Speedos & Tachs

[Rex Piekarczyk]

How does a mechanicaly driven speedometer or tachometer convert a rotational input into a static needle position?Magnets & springs?

[s100]

Rex,

You are right, the basic idea is that the driving cable spins a little magnetic rotor. The needle is attached to a shaft, coaxial to the rotor shaft, that has a couple of steel poles on it. The poles are in close proximity to the magnets. When the magnets turn, the steel poles are attracted to the magnets and want to follow them around. They can't, though, because the magnetic force is too weak. A small clockspring attached to the needle shaft serves two functions, one, to increase the resistance the needle encounters when trying to follow the magnets around and two, to dampen shocks. If you have ever seen an old car with the speedometer needle bouncing all over the place, the likely cause is a broken clockspring.

Hope this helps,

Steve

[Rex Piekarczyk]

Yes it does help. I had a '62 Chevy C10 that had a bouncing speedo that turned out to be a dry cable. Same with my brother's Royal Enfield motorcycle. Better that than a busted spring.
What I'd like to do is make a period correct looking tachometer for my Economy 5E tractor project. I've found a few that would look right, but are scaled @ 0-8000 rpm with the dial face showing 0-80 or 0-8. Might be easier to increase the drive speed x10 to indicate the 400 to 600 rpm of this engine.

[K D Redd]

The metal that is close to the magnet IS NOT steel but aluminum. If it were steel the neddle would slam the end of the gauge. When the magenets are rotated near a conducter, the aluminum, there is a voltage generated. when you have a voltage generated in the aluminum you will have current flow in the aluminum. When you have current flow, you magnetic field. Then you have one magnetic field atracting the other and the needle on the speedo or mechanical tach will move up smoothly. The faster the cable turns the attached magenet the more current is induced in the aluminum and the higher the needle moves.

Kent

[Rex Piekarczyk]

That sounds like an eddy current clutch. Rotating magnets in close proximity to thick copper disks. Eddy currents induced by the magnets in the copper disks caused the disks to spin the load. No physical contact between the drive plates and driven plates.