1144 Sliding cylinder body type brake caliper

This type of caliper unit consists of a carrier bracket bolted to the suspension hub carrier and a single piston cylinder bridge caliper which straddles

the disc and is allowed to slide laterally on guide keys positioned in wedge-shaped grooves machined in the carrier bracket.

Operation (Fig. 11.22) When the foot brake is applied, the generated hydraulic pressure enters the cylinder, pushing the piston with the direct acting pad onto the inside disc face. The cylinder body caliper bridge is pushed in the opposite direction. As a result, the caliper bridge reacts and slides in its guide groove at right angles to the disc until the indirect pad contacts the outside disc face, thereby equalling the forces acting on both sides of the disc.

A pad to disc face working clearance is provided when the brakes are released by the retraction of the pressure seal, drawing the piston a small amount back into the cylinder after the hydraulic pressure collapes.

To avoid vibration and noise caused by relative movement between the bridge caliper and carrier bracket sliding joint, anti-rattle springs are normally incorporated alongside each of the two-edge-shaped grooves.

11.4.5 Twin floating piston caliper disc brake with hand brake mechanism (Fig. 11.23) This disc brake unit has a pair of opposing pistons housed in each split half-caliper. The inboard half-caliper is mounted on a flanged suspension hub carrier, whereas the other half straddles the disc and is secured to the rotating wheel hub. Lining pads bonded to steel plates are inserted on each side of the disc between the pistons and disc rubbing face and are held in position by a pair of steel pins and clips which span the two half-calipers. Brake fluid is prevented from escaping between the pistons and cylinder walls by rubber pressure seals which also serve as piston retraction springs, while dirt and moisture are kept out by flexible rubber dust covers.

Foot brake application (Fig. 11.23) Hydraulic pressure, generated when the foot brake is applied, is transferred from the inlet port to the central half-caliper joint, where it is then transmitted along passages to the rear of each piston.

As each piston moves forward to take the clearance between the lining pads and disc, the piston

Fig. 11.23 Twin floating piston caliper disc brake with hand brake mechanism

pressure seals are distorted. Further pressure buildup then applies an equal but opposite force by way of the lining pads to both faces of the disc, thereby creating a frictional retarding drag to the rotating disc. Should the disc be slightly off-centre, the pistons will compensate by moving laterally relative to the rubbing faces of the disc.

Releasing the brakes causes the hydraulic pressure to collapse so that the elasticity within the distorted rubber pressure seals retracts the pistons and pads until the seals convert to their original shape.

The large surface area which is swept on each side of the disc by the lining pads is exposed to the cooling airstream so that heat dissipation is maximized.

Hand brake application (Fig. 11.23) The hand brake mechanism has a long and short clamping lever fitted with friction pads on either side of the disc and pivots from the lower part of the caliper. A tie rod with an adjusting nut links the two clamping levers and, via an operating lever, provides the means to clamp the disc between the friction pads. Applying the hand brake pulls the operating lever outwards via the hand brake cable, causing the tie rod to pull the short clamp lever and pad towards the adjacent disc face, whilst the long clamp and pad is pushed in the opposite direction against the other disc face. As a result, the lining pads grip the disc with sufficient force to prevent the car wheels rolling on relatively steep slopes.

To compensate for pad wear, the adjustment nut should be tightened periodically to give a maximum pad to disc clearance of 0.1 mm.

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