1142 Sliding yoke type brake caliper

With this type of caliper unit, the cylinder body is rigidly attached to the suspension hub carrier, whereas the yoke steel pressing fits over the cylinder body and is permitted to slide between parallel grooves formed in the cylinder casting.

Operation (Fig. 11.20) When the foot brake is applied, hydraulic pressure is generated between the two pistons. The hydraulic pressure pushes the piston apart, the direct piston forces the direct pad against the disc whilst the indirect piston forces the yoke to slide in the cylinder in the opposite direction until the indirect pad contacts the outstanding disc face.

Further pressure build-up causes an equal but opposing force to sandwich the disc between the friction pads. This pressure increase continues until the desired retardation force is achieved.

During the pressure increase the pressure seals distort as the pistons move apart. When the hydraulic pressure collapses the rubber pressure seals retract

Fig. 11.19 Swing yoke type brake caliper

and withdraw the pistons and pads from the disc surface so that friction pad drag is eliminated.

Yoke rattle between the cylinder and yoke frame is reduced to a minimum by inserting either a wire or leaf type spring between the sliding joints.

11.4.3 Sliding pin type brake caliper (Fig. 11.21) The assembled disc brake caliper unit comprises the following; a disc, a carrier bracket, a cylinder caliper bridge, piston and seals, friction pads and a pair of support guide pins.

The carrier bracket is bolted onto the suspension hub carrier, its function being to support the cylinder caliper bridge and to absorb the brake torque reaction.

The cylinder caliper bridge is mounted on a pair of guide pins sliding in matched holes machined in the carrier bracket. The guide pins are sealed against dirt and moisture by dust covers so that equal frictional sliding loads will be maintained at all times. On some models a rubber bush sleeve is fitted to one of the guide pins to prevent noise and to take up brake deflection.

Frictional drag of the pads is not taken by the guide pins, but is absorbed by the carrier bracket. Therefore the pins only support and guide the caliper cylinder bridge.

As with all other types of caliper units, pad to disc free clearance is obtained by the pressure seals which are fitted inside recesses in the cylinder wall and grip the piston when hydraulic pressure forces the piston outwards, causing the seal to distort. When the brakes are released and the pressure is removed from the piston crown, the strain energy of the elastic rubber pulls back the piston until the pressure seal has been restored to its original shape.

Operation (Fig. 11.21) When the foot brake is applied, the hydraulic pressure generated pushes the piston and cylinder apart. Accordingly the inboard pad moves up to the inner disc face, whereas the cylinder and bridge react in the opposite sense by sliding the guide pins out from their supporting holes until the outboard pad touches the outside disc face. Further generated hydraulic pressure will impose equal but opposing forces against the disc faces via the pads.

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