If the shaft whose speed is being measured rotates counterclockwise, the tachometer shaft with axis A is linked to the shaft being tested. If the tested shaft rotates clockwise, it is linked to the tachometer shaft with axis D and rotation is transmitted through gears 1 and 2 to impeller 3. Thus, impeller 3 always rotates counterclockwise. The vanes of impeller 3 deliver air in the direction of the tangential outlet of housing 4, lifting shutter 5 and overcoming the resistance of flat spring 6. The pressure of the air against shutter 5 and the angle through which it is turned depend upon the angular velocity of the shaft being tested. The shaft speed is indicated by the hand of an instrument linked to shutter 5.
$4017$THP,M$