Cold-worked piston for hydraulically operating brake

For the elimination of distortion of a thin walled cold formed brake piston the piston (1) perimeter wall (5) is stiffened by one or more formed grooves (3) and one or more inwardly projecting ridges each formed by a corresponding groove. The shaping of the groove is carried out so that the wall thickness within the critical range (2) is at least equal to the wall thickness of the piston wall (4). The grooves may be used for securing a sealing cup, a rubber seal and/or a retaining spring for the brake pads.

BACKGROUND OF THE INVENTION 
Pistons which act on the carrier plate of brake pads are required for the 
actuation of hydraulically operated brakes. As a rule, such pistons are 
cast and then machined. In order to lower the cost of manufacture of such 
pistons, which are required in huge numbers, it has been proposed to 
fabricate them without machining, for example by extrusion processes or by 
hydromechanical forming operations. In this context, the European patent 
application published without examination, No. 304,103 has, for example, 
become known. Such pistons have comparatively thin wall thicknesses, and 
when subjected to the considerable hydraulic forces required to brake 
larger vehicles, the perimeter wall of the piston may very well be 
radially distorted. This distortion may cause improper sealing of the 
piston or improper guidance of the piston in its travel within the 
cylinder. 
The invention has as its object to improve the loading capacity of a piston 
of this type and to enable a reduction in weight. 
SUMMARY OF THE INVENTION 
This object is attained for a piston of the type described by a piston 
having an inwardly projecting circumferential stiffening ridge formed by a 
cold worked groove formed into the perimeter of the piston. The 
groove-ridge feature substantially stiffens the perimeter wall of the 
piston at a point along the length thereof particularly subject to 
distortion by the axially applied hydraulic forces. 
It is preferred in many instances to additionally reinforce the piston 
perimeter wall at the point along the length thereof which is critical as 
regards distortion by forming the radius of a curvature between the ridge 
and the inner surface of the perimeter wall to be larger by the thickness 
of the piston perimeter wall than the radius of a curvature at the bottom 
of the groove. 
The circumferential groove may be formed by roller burnishing, in which 
case it may, then, be convenient to provide a plurality of grooves 
disposed side by side. 
The grooves may be utilized to simultaneously accommodate one end of a 
protective cup and/or the sealing rubber. The ridge may be used as a 
feature to secure a pad retaining spring. 
In order to further reduce any distortion of the piston, the end wall of 
the piston may be convexly shaped towards the open end of the piston, 
extending in a direction away from the axially acting hydraulic force.

DETAILED DESCRIPTION 
The piston 1 in FIG. 1 constitutes in essence a hollow cylinder with a 
perimeter wall 5 and closed at one end with an endwall 10. The hydraulic 
forces of the brake act within a chamber 12 of a housing 14 (FIG. 3) on 
the piston endwall 10 which in the position shown in FIGS. 1 and 3 will, 
as a result, move to the left in a bore 13 of the housing 14, and with its 
lefthand open end as viewing FIG. 1 and engage a carrier plate 16 of a 
brake pad 18 (FIG. 3). The pad 18 is thereby forced against the brake disc 
20 in the manner well known in the art. 
The piston perimeter wall 5 has a generally uniform wall thickness "b", 
which perimeter wall thickness is comparatively thin. Hydraulic forces 
acting on the piston endwall 10 cause the piston perimeter wall 5 to 
distort to a pronounced extent in a critical range 2 along the length of 
the piston 1, range 2 lying between axial location 8 and 9 in the example 
of FIG. 1. This distortion may cause the cylinder perimeter wall 5 to 
bulge outwardly in that range. As a consequence, the bulging may interfere 
with the fit of the piston within the brake cylinder, which may cause the 
piston to jam. The distortion may also cause inward movement of the piston 
perimeter wall 5, creating a problem of improper sealing. 
The cylindrical piston 1 may be fabricated by extrusion or by deep-drawing 
and consequently may have the comparatively thin weight-saving wall 
thickness already described above. 
In order to eliminate the problems described, a circumferential groove 3 is 
formed into the outer surface 4 of the perimeter walls at an axial 
location within the critical range 2, in such a way that an inwardly 
projecting stiffening ridge is formed extending into the interior space 
within the piston 1. The forming of the circumferential groove 3 leads to 
a stiffening of the piston 1 by the resultant thickened wall adjacent the 
groove 3. The groove 3 is preferably formed by roller-burnishing, and it 
is important that this step does not lead to a thinning of the wall 
thickness "a" within the critical range 2, but rather to locally increase 
the wall thickness by an appropriate shaping of the tool used, so that the 
wall thickness "a" through the bottom of the groove in the critical range 
2 comes to be at least equal to the wall thickness "b" of the remaining 
part of the piston. In this connection, the wall thickness should be 
measured at right angles to the direction of extension of the piston 
perimeter wall 5. As a consequence, also the radius R of the curvature 
between the ridge 6 and the inside surface of the piston perimeter walls 
should be larger at least by the amount "b" than the radius "r" of the 
curvature at the bottom of the circumferential groove 3. 
The groove 3 which is provided for reasons of stiffness may, at the same 
time, be utilized in a multiple way insofar as, for example, to receive 
and mount one end of a protective cup; or, the rubber seal 22 sealing the 
piston 1 in respect of the associated cylinder bore 13 may be secured in 
the groove 3, as shown in FIG. 3 depending on the specific axial location 
of the critical range 2. 
Optionally, the ridge 6 may be utilized in such a way that the spring legs 
of a retaining spring being secured to the rear side of the pad carrier 
plate can be engaged behind the ridge 6, thus retaining the brake pad in 
respect to the piston 1. 
The convex configuration of the piston endwall 10 serves as an additional 
safeguard against any distortion of the outer piston perimeter wall 5 in 
the event of elevated forces acting on the piston bottom 10. Preferably, 
the curve has a parabolic shape. 
One or a plurality of grooves 3 may be formed in the piston as shown in 
FIG. 4.