Most piston-type actuating cylinders are double-acting, which means that fluid under pressure can be applied to either side of the piston to apply force and provide movement.
One design of the double-acting cylinder is shown in figure 10-6. This cylinder contains one piston and piston rod assembly. The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder, alternate as inlet and outlet ports, depending on the direction of flow from the directional control valve. This actuator (fig. 10-6) is referred to as an unbalanced actuating cylinder because there is a difference in the effective working areas on the two sides of the piston. Therefore, this type of cylinder is normally installed so that the blank side of the piston carries the greater load; that is, the cylinder carries the greater load during the piston rod extension stroke.
A four-way directional control valve is normally used to control the operation of this type of cylinder. The valve can be positioned to direct fluid under pressure to either end of the cylinder and allow the displaced fluid to flow from the
opposite end of the cylinder through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems.
There are applications where it is necessary to move two mechanisms at the same time. In this case, double-acting piston-type actuating cylinders of different designs are required. See figures 10-7 and 10-8.
Figure 10-7 shows a three-port, double-acting piston-type actuating cylinder. This actuator contains two pistons and piston rod assemblies. Fluid is directed through port A by a four-way directional control valve and moves the pistons outward, thus moving the mechanisms attached to the pistons’ rods. The fluid on the rod side of each piston is forced out of the cylinder through ports B and C, which are connected by a common line to the directional control valve. The displaced fluid then flows through the control valve to the return line or to the atmosphere.
When fluid under pressure is directed into the cylinder through ports B and C, the two pistons move inward, also moving the mechanisms attached to them. Fluid between the two pistons is free to flow from the cylinder through port A and through the control valve to the return line or to the atmosphere.
The actuating cylinder shown in figure 10-8 is a double-acting balanced type. The piston rod extends through the piston and out through both ends of the cylinder. One or both ends of the piston rod may be attached to a mechanism to be operated. In either case, the cylinder provides equal areas on each side of the piston. Therefore, the same amount of fluid and force is used to move the piston a certain distance in either direction.