Mechanical Speedometers

Speedometers are very interesting component of vehicles. Today we will look at the working of mechanical speedometers. Mechanical speedometers are those which do not use electronic sensors for sensing speed. They rely only on mechanical and electromagnetic components for showing the speed. The main components of mechanical speedometers are

1. Speedometer gear

2. Speedometer cable

3. Magnetic plate

4. Speed cup

5. Coil spring

6. Damper

7. Speedometer needle

1. The wheels of automobile are connected to speedometer cable through appropriate gear arrangement.

2.The speedometer cable is square in cross section so as to transfer rotary motion without slipping.

3.The cable spins a magnetic plate around at the same speed as that of wheels, inside the speed cup. 

4.The spinning magnet creates eddy currents in the speed cup.

5.The eddy currents make the speed cup rotate in same direction, in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together in any way—there's air in between them.

6. The hair spring is a spiral spring fitted on shaft connecting speed cup and it tightens, restraining the speed cup so it can turn only a little way.A

7.As the speed cup turns, it turns the pointer up the dial, indicating the vehicle's speed.

8. The shaft is surrounded by a cylindrical arrangement containing damping fluid. This viscous fluid damps the vibrations in speedometer needle.