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?
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 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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