High-Pressure Cylinder Selection: Don't Get the Pressure Rating and Peak Pressure Wrong

The criterion for pressure rating of an in-cylinder magnetostrictive displacement sensor is the pressure peak the system can produce, not the working pressure set on the relief valve. Hydraulic systems generate pressure spikes well above the steady-state value at valve reversal, actuator emergency stop, external load impact and accumulator cut-in. The duration is short, but that is the real duty of the seal and the pressure-bearing structure. Align the system peak with the series rating rather than selecting “just enough” on steady-state pressure: Series 16/16R is 350 bar working / 530 bar peak, Series 17/17EX is 350/600 bar, Series 19 in-cylinder is 300/600 bar. The overall background of hydraulic position control is in Why Choose Magnetostrictive Sensing for Hydraulic Cylinder Position Control.

Where pressure spikes come from

• Reversing shock: oil flow is cut off suddenly when the directional valve switches, and the kinetic energy of the inertial load is converted into pressure. This is the most common source of spikes and is especially marked on heavily loaded, high-speed machines.
• Emergency stop and hitting a hard stop: pressure in a trapped volume rises sharply on a safety stop or when the mechanism hits a hard stop.
• Reverse impact from the external load: a forging-hammer blow, press breakthrough (breakthrough shock), a construction-machine bucket strike, and similar events send the shock into the oil chamber through the piston.
• Intensification and accumulators: pressure on the output side of an intensifier cylinder is a multiple of the input side; a sudden accumulator cut-in produces a pressure wave.
• Thermal pressure rise in a trapped volume: after both ends of a cylinder are closed, a rise in oil temperature expands the trapped fluid and raises static pressure — often completely overlooked.

These spikes do not show on a steady-state pressure gauge; they must be captured with a high-sample-rate pressure sensor or a peak-pressure recorder. When measured data are lacking, the prudent engineering practice is to take one pressure class above the system rated pressure.

How to align the pressure class

The bands in the table are qualitative; the actual class must be taken from the measured or calculated peak of this system. How to read the pressure parameter on the datasheet is in How to Read a Datasheet: The Parameter Traps in Range, Accuracy and Resolution.

Pressure-bearing parts and mechanical fit

The pressure-bearing boundary of an in-cylinder sensor is at the mounting flange/thread and its sealing face: the rod is immersed in the oil chamber and does not itself see the full differential pressure; what actually takes the pressure is the seal structure of the mounting section and the mating face of the cylinder-base bore. Besides the pressure class, therefore, also check:

• Mounting-thread specification and tightening torque: too little torque leaks, too much can damage the sealing face;
• O-ring material compatibility with medium and temperature: confirm the material for high temperature plus special media;
• Machining quality of the cylinder-base mounting bore: roughness and perpendicularity of the sealing face directly determine whether high-pressure leakage occurs;
• Piston-rod clearance bore: minimum ≥12.7 mm, with coaxiality maintained so that piston deformation under high pressure does not impose a radial load on the rod;
• Space at the cylinder base: electronics-head envelope and connector plug-in space.

Machining detail is in Cylinder Bore Machining Requirements: 12.7 mm Minimum Bore, Depth Allowance and Concentricity Control. Products include Series 16 cylinder-integrated, Series 17 hydraulic-cylinder integrated and Series 19H cylinder-integrated; for safety-related duty needing dual-channel redundant output see Series 16R redundant in-cylinder.

Associated selection items in high-pressure duty

High-pressure duty usually appears together with shock, vibration and oil temperature; consider them as a package:

• Vibration and shock resistance: for mobile machinery and impact equipment prefer products aimed at severe duty such as Series 13 mobile hydraulics, rated 25 g vibration and 100 g shock;
• Oil temperature: high-pressure systems generate a lot of heat; oil temperature and ambient at the electronics head must be calculated separately — see High-Temperature Selection: The Dual Constraint of Oil Temperature and Ambient Temperature;
• Oil cleanliness: particle contamination abrades sealing faces faster under high pressure; see Hydraulic Oil Cleanliness and Filter Management;
• Typical industry applications: construction-machinery cylinder schemes are in Construction Machinery Cylinders: Position Solutions for Excavators and Loaders.