The speedometer, or speed meter, is an instrument used to measure and display the instantaneous speed of a vehicle. It was first introduced in the early 1900s as an optional device and became standard equipment around 1910. Today, speedometers are universally installed in automobiles, motorcycles, and adapted for other vehicles such as boats (pit log) and aircraft (airspeed indicator).
Magnets play a key role in the functioning of traditional analog speedometers, making them accurate, durable, and reliable.
How Does a Magnet Work in a Speedometer?
In mechanical speedometers, a permanent magnet is attached to the rotating speedometer cable, which is driven by the transmission. As the vehicle moves, the magnet rotates near an aluminum cup, also called a speedcup.
- The rotating magnet generates a changing magnetic field, which induces eddy currents in the aluminum cup.
- These eddy currents create a secondary magnetic field, exerting torque on the cup and causing it to rotate.
- The cup is connected to the pointer shaft, which moves across the dial to indicate speed.
To ensure accuracy, a torsion spring resists the torque. The balance between the spring’s resistance and the magnetic drag ensures that the pointer’s deflection is directly proportional to vehicle speed.
This principle allows a speedometer to measure speed without direct mechanical contact, reducing wear and improving longevity.
Why Use Alnico Magnets in Speedometers?
Alnico magnets (mainly Alnico 2 or Alnico 5) are widely used in analog speedometers due to their:
- High thermal stability: Reliable performance in varying engine and environmental temperatures.
- Strong magnetic properties: Ensures consistent eddy current generation.
- Long-lasting magnetism: Minimal loss of magnetic field strength over time.
- Machinability: Can be manufactured in precise shapes and sizes required for speedometer assemblies.
Common Specifications of Alnico Magnets in Speedometers
- 23 × 13.5 × 3 mm without injection molding
- 2.3 × 6 × (3.5–4) mm with 3, 4, or 6 molding holes
- 23 × 6 × 3.47 mm
These sizes are optimized for compact instrument assemblies and can be customized based on OEM requirements.
Advantages of Magnetic Speedometers
- Durability – fewer mechanical contacts reduce wear and tear.
- Accuracy – proportional movement ensures precise readings.
- Reliability – stable magnetism guarantees consistent performance.
- Cost-effectiveness – simple, proven design widely used in the automotive industry.
Conclusion
Magnets, particularly Alnico magnets, are the backbone of traditional analog speedometers. Their role in generating eddy currents and translating rotational speed into accurate readings ensures that speedometers remain both precise and durable. For automotive manufacturers and instrument suppliers, selecting the right magnet specifications is essential to maintaining performance and reliability.
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