GB2357268A

GB2357268A - Steering mechanism for variable-wheelbase counterbalance fork-lift trucks

Info

Publication number: GB2357268A
Application number: GB0030543A
Authority: GB
Inventors: Hans-Peter Claussen; Ernst Peter Magens
Original assignee: Jungheinrich AG
Current assignee: Jungheinrich AG
Priority date: 1999-12-17
Filing date: 2000-12-14
Publication date: 2001-06-20
Anticipated expiration: 2020-12-14
Also published as: DE19960946A1; DE19960946C2; GB2357268B; GB0030543D0; FR2802494B1; FR2802494A1

Abstract

A counterbalance fork-lift truck has a rigid front axle supporting two wheels and two rear steering wheels 38, 40 each supported on a pivoted bolster (Fig. 5) with each bolster being pivoted for steering by a respective electric motor. The wheelbase Y1, Y2 varies with the operation of the lift frame. A steering controller is supplied with signals from sensors which determine the wheelbase, the positions of the steered wheels 38, 40 and the position of the driver-operated steering wheel such that the angles to which the steered wheels 38, 40 are set will take the wheelbase into account, e.g. to ensure that at the minimum steering radius 62 the axes of the steered wheels intersect at the centre of the rigid axle. The steering controller may also be influenced by the speed of the vehicle.

Description

2357268 STEERING MECHANISM FOR COUNTERBALANCE FORK-LIFT TRUCKS This

invention relates to a steering mechanism for a counterbalance fork- lift truck according to claim 1.

Four-wheel vehicles generally have a rigid axle. In passenger cars and autotrucks, it is positioned in the rear. In counterbalance fork-lift trucks, the front load-bearing axle mostly is the rigid drive axle with the steering axle always being disposed at the other end of the vehicle. Chassis of this type can only reach relatively large turning radii because the most frequently adopted principle of double-pivot steering with its mechanics of steered wheels and its swivel only admits of relatively small steering angles. For the field of industrial trucks, special purpose steering axles are employed, which make it possible that the instantaneous roll center of the rotary motion, if the steering angle is maximal and, hence, the turning radius is minimal, approximately is at the outer border of the wheels of the rigid drive axle, i.e. the load-bearing axle.

US 4 754 837 has made known a vehicle in which the off-center drive wheel is swiveled via a chain-driven pivoted bolster for steering the vehicle. Likewise, in order to equally steer the off-centre supporting roller at the same vehicle end this one is also designed as a pivoted bolster and is coupled to the driving wheel via a lever mechanism. Thus, it is forced to perform the steering motion necessary for a normal driving mode.

A similar principle has become known from the company publication "News Carer" of June I't, 1996. In the known steering mechanism, a drive is accomplished by turning one of the suspension links which is connected to the two pivoted bolsters via more suspension links. The company publication "Drehen und Wenden in jeder Lage" of the Linde AG company has made known a steering axle which also .. /2 includes two pivoted bolsters each of which is coupled to a joint hydraulic cylinder via a lever mechanism. The configuration of the individual suspension links of the lever mechanism achieves that the pivoted bolsters, in a coupled motion, rotate so that the instantaneous roll center of the rotary motion of the whole vehicle always is approximately on the load-bearing axle centerline and, in an extreme case, migrates up to the load-bearing axle center if the vehicle is steered at a minimal turning radius. In all of the known solutions, the design of the lever and chain- driven mechanisms is dependent on a defined, fixed wheelbase of the vehicles. To improve the running characteristics, a steering mechanism may be purposively adjusted in the form of a toe-in or toe-out. This "trimming" of the steering mechanism, however, mostly has a positive effect on one direction of travel only and, at the same time, a negative effect on the other. Therefore, it is inappropriate for industrial trucks which are driven in either direction approximately to an equal degree. Also, an automatic swivel of the steered wheels back to the straight position as is known from a passenger car will positively act in one direction only whereas it has a contrary effect in the other direction. As was mentioned, since industrial trucks and counterbalance fork-lift trucks, in particular, mostly run forwards as frequently as they run backwards the direction-dependent adjustments are made to these vehicles only to a limited extent or are not performed at all.

If the industrial truck is fitted with a variable wheelbase as is the case, for example, if the load-bearing axle is rigidly connected to the lift frame and this one is designed to be tilted both forwards and backwards another wheelbase will adjust itself depending on the angle of inclination of the lift frame. The geometry of the steering mechanism, however, has been designed for a defined wheelbase and any deviation therefrom leads to a deviation from the ideal geometry and, thus, results in heavier wear to the tires and a stronger rolling drag because the rolling motion proper is overlain by a sliding motion.

/3 It is the object of the invention to provide a steering mechanism for a counterbalance fork-lift truck in which the steering geometry is adapted to the respective wheelbase.

The object is attained by the features of claim 1.

In the inventive counterbalance fork-lift truck, the two wheels of the steering axle are pivotally supported separately about a vertical axis in a pivoted bolster. The essential feature of the invention is that an electric motor is associated with each pivoted bolster in order to pivot it through a desired angle. Further, the invention provides a wheelbase sensor which measures the distance between axles and generates a respective signal. Associated with each pivoted bolster is a steering angle sensor and associated with the steering element of the vehicle, e.g. a steering wheel, is a set-point sensor. Finally, a steering controller is provided for the steering motors which, depending on the deviation of the steering set-point from the steering angle actual value and the axle base, generates a positioning signal for the steering motors.

As explained above, each wheelbase includes a certain steering geometry.

The individual geometries are stored in the steering controller and may be employed depending on the respective wheelbase measured in order to ensure that the axle centerlines of the steered wheels intersect on the centerline of the rigid axle regardless of the respective wheelbase. Therefore, the invention ensures that optimal steering conditions always exist and do not depend on the wheel base. The invention is particularly advantageous if a sliding axle is employed.

The steering controller may also be responsive to other influencing variables such as the speed, and also the magnitude of the steering angle. The steering wheel presets the steering-angle set-point. If the steering-angle set-point is appropriately modified, e.g. in dependence on the speed or in dependence on the extent of turning the steering wheel and on the direction of travel, the set-point of the individually steered wheels may be modified to obtain a desired trimming (toe-in angle).

.. A The invention will now be explained in detail with reference to the embodiments shown in the drawings.

Fig. I shows a side view of a counterbalance fork-lift truck.

Fig. 2 shows a plan view of the vehicle of Fig. I with a known doublepivot steering mechanism.

Fig. 3 shows a plan view of the steering axle of the representation of Fig. 2.

Fig. 4 shows a representation of a minimal turning circle of the vehicle of Figures I and 2.

Fig. 5 shows a rear view of an inventive steering axle.

Fig. 6 shows a plan view of a counterbalance fork-lift track including a steering mechanism according to the invention.

Fig. 7 shows a representation similar to Fig. 6 with a minimal turning circle for the vehicle.

Fig. 8 shows a block connection diagram of the controller for the inventive steering mechanism.

Fig. 9 shows a diagram of the respective steering angles of the individually steered wheels of the inventive steering mechanism as a function of the effective steering angle for two different wheelbases.

Fig. I illustrates a side view of a conventional counterbalance fork-lift truck with a vehicle base frame 12, a rigid axle 14, and a steering axle 16. The lift frame 18 is supported on the rigid axle 14. Since the lift frame is adapted to be pivoted forwards and backwards (not shown in detail) the wheelbase Y will change in dependence on the extent of pivot. The steering axle 16 of the vehicle is provided in the way shown in Fig. 3. The steered wheels 15 are coupled via a lever mechanism 20 in such a way that the axle centerlines 22, 24 of the steered wheels 15 intersect on a centerline 26 of the axle 14. The point of intersection is indicated by 28. In this conventional steering mechanism, the instantaneous roll center of the rotary motion of the vehicle 10 can be moved, at the most, up to the outer border of .. 15 the wheels of the rigid axle 14. This will lead to a turning radius 30 of Fig. 4. If the axle base Y is changed the axle centerlines of the steered wheels Vill no longer intersect on the centerline of the rigid axle. The embodiment of Figures 5 through 8 serves for an adaptation to the wheelbase.

Referring to Fig. 5, a steering axle 32 is shown which rotatably supports two pivoted bolsters 34, 36 about a vertical axis. Each pivoted bolster 34, 36 includes a steered wheel 38, 40. The pivoted bolsters 34, 36 are pivoted by means of electric motors 42, 44 with the motors having integrated therein a transmission. Coupling between the pivoted bolster 34, 36 and the motor 42, 44 is performed by a chain and-sprocket drive 46 and 48, respectively. Associated with each pivoted bolster is a steering-angle sensor 50 and 52.

Fig. 6 shows a plan view of a counterbalance fork-lift truck 52 similar to that of Figures I and 2 wherein the varying wheelbase is outlined, too. The actual wheelbase Y is measured by a sensor 54 (Fig. 8). The steering system receives a preset value for the effective steering angle via the steering wheel 56 actuated by the driver of the fork-lift truck 52 and the set-point sensor 58 which is connected thereto. This preset value is passed into a steering controller 60 which will then provide a positioning value for the steering angles to the steering motors 42, 44 in dependence on the wheelbase Y. The motors are energized according to the preset values for a time until the respective set-point steering angle sensors 50, 52 signal back the required value.

Referring to Fig. 7, it can be seen that a minimal steering radius 62 is obtained where, in i ts minimal case, the axle centerlines of the steered wheels intersect in the center of the rigid axle.

Fig. 9 diagrammatically shows the values of the steering angles of the internal-curve wheel alphal,i and alpha2,i for the wheelbases; Y1 and Y2, and those of the external-curve wheel alphal,a and alpha2,a as a ftmction of the steering angle /6 for a range of from 0 to +90'. An equal diagram will result for the range of from 0 to -900.

Normally, there is a linear interrelationship between the steering-angle set- point and the preset value of the effective steering angle. However, it is also possible to produce a non-linear interrelationship, e.g. to make it depend on or to modify it in dependence on the travelling speed or the magnitude of the effective steering angle.

Target-related parameters may be fed to the steering controller via the block 64.

Likewise, it is possible to provide a trimming of the steered wheels by changing their "toe-in angle". Such a trimming, which increases their stability in running, may be carried out depending on the direction of travel and/or their speed with the set-points being individually variable for each wheel.

/7 t 7

Claims

1. A steering mechanism for counterbalance fork-lift trucks, comprising:

a rigid front axle supporting two wheels, two steered wheels on a rear steering axle, each of which is supported in a pivoted bolster pivotally supported about a vertical axle, an electric steering motor for each pivoted bolster, a sensor for determining the axle base between the front wheels and the steered wheels, a steering-angle sensor for each pivoted bolster, a set-point sensor coupled to a steering element, and a steering controller for the steering motors which, is depending on the deviation of the steering set-point of the set-point sensor from the steering-angle actual value of the steering-angle sensors and the axle base, generates a positioning signal for the steering motors.

2. A steering mechanism according to claim 1, in which a speed sensor is provided the signal of which is delivered to the steering controller to influence the positioning signal depending on the speed of the vehicle.

3. A steering mechanism according to claim 1 or claim 2, in which the signal delivered to the steering controller depends on the steering angle.

4. A steering mechanism according to any preceding claim, in which the positioning signal, depending on the speed and/or the direction of travel, is modified so that the steered wheels undergo trimming.

5. A steering mechani.sm substantially as hereinbefore described with reference to Figures 5 to 9 of the accompanying drawings.