Re: 300U Tachometer Problems

Posted by Janicholson on July 29, 2006 at 15:49:35 from (66.173.50.148):

In Reply to: Re: 300U Tachometer Problems posted by Kerwin on July 29, 2006 at 12:32:28:

There is a cup shaped rotor made from aluminum in the middle of the guts of the beast. This cup is driven around and around by the tach shaft. there is a bearing controlling the location (accurately) of this cup. The cup is a eddy current driver. The needle is connected to a hair spring just behind the face plate. Under this hair spring there is a magnet that sits inside the cup. This magnet is located by a small bearing in the inside cf the cup. This, and the cup bearing need a light synthetic lube Found at good hardware stores, and other places, ___not 3-n-1 or other plain petroleum oil___ . Lube sparingly and put it back together.
The magnet sets up a field in the cup. This field is intercepted by the cup. when the cup turns, the field moves in the cup setting up what are called eddy currents in the metal of the cup. These currents oppose the magnetism in the magnet, causing the magnet to be dragged in the direction of the cup's rotation. The faster the rotation, the more current, and the greater the drag on the magnet. This rotation on the magnet is opposed by the hair spring, and the opposition is what causes the needle to point to the correct rpm. I hope this helps, JimN

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