233Metallic friction materials

Metallic and semi-metallic facings have been only moderately successful. The metallic linings are normally made from either sintered iron or copper-based sintered bronze and the semi-metallic facings from a mixture of organic and metallic materials. Metallic lining materials are made from a powder produced by crushing metal or alloy pieces into many small particles. They are then compressed and heated in moulds until sufficient adhesion and densification takes place. This process is referred to as sintering. The metallic rings are then ground flat and are then riveted back to back onto the driven plate.

Generally the metallic and semi-metallic linings have a higher coefficient of friction, can operate at higher working temperatures, have greater torque capacity and have extended life compared to that of the organic asbestos based linings. The major disadvantages of metallic materials are their relatively high inertia, making it difficult to obtain rapid gear changes; high quality flywheel and pressure plate. Cast iron must be used to match their friction characteristics and these facings are more expensive than organic materials.

2.3.4 Cerametallic friction materials (Fig. 2.9) Cerametallic button friction facings are becoming increasingly popular for heavy duty clutches. Instead of a full annular shaped lining, as with organic (asbestos or substitute) friction materials, four or six cerametallic trapezoidal-shaped buttons are evenly spaced on both sides around the driven plate. The cerametallic material is made from a powder consisting mainly of ceramic and copper. It is compressed into buttons and heated so that the copper melts and flows around each particle of solid ceramic. After solidification, the copper forms a strong metal-ceramic interface bond. These buttons are then riveted to the clutch driven plate and then finally ground flat.

The inherent advantages of these cerametallic-lined driven plates are:

1 A very low inertia (about 10% lower than the organic disc and 45% lower than a comparable sintered iron disc). Consequently it will result in quicker gear changes and, in the case of synchronized transmission, will increase synchronizer life.

2 A relatively high and stable coefficient of friction, providing an average value in the region of

Drive plate Limiting stop

Torsional damper spring

Splined hub

i ---Ceramic button ol

Fig. 2.9 Clutch driven plate with ceramic facings

0.4, which increases the torque capacity of clutches using these driven plates.

3 The capability of operating at high working temperatures of up to 440 ° C for relatively long periods without showing signs of fade.

4 Button type driven plates expose more than 50% of the flywheel and pressure plate surfaces to the atmosphere during clutch engagement, so that heat transfer to the surrounding by convection may be improved by as much as 100%.

5 Button type friction pads do not suffer from warpage as do full ring metallic or organic linings and therefore are less prone to distort and cause clutch drag.

6 Button type friction pads permit the dust worn from the friction surfaces to be thrown clear of the clutch areas, thus preventing the possibility of any trapped work-hardened particles from scoring the friction faces.

7 Cerametallic materials are not as sensitive to grease and oil contamination as organic asbestos based linings.

8 The early ceramic-metallic friction buttons had a poor reputation as they tended to wear tracks in flywheel and pressure plate facings. A prolonged development programme has virtually eliminated this problem and has considerably extended the driven plate life span compared to driven plates using organic (asbestos-based) annular disc linings.

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