Technical-Services - Note Sheets

Bedding - Normal Condition - Glazing

When a piece of friction material is first put into service, two things start to happen:-

(i) It beds in to the mating surface at a rate which depends on the roughness and hardness of the metal, the wear resistance of the friction material and the duty to which the friction device is subjected;

During bedding there may be a variation from the expected value of; the effective radius, in the case of a clutch facing or disc brake pad, or, the circumferential pressure distribution, in the case of a drum brake lining.

In either case, any such variation will cause the torque generated to vary from the expected value, the deviation diminishing as bedding approaches 100%.

(ii) The friction material is conditioned on the surface by the heat generated (the surface temperature should not, during this process, exceed the maximum recommended intermittent value for the material) at a rate related to the rate of bedding and the nature of the material; as conditioning takes place there is likely to be some variation of the friction level, depending on the material concerned, before a stable value is achieved (some materials, particularly 'those for competitions use, need a special conditioning procedure which is advised on the data sheet).

Since the mating surface should be machined to a good standard of finish, the friction material should itself have a smooth or polished surface finish when it is bedded and this will permit the development of true friction; a rough surface might promote greater resistance to motion, because of ploughing (not true friction), but at the expense of a greater wear rate.

The colour of the surface of a piece of friction material will darken during conditioning, its appearance then depending on the original colour, but should not generally be black as this indicates that carbonisation of the resin is taking place, due to overheating; considerable experience is needed to be able to interpret observations of surface condition.

Once bedded and conditioned, the normal duty on a friction material (i.e. the heat generated) causes a very gradual breakdown of the surface layer, small particles of which are then ejected as wear products together with material from the mating surface. Because the wear products are so small and the breakdown so gradual, the smooth surface is maintained and the conditioning is continuously regenerated.

If the duty on the friction material becomes more severe (i.e. the maximum recommended intermittent temperature is exceeded more often and for longer periods), progressive charring of the resin and some other constituents will occur, with blackening of the surface as noted above (in extreme cases, all but the fibre in an asbestos based lining may be lost from part of the surface); because of the changes in the surface layer, the friction level will change (usually.it will fall and be recognised as 'fade') and the wear rate will increase.

If excessive duty continues, the penetration by heat may ruin the whole lining; if it is only of short duration, the penetration will be limited and the damaged surface can be worn away with a progressive return to normal friction and wear resistance (recovery from fade).

If the duty level of the friction material should be unusually low (rarely the case with clutch facings), the surface temperature may not be sufficient to complete the breakdown and release wear particles; it then becomes 'clogged' with partly degraded material (together with any contaminating matter which may intrude) which polishes into a glassy, hard surface which has lower friction than normal.

This condition is known as glazing and has largely been eliminated by careful formulation; to overcome it in service it is necessary to subject the linings to severe duty for a sufficient period to cause breakdown and wear of the glazed surface until a normally conditioned surface is achieved.

Abrasion of the surface (?roughing up') will also remove glaze, but will also necessitate extensive re-bedding and reconditioning, so it is not recommended.

In friction terms linings (except any that are produced with reinforcing or special backing layers) will give consistent performance until they are worn right away; their physical strength and their resistance to heat flow diminish, however, as their thickness is reduced and the chance of rivet heads or the supporting metal part damaging the mating surface increases.

Linings should therefore be replaced when they reach the limit of wear recommended by the manufacturer of the equipment to which they are fitted.

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"

home | profile | products | services | mining | marine | offshore | location | contact us

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