It is generally known that DC Motor has to work with the carbon brushes, but the carbon brush will wear and the length will be short with the working time increasing. In order to make sure the carbon brush can touch the rotor, we can add a spring into the carbon brush. In such case, no matter what’s the length of the carbon brush, it can touch the rotor of motor, so the motor can work all the time.

But the spring tension  will change with the spring length change. This will reduce the friction between carbon brush and rotor once the length of carbon became shorter. Then this will cause the later work of motor is not stable, the working life of carbon brush will be reduced dramaticlly.

To solve this problem, the engineer use the carbon brush spring instead of spring, because the tension of carbon brush spring keep constant. No matter what’s the length of the carbon brush, the friction between carbon brush and rotor will keep same, the working stability of motor is guaranteed before being scrapped, and the working life of carbon brush increase substantially, especially the motor which difficult to replace the carbon brush spring, such as automobile starting motor.

According to the working methods, the carbon spring can be divided into single coil spring and twin coils spring.Following shown:

     

The differences between single coil spring and twin coils spring:
1, If the tension is same, the volume of twin coil spring will be smaller.
2, The force is more stable due to the twin coil spring is bilateral symmetry,
3, The length of carbon brush can be longer if you use twin coil spring.
4, The outer diameter of motor can be smaller if you use single coil spring.

     
     

The advantage of carbon brush spring:
1, Because the force is constant, no matter what’s the length of carbon brush, the friction between carbon brush and rotor will keep same.
2, The carbon brush spring can reduce the loss, increase the working life of carbon brush.
3, It’s suited to small design, can reduce the diameter of motor.
4, It can increase the length of the carbon brush.
5, It can reduce the spark and the electromagnetic interference.