GB2276607A

GB2276607A - Lift trucks

Info

Publication number: GB2276607A
Application number: GB9405985A
Authority: GB
Inventors: Marcel Krenzin
Original assignee: Jungheinrich AG
Current assignee: Jungheinrich AG
Priority date: 1993-03-30
Filing date: 1994-03-25
Publication date: 1994-10-05
Anticipated expiration: 2014-03-25
Also published as: FR2703320B1; GB2276607B; IT1271170B; GB9405985D0; ITMI940573A0; DE4310257C2; ITMI940573A1; DE4310257A1; FR2703320A1

Description

Ir l 2276607 1 LIFT TRUCKS A substantial category of lift trucks relates

to wheeled arm supported four-wheel trucks having an asymmetric drive.

These lift trucks comprise a drive section and a load section, with the drive section having a steerable drive wheel on one side of the longitudinal axis of the truck and a caster roller on the other side thereof. The fork-like wheeled arms are supported at the ends by at least one load wheel. Furthermore, it is known to arrange the load section so as to perform a so-called initial lift sufficient to raise a pallet from the ground while the lifting forks of the lift truck adapted to the load arms engage into the pallet. For providing said initial lif t it has become known to mount a linkage for each wheeled arm to the drive and load sections, the linkage serving to exert a tension force or a compression force onto the wheel bearings of the load wheels in order to raise the wheeled arm relative to the load wheels for a limited distance. To this end, the load wheels are supported in rockers which are pivoted by means of the drawing bars or compression bars in order to provide for the above-mentioned lifting of the wheeled arms. With such linkages it has become known to provide a rocker at the drive end of the load section, which rocker has one end supported against a surface of. the drive section at the underside thereof, preferably via a roller, and has its other end pivotally mounted to the drawing bar or compression bar.

A four-wheel vehicle of the above-described type is statically overdetermined which results in undefined wheel normal forces which are non-uniform in operation. The parameters determining the amount of the individual wheel normal forces are the position of the overall center of gravity, the geometry of the undercarriage, the stiffness 2 of the f rame and last but not least the wear of the wheels and rollers.

In order to explain the problems of the abovedescribed undercarriage concept, it is assumed for simplicity that the undercarriage is symmetrical, i.e. the wheels of the drive axle and, respectively, the load axle are coaxial and are spaced from the longitudinal axis of the truck by the same amount (which is not readily true f or a number of lifting trucks). Now it is not only the axle loads but also the individual wheel normal forces which are clearly defined provided the center of gravity is along the truck longitudinal axis. The axle loads are distributed between the wheels of the respective truck axle 50t to 50k. The amount of the axle loads depends on the total weight of the truck and the position of the overall center of gravity.

Due to the truck structure and the respective load classes of the known four-wheel lift trucks these two parameters can be varied only insignificantly without adversely affecting its effectiveness. In order to be able to stop the truck in accordance with the legal regulations, and in order to ensure safe operation in curves, a certain wheel normal force at the steered wheel and at the braked wheel is required. The amount of this force is determined by the truck velocity and the truck total mass. Since variation of the truck total mass and the position of the center of gravity is only small, the maximum admissible travel velocity is determined by the drive axle distribution between the drive wheel and the caster roller.

When the travel velocity is determined, it is also necessary to consider the required drive wheel normal force and to calculate wear of the wheels. The four wheels wear differently during operation of the truck. In order to be able to realize the velocities a"s required in practice, a 50t distribution of the drive load to the wheels, without considering wear, would not be sufficient. Accordingly, t 3 measures for increasing the drive wheel normal force are required.

It has become known to provide the required wheel normal force by withdrawing the caster roller into the truck contour so that the required normal force at the drive wheel will result. If it is assumed that the undercarriage is completely stiff, the caster roller while being under load could be raised so that conditions of a three wheeler would result. In this case the total drive load would be taken up by the drive wheel. However, due to unavoidable resiliency of the frame the undercarriage is twisted so that the truck frame linearly inclines towards the caster roller side when the caster roller is raised. This is why there are limits to trimming of the drive load is towards the drive wheel by nclining the truck and retaining ground clearance. Furthermore, such inclination of the total drive axle results in additional steering torque which can be compensated only by the user. The reason is that the steering axle which is normal to the wheel axle is inclined when the drive axle is trimmed for the same amount so that a lever arm between the wheel transverse force and the steering axle is provided. Such lever arm is further adversely affected by the shape and width of the so-called wheel bandage. This is so because an extremely narrow and spherical shape of the bandage is aimed at.

The problem to be solved by.the present invention is to provide a wheeled arm supported lift truck in which the drive wheel normal pressure can be increased in a simple manner without adversely affecting the steering conditions.

According to the present invention, a lift truck comprises: a drive section, which includes a steerable drive wheel on one side of the truck's longitudinal axis and a caster roller on the other side of the truck, s longitudinal axis; a load section, which is guidable along the drive section so as to be adjustable in height and which includes at its underside load wheels having 4 respective bearings adjustably mounted relative to associated wheeled arms; a lifting device between the drive section and the load section; and a linkage, for each load wheel, which cooperates with the drive section and the load section and engages the wheel bearings to raise the wheeled arms for a predetermined lift when the lifting device raises the load section; with the linkage associated with the drive wheel initially performing a limited lost motion when the load section is raised.

In other words, with the present invention the linkage associated with the drive wheel is arranged so as to provide for limited lost motion when the load section is raised. While the load section is raised, the wheeled arm on the caster roller side moves in advance of the wheeled arm on the drive wheel side. on the caster roller side the linkage is rigid as usual so that the associated wheeled arm is raised immediately when the so-called initial lift is started. However, the wheeled arm on the drive wheel side initially remains in its lowermost position as long as the lost motion has not been overcome. only when the lost motion has been overcome does the wheeled arm on the drive wheel side begin to rise. Such asymetric raising of the load results in provision of a main load axis between the drive wheel and the load wheel on the caster roller side.

The invention thus provides for an improvement of traction of the drive wheel which does not result in any inclination of the drive axle and accordingly does not have the above-described disadvantages.

The trailing of the lifting linkage on the drive wheel side may be obtained by incorporation of a defined play. One embodiment of the invention provides that a compression or draw bar performs the lost motion. To this end ' according to a further development of the invention, it may be provided that the compression or draw bar of the linkage has one end extending into a sleeve which is pivotally mounted to an adjacent portion of the linkage associated with the drive wheel. It is particularly simple to provide 17 that the compression or draw bar includes a radial flange which is mounted within the sleeve for limited movement.

As an alternative, supporting of the linkage at the drive section may be obtained via a play by arranging a support surface at a member which is movable for a limited distance.

Another alternative, to provide f or a play of the lifting linkage associated with the drive wheel, is that a f urther roller of resilient material is mounted on the same axis as a support roller and has a diameter which exceeds the diameter of the support roller. The material of the greater roller may be f or example an elastomeric material, e.g. rubber. During the lifting operation, the resilent roller is initially deformed until the support roller which is consists e.g. of steel comes into engagement with the support surface of the drive section.

The described structural measures provide for a trimming of the wheels or rollers independently of the amount of the load. If, according to an embodiment of the invention, the lost motion is to be overcome against the force of a spring, the amount of the lost motion or, even better, of the resulting lifting differential will be proportional to the load wheel normal force and accordingly the vehicle load. The load responsive distribution of the drive load between the caster roller and the drive wheel will now be determined by the spring stiffness. The trimming of the undercarriage is no longer uniform but rather load responsive. A proper drive wheel normal force with respect to the load will have the result of minimizing the steering forces. The resulting difference in height of the wheel arms in the area of the load axle is substantially less as compared to trimming of the caster roller (support roller trimming) and does not adversely affect the travelling conditions. However, the individual load wheel normal forces and the resulting fork tine kinematic loads are substantially varying. This is why these components must be dimensioned accordingly.

6 The invention will now be explained in more detail with reference to the accompanying drawings in which:- Fig. 1 shows schematically a side elevation of a lift truck according to the invention; Fig. 2 shows the underside of the lift truck of Fig.

Fig. 3 shows a detail of the lif t truck of Figs. 1 and 2; Fig. 4 shows an alternative of the embodiment of Fig.

3; Fig. 5 shows, at an enlarged scale, a part of the lif t truck of Fig. 1 with an alternative embodiment of the lifting linkage; Fig. 6 shows a side elevatipn of another alternative embodiment of the lifting linkage; and Fig. 7 shows a section along line 7-7 of Fig. 6.

The lift truck 10 shown in Figs. 1 and 2 is a so-called four-wheeled arm supported lifting truck comprising a drive section 12 and a load section 14. The drive section includes a drive wheel 16 which is driven by a motor 18. It is positioned on one side of the truck longitudinal axis 20. On the other side there is positioned a caster roller 22. The load section includes a pair of parallel wheeled arms 24,26 which are connected to each other by a load carriage 28 which, as indicated at 30, is guided by suitable means at the drive section 12 so as to be adjustable in height. The load carriage 28 is connected to the rod of a lifting cylinder 32 in the drive section 12.

A lifting linkage 34 and, respectively, 36 is operatively connected between the drive section 12 and the load section 14. Each lifting linkage comprises a first rocker 38 which is pivotally mounted about a horizontal axis in the forward area of the wheeled arm 24, 26. One arm of the rocker 38 supports a load wheel 40 and, respectively, 42. A second rocker 44 is pivotally mounted at 46 to the underside of the load section 14. One arm of t 7 the rocker 44 is pivotally mounted to one end of a draw bar 48 which has its other end pivotally mounted to the other arm of the rocker 38. The other arm of the rocker 44 includes a roller 50 which is supported against the 5 underside of the drive section 12.

As so far described it is a conventional lift linkage for generating a socalled initial lift for the wheeled arms 24, 26 when the load section 14 is raised by actuation of the cylinder 32.

As may be seen from Fig. 3, however, the draw bar 48 here comprises a rigid bar 52 having one end extending into the sleeve-like portion 54 of a holder 56. The inner diameter of the bore of the sleeve 54 exceeds that of the bar 52 so that a radial flange 58 on the bar 52 is seated with a close f it. The bore is closed at its open side by an annular member 60 which is fixed by a snap ring 62. Between the annular member 60 and the f lange 58 there is positioned a spring element 64. Such a structure of the lif ting linkage is provided only for the lifting linkage 36 on the drive wheel side, whereas the draw bar 48 for the lifting linkage 34 is rigidly pivotally mounted at both ends.

In use, when the lifting cylinder 32 exerts a lifting force onto the load section 14, the lifting linkage 34 responds promptly so as to give the usual initial lift to the associated wheeled arm 26. However, the tension force which is exerted in the same manner onto the holder 56 does not yet result in actuation of the bar 52. Rather the spring 64 yields, and actuation of the bar 52 occurs only when a sufficient force has been transmitted to the bar, a force which is sufficient to pivot the associated rocker 38. As a result, the wheeled arm 24 is raised somewhat less than the wheeled arm 26, by which asymetric load distribution the drive wheel 16 experiences an increased wheel normal force, irrespective of the amount of the load which rests on the load section 14.

8 It is to be understood that the spring 64 can be omitted and in this case the amount of the trailing lif t is determined by the play s.

As an alternative there are known lifting linkages in which the bars comprise compression bars. Fig. 4 shows this case. Components in Fig. 4 which are similar to those in Fig. 3 have been designated by the same reference numerals with the addition of an apostrophy. As may be seen the connection of the bar 481 to the holder 56' is somewhat similar to that of Fig. 3. A spring 66 is positioned bet ween the radial flange 581 and the bottom of the bore of the sleeve portion 541. If pressure is exerted against the bar 481 via the holder 561, the spring 66 yields until the spring force is sufficient to obtain the desired lift ad justment. It is to be understood that also in this case the spring 66 may be omitted so that a play s' will result when the lifting linkage 36 on the drive wheel side is actuated.

Fig. 5 indicates a drive section 12 11 and a load section 1411 as well as a lifting linkage 3611 including a bar 4811 and a second rocker 4411pivoted at 46n. To the underside of the drive section 1211 is mounted a small cylinder 70 in which a small piston 72 is reciprocable. It is connected to a bar 74 which extends from the cylinder 70 downwards and is connected to a reaction plate 76. The reaction plate 76 is engaged by the roller 5011 of the linkage 3611. Between the reaction plate 76 and the under side of the cylinder 70 there is positioned a spring 78.

When the load section 14 11 is raised, the plate 76 is upwardly displaced against the spring 78 until the spring force of the spring 78 is sufficient to raise the associated wheel arm. When the spring 78 is omitted, the piston 72 moves initially to the underside of the drive section 1211 before the wheeled arm on the drive side performs an initial lift. This play is indicated by s11.

Pigs. 6 and 7 show a lifting linkage similar to the lifting linkages 36 of Figs. 1 and 2 and 3611 of Fig. 5. The fork-like rocker 44a is pivotally mounted to the load 9 section at 46a. To the shaft of the fork-like rocker 44a is mounted a bearing pin 70a rotatably mounting a support roller 50a similar to the support roller 5011 of Fig. 5. On the pin 70a, however, there is rotatably mounted a rubber roller 72a having a diameter which substantially exceeds that of the roller 50a. Both rollers 50a, 72a are associated with a support surface of the drive section (not shown). When the load section is raised, the pin 70a moves towards the support surface so that initially the roller 10 72a comes into engagement therewith. Due to the resulting force the roller 72a is deformed until the roller 50a which is made e.g. of steel engages the support surface. In this manner a resilient lost motion is provided until the linkage 36a actuates the associated load wheel and raises is the load section.

1

Claims

1. A lift truck comprising: a drive section, which includes a steerable drive wheel on one side of the truck's longitudinal axis and a caster roller on the other side of the truck, s longitudinal axis; a load section, which is guidable along the drive section so as to be adjustable in height and which includes at its underside load wheels having respective bearings adjustably mounted relative to associated wheeled arms; a lifting device between the drive section and the load section; and a linkage, for each load wheel, which cooperates with the drive section and the load section and engages the wheel bearings to raise the wheeled arms for a predetermined lift when the lifting device is raises the load section; with the linkage associated with the drive wheel initially performing a limited lost motion when the load section is raised.

2. A lift truck according to claim 1, in which a compression or draw bar performs.the lost motion.

3. A lif t truck according to claim 2, in which the compression or draw bar has one end extending into a sleeve which is pivotally mounted to an adjacent portion of the linkage associated with the drive wheel.

4. A lif t truck according to claim 3, in which the compression or draw bar includes.a radial flange which is mounted within the sleeve for limited movement.

S. A lift truck according to any one of claims 1 to 4, in which the lost motion is to be overcome against a spring force.

6. A lift truck according to claim 3 or claim 4, in which a spring is mounted in the sleeve and engages the end of the bar extending into the sleeve so that the lost motion is to be overcome against the force of the spring.

7. A lift truck according to any one of claims 1 to 4, in which the linkage associated with the drive wheel is slidably supported on a lower surface, of the drive 11 section, which lower surface is. arranged on a component which is adjustable in height for a limited amount.

8. A lift truck according to claim 7, in which the linkage associated with the drive wheel includes a roller which is in contact with the component which is adjustable in height.

9. A lift truck according to claim 7 or claim 8, in which the component is acted upon by a spring so that the lost motion is to be overcome against the force of the spring.

10. A lift truck according to any one of claims 1 to 4, in which the linkage associated with the drive wheel is slidably supported on a lower surface of the drive section, via a support roller, with a further roller of resilient material being mounted on the same axis as the support roller and having a diameter which exceeds the diameter of the support roller.

11. A lift truck substantially as hereinbefore described with reference to Figures 1 to 3, or Figures 1 to 3 as modified by Figure 4, Figure 5, or Figures 6 and 7, of the accompanying drawings.