Hydraulic Cylinder

What is a Hydraulic Cylinder

A hydraulic cylinder is a mechanical actuator that uses pressurised hydraulic fluid, almost always a mineral or synthetic oil, to produce a unidirectional linear force through a unidirectional stroke. It is the most direct way to turn the high power density of a hydraulic system into useful work: a pump and motor supply pressure and flow, valves direct that flow, and the cylinder is the muscle that actually moves the load. Alongside the hydraulic motor, which produces rotary output, the cylinder is the primary output device of fluid power.

The physics is captured in one equation: output force equals system pressure multiplied by the effective area of the piston, F = p x A. On the extend stroke the full bore area is pressurised; on the retract stroke the cross-section of the rod is subtracted, leaving the smaller annulus area, so a double-acting cylinder always pulls with less force than it pushes at the same pressure. Output speed follows the other half of the relationship: piston velocity equals supplied flow divided by the working area, v = Q / A. These two relationships, together with the area ratio between bore and annulus, govern almost every selection decision.

A typical industrial cylinder consists of a steel barrel (tube), a piston carrying piston seals and wear bands, a chrome-plated piston rod sealed at the gland by a rod seal and wiper, a rod-side head (gland) and a cap-side end, and port connections for oil. Mounting features such as clevises, flanges, trunnions or foot brackets transfer the reaction force into the machine frame. The hydraulic medium is incompressible for practical purposes, which gives the cylinder its characteristic stiffness and precise position holding under load, a property that pneumatic cylinders, working on compressible air, cannot match.

Hydraulic actuation is old technology with a continuous engineering lineage. The principle traces to Joseph Bramah, who patented the hydraulic press in 1795 and exploited Pascal's law that pressure applied to a confined fluid transmits equally in all directions. Through the nineteenth century, water-hydraulic accumulators powered cranes, lock gates and forging presses in industrial cities. The shift to oil as the working fluid in the early twentieth century brought lubrication, corrosion resistance and higher pressures, and by the mid-century the modern oil-hydraulic cylinder, with elastomer seals and chrome-plated rods, had become standard on machine tools, presses and the first hydraulic excavators.

The application range is enormous. Excavators, wheel loaders and cranes rely on welded mobile cylinders; metal-forming and injection-moulding presses use large-bore, high-pressure cylinders; steel-mill roll-adjustment and continuous-casting equipment use mill-type cylinders built for thermal shock and heavy cycling; dump trucks and aerial platforms use multi-stage telescopic cylinders for long reach from a short retracted length. Operating pressures run from about 70 bar in light equipment to 160 to 250 bar for most industrial duty, and to 700 bar or more in high-tonnage bolt-tensioning and workholding tools. No single cylinder spans this range, so selection is fundamentally about matching construction, size and materials to the duty.

Construction Types and Mounting

By how the ends are joined to the barrel and how the stroke is staged, hydraulic cylinders fall into four mainstream constructions: tie-rod, welded, mill-type and telescopic. A second axis is whether the cylinder is single-acting (pressure extends, gravity or a spring returns) or double-acting (pressure both extends and retracts). Choosing the wrong construction is a costly mistake because it dictates serviceability, pressure ceiling and how the unit bolts to the machine. The table below contrasts the four constructions on the metrics that matter to a buyer.

Tie-rod cylinders clamp the two end caps to the barrel using high-tensile steel rods, usually four, torqued to a defined preload. The construction is modular and demountable, so seals can be replaced in the field and the same unit reused for years, which keeps total cost of ownership low. Tie-rod cylinders built to ISO 6020-2 or NFPA dimensions are the default for factory machinery up to roughly 160 to 210 bar. Their weakness is tie-rod stretch: at very high pressure or long stroke the rods elongate and the joint relaxes, so tie-rod designs are not the first choice for the highest-pressure mobile work.

Welded cylinders fuse the cap and ports directly to the barrel, eliminating tie rods entirely. The result is a compact, rugged body that tolerates higher pressure (frequently above 350 bar) and long strokes without joint relaxation, which is exactly what mobile equipment needs in cramped, shock-loaded environments such as excavator booms and loader arms. The trade-off is serviceability: because the cap is welded, reseal usually means cutting the head off or using specialised gland-removal tooling, so welded cylinders favour rebuild at a shop over field repair.

Mill-type cylinders are a heavy industrial class designed for steel mills, foundries and presses. They use a bolted, flanged head that can be opened for reseal, thick barrel walls, and rugged glands sized for continuous high-cycle duty and thermal load. Bosch Rexroth's CDH1 / CGH1 series is a representative example, rated 250 bar with bores from 40 to 320 mm. Telescopic cylinders nest several tubes of decreasing diameter so that a long stroke retracts into a short body; they are usually single-acting with two to five stages and dominate dump-truck bodies, refuse vehicles and crane outriggers where reach matters more than pull force.

Mounting style transfers the reaction force into the machine and is standardised so that any maker's cylinder bolts to the same frame. The three families are fixed centreline mounts (flanges and tie-rod extensions such as NFPA MF1, MF2 and MX2 that take the load straight along the axis), fixed non-centreline mounts (foot or side-lug brackets like MS2 and ML1 that create a bending moment), and pivot mounts (clevis MP1, eye and trunnion MT1 that allow the cylinder to swing as the load arcs). NFPA T3.6.7 and ISO 6020 together define this envelope, and heavy-duty makers typically offer up to about nineteen mounting styles per bore. Selecting a centreline mount wherever possible minimises side load on the rod bearing and extends seal life.

Dimensional and Pressure Standards

Interchangeability between manufacturers rests on a small family of ISO standards plus the North American NFPA / ANSI envelope. These standards fix the bore and rod diameters, the area ratios, the rated pressures, the stroke series, the rod-end threads and the mounting dimensions, so that a buyer can specify a cylinder by class and bolt in a substitute from another maker. The table below maps the principal standards to what each one controls and the pressure class it implies.

ISO 3320 is the dimensional backbone. It defines the preferred metric bore series of 50, 63, 80, 100, 125, 140, 160, 180, 200, 250 and 320 mm, the matching rod diameters, and the area ratio phi between bore area and annulus area. Standard area ratios are 1.06, 1.25, 1.4, 1.6, 2.0 and 2.5; the ratio determines how much slower and weaker the retract stroke is than the extend stroke, so it is chosen deliberately, for example a 2:1 ratio to balance force or speed between strokes. Specifying a bore and rod from this series guarantees that spare pistons, seals and rods are available across suppliers.

ISO 6020-1, 6020-2 and 6022 fix the external mounting dimensions for three pressure classes. ISO 6020-1 covers a medium 160 bar (16 MPa) series, ISO 6020-2 covers a compact 160 bar series, and ISO 6022 covers a heavy 250 bar (25 MPa) series for mill and press duty. Because these standards lock the mounting interfaces, a 250 bar ISO 6022 cylinder from one maker has the same flange pattern, pin diameter and port location as another's, which is the practical reason large plants standardise on a single ISO class across hundreds of machines.

NFPA T3.6.7, published through ANSI as B93.15, is the North American counterpart. It codifies mounting styles with two-letter-plus-number designations: MF1 and MF2 head and cap rectangular flanges, MX2 extended tie rods, MS2 side lugs, MP1 clevis and MT1 / MT4 trunnions, among roughly nineteen options. NFPA heavy-duty industrial cylinders are commonly rated to 3,000 psi (about 207 bar), with class designations distinguishing service severity. Pressure classes such as the European DIN-based 160 and 250 bar ratings and the imperial 2,000 and 3,000 psi ratings are not directly interchangeable, so a global project must confirm which envelope a machine frame was drilled for before sourcing a replacement.

Two further dimensions complete a specification: stroke and stop-tube. ISO 4393 standardises the preferred stroke series so that catalog units cover common travels without custom machining, while long-stroke cylinders add a stop tube, a spacer that increases the distance between the piston and rod bearing at full extension, reducing the effective buckling length and the bearing load. Specifying stroke, area ratio, mounting style and pressure class together is what turns an abstract force requirement into an orderable, interchangeable part number.

Rod, Barrel and Seal Materials

Three material systems decide how long a cylinder lasts: the piston rod surface, the barrel bore, and the sealing package. The piston rod is the most safety-critical because it reciprocates through the rod seal under load, exposed to dirt, weather and side load. A pitted or worn rod tears the seal and leaks; a buckled rod fails the actuator entirely. The barrel must hold pressure without bulging and present a smooth bore for the piston seals. The seals must contain pressure for millions of cycles across the service temperature band.

Piston rod steel is selected for strength, then surface-treated for hardness and corrosion resistance. The standard surface is hard chromium electroplate, typically 20 to 50 micrometres thick (about 0.001 inch minimum per side), with a chrome hardness above 800 HV, applied over a core that is induction-hardened to roughly 50 to 55 HRC for indentation resistance. The plated rod is honed or super-finished to a surface roughness of about Ra 0.1 to 0.3 micrometre so the seal lip rides on a smooth, oil-retaining surface. Common base steels are C45 / AISI 1045 for general duty, 42CrMo4 / AISI 4140 quench-and-tempered for higher strength, and 17-4PH stainless or nickel-chrome (Ni-Cr) double-layer plating for marine and chemical exposure.

Barrel tube is usually seamless cold-drawn or honed-bore steel such as E355 (St 52), supplied as skived-and-roller-burnished (SRB) tube with a bore roughness around Ra 0.2 to 0.4 micrometre. The smooth, work-hardened bore both seals well and resists piston-seal wear. Wall thickness is sized to the burst pressure with a safety factor, typically giving a burst rating several times the rated working pressure. For corrosive or food service the bore may be hard-chrome plated or the whole cylinder built in stainless steel.

The sealing package combines several elements: a rod seal that contains pressure at the gland, a wiper (scraper) that excludes dirt and moisture as the rod retracts, piston seals that contain pressure across the piston, and bearing or wear bands of bronze-filled PTFE or polyamide that carry side load and keep metal off metal. The table below summarises the mainstream seal elastomers, the temperature band each covers, and where it fits, so a buyer can confirm the offered package matches the fluid and climate.

Matching the elastomer to the fluid is as important as matching it to temperature. Standard NBR and PU suit mineral oils and most synthetic esters, but phosphate-ester fire-resistant fluids of the HFD class attack them and require EPDM or FKM seals. Water-glycol HFC fluids tolerate NBR but demand attention to bearing materials and corrosion. When a cylinder is quoted, the seal kit material and the fluid compatibility should both appear on the datasheet; a mismatch is a common cause of early field leaks that no amount of rod quality can prevent.

Key Specification Parameters

A cylinder datasheet can list twenty or more lines, but a manageable set of parameters drives the selection decision: bore, rod diameter and area ratio, stroke, rated and test pressure, output force at pressure, piston speed, mounting style, cushioning, and the seal and rod surface. Each is decoded below so a buyer can read across competing quotes on a like-for-like basis.

Bore, rod diameter and area ratio together fix both force and speed. The bore sets the extend force (F = p x A), the rod diameter sets how much area is lost on retract, and the area ratio phi (bore area divided by annulus area, standardised in ISO 3320 at 1.06 to 2.5) sets the imbalance between strokes. A larger rod raises buckling resistance and retract speed but lowers retract force; the choice is a deliberate engineering trade, not a default. Worked example: a 100 mm bore at 160 bar gives about 125 kN extend force, and with a 56 mm rod (a 1.4 ratio) about 86 kN retract force.

Rated, test and burst pressure describe three different limits. Rated (nominal) pressure is the continuous working pressure the cylinder is designed for, such as 160 or 250 bar per the ISO class. Test (proof) pressure is the factory hydrostatic test level, commonly 1.5 times rated. Burst pressure is the mechanical rupture point of the barrel and is several times the rated pressure. Sizing always works to rated pressure, while pressure spikes from valve closure or load impact must stay below the test level to avoid permanent deformation. Always confirm which figure a supplier quotes, because a high "maximum pressure" number sometimes refers to test rather than continuous duty.

Stroke and stop-tube set the travel and the buckling margin. Stroke is the usable piston travel; the stop tube is an internal spacer added to long-stroke cylinders to keep the rod bearing and piston farther apart at full extension, cutting the effective unsupported length and the side-load on the bearing. Piston speed follows v = Q / A and is typically 0.05 to 0.5 m/s in industrial service; above roughly 0.1 m/s, or with high inertial loads, end cushioning becomes necessary to avoid slamming the end caps.

Cushioning, mounting and surface finish the picture. Cushioning options range from none, through fixed cushions, to adjustable needle-valve cushions at one or both ends; for very high kinetic energy, external deceleration valves or shock absorbers supplement the internal cushion. Mounting style (flange MF, clevis MP, trunnion MT, foot MS) must match the machine and is the line most likely to cause a non-fitting delivery if mis-specified. Surface and protection covers rod plating thickness and hardness, optional Ni-Cr or stainless rods for corrosion, and the paint or coating on the body for the operating environment.

The output that ties these together is force across the working pressure range. The list below shows how the same 80 mm bore cylinder scales, illustrating why pressure class and bore are chosen jointly rather than independently:

• 80 mm bore at 100 bar: extend area about 0.00503 m squared, push force about 50 kN.
• 80 mm bore at 160 bar: same area, push force about 80 kN (ISO 6020 class).
• 80 mm bore at 250 bar: same area, push force about 126 kN (ISO 6022 class).
• Retract at 160 bar, 45 mm rod: annulus area about 0.00343 m squared, pull force about 55 kN.
• Burst margin: barrel typically rated several times the working pressure, with a 1.5x hydrostatic proof test.

Selection Decision Factors

To turn a load requirement into an orderable cylinder, work the steps below in order. Most selection errors come not from a single bad number but from deciding a downstream parameter (mounting, rod size) before an upstream one (force, pressure, buckling) is settled. These nine steps double as a fixed RFQ template.

1. Force and direction: Establish the worst-case force and whether it occurs on extend or retract. Size on that stroke, remembering that pull force is always lower than push at equal pressure because of the rod annulus.
2. System pressure and bore: Choose a working pressure consistent with the power unit (commonly 160 or 250 bar), then compute bore area = force / pressure and round up to the next ISO 3320 bore (50, 63, 80, 100, 125, 160, 200, 250, 320 mm).
3. Rod diameter and area ratio: Select rod size for buckling resistance and the desired area ratio phi; a 2:1 ratio is common. A larger rod raises retract speed and column strength but cuts retract force.
4. Stroke and buckling check: Set the stroke, then verify the rod against Euler buckling, critical load = pi squared x E x I / (K x L) squared, E = 210 GPa, K from the mounting (0.5 fixed-fixed to 1.0 pinned-pinned), with a safety factor of at least 3.5. Add a stop tube or larger rod if marginal.
5. Mounting style: Pick a fixed centreline mount (flange MF, MX) where possible to avoid side load; use clevis MP or trunnion MT only where the load arcs. Confirm the pattern matches the machine frame.
6. Construction and pressure class: Tie-rod ISO 6020-2 for serviceable factory machinery, welded for compact high-pressure mobile duty, mill-type ISO 6022 for steel-mill and press service, telescopic for long reach from a short body.
7. Speed and cushioning: Compute piston speed v = Q / A; if above roughly 0.1 m/s or moving large inertia, specify adjustable end cushioning, and consider external deceleration valves for high energy.
8. Fluid, seals and temperature: Match the seal material to the fluid and climate (PU / NBR for mineral oil to +100 C, FKM to +200 C, EPDM for HFD fire-resistant fluid), and the rod plating to the corrosion environment.
9. Ports, certifications and protection: Specify port type and size (BSP, SAE, metric), any pressure-equipment or functional-safety requirements, ingress and paint for the environment, and position sensing if needed.

One dimension that is easy to overlook at quotation time but decisive over a ten-year service life is serviceability: whether the cylinder can be resealed in the field, whether seal kits and rods are stocked locally, and whether the maker conforms to ISO 6020 / 6022 or NFPA dimensions so a substitute can be fitted without re-drilling the machine. Established makers including Bosch Rexroth, Parker Hannifin, Eaton, Bucher Hydraulics, Hanchen and Enerpac, alongside ISO-conforming suppliers such as Brant Hydraulics and HENGLI, publish full dimensional datasheets that let a buyer confirm interchangeability before committing. Choosing a standards-conforming, serviceable unit usually beats a marginally cheaper proprietary cylinder once downtime and spare-part lead time are priced in.