Brake Drum Service Guide

WEAR

The maximum diameter of a brake drum is dependant upon the braking system and the drum’s design. To check the diameter use a micrometer, steel rule or a Wingauge. The maximum diameter should not exceed 1% of it’s starting diameter. Diamter examples:

NEW DIAMETER	MAX. SKIM	MAX. WEAR
350	353	354
394	397	398
413	416	417

HEAT CRAZING

Heat crazing, as illustrated in the image opposite, is the result of normal heating and cooling of the brake drum during service. These fine heat cracks are not detrimental to the performance of the brake and will frequently wear away and reform as a result of normal braking. However, heat crazing can develop into deeper cracks therefore the sur-face condition should be examined to ensure the cracks are not too severe.

HEAT CRACKING

Deep surface cracks, as illustrated in the image opposite, are usually the result of excessive heat. Brake drums with cracks going through the casting wall must be replaced immediately. Repeated cracking may indicate a system, balance or geometric problem.

HARD SPOTS

Hard spots are slightly raised coloured areas on the braking surface and are created by non-uniform contact cycles creating localised graphite crystallisation in the surface of the cast iron. This condition indicates that the brake drum has been operating at extremely high temperatures. It may be possible to remove these hard spots by skimming the braking surface as long as the diameter is within allowable limits. However, continuation of this condition can, under extreme conditions, result in a thermal transformation of the casting structure "martensite". If this does occur we would recommend immediate replacement.

GROOVED

A scored brake drum, with a grooved tramline type wear pattern, is the result of an abrasive foreign body working be-tween the brake lining and the braking surface. It can also be the result of excessive brake lining wear, creating a metal on metal contact situation. Scoring will reduce braking performance and should be removed through skimming as long as the drum is within maximum diameter limitations. Brake drums with severe scoring must be replaced and re-fitted with new brake linings.

CRACKING OF MOUNTING FACE

All brake drums should be inspected prior to fitting to ensure there are the same parts as being removed, if there is any doubt contact your supplier or our Technical Sales Department for guidance. Drums should also be inspected to ensure there are no cracks in the casting wall or across any of the stud holes. Damage can be caused by miss-handling or incorrect fitment.

STORAGE

Brake drum ovality is a condition where the drum is out of round i.e. egg shaped. The maximum tolerance we allow for for ovality is 0.005” (0.127mm). Ovality is usually caused by by improper handling but can also be affected by incorrect storage or extreme operating conditions. Therefore, brake drums should always be handled with care and stacked correctly (see image). It may be possible to re-machine the braking diameter as long as it does not exceed the drums allowable limitations.

OVALITY

Ovality is term used to represent the drums roundness. For correct operation we would recommend that the ovality does not exceed 0.005" ( 0.127mm). The main cause of ovality is miss-handling or incorrect storage, see above. Oval drums can be re-machined as long as the skimmed diameter does not exceed the drums maximum diameter limit, see wear..