Brakes – Wheel – Axially movable brake element or housing therefor
Patent
1998-06-09
1999-10-26
Butler, Douglas C.
Brakes
Wheel
Axially movable brake element or housing therefor
188 728, 188158, 188 718, F16D 6516
Patent
active
059711108
DESCRIPTION:
BRIEF SUMMARY
This invention relates to improvements in electrically-operated disc brake assemblies for vehicles of the kind in which a friction member is adapted to be applied to a rotatable brake disc by a brake-applying means including an electric motor which forms part of the assembly, and transmission means in the form of a screw device adapted to apply an axially-directed brake-applying force to the friction member in response to operation of the motor.
A brake of the kind set forth is known from EP-A-0 339 024. In the brake of EP-A-0 339 024 the electric motor is used to drive an input to a reduction gearbox, and the gearbox is adapted to translate the high speed movement of the motor into a controlled high torque output to the screw device.
In the brake disclosed in EP-A-0 339 024, the reduction gearbox typically has a ratio of 115:1. Such a high gear reduction is required in order to provide a suitable control of the clamp force from the high speed low torque, electric motor. Gearboxes on this type are by their nature very large and complex. The gearbox disclosed in EP-A-0 339 024 is of an epicyclic design having many individual components. The output from the gearbox is operatively connected to the screw device, disclosed in EP-A-0 339 024 as a ball screw. The advantage of the ball screw over a typical acme thread is the improved efficiency of the drive. However all screws by virtue of their design are relatively large in diameter and relatively long in pitch for a given axial load. The large diameter required for the high axial load produces problems in terms of packaging, particularly when the device has to be installed in a restricted operating envelope, namely the limited space available within the confines of the wheel/disc assembly. The long pitch further compounds the gearbox reduction requirements to effect suitable control of the clamp loading from the high speed motor. The relatively large pitch of the ball screw has the disadvantage that it is then necessary for the gearbox to be of suitable high reduction in order to produce the overall fine control required to actuate the brake.
In GB-A-2 199 097 a motor is used via a gearbox, again of declared high reduction, to drive the input of a roller screw actuator. This has the advantage that a roller screw is able to support an equivalent axial load to that of the ball screw but on a screw diameter of small proportions. Therefore the packaging of the device into the above mentioned restricted operating envelope becomes significantly simpler.
We are also aware of U.S. Pat. No. 4,596,316 which discloses an aircraft brake assembly in which brake actuation is achieved by the operation of a roller screw drive mechanism. In this brake assembly a single stage gearbox in the form of a high reduction gearbox is used in conjunction with a ring gear which is extremely large in diameter. Such a mechanism is therefore entirely unsuitable for incorporation into, or for use in, a brake assembly of the kind set forth.
According to our invention in a brake assembly of the kind set forth the screw device comprises a recirculating roller screw assembly which is adapted to be driven from the electric motor through a gear train, and the roller screw assembly comprises a fine pitch recirculating roller screw, and in which the screw device further comprises inner and outer screw threaded members with the roller screw acting therebetween, and the inner member is constituted by two separate components comprising an inner tappet which is keyed against rotation, and a hollow screw keyed against relative movement in an axial direction with respect to the tappet.
Utilising a fine pitch recirculating roller screw lowers the inertia of all components for a given response for the brake assembly, and enables the gear ratio to be reduced since one turn of the screw represents a relatively small axial displacement in a brake-applying direction.
Using a recirculating roller screw of relatively fine pitch has the advantage that the small pitch adds to the overall gear reduction between the m
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Butler Douglas C.
Lucas Industries public limited companuy
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