Basic Seal Knowledge of Hydraulic Cylinders

Many people would think that the purpose of the sealing ring on the piston is to prevent the piston rod from falling (when the cylinder is placed vertically). This idea is wrong.

First of all, we shall understand the basic structure of the typical hydraulic cylinder, mainly composed of the following four parts:
Cylinder Tube
Cylinder Rod
Piston and Piston Seal

hydraulic cylinder standard structure

The hydraulic cylinder produces linear motion. It consists of a cylinder block and a piston rod which reciprocates in and out of the tube. The cylinder length limits the piston movement range.

The cylinder tube is divided into two chambers by piston seal: rod cavity and rodless cavity. If the oil falls into the rodless chamber, the piston rod will be extended. The piston rod will recoil when oil pours into the rod chamber.

Remember one thing, I choose hydraulic oil as the working medium of the hydraulic system, mainly because the hydraulic oil is approximately incompressible. (In fact, hydraulic oil under high pressure will have a certain amount of compress, but it is too tiny to notice it). The piston and piston rods indeed occupy only a portion of the barrel—and other parts are filled with hydraulic fluid.
Imagine that we push the piston and piston rod into the tube full of hydraulic oil, there will be a volume of oil overflow the cylinder. As shown in the following figure:

Hydraulic push

Since the total cylinder tube capacity is fixed, the oil must flow out of one chamber, as you continue to fill the other. 
For an example, if I plug up both ports of a hydraulic cylinder, the piston will not be able to move down even if a heavy object is placed on the piston rod (the pressure is within the pressure range of the cylinder) :

So what role does the piston seal ring play?
Through the above study, we learned that the only way to make the piston rod move is to provide a means for the oil to flow into and out of the hydraulic cylinder. The role of the sealing ring is to ensure a constant moving speed of the piston.
Why do you say that? Without seal rings, some hydraulic oil will bypass the piston through the gap, and flow to the other side of the chamber. As a result, the flow to push the piston is reduced, and the piston’s speed must not be constant.

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