Cylinders undergoing pressure and temperature changes elongate and contract. In addition, flexing and rocking makes the mounting head sway under load. The type of mount to specify depends on the application, but the effect of pressure and temperature changes must be provided for or the cylinder will leak. Consider these factors:
- Cylinders with non-centerline-type mountings, Figure 15, tend to change length and sway under load and temperature change. Any rigid tubing connected to a cylinder cap port will be subject to that resulting force and motion, Figure 16. If a cylinder is rigidly plumbed, the question is not whether it will leak, but when.
- Cylinders with non-centerline mountings often require stronger machine members to resist bending, so consider the rigidity of the machine frame. For example, where one end of a cylinder must be overhung, Figure 17, an additional supporting member should be provided.
- In most cases, a layout of the rod-end path will determine the best type of pivot mounting.
- Fixed, non-centerline mounted cylinders with short strokes add another strength problem because mounting bolts will be subjected to increased tension in combination with shear forces.
- Do the major applied forces result in cylinder rod thrust or tension? Cap-end flange mounts are best for thrust loading; rod-end flange mounts are best with the rod is in tension.
- If misalignment occurs between the cylinder and its load, the mounting style may have to be altered to accommodate the skewing movement. A simple, pivoted centerline mounting, such as a clevis and trunnion, compensates for single-plane misalignment. If multiple-plane misalignment is encountered, the cylinder should have self-aligning ball joints on the cap and rod ends of a clevis-mounted cylinder - and fluid-line connections should be able to accept the movement.
Spare the rod, double the stroke
The continuing trend to make machines smaller by taking advantage of more compact components is the driving force behind the increasingly widespread use of rodless cylinders. The three main types of rodless cylinders are the piston-lug version, the cable cylinder, and the flexible-wall cylinder. Most of these are designed for use with pneumatics, but some manufacturers provide ratings for low-pressure hydraulic service.
The piston-lug design works in a fashion similar to that of a conventional cylinder, but does not move the load through a rod. Instead, a bolt extends from the side of the piston out through a longitudinal slot in the barrel. A drive lug is attached to the end of the bolt and moves directly with the piston.
To seal the slot between the piston and lug, steel bands pressing against each other separate when the bolt passes by. Different piston widths are available to meet any bending moments imposed by a load. Stroke lengths of piston-lug cylinders can exceed 30 ft. Options include position switches, brakes, and carriages to support loads and maintain alignment.
A variation of the piston-lug cylinder uses a permanent magnet in the piston to create a magnetic field that links the piston to the lug through the cylinder barrel. This eliminates the need for a longitudinal slot in the barrel and, therefore, the need for any dynamic seals. Breakaway forces of the magnetic field can exceed 200 lb.
A new type of rodless cylinder uses a magnetic brake for precise motion and position control. This recently introduced cylinder is designed to achieve the precise control of proportional hydraulics using simple pneumatic directional control.
In a cable cylinder, as the piston moves inside the cylinder barrel, it pulls a cable attached to both sides of the piston. The cable wraps around a pulley mounted at each end and attaches to a yoke. As the double-acting piston moves in one direction, the yoke travels in the opposite direction because of the wrap around the pulleys.
Options include automatic cable tensioning, single-acting models, cable tracks for greater load stabilization and capacity, a pulley arrangement to double the stroke and speed, caliper disc brakes on the cable pulley, and reed switches. The cable also can be wound around a drum to provide rotational motion.
A variation of the cable cylinder relies on a metal band running over pulleys instead of a cable. Each end of the the upper yoke rides on the cylinder barrel for greater load stability and capacity, negating the need for a separate load carriage in many applications. These cylinders may be fitted with a brake that stops and holds the load anywhere along the stroke.
Flexible wall cylinders have evolved from designs that were originally made for vibration and isolation mounting. They consist of metal mounting plates fixed to a reinforced rubber chamber that extends and collapses, respectively, as it is pressurized and vented. They have generous lateral misalignment allowances and can actuate through an arc without a clevis mount.
Some precautions should be exercised when applying these cylinders. First, mechanical stops should be provided to limit the length of extension. Otherwise, an overrunning load could pull an end plate off the cylinder. Mechanical stops should also limit retraction, thereby preventing crushing the elastomeric portion of the cylinder between end plates. Alignment of these cylinders is much less critical than with conventional cylinders. However, relative torsional rotation between the end caps should be prevented to keep from having the elastomeric portion fail due to excessive shear stress.
An alternate design resembles a length of flexible hose sealed at both ends. With no pressure, the hose is flat; pumping air into it expands the hose into a tubular shape. Maximum stroke is approximately the ID of the inflated hose. Using a long length of such hose can generate very high force from a relatively low pressure. However, actuation force decreases with stroke length. This is because as the hose expands, it becomes more round, so a smaller area is in contact with the load to apply the force.