FORM 2
The Patent Act 1970
(39 of 1970)
AND
The Patent Rules, 2005
COMPLETE SPECIFICATION (See Section 10; rule 13)
TITLE OF THE INVENTION A VARIABLE RATIO STEERING SYSTEM FOR A VEHICLE
APPLICANT (S)
1) TATA MOTORS LIMITED
an Indian Company Bombay House, 24 Homi Mody Street, Hutatma Chowk, Mumbai 400 001, Maharashtra, India.
2) TATA MOTORS EUROPEAN TECHNICAL CENTRE Plc
A British Company
18 Grosvenor Place, London, SW1X 7HS,
London, United Kingdom.
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[0001] The present disclosure relates to a variable ratio steering system capable of varying a steering ratio between an input from a steering wheel and an output steering angle of wheels of a vehicle.
BACKGROUND OF THE INVENTION
[0002] Conventional rack and pinion steering mechanism for motor
vehicles is provided either with hydraulic assistance or with an electric motor
for reducing the effort required for turning the steering wheel. A
few conventional steering systems provide variable ratio steering. Such
variable ratio steering systems are typically formed as illustrated in FIG. 1, by using a pinion (1) and a corresponding rack (2) with non-uniform rack teeth (3)In an arrangement shown in FIG. 2(a) and 2(b), a rate of rack (2) travel changes, as pitch P of the rack teeth (3) is progressively increased and as effective gear radius is increased from r to R. Rack tooth profiles change with rack travel to change the effective gear ratio. This “non-linear” capability is used to balance vehicle dynamics, steering feel and steering effort. Although desirable, this approach has limitations. The pinion nearly always operates through several rotations (typically 3 turns of the steering wheel are expected by a driver to steer from full left lock to full right lock), and so, it must be rotationally symmetrical and concentric. This limits the range of the pinion radius change to within the height of the gear teeth, meaning the ratio change is limited to typically < 1:1.2. Also the rack gear teeth vary in both pitch radius and pressure angle, which is expensive to manufacture with repeated accuracy.
[0003] The power assistance required for a steering system is normally minimal when the vehicle is traveling near straight or at cruising speeds but becomes more necessary when maneuvering at low speeds. Suspension geometry is also arranged so as to promote self-return to center.

[0004] The present subject matter is aimed to address the above discussed problems by increasing the steering ratio as lock is applied so as to counter the increase in steering effort required with increasing lock, which is inherent in steering systems.
OBJECT OF THE INVENTION
[0005] The principal object of the embodiments herein is to provide a variable ratio steering system, where the steering ratio increases with steering angle, thus reducing steering effort while maneuvering at lower speeds. This reduces or eliminates the requirement for steering assistance. In addition, the steering ratio may be reduced near center to improve steering responsiveness at higher speeds. The main embodiment for this variable ratio steering system comprises a pinion, a rack, and a ratio modifying gear meshing a uniform gear form with the pinion and meshing a single rotation eccentric or non-circular external gear form with a corresponding rack to modify the steering ratio of the vehicle as lock angle is applied in either direction.
[0006] Another object of the embodiment illustrated herein is to use multiple planetary gears between the pinion and the ratio modifying gear to provide multiple load paths reducing stresses in the pinion teeth, thereby reducing the overall size of the steering system. This embodiment also reduces free play and remove the need for the ring gear to have a spindle.
[0007] Another object of the embodiment herein is to use a spur gear stage with the pinion to enable greater packaging freedom.
SUMMARY OF THE INVENTION
[0008] In one aspect, the object is satisfied by providing a variable ratio steering system for a vehicle. The variable ratio steering system comprises a

pinion, functionally coupled to a steering wheel of the vehicle, and meshing with a ratio modifying gear, which in turn meshes through an eccentric or non-circular gear with a corresponding wave shape rack producing a variable ratio steering system for the vehicle.
[0009] In an embodiment, the pinion (1) rotates three turns to produce a single rotation of the variable ratio ring gear (2), to produce the non-uniform rack (3) travel from lock to lock.
[0010] In an embodiment, the ratio modifying gear (2) is a ring comprising an internal gear race meshing with the pinion (1) and an external gear race meshing with the non-uniform rack (3).
[0011] In an embodiment, the pinion (1) meshes with the ratio modifying gear (2) via a plurality of planetary gears (4).
[0012] In an embodiment, the pinion (1) meshes with the ratio modifying gear (2) via a spur gear stage (5).
[0013] In an embodiment, engagements of the ratio modifying gear (2) with the non-uniform rack (3) and the pinion (1), respectively, are aligned in a single axis such that a single pre-load device acts along the single axis to reduce free-play in the engagements, simultaneously.
[0014] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of

the embodiments herein without departing from the scope thereof, and the embodiments herein include all such modifications.
BRIEF DESCRIPTION OF FIGURES
[0015] This system is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
[0016] FIG. 1 illustrates a variable gear ratio steering system, according to prior art;
[0017] FIGS. 2(a)-2(b) illustrate a sectional view of central pinion and rack teeth, both at center and off center, according to prior art;
[0018] FIG. 3 illustrates a variable gear ratio steering system, according to an embodiment as disclosed herein;
[0019] FIG. 4 illustrates use of multiple planetary gears, according to an embodiment as disclosed herein;
[0020] FIG. 5 illustrates a ratio modifying gear where the internal gear race is replaced with an external spur gear stage, according to an embodiment as disclosed herein;
[0021] FIG. 6 is a graph illustrating a lock angle versus steering effort modified with the proposed variable ratio steering system, according to an embodiment as disclosed herein; and

DETAILED DESCRIPTION OF THE INVENTION
[0022] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The term “or” as used herein, refers to a non-exclusive or, unless otherwise indicated. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
[0023] The accompanying drawings are used to help easily understand various technical features and it should be understood that the embodiments presented herein are not limited by the accompanying drawings. As such, the present disclosure should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawings. Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another.
[0024] Referring now to the drawings, and more particularly to FIGS. 3 through 6, there are shown preferred embodiments.

[0025] FIG. 3 illustrates a variable ratio steering system, according to an
embodiment as disclosed herein. The variable gear ratio steering system (100) of a vehicle comprises a pinion (1) meshing uniformly with a ratio modifying gear (2), which in turn, meshes non-uniformly through an eccentric or non-circular gear with a corresponding wave shape rack (3). The pinion (1) is functionally coupled to the steering wheel of the vehicle, and is designed to mesh in a uniform way with the ratio modifying gear (2) with a fixed gear reduction of approximately 4:1 to translate multiple rotations of the steering wheel into a single (or partial) turn of the ratio modifying gear (2). The ratio modifying gear (2) has an external eccentric or non-circular rear form which engages with the corresponding wave shaped rack (3), in order to produce a non-uniform translation ratio, throughout the single (or partial) rotation of the ratio modifying gear, to produce the variable ratio steering for the vehicle. Because the ratio modifying gear (2) operates over a single (or partial) rotation it allowing aggressive non-uniform gear profiles to generate much higher ratio changes.
Unlike conventional steering system, the proposed variable gear ratio steering system (100) uses a wide range variable ratio steering system to reduce or eliminate the requirement for assistance. This would save fuel in a conventional powertrain vehicle but would also be particularly attractive for electric cars where the reduced power consumption and weight would increase range. In brief, the present invention discloses a variable ratio steering system (100) for a vehicle, which comprises of: a pinion (1) functionally coupled to a steering wheel; and a ratio modifying gear (2) meshing in a uniform gear form with the pinion (1) and meshing in an eccentric or non-circular external gear form with a corresponding non-uniform rack (3), wherein a rate of travel of the non-uniform rack (3) with rotation of pinion (1) varies with travel of the non-uniform rack (3).
[0026] The core of the proposed variable ratio steering system (100) is to achieve a more significant ratio change in order to reduce or eliminate steering

assistance. The solution allows for conventional ratios while driving predominantly straight (which covers the majority of most vehicles usage) but then increases steering ratio with increasing steering angle (typically when maneuvering at lower speeds) to counteract the inherent increase in steering effort, thus reducing the need for assistance. Alternatively, slightly lower ratios may be used when driving predominantly straight to improve responsiveness at higher speeds with more modest increases in ratio at higher steering angles.
[0027] The solution uses a mechanical gear system to increase steering ratio (steering wheel angle / hub steer angle) in order to reduce steering effort, as lock is applied in either direction. In its simplest arrangement, a steering system is modified with the addition of a ratio modifying gear (2) operating between the pinion (1) and a wave shape rack (3) as illustrated in the FIG. 3. In an embodiment, the ratio modifying gear (2) rotates about a central and essentially fixed axis. A gear ratio between the pinion (1) and the ratio modifying gear (2) is constant and is set to provide the desired total number of turns of the steering wheel while providing only a single (or partial) rotation of the ratio modifying gear (2). In this embodiment, the ratio modifying gear (2) comprises an internal gear race and an external gear race. The internal gear race meshes with the pinion (1) and the external meshes with the wave shape rack (3). The external gear race comprises an eccentric or non-circular gear profile. The solution is simple, requiring only one additional element and can use conventional gear tooth engagement loading to reduce backlash.
[0028] The pinion (1) is a circular gear rotating about a fixed axis. It receives input directly from a steering wheel via a steering column (not shown for simplicity) or, alternatively, the steering actuator of a steer by wire system. The pinion (1) does not engage directly with the rack (3), instead it engages with the internal gear race of the ratio modifying gear (2). The ratio modifying gear (2) has the external gear race, which engages with the wave shape rack (3). Numerous ratio profiles are possible.

[0029] Additionally, it is important to minimise free play in the steering system (100). This is normally achieved by managing the engagement between the rack (3) and the pinion (1) with a pre-load acting along the axis of engagement. In this invention, in any of the 3 embodiments covered herein, it is efficient to align both the engagements between the ratio modifying gear (2) with the pinion (1) and the wave shape rack (3) along a single axis so that a single pre-load device can manage clearance for both engagements. FIG. 4 illustrates use of multiple planetary gears, according to an embodiment as disclosed herein. In an embodiment, the pinion (1) meshes with the ratio modifying gear (2) via a plurality of planetary gears (4) Here, multiple planetary gears (4) (for example, more than 3) provide multiple load paths through the first gear stage, reducing the stresses in the pinion teeth, thereby reducing the overall size of the steering system. This also reduces free play and can remove the need for the ratio modifying gear to have a spindle.
[0030] FIG. 5 illustrates an embodiment where the internal gear race of the ratio modifying gear is replaced with an external spur gear stage as disclosed herein. This may add complexity but provides greater packaging freedom.
[0031] FIG. 6 is a graph illustrating a lock angle versus steering effort modified with the proposed variable ratio steering system, according to an embodiment as disclosed herein. The graph shows how the proposed variable ratio steering system (100) changes the steering effort with the lock angle. A typical example would be a configuration, which varies from 3 turns lock to lock on centre to 5 turns at full lock produces a 40% reduction in steering assistance for an overall total of 4.1 turns lock to lock. This would deliver 1 to 2% increase in range for an electric vehicle on an urban cycle. Importantly, the additional turns of the steering wheel are only experienced during high steer angle maneuvers, typically only during low speed manouvers.

[0032] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the scope of the embodiments as described herein.
List of Referral Numerals:

Description Referral Numerals
variable gear ratio steering system 100
pinion 1
ratio modifying gear 2
wave shape rack 3
planetary gears 4
spur gear stage 5

We Claim:
1. A variable ratio steering system (100) for a vehicle, comprising:
a pinion (1) functionally coupled to a steering wheel; and a ratio modifying gear (2) meshing in a uniform gear form with the pinion (1) and meshing in an eccentric or non-circular external gear form with a corresponding non-uniform rack (3), wherein a rate of travel of the non-uniform rack (3) with rotation of pinion (1) varies with travel of the non-uniform rack (3).
2. The variable ratio steering system (100) as claimed in claim 1, wherein the pinion (1) rotates three turns to produce a single rotation of the variable ratio ring gear (2), to produce the non-uniform rack (3) travel from lock to lock.
3. The variable ratio steering system (100) as claimed in claim 1, wherein the ratio modifying gear (2) is a ring comprising an internal gear race meshing with the pinion (1) and an external gear race meshing with the non-uniform rack (3).
4. The variable ratio steering system (100) as claimed in claim 1, wherein the pinion (1) meshes with the ratio modifying gear (2) via a plurality of planetary gears (4).
5. The variable ratio steering system (100) as claimed in claim 1, wherein the pinion (1) meshes with the ratio modifying gear (2) via a spur gear stage (5).
6. The variable ratio steering system (100) as claimed in claim 1, wherein engagements of the ratio modifying gear (2) with the non-uniform rack (3) and the pinion (1), respectively, are aligned in a single axis such

that a single pre-load device acts along the single axis to reduce free-play in the engagements, simultaneously.