Intro to Linear Actuators

What are Linear Actuators?

A linear actuator is a device that uses a form of energy or power to create motion in a straight line. It can be found everywhere from construction equipment to ailerons on aircraft to a store counter. For example, it could be used to raise and lower the arm of an excavator!

What type of Linear Actuators does CAT specialize in?

There are many different types of linear actuators (all with unique energy sources) including electro-mechanical, mechanical, hydraulic (liquid), and pneumatic (gas) linear actuators. At CAT, we focus on hydraulic linear actuators. A hydraulic linear actuator is a type of linear actuator that uses incompressible fluids (fluids whose density remains constant during flow) from a pump to move a piston in a linear motion.

How does a Hydraulic Linear Actuator work?

Figure 1: Hydraulic Linear Actuators in CAT Excavators

Most hydraulic linear actuators utilize a piston-cylinder configuration. This means that inside a cylinder (or piston chamber), there is a piston, piston rod, and inlets on both sides for an incompressible liquid to enter. (Figure 1)

Figure 1: A Hydraulic Linear Actuator

Extending the Arm

To extend the linear actuator, an incompressible liquid from an external pump enters the cylinder through the fluid inlet and becomes pressurized. Since force is the product of pressure and the cross-sectional area, greater pressure translates into greater applied force (as the cross-sectional area is constant). Thus, the induced pressure from this added liquid applies a force onto the piston and the connected rod, pushing both components forward in a straight line. (See Figure 2)

Figure 2: A Hydraulic Linear Actuator Being Extended.

The key to the operation of a hydraulic linear actuator is pressure. The unbalanced pressure from the fluid entering one side of the cylinder applies a force on the linear actuator's inner piston, pushing it forward and forcing it to move. The speed at which the piston extends can also be adjusted by altering the flow rate of the fluid.

Retracting the Arm

The piston can be retracted back to its original position by sucking out the fluid that was originally added, having a spring push the piston backwards (see figure 1), or supplying fluid on the opposite side (see figure 3).

Figure 3: A Hydraulic Linear Actuator Being Retracted Back to its Resting Position.

In this case, the pressure on the fluid on the left side of the piston is released, causing the fluid to return back to the reservoir. At the same time, fluid is being supplied to the opposite side of the piston, increasing the pressure on that side and causing a force to be exerted towards the resting position of the piston. In addition, for a hydraulic linear actuator utilizing a spring (see figure 1), the spring mechanism in the piston chamber expands due to the release of pressure that once held the rod in its extended position. These forces move the piston towards the left (in this example), forcing the piston back into its original position. Overall, hydraulic linear actuators use unbalanced pressures in fluids to move the piston, exert force, and create linear displacement.

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