A cylinder is one of the devices that create movement. When pressure is
applied to a port it causes that side of the cylinder to fill with fluid. If the
fluid pressure and area of the cylinder are greater than the load that is
attached, then the load will move. If the pressure remains constant a larger
diameter cylinder will provide more force because it has more surface area
for the pressure to act on. Cylinders are sometimes called linear actuators or
linear motors.
CLASSIFICATION OF CYLINDERS
There are many ways of classifying cylinders but, broadly, cylinders or linear actuators can be classified on the basis of:
1.Construction
2.Working fluid
CLASSIFICATION OF CYLINDERS ON THE
BASIS OF CONSTRUCTION
Depending on the construction, cylinders can be classified as:
1. Single-Acting Cylinders
2. Double-Acting Cylinders
The basic difference between the two single cylinders is that single acting cylinders can be pressurized from one end, while the double acting cylinders can be pressurized from both ends.
SINGLE-ACTING CYLINDER
A single-acting cylinder is shown in Figure 6.3 (a). The cylinder is only powered in one direction and needs another force to return it such as an external load or a spring. This cylinder only has a head-end port and is
operated hydraulically/pneumatically in one direction. When pressurized fluid is send into a port, it pushes the plunger/piston rod thus extending it. To return or retract a cylinder, fluid must be exhausted out. A plunger returns
either because of the weight of a load or from some mechanical force such as a spring. In mobile equipment, flow to and from a single-acting cylinder is controlled by a reversing directional valve of a single-acting type. This type of cylinder is commonly used for jacks and lifts. Figure 6.3 (b) shows the cut
section view of a spring returned pneumatic single-acting cylinder
Double-acting cylinders are the most commonly used cylinders in hydraulic and pneumatic applications. In this design, pressure can be applied to either side of the cylinder. The symbol for double-acting cylinder shows two ports where fluid pressure can be applied as shown in Figure 6.4. The
extension and retraction of cylinder is due to hydraulic/pneumatic pressure.
A cylinder is constructed of a barrel or tube, a piston and rod (or ram), two end caps, and suitable oil seals. A barrel is usually seamless steel tubing, or cast, and the interior is finished very true and smoothly. The moving part inside the cylinder body is highly polished; chrome plated steel piston rod and the solid cast iron piston assembly. The body and the heads are held together by steel tie rods and nuts. The heads on either end contain ports for fluid flow. It is supported in the end cap by a polished surface. Seals and wipers are installed in the rod’s end cap to keep the rod clean and to prevent external leakage around the rod. Other points where seals are used are at the end cap and joints and between the piston and barrel. Mounting provisions often are made in the end caps, including flanges for stationary mounting or clevises for swinging mounts. Internal leakage should not occur past a piston. It wastes energy and can stop a load by a hydrostatic lock (oil trapped behind a piston).
Figure 6.6 shows the cut section view of a double-acting hydraulic
cylinder.
