Clutch Frictional Material
Clutch friction linings are subjected to severe rubbing so that generation of heat in relatively short periods takes place. Therefore, the lining material should have a combination of the following properties to withstand the operating conditions.(a) Relatively high coefficient of friction under entire operating conditions.
(b) Maintenance of friction properties during entire working life.
(c) Relatively high energy absorption for short periods.
(d) Withstanding high pressure plate compressive loads.
(e) Withstanding high impacts of centrifugal force during gear changing. (/) Adequate shear strength to transmit engine torque.
(g) High level of endurance in cyclic working without effecting friction properties.
(h) Good compatibility with cast iron facings over the entire range of operating temperature.
(i) A high degree of tolerance against interface contamination without affecting its friction take-up and grip characteristics.
24.12.1.
Asbestos-based Linings
Asbestos-based linings are, generally, of the woven type. These woven linings are made from asbestos fibre spun around lengths of brass or zinc wire of make lengths of threads, which are both heat resistant and strong. Advancement in weaving techniques has also eliminated the use of wire coring. This has enabled to offer asbestos woven lining as either non- or semi-metallic to suit a variety of working conditions. The woven cloth can be processed in one of two ways :In one method, the fiber wire thread is woven into a cloth, which is then pressed into discs of the required diameter. Several of these discs are stitched together to obtain the desired thickness. The final disc is dipped into resin solution so that the woven asbestos threads are bonded together. In the other method, the asbestos fiber wire is woven in three dimensions in the form of a disc to obtain the desired thickness in a single stage. It is then pressed into required shape and bonded together by again dipping it into a resin solution. Finally the rigid lining is machined and drilled for reverting to the driven plate.
These asbestos linings operate satisfactorily bellow 533 K providing a reasonable life span and uniform coefficient of friction between 0.32 and 0.38. However, most manufacturers of asbestos-based linings quote a maximum safe temperature of 633 K.
As a substitute to asbestos lining material, the DuPont Company has developed a friction material with the trade name Kevlar aramid derived from automatic polyamide fibers belonging to the nylon family of polymers. It possesses high endurance over its normal working pressure and temperature range. Since it is lighter in weight than asbestos material, the time required for gear changing is less due to a reduction in driven plate spin. It exhibits good take-up characteristics. It has good centrifugal strength to withstand lining disintegration due to sudden acceleration caused during the changing of gears. It has stable rubbing properties at high operating temperatures. At a temperature of 698 K it begins to break down steadily changing to carbon, and the disintegration process is completed at about 773 K.
24.12.2.
Metallic Friction Materials
The metallic linings are normally made from either sintered iron or copper-based sintered bronze. The semi-metallic linings are made from a mixture of organic and metallic materials. Metallic lining materials in a powder form are compressed and heated in moulds until sufficient adhesion and densification take place. This process is known as sintering. The metallic rings are then ground flat and are then riveted back to back onto the driven plate.When compared to the organic asbestos based linings, the metallic and semi-metallic linings -generally have a higher coefficient of friction, can operate at higher working temperatures, have greater torque capacity and have extended life. The metallic materials have relatively high inertia, making it difficult to obtain rapid gear changes, and these facings are more expensive than organic materials. Therefore, metallic and semi-metallic linings have been only moderately successful.
24.12.3.
Cerametallic Friction Materials
Cerametallic button friction linings are becoming popular for heavy duty clutches. Instead of a full annular shaped lining, like organic friction materials, four or six cerametallic trapezoidal-shaped buttons are evenly spaced on both sides around the driven plate. The cerametallic material in a powder form containing mainly of ceramic and copper, is compressed into buttons and heated. This causes the copper to melt and flow around each particle of solid ceramic. Once solidified, the copper forms a strong metaceramic interface bond. These buttons are then riveted to the clutch driven plate and then finally ground flat. The cerametallic lined driven plates offer the following advantages.(a) They have a very low inertia (about 10% lower than the organic disc and 45% lower
than a comparable sintered iron disc). Therefore it results in quicker gear changes. (6) They have a relatively high and stable coefficient of friction with an average value of 0.4 so that the torque capacity of clutches is increased.
(c) These can satisfactorily operate at high working temperatures of up to 713 K for relatively long periods without causing fade.
(d) Button type driven plates use only about 50% of the flywheel and pressure plate surfaces during clutch engagement. This improves convective heat transfer to the surrounding by as much as 100%.
(e) Button type friction pads do not warp during use unlike full ring metallic or organic linings.
(/) Button type friction pads prevent scoring of the friction faces, as they permit the dust worn from the friction surfaces to be thrown clear of the clutch areas.
ig) Cerametallic materials are less sensitive to grease and oil contamination than organic asbestos based linings. Also present time ceramic-metallic friction buttons do not wear tracks in flywheel and pressure plate facings, which was a problem is early days of application.