The term “high pressure” in hydraulics is quite subjective. Among the strictest definitions is any system with operating pressure exceeding 10,000 psi. Perhaps a more general interpretation is 6,000 psi, which will be assumed for this discussion. For this article, high system pressure will be defined as 6,000 psi (410 bar). This is the pressure setting on some newer excavator functions. A system that operates at 6,000 psi will see brief pressure spikes that are two to three times the 6,000-psi working pressure. This means the components need to accommodate 12,000 to 18,000 psi, 827 to 1,241 bar pressure spikes, for a reasonable service life of 8,000 hours (about four years), and should not fail in less than 2,000 hours, about one year of service.

Because most of my extensive learning experience is from working with logging and construction equipment attachments, the examples I have set forth are from those industries. I chose not to blame the customer for using the cylinders differently than I specified, and I accepted full warranty responsibility for any poor design choices. Several cylinders were repaired under warranty several times, and a few cylinders were scrapped and completely new designs implemented to meet the self-imposed one-year or 2,000-hour warranty.

The concepts and areas of focus being presented here can be applied to most hydraulic cylinder applications.

Cylinder Design Considerations

What is actually being done with the cylinder and how is the cylinder being loaded? When a machine or attachment is being designed, the designer has an idea of what the unit should do and how it should be done. As soon as you hand the machine to a different operator, the application, duty cycle, and operating parameters have changed. In some cases the change is enough that things start to fail. Frequently, the cylinder will be subjected to a work-induced load that is much greater than what is possible by direct control.

  • Will the cylinder need load-holding valves? Both counterbalance valves and pilot-operated check valves are used to hold a cylinder in position, and both valves can produce additional loads on the cylinder components when in operation. Some load-holding valve settings are increased by back pressure on the pilot port.
  • What is the cylinder stroke? A short-stroke cylinder does not typically need to be concerned with buckling load design. A long-stroke cylinder will require running a buckling load calculation. Use standard NFPA standard T3.6.37 R1-2010 for buckling calculations.
  • How fast is the cylinder going to move? The speed the cylinder will move, or the flow going into the cylinder, must be considered to determine port and plumbing size. This speed is also important when designing cushions if they are required.
  • What are the appropriate bore and rod diameters? Choosing the correct bore and rod size can be the single most difficult aspect of the cylinder to determine because of the forces required when extending and retracting, the stroke required, and the pin centers’ distance needed to be met. If the cylinder needs cushioning, this could change the bore or rod size requirements. Sometimes a larger bore and rod will be required to meet all the design needs. One side of the piston may need to run at a lower pressure to meet the force requirements as well as structural requirements, such as the piston-to-rod interface, the piston-fastening needs, and buckling of the rod.

Read more: Avoiding Problems with Hydraulic Cylinders