Automatic slack adjusters for vehicle shoe-drum brakes

A vehicle shoe-drum brake of the one-leading, one-trailing shoe-drum simplex type comprising a rotatable drum, a relatively stationary torque-plate, brake shoes mounted on said torque plate and carrying friction linings for engagement with said rotatable drum, an actuating cam disposed between adjacent ends of said shoes, a cam shaft carrying said cam and which is journalled in said torque plate for rotation about a fixed axis to enable said cam to separate said adjacent ends of said shoes, and a slack adjuster, said slack adjuster comprising a housing mounted on said torque plate and fixed relative to said shoes, a pair of thrust assemblies guided to slide axially in bores in said housing, each said thrust assembly being adapted to act between said cam and said end of a respective one of said shoes, and each said thrust assembly comprising first and second interengaged screw-threaded members which are relatively rotatable to increase the effective length of said thrust assembly to compensate for wear of said friction lining of said respective one shoe, a rigid strut interposed between said cam and each said second member and through which said cam acts on said thrust assemblies to separate said adjacent ends of said shoes in response to rotation of said cam shaft, complementary recesses of part-spherical outline in said cam and in said second members and in which opposite ends of said struts have rocking engagements, means for holding each said first member against rotation with respect to said end of said respective shoe, and gear teeth provided on each said second member, a drive mechanism in constant meshing engagement with said gear teeth of both said second members so that both said second members are adapted to rotate together whereby the effective lengths of both said assemblies are increased by equivalent amounts, and means for rotating at least one of said second members when adjustment to compensate for wear is required.