Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-12-14
2002-12-10
Hurley, Kevin (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C310S07500D
Reexamination Certificate
active
06491131
ABSTRACT:
This invention relates to improvement in gear assemblies, and in particular to electrical power assisted steering assemblies which incorporate a worm and wheel gear assembly for transferring torque from an electric motor to a steering column or output shaft operatively connected thereto.
It is known to provide a power steering system for a vehicle comprising an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering column, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of torque in the output shaft produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying an assistance torque which helps to steer the vehicle.
In a simple arrangement, the input shaft defines a worm gear, and the output shaft is provided with a wheel gear which is adapted to mesh with the worm gear. Whilst such a system is relatively effective, there exists a problem with noise and vibration due to incorrect meshing between the worm and gear wheel. This incorrect meshing may arise due to manufacturing tolerances, thermal changes in dimensions, distortion due to torsional loads and wear during service.
We are aware of U.S. Pat. No. 4,967,859 which discloses an electric power apparatus comprising all the features contained in the pre-characterising clauses of claims 1 and 18. Specifically, the apparatus of U.S. Pat. No. 4,967,858 does not include any form of biasing for the shaft.
We are also aware of EP-A-0420131 which discloses a rear wheel steering apparatus in which a worm shaft extending from a motor is biased towards a worm wheel by a resilient means in the form of helical spring.
In J-A-62255618 a resilient element constituted by leaf springs has a plurality of drive faces which co-operate with drive faces defined on a rotor and on an input shaft.
According to a first aspect, the invention provides an electric power assisted steering system comprising a housing, an electric motor fixed relative to the housing having a stator and a rotor, an input shaft operatively connected to the rotor, an output shaft operatively connected to a steering column, and a torque sensor adapted to produce an output signal indicative of the torque in the output shaft, the motor being adapted to apply a torque to the output shaft dependent upon the output signal from the torque sensor through a worm gear provided on the input shaft which is adapted to mesh with a wheel gear on the output shaft, the steering system being characterised by further comprising a first bearing means which supports the input shaft relative to the housing at its end distal from the motor and a resilient biasing means adapted to act upon the first bearing means to bias the input shaft towards the wheel gear.
Preferably, the input shaft is biased in a tilting movement which is centred at a second bearing means which supports the input shaft relative to the housing at its end adjacent to the motor.
The biasing means may be adapted to apply a sufficient biasing force to the first bearing means to maintain a fully meshed engagement between the teeth of the worm gear and the teeth of the wheel gear over a predetermined range of torque values carried by the wheel gear. This helps to prevent gear rattle when driving straight ahead or on rough roads by ensuring both sides of the engaging teeth on the worm and wheel are in contact at substantially all times over this range of torques. Because the arrangement increases quiescent friction in the gearbox it is important to maintain control of the force applied by the biasing means over the full range of the input shaft that is required. Therefore the biasing means must have a low spring rate.
The provision of the biasing means allows a controlled biasing force to be applied whilst permitting sufficient tilting movement of the input shaft to compensate for changes in dimensions due to manufacturing variations and temperature changes etc. The maximum torque value up to which the fully meshed engagement is effective is carefully chosen (by compromise) to avoid excessive friction.
The biasing means may comprise a resilient spring of any type adapted to act between a portion of the housing and the first bearing means.
In some configurations, it is preferred that the resilient spring comprises a leaf spring which may be attached to the housing at a first end and act upon the first bearing means at its second end. This may engage the first bearing means at the opposite side of the input shaft to the wheel gear so as to bias the worm into contact with the wheel gear.
The leaf spring may be provided outside of the housing and the second end of the leaf spring may pass through an opening in the housing to engage with the first bearing means. The second end of the leaf spring may carry a seal which seals with the opening through which it passes.
The input shaft may be directly connected to the motor rotor. It may extend continuously through the rotor.
The input shaft may be operatively connected to the rotor through a flexible coupling which allows the worm to tilt without movement of the rotor.
The flexible coupling may comprise a resilient element, for example of rubber. The motor rotor may be adapted to apply a drive force to the resilient element through one or more circumferentially spaced radially extending surfaces of the resilient element. The resilient element may in turn be adapted to apply a drive force to the input shaft through one or more other circumferentially spaced radially extending surfaces of the element. The element may comprise a spider shape having a multiple of arms presenting a number of radially extending, circumferentially spaced drive surfaces.
Where the input shaft is connected to the motor rotor by a flexible coupling, a first compression means may be provided between the housing and the first bearing means at the end of the input shaft distal from the motor which applies a compressive force onto the input shaft to bias it towards the motor rotor. It may comprise a coil spring. Its function is to prevent noise and vibration due to axial free play in the second bearing means. In its compressed state, there should be free space between adjacent coils in order to avoid frictional resistance to the tilting motion of the input shaft.
A second compression means (such as a coil spring) may also be provided between the end of the input shaft adjacent the motor rotor and the motor rotor. This may be provided in a cup formed on an end of the input shaft whilst a pin extending about the rotational axis of the motor rotor and forming a part of the rotor projects into the cup to engage the spring.
Thus, whilst the first compression means biases the second bearing means through the flexible coupling, the second compression means biases the rotor directly through the pin. There must be a difference between the forces provided respectively by the first and second compression means which is sufficient to bias the second bearing means in the direction of the input shaft axis by the desired amount.
In an alternative, the biasing means may comprise an annular O-ring provided between the first bearing means and the housing. The O-ring may be of rubber, and may contact an outer circumference of the first bearing means and a portion of the housing.
The first bearing means may therefore move relative to the housing against a resistant force applied by the O-ring as at least part of the O-ring is compressed. In such an arrangement, the wheeled gear or the worm gear may deliberately be made slightly oversize relative to the dimensions which are calculated according to the distance between their respective shaft axes. This ensures the O-ring is always under a small amount of residual compression.
In an alternative to an O-ring, the resilient biasing means may comprise a resilient element which is accommodated between the first bearing means and the hou
Hurley Kevin
Tarolli, Sundheim, Covell Tummino & Szabo L.L.P.
TRW LucasVarity Electric Steering Ltd.
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