is in its neutral intermediate position.

The sliding control valve 59 is connected to the connecting line 60 and serves for a simultaneous feeding ofthe lifting cylinderwhen the sliding control valve 56 is in the corresponding position. The actuation may depend on the cross-section of the piston and on the setting ofthe throttle valves 50,51 to override the influence ofthe cross-section ofthe piston. In another position of the sliding control valve 59 the working chamber of the lifting cylinder 6 is shutoffand the discharge pressure ofthe pump is applied directly to the sliding control valve 59 via the connecting line 60.

The sliding control valve 59 is movable by an actuating solenoid 62 against the action of a spring 63.

Athird sliding control valve 67 is connected by a junction 66 to the supply line 64 coming from the pump 65 and is movable by an actuating solenoid 68 against the action of a spring 69. In the illustrated position the sliding control valve 67 effects a shut-off by means ofthe built-in check valve 70 orwhen actuated opens a passage 71 to permit a draining through a throttle valve 72to a tank73,from which fluid is sucked also by the pump 65.

Owing to that arrangement the lifting cylinder 6 can initially be extended when the system has been started. In dependence on the pressure the bias of the check valve 61 can be overcome so that pressure can be applied to the control cylinder29 in dependence on the position of the sliding control valve 56. In that case a setting may be adopted in which the swivel lever or carrier 36 can be retracted to a special position so that the functions described hereinbefore can be performed.

The sliding control valve 67 servesto lockorto eliminate a specific position. The sliding control valve 59 perse permits any lifting operation to be performed. An initiation is possible only in a lifting sense. For a lowering, the sliding control valve 67 must be reversed so that fluid can be drained through the junction 66. Fluid is not drained through the connecting line when the sliding control valve 59 is in the illustrated position but only when the sliding control valve 59 has also been actuated to permit a draining of fluid through a passage74.

Figure 7 is a diagram of a circuit which comprises an inclination-sensing switch 75, which is formally shown also in Figure 2 and in alternation precedes the actuating solenoids 68 and 58. The motor 77 for the pump 65 is also included in the series branch 76.

That arrangement provides a certain requirement for switching or interlock in that in case of an inclination in one sense the actuating solenoid 68 can be energized to open a path for a return flowthrough the passage 71 whereas the pump 65 is de-energized and the circuitfor energizing the actuating solenoid 57 is opened atthe sametime.

In case of an inclination in the other sense the actuating solenoid 68 is activated but the motor 77 forthe pump 65 remains energized just as the actuating solenoid 58 so that the sliding control valve 56 can be extended in the direction in which the control cylinder 29 extends its piston rod. This results in the operating position shown in Figures 3 and 4 so that the inclination of the riding surface 42 from the horizontal is compensated and it is ensured thatthe load-carrying arm orfork carrier 39 will automatically assume a horizontal position.

In the embodiment shown in Figures 8 to 12the load-handling section is shown in a raised position relative to the power section 1 in Figure 8 and in a lowered position in Figure 9.

A rod 35 is pivoted on a pivot pin 40, which is mounted in the swivel lever at a distance from the pivot pin 38. The other end of the rod 35 is pivoted by a pivot pin 34to a link 32, which is pivoted on a pin 33, which is mounted on the load-handling section 11 and may be disposed adjacent to the load carrier 25. The link 32 carries at its other end a backing roller 78,which is supported in a crosspiece 79, which is secured in the power section 1. Because the load-handling section 11 is raised or lowered relative to the power section 1 in dependence on the operation ofthe lifting cylinder, the elevation ofthe pin 33 relative to the crosspiece 79 is altered so that the link is pivotally moved and the friction is reduced by the backing roller 78.

In Figure8the load-handling section 11 is in a raised position so that the swivel lever 36 has been swung outto support the load carriers 25 at their forward ends in the stated position. In Figure 9the load-handling section 11 is shown in a lowered position so that, e.g., the backing roller 78 is disposed above the pivot pin 34. In that position the Iink32protrudestothe Ieftfromthecrosspiece79to a larger extent so thatthe swivel lever 36 and the roller 37 have been swung in underthe load carrier 25, which is now in its lower position over the riding surface.

It will be understood that one of the assemblies comprising the swivel lever 36, the rod 35 and the link 32 is provided on either side of the fork lift truck, which comprises two fork arms as load carriers 25.

A particularly desirable design of a rod which is variable in length is apparent from Figures 8 and 9.

That rod consists of a cylinder-piston unit. A cylinder 80 is pivoted on the pivot pin 34. The rod 35 constitutes a piston rod, which is provided at one end with a piston 81 that is disposed in the cylinder 80. The rod 35 is guided by the sealing means 82, which are provided on the piston and may consist of piston rings, and by a sealed lead-through 83 atthe entrance to the cylinder. It will be understood that additional guiding means may be provided.

Figures 8 and 9 illustrate the normal mode of operation, in which the rod 35 has a constant length.

This is ensured by fluid that has been supplied to the cylinder chamber 84.

In the embodiment which has been described the piston may be single-acting so that it is sufficientto supplyfluid to the chamber 84. As the assembly is biased, particularlywhen the swivel lever 36 has the inclination shown in Figure 8, the assembly will yield or will be reversed in response to a pressure relief of the chamber 84.

Figures 10 and 11 illustrate the change of the position of the load carriers 25 relative to the riding surface 85,86. The riding surface shown in Figures 8 and 9 is horizontal. The riding surface 85 is a rising ramp, which is continued at one end by a platform 87, on which a pallet 88, for instance, may be disposed.

In Figure 10 the load-handling section 11 is in the raised position shown also in Figure 8. This is apparent from the position of the arcuate link 32. But the cylinder chamber 84 is pressure-relieved so that the weight bias imparts to the swivel lever 36 a pivotal movementtothe undersideoftheforward end of the load carrier 25 and the piston 81 is thus pulled in the cylinder chamber asfaras tothe lead-through 83. As a result, the tips 89 of the load carriers are lowered onto the riding surface 85 or onto the platform 87 and can satisfactorily be inserted into the pallet 88. When it is imagined that the riding surface 85 is horizontal, it will become apparent that the tips of the load carriers 25 are lowered to the riding surface from the position shown in Figure 8.

In Figure lithe drive wheel 3 is shown on a rising portion ofthe riding surface. The load carrier 25 is disposed over an adjoining riding surface 86, which is substantially horizontal. In that position the load-handling section 11, which is movable in height, has been raised by means ofthe lifting cylinder 6 to a larger height above the other parts of thetruckthanisshownin Figure9.Asareulst,the link 32 protrudes to the left to a somewhat larger elevation than in the position shown in Figure 9 on a horizontal riding surface.

Owing to that larger overhang, the load carriers 25 can be adjusted to be parallel to the area 86 ofthe riding surface even when the drive wheel 3 is in a lower position than in Figure 9. The compensation of the inclination is assisted by the load on the roller37 so that a pressure relief of the cylinder chamber 84 will cause the piston 81 to move in adaptation to the differential inclination of the riding surface. I.e., the piston can move to the left in Figure 11.

In said embodiment the adaptation is effected with simplified means.

The control circuit shown in Figure 12 comprises the lifting cylinder 6 and two cylinders 80 having cylinder chambers 84, also the piston 81 in its left-hand position, and the piston rod 35.

The hydraulic control circuit comprises a pump 65, which is driven by a motor 77. The suction port ofthe pump 65 communicates with atank73.

The feed line 64from the pump leads through a check valve 90 to the lifting cylinder 6.

Branch lines 91,92 are connected to the feed line 64 between the check valve 90, which opens to the lifting cylinder 6, and the lifting cylinder. Through a check valve 93, which closes to the feed line 64, the branch line 91 leads to a sliding control valve 94, which controls ports 95,96 connected in parallel to thecylinderchambers84.

The sliding control valve 94 is movable in one direction by an actuating solenoid 97 against the action of a spring 98. In the position shown the actuating solenoid 97 is de-energized, i.e., disconnected from its power source, so that the spring 98 urges the sliding control valve 94to the illustrated position, in which the cylinder chambers 84 are filled with fluid. This is applicable to the various operating positions shown in Figures 8,9 and 11.

Those cylinder chambers which are defined by that end ofthe cylinder which is opposite to the lead-through 83 communicate through lines 99, 100 with a return line 101,which leadstothetank73so that the cylinders 80 can easily be controlled by a supply offluid to one chamber. When the actuating solenoid 97 is energized to reverse the sliding control valve 94, the cylinder chambers 84 are connected through an adjustable throttle valve 102 to the return line 101 sothatthe cylinder chamber 84 can gradually be emptied.The constant which has been adjusted at the throttle valve 102 will ensure in any case that the position ofthe load carriers will not be abruptly changed but a constant will be effective which corresponds to the instantaneous operating speed. That state in which the actuating solenoid 97 is connected to its powersource, e.g., to an accumulator, will be obtained in the operating position which is shown in Figure 10 and in which the truck is travelling on a ramp and is being adapted to a horizontal platform 87.

Through a sliding control valve 103, provided with an actuating solenoid 104 and a backpressure spring 105, and through an adjustable throttle valve 106, the branch line 92 communicates with another return line 107, which leads to the tank.

In the position shown the actuating solenoid 104 is de-energized so that pressure is applied to the lifting cylinder 6. This is applicable to the operating positions shown in Figures 8 and 11.

In response to a reversal of the sliding control valve 103,the lifting cylinder 6 can be drained into thetank73. This is applicable to the operating positions shown in Figures 9 and 11. In Figure 11, an adaptation will be obtained because the sliding control valve 94 is in the illustrated position atthe sametime.

It is appa rent from the description that the circuit is simple because the pump pressure is applied in a reasonable manner and a reliefviathe branch line 92 and the throttle valve 106 is restricted to ensure the appiication of an adequate pressure forfeeding the cylinder spaces 84.

108 designates an adjustable relief throttle valve for avoiding an overloading of the pump.

For the operation it must be borne in mind that even when the lifting cylinder 6 has been relieved by a reversal of the sliding control valve 103 an adequate feeding may be effected through the branch line 91, if required, provided that the sliding control valves 94 are in the illustrated position. It must be borne in mind that in the position shown in Figure 9 the swivel levers 36 are in their end position and that the swivel levers 36 are always biased at least by the truck. Besides, the throttle valve 106 ensures a certain metering of the return flowthrough the branch line 92. The piston 81 has a smaller cross-section area than the piston of the lifting cylinder 6.

Claims (15)

1. A lift truck comprising a steerable power section and a vertically movable load-handling section, which is guided on the power section and which by means of a lifting cylinder in the power section is adjustable in height relative to the power section,wherein rollerswhich are movable in height and are particularly mounted on a swivel lever are disposed under a load carrier, which extends ata fixed angle to the load-carrying section and particularly consist of load-carrying fork arms, and the carriers for said rollers consist particularly of swivel levers and are movable in height in dependence on an elevation control means, which operates in correspondence with the operation of the lifting cylinder,wherebythe load carrier can be adjusted in height while it is held parallel to the riding surface, characterized in that the linkages for connection to the rollers, which are adjustable in height, comprise a selectively operable adjusting device and the angular position of the load carrier relative to the riding surface can be selectively altered.

2. A lifttruckaccording to claim 1, wherein the swivel levers are pivotally movable by means of rods which extend between respective swivel levers and a pivoted link, characterized in that the rods constitute links between the swivel levers and the other linkage members and can be adjusted in length by the adjusting device.

3. Alifttruckaccordingtoclaiml or2, characterized in that the adjusting device consists of a cylinder-piston unit, the rods consist of piston rods, which extend into respective cylinders, which are pivoted to the linkage members, and an adjustment ofthe piston rod can be effected by a controlled supply of pressure fluid to at least one end ofthe piston.

4. Alifttruckaccording to claim 1 or2, characterized in that the adjusting device is a mechanical device comprising a turnbuckle nut having end portions formed with mutually oppositely handed screw threads in screwed engagement with rod portions, and an adjusting drive for each turnbuckle nut.

5. A lift truck according to claim 1, wherein the linkage consists of a lever arrangement which is disposed between the load-handling section and the power section and by means of connecting members consisting particularly of rods is linked to the roller carriers consisting particularly of swivel levers, characterized bya mechanical adjusting device consisting of a rope drive or a system of levers.

6. A lift truck according to claim 1 wherein the linkage consists of a lever arrangement which is disposed between the load-handling section and the power section and by means of connecting members consisting particularly of rods is linked to the roller carriers consisting particularly of swivel levers, characterized in thatthe adjusting device comprises at least one control cylinder, which is pivoted in the power section and has a piston rod that is pivoted to a lever in the at least one linkage.

7. A lifttruck according to any of claims 1,5 and 6, characterized inthata bell-crankleveris mounted in the power section and by means of a coupling link that is pivoted at both ends is connected to a particularly arcuate swivel leverthat is pivoted in the load-handling section and to a rod, the control cylinders are movably mounted on the bell-crank lever, and a dual constraint is provided between the power section and the load-handling section bythe coupling link which connects the bell-crank lever and the swivel lever and by a backing roller, which is movably mounted atthe free end ofthe bell-crank lever and is engageable with a track in the load-handling section and can be moved along and lifted from the track.

8. A lifttruck according to any of claims 1 to 7, characterized in thatthe load-handling section which is movable in height is guided on parallel on the power section.

9. A lift truck according to any of claims 1 to8, characterized in that the upperjoint is non-displaceable and non-pivotal,the lifting cylinder and the control cylinder are connected by sliding control valves to a hydraulic pump and a connecting line comprising a check valvewhich closes to the lifting cylinder is provided between the lifting cylinder and the sliding control valves and associated with the control cylinders and is adapted to be selectively connected in a sense to retract or extend the control cylinder.

10. A lifttruckaccording to any of claims 7 to 9, characterized by a third sliding control valve for providing for a return flow from the pump.

11. A lifttruck according to any of claims7to 10, characterized in thatthe lines leading to the working chambers ofthe control cylinders are connected thereto by adjustable throttle valves and by-pass check valves, and that of said check valves which is connected to the valve space for effecting a retraction of the control cylinders opens to said valve spaceandthe other checkvalve connected to the other valve space closes to that valve space.

12. Alifttruckaccording to any of claims 1 toll, characterized in that the sliding control valves are actuated by solenoids, two solenoids are provided for each of the sliding control valves for controlling the supply and discharge of the hydraulicfluid to and from the control cylinders, one solenoid is provided for each of other sliding control valves and is opposed buy a backpressurespring.

13. Alifttruckaccording to claim 1,2 or any of claims 3 to 12, characterized in that a lifting cylinder is pivoted for a lateral pivotal movement on a supporton a stiffening member by means of a pin.

14. A lifttruck, substantially as described hereinbeforewith referenceto and as shown in Figures 1 to 7 ofthe drawing.

15. A lifttruck substantially as described hereinbefore with reference to and as shown on Figures8to l2ofthedrawings.