1129 Properties of friction lining and pad materials

Friction level (Fig. 11.6) The average coefficient of friction with modern friction materials is between 0.3 and 0.5. The coefficient of friction should be sufficiently high to limit brake pedal effort and to reduce the expander leverage on commercial vehicles, but not so high as to produce grab, and in the extreme case cause lock or sprag so that rotation of the drum becomes impossible. The most suitable grade of friction material must be used to match the degree of self-energization created by the shoe and pad configuration and applications.

Resistance to heat fade (Fig. 11.6) This is the ability of a lining or pad material to retain its coefficient of friction with an increase in rubbing temperature. The maximum brake torque the lining or pad is to absorb depends on the size and type of brake, gross vehicle weight, axle loading, the front to rear braking ratio and the maximum attainable speed. A good quality material should retain its friction level throughout the working temperature range of the drum and shoes or disc and pads. A reduction in the frictional level in the

Fig. 11.6 Effects of temperature on the coefficient of friction

higher temperature range may be tolerated, provided that it progressively decreases, because a rapid decline in the coefficient of friction could severely reduce the braking power capability when the vehicle is being driven on long descents or subjected to continuous stop-start journey work. The consequences of a fall in the friction level will be greater brake pedal effort with a very poor retardation response. It has been established that changes in the frictional level which occur with rising working temperatures are caused partly by the additional curing of the pad material when it heats up in service and partly because chemical changes take place in the binder resin.

Recovery from fade (Fig. 11.6) This is a measure of the ability of a friction material to revert to its original friction level upon cooling after brake lining or pad temperature fade has taken place. The frictional characteristics of a good quality material will return on cooling, even after being subjected to repeatedly severe heating, but an inferior material may have poor recovery and the friction level may be permanently altered. Poor recovery is caused principally by a chemical breakdown in the ingredients. This may cause hardening, cracking, flaking, charring or even burning of the linings or pads. If the linings or pads are using thermoplastic binder resins a deposit may form on the rubbing surfaces which may distort the friction properties of the material.

Resistance to wear (Fig. 11.6) The life of a friction material, be it a lining or pad, will depend to a great extent upon the rubbing speed and pressure. The wear is greatly influenced by the working temperature. At the upper limits of the temperature range, the lining or pad material structure is weakened, so that there is an increase in the shear and tear action at the friction interface resulting in a higher wear rate.

Resistance to rubbing speed (Fig. 11.7) The coefficient of friction between two rubbing surfaces should in theory be independent of speed, but it has been found that the intensity of speed does tend to slightly reduce the friction level, particularly at the higher operating temperature range. Poor friction material may show a high friction level at low rubbing speeds, which may cause judder and grab when the vehicle is about to stop, but suffers from a relatively rapid decline in the friction lever as the rubbing speed increases.

Resistance to the intensity of pressure (Fig. 11.8) By the laws of friction, the coefficient of friction should not be influenced by the pressure holding the rubbing surfaces together, but with developed friction materials which are generally compounds held together with resin binders, pressure between the rubbing surfaces does reduce the level of friction. It has been found that small pressure increases at relative low pressures produce a marked reduction in the friction level, but as the intensity of

Fig. 11.7 Effects of rubbing speed on the level of friction over the temperature range

1 .

J 1 J

0 200 400 600 800 1 000

Rubbing pressure ikN/m2)

0 200 400 600 800 1 000

Rubbing pressure ikN/m2)

Fig. 11.8 Effects of rubbing pressure on the coefficient of friction pressure becomes high the decrease in friction level is much smaller. A pressure-stable lining will produce deceleration proportional to the pedal effort, but pressure-sensitive materials will require a relatively greater pedal force for a given braking performance. Disc brakes tend to operate better when subjected to high rubbing pressures, whereas shoe linings show a deterioration in performance when operating with similar pressures.

Resistance to water contamination (Fig. 11.9) All friction materials are affected by water contamination to some extent. Therefore, a safe margin of friction level should be available for wet conditions, and good quality friction materials should have the ability to recover their original friction level quickly and progressively (and not behave erratically during the drying out process). A poor quality material may either recover very slowly or

Table 11.3 Shoe factor, relative braking power and stability for various brake types

Type of brake

Shoe

Relative

Stability

factor

braking

power

Single trailing shoe

0.55

Very low

Very high

Two trailing shoes

1.15

Very low

Very high

Disc and pad

1.2

Low

High

Single leading shoe

1.6

High

Low

Leading and trailing

shoes

2.2

Moderate

Moderate

Two leading shoes

3.0

High

Low

Duo-servo shoes

5.0

Very high

Very low

Number of stops

Fig. 11.9 Effects of water contamination on the material's friction recovery over a period of vehicle stops

Number of stops

Fig. 11.9 Effects of water contamination on the material's friction recovery over a period of vehicle stops may develop over-recovery tendency (the friction level which is initially low due to the wetness rises excessively during the drying out period, falling again as the lining or pad dries out completely). Over-recovery could cause brake-grab and even wheel-lock, under certain driving conditions.

Resistance to moisture sensitivity The effects of atmospheric dampness, humidity or dew may increase the friction level for the first few applications, with the result that the brakes may become noisy and develop a tendency to grab for a short time. Moisture-sensitive friction materials should not be used on brakes which have high self-energizing characteristics.

Friction materials Materials which may be used for linings or pads generally have their merits and limitations. Sintered metals tend to have a long life but have a relatively low coefficient of friction. Ceramics mixed with metals have much higher coefficient of friction but are very rigid and therefore must be made in sections. They tend to be very harsh on the drums and disc, causing them to suffer from much higher wear rates than the asbestos-based materials. There has been a tendency to produce friction materials which contain much less asbestos and much more soft metal, such as brass zinc inserts or aluminium granules. Nonasbestos materials are now available which contain DuPont's Kevlar, a high strength aramid fibre. One manufacturer uses this high strength fibre in pulp form as the main body for the friction material, whereas another manufacturer uses a synthetically created body fibre derived from molten blastfurnace slag reinforced with Kevlar for the main body. Some non-asbestos materials do suffer from a drastic reduction in the coefficient of friction when operating in winter temperatures which, if not catered for in the brake design, may not be adequate for overnight parking brake hold.

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