What is the purpose of a bell crank linkage?

The purpose of a bell crank linkage is to change the direction of movement through 90°. This mechanism is specifically designed to redirect motion, allowing an input force applied in one direction to create an output force that moves at a right angle to the input. This characteristic is particularly useful in various applications, such as in mechanical systems where the available space or design requires the redirection of linear or rotary motion.

When considering the other options, while it is true that a bell crank can be involved in systems that change motion or provide some mechanical advantage, its primary function is focused on the direction change. For example, converting rotational movement to linear is more characteristic of other mechanisms, such as a cam or a lever, and increasing speed or reducing effort typically involves different setups that provide mechanical advantage or gear ratios rather than the basic operation of a bell crank linkage.