This self-contained unit contains a bell crank lever pivoted around a pillar. Different loading points allows the variation in length of mechanical arm, hence, varying the mechanical advantage. A spring balance mounted horizontally at the base of the bell crank lever measures the load transferred. The bell crank lever offers the typical mechanical advantage of a lever, and in addition it turns the line of action of the effort through 90°.

The model is built with acrylic to simulate real crank. This traditional item enables the reaction force of a 90° bell crank to be measured by a spring balance when a load is applied at one of five leverage ratios.