GB2094727A

GB2094727A - Lift truck comprising centrally disposed drive means which are automatically adjustable in height

Info

Publication number: GB2094727A
Application number: GB8133394A
Authority: GB
Original assignee: Jungheinrich Unternehmensverwaltung KG
Current assignee: Jungheinrich AG
Priority date: 1981-02-19
Filing date: 1981-11-05
Publication date: 1982-09-22
Also published as: SE8106044L; GB2094727B; SE437652B; DE3106027C2; DE3106027A1

Abstract

A lift truck comprises wheel hub drive means disposed between caster wheels or rollers and automatically adjustable in height, ground contact pressure control means which are adjustable in the direction of the height adjustment, and a load carrier. The wheel hub drive means are supported on the chassis by spring element, which has an initial stress that is adapted to be controlled in dependence on the elevation and/or weight of the load. An abutment for at least one spring element constitutes a part of a hydraulic cylinder-piston unit, which is connected to the working hydraulic system by a control valve. The abutment is particularly so dimensioned that the ground contact pressure is decreased as the weight of the load increases. <IMAGE>

Description

SPECIFICATION Lift truck comprising centrally disposed drive means which are automatically adjustable in height Description This invention relates to a lift truck having wheel hub drive means for driving a wheel which is disposed between casters, means for automatically adjusting said drive means in height, contact press ure-applying means which are yieldable in the direction of the height adjustment, and a load carrier. The wheel hub drive means are generally centrally disposed.

Included within the scope of the invention is a lift truck having a load carrier which is movable in height by a working hydraulic system including a pump. It is also known to provide a sensor, which is responsive to the elevation of the load on the lifting frame, and to derive control actions from an output signal of the sensor. In that connection it is known to limit the steering angle orthe speed of travel in dependence on or proportional to the elevation of a load on the lifting frame.

In a lift truck which is of the kind described first hereinbefore and which in addition to the centrally disposed drive means and the caster wheels or caster rollers disposed in the same transverse plane has also load-supporting wheels or load-supporting rollers, which are disposed at the other end of the vehicle and may be arranged at prongs of the load carrier, the problem of ensuring a continuous ground contact of the steerable driven wheel of the centrally disposed drive means arises particularly when caster wheels or caster rollers are provided to increase the lateral stability of the lift truck.Owing to the provision of laterally disposed caster wheels or caster rollers as well as of the load-supporting wheels or load-supporting rollers, the vehicle is supported on the ground at five points so that a centrally disposed driven wheel which is fixedly mounted might disengage the ground. In order to avoid such disengagement it is known to provide for an automatic adjustment of the centrally disposed drive means in height in that height-adjusting means are provided which always urge the steerable driven wheel of the centrally disposed drive means against the ground regardless of depressions and elevations of the ground and regardless of the arrangement of the wheels or rollers of the lift truck.It is known to effect the height adjustment by a compression spring, by which a steering mechanism that is rotatable about a vertical axis is urged downwardly towards the ground. The steering mechanism of the centrally disposed drive means is mounted and guided in a drive housing of the lift truck. With such an arrangement, defined driving and braking tor ques can be exerted on the ground even where laterally disposed caster wheels or caster rollers are provided.

In the operation of lift trucks for carrying heavy loads, particularly of lift trucks having a lifting frame along which the load can be lifted to substantial elevations above the ground or above the chassis, an elevation of the centre of gravity, which depends on the load, will decrease the lateral stability particularly in case of an unsymmetry or as the lift truck is cornering.If an unsymmetry or the reaction forces which are due to a cornering tend to promote a lateral inclination, centrally disposed drive means which are automatically adjusted in height, e.g., by a compression spring will be increasingly relieved as a result of the lateral inclination and the compression spring or another pressure-applying element associated with the centrally disposed drive means will promote the lateral inclination so that the danger of a tiiting of such lift truck will be increased.

From another aspect a compression spring will permit an adaptation only within an intermediate range of conditions. It must be taken into account that elevations and depressions of the ground and the weight of the load may have cumulative effects so that an optimum operation as regards the pressure applied by the wheel is not necessarily ensured.

This will be particularly undesirable if such lift truck is operated automatically without a driver.

It is an object of the invention so to improve a lift truck of the above-mentioned kind that the ground contact pressure exerted by the driven wheel is variable in dependence on the elevation of the load carrier and so as to decrease the tendency of the lift truck to tilt is decreased, or the influence of yieldable pressure-applying means associated with the centrally disposed drive means can be eliminated.

This object is accomplished according to the invention in that the wheel hub drive means are supported on the chassis by means of at least one spring element having an initial stress that is adapted to be controlled in dependence on the elevation of the load carrier and/or the weight of the load. Specifically, the pressure exerted by the ground contact pressure control means associated with the centrally disposed drive means is adapted to be controlled in dependence on the elevation of the load carrier.

With such an arrangement, the ground contact pressure exerted by the wheel of the centrally disposed drive means will be high in case of a iow elevation of the load carrier and lower in case of a higher elevation of the load carrier so that a ground contact which is sufficient for braking will be ensured.

The term "controlled" is also applicable to an arrangement in which particularly in case of a lift truck having a lifting frame carrying a lifted load a lateral inclination of the vehicle in excess of a predetermined extent will have the result that the action promoting the tendency to tilt is reduced. For this purpose a limiting abutment for controlling the pressure may preferably be provided.

The ground contact pressure control means may be designed in various ways and if hydraulically operable may comprise a device for re-adjusting the vertically movable, centrally disposed drive means by an increasingly sensitive control in response to a scanning of the ground. An adaptation may also be effected by compressible ground contact pressure control means, which are controllable. It is essential that the pressure can be adapted by a supply of additional pressure fluid.

In the particularly preferred embodiment of the invention the initial stress of the spring element is adjustable particularly by an adjustment of an abutment. This will result in a particularly simple design, particularly with spatially defined supporting means, which include a limiting abutment. The term "spring element" includes coil springs, in the preferred embodiment, but includes also pressure bags or resilient pressure sleeves of rubber or a similar material as well as an arrangement of, e.g., concentrically disposed spring elements, which may also consist of torsion springs orthe like.

In connection with the above-mentioned reference to an adjustment in dependence on the weight of a load that has been picked up and/or the ground contact of the centrally disposed drive means, a preferred additional feature resides in that the adjustable ground contact pressure control means or the abutment are or is adjustable in dependence on the elevation of the load. This adjustment may be an overriding adjustment, which in connection with ground contact pressure control means associated with centrally disposed drive means that are movable in height affords important advantages as regards the speed at which the lift truck can be operated.

In a particularly preferred embodiment, at least two spring elements are provided and at least one of them is under an adjustable initial stress. This will permit a particularly desirable adaptation. The use of a plurality of spring elements has inherently the advantage that an exact adaptation to certain different operating conditions can be ensured in a manner which is specific to a given type of truck as one or more of a plurality of spring elements can be disabled. Besides, risks arising from a sudden failure of a spring element can be more easily avoided. In the preferred embodiment, the adjustability of only one spring element will be sufficient for the desired adjustment.

In a simple arrangement which can easily be controlled, the abutment constitutes a part of a hydraulic cylinder-piston unit, which is connected to a control device, and the control device is controlled in dependence on the elevation of a load carrier or in dependence on the pressure in the working hydraulic system. This arrangement will be structurally simple and reliable in operation and can easily be serviced and will permit an automatic adaptation to the elevation of the load. The control device may consist of a connecting and disconnecting sliding valve.

In accordance with a preferred additional feature the above-mentioned sensor is arranged to control the prestressing device associated with the ground contact pressure control means and particularly the at least one spring element under initial stress and the prestressing device consists particularly of a hydraulic cylinder unit. Such an arrangement will permit an automatic adaptation. In a preferred arrangement, a control valve is connected between a cylinder-piston unit for controlling the initial stress of a spring and a pressure-loaded part of a working hydraulic system and is adapted to be controlled by at least one sensor in dependence on the weight of a load that has been picked up and/orthe elevation of that load on the lifting frame and said control valve is particularly adapted to establish a connection to a drain in response to the at least one sensor.In such an arrangement the abutment, which may consist of a piston, and the spring elements may be so dimensioned that the ground contact pressure can be adjusted in dependence on the load so as to decrease the ground contact pressure as the load increases.

The use of the invention permits always to maintain optimum driving and braking conditions so that the safety in operation is significantly increased.

In a particularly desirable arrangement, a helical compression spring is disposed between a supporting plate of a drive housing and a memberfor moving the wheel hub drive means in height and an adjustable additional compression spring bears at one end on the carrying plate and at the other end on a piston, which defines at one end a pressure chamber, which is defined at its other end by means fixed to the carrying plate, whereas the piston is supported at its other end by an abutment connected to the wheel hub drive means. This arrangement will be compact and consists of simple relatively movable components which can be guided or adjusted relative to each other in a desirable manner even when the vehicle is under relatively high load.

It will be particularly desirable to provide for this purpose a plurality of telescopic cylindrical elements, one of which consists of a tubular shell, which is rotatably mounted on the drive housing, the next inner element consists of a bushing, which is axially slidably and rotatably mounted in the tubular shell and carries the wheel hub drive means and contains and guides a first compression spring, and the innermost element consists of a guide tube, which is connected to the tubular shell and has an inner end portion which consists of a cylinder confining the pressure chamber which is defined by the piston and which is adapted to be supplied with pressure fluid from the outside in orderto movethe piston.

Within the scope of the invention the pressure chamber is preferably closed at its lower end by a bottom portion of the guide tube, and the piston is fixed to an abutment carrier, which adjacent to the neck connected to the steering handle and the wheel hub drive means is guided to be movable in height and which carries an abutment that is engageable with the wheel hub drive means. The abutment carrier may particularly carry an upper abutment for limiting the initial stress of the adjustable compression springs.

Additional sensors are suitably mounted on abutments associated with the compression springs.

The invention will now be described with reference to embodiments shown by way of example on the accompanying diagrammatic drawings, in which Figure t is a side elevation showing a known lift truck provided with centrally disposed drive means which are not shown in detail in the side elevation and automatically adjustable in height.

Figure 2 is a top plan view showing that lift truck, Figure 3 is a fragmentary vertical longitudinal sectional view showing a preferred embodiment of centrally disposed drive means which are automatically adjustable in height and held on the vehicle frame, particularly on the drive housing of a vehicle frame, and Figure 4 is a diagrammatic showing of adjustable ground contact pressure control means which cooperate with a working hydraulic system of the lift truck and comprise ground contact pressure-exerting elements consisting of coil springs.

Figures 1 and 2 show the design of a known lift truck, which is generally designated 1 and comprises centrally disposed drive means 3, which are pivotally movable by means of a steering handle 2 and are automatically adjustable in height. The centrally disposed drive means comprise the steerable driven wheel 4. The means for an automatic adjustment in height are symbolized by a spring 5.

The lift truck comprises a drive housing 6, in which the steering mechanism of the centrally disposed drive means 3 is mounted. The centrally disposed drive means are disposed between caster rollers 7, which are mounted on the drive housing 6.

Wheel-carrying arms 9 extend from the drive housing 6 to positions under a load-carrying fork 8 and at their forward ends are provided with loadsupporting rollers 10.

The load fork 8 is carried by a fork carrier 11, which is mounted on and movable up and down along a telescopic lifting frame 12 and is moved by loadcarrying chains 58 connected to a lifting cylinder 13.

The centrally disposed drive means generally designated 3 are rotatably mounted on an upper carrying plate 14 of the drive housing 6 by means of a steering bearing 17, which is held in retaining rings 15, 16. The centrally disposed drive means 3 are steered by means of a steering handle 2, which is non-rotatably connected to the drive means 3 at a connecting neck 18.

A tubular shell 19 is rotatably mounted in the steering bearing 17 and is fixed in elevation relative to the carrying plate 14. A sleeve 20 is axially slidably mounted in the tubular shell 19 and carries a slider 21, which is movable in an axial guide slot 22 formed in the tubular shell 19.

The tubular shell 19 is closed at its top end by a cover 25, which is secured by means of bolts 23, 24 and which constitutes an abutment for a helical compression spring 26, which at its other end bears on a stepped abutment 27 provided on the slidable sleeve 20. The latter is fixed to the centrally disposed drive means 3, which in addition to the steerable driven wheel 4 comprises wheel hub drive means 28.

A guide tube 29 is centrally secured in the cover 25. The neck 18 for connection to the steering handle is secured to the top of the guide tube 29 and serves also as an abutment for an adjustable second helical compression spring 30, which at its other end bears on a piston 31, which is fixed to an inner abutment carrier 39, which is guided in the guide tube 29.

The guide tube 29 is closed at its lower end by a pistonlike buttom member 33, which has a neck 62, that is fixed to the guide tube 29 by retaining means 32. The chamber between the piston 31 and the bottom member 33 constitutes a pressure chamber 35, which is adapted to be supplied with pressure fluid through a pipe 38, which extends through the bottom member 35 and though openings 37,36 formed in the tubular shell 19 and the sleeve 20, respectively. The pressure chamber 35 communicates via the pipe 38 with the working hydraulic system of the lift truck so that the piston 31 is adapted to be moved upwardly in the guide tube 29 in order to increase the initial stress of the second compression spring in dependence on the pressure of the fluid that has been supplied.

The abutment carrier 39 extends through the bottom member 33 and is sealed therein and movable relative thereto in height and is guided in the connecting neck 18 by means of a pin 40 provided at the top end of the abutment carrier 39 and is guided adjacent to the wheel drive means 28 by a pin 41 secured to the abutment carrier 39. The abutment carrier 39 carries a lower abutment 34, which is engageable with an abutment 59 carried by the wheel hub drive means 28. In that position a limiting upper abutment 60 carried by the abutment carrier 39 is spaced from an associated abutment 61 carried by the connecting neck 61.

The pressure in the pressure chamber 35 acts on the piston 31, which is disposed under the second helical compression spring 30 and raises the abutment carrier 39 so that the lower abutment 34 tends to disengage the associated abutment 59. But the engagement will be maintained if the helical compression spring 36 is compressed by the load; in that case the initial stress of the second helical compression spring 30 will be increased. That operation will be terminated when the limiting upper abutment 60 reaches the associated abutment 61.

In the design shown in Figure 3, the connecting neck 18 is fixed in height to the carrying plate 14.

That design affords the advantage that the steering handle 2 is vertically fixed to the drive housing 6.

In Figure 4, corresponding parts are designated with the same reference characters and the connection to the working hydraulic system is shown. But Figure 4 indicates also that the spatial association of the parts may be changed.

In accordance with Figure 4the lifting frame 10 constitutes a cylinder of the working hydraulic system and a lifting piston 42 is movable in height in said cylinder. A piston rod 43 protrudes from the cylinder and is provided at its top end with reversing sprockets 44 associated with load-carrying chains 58. It will be understood that parallel twin arrangements may be provided on both sides of the lift truck. The working chamber 45 of the lifting cylinder communicates with a pump 46 of the working hydraulic system via a passage 47, from which a branch passage 48 leads to the pipe 38. The branch passage 48 incorporates a valve 49, which has an actuating member 50 that is controlled by a sensor 51, which is mounted on the lifting frame 10 and delivers signals in dependence on the elevation of the fork carrier 11.

Figure 4 indicates on a functional link 52 used to transmit signals from the sensor 51 to the actuating element 50. The signals may be directly mechanically transmitted or may be indirectly transmitted electrically by means not shown and serve to control the initial stress of the second helical compression spring 30 in dependence on the elevation of the load.

Initially, the valve 46 is open to maintain the ground pressure contact of the wheel 4. The valve 49 is shiftable to establish a drain connection to the reservoir 53, as shown, particularly to preclude a promotion of a tendency to tilt in case of a high elevation of the load.

It will be understood that the sensor 51 is only symbolized and that intermediate stages may be provided for, particularly for a control of the travel drive.

Whereas helical compression springs 26,30 are shown one beside the other in Figure 4, the spring 30 tends also in this embodiment to urge the abutment carrier 39 into engagement with the sleeve 20 connected to the hub drive means 28.

In the design shown in Figure 4, additional sensors 56,57 are mounted on the abutments 54, 55, which are carried by the tubular shell 19 and associated with the compression springs 30,36. These sensors 56, 57 to control and particularly disable the travel drive in response to spring breakage.

In the arrangement shown in Figure 4, the two springs 26 and 30 preferably consisting of helical compression springs act directly or by means of the abutment carrier 39 when the load-carrying form 8 is in its lowermost position in the initial position of the lift truck. The highest ground contact pressure will thus be applied by the wheel 4 of the centrally disposed drive means in this position. When a load has been picked up and is being lifted, the sensor 51 will respond when the load has reached a predetermined elevation or one of several predetermined elevations and will then cause hydraulic fluid to be supplied to the pressure chamber 35. The pressure in said pressure chamber 35 will depend on the operating conditions, particularly on the weight of load that has been picked up.For instance if the weight of a load that has been picked up is about 10% of the largest permissible weight, the piston 31 in the preferred embodiment will not disengage the spring 30 but only the ground contact pressure applied by the centrally disposed drive means will be reduced so that the helical compression spring 26 will be somewhat relieved.

Similar actions will be performed under different loads, so that the pressure in the pressure chamber 35 will depend on the weight of the load that has been picked up and the ground contact pressure applied by the driven wheel 4will be reduced in accordance with the weight of the load.

In dependence on the type of the vehicle the diameter of the piston 31 will be selected so that the spring 30 consisting preferably of a helical compression spring will be compressed in case of a predetermind weight of the load to such an extent that the abutment carrier 39 no longer provides a support and the ground contact pressure then depends only on the action of the spring 26, which consists of a helical compression spring. It is apparent that a load-dependent function will be obtained if the diameter of the piston 31 and the dimensions of the springs 26 and 30 consisting preferably of helical compression springs are properly selected.

Claims

1. A lift truck having drive means for driving a wheel which is disposed between casters, means for automatically adjusting said drive means in height, contact pressure-applying means which are yieldable in the direction of the height adjustment, and a load carrier, characterized in that the wheel hub drive means are supported on the chassis by means of at least one spring element having an initial stress that is adapted to be controlled in dependence on the elevation of the load carrier and/or the weight of the load.

2. A lift truck according to claim 1, comprising a spring element for an automatic adjustment in height, characterized in that the initial stress of the spring element is adjustable, preferably by an adjustment of an abutment.

3. A lift truck according to claim 1 or 2, comprising a lifting frame along which the load carrier is movable in height by a working hydraulic system comprising a pump, characterized in that the adjutable ground contact pressure control means or the abutment are or is adjustable in dependence on the elevation of the load.

4. A lift truck according to claim 2 or 3, characterized by at least two parallel spring elements are provided and at least one of them is under an adjustable initial stress.

5. A lift truck according to claim 4, characterized by an abutment for limiting the adjustable initial stress.

6. A lift truck according to claim 2 or 3, characterized in that the abutment constitutes a part of a hydraulic cylinder-piston unit, which is connected to a control device, and the control device is controlled in dependence on the elevation of a load carrier or in dependence on the pressure in the working hydraulic system.

7. A lift truck according to claim 6, characterized in that the control valve consists of a connecting and disconnecting sliding valve.

8. A lift truck according to any of claims 3 to 7, comprising at least one sensor on the lifting frame, characterized in that a control valve is connected between a cylinder-piston unit for controlling the initial stress of a spring and a pressure-loaded part of a working hydraulic system and is adapted to be controlled by at least one sensor in dependence on the weight of a load that has been picked up and/or the elevation of that load on the lifting frame and said control valve is particularly adapted to establish a connection to a drain in response to the at least one sensor.

9. A lifttruck according to any of claims 1 to 8, characterized in that the abutment, which may consist of a piston, and the spring elements are so dimensioned that the ground contact pressure can be adjusted in depandence on the load so as to decrease the ground contact pressure as the load increases.

10. A lift truck according to any of claims 1 to 9, characterized in that a helical compression spring is disposed between a supporting plate of a drive housing and a member for moving the wheel hub drive means in height and an adjustable additional compression spring bears at one end on the carrying plate and at the other end on a piston, which defines at one end a pressure chamber, which is defined at its other end by means fixed to the carrying plate, whereas the piston is supported at its other end by an abutment connected to the wheel hub drive means.

11. A lift truck according to claim 10, characterized in that a plurality of telescopic elements are provided one of which consists of a tubular shell, which is rotatably mounted on the drive housing, the next inner element consists of a bushing, which is axially slidably and rotatably mounted in the tubular shell and carries the wheel hub drive means and contains and guides a first compression spring, and the innermost element consits of a guide tube, which is connected to the tubular shell and has an onner end portion which consists of a cylinder confining the pressure chamber which is defined by the piston and which is adapted to be supplied with pressure fluid from the outside in order to move the piston.

12. A lift truck according to claim 11, characterized in that the pressure chamber is closed at its lower end by a bottom portion of the guide tube, and the piston is fixed to an abutment carrier, which adjacent to the neck connected to the steering handle and the wheel hub drive means is guided to be movable in height and which carries an abutment that is engageable with the wheel hub drive means.

13. A lift truck according to claim 12, characterized in that the abutment carrier carries an upper abutment for limiting the initial stress of the adjustable compression spring.

14. A lift truck according to any of claims 4 to 13, characterized in that additional sensors are provided on abutments associated with the compression springs.

15. A lift truck substantially as herein described with reference to the accompanying drawings.