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Specifying Hydraulic Cylinders Is A Balancing Act

Update:Nov 22, 2019
Summary:

Cylinders allow hydraulic systems to apply linear motio […]

Cylinders allow hydraulic systems to apply linear motion and force without mechanical gears or levers by transferring the pressure from fluid through a piston to the point of operation.

Hydraulic cylinders are at work in both industrial applications (hydraulic presses, cranes, forges, packing machines), and mobile applications (agricultural machines, construction equipment, marine equipment). And, when compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable, and offer greater power. For example, a hydraulic pump has about ten times the power density of an electric motor of similar size. Hydraulic cylinders are also available in an impressive array of scales to meet a wide range of application needs.

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 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.