Hydraulic cylinders essentially convert fluid pressure and flow into force and linear motion. They are available in a wide range of styles, sizes, materials and configurations. Many OEM engineers play it safe by over-engineering cylinder specifications. But less can sometimes be more when it comes to complex hydraulics. Let’s first identify some of the key factors that should be considered when specifying hydraulic cylinders.
Design Factors for Hydraulic Cylinders
Specifying hydraulic cylinders is essentially a balancing act or a cascade of compromises, as each design factor influences one or more of the other design factors. Designers must weigh each positive effect against potential negatives to get the best performance. Here‘s a concise look at those factors:
Capacity. Most industrial hydraulics are considered medium-duty and operate at 1,000 psi. Heavy-duty hydraulics, which are common in presses and automotive machinery applications, can handle pressures up to 3,000 psi. If loads are extremely high, tandem cylinders are preferred to larger-bore or custom high-pressure cylinder designs.
Stroking distance. Stroke distances can exceed 10 ft (5.05 m) for custom-built cylinders, but pressure ratings can be a problem with such long strokes. The rod diameter needs to be determined in order to gauge its ability to handle the load. If necessary, a pressure rating on load in thrust (push mode) must be specified. In horizontal applications, which are common, premature rod bearing wear can lead to rod sag over long strokes. Although custom stroke distances above 10 ft (3.05 m) are possible, pressure rating can be a concern.
Speed. Every application engineer has their own definition of “excessive speed.” A good rule of thumb is that standard hydraulic cylinder seals can easily handle speeds up to 3.28 ft/sec. (1 m/sec.). The tolerance threshold for standard cushions is roughly two-thirds of that speed. Standard low-friction seals are often a better choice for high-speed applications, but what you gain in one aspect of performance you lose in another. The higher the fluid velocity, the higher the fluid temperature. So, when opting for higher speeds, it is essential to consider how higher temperatures will affect the entire hydraulic system. In some hydraulic systems, over-sizing the ports may eliminate concerns over higher temperatures.