Join me, for a moment, in a thought experiment. Imagine that you’re holding a straw in your hands. Imagine also that on one end of the straw is a piece of tape that covers one opening. Now imagine that you’ve partially filled the straw with water and you’ve tipped it so that the water is being contained in the end of the straw covered with tape. Imagine now that you’re squeezing the straw just where the waterline starts. If you squeeze hard enough, you’ll notice that the seal formed by the tape over the straw’s opening will break, and the water will come squirting out. It’s a bit of an oversimplification to describe it this way, but this is the basic principal behind the operation of a hydraulic linear actuator.

What just happened here? What caused the seal at the end of the straw to break? Well, it’s probably clear to anyone who thinks through that thought experiment that the conclusion is simple: the accumulation of pressure caused the force of the water to accumulate to the extent that it overpowered the seal. This is correct. The water experienced overwhelming pressure from one side, and since its pushback wasn’t strong enough to prevent that side from giving way, the pushback on the other side was. After all, a little tape isn’t as resilient as your fingers. A hydraulic linear actuator is basically like a straw with a rod inside. Instead of water, some variety of hydraulic fluid is inserted, and instead of fingers, another force causes the fluid to become pressurized. When this happens, the rod within the actuator cylinder is pushed out. When the pressure is reduced, or when pressure is exerted from the other direction, the rod moves in the opposite direction. This is the simplest way to analogize the operation of hydraulic linear actuators.