What is in this article?:
Dont just select the cylinder design, size, and pressure. A little attention to mounting style can dramatically improve performance and life.
Pivot mounts also absorb force on their centerlines and should be used on applications where a dynamic machine member travels in a curved path. The two basic types are clevis and trunnion mounts. Both can be used on tension (pull) or thrust (push) applications at full rated pressure, except on long-stroke thrust applications where the piston rod’s column strength becomes the limiting factor.
Clevis mounts can be fixed to the cap or detachable. They provide a single pivot point for mounting the cylinder and, therefore, need a pivot pin with sufficient diameter to withstand the application’s maximum shear load. Clevis mounts can be used with the cylinder centerline vertical, horizontal, or any angle in between.
On longer-stroke thrust applications, it may be necessary to oversize the piston rod to prevent buckling, add a stop tube, or both. These steps minimize side loading of the gland and piston bearings to prevent a jackknife action of the fully extended rod. (A stop tube is installed around the piston rod adjacent to the piston. It lengthens the distance between the piston and gland to reduce bearing loads when the cylinder fully extends.)
If the piston rod’s travel path is in more than one plane, misalignment can cause gland and piston bearing wear. For such applications, consider a spherical-bearing mounting. This type of mounting allows for up to 3° misalignment. A sphericalbearing rod eye should also be attached at the rod end.
The thrust (push) or tension (pull) force that a pivot-mounted cylinder transmits to the load is affected by the angle and length of the lever arm it operates, as seen in the “Pivot mount” graphic. A quick way to calculate effective thrust T from cylinder thrust F is by: T = FsinA.
Clevis-mount cylinders often are used with a clevis or knuckle at the rod end. The pivot-pin centerline for the rod should be in the same axis as the pivot-pin centerline on the cylinder. The diameter of the rod pivot pin also should at least equal the diameter of the cylinder pivot pin. Finally, ensure that the centerline of the clevis-mount cylinder pivot pin rotates on an axis parallel to that of the pivot pin of the rod clevis or knuckle.
Trunnion mounts are also pivot mounts. Three styles are: head, cap, and intermediate-fixed trunnion. All three can be used in thrust (push) and tension (pull) at full rated pressure, except where longstroke thrust cylinders are limited by piston-rod column strength. Trunnion pins are designed for shear loads only, so they should not be subjected to bending stress. Pillow blocks, rigidly mounted with bearings at least the length of the trunnion pins, should be used to reduce bending stress. The support bearings should be placed as close to the trunnion shoulder faces as possible. Cap-end trunnion mounts are used on applications similar to fixed clevis mounts.
Head or rod-end trunnion mounts can generally be specified with smaller diameter piston rods than cylinders with the pivot at the cap or intermediate position. On head trunnion mounts with long strokes, designers should consider the overhanging weight at the cylinder cap end. One rule of thumb: limit the stroke to five times the bore diameter and keep piston rod speed at less than 35 ft/min.
Intermediate fixed-trunnion mounts are generally considered to be the best of this mounting type. This trunnion can be located to balance the weight of the cylinder, or at a point between the head and cap to suit the application.