Properly applied pneumatic and hydraulic cylinders provide outstanding linear-actuation performance in a wide variety of applications. But if applied improperly, a cylinder in short order may not only ruin itself but also the equipment on which it is installed.
In a perfect world, cylinders would align exactly with their loads, but that’s often not the case. By far the major cause of premature cylinder failure is side-loading away from the cylinder centerline which wears the bearings and seals. Yet this type of failure can be minimized by selecting the correct cylinder and installing it properly. Too often designers overlook simple but important steps — such as specifying the mounting style that best suits the cylinder and its installation.
Cylinder mounting styles fall into three major groups. Here’s a closer look at each.
The best way to support a cylinder is along its centerline. Fixed mounts that absorb force along the cylinder centerline, including extended tie rod, centerline lug, and flange mounts, should be used whenever possible.
Extended tie-rod mounts are available at the rod or cap end of the cylinder. Because they are symmetrical, any thrust or tension forces on the piston rod are uniformly distributed about the cylinder centerline. The rods are designed to withstand maximum internal rated pressure, so this mounting style can handle thrust or tension forces at full rated pressure. While considered to be one of the best mounting styles, extended tie-rod mounts often are passed over by designers.
Centerline lug mounts are another style of fixed mounts. The mounting lugs hold the cylinder in place while friction between the lug underside and the machine surface on which it rests prevents the cylinder from moving laterally. When used at high pressure or when subject to shock loading, the lugs should be dowel pinned to the machine surface. For thrust loads, pin both lugs at the cap end. For tension loads, pin both lugs at the rod end. Never pin both ends.
Flange mounts are also considered to be fixed-centerline mounting styles and are among the best mounts for use on straight-line, force-transfer applications. Three types of flange mounting styles at each end of the cylinder are the head (rod end) or cap rectangular flange, head or cap square flange, and rectangular head or cap. Selecting the proper flange depends in part on whether the major force applied by the load will result in compression (push) or tension (pull) stresses to the piston rod. Cap-end flange mounting styles are recommended for thrust (push); rod-end flange mountings are recommended for tension (pull).
Flange mounts work best when the mounting face is attached to a machine support member. This is important when a rectangular head flange is used in tension; it avoids flexing or bending stress on the flange. At times, an application may require a head-flange mount for a thrust load. In general, head or cap rectangular flanges for pneumatic cylinders will handle any forces that the actuator develops. For hydraulic cylinders, flange mounts will handle full rated pressure, but care must be taken not to mount the cylinder so that the mounting bolts are exposed to full cylinder forces. Head and cap rectangular flange mounts are usually rated at a pressure below the nominal rating because bending forces on the rectangular shape may damage the flange.