StepLock Piston Seal Design Information
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When designing a hydraulic cylinder for working pressures over 4,000 psi several things should be considered. The following data is to assist in designing long life cylinders. Each application will be different so some things will not apply, or work well. Testing every design will ensure safe and reliable operation.
- Check the barrel wall thickness. Using Barlow’s formula will determine if the barrel will yield or burst, but some working pressures will expand the barrel enough to reduce seal life. Page 2 of our StepLock patent information gives an example.
- Check the piston to rod interface for sufficient capacity. This interface has two problem areas; one created by extend pressure and one created from retract pressure. While extending the piston is pressing against a shoulder on the rod. This bearing area should be large enough to stop either part from yielding. While retracting the piston is pressing against a nut, the bearing area between the nut and piston must be large enough to stop either part from yielding and the tensile stress area of the male thread must be large enough to not yield. Remember the nut torque will create an axial force and it will be additive to the hydraulic forces. This additional force will effect the bearing areas only. We do not recommend having the piston threaded onto the rod unless the lock nut is large enough to hold all the forces.
- Using step cut wear ring will buffer pressure spikes.
- Allow for running clearance between wear rings and rod or barrel. Nylon wear rings will absorb water and expand 1+% or more.
- Long stroke cylinders may require spacers between the piston and the head to increase column strength.
- Welding should be done with high strength filler material. We recommend an E80SD2 solid wire and multiple passes for large welds.
- Do not use any free machining steel in ports, flanges, butt plates (blind ends) or any other part that will be welded. C1018 and 8620 are the best choices.
- Seamless schedule 80 pipe will work for port pipes. This material can be cold or hot bent and welds to the ports and barrels well.
- Work induced loads may cause the cylinder to see higher internal pressures than the hydraulic system can produce.