Technical Services Notes Sheets

Getting the best out of Truck Brake Linings

Even when a brake lining material has been made by a reputable manufacturer, and the correct size and shape of product for the vehicle has been obtained, many things can prevent the desired end result - satisfactory braking for a long period - from being achieved, things which have nothing to do with the quality of the brake linings concerned.

There are so many of these factors than they cannot all conveniently be listed and such a list would not make easy reading; however, this sheet sets out many of the more important ones under headings which illustrate the kind of feature for which one must be on the alert.

Riveting

A smart paint finish makes a good impression, but if the shoe is sub standard, it is a waste of money putting a good lining on it. Check that the shoe is not damaged, that the rivet holes are not oversize and that the platform is not excessively concave; remove any scale from the platform surface before relining. Ideally, clamps should be used to hold the linings tightly against the shoe for riveting, or gapping may occur; be sure that half blocks are fitted in the correct relative positions.

The catalogue will advise the correct diameter of rivet but the length advised should be checked with the lining on the shoe; for a satisfactory clench to be achieved, using a semi-tubular rivet, the end of the rivet should project through the shoe by half to three quarters of the hole diameter.

An appropriate rim punch should be used - preferably in an air riveting machine - and should achieve two things; the shank of the rivet should be expanded so as to fill the hole in the shoe platform (the hole in the lining is a little larger) and the tube should be rolled over to press against the underside of the platform.

If a piece of metal rod is dropped a short distance onto the head of a clenched rivet, the dull sound from a loose rivet is easily distinguishable from the brighter ring of a tight one. A check with feeler gauges around the end of the lining should reveal no more than O.006 inches (0.15mm) gap, an that only as far as the line of rivets. i

Loose riveting can cause noise and pulling, with eventual movement of the linings leading to breakage; linings riveted to badly concave shoes may be over stressed and may fracture. Excessive force when riveting can cause cracking.

Drums

Newly relined shoes used with drums worn oversize will make initial contact only at the crown; bedding in will take longer than usual and, during this period, brake output will be somewhat reduced and there may be overheating of the reduced area in contact.

Linings used in oversized drums will necessarily wear more at the crown than should be the case. If the drums are scored, this also will reduce the initial contact area with resulting problems.

Shoe Installation

When relined shoes are fitted, all metal to metal contact points should be greased and correct functioning of working parts must be assured; any shoe steady posts or springs must be correctly set and shoe return springs must be in good condition.

Manual adjustment must be set according to the appropriate instructions and automatic adjustment must be activated sufficiently for correct clearances to be achieved. Poor maintenance with sliding shoe brakes can cause taper lining wear, noise, pulling or dragging; uneven adjustment can cause brake unbalance, pulling and excessive air consumption.

When the brakes on axle are religned, it is desirable that those elsewhere on the vehicle are serviced to ensure that they are functioning correctly; in a bad case, the new linings may have to do excessive work and wear prematurely. Premature wear can also be caused by a faulty or wrongly set load sensing value or by unbalance in front and rear air system pressures caused by a faulty foot valve.

Actuation

With air / hydraulic actuation, the hydraulic system pressures front and rear should always be equal, even if the two air pressure inputs to the actuator are not; with straight air braking, however, the two pressures delivered by a dual foot valve may differ sufficiently to affect brake balance and lining wear. Any test of these two pressures should be made at a number of pressure levels to be sure of showing up any differences that exist.

Air chamber output can vary with stroke, so correct adjustment is important; output can also vary between equipment of different makes. Spring brake output decreases as stroke increases, so adjustment is particularly important.

The control signal for a trailer air braking system is sometimes passed through a trailer control valve, which can increase the air pressure in the trailer by an amount which can be varied (predominance); is this bias is overdone, the trailer brakes will do an excessive share of the work and the linings may wear out prematurely.

Because trailers may not be always serviced at the same depot, their brakes are particularly likely to be neglected by comparison with those on tractor units.

Contact us now for guidance and advice, or to discuss a specific project, by telephone on 01254 397561, by fax on 01254 389722 or by email at david@frictioncomponents.co.uk

"Solving Industries Braking Point"

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Friction Components and Systems Ltd - Solving Industries Braking Point
Unit 103, 25 Clydesmill Road, Clydesmill Industrial Estate, Cambuslang, Glasgow, G32 8RE

part of the Friction Components and Systems group of companies