Inner workings of the speedo meter
Description: Inner workings of the speedo meter
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Many people do not know how "magnetic" speedometers work. In the above picture the dark ring is a magnet. It is driven by the speedo cable and spins. The next shot shows a housing with a suspended metallic cup ( the lighter silver object visible in the notch). When the magnet spins within the cup the resulting magnetic attraction creates drag and the cup moves in the direction of the spinning magnet. There is NO physical connection between the two. The cup has a shaft that extends and the needle is attached to it. As it spins faster the needle points to the appropriate speed. By calibrating the speedo (adding weight to make the needle read correctly for the RPM the magnet is spinning) the instrument is fairly accurate. The numbered wheels are a direct connection to a skew gear and register mileage traveled. Most common failures are caused by lack of lubrication (or dried up lube from time) so often a good cleaning and lube is the cure. However the bezel is crimped on so specialty tools are best used to dismantle. And a calibration machine is needed to get it all working correctly.
What's most interesting to me is that the cup is usually aluminum-usually considered to be a non-magnetic material.
But that is at rest. That is, when the magnet is not spinning(or when the cup and the magnet are spinning at the same speed), there is no attraction between the magnet and the cup.
However, when there is a speed difference between the magnet and the cup, the magnetic field moving through the aluminum actually causes an electrical current to flow in the aluminum, with the result that the cup tries to remain stationary with respect to the magnet-the spring counteracts this force, and the end result is that the faster you spin the magnet, the further the cup is moved against the force of the spring.