Power-assisted steering assembly

Abstract

A power-assisted steering assembly for a vehicle comprises: a motor shaft for transmitting torque from a motor; a steering rack; a primary transmission mechanism; a secondary transmission mechanism; a motor coupling; and a rack coupling. The primary transmission mechanism is configured to transmit power from the motor shaft to the steering rack and includes a drive belt. The secondary transmission mechanism is configured to transmit power from the motor shaft to the steering rack upon a failure of the primary transmission mechanism. The motor coupling is configured to transmit power from the motor shaft to the primary transmission mechanism and the secondary transmission mechanism. The rack coupling is configured to transmit power from the primary transmission mechanism and the secondary transmission mechanism to the steering rack. Prior to the failure of the primary transmission mechanism, the motor coupling and rack coupling are configured to transmit a greater amount of power to the primary transmission mechanism than to the secondary transmission mechanism.

Claims

1. A power-assisted steering assembly for a vehicle, the power-assisted steering assembly comprising: a motor shaft for transmitting torque from a motor; a steering rack; a primary transmission mechanism configured to transmit power from the motor shaft to the steering rack, the primary transmission mechanism including a drive belt; a secondary transmission mechanism configured to transmit power from the motor shaft to the steering rack upon a failure of the primary transmission mechanism; a motor coupling configured to transmit power from the motor shaft to the primary transmission mechanism and the secondary transmission mechanism; and a rack coupling configured to transmit power from the primary transmission mechanism and the secondary transmission mechanism to the steering rack; wherein, prior to the failure of the primary transmission mechanism, the motor coupling and rack coupling are configured to transmit a greater amount of power to the primary transmission mechanism than to the secondary transmission mechanism.

2. The power-assisted steering assembly according to claim 1, wherein one of the motor coupling and rack coupling comprises a first couple part in communication with the primary transmission mechanism, a second couple part in communication with the secondary transmission mechanism, and a lost-motion mechanism connecting the first couple part to the second couple part, the lost-motion mechanism allowing relative rotation of the first couple part and second couple part by an amount limited by the function of the lost-motion mechanism.

3. The power-assisted steering assembly according to claim 2, wherein the lost-motion mechanism comprises a tooth and a detent engageable by the tooth, movement of the first couple part causing relative movement of the tooth and detent as the motor shaft rotates.

4. The power-assisted steering assembly according to claim 3, wherein the lost-motion mechanism further comprises a compressible element that is compressed by movement of the tooth relative to the detent.

5. The power-assisted steering assembly according to claim 2, wherein the lost-motion mechanism includes a spring-pin coupling.

6. The power-assisted steering assembly according to claim 2, wherein the other of the motor coupling and rack coupling fixedly couples the primary transmission mechanism and the secondary transmission mechanism.

7. The power-assisted steering assembly according to claim 6, wherein the motor coupling includes the first couple part and second couple part and the rack coupling fixedly couples the primary transmission mechanism and the secondary transmission mechanism.

8. The power-assisted steering assembly according to claim 6, wherein the rack coupling includes the first couple part and second couple part and the motor coupling fixedly couples the primary transmission mechanism and the second transmission mechanism.

9. The power-assisted steering assembly according to claim 1, further comprising a housing configured to separate the primary transmission mechanism from the secondary transmission mechanism over at least a part of a path of the drive belt.

10. The power-assisted steering assembly according to claim 1, wherein the secondary transmission mechanism comprises an additional drive belt.

11. The power-assisted steering assembly according to claim 1, wherein the secondary transmission mechanism comprises an idler gear.

12. The power-assisted steering assembly according to claim 11, wherein the idler gear is sized such that it remains in mesh over a full range of adjustment of the primary transmission mechanism.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a vehicle including a power-assisted steering assembly in accordance with the present invention;

(2) FIG. 2 is a detail view of the power-assisted steering assembly of FIG. 1;

(3) FIG. 3 is a plan and cross-sectional view of the motor coupling of the power-assisted steering assembly of FIG. 1;

(4) FIG. 4 is a detail view of a second embodiment of a power-assisted steering assembly.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring firstly to FIG. 1, there is shown a schematic view of a vehicle 100. The vehicle 100 includes a steering mechanism 102 in communication with a steering rack 104 and through this to the wheels 106 of the vehicle 100, of which only two are shown. The wheels 106 to which the steering rack 104 and steering mechanism 102 are connected may therefore be steered by a driver of the vehicle 100. The steering mechanism 102 communicates with a power-assisted steering assembly 108 that provides power-assistance to assist in the steering of the wheels 106. The power-assistance is provided directly to the steering rack 104.

(6) The power-assisted steering assembly 108 of the depicted embodiment includes an electric motor 110 that rotates a motor shaft 112. The motor shaft 112 transfers power to a motor coupling 114, through a primary transmission mechanism 116 including a drive belt 118, to a rack coupling 120 disposed on the steering rack 104. A secondary transmission mechanism 122 that includes an additional drive belt 124 is also located between the motor coupling 114 and the rack coupling 120.

(7) An enlarged view of the motor coupling 114, rack coupling 120, primary transmission mechanism 116 and secondary transmission mechanism 122 is shown in FIG. 2. The rack coupling 120 includes a ball-screw mechanism 126 which engages with the steering rack 104, rotation of the rack coupling 120 therefore causing lateral motion of the steering rack 104.

(8) Both the drive belt 118 of the primary transmission mechanism 116 and the drive belt 124 of the secondary transmission mechanism 122 are toothed belts and engage with pulley teeth on both the motor coupling 114 and rack coupling 120. Thus, the drive belts 118, 124 have no slippage and transfer power efficiently. However, if desired it will be known that drive belts operating through friction alone may instead be used for either or both of the drive belts 118, 124.

(9) The motor coupling 114 includes a first couple part 128 and a second couple part 130. As can be seen in FIG. 2, the first couple part 128 engages with the drive belt 118 of the primary transmission mechanism 116 and the second couple part 130 engages with the drive belt 124 of the secondary transmission mechanism 122. Both the motor coupling 114 and rack coupling 120 include flanges 132 that separate the drive belts 118, 124 from one another and also act to retain the drive belts 118, 124 on the motor coupling 114 and rack coupling 120.

(10) FIG. 3 shows both a plan and cross-sectional view of the motor coupling 114 of FIG. 2, although the flanges 132 are omitted for clarity.

(11) Whilst the motor shaft 112 is rigidly connected to the first couple part 128, it can be seen that the first couple part 128 and the second couple part 130 are inter-engaged by a dog-tooth arrangement 134. The opposed faces of the first couple part 128 and the second couple part 130 each comprise a number of teeth 136in this case four teeth 136 being provided on each of the first couple part 128 and the second couple part 130. The teeth 136 of each of the couple parts 128, 130 are received within the detents 138 defined between the teeth 136 of the other of the couple parts 128, 130. In addition, a compressible element 140, provided in the present embodiment as a rubber damper, is disposed between the teeth 136 and within the detents 138. Relative movement of the teeth 136 is therefore enabled with some damping provided by the compressible element 140. The dog-tooth arrangement 134 therefore provides a lost-motion mechanism.

(12) Because the compressible element 140 is designed to be always under slight compression, backlash noise is virtually eliminated during normal operation. Even after wear of the compressible element 140, which may occur over time, the dog-tooth arrangement will still provide a lost-motion mechanism.

(13) In the present embodiment, it will be apparent that the second couple part 130 does not transmit power directly from the rotation of the motor shaft 112 but instead rotates due to the motion of the first couple part 128 and the engagement through the lost-motion mechanism. However, in practiceas will become apparentthe second couple part 130 only transmits a significant amount of power to the secondary transmission mechanism 122 when the primary transmission mechanism 116 has failed through breakage, slippage, or otherwise.

(14) The operation of the power-assisted steering assembly 108 will now be discussed.

(15) Energisation of the motor 110, most commonly associated with a torque detected within the steering mechanism 102 of the vehicle 100, causes rotation of the motor shaft 112, this rotation being passed to the motor coupling 114 and more particularly to the first couple part 128. The first couple part 128 therefore begins to rotate. The rotation of the first couple part 128 causes motion of the drive belt 118 of the primary transmission mechanism 116, this motion being passed to the rack coupling 120, causing movement of the steering rack 104. Thus, when the primary transmission mechanism 116 is functional, power is transmitted from the motor shaft 112 to the steering rack 104. Movement of the secondary transmission mechanism 122 will now be considered.

(16) Whilst the primary transmission mechanism 116 is functional, the secondary transmission mechanism 122 is prevented from being placed under a substantial load or significant tension due to the action of the motor coupling 114 and rack coupling 120. Considering firstly the motor coupling 114, the dog-tooth arrangement 134 ensures that relative motion is provided between the first couple part 128 and the second couple part 130. As the first couple part 128 rotates, this motion is absorbed by the relative movement of the teeth 136 and the compression of the compressible element 140. By this mechanism, the secondary transmission mechanism 122 is subjected to a significantly lower torque than the primary transmission mechanism 116.

(17) Additionally, the rack coupling 120, to which the primary transmission mechanism 116 transmits torque, is formed such that the drive belts 118, 124 of the primary transmission mechanism 116 and the secondary transmission mechanism 122 are fixed relative to one another. Therefore, as the primary transmission mechanism 116 transmits power from the motor shaft 112, the drive belt 124 of the secondary transmission mechanism 122 is forced to turn. However, the drive belt 124 of the secondary transmission mechanism 122 is not under load at the end engaging the motor coupling 114 and therefore the drive belt 124 is not put under significant dynamic tension. It can therefore be understood that the secondary transmission mechanism 122 rotates with the motion of the primary drive mechanism 116 but does not transmit significant power and is not put under significant dynamic tension whilst the primary transmission mechanism 116 is functional.

(18) Upon a failure of the primary transmission mechanism 116for example by the drive belt snapping or otherwise failing to transmit powerthe rack coupling 120 will no longer move the secondary transmission mechanism 122. Therefore, the lost-motion mechanism of the motor coupling 114 will be forced to transmit all power through to the second couple part 130 and thus to the drive belt 124 of the secondary transmission mechanism 122. Upon a reversal in direction of rotation of the motor shaft 112, the lost-motion mechanism will again operate, but power will still be transmitted to a sufficient extent to allow proper operation of the steering. Therefore, with the drive belt 118 of the primary transmission mechanism 116 in a non-functioning state, the secondary transmission mechanism 122 will provide a back-up pathway for the transmission of power.

(19) A housing 142 is shown in cross-section at the top of FIG. 2, but in actuality extends around the path of the primary transmission mechanism 116 and the secondary transmission mechanism 122. The housing 142 is provided with small clearances around the flanges 132 such that the drive belts 118, 124 may not pass through the clearances if they were to become dislodged from the motor coupling 114 or rack coupling 120 or if they were to break. Although the housing 142 is, in the present embodiment, shown around both the primary transmission mechanism 116 and the secondary transmission mechanism 122, it may only be provided around one or other of the mechanisms 116, 122, for simplicity.

(20) An alternative embodiment of a power-assisted steering assembly 208 is shown in FIG. 4, in both side-on and plan view. The majority of the components of the power-assisted steering assembly 208 are the same as those of the first embodiment and therefore further description is omitted, for brevity.

(21) The embodiment of FIG. 4 differs from that which has previously been described in that the secondary transmission mechanism 222 takes the form of an idler gear 244 that engages with teeth on the motor coupling 214 and rack coupling 220. The idler gear 244 operates in much the same way as the drive belt 124 of the first embodiment, in that it whilst the primary transmission mechanism 216 is functional it does not transmit significant power due to the action of the motor coupling 214 and the rack coupling 220 but does so if the primary transmission mechanism 216 becomes non-functional.

(22) Beneficially, the use of an idler gear 244 further prevents any failure of the secondary transmission mechanism 222 due to rupture or other breakage of the drive belt 124. Whilst an idler gear 244 may provide additional resistance or power-loss whilst operating the power-assisted steering mechanism 208, this would be acceptable in the case that the primary transmission mechanism 216 was non-functional, at least until a repair could be effected. With the primary transmission mechanism 216 functional, the idler gear 244 would not add any significant resistance to operation as it would not be under loading, as discussed in relation to the first embodiment.

(23) Referring again to FIG. 2, a failure detector 146 is provided that detects the failure of the drive belts 118, 124 of the primary transmission mechanism 116 and the secondary transmission mechanism 122. The failure detector 146 of the present embodiment includes two optical sensors 148, each of which is configured to detect the presence of its respective drive belt 118, 124. Therefore, if either drive belt 118, 124 fails, for example by snapping, the respective optical sensor 148 will detect this failure and can transmit this to an ECU 150 of the vehicle 100 or steering assembly 108.

(24) The ECU 150 can then alter the operation of the vehicle 100 or steering assembly 108, if required, in order to put the vehicle 100 and/or steering assembly 108 into a limp-home mode. For example, upon the failure of the primary transmission mechanism 116, the steering assembly 108 may be programmed to provide less power from the motor 110, in order that the secondary transmission mechanism 122 is not forced to transfer a degree of power that may result in failure of the drive belt 124 of the secondary transmission mechanism 122.

(25) Although the present embodiment uses optical sensors 148, it will be apparent to the skilled person that other sensors may be used, such as magnetic sensors that detect magnetic elements within the transmission mechanisms. Alternative sensing mechanisms will be readily apparent to the skilled person.

(26) Although it is stated in the above embodiments that the secondary transmission mechanism does not transmit significant power whilst the primary transmission mechanism is functional, it is possible for the system to be designed such that a chosen amount of tension is provided through the primary transmission mechanism. This could be provided by adjusting the amount of lost-motion available in the motor coupling and through selection of the compressible element to provide a set level of resistance. Such adaptations would be readily achievable by the skilled person in view of the present disclosure.

(27) The drive belts of the primary transmission mechanism and the secondary transmission mechanism should be manufactured separately from one another rather than being two parts from the same larger belt. By doing so, it is less likely that both drive belts could be subject to a common cause failure due to manufacturing defects.

(28) A further advantage may be gained by providing independent tensioning mechanisms for each drive belt. Although not shown, such mechanisms would ensure that there is no common failure mode associated with loss of tension and associated belt slippage of the primary and secondary transmission mechanisms.

(29) Although described in relation to the motor coupling, it may also be possible to provide the first coupling part and second coupling part as parts of the rack coupling. Where this is the case, the first coupling part will be held stationary relative to the ball-screw mechanism with the second coupling part transmitting power through the lost-motion mechanism and the first coupling part. The motor coupling may then be configured such it holds the primary transmission mechanism and secondary transmission mechanism fixedly relative to one another. Thus, the transmission is effectively reversed.

(30) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained.