Actuators can be classified into two types based on their motion. Two primary actuator types are rotary and linear. Rotary actuators will turn something a number of degrees in a circle – it might be a limited number or an infinite number.  Linear actuators, in essence, move something along a straight line, usually back and forth.

In the pure sense, an example of a rotary actuator is an electric motor, which is an actuator that converts an electrical signal into a rotating motion of its shaft. When current is applied to a basic motor, the motor rotates.  Connect a motor directly to a load – you create a direct drive rotary actuator.  Many rotary actuators are combined with mechanisms that are used as mechanical levers (an advantage) to decrease rotational speed and increase torque.  The output of this assembly is still a rotary actuator if the end result is rotational.

On the other hand, rotary actuators also connect to mechanisms that translate the rotational motion to a back and forth motion and are called – linear actuators.

What are these mechanisms I’m talking about? Such as ball screws, belt and pulleys, rack and pinions. All these mechanisms are mechanical devices that translate energy in some way. Ball screws and roller screws are typically used to translate rotary motion into a precision linear motion, such as on machining centers. Belts and pulleys can be used to translate the rotary motion of a motor into linear motion (e.g. conveyor).  Rack and pinions are typically multiplying torque, reducing the speed of a rotary motion – but also can be used in combination with a mechanism that will translate the rotary motion into linear motion. 

One word of caution – each time some translation of motion occurs between multiple bodies, an element of compliance is introduced, which affects the ability of that actuator to respond quickly – to achieve high bandwidth. Hence getting as close to the load as possible, such as a stepper motor or linear stepper motorsolution typically provides the highest bandwidth system.