How Hydraulic Cylinders Work

Hydraulic cylinders are remarkably simple, nothing more than a steel tube & rod and other bits holding it all together, but so incredibly powerful. They are truly the workhorse of our world, provide the muscle that moves & forms the earth & structures around us. But do you know how hydraulic cylinders work? How does something so simple & relatively small do such an amazing amount of work?
Hydraulic Cylinders

It’s an old principle. The amount of pressure exerted on anything increases in force as the area of the object increases. If you put 1 pound of pressure on a 1 inch object, you get 1 pound of pressure. If you put 1 pound of pressure on 2 inches, you get 2 pounds per square inch. The formula for this is Area X PSI (Pounds per square inch) = Force.
So, the larger the cylinder piston, the more lifting (or pushing) it can do.

The PISTON is the part inside the cylinder that the fluid pushes against. The diameter of the cylinder piston is called the BORE. The larger bore cylinders have more force exerted upon them, therefore a higher lifting capacity. The hydraulic fluid is contained by the piston seal. That’s why a cylinder with a defective piston seal will not lift as much as it should. Even though the cylinder may not be leaking on the outside, a damaged piston seal will allow oil to bypass the piston, so the pressure will not reach the required level to do the lifting that is expected.

The ROD (or shaft) of the cylinder is the part that travels through the GLAND (or head) of the cylinder and attaches the piston to the end fitting (usually a clevis, cross-tube or tang). The diameter & hardness of the rod are important because the further out it is extended, the more “side-load” is exerted on it, increasing the risk of bending. That’s why higher pressure cylinders have stronger rods, so that if they are lifting a heavier load they are less likely to bend. This is commonly know in the industry as COLUMN LOAD. Welded hydraulic cylinders typically have an “induction-hardened” rod, which is harder to bend.

The STROKE is the difference fully retracted length and the fully extended length of the rod. This is the total travel of the cylinder.
Hydraulic Cylinders

The GLAND (or head) of the cylinder is part of the cylinder that the piston rod travels through. The rod seal is contained inside the gland and is the most common cause of cylinder leaks, since it is exposed to the elements and is in charge of removing debris from the rod as it retracts into the body of the cylinder.

The BUTT is the base (or cap) of the cylinder. On tie-rod cylinders it is a separate piece that also has an o-ring seal, which is a point of potential leaks. On welded cylinders, it is welded to the hydraulic cylinder tube, so no seal is required.

Some cylinders are made single-acting (push under pressure, gravity return), but most are double-acting which means that the piston is under pressure on both the push and pull side of the cycle. Double-acting cylinders can be easily used in single-acting applications, because a breather fitting can be fitted in the unused port to allow air to be displaced on that side.

So, that’s how it works in a nutshell. It’s remarkably simple for a device that does such an amazing amount of work.

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Any Questions? We’ll be here all week…

  1. Yeah your right, “provide the muscle that moves & forms the earth & structures around us hydraulic”. Without the help of hydraulic cylinders, machine will not work.

  2. “Side-Load,” as you have called it, and “Column Load,” are not the same. A column load is applied axially and induces the condition known as buckling. A bending load (side-load) is an off axis load which causes pure bending. Most cylinder rods are only designed for column load, which is why most rod failures are due to the superposition of a column and a bending load. Furthermore, whether or not a cylinder is welded or has tie-rod ends should be independent of whether or not the rod is induction hardened. Apologies, I do realize this article was intended as an introduction, not a case study on rod design.

  3. I appreciate you explaining how hydraulic cylinders work. It does make sense that the larger the cylinder the more pressure there will be, but I am still having a hard time getting my head around it. I keep thinking that even a narrow tube should be able to get the same amount of force, because the hydraulic power is based on the amount of pressure and not the size. So, could you explain in a little more detail why having a bigger bore cylinder is better, rather why there is more force exerted on them?

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