Device for adjusting the clearance of a drum brake

A device for adjusting the clearance of a drum brake arranged between two shoes which cooperate with a braking surface of a drum, said shoes are moved by an actuation device which stresses them from a resting position, in which the shoes are away from said braking surface at least by a minimum operating clearance, to a braking position, in which the shoes abut against said braking surface, wherein:

• said clearance adjustment device comprises an extensible strut assembly arranged between the shoes, which creates a reaction action on the shoes when stressed by a braking action applied by the actuation device;
• said extensible strut assembly comprising:
• a piston rotationally constrained to a first shoe;
• a support rotationally constrained to a first shoe and slidingly connected to said piston making the strut extensible to adjust the distance of said first and second shoe;
• a toothed ring rotational with respect to said piston and abutting against said support, by rotating said toothed ring applies an action on said piston and on said support thus extending said strut and moving away said shoes;
• and wherein said clearance adjustment device further comprises:
• a lever assembly having a first lever end arranged in contact with one of said shoes and biased in rotation by said shoe when said shoe is stressed by said actuation device;
• said lever assembly rotating about a lever pin;
• and wherein said clearance adjustment device further comprises:
• a sliding guide associated with said lever assembly with at least one sliding guiding portion thereof inserted in the extensible strut assembly;
• said sliding guide sliding with respect to said extensible strut assembly during the rotation of said lever assembly;
• and wherein
• said sliding guide comprises a ratchet mechanism which acts on said toothed ring so as to turn said toothed ring when said sliding guide slides with respect to said extensible strut assembly and when it exceeds a predetermined idle stroke thereof of said ratchet mechanism with respect to said toothed ring;
• and wherein
• said coupling of said sliding guide and said extensible strut assembly makes the sliding of said sliding guide possible when the reaction action, equal and contrary to the braking action, is lower than a predetermined threshold, thus allowing the ratchet mechanism to run said idle stroke and possibly actuate said toothed ring extending said extensible strut assembly;
• and wherein
• said coupling of said sliding guide and said extensible strut assembly prevents the sliding of said sliding guide when the reaction action, equal and contrary to the braking action, is higher than a predetermined threshold, thus preventing the ratchet mechanism, when said threshold is reached, from actuating said toothed ring and preventing the extension of said extensible strut assembly .

A method for adjusting a clearance between the shoes of a drum brake, comprising the steps of:

• providing a device for adjusting the clearance between the drum brake and two shoes;
• applying a braking force of lower magnitude than a predetermined threshold force on the shoes and rotating the shoes;
• rotating the lever assembly;
• making the sliding guide and the ratchet mechanism slide, thus recovering the idle stroke;
• alternatively
• if the braking force continues remaining lower than the predetermined force threshold, engaging the ratchet mechanism in the toothed ring and rotating the ring extending the extensible strut;
• or
• if the braking force exceeds the predetermined threshold force, blocking the sliding guide, thus preventing the rotation of the toothed ring and avoiding to increase the distance between the shoes.