Hydraulic cylinder glossary

Core terms of hydraulic cylinder engineering: bore, rod, stroke, stage, sealing, buckling and more. Short, accurate, quotable definitions.

Bore: The inner diameter of the cylinder tube in which the piston travels. The bore area determines the push force at a given pressure (F = P x A). HPS manufactures cylinders with bores from Ø25 to 320 mm.
Piston Rod: The steel shaft that transmits the cylinder force to the load. HPS piston rods are made of C45E (Ck45) or 42CrMo4 steel in diameters from Ø15 to 250 mm and hard-chrome plated to min. 20 µm per side, typically 30 µm. Pull force is lower than push force because the rod cross-section reduces the effective area.
Stroke: The distance the piston rod travels between the fully retracted and fully extended positions. HPS builds industrial cylinders with strokes of 100–7,000 mm, with a buckling check on long strokes, and telescopic cylinders with 100–12,600 mm total stroke.
Single-Acting Cylinder: A cylinder that is powered by hydraulic pressure in one direction only. The return movement is produced by the external load, gravity, or a spring, so only one supply line is required.
Double-Acting Cylinder: A cylinder that develops hydraulic force in both extension and retraction. The two sides of the piston are pressurized alternately through two ports. It is the most common configuration in industrial applications.
Telescopic Cylinder: A cylinder with multiple nested stages that extend sequentially, largest stage first. It delivers a long stroke from a short closed length and is used in tippers, refuse vehicles, and mobile platforms. HPS builds telescopic cylinders with 2 to 7 stages and 100–12,600 mm total stroke.
Stage: One of the nested moving sleeves of a telescopic cylinder. As each smaller stage extends, the effective area decreases, so at constant flow the speed rises and the force falls.
Closed Length: The overall length of the cylinder when fully retracted. Where closed length is limited but a long stroke is required, telescopic cylinders are the typical solution.
Working Pressure: The pressure at which the cylinder is designed to operate continuously. HPS cylinders have a nominal pressure of 250 bar as standard, with higher pressures to drawing.
Test Pressure: The pressure applied during factory acceptance testing, above the rated working pressure. At HPS, test pressure follows the drawing, otherwise 1.5 × nominal pressure (ISO 10100). Our test bench works up to 400 bar as standard. A test record is available on request.
Seal: A component that prevents fluid leakage between the moving and static parts of a cylinder. HPS uses NBR up to about 90 °C, HNBR to about 120 °C, PU for heavy-duty wear and high pressure, FKM for high temperature and aggressive fluids, and PTFE for low friction and high speed. Final seal selection is verified by engineering.
Wiper (Scraper): A scraper seal fitted at the outer end of the gland that cleans dirt, dust, and moisture from the rod as it retracts. It protects the rod seals and extends service life.
Gland: The head assembly at the rod end of the cylinder that guides the piston rod. It houses the rod seal and the wiper.
Cushioning: A built-in damping feature that slows the piston near the end of the stroke by restricting the outlet flow. It reduces impact, noise, and shock loads at the end of travel.
Port: The threaded opening through which hydraulic fluid enters and leaves the cylinder. A double-acting cylinder has two ports, one on each side of the piston.
Mounting: The mechanical interface that attaches the cylinder to the machine, such as a clevis, trunnion, flange, or foot. The mounting style determines the free buckling length. Mounting types are designated per ISO 6099 and NFPA T3.6.7R.
Clevis: A fork-shaped mounting with a pin, fitted at the cap or rod end. It lets the cylinder pivot in one plane and accommodates angular movement during the stroke.
Trunnion: A mounting with two cylindrical pivot pins on the cylinder body, at the head, the cap, or an intermediate position. It allows the cylinder to swing in one plane while the pins carry the load.
Buckling: Lateral instability of the rod or cylinder under compressive load, evaluated with Euler analysis. The critical load follows Pcr = pi^2 x E x I / Lk^2, where the free buckling length Lk depends on the mounting configuration. Remedies include a thicker rod, a larger bore, a stop tube, or a different mounting.
Stop Tube: A spacer inside the cylinder that keeps the piston away from the gland at full extension. The longer bearing distance reduces side loading and improves buckling resistance on long-stroke cylinders.
Honing: A precision finishing process for the inside of the cylinder tube. It produces an accurate bore with the surface finish required for reliable sealing.
Hard Chrome Plating: An electroplated chromium layer on the piston rod that provides wear and corrosion resistance and a smooth sealing surface. At HPS, piston rods and the outside of telescopic stage tubes are hard-chrome plated to min. 20 µm per side (40 µm on the diameter), typically 30 µm per side; stage tubes are plated on our own line, with approved platers for extra capacity.
Drift: Slow, unintended movement of the rod under load when the cylinder should hold its position, caused by internal leakage. At HPS, internal leakage is checked on the test bench before dispatch.
ISO 6020-2: An international standard defining the mounting dimensions of compact series hydraulic cylinders, ensuring interchangeability between manufacturers.