FLUID POWER HANDBOOK
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HYDRAULIC
CYLINDERS IMAGE COURTESY OF RAM INDUSTRIES
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things represent a fluid power system the way a hydraulic cylinder does. These true workhorses operate in industrial and mobile applications. When compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable and also offer greater power density. For example, a hydraulic cylinder has about ten times the power density of an electric linear actuator of similar size. Selecting the right cylinder for an application is critical to attaining maximum performance and reliability, which means taking into consideration several design and performance parameters. Fortunately, an assortment of cylinder types, mounting methods and “rules of thumb” are available to help select the appropriate cylinder.
CYLINDER TYPES The three most common types of cylinders are tie-rod, welded and ram, the latter of which is single acting, meaning it is powered in one direction only. Tie-rod cylinders can be single acting, although they are most often powered in both directions. They have machined, square caps and heads being forced together against the 10
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barrel by high-tensile steel tie rods fastened by nuts, making them easy to disassemble and repair in the field. Welded cylinders employ a steel barrel with a cap welded to the bottom and the end treatment subsequently welded to the cap. The rod and piston assembly then has to be assembled around the head — which uses a buttress thread for strength — and is tightened into the barrel. Finally, the singleacting ram is typically just a rod inside a barrel with a single port and requires either a spring or mass to retract. For all cylinders, the critical measurements include stroke length and bore and rod diameter. Stroke lengths vary from less than an inch to several feet or more, depending on the requirement of the machine. Bore diameters can range from 1 in. up to more than 24 in., and piston rod diameters range from 1-2 in. to more than 20 in. In practice, however, the choice of stroke, bore and rod dimensions may be limited by environmental or design conditions.
CYLINDER MOUNTING METHODS Mounting methods also play an important role in a cylinder’s performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer, ideal column loading and avoiding excessive wear. Pivoting mounts, such as clevis or trunnion, require care in application, because of their capacity to
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move as the cylinder is stroked, resulting in a possible bent rod or excessive wear.
Common types of mounting include: Flange mounts — Strong and rigid, but have little tolerance for misalignment. It is recommended to use cap end mounts for thrust loads and rod end mounts for loads under tension.
Side-mounted cylinders — Easy to install and service, but the mounts can sometimes create a bending moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts, such as side lugs, need to be well aligned and the load supported and guided. Centerline lug mounts — Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or as a result of shock loads. Pivot mounts — Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with
