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Depending on the nature of the output motion, actuators can be divided into linear and rotary. Each of these is available in different versions and configurations that help deliver the desired outcome.

Peter Jacob, Senior Director of Marketing | CNC Masters

An actuator is a machine element that converts energy into physical movements. This energy source could be electric, pneumatic, hydraulic, or mechanical forces, to name a few. You may have heard of these in tabletop CNC mills, different types of valves, or even in automated blinds! Depending on the nature of the output motion, actuators can be divided into linear and rotary. Each of these is available in different versions and configurations that help deliver the desired outcome.

Let us examine this closely in the following sections:

As the name indicates, linear actuators move an object along a straight line - usually in a back and forth manner. It uses a belt and pulley, rack and pinion, or ball screw mechanisms to convert the motion of the electric motor from rotary into linear. They are designed to traverse a fixed linear distance and then come to a stop.

The up and down motion of pistons represents linear actuator in action.

Linear actuators enjoy the following features:

High repeatability and positioning accuracy

Effortless mounting, integration, and operation

Can withstand harsh and adverse weather conditions and environments

Compact design with higher reliability and fail-proof operation

Linear actuators are usually used in applications where loads need to be pushed, pulled, lifted, lowered, positioned, or rotated. Such movements are often required in the following industries:

Rotary actuators convert energy into a rotary motion through a shaft that controls the attached equipment's speed, position, and rotation. These offer continuous rotational motor, intermittent feeding, mixing, screw clamping, turning over, and dumping actions. Since the direction of the force applied is away from the axis of rotation, the resultant motion is not restricted by the distance they travel, thereby granting them greater versatility in usage.

An electric motor is a classic example of a rotary actuator. The electric signal powers a magnetic field in the stator of the motor, and the rotor turns around in response to this input.

Rotary actuators offer the following features:

High torque even with compact configurations

Constant torque during full angle rotation

Compatibility with a variety of diameters

Hollow shaft with zero backlashes

Twice the output as holding toque is double of the forward driving torque

Any degree of rotation, from 90-degrees to full 360-degrees, is achievable

Rotary actuators find use in diverse applications where the rotary action and the corresponding torque are used to facilitate action. Resultantly, rotary actuators find use in:

One of the clearest distinctions between linear and rotary actuators is the direction of movement. Linear actuators move in a straight line, usually in a back and forth motion. In contrast, rotary actuators move in angular degrees with reference to a mid-point, that is, along a circle.

However, it is worth pointing out herein that in their simplest form, linear actuators are an extension of rotary actuators. These contain an additional motion converter that translates rotational motion to linear motion. As stated above, ball screws, roller screws, belts and pulleys, rack and pinion convert rotary movement into its linear counterparts.

A few other differences between linear and rotary actuators are outlined below:

A linear actuator delivers a push-pull motion, while a rotary actuator helps with a rolling motion.

For linear actuators, the end of arm tooling of the CNC mill slides between two points on a rule-like line. As for rotary actuators, the end of arm tooling goes in circles.

Linear actuators consist of a motor and a gear axle thread. Rotary actuators contain hollow, cylindrical chambers with stationary barriers and a central shaft.

The output motion of linear actuators is in line with the output shaft. As for the rotary actuators, the output motion is perpendicular to the shaft. 

Since obtaining linear motion is one of the commonest design functions, linear actuators have a compact build and design. 

Rotary actuators follow an angular path, and the distance traveled can be infinite and repeated for continuous spinning action. In contrast, linear actuators only travel a limited distance and come to a standstill.

The use of motion conversion for linear actuators affects the bandwidth and responsiveness of the setup. This is because some amount of tradeoff takes place during the translation.

Mounting of linear actuators is easier as one does not have to calculate the rotation angle as in the case of rotary actuators.

Now that you understand the difference and impact of the two, you can make a smarter decision while choosing one from the other. In some instances, you may find a combination of linear and rotary actuators. For instance, the drill bit in a tabletop milling machine offers top-down movement while the drill is under rotary motion.

The challenge and the proposed solution ultimately govern your choice of the actuator.

Peter Jacobs is the Senior Director of Marketing at CNC Masters. He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general.

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