Cylinders, fluid power motors, and rotary actuators

Fluid power actuators

Fluid power actuators receive fluid from a pump (typically driven by an electric motor). After the fluid has been pressure, flow, and directionally controlled, the actuator converts its energy into rotary or linear motion to do useful work. Cylinders account for more than 90% of the actuators used in fluid power systems for work output. Of the approximately 10% of actuators that produce rotary output, more than 90% are hydraulic motors, while the rest are some form of rotary actuator.

Single-acting ram cylinders

The symbols and cutaway views in Figure 15-1 show single-acting ram cylinders in push and pull types. Rams can be as small and simple as a service station lift operated by air over oil, or as big and complex as a 100,000-ton extrusion press.

Single-acting rams often are mounted vertically up and are weight returned. When a ram cylinder is mounted vertically down or horizontally, it must have some method of retracting it to the home position. Figure 15-1 shows one method. Small single-acting pull rams -- mounted alongside the large working ram -- raise and hold it in the up position with a counterbalance valve (not shown). A directional valve or a bi-directional pump directs fluid to the push or pull rams to make them cycle. Another retraction method uses single-acting push rams that oppose the platen movement from the opposite side. (For a circuit that uses a large-diameter vertical down acting ram cylinder, see Figure 10-9.) Small ram cylinders may be returned manually or via a spring.

Ram cylinders only have seals where the ram passes through the body. Anytime a ram cylinder drifts from its stopped position, the cause is valve or pipe leakage if no fluid is coming out around the ram seal.

As the ram moves, stops and guide protrusions on it keep it aligned and indicate maximum stroke. Usually on large-area rams, the stops tear off the packing gland and bushing retainers if the ram is not stopped some other way. Most machines using rams have other methods to keep them from overstroking. (Some only have warning placards about problems if the ram is powered beyond certain limits.) The guide protrusions and bushing align the ram in its housing so it runs true.

Figure 15-2 shows another type of ram cylinder. When there is a need for a long-stroke actuator with a short retracted length, one option is a telescoping cylinder. Although the majority of telescoping cylinders are single acting, double-acting models are available. Most telescoping cylinders stroke slowly and cycle infrequently because their construction is not robust enough for high production applications.

Telescoping ram cylinders

The cutaway views and symbols in Figure 15-2 depict typical multi-stage telescoping cylinders. The one on the left is single acting; the one on the right is double acting.

Single-acting telescoping cylinders are usually mounted vertically with the small ram up. The cylinder then can be weight returned. This arrangement leaves the large ram with its ports attached to a stationary machine member.

Double-acting telescoping cylinders can be mounted vertically with the small ram down or horizontally when required. The best mounting position for any double-acting telescoping cylinder is with the small ram attached to a stationery machine member so the ports do not move. Long-stroke double-acting telescoping mounted horizontally need some sort of carrier to support the center section during extension so they will not sag and wear out seals and bushings prematurely. Also note: the return area may only be only 10% of the extend area, so the return force is not capable of doing much work. This small area also requires very little fluid to give maximum retraction speed without excessive backpressure at the extend port. Another possible problem with double-acting telescoping cylinders occurs if the retract port is blocked while the cylinder is trying to extend. Up to a 10:1 intensification can result and the high pressure may damage the housing or rams. Installing a safety relief valve at the retract port may be necessary if this port can be blocked or restricted for any reason.

For all telescoping cylinders, make sure the small ram can do the work required. As a telescoping cylinder starts to extend, the large ram always moves first at a lower pressure. When the first and subsequent rams bottom out, pressure and speed increase due to the decreased ram area. If the small ram produces insufficient force, the unit stops before making a full stroke.

Several suppliers build double-acting pneumatic telescoping cylinders in small sizes, with up to three stages.

One manufacturer makes a single-acting telescoping cylinder with internal porting and matching areas that cause all rams to move in unison as they extend and retract. These cylinders come in a maximum of three stages because the area staging would make any more rams into a vastly oversized package. An integral combination check and relief valve allows the rams to be filled and bled and to stroke fully in case of bypass at the seals. This design’s main advantage is smooth extension and retraction without the bumps of a typical telescoping unit.