Expander for internal shoe drum brakes

Claim 1

An expander for mutually displacing adjacent ends of diametrically opposed brake shoes of internal shoe drum brakes of the type consisting of an axially extending stem with a longitudinal axis and of a pair of transversely extending curved lever arms on said stem carrying convex axial involute working surfaces the involutes of which have similar base circles and define each an angular range around said longitudinal axis between a first limit line and a second limit line both limit lines passing through said longitudinal axis, a central circle with its center on said longitudinal axis, the centers of the base circles lying diametrically opposed on the circumference of said central circle and defining a control line passing through said longitudinal axis within said angular range, said control line enclosing a control angle with said first limit line, the centers of curvature associated with involute points on said first limit line and lying on the circumferences on said base circles defining a reference line which encloses a reference angle with said first limit line, said control angle being at least equal to said reference angle, the center of each base circle being disposed on the same side of said longitudinal axis as its associated involute working surface.