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Specifying Hydraulic Cylinders

When specifying hydraulic cylinders, there are several factors that need to be taken into consideration:

Capacity. Most industrial hydraulics are considered medium-duty and operate at 1,000 psi. Heavy-duty hydraulics, which is 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.

Hydraulic Cylinder, Mill

Hydraulic cylinder from a mill (Wikimedia).

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.

Temperature. As previously noted, hydraulic cylinders using standard components can be designed to meet application temperatures as high as 500°F (260°C) and as cold as −65°F (−54°C). But temperatures affect both the “hard” and “soft” design components of cylinders. That means engineers designing applications that will see high and/or low temperature extremes must be aware of individual components interdependency to best balance short- and long-term performance. For example, seals and metal parts used in cold environments will contract.

Mounting styles. There are basically three mounting styles. Load, speed and cylinder motion are the parameters that help determine which is best. For example, fixed and pivot mountings absorb forces on the cylinder’s centerline, the preferred way for mountings to handle the thrust or tension. These two types of mountings can usually be medium- or heavy-duty. A third style, fixed, supports the entire cylinder on its mounting surface below the cylinder’s centerline rather than absorbing forces only along the centerline.

Read the full article at Hydraulics and Pneumatics »

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