Many a good designer has shot himself in the proverbial foot, because they were overlooking the basic and sometimes the obvious, when applying cylinders. We hope the following will bring to your attention some of these pitfalls and save you from wondering; “How did I go wrong?”
Just because a cylinder manufacturer’s catalog states a maximum pressure on the outside of their catalog, it doesn’t mean that all cylinders will operate at that pressure. Always check the specific mounting, and bore sizes to be sure it hasn’t been de-rated in pressure.
Know the manufacturer’s safety factor before applying the cylinder. What you may assume is a 4:1 safety factor may actually be 3:1 or even 2-1/2:1.
Most manufactures use internal seals compatible with mineral based fluids. Glycols, synthetics, and some emulsions require enhanced compatibility of the internal seals.
Be aware of the environment the cylinder will be exposed. Although the cylinder may be compatible with the operating fluid, external fluid may degrade the rod seal. Boots or special shrouds may be required to offer additional cylinder protection.
Manufacturer’s following National Fluid Power Association (NFPA) standards will provide port sizes based on a fluid flow velocity of 15 ft./sec. To attain cycle times, designers often exceed 15 ft./sec. Oversized, and sometimes multiple ports, should bring elevated flow rates back in line. Undersized porting will effect cycle time, and create noise and heat.
Many designers do not consider the flow rate out of the cap end (piston side) of the cylinder. They are to busy watching the extend portion of the cycle. The ratio of the area between the rod side and piston side may be as high as 2:1, causing a 2:1 intensification of flow coming out of the rod side during extension. Larger valves may be required because of return flow exceeding the extend flow requirements.
Thrust keys are used to absorb cylinder loads and eliminate the transfer of force to the cylinder mounting bolts. If possible, don’t let the mounting bolts handle the loads created by the cylinder force.
Intermediate or additional mountings should be specified when a long cylinder with fixed mounts or extended tie rods require additional support.
Stop Tubing & Double Pistons
Long stroke applications may result in elevated loading on the rod gland or piston. Long strokes may also tend to buckle the piston rod when pushing loads. Stop tubes or even double pistons may be required to eliminate the possibility of rod buckle or elevated rod gland and piston wear.
Most cylinder catalogs provide a piston rod/stroke selection chart. For the designer who rarely specifies long stroke cylinders, consulting you cylinders’ manufacturing representative for engineering expertise may be advisable.
Piston Rings vs. “U” Cups or reloaded seals
Piston rings are “step cut” and not continuous; therefore, leakage is always a concern. Piston rings may also introduce fine contaminants to the system which may be detrimental should you be using proportional or servo valves. A unique phenomenon can also occur to even the experienced designer when piston rings are specified.
If a vertical cylinder is supporting a load, and a pilot operated valve or counterbalance valve is chosen to eliminate cylinder drifting, drift may still occur if piston rings are used. This is due to the internal leakage of the step cut style piston rings.
Most cylinder manufacturers offer optional piston “seals”. Make sure you are aware of the type of internal piston sealing method used in the cylinder you’ve specified, or you may waste time suspecting your load holding valve should cylinder drift appear.
Cushions are necessary to control kinetic energy. They minimize excessive deceleration of loads, internal and external shock, and machine noise. Make sure you haven’t ignored the linear velocity of the cylinder. It may be advisable to use a proportional valve or even an external shock absorber to handle excessive deceleration or to absorb kinetic energy.
Many designers specify cushions on short stroke cylinders, without considering the length of the cushion spear travel. The result is loss in cycle time, and difficulty or non-attainable cushion adjustment. Cushioning is a costly option. Only specify cushions when necessary.
If you weren’t aware of some of the cylinder design nuances we’ve pointed out, we hope you’ve found this information useful. Remember, mistakes happen, we’re human. It seems like we always have time to do a job over if it doesn’t turn out right. So why not take the time to do it right the first time?
When in doubt, consult your local fluid power professional.