CYLINDERS

Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically oil. The cylinder consists of a cylinder barrel, in which a piston connected to a piston rod is moving. The barrel is closed by the cylinder bottom and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber and the piston rod side chamber. The hydraulic pressure acts on the piston to do linear work.

A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of the hydraulic system is the hydraulic pump, that brings a fixed or regulated flow of oil into the system. Mounting brackets or clevisses are mounted to the cylinder bottom as well as the piston rod.

By pumping hydraulic oil to the bottom side of the hydraulic cylinder, the piston rod starts moving upward. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is zero, the force on the piston rod equals the pressure in the cylinder times the piston area. If the oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure, the pressure in the piston rod area chamber is Pull Force/(piston area - piston rod area). In this way the hydraulic cylinder can both push and pull.

Parts of a hydraulic cylinder

A hydraulic cylinder consists out of following parts:

Cylinder barrel

The cylinder barrel is mostly a seamless thick walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally.

Cylinder bottom

In most hydraulic cylinders, the barrel and the bottom are welded together. This can damage the inside of the barrel. Therefore it is better to have a screwed or flanged connection. In that case also the barrelpipe can be maintained and/or repaired in future.

Cylinder head

The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general however the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.

Other parts

• Piston
• Cylinder bottom connection
• Piston rod connection
• (sometimes) Feet for mounting of the barrel

A hydraulic cylinder should be used for pushing and pulling and no bending moments should be transmitted to the cylinder. For this reason, the ideal connection of a hydraulic cylinder is a single clevis with a ball bearing.