Shaft A of the tachometer is driven by the shaft whose speed is to be measured. The centrifugal force of weights 1 move piston 2 to the left in cylinder 3 which is filled with compressed air. The air is delivered into the cylinder by pump 4. The higher the angular velocity of the shaft being tested, the more piston 2 uncovers ports a of cylinder 3 and the less, consequently, the pressure in the cylinder and pressure gauge tube 5. Thus, any variation in the angular velocity of the shaft being tested is indicated on a scale by hand 6, which turns about fixed axis B.
$4054$EHP,M$