Automatic adjustment
The 1.6L eight-valve Vectra has an automatic adjustment device on the rear drum brakes. Thus, there is no need for manual adjustment. The adjustment module consists of an adjustment lever, an adjustment spring and a two-piece pressure rod, the length of which can be changed using an adjustment gear.
When braking, the pistons of the wheel brake cylinder move the pads in the direction of the brake drum. The front brake shoe adjusting lever is preloaded by the adjusting spring and thus constantly rests on the pressure rod, while engaging with the sawtooth profile of the adjusting gear.
As a result, an increase in the distance between the brake shoes causes a rotational movement of the adjusting lever. As pad wear progresses, the rotational movement becomes large enough for the lever to engage the next tooth in the adjuster gear. The adjusting gear continues to rotate, the push rod lengthens. Now the brake pads are approaching the drum again - the brake is adjusted.
Left: to adjust the brake pads, insert a screwdriver through the hole and turn the adjusting gear in the direction of the arrow with forcing movements until the brake pads are in contact and the brake disc no longer rotates. Then rotate the adjusting gear back until the brake disc turns freely.
On right:With the brake disc removed, the following details are visible:
1 - adjusting device;
2 - lower return spring;
3 - clamping spring;
4 - upper return spring;
5 - spacer lock (when the handbrake is applied, it presses the pads against the drum);
6 - rear hand brake shoe.
This is how automatic adjustment works: when you press the brake pedal, the brake pads are pressed outward towards the drum. The translational-return movement of the incoming and outgoing brake pad bridge resulting from this is pulled by the adjusting lever in the direction «1», rotates around the fulcrum «2» and with the end of the blade turns the gear on the pressure rod. But this process is carried out only if there is a corresponding wear of the lining. To prevent adjustment due to thermal expansion (e.g. when driving downhill) of the brake drum, there is a thermal clamp between the pressure sleeve and the pressure rod (IN), which causes automatic adaptation to the gap caused by heating.
Automatic adjustment and temperature
When the brake is released, there must be a small distance between the shoes and the brake drum for the wheels to rotate freely. With automatic adjustment, this distance must always remain the same. Continuous braking, for example, when descending a slope, with a fully loaded vehicle, can increase the temperature of the drum brakes up to 350°C. Under the influence of this temperature, an increase in the diameter of the brake drum can reach 1 mm. The automatic adjustment device can perceive this as pad wear and adjust with a heated oversized drum brake. Consequence: After the brakes have cooled down, the wheels lock up.
With increased thermal expansion, the adjustment should be suppressed. This is done by the so-called thermal clamp inserted into the pressure rod. The bimetal part expands strongly under the influence of temperature and in this way compensates for the change in the diameter of the brake drum. The effect: The distance between shoe and drum remains constant and false adjustment does not occur.
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