WO2008120231A2

WO2008120231A2 - Device for steering returnability and centering for a vehicle and method of its assembly

Info

Publication number: WO2008120231A2
Application number: PCT/IN2008/000184
Authority: WO
Inventors: Soheb Ahmed; Abhay Deshpande
Original assignee: Tata Motors Limited
Priority date: 2007-03-29
Filing date: 2008-03-25
Publication date: 2008-10-09
Also published as: WO2008120231A3

Abstract

This invention relates to a device for steering returnability and centering for a vehicle comprising at least two spiral springs (9, 10), an arbor (13), case assembly (11 ) and mounting means. Wherein said spiral springs (9, 10) are provided on said arbor (13) coaxially and the unit of said springs (9, 10) and arbor (13) is assembled inside the said casing assembly (11 ). Said arbor (13) is adopted to be rotatably engaged with steering column (7) / wheel (3) and said casing assembly (11 ) is mounted on mounting means. When steering wheel (3) is turned in either direction the springs (9, 10) provide return torque so that the wheel comes back to neutral position. The invention thus provides steering returnability in response to large steering wheel (3) inputs and steering centering in response to spurious steering inputs, thus eliminating the need of providing positive caster in the suspension. The invention is compact and has least number of parts. Apart from that the device is easy to assemble and manufacture.

Description

A DEVICE FOR STEEREVG RETURNABILITY AND CENTERING OF VEHICLE

FIELD OF INVENTION

This invention relates to a device for steering retumability and centering of vehicle and more particularly relates to a mechanical device for steering retumability and centering of vehicle.

BACKGROUND OF INVENTION

Primary function of the steering systems in automobiles is to provide driver control on the vehicle. These systems may be conventional mechanical systems or steer-by- wire systems. In addition to the primary requirement of driver control, these systems have provisions for steering retumability and centering.

Steering retumability is desired because after executing a turn, the vehicle should move in straight path or the steering wheel should come to neutral position for straight driving. In other words, when driver releases the steering wheel after turning the steering wheel to change the direction of the vehicle, the steering wheel should automatically return to the center position.

Steering centering or center feel is also essential, because wide variety of extraneous forces act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise precise and careful driving control. The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in straight line unless the driver intentionally turns the vehicle. The ideal steering system should therefore require little attention from the driver as the vehicle progresses along a straight line path down the roadway.

Conventionally, automobile steering systems have steering wheel returnability by slanting the king pins of the steer wheels. Such positive caster is provided by positive caster offset, which is also known as mechanical trail. Caster offset is the distance from the ground intersection point of a pivot line drawn through the pivot axis of a steerable wheel to a contact point at the center of the area over which the wheel contacts the ground. The pivot axis of a steerable wheel of a motor vehicle is usually provided by a "king pin". Because the contact point of a steerable wheel with positive caster trails the pivot line point of the wheel, side forces cause the wheel to turn in the direction that the force is being applied.

But this provision has its disadvantages like effects of cross winds are more pronounced with large amounts of positive caster offset. The vehicle tends to caster towards the side of the roadway to which it is being pushed by the wind. Thus castering has destabilizing effect when driving in a crosswind. Similarly, a high crown at the center of the roadway or a slanted roadway tends to cause vehicles with castered steer wheels to turn toward the edge of the roadway, that is, in the downhill direction. Castered steer wheels also allow steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander. Keeping a vehicle tracking straight and under control currently requires an inordinate amount of driver steering corrections to counteract the adverse side effects of castered steer wheels. The repetitive task of making numerous precise steering corrections mile after mile weighs heavily on a driver's physical and mental well-being, and may result in extreme driving fatigue. In addition, due to increased turning-lift effects, generous positive king pin angles provide significant resistance to small radius turns, which can make driving quite fatiguing. Moreover, positive caster generates camber gain in a corner which may also cause the tires to heat up, lose grip, and wear out prematurely.

These adverse effects are some of the negative aspects of attempting to achieve steering system stability through castering. Hence, there is critical need to provide directional stability by replacing castered wheels with another method of achieving steer wheel returnability. In other words there is need of providing directional stability of steer wheels with no positive caster, i.e., a caster angle of zero degree. The replacement of steerable wheel castering with the mechanical devices for steering returnability and centering have a number of additional advantages, these include precision on-center directional stability of the steerable wheels that make the vehicle more comfortable to drive, improvement in the tire wear pattern of the steerable wheels that increases tire life and a stable non- swaying ride along with enhancing overall safety of vehicle.

Instead of relying on positive castering, steering wheel returnability after a turning movement is provided by steering centering/returning assemblies. Some of the relevant ones are discussed as under.

US 5050443 describes a spring operated return assist device which permits a limited degree of free rotation of the steering shaft before activation of the mechanism. The return assist device urges the shaft and the steering wheel to a near central neutral position after rotation of the steering wheel. The invention suffers from the shortcoming that it will have poor center feel due to presence of aperture. Apart from that the system is backlash prone. In addition the system has great many parts and the assembly is complicated and costly to manufacture. U.S. 6694839 describes a springs designed to apply a return torque on the rotating ring to return it to the neutral point and means for limiting in two directions the rotation of the rotating ring. The device comprises a plate mounted rotatable relative to the fixed element and two helical springs, a first spring being arranged on one side of the plate and fixed thereon by its internal end, a second spring being arranged on the other side of the plate and fixed by its internal end on the fixed element, the external end of the first spring being linked, in a first direction, to the rotation of the rotating ring and the external end of the second spring being linked, in a second direction opposite to the first, to the rotation of the plate. The invention suffers from the shortcoming that there are no provisions to address the issue of permanent set of springs, resulting in backlash over the period of use resulting in poor center feel.

U.S. 6705419 describes a steering shaft and a center feel mechanism. The center feel mechanism comprises a cam face, an urging member and a second geared portion. The cam face and the urging member are configured to generate a retiming torque to the center feel mechanism. The returning torque has a direction opposite the second angular displacement. Similarly, U.S. 6820703 also describes a mechanical steering feel enhancement device which provides a return-to-center function. The invention suffers from the drawback that the system has great many parts and the assembly is not compact as well as complicated and costly to manufacture. Secondly the cam and roller assembly in conjunction with gears will result in friction related issues.

A study of the prior art related to steering returnability and steering centering devices reveals a long felt need for a system that,

1. Comprises of less no. of parts

2. Is coaxial with the steering wheel / steering column and compact 3. Is easy to manufacture and assemble

4. Eliminates friction arising due to mechanical interaction between two or more parts

5. Self compensates any loss in spring characteristics or permanent set

OBJECTS OF INVENTION

The main object of this invention is to provide a device for steering returnability of vehicle which eliminates the requirement of positive caster in the suspension. Another object of the invention is to provide steering centering (improved center feel) in response to small/spurious steering inputs.

Another object of the invention is to provide steering returnability and steering centering device with least number of parts.

Yet another object of the invention is to provide steering returnability and steering centering device which is compact, easy to assemble and manufacture and cost effective.

Yet another object of the invention is to provide steering returnability and steering centering device with least system level friction.

Yet another object of the invention is to provide steering returnability and steering centering device which self compensates any loss in spring characteristics or permanent set.

Yet another object of the invention is to provide steering returnability and steering centering device which substantially matches the return torque requirements vis-a-vis steering wheel angle. BRIEF DESCRIPTION OF INVENTION

A Device for steering returnability and centering of vehicle in accordance with this invention basically comprises of at least two spiral springs, an arbor, case assembly and mounting means. Wherein said spiral springs are provided on said arbor coaxially and the unit of said springs and arbor is assembled inside the said casing assembly. Said arbor is adopted to be rotatably engaged with steering column/wheel and said casing assembly is mounted on mounting means.

When steering wheel is turned in either direction the springs provide return torque so that the wheel comes back to neutral position. The invention thus provides steering returnability in response to large steering wheel inputs and steering centering in response to spurious steering inputs, thus eliminating the need of providing positive caster in the suspension. The invention is compact and has least number of parts. Apart from that the device is easy to assemble and manufacture. Moreover, the design of the system is such that it self compensates any loss in spring characteristics or permanent set.

BRIEF DESCRIPTION OF DRAWINGS

Fig.1 shows a device for steering returnability and centering of vehicle mounted on steering system.

Fig.2 shows a device for steering returnability and centering of vehicle with steering wheel assembly.

Fig. 3 shows the exploded view of a device for steering returnability and centering of vehicle.

Fig. 4 shows the isometric view of steering returnability and centering device.

Fig. 5 shows isometric and plan view of spiral spring

Fig. 6 shows constructional details of arbor Fig. 7 shows constructional details of casing assembly

DETAILED DESCRIPTION OF INVENTION

Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting the same,

Referring to Fig. 1 and Fig. 2 the device for steering returnability and centering of vehicle (2) is mounted on plate (8) using bolts (14). The plate (8) is welded to column tube (4) which is mounted on steering^' system cross member (6) using mounting bracket 5.

Referring to Fig. 3 and Fig. 4 it can be noticed that invention comprises of unitary arbor (13), unitary casing (11) and a set of similar spiral springs (9) and (10). The unitary casing (11) is mounted on plate (8) using assembly bolts (14) and mounting legs 12). The plate (8) is welded to steering column tube (4). The steering column (7) consists of multiple keys (15) on its outer periphery. The spiral spring (9) is assembled in the unitary casing (11) and unitary arbor (13) such that the outer ends of the spiral spring (9) are seated in the rectangular slots (18) and (20). Similarly, the spiral spring (10) is assembled in the unitary casing (11) and unitary arbor (13) such that the outer ends of the spiral spring (10) are seated in the rectangular slots (17) and (19). The unitary arbor (13) is rotatably engaged with steering column (7) by seating steering column keys (15) in key slots (16) of unitary arbor. The unitary casing (11) is bolted to the plate (8) using bolts (14).

Fig.5 shows isometric and plan view of spiral spring (9) and (10) Referring to Fig.6, unitary arbor (13) has multiple key slots (16) on its inner periphery and two rectangular cut slots (17) and (18) on its outer periphery. Referring to FIG 7, unitary casing (11) comprises of two rectangular cut slots

(19) and (20) on its inner periphery, and bolting legs (12) welded on outer periphery.

Assembly procedure for the steering returnability and centering device

Referring to Fig. 3, spiral spring (9) is first assembled in free condition such that outer ends of spiral spring (9) are seated in the rectangular slots (18) of arbor and

(20) of unitary casing a. This yields an assembly of spiral spring (9), unitary casing (11) and unitary arbor (13). Now the unitary casing (11) is held in a fixture/vice and unitary arbor (13) is rotated such that spiral spring (9) is wind to solid state. After this the spiral spring (10) is assembled in free condition such that outer ends of spiral spring (10) are seated in the rectangular slots (17) and (19). When the unitary casing (11) is released from fixture, the spiral spring (9) unwinds half the no of turns to solid and spiral spring (10) winds for half the number of turns to solid. At equilibrium state both spiral springs (9) and (10) exert equal torques and achieve steady state.

The invention comprises of unitary arbor (13), unitary casing (11) and a set of similar spiral springs (9) and (10). The unitary casing (11) is rigidly fixed onto the steering mounting frame (6) via plate (8). The unitary arbor (13) is rotatably engaged with steering column (7), Hence in the unitary arbor (13) rotates in response to steering wheel movement. When steering wheel (3) is turned in clockwise direction by vehicle driver as viewed from driver side unitary arbor (13) rotates such that spiral spring (10) winds and spiral spring (9) unwinds. This creates torque imbalance in the steering returnability and centering device (2). The device (2) exerts reaction torque onto the steering wheel (3). When the driver releases steering wheel (3) the reaction torque exerted by steering returnability and centering device (2) returns the steering wheel (3) to its central/neutral position. When steering wheel (3) is turned in anti-clockwise direction by vehicle driver as viewed from driver side unitary arbor (13) rotates such that spiral spring (9) winds and spiral spring (10) unwinds. This creates torque imbalance in the steering returnability and centering device (2). The device (2) exerts reaction torque onto the steering wheel (3). When the driver releases steering wheel (3) the reaction torque exerted by steering returnability and centering device (2) returns the steering wheel (3) to its central/neutral position. Thus the steering returnability and centering device (2) not only provides steering returnability in response to large steering wheel inputs but also ensures steering centering (improved center feel) in response to small/spurious steering inputs. In case either of the spiral springs (9) or (10) has permanent set or load loss after usage over a period of time the torque balance point shifts which can be corrected by resetting toe characteristics of the vehicle by method known in the art. It is evident in the description that this system has no friction losses.

The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

Claims

1. A device for steering returnability and centering of vehicle comprises of at least two spiral springs, an arbor, casing assembly and mounting means wherein said mounting means is adopted to be fitted on steering column tube, said casing assembly is rigidly mounted on said mounting means, said at least two spiral springs are provided between said arbor and casing assembly coaxially.

2. A device for steering returnability and centering of vehicle as claimed in claim 1, wherein said arbor is having provisions at its inner surface to rotatably engage with said steering column.

3. A device for steering returnability and centering of vehicle as claimed in claims land 2 wherein said provisions of arbor are one or more slots.

4. A device for steering returnability and centering of vehicle as claimed in claims lto 3 wherein said arbor is unitary arbor.

5. A device for steering returnability and centering of vehicle as claimed in claims lto 4 wherein said casing is unitary casing.

6. A device for steering returnability and centering of vehicle as claimed in claims lto 5 wherein said casing is provided with at least two slots on its inner surface to engage said springs.

7. A device for steering returnability and centering of vehicle as claimed in claims lto 6 wherein said arbor is provided with at least two slots on its outer surface to engage said springs.

8. Method of manufacturing device for steering returnability and centering of vehicle as claimed in claims 1 to 7 comprises of

1. Assembling one spring in free condition between arbor and casing assembly;

2. Winding said spring ;

3. Assembling second spring in free condition between arbor and casing assembly on opposite side of first spring;

4. Mounting the assembled unit of said step (c) rigidly on mounting means provided on steering column such that arbor engages rotatably with steering column.

9. Method of manufacturing device for steering returnability and centering of vehicle as claimed in claim 8 wherein said second spring is assembled only when said first spring is held in wound state.

10. A device for steering returnability and centering of vehicle and method of manufacturing thereof substantially as herein described with reference to accompanying drawings.