Brake Lining and Brake Pad

1.14. Brake lining

Brake linings are the consumable surfaces in brake systems, such as drum brakes and disc brakes used in automobiles asbestos brake linings.

Fig 1.24: Brake liner (Ferodo 2629 F) for drum brake

Since the lining is the portion of the braking system which converts the vehicle's kinetic energy into heat, the lining must be capable of surviving high temperatures without excessive wear (leading to frequent replacement). A Fig 1.24 shows the brake lining Ferodo 2629 F material.

1.14.1. Structure of brake linings:

Brake linings are composed of a relatively soft but tough and heat-resistant material with a high coefficient of dynamic friction (and ideally an identical coefficient of static friction) typically mounted to a solid metal backing using high-temperature adhesives or rivets. 

The complete assembly (including lining and backing) is then often called a brake shoe (or lining) in case of drum brake, but in disc brake it is an individual and name brake pad. The brake lining is that part of the brake pad which actually contacts the metal brake disc (rotor) when the brake is engaged.

The lining has to be tough. During hard braking, the shoe may be pressed against the drum by force of 4500 N or higher. Since friction increases as the applied force increases, a strong frictional drag is produced on the brake drum. This produces the braking effect at the' wheel.

The braking linings are attached with shoe brakes by synthetic resin adhesives or and by riveting.

1.14.2. The usual advantages of synthetic resin adhesives lining due to following reasons:

 Absence of riveting holes.
 More contact surface.
 Free from scoring action.
 More effective wearing thickness.

1.14.3. There are of two main varieties (of brake lining):

  1. Solid -woven type
  2. Moulded type

1. Solid -woven type: The asbestos -base non-metallic linings have in general, an average coefficient of friction of 0.4 up to around 260° C and fade is not serious up to 300°C. Their maximum temperature resistance is around 350°C.

A further improvement is the zinc wire inclusion. Zinc serves to conduct some heat away from the working surface and so reduces slightly the maximum temperature reached by the particular brake mechanism. Zinc -wire linings have also better anti -fade characteristics and resistance to wear than the non-metallic type.

2. Moulded type: This type is moulded directly from the "mix", which contains asbestos fibres, together with resin powders and fillers. The average coefficient of friction with this type of linings is 0.4 and temperature resistance varies from 400° to 450°C. They have good anti -fade and anti-wear characteristics.

Types of moulded friction material for brake linings:
 (i) Ferodo 2629 F friction material – for drum brake lining shoe.
 (ii) Ferodo 2449 F friction material – for disc brake pad.

(i) Ferodo 2629 F: -

This is for use on drum type brakes. It is fawn in colour, having random fibre asbestos base and containing while metallic particles (Fig. 1.24).

Recommended operating range:
 Unit pressure 70-690 kN/m²
 Maximum rubbing speed 18m/sec
 Maximum temperature 400° C
 Maximum continuous temperature 175° C

Bonding Common adhesives may be used for bonding, but for better results thermosetting adhesives should be employed.

Recommended mating surface: - Good quality, fine grained cast iron. Cast steel is not suitable as a mating surface, but forged or cold rolled steel with a Brinell hardness of 200 or more may be used.

(ii) Ferodo 2449 F: -

This rigid moulded friction material is for disc brake pads for cars. It is light brown in colour, having a random fibre asbestos base and containing copper particles. The material has good resistance to fade and to wear. Ferodo 2449 F is only supplied integrally moulded to steel back plates in specified sizes (Fig. 1.25).

Recommended operating range:
 Unit pressure 0.35-5.2 MN/m 2
 Maximum rubbing speed 24m/sec
 Maximum temperature 550° C
 Maximum continuous temperature 250°C

Recommended mating surface: Good quality fine grained pearlitic cast iron. Cast steel is not suitable but forged or cold rolled steel with a Brinell hardness of 200 or more may be used.

1.14.4. Maintenance of brake linings:

When the lining is worn out, the backing or rivets will contact the rotors or drums during braking, often causing damage requiring re-machining or replacement of the drums or rotors. An annoying squeal caused by the warning tang is the typical alert that the pads need to be replaced; if the squeal is ignored for too long, drum or rotor damage (usually accompanied by an unpleasant grinding sound or sensation) will be the typical result.

The lining may also become contaminated by oil or leaked brake fluid. Typical symptoms will be brake chatter, where the pads vibrate as the lining grabs and releases the rotor's surface. The solution is to repair the source of the contamination and replace the damaged pads.

1.15.    Brake pad

The brake pads are made of T -section and are curved to match the surface curvature of the disc.  The expander end of the pad is called the 'toe' and the anchor end is termed the 'heel'. The ends may be flat, curved or have a semi-circular groove to locate the expander or anchor. They are made of cast iron or steel. The modern vehicles employ pressed steel brake pad. The dynamic friction coefficient "µ" for most standard brake pads is usually in the range of 0.35 to 0.42. There are some racing pads that have a very high µ of 0.55 to 0.62 with excellent high temperature behavior.

Fig: 1.25: Brake pad (Ferodo 2449 F)

Brake pad service:- 

Brake pads must always be replaced simultaneously on both ends of a vehicle's wheel, as the different pad thicknesses (and possibly material types) will cause uneven braking, making the vehicle pull in the direction of the more effective brake. For most vehicles, replacing pads is very easy, requiring a minimum of tools and time. Pads and lining are designed to be consumable and should therefore be easy to service.