Rotation is transmitted from the shaft whose speed is being measured through gear 1 to shaft 2 on which weight 3 is pivoted. When shaft 2 rotates, weight 3 tends to turn to the position perpendicular to the axis of the tachometer. This distorts flat spring 4 which moves sleeve 5, turning segment gear 6 and hand 7 to indicate the angular velocity of the shaft being tested.
$2531$LrG,M$