Patent ID: 7827868

Claim:
Apparatus for use in a materials testing machine for securely holding a specimen grip in a fixed position relative to a jaw housing, the apparatus comprising: the jaw housing containing the specimen grip, the grip having two external opposing inclined faces, each of the faces abutting against a corresponding one of two complementary interior faces of the jaw housing, the two complementary faces and a bottom interior surface of the housing collectively forming a cavity in the jaw housing which accommodates the specimen grip; and a cam jack situated in a recess in the housing below the cavity, wherein the cam jack abuttingly engages with an underside of the specimen grip and an opposing interior surface of the housing and generates an increasing mechanical force which moves the faces of the specimen grip against the complementary faces of the housing so as to frictionally lock the specimen grip into the fixed position in the jaw housing, the cam jack having: a cylindrical cam having an eccentric, with a contact surface having a predefined raising rate, the cam being oriented for rotational movement about a longitudinal axis thereof; and a resilient push block having first and second opposing surfaces and oriented transverse to the longitudinal axis of the cam, the first surface of the push block oriented to transmit the mechanical force to the underside of the specimen grip, and the second surface of the push block having a first channel formed therein with a predefined shape and against which the eccentric rotatingly abuts, with the push block also having and first and second end portions situated between the first and second opposing surfaces and oppositely and transversely extending beyond the cam; and wherein, the eccentric, as a result of the rotational movement of the cam in the first channel, to a predefined angular position, causes the push block to apply said mechanical force, through the first surface of the push block, to the specimen grip to lock the specimen grip in the fixed position and also causes bending moments to appear in the first and second end portions of the push block which, in turn, cause the first channel to undergo increasing elastic deformation such that a contact surface of the first channel increasingly deflects around the contact surface of the eccentric until said predefined angular position is reached and thereby, through increased friction occurring between the contact surface of the eccentric and the contact surface of the first channel as a result of the deflection, prevents the cam from rotating from said predefined angular position.