Deglaze linings in service?
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Only in one case, says Ferodo
• We've had a friendly letter from Alan Baker, Ferodo's technical services manager, who spent three days with Commercial Motor many years ago before becoming responsible for Ferodo International Technical News. A reference in Workshop Ways last Decembet, referring to "deglazing" linings and drums on a Sherpa service, caught his attention. Why are they glazed?
• In fact, glazing is a rare condition with modern brake linings and typical driving conditions, he writes, arising as it does from prolonged light duty and being accompanied by a falling off in friction level.
The fact that linings and drums (or discs) in service are smooth and shiny is as it should be, as, simply speaking, dull, rough surfaces do not equate with long life; the two surfaces have not only bedded in to each other, but have also become mutually conditioned.
The lining surface has, as a result of working at elevated temperatures, changed somewhat from the "as supplied" condition; although the performance changes during this process should be minimal, there will be some variation until a stable condition is reached. Meantime, the drum acquires a smear of matter from the lining, which contributes to the subsequent stable output of the lining and drum (or disc) working together.
If the two surfaces are then abraded, the bedding previously achieved is slightly impaired and their conditioning is lost; it is then necessary to again achieve full contact and to recondition before the linings will give of their best.
A truly glazed lining surface has a glassy appearance, which contrasts with the superficial shine of the well bedded and conditioned lining; the surface is hard and removing it by abrading will leave it rough, and in need of rebedding. The best way to overcome glazing and restore performance is to use the brakes harder for a sufficient time to heat up and wear away the glazed surface and achieve normal conditioning of the fresh surface. If the particularly light-duty operating conditions are normal, reconditioning may have to be repeated occasionally.
The only time that we would suggest abrading linings in service is to help in achieving full contact, if bedding is initially uneven. Once friction surfaces are working well, they need no maintenance, but should merely be protected against contamination.
I hope that these comments may be of interest.
Alan Baker Technical Services Manager Ferodo Stockport
• From Sheffield, Ian Pedler writes to ask who did the bodywork on the Transit Mark 3 recovery vehicle featured in November's Workshop. The answer is Recovery Ltd of Middlesbrough. -Thank you for a great magazine," writes our correspondent. Thank you, Sir.