Cylinders
Introduction
Cylinders are linear actuators. The cylinder outputs are straight line motion or force. Two of the most common types are single acting cylinders and double acting cylinders.
Effective Area of a Cylinder
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| Illustration 1 | g01064834 |
The bore size of the cylinder refers to the inside diameter of the cylinder. A cylinder with a large bore size creates a larger volume per unit of length than a cylinder with a small bore size. The large bore size cylinder uses more oil to move the piston the same distance than the small bore size cylinder. Therefore, for a given flow rate, a large bore size cylinder will move slower than a small bore size cylinder.
The effective area of a cylinder is the surface area of the piston and piston seal, upon which the oil pushes. Because one end of the rod attaches to the piston and the opposite end extends out of the cylinder, the rod end effective area is less than the head end effective area. The oil can not push against the area of the piston that is covered by the rod.
The volume of oil that is needed to fill the rod end of the cylinder is less than the volume of oil that is needed to fill the head end of the cylinder. Therefore, the cylinder rod retracts faster than the cylinder rod will extend for a given flow rate.
Seals
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| Illustration 2 | g01064835 |
Seals are used in various places throughout the cylinder, as shown in Illustration 2.
The piston seal is used between the piston and the cylinder wall. Oil pressure spreads the seal against the cylinder wall. The greater the pressure is the greater is the sealing force.
The head seal (O ring) prevents oil from escaping between the rod gland and the cylinder wall.
The rod seal is a U shaped seal or a U-cup shaped seal that prevents oil from escaping between the rod and the rod gland. The rod seal wipes the oil off the rod as the rod is extended from the cylinder.
The wiper seal is fitted to the cylinder and prevents dirt or grit from being drawn into the cylinder as the cylinder rod retracts.
Seals are made of polyurethane, nitrile, or viton. The material should be verified in order to be compatible with the fluid that is used and the operating conditions.
Cylinders Equipped with Snubbers (Dampers)
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| Illustration 3 | g01064836 |
Illustration 3 shows a cylinder that is equipped with snubbers (dampers).
When a moving cylinder runs into a dead end, the concussion is called shock loading. When cylinders are subject to shock loading, snubbers are used to minimize the effect.
When the piston approaches full stroke, the snubber moves into the return oil. Then, the snubber restricts the return oil flow from the cylinder. The restriction causes an increase in the return oil pressure between the return oil passage and the piston. The increase in oil pressure provides a cushioning effect which slows the piston. This minimizes the shock that occurs at full stroke.
Some cylinders may require a head end snubber. Some may require a rod end snubber. Others may require both head end snubbers and rod end snubbers.