Why Control the End of the Stroke?

In the previous content pages, we talked about how to control actuator speed. We can use different pressures and different valves, like a flow control valve, a needle valve, or a quick exhaust valve. We can also control the end of the stroke of a cylinder, either the return stroke or the extend stroke, with devices called cylinder cushions.

A machine operates, and a cylinder extends and retracts with a lot of speed and velocity. When that cylinder reaches the end of its stroke, the piston in the cylinder can have the tendency to slam as the piston bottoms out. When that piston slams against the end of the cylinder, it could have the potential to harm the cylinder body, the bracketing that the cylinder is attached to, or the machine that the cylinder is working on. All the jarring that occurs when the piston bottoms out can send unwanted vibrations throughout the machine that can cause damage to the operation of that machine. To minimize this, we can buy cylinders that have cushions in them. See the picture below.

As you can see in the picture, there are pre-installed flow control valves on each end of this double-cushioned cylinder. As the cylinder extends or retracts and as the cylinder reaches the last inch of its cycle, the cushioning piston blocks off the normal escape route of the flow (the work ports of the cylinder). It then forces the air to escape through the flow control valve port. This flow control valve allows you to adjust how much air escapes at a certain rate. This causes system friction, which slows down the last inch or so of the stroke of the cylinder. Cushions are critical in applications where the cylinders have a high speed of operation or the actuator load can cause the cylinders to move at an increased speed.

You will learn how to use a cushioned cylinder in Trainer Lab #12.

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