Do you know how the speedometer works?

The function of the speedometer is to display the speed of the vehicle at any time, so that the driver can grasp the driving speed and ensure driving safety. It records vehicle mileage and provides reliable data for timely vehicle maintenance. The speedometer is mainly composed of a worm gear pair, a flexible shaft device, a speedometer device and a counter. The worm gear pair is combined with the countershaft in a motorcycle gearbox or the front hub of a moped. One end of the soft auxiliary device is connected with the worm gear pair, and the other end is connected with the drive shaft.


The permanent magnet is fixed on the drive shaft and rotates with the drive shaft, which is equivalent to a set of coils with a metal cover covering the outer periphery of the permanent magnet. There is no connection between them, and there is a certain gap. The speedometer pointer is fixed on the upper end of the pointer shaft, and the lower end is fixed with the rotation center of the metal cover. The hairspring is installed under the indicator plate. The hairspring allows the speedometer needle to swing smoothly and return to its original position in time. The worm gear housing prevents mud, sand, water and dust from falling into the worm gear pair.


When the moped is running, the front wheel drives the worm gear pair, the flexible shaft, the drive shaft and the permanent magnet to rotate. At this time, the magnetic line of force cuts the metal coil, causing the coil to generate an induced current to form a magnetic field, which interacts with the magnetic field of the permanent magnet, causing the metal coil to rotate, and the metal rotating coil is damped by the hairspring. When the torque of the metal coil is equal to the damping torque of the hairspring, the speedometer needle displays a specific value to indicate the driving speed. The higher the speed, the faster the permanent magnets spin, the stronger the torque produced, and the greater the swing angle of the speedometer needle driven by the metal coil. When the vehicle speed decreases, the rotation speed of the permanent magnet decreases, the induced magnetic field generated in the coil weakens, the torque becomes smaller, and the swing angle of the speedometer pointer becomes smaller under the action of the hairspring. If the car stops, the permanent magnets also stop rotating. The induced magnetic field in the coil disappears and the speedometer needle returns to the zero position driven by the hairspring.