Requirement of a good clutch facing :-

1. Good wearing properties the wear of clutch facing depends upon the rubbing speed and intensity of pressure. For maximum life the rubber speed at the time of engagement should not exceed 30 m/s while pressure intensity should not exceed 100 kPa.

2. Presence of good binder in it.

3. Cheap and easy to manufacture.

4. High coefficient of friction.
5. High resistance to heat. A good clutch facing can withstand temperature of about 330°C.

Types of friction materials :-

1. Millboard type :- This is only asbestos sheet treated with certain impregnants. From this sheet are than the facing discs cut according to different size requirements. This is the cheapest available type but is quite satisfactory.

2. Molded type :- This is made by mixing asbestos fibres with suitable binding material, heating to a certain well defined temperature and then moulding in discs under pressure. Metallic wires are also sometimes inserted to improve wearing qualities.

This type of facing is more dense and capable of taking heavier working loads. However there is one disadvantage that each clutch facing has to be molded separately.

3. Woven type :- This type consists of a cloth impregnated with certain binders. The cloth made be either woven like ordinary cloth with wrap and weft or by winding with the fibres in circumferential direction only.

In the solid woven variety, the cloth is woven just to the required thickness. In the case of laminated variety, the layers of cloth one upon the other are held together by a binder. Stitches are provided in addition to the binder.

Common clutch facing materials :-

Organic friction materials are the most common types of clutch facing materials example are :-

1. Asbestos :- Asbestos facing has coefficient of friction of about 0.2. However it has got anti heat characteristics. But due to its polluting nature, its use has been banned worldwide.

2. Raybestos and Ferodo :- This asbestos based facing, now obsolete, have a coefficient of friction about 0.35 and have been widely used for clutch facing. However due to health risk posed by asbestos, these are being steadily replaced by non asbestos facing materials.

Asbestos tends to break down into a dust of microscopic size fibre. Because of their size and shape, these tiny fibres remain suspended in the air for long time periods and can easily penetrate body tissues after being inhaled or ingested. Because of the durability, this fibres remain in the body for many years and thereby become the cause of asbestos related disease. As asbestos clutch facing materials wear down in normal use, asbestos containing dust in the form of tiny fibres is created and released to the outside environment. Much of this material is entrapped within the clutch space, ready to be released in concentrated form when repair or replacement work is done.

3. Non asbestos clutch facing :- These are made of man made fibre yarn such as glass fibre, mixed with special rubber compound to improve frictional performance, firmly locked with elastomer based novolac binder and moulded with pressure and heat. Some materials may also be woven and may contain brass wire.

SW3-AF is one typical non asbestos clutch facing from super circle auto limited. It is woven type facing having coefficient of friction varying linearly from about 0.28 at 50°C to 0.36 at 250°C. It is non aggressive to mating surface, has high strength and burst speed coupled with excellent modulus of elasticity for comfortable driving. It can be used in dry clutches for all passenger cars, light commercial vehicles and tractors and is expected to give best performance with mating surface of fine grained pearlitic cast iron with a Brinell hardness of 179-229

HWK 200 from Hawkhead Automotive, Inc. is a rigid woven laminated composite containing Kevlar and brass wire. It has excellent strength characteristics, exhibits good fade and wear resistance and is easily machined. This material can be used for both dry and wet applications. Its coefficient of friction is 0.39 at normal temperature and 0.40 when hot.

Graphite materials are used in clutches with high operating temperatures due to high energy absorption or low cooling fluid flow rates. They work better under this conditions since more of the head is held in the friction material and moreover, the heat is held for a longer period.

For more severe applications, sintered metal friction materials are sometimes used. These are made by combining metallic powder and friction modifiers and compacting them into a solid form. Since such materials can withstand extremely high temperatures, these are quite suitable for wet clutches with very low on intermittent oil flow. However, the disadvantage of such material is that it tends to weld itself to the mating pressure plate and flywheel surfaces at high temperatures. For very heavy vehicles operating under extreme conditions, combined metal ceramic friction surface material can be used. However, these materials are satisfactory only when operating under very high temperature rather than under light duty and low temperature when they tend to have an abrasive action on the mating plate surfaces.