This is a study in contrasts. In a vertical row on the left, parts of a British Smith's tachometer. The vertical row on the right shows the similar parts of a U.S. Stewart-Warner tachometer. Both are cable driven. The Smith's unit used what was called a "chronometric" internal mechanism to motivate the needle around the dial. The S-W unit used a common, and very simple, rotating magnet to move the needle. The Smith's tach was built around 1950, before the Brits had discovered magnetism, I think; the S-W unit around 1960.
You want contrasts, here's more. The Smith's tach [left] has a brass face and cast aluminum frame. The S-W tach a thin steel face and plastic frame. But the BIG contrast is in the internal gubbins. Just look at all that "stuff" in the Smith's unit. Levers and gears and balance arms and a couple of crude camshafts, and some other items identifiable only if you live in the outskirts of Coventry, England. The S-W uses, simply, a rotating magnet in sort of a modified half cup shape that rotates around the round "can" that is connected to the speed needle.
There was at least SOME justification for Smith's use of the monkey-motion mechanism. First they used the chronometric jazz in their speedometers. If you look closely at the frame of the Smith's unit you will see holes in the legs of the frame where--if this were used in a speedometer--the odometer registers would be fitted. Take out the odometer gubbins and Presto! Instant tachometer! Clever, aye, mate?
Well....yes and no. There is this thing called complexity. Murphy's law says that the more "stuff" in a unit, the more apt the unit is to screw up. When I received the Smith's tach, the needle would go to maybe 15 [x100] rpm and stop. It looked like it had been Arabia with Sir Lawrence [who wasn't a "Sir" then] and needed TWO baths in the ultrasonic unit with clock cleaning fluid. Then a rinse, then careful oiling with clock oil. I didn't have high hopes for the sucker.
But I put it on the test bed and started the [anti-clockwise] rotation and th' blinkin' blighter started working! Chronometric tachs sort of step up, in about 500 rpm jerks. This one settled down at 6800 rpm, steady as a Spitfire in a dive after an ME-109. When the input rpm is slowed to a stop, the chrono tach backs down in about 2500 rpm steps.
Amazing what can happen when you get 65 years of dirt, dust and sand out of all of the monkey-motion bits in one of these critters.
But I look forward to peering into the bowels of a Smith's tach after they discovered magnetism....that happened in about 1990, if memory serves me correctly....
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