A speedometer is a special gauge which measures and displays the instantaneous speed of a car. Automobile speedometers differ in gear ratios of the mechanism that drives the counting unit, and so in size and in appearance.
Usually all speedometers have a mechanical drive, but with a cable length of more than 3.5 meters, an electric drive is used. The speed measurement range is from 0 to 200 mph.

Speedometer Construction

Speedometer consists of two devices: speed indicator and counter of total traveled, that converts the input shaft speed into mileage indication. There is used a tachometer with a drive from engine camshaft too.

a – speed indicator scheme; b – speed counter; c – speedometer drive scheme; 1 – magnet; 2 – pinion; 3 – speed screen; 4 – roller; 5 – arrow; 6 – spring-hair; 7 – pinion; 8 – drums; 9 and 10 – teeth of speed drum.

Speedometer Scheme

The operation of speedometer speed nodes is based on the magneto-induction principle. The field lines of the magnet field 1 penetrate the non-magnetic material (aluminum) of the card 2 mounted on the roller 4 and rotating freely on two bearings. The magnetic field is concentrated by a screen 3 of soft magnetic material. The magnetic field is created by eddy currents induced in the card body during rotation of the magnet 1.

How a speedometer works

As a result of the interaction of the fields of the card and the magnet, a torque arises that turns the card in the direction of rotation of the magnet. The spiral spring-hair 6 prevents the rotation of the card. In the case of a constant frequency of rotation of the magnet, the moment of the spring-hair and the moment of interaction of the magnetic fields are balanced when the card rotates by a certain limiting angle proportional to the frequency of rotation of the poles relative to the card. Therefore, the displacement of the arrow 5 of the device is proportional to the frequency of rotation of the magnet.

The counting unit of the speedometer has pinion 7 with six teeth engaged with the drums 8. Each counting drum has twenty teeth 10 on the drive side, and two teeth 9 on the other side. On the side of the pinion that is connected to the two teeth of the drum, three of the six teeth are shortened through one by width.

The rotating initial drum rotates the pinion with two teeth by a third of a revolution, and the pinion rotates the next drum by two teeth, i.e. 1/10 of its turnover.

The pinion remains stationary until the side of the first drum with two teeth makes a complete revolution. Thus, each subsequent drum rotates 1/10 of a turn, while the previous drum makes one full revolution. The speedometer usually has six drums, and therefore, the countdown goes up to 100,000 revolutions of the first drum, after which everything starts again. The numbers from 0 to 9, moved on the outer surface of the drums, show the car’s mileage in tenths of kilometers, etc. ranging from 000 000, 0 to 99999. 9 km. The accuracy of the readings depends on the condition of the tires of the car.

The main part of the drive of the speedometer is a flexible shaft, connected at one end to the device installed in the cab on the shield, and the other to the place of the drive in the gearbox. The flexible shaft is made in the form of flexible armor from profiled wire with nipples and nuts at the ends. On top there is a sealing plastic shell. The flexible shaft rotates freely in the containment shell and also has some axial clearance. A flexible shaft is wound from six to seven layers of wire with the opposite direction of winding of adjacent layers.