GB2279305A

GB2279305A - Vehicle power steering system

Info

Publication number: GB2279305A
Application number: GB9412425A
Authority: GB
Inventors: Paul Rickers; Andreas Klatt; Rainer Bruns; Peter Dibbern; Bodo V Wartenberg
Original assignee: Jungheinrich AG
Current assignee: Jungheinrich AG
Priority date: 1993-06-30
Filing date: 1994-06-21
Publication date: 1995-01-04
Also published as: ITMI941332A1; IT1271650B; DE4321670C1; FR2708246A1; GB9412425D0; ITMI941332A0

Description

1 - h t POWER STEERING SYSTEKS 2279305 The invention relates generally to

power steering systems for motor vehicles and in particular for lift trucks.

DE-A-3933771 discloses a power steering system or steering effort assist system for motor vehicles, in particular for the parking traffic speed range, including at least one clutch pad which effects a drive connection between a servo motor and a steering gear mechanism side of a steering column when a drive member connected to a steering wheel side of the steering column generates the is necessary clutch engagement pressure by mechanically acting upon the clutch pad as torque is manually applied to a steering wheel. The clutch pad co-operates with co-axial gears which are driven by the servo motor in opposite directions and which drive the steering gear mechanism side portion of the steering column via the clutch pad upon clutch engagement. This is caused by the drive member being laterally mounted to the steering wheel side portion of the steering column, the drive member causing the clutch pad to engage the respective gear when the steering wheel is rotated.

An advantage of the described steering effort assist system is that a sensor for a torque dependent steering effort assist operation is not required. Engagement of the clutch pad with the respective gear depends on the torque at the steering wheel which provides the steering effort assistance in a torque responsive manner.

With lift trucks including a steering wheel excessive steering torques result either in a large number of steering wheel rotations in order to obtain acceptable forces f or the user of the steering wheel, or the user must exert excessive steering f orces. Nevertheless, a servo 2 assist system must be able to take up the relatively big torques acting upon the steering gear mechanism.

The problem to be solved by the invention is thus to provide a power steering system, in particular for lift trucks, which enables, at minimum expenditure, a steering torque responsive steering effort assistance, even if substantial forces are exerted upon the wheel to be steered.

According to the present invention, a power steering system for use with a motor vehicle comprises:

a servo motor acting upon a steering column via a clutch element when torque is manually applied to a steering wheel to rotate a steering wheel side portion of the steering column relatively to a steering gear mechanism side portion of the steering column; and the clutch element being mounted on the steering wheel side portion of the steering column so as to be axially movable relatively thereto, being displaceable via a guide connected to the steering wheel side portion of the steering column either towards or away from the steering wheel depending on the direction of steering wheel rotation, being arranged on the steering gear mechanism side portion of the steering column so as to be both axially movable and non-rotatable relatively thereto, and including a pair of axially spaced, co-axial, annular friction surfaces adapted, when the clutch element is axially displaced, to be selectively engaged with and driven by at least one friction drive surface associated with and driven by the servo motor.

In the power steering system of the present invention, the clutch element driven by the servo motor is non-rotatably mounted on the steering gear mechanism side portion of the steering column. Accordingly, the clutch element is able effectively to exert the necessary servo force upon the steering column irrespective of whether high steering forces are to be developed. Adjustment of the axially movable clutch element is obtained by a suitable is 1 1 1 3 ?I guide provided at the steering wheel side portion of the steering column. The clutch element may thus have either of its two friction surfaces selectively engage the at least one friction drive surface associated with the servo motor, which may be of any conventional construction e.g. electromotive. - The steering column portions may be completely separate. However, it is preferred to connect a torsion spring or the like therebetween so that the friction surfaces are moved into contact with each other only when the steering torque exceeds a predetermined value. As a result the steering effort assistance cannot be operated too easily so that there will be always a feeling of "drive lane contact".

Electronic or hydraulic means are not required in the steering power system of the present invention. The mechanical actuating force is transmitted directly to the steering column. The amount of steering effort assistance depends on the steering force dependent pressure between the friction surfaces.

There are various structural solutions for practizing the teaching of the invention.

In one solution, the clutch element includes a pair of conical concentric friction surfaces facing each other, which friction surfaces are adapted to be engaged with a conical friction member connected to the servo motor.

In an alternative solution, rotatably mounted wheel disks are disposed on opposite sides of the clutch element, which wheel disks are adapted to be driven in opposite directions by the servo motor.

Furthermore, various possibilities for displacing the clutch element may be envisaged. In one possibility, the steering wheel side portion of the steering column includes a thread co-operating with an internal thread of the clutch element.

To provide resilient resistance to axial displacement of the clutch element opposite sides of the clutch element 4 1 may be engaged by respective springs each supported with respect to a respective one of the steering column portions.

Preferably, the servo motor acts upon the steering column via a reduction gear mechanism.

The invention will be explained in more detail with reference to the accompanying drawings wherein:- Fig. 1 shows schematically a f irst embodiment of a power steering system according to the invention; and Fig. 2 shows a cross-section through a second embodiment of a power steering system according to the invention.

In Fig. 1, a steering column 10 comprises a steering wheel side portion 12, to which a steering wheel 14 is connected, and a steering gear mechanism side portion 16 acting via a steering gear mechanism 18 upon a steerable wheel 20 of a lift truck.

A clutch element 22 comprises a first disk portion 24 and a second disk portion 26 which are connected by a sleeve-like web 28. The disk portion 26 is mounted on the steering column portion 16 so as to be both non-rotatable and axially movable relatively thereto, with the disk portion 24 being mounted on the steering column portion 12 so as to be axially movable relatively thereto. A guide 30, not shown in detail, is provided on the steering column portion 12 and acts to displace the clutch element 22 axially either towards or away from the steering wheel 14, when the steering wheel 14 is rotated. The disk portions 24, 26 are provided, on their sides facing each other, with friction surfaces 32, 34 adapted to co-operate with a conical friction member 36 which may be driven by a servo motor 38.

When the clutch element 22 is displaced, upon a rotation of the steering wheel 14, either the friction sur- face 32 or the friction surface 34 is moved into engagement with the conical friction member 36. The servo motor 38 then assists the steering effort with the amount of 1 1 1 steering effort assistance depending on the amount of friction engagement and the force and torque which are applied to the steering wheel 14.

In Fig. 2, the ends of a steering wheel side portion 12a and of a steering gear mechanism side portion 16a of a steering column 10a are shown mounted in a housing 44 by anti-friction bearings 40 and 42 so as to be rotatable and axially fixed.

An output shaft 48- of a servo motor, not shown, is mounted about an axis perpendicular to the steering column 10a by means. of a ball bearing 46. Ball bearings 50 rotatably mount the shaft of a conical gear 52 which is driven by the shaft 48. The conical gear 52 meshes with the conical toothings of a pair of friction disks 54, 56 which are rotatably mounted in the housing 44 in co-axial relationship with respect to the axis of the steering column 10a. Between the friction disks 54, 56, which are driven in opposite directions, there is disposed a clutch disk 60 which has an internal thread engaging a trapezoidal thread 62 of the steering column portion 12a. The clutch disk 60 is engaged with the steering column portion 16a by spline key means 64 so as to be axially movable relatively thereto. opposite sides of the clutch disk 60 are engaged by Belleville springs 66, 68, each of which is supported against the steering column mount and resiliently resists axial displacements of the clutch disk 60.

When the steering column portion 12a is rotated by the steering wheel, the clutch disk 60 is axially displaced either towards the steering wheel or away therefrom.

Accordingly the clutch disk 60 is moved into frictional engagement with a friction layer 70 or 72 of the friction disks 54 or 56 which are permanently driven at this time. As a result the clutch disk 60 is driven so as to drive the steering gear mechanism side portion 16a. The steering effort assistance depends on the pressure urging the clutch disk 60 against the associated friction layer 70 or 72.

h t 6

Claims

1. A power steering system, for use with a motor vehicle, comprising:

a servo motor acting upon a steering column via a clutch element when torque is manually applied to a steering wheel to rotate a steering wheel side portion of the steering column relatively to a steering gear mechanism side portion of the steering column; and the clutch element being mounted on the steering wheel side portion of the steering column so as to be axially movable relatively thereto, being displaceable via a guide connected to the steering wheel side portion of the steering column either towards or away from the steering is wheel depending on the direction of steering wheel rotation, being arranged on the steering gear mechanism side portion of the steering column so as to be both axially movable and non-rotatable relatively thereto, and including a pair of axially spaced, co-axial, annular friction surfaces adapted, when the clutch element is axially displaced, to be selectively engaged with and driven by at least one friction drive surface associated with and driven by the servo motor.

2. A power steering system according to claim 1, in which 25 the steering column portions are separate.

3. A power steering system according to claim 1, in which a torsion spring is disposed between the steering column portions.

4. A power steering system according to any one of claims 1 to 3, in which the clutch element includes a pair of conical concentric friction surfaces facing each other, which friction surfaces are adapted to be engaged with a conical friction member connected to the servo motor.

5. A power steering system according to any one of claims 35 1 to 3, in which a pair of rotatably mounted wheel disks are disposed on opposite sides of the clutch element, which 1 7 wheel disks are adapted to be driven in opposite directions f by the servo motor.

6. A power steering system according to any preceding claim, in which the steering wheel side portion of the steering column includes a thread co-operating with an internal thread of the clutch element.

7. A power steering system according to any preceding claim, in which opposite sides of the clutch element are engaged by respective springs each supported with respect to a respective one of the steering column portions.

8. A power steering system according to any preceding claim, in which the servo motor acts upon the steering column via a reduction gear mechanism.

9. A power steering system substantially as hereinbefore is described with reference to Figure 1 or Figure 2 of the accompanying drawings.