If an ‘L’-shaped lever is pivoted at its centre, the direction of the input movement or force would be turned through 90° at the output. This linkage is known as a Bell Crank (so called because it was used in Victorian times in linkages used to operate doorbells and servants' bells). They can be used to change the angle of motion through any angle but 90° is common.
A common device which uses this mechanism is the brake caliper on a bicycle. Here the force from the handlebar lever is turned through 90° to squeeze the brake block against the wheel rim.
If the fulcrum or pivot is at an equal distance from the input and output then the movement of the output will be equal to the movement of the input. Otherwise the movement will be different and the system will have some degree of Mechanical Advantage.
|90° Bell Crank||Robot Gripper - a hydraulic or pneumatic cylinder opens and closes the jaws, permitting it to grasp and release objects. The blue jaw is a Bell Crank which, together with the green con-rod and pink slider, creates a slider crank mechanism.||Toggle Clamp - the adjustable clamp head is held on a Bell Crank which, together with green con-rod and violet lever, form a four bar linkage coming into toggle as the green handle of the con-rod is pushed down.|
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