An Introduction To Actuators: Part II

An Introduction To Actuators: Part II

Gregory Kimbell
Gregory Kimbell
PA Engineer
Learn everything you need to know about linear actuators.

Last time we talked about the basic definition and uses of simple actuators, and introduced the concept of the electric linear actuator (screw based). To review, a linear actuator is called such because it transforms energy into a linear direction; that is, straight back and forth. This is valuable because most actuators derive their motive force from a rotational source, such as a gear, a pulley, or a wheel and axle.


By taking rotational motion and converting it into linear motion, a linear actuator efficiently and predictably puts a lot of force into a relatively small area. This is important because the majority of uses of actuators (automobiles, robotics, electronics, etc.) put a premium on both space and weight. Pneumatic and hydraulic actuators (where the force is created by pressure differentials in a liquid or gas) are powerful and varied, but they tend to take more space than an electric linear actuator of the same force/rating.

To give you a better understanding of how these electric devices work, let's take a look at the anatomy of one:

Electric Motor

The electric motor serves as the power source for the actuator, in this case usually an electric 12 volt DC motor. The energy is transmitted via a belt to:

Gears
The gear turns from the motor above, and in turn rotates the screw or bolt

Screw/Bolt
This is where the magic happens. The screw is usually the biggest piece of the actuator. The screw turns from the force of the electric motor and forces itself through a nut. This motion pushes the screw forward, extending it.

Internal Seal
The internal seal prevents dust, liquids, and other matter from getting inside the actuator and gumming up the screw, nut, or gears.

Rod or Plunger
This is where the actuator interacts with the outside world. Nearly anything can be connected to the plunger; a rod, a wire, a valve, a button. The rod is what manipulates an external object, and harvests the force provided by the actuator.

A note about electric motors: All ProgressiveAutomations electric linear actuators have a built in limit switch at the end of each extension (e.g. fully retracted and fully extended). This turns off the motor (without disabling the power) automatically when the actuator is 'locked' at full extension. This prevents the motor from over-running (extending the life of the actuator) and saves on electricity consumed.

Now that you know the parts, you should know what strengths and kinds of linear actuator are available. There are four main criteria to consider when classifying an actuator:

Stroke

The Stroke is the amount of distance the screw can travel before it is fully extended; usually measured in inches. Small actuators can have a stroke of 1 or 2 inches, but it can go up to 12 inches, 24 inches, or even more for custom actuators. There is a trade off though, between stroke length, speed, and force.

Speed

The Speed is how fast the screw travels through the nut, or how quickly the rod/plunger 'moves'. This can vary depending on the application. Actuators that have to push a lot of force are generally slower (e.g. 0.24 inches per second) while lighter weight actuators can be much faster (e.g. anywhere from 1 inch to 9 inches per second).

Force

The Force (or push/pull force) of an actuator is how much force it outputs at the rod/plunger end, and is usually measured in pounds. This is basically how hard a linear actuator can 'push' something. Force varies between models, and can be as low as 15 pounds going all the way up to 2000 pounds or more for certain heavy duty industrial applications.

Lock-Up Force

This is similar to Force, but refers instead to how much weight the actuator can support by itself before the screw is pushed back in. So an actuator may be able 'push' 100 pounds of force, but can only support the weight of a 50 pound object directly resting on it. This is usually measured as the amount of pounds the actuator can support when not in motion, e.g. in 'locked' mode. This also varies from model to model but at minimum is usually the same as the Push/Pull force, and in some cases can support four times as much as weight.

So now you know a bit more about how linear actuators are put together, and what attributes define them. One thing to keep in mind is that the Stroke, Force, Speed, and Weight Capacity all influence each other, and can cause diminishing returns in one area depending on the value of another. For example, an actuator with a high Force/Weight Capacity generally doesn't have a high speed; this is because the nut and screw have to be designed to exert the maximum amount of force, and speed is usually what is sacrificed. This doesn't mean that heavy load, high speed actuators don't exist (ask us about them in our custom orders section), but rather they're not commonly used.

So ends Part II of our Introduction to Actuators. Next time, we'll go into what other parts you'll need to actually fully utilize an actuator in your project or work. Remember, if you missed Part I you can read it here.

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