GB2190060A

GB2190060A - Improvements in vehicle tail lifts

Info

Publication number: GB2190060A
Application number: GB08611254A
Authority: GB
Inventors: Michael Ashcroft Buzzard; Peter John Keilty
Original assignee: Ratcliff Tail Lifts Ltd
Current assignee: Ratcliff Tail Lifts Ltd
Priority date: 1986-05-08
Filing date: 1986-05-08
Publication date: 1987-11-11
Also published as: GB8611254D0; GB2190060B

Abstract

A balance mechanism for a vehicle tail lift which is arranged to provide on the pivotable platform (11) a counterbalancing moment which accurately compensates for the effect of the weight of the platform over its full range of pivotal movement. The counterbalance is effected by a stack of disc springs located between a plunger (18) and a collar( 19). A damper (20) is provided on the plunger rod (18). <IMAGE>

Description

SPECIFICATION Improvements in vehicle tail lifts This invention relates to a vehicle tail lift of the kind comprising a runner assembly to be mounted for movement up and down relative to the vehicle, with a load carrying platform pivotally mounted on the runner assembly and rotatable between a substantially vertical, stowed position and a substantially horizontal, load carrying position.

It is known to use a torsion spring in a vehicle tail lift of this kind to oppose the effect ofthe weight of the platform itself in an attemptto make iteasierto move the platform between its end positions.

However, this known arrangement is a compromise because the effect ofthe weight of the platform is a torque which varies as the sine of the angle of its inclination, whereas the opposing effect from the torsion spring is a counter-torque which varies simply as the angle of inclination ofthe platform.

Also, this known arrangement lacks any damping device to control the speed of movement of the platform.

According to the present invention there is provided a balance mechanism for a vehicle tail liftof the kind comprising a runner assembly to be mounted for movement up and down relative to the vehicle, with a load carrying platform pivotally mounted on the runner assembly and rotatable between a substantially vertical, stowed position and a substantially horizontal, load carrying position, the balance mechanism comprising spring means arranged for biasing the platform towards its stowed position, means connecting the spring means between the platform and runner assembly such that the force from the spring means is caused to provide on the platform a balancing moment which varies as or at least substantially as the sine of the angle of inclination ofthe platform.The balance mechanism preferably includes a damper connected between the platform and runner assembly.

In the preferred embodiment, the means connecting the spring means between the platform and runner assembly includes a strut which is pivotally connected at one end to the spring means and pivotally connected at the other end to the platform at a position spaced from the pivotal mount ofthe platform on the runner assembly. The balance mechanism in this preferred embodiment is abie to provide a counter-torque which substantially matches the platform torque over the full range of movement of the platform, and the damper is preferably included to control the speed of movement of the platform.

By way ofexample, an embodimentofthe invention will now be described with reference to the accompanying drawings, in which: Figure 1 is view showing details of a balance mechanism for a vehicle tail lift according to the invention.

There is seen in Figure 1 a runner 10 of a vehicletail lift. The tail lift is ofthe known kind having a pairof such runners 10 which are movable up and down relative to the vehicle on a pair of upright columns (not shown). A load carrying platform 11 is pivotally mounted to the runners 10 by means of a hinge 12 provided in plates 31 attached to the bottom of the runners. The runner 10 shown in Figure 1 houses a preferred form of balance mechanism, generally indicated by 13. One end of a strut 14 is pivotally connected to the lower end of the balance mechanism 13 buy a pivotjoint 15 and the other end of the strut is pivotally connected to the platform 11 by a hinge 32 which is spaced from the platform hinge 12. The hinge 32 is provided in plates 33 attached to the platform 11.The platform 11 is rotatable about the hinge 12 between a substantially vertical, stowed position (shown in full lines in Figure 1) and a substantially horizontal, load carrying position (shown in dotted lines in Figure 1). Rotational movement of the platform 11 causes, via the strut 14, a change in length ofthe balance mechanism 13, as to be described.

The pivot joint 15 by which the strut 14 is pivotally connected to the balance mechanism 13 is provided on a plunger 16. The plunger 16 is fitted with awear ring 17,which in this case is rectangular, to be slidably movable within the runner 10, which in this case is of rectangular hollow section. The plunger 16 is fitted to the lower end of a plunger rod 18 which passes through a collar 19 and is connected at its upper end to the lower end of a damper20.A compression spring 21, in this case a series of disc springs, is interposed between the plunger 16 and the collar 19. The collar 19 is attached to the lower end ofatube 22.Atthe upper end ofthetube 22 there is attached anothercollar23,which hasatapped hole 24therethrough.A pair offlat plates 30 are attached to the collar 23 to prevent its rotation in the runner 10. Athreaded shank25 is engaged in the tapped hole 24 in the collar 23. The lower end ofthe threaded shank 25 is connectd with the upper end of the damper 20. The upper end of the threaded shank 25 is attached to an extension tube 26 which abuts an end cap 27. The end cap 27 is fastened over the upper end ofthe runner 10to hold the balance mechanism 13 captive within the runner. The upper end ofthe extension tube 26 is fitted with a boss 28 with a hexagonal form 29 which protrudes through a clearance hole in the end cap 27.This arrangement allows adjustment of the preload on the compression spring 21 because, by rotating boss 28 by its hexagonal form 29, the extension tube 26 with the threaded shank 25 also rotates which, because the tube 22 is prevented from rotating, causes longitudinal movement ofthe collar 23 and tube 22, thus lengthening or shortening the compression spring 21.The preload is adjusted to suit the weight ofthe platform 11.

It will be seen that as the platform 11 is rotated clockwise from its stowed position, shown in full lines in Figure 1 towards its load carrying position, shown in dotted lines in Figure 1,the strut 14is caused to force the plunger rod 18 upwards. As the plunger 1 6travels towards the collar 19 at the bottom of tube 22, the compression spring 21 is compressed and the damper is shortened by the same amount, since the spring and damper are effectively working in parallel. ltwill be understoodthatasthe platform 11 rotates clockwise, the effective torque from the weight of the platform increases upto a maximum when the platform is substantially horizontal. The rate of increase in platform torque varies as the sine of the angle of inclination of the platform.The torque applied to the platform by the compression spring 21 fora given platform angle is governed by spring stiffness.

ii. Spring compression.

iii. Offset of spring from platform pivot 12.

iv. Position of the hinge pin 32 with respectto pivot 12.

It can be shown that by using appropriate values for the above variables the torque angle relationship can be tuned to give the desired characteristic i.e. a lightforce applied to the end of the platform to move the platform from the vertical to the horizontal position and also the reverse.

When the spring system is correctly adjusted the platform can be easily opened orclosed.

The damper has been included to control the rate of change in length ofthe balance mechanism 13, i.e.

to control the speed of opening and closing the platform. It enhances the safety ofthe tail lift in that should the springs fail, it will prevent the platform falling rapidly from the vertical position and will reduce noise and wear on the pivots etc. due to sudden deceleration.

It will be appreciated that many different arrangements of the balance mechanism can be devised which retain the advantages ofthat described above. For example, the compression spring may be in some other suitable form, e.g.

helical spring, or the balance mechanism could have a tensile spring and be arranged to operate in a pulling mode, rather than a compression mode. A balance mechanism could be provided in each runner ofthetail lift or the spring mechanism and damper could be incorporated independently of each other, e.g. by housing each separately in a different runner.

Claims

1. A balance mechanism for a vehicle tail lift of the kind comprising a runner assembly to be mounted for movement up and down relative to the vehicle, with a load carrying platform pivotally mounted on the runner assembly and rotatable between a substantially vertical, stowed position and a substantially horizontal, load carrying position, the balance mechanism comprising spring means arranged for biasing the platform towards its stowed position, means connecting the spring means between the platform and runner assembly such thatthe force from the spring means is caused to provide on the platform a balancing moment which varies as or at least substantially as the sine of the angle of inclination ofthe platform.

2. A balance mechanism as claimed in claim 1 and including a damper connected between the platform and runner assembly.

3. A balance mechanism as claimed in claim 1 or claim 2 wherein the means connecting the spring means between the platform and runner assembly includes a strut which is pivotally connected at one end to the spring means and pivotally connected at the other end to the platform at a position spaced from the pivotal mount ofthe platform on the runner assembly.

4. A balance mechanism as claimed in claim 1, claim 2 or claim 3 and further comprising means for adjusting the preload of the spring means.

5. A balance mechanism as claimed in any preceding claim wherein the spring means is housed within a runner of the tail lift.

6. A balance mechanism as claimed in any one of claims 2 to 5 wherein the damper is housed within a runnerofthetail lift.

7. A balance mechanism as claimed in Claim 6 wherein the spring means and damper are incorporated as a unit, with the spring means and damper acting in parallel, and the unit is housed within a runner ofthe tail lift.

8. A balance mechanism as claimed in any preceding claim wherein the spring means is a compression spring and the balance mechanism operates in compressive mode.

9. A balance mechanism as claimed in anyone of claims 2 to 8 wherein the balance mechanism spring and/or damper components are mounted in the platform and act against the runner.

10. A balance mechanism for a vehicle tail lift substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

11. Avehicletail lift having a balance mechanism as claimed in any preceding claim.