The principal of braking in vehicles involves conversation of kinetic energy to heat (thermal energy) by friction. Brakes which directly connected to the wheels carry out two main functions in a very short periods of time.
- Generation of braking torque
- Absorption of heat (thermal energy) as a result of energy conversation
Two major kinds of friction brakes according to the brake system design are disc brakes and drum brakes. In conventional braking systems, brake cylinder which is directly connected to the brake pedal converts driver’s foot pressure via hydraulic, pneumatic or hydro pneumatic system, providing friction material to contact with disc/drum. Frictional forces between friction material and disc/drum enable car to slow down and stop.
The performance of a disc brake pad or brake lining is determined by frictional forces emerged by braking, namely friction coefficient (µ) is the major characteristic of a friction material.
If the friction coefficient of the brake pad/lining is higher than the coefficient set by brake system designer, the brakes may grab causing wheels to lock up and slide even in low braking pressures. Contrary to this; if the friction coefficient is lower than design µ, braking distance will be longer due to lower frictional forces. Depending on circumstances, both cases may emerge dangers so priority for a brake pad/lining is to design a friction material with friction coefficient values (µ) as indicated by brake system designer.
How is the required coefficient of friction determined for the brake pad/lining?
When determining a vehicle’s brakes friction coefficient, brake force distributions are calculated based on technical data of the vehicle and internal brake factor (C*) which provide the best fit to the ideal curve are settled.
Internal Brake Factor C*
This parameter defines the relationship between braking force and actuating force. In other words it sets the brake torque, deceleration and stopping distance for different brake pressure values as designed by manufacturer; hence determines friction coefficient values requested from brake pad/lining producer.
WHAT DO WE EXPECT FROM THE BRAKE LININGS?
SAFETY
COMFORT
ECONOMY
SAFETY
Friction coefficient ensures the required internal brake factor.
Stability of friction coefficient in
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Total life span |
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Cold/Hot braking conditions |
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Braking after heating up |
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Braking with changing pressures |
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Braking with changing speeds |
Short bedding time
Stability of dimensions after heating up
High shear forces
High resistance to igniton
COMFORT
Low Pedal Forces.
Quite braking
No environmental effects
No smell.
ECONOMY
Reduced wear
Minimal disc or drum wear
Durability of friction material & disc/drum against weight loss
It should be understood that some of the criteria of brake pads/linings may effect each other negatively; so an optimization of requested criteria should be observed.
Evaluation of a brake pad/lining according to a single property would not be a reliable assessment; all criteria should be taken into consideration.
Additionally, a brake pad / lining is just one of the parts in whole braking system and proper functioning of pad / lining strictly depends on the condition of these parts. Hence problems because of other parts in braking system (improper disc / drum surface, shoes, brake cylinders, covers, s-cam etc) as well as nonconformance to terms of use will affect our performance expectations negatively.
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