Brakes – Wheel – Transversely movable
Patent
1992-01-17
1994-09-06
Graham, Matthew C.
Brakes
Wheel
Transversely movable
188196D, F16D 6509, F16D 6556, F16D 5118
Patent
active
053439872
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a hydraulically and mechanically actuable drum brake, in particular for rear wheels of a motor vehicle, comprising includes a piston, remote from the wheel brake cylinder, and in the vicinity of the wheel brake cylinder, that on hydraulic actuation of the brake during forward travel both brake shoes are secondary shoes.
BACKGROUND OF THE INVENTION
In a known drum brake of this type (DE 3021794 A1) the intermediate support is mounted for oscillation or is displaceably guided on the anchor plate in such a manner that the ends of the two brake shoes supported thereon can execute a floating movement substantially parallel to the wheel brake cylinder. When this brake is mechanically actuated whilst the associated vehicle is travelling forwards or for example tends to roll forwards due to a gradient, the brake drum entrains the first brake shoe so that the end thereof normally supported at a closed end face of the wheel brake cylinder moves away from the latter and the other end thereof exerts a peripheral force on the adjacent end of tile second brake shoe via the floating intermediate support. The second brake shoe is pressed by this force and by peripheral forces acting from the brake drum directly thereon against the piston of the wheel brake cylinder so that the latter is displaced into the wheel brake cylinder and displaces brake fluid until it comes to bear on a stop in the wheel brake cylinder.
The displaced brake fluid must be replenished by the master brake cylinder on the next hydraulic brake actuation. This requires a travel of the brake pedal which is greater than the usual pedal travel. In unfavourable cases, the travel of the brake pedal necessary for refilling the wheel brake cylinders can be greater than the maximum possible pedal travel so that adequate hydraulic braking is possible only after repeated actuation of the brake pedal, if at all. The known drum brake thus did not acquire any practical significance although in a desirable manner it has a braking coefficient which in hydraulic braking is comparatively small and has a degressive characteristic but on mechanical actuation is comparatively large and has a progressive characteristic.
The braking coefficient is understood as the ratio of the sum of the peripheral forces acting on the brake shoes during braking to tile clamping force with which the brake shoes are pressed against the brake drum. Characteristic means the dependence of the brake coefficient on the friction coefficients of the pair comprising brake lining and brake drum.
SUMMARY OF THE INVENTION
The invention is based on the problem of providing a hydraulically and mechanically actuable drum brake which on hydraulic actuation has a degressive characteristic but on mechanical actuation a progressive characteristic and which avoids or at least reduces the disadvantageous displacement of brake fluid from the wheel brake cylinder on mechanical actuation.
The problem is solved according to the invention proceeding from a drum brake of the type set forth in the preamble in that
the intermediate support comprises a housing which is secured to the anchor plate and has a guide at least approximately parallel to the wheel brake cylinder and a stop,
in the guide an adjusting member is axially displaceable to a limited extent and
a push member which with one of its two ends supports the first brake shoe and on hydraulic actuation of the brake is displaceable by a force acting on its second end in the direction towards the first brake shoe and is connected to the adjusting member in a manner free of self-locking with respect to said force but self-locking in the opposite direction.
This achieves that hydraulic brake actuating forces, irrespective of the state of wear of the friction linings of the brake shoes, are transmitted substantially unobstructed by the second brake shoe, which is actuated directly by the piston of the wheel brake cylinder, via the intermediate support to the first brake shoe but that mechanical brake actuating
REFERENCES:
patent: 2619198 (1952-11-01), Freund
patent: 2748901 (1956-06-01), Brooks
patent: 3762510 (1973-10-01), Newstead
patent: 4706783 (1987-11-01), Rath et al.
patent: 4742897 (1988-05-01), Hiroshi et al.
Graham Matthew C.
Lucas Industries Public Limited Company
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