Pneumatic cylinder cushions
Pneumatic cylinder cushions have similar designs but operate differently because the fluid is compressible. When the cushion plunger enters a cushion chamber, the trapped air starts compressing according to Boyles’ Law discussed in Chapter 1. When the load is light and the starting trapped pressure is high enough, the cylinder stops smoothly without slamming. When the load is heavy and/or starting trapped pressure is low, the cylinder slows but still may bang the end of stroke. This situation requires the addition of external deceleration in the form of valves and/or shock absorbers. Often the rod cushion is ineffective due to less area for the trapped fluid to work against. As with hydraulic cylinders, oversize rods exaggerate the problem.
Most suppliers offer standard cushions on both air and hydraulic cylinders. This option could smooth the operation of certain heavy loads that need extra deceleration distance. These longer cushions only work well for a fixed load and speed condition. If the machine has changing loads and/or speeds a cushion is not the way to go. Using external shock absorbers or proportional valves makes control easy to setup.
Cylinder rod column strength and buckling
Most cylinders are designed to have ample rod strength regardless of mounting style to rated pressure and strokes to 20 in. Above 20-in. strokes, standard rod size may be too small to keep it from bending in compression. Some mounting styles are more prone to buckling problems and need special attention. When an order is placed for cylinders with long strokes that operate above certain pressures, cylinder suppliers usually offer engineering help on design changes and/or rod modifications. Most cylinder catalogs have formulas and/or charts that show how to figure rod column strength for different mounting styles. In many cases the life of a cylinder is dependent on cylinder mounting type and rod size.
Another problem encountered in cylinder design is with bearing loads. A cylinder has bearing points at the rod and its bushing and the piston in the bore. Neither area is strong enough to carry external loads such as a machine member or other parts. Always use the cylinder to push and pull -- not as a guide.
Figure 15-14 shows column strength and bearing load problems on long stroke cylinders and how to overcome them. The vertical cylinder at top left with the heavy load does not have a large enough rod to lift it. When pressure is applied the undersized rod will bend and the load will not move. If the load does move, rod flex soon wears the bushing and allows leakage. Matching rod size to load and pressure makes a workable design.
The lower horizontally mounted long-stroke cylinder with a standard rod has two problems -- column strength and high bearing loads. The weight of the cylinder as the cylinder extends wears on the bearing points and high force will finally bend the rod instead of moving the work. The oversize rod on the lower, horizontally maintained cylinder is sized to handle the load at maximum force but does little to help bearing load problems.
Adding a stop tube to the piston rod keeps the bearing points farther apart and reduces wear. Notice the overall package is longer now because the stop tube takes up space inside the cylinder. Always check out bearing side-loading problems before building the machine. It can be difficult to fit the longer stroke cylinder or more costly to constantly change rod bushings and seals.
Cylinder mounting styles
The mounting styles shown in Figure 15-15 depict the standard NFPA-approved ways to mount cylinders. Up to 25 different companies make cylinders that match these styles in every dimension. This means no supplier has to be the sole source for any cylinder on a machine. Starting at top left are the least expensive mounting styles. Tie-rod mounts are extensions of the tie-rod threaded section. These extensions go through a machine member with nuts installed and tightened to hold the cylinder in place.
The standard offerings are designated two tie rods extended both ends, MX4, four both ends, MX1, four cap end, MX2, four head end, MX3. At best, these mounting styles hold a cylinder in place but not necessarily in a precise location. They should only be considered when what they are attached to does not travel a rigid path.
The tapped mount, top right, is another inexpensive mount but may be hard to locate and hold cylinder position. It can create problems on long-stroke cylinders that need to stretch when pressure is applied. With both ends tied down, the cylinder is not free to grow from pressure or external force without binding. They are designated by the MS heading.
The side and end lug-mounted cylinders are another way to rigidly mount a cylinder but also can be a problem on long strokes. The lugs are extra long so dowel pins can be installed after a precise position is determined. It is not good practice to have dowels in both ends of the cylinder or on opposite corners especially on long strokes. They are designated by the MS heading.
Other rigid mountings are flange mounts in rectangular and square style. They are designated by the MF and ME headings. The bolt-on flanges are for light-duty operation or where the flange is in compression. The extended head and cap flanges are extremely heavy duty and directly replace the bolt on flanges.
The most versatile mounting styles are pivot mounts. They allow freedom of movement in one or both planes and keep the cylinder in perfect alignment. The intermediate trunion is very good on long stroke applications since cylinder weight can be balanced and reduce bearing loads. Care in choosing cylinder mounting style can add years to its life and eliminate rod seal leaks for long periods. Interchangeable NFPA tie-rod design cylinders come in bore sizes of 1 1/2, 2, 2 1/2, 3 1/4, 4, 5, 6, 7, 8, 10, 12, 14, 16, 18, and 20 in. Rod sizes available are 5/8, 3/4, 1, 1 3/8, 1 3/4, 2, 2 1/2, 3, 3 1/2, 4, 4 1/2, 5, 5 1/2, 7, 8, 9, and 10 in. The largest rod diameter for any bore is equal to or slightly under half the area of the piston. This is commonly known as a 2:1 area ratio cylinder. Oversize rods give fast return at low force, allow for regeneration of rod flow, and allow for higher push force capabilities due to greater column strength. Many manufacturers make cylinders in smaller and larger bores but they are not necessarily interchangeable.
Steel mills often use what are called mill cylinders that have bolted-on heads and caps. They are special to a machine in most cases with little or no interchangeability. Mobile equipment uses cylinders with welded heads and caps that are designed to be thrown away when they fail. However, many repair shops cut them apart and repair them. Mobile cylinders also have screwed-on heads or special snap ring arrangements in their assembly.
The cylinder in Figure 15-16 has such a small diameter and is so far from the directional valve that the lines to and from it hold more fluid than it does. In this situation fresh filtered fluid never cycles through the cylinder so it overheats and all wear particles and rod ingestion stay in it until it fails. Failure is always premature and the amount of contamination present is excessive when it is repaired. Changing the lines to the dual flow path shown in Figure 15-16 eliminates the contamination problem. Cylinder operation and longevity is greatly increased because filtered fluid constantly flushes it.
A tee is installed at A and B ports and check valves facing opposite directions are placed at each end. Separate lines run from the check valves to a tee at each cylinder port. Now all fluid must go to the cylinder through one line and return through the opposite one. Fresh fluid is continually being fed to the cylinder while used fluid returns to be cooled and filtered.
The dual flow path circuit is more important when the cylinder is mounted vertically with long lines. Contamination in this configuration lays on the piston and seals and can score cylinder tube and cause premature seal wear when allowed to collect.