Brakes – Wheel – Transversely movable
Patent
1995-11-03
1997-05-20
Butler, Douglas C.
Brakes
Wheel
Transversely movable
188106F, F16D 6522
Patent
active
056304862
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to drum brakes, intended in particular to provide braking for motor vehicles. These drum brakes generally include a hydraulic actuation device constituting the main brake motor, and a mechanical actuation device, constituting an auxiliary brake motor, to fulfil, for example, the parking brake or emergency brake functions.
Conventionally, drum brakes include a carrier plate integral with a stationary part of the vehicle such as a stub-axle holder, and a drum integral with the wheel to be braked. Two shoes, each one including a web and a rim part provided with a friction lining, are slidably mounted on the carrier plate.
A hydraulic actuation device generally consists of a wheel cylinder which includes two pistons sliding in a bore and defining between them a sealed chamber in which a rise in pressure causes separation of the pistons, and therefore of the shoes, the webs of which are kept bearing on the pistons by springs.
A mechanical actuation device generally consists of a lever, mounted so that it can pivot on one of the webs, the end of one of the arms is connected to an operating cable, and the other arm of the lever acts on a strut bearing on the other shoe by means of springs, so that rotating this lever causes the separation of the shoes.
According to one design of drum brake, the actuation device is located between a first end of the webs of the shoes, while a second end of the webs of the shoes bears on a bearing piece integral with the carrier plate. Such a design is known in the art by the expression "floating shoe brake", due to the fact of absence of anchorage of the webs of the shoes on the bearing piece. It gives high stability of the drum brake, but requires a very high actuating force.
According to another design, known in the art under the term "duo-servo", the actuation device is located between a first end of the webs of the shoes, the other ends of the webs of the shoes being articulated to one another, the carrier plate not having a bearing piece. This design gives the drum brake very high effectiveness, but very mediocre stability.
There are known, for example from documents FR-A-2,697,599, FR-A-2,697,600 or EP-A-0,419,171, drum brakes which combine these two designs, that is to say in which the hydraulic actuation device is of the floating-shoe type, and in which the mechanical actuation device is of the duo-servo type.
The drum brakes thus produced thus combine the advantages of the two technologies while avoiding their drawbacks. However, such a combination gives rise to another drawback.
What happens is that in any drum brake, the combination of hydraulic and mechanical actuating devices means that a system for automatically taking up wear has to be provided so that the travel of the mechanical actuation device is constant regardless of the state of wear of the friction linings.
A system for automatic take-up of wear conventionally consists of a screw-nut system mounted on the strut of the mechanical actuation device, located between the webs of the shoes, generally in the vicinity of the hydraulic actuation device, so that the length of the strut is increased, upon hydraulic actuation, if the webs of the shoes separate by a distance greater than a predetermined value, following wear of the friction linings. The travel of the mechanical actuation system thus keeps a substantially constant value.
When the drum brake is dismantled, for example for operations of maintenance or for changing the shoes or the drum, it is sufficient to cause the lever of the mechanical actuation device to pivot in the opposite direction from that of its normal actuation so that the shoes come together under the action of the various tension springs used in the drum brake. In that way, the shoes and in particular the friction linings, radially leave the track which they have made in the internal face of the drum, and it is therefore possible to extract them axially from the drum brake.
Now, in the drum brakes mentioned earlier, where the hydraulic actuation device is of the
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AlliedSignal Europe Service Techniques
Butler Douglas C.
McCormick Jr. Leo H.
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