Hydraulic Cylinder Design Types

Conventional double-acting cylinders are the most common hydraulic actuators because they are especially controllable and can develop both compressive and tensile forces.

Double-acting cylinders have ports at both cap and rod ends. The cylinder is extended by pumping hydraulic fluid into the cap end of the cylinder, which moves the piston and rod. Fluid in the rod end of the cylinder is exhausted out of the rod-end port. Reversing the flow of oil through the cylinder retracts the cylinder rod.

Conventional single-acting cylinders have high-pressure fluid ports only at their cap-ends. Fluids pumped into their cap-end ports push on their pistons and cause their rods to extend. Rods are subsequently retracted by allowing fluids to drain out of their cap-ends. Gravitational, spring, or other external forces are necessary to retract their rods and move their connected structures. The volumes on the rod sides of these cylinders can be vented to atmosphere, isolated with expansion bladders, or drained back to reservoirs through low-pressure rod-end ports.

Single-acting hydraulic cylinders can be ideal when significant gravitational or other external forces act consistently upon structures with simple movements. Mechanical springs can be installed in these cylinders on the rod-sides of their pistons. In these case, the cylinders are termed “Spring Return Cylinders”.

Double-rod cylinders feature rods that extend through both their cylinder heads and caps. When these cylinders employ rods of identical diameters, their extend and retract forces and speeds are identical given fixed pressures and flows. Precision, high-speed servo-cylinders for high-cycle tests often employ double-rod cylinders to capitalize on the performance and tuning benefits enjoyed by cylinders with equal effective areas on both sides of their pistons.

Telescoping ram-type cylinders are comprised of a series of nested rams. Except for the smallest stage, telescoping ram stages are hollow and serve as cylinder bodies for smaller stages.

These cylinders provide exceptionally long strokes relative to their collapsed lengths. As with other single-acting cylinders, telescoping ram type cylinders can only provide a thrust force upon extension. External forces, such as gravity, are necessary to effect cylinder retraction.

Telescoping cylinders are especially useful in applications where precision motion and force control are not essential.

Considerably more complicated than their single-acting ram counterparts, telescoping double-acting cylinders can develop both compressive and tensile motive forces. These cylinders provide longer strokes than conventional cylinders of similar collapsed lengths. One typical design arrangement locates both the extend and retract ports at the rod end of the cylinder.

Telescoping cylinders are especially useful in applications where precision motion and force control are not essential.

Piggyback cylinders are pairs of cylinders rigidly coupled together to provide long working strokes and relatively short collapsed lengths. These cylinders provide both compressive and tensile forces and can compare favorably to two-stage telescoping double-acting cylinders in certain applications. Piggybacked cylinder ports can be factory cross-plumbed to reduce the number of field connections or they can be left independent to provide up to four discreet machine positions.

Special attention to bearing loads is necessary when these cylinders are designed to safely accommodate their internal eccentric loads and preclude premature wear.

Tandem cylinders consist of two or more in-line cylinders constructed together as an integral unit. Each individual cylinder in a tandem cylinder set features its own piston and ports, but shares a rod common to the assembly. Although the bores of individual cylinders in a tandem set are often identical, dissimilar bores can be provided.

Tandem cylinders are useful in a number of areas:

• Produce greater axial forces than conventional cylinders of similar bores at any given pressure.
• Permit rapid, low-force cylinder motions in presses when the tandem cylinder bores are sized differently.
• Actuate machines with two or more independent hydraulic systems.
• Provide the superior motion control of hydraulic systems on machines that are powered with compressed air.
• Accurately synchronize the strokes of two cylinder assemblies.