All engine types are fitted with 228 mm diameter rear drum brakes.the main drum brake components are illustrated in the diagram below.
1. Shoes2. Brake back plate3. Shoe recovery clearance device4. Shoe control cylinder
CLEARANCE RECOVERY SELF-ADJUSTING DEVICE BETWEEN SHOES AND DRUM
The adjustment of the clearance between the shoes and the drums takes place automatically and continuously, as required, every time the brakes are applied by means of the self-adjustment device.The device comprises the rod (1) inside which the adjustment screw (2), to which the toothed ring nut (3) is bolted, slides freely.In the rest position, the front shoe return spring (6) keeps the device compressed and, as a result, the ring nut (3) pushes the square plate (4) in contact with the end of the rod (1).The square plate (4) is also subject to a thrust force from the flexible spring (5). When the brakes are operated the two shoes move apart and come into contact with the drum; the two ends of the device are kept in contact with the shoes through the springs (7) and (8).The square plate (4), thrust by the flexible spring (5), reacts with the toothed ring nut (3) and causes it to rotate by means of the clip (9) in permanent contact with the toothed ring nut.
During the release stage, the self-adjusting device is compressed once again through the action of the shoe upper return spring: the toothed ring nut (1) remains in the angular position assumed during braking.This stop during the rotation stage is due to the friction between the plate (2) and the toothed ring nut (1). With the toothed ring nut (1) stopped during the rotation stage, if the linings are not too worn as a result of previous braking, the clip (3) moves and engages with the next tooth.
The maximum travel of the toothed ring nut (1) on the adjustment screw (2) is one tooth (0.020 to 0.025 mm); the bedding in of the brakes following dismantling is an exception: the travel is two teeth.If the brakes overheat after excessive braking and the temperature reaches between 100° and 110°C, the flexible spring device (3) starts working and bends to block the plate (4) in a neutral position.
During braking the toothed ring nut is no longer subject to the thrust action of the flexible spring and, as a result, the clip is at the same angle as the ring nut tooth which is free to slide with the adjustment screw on the clip without recovering the clearance due to the expansion of the drum.In the case of overhauling before fitting the brake linings, the toothed ring nut must be placed in contact with the spring and loosened by half a turn.
