Specifying a hydraulic cylinder starts with six hard numbers: bore diameter, rod diameter, stroke length, working pressure, tonnage, and duty cycle. Get those on paper before you look at brands, because a hydraulic cylinder is essentially a force converter, and its real output is a function of those six, not of the vendor name on the nameplate [S1][S2].
Working pressure is the dominant input. Most industrial mobile and stationary machinery sits in the 160–250 bar (16–25 MPa) band, with heavy presses and offshore tooling running 350–700 bar; a 100 mm bore at 200 bar delivers roughly 157 kN of thrust, while the same bore at 350 bar lifts that to 275 kN, which is why a single spec sheet covers a wide duty range [S2].
Define the duty before the brand
Tonnage and stroke both follow the same engineering 80% rule: spec to 80% of the manufacturer's published maximum, not the headline number. A 100 t cylinder is treated as an 80 t working cylinder, and a 1000 mm stroke is treated as an 800 mm working stroke, leaving margin for sideload, seal swell, and pressure spikes [S2].
Single-acting cylinders use hydraulic pressure to extend and a spring (or gravity) to retract, which keeps cost and complexity down for lift-and-hold applications. Double-acting cylinders use hydraulic force for both directions, giving controlled extension and retraction speeds, which is mandatory for pressing, pulling, and any job where mid-stroke positioning matters. If you need the load held indefinitely, the correct answer is a locknut cylinder or a check-valve held cylinder, not just a single-acting ram [S2].
Construction style: tie-rod, welded, telescopic
Tie-rod cylinders use through-bolts to clamp the end caps to the barrel, are easy to disassemble in the field for seal service, and dominate general factory and mobile equipment up to about 250 bar. Welded (or welded-body) cylinders have the end caps welded directly to the barrel, are more compact, tolerate higher pressure spikes, and are harder to repair; they are the default on construction machinery where shock loading is normal [S1][S2].
Telescopic cylinders pack multiple stages inside each other, so a short collapsed length produces a long extended reach. They are the only practical option for dump trucks, refuse packers, and reach-limited mobile booms; if a standard cylinder cannot collapse into the available envelope, a hydraulic actuator in telescopic form is the answer, not a longer-stroke single-stage unit [S4].
Body material and surface treatment
Steel bodies are inherently stronger than aluminium, while aluminium bodies cut weight by roughly 40–50% and are easier to handle on site, useful for portable jacking and rescue tools. For corrosive or wet environments (marine, washdown, chemical plants), a nitrocarburized surface treatment adds measurable corrosion protection on top of the base steel and is the default spec for harsh-service cylinders [S2].
Seal package drives service life more than the metalwork. Polyurethane (PU) seals cover most general industrial duty at -20 to +80 °C; nitrile (NBR) handles petroleum-based fluids at lower cost; PTFE-based compounds with O-ring energizers cover high-temperature or chemically aggressive circuits above 100 °C. The seal stack, not the bore size, is what determines whether a cylinder reaches 50,000 cycles or 500,000 [S1].
Mounting, porting, and envelope constraints
Mounting style controls how the load is reacted into the frame. Common mobile and industrial mounts include clevis, cross-tube, trunnion, flange (head, cap, intermediate), and side-lug; mismatch between mount and frame is the single most common cause of premature rod bearing failure. Collapsed height and stroke are a coupled constraint: a 16.5 cm (6.5 in) collapsed height simply will not work where 12.7 cm (5 in) is available, and low-height (pancake) cylinders exist specifically for these confined-space cases [S2].
Porting (SAE O-ring boss, BSPP, NPT, flanged) and port size must match the existing hydraulic valve manifold or directional control block; an undersized port chokes flow, raises case pressure, and turns a well-sized cylinder into a slow, hot one. Pilot ports for position sensing or load-hold valves need to be planned in at the spec stage, not drilled later [S1].
Who a generic catalog cylinder is NOT for
A standard tie-rod catalog cylinder is the wrong pick for: high-cycle presses (above roughly 30 cycles/min, where rod packing heat builds up); subsea or sub-zero service below -30 °C (where seal compounds and impact toughness change); and hygienic or pharmaceutical lines, where surface finish, seal materials, and cleanability rules from ISO 4413 and EHEDG apply rather than general industrial norms. For those, the specifier needs custom rod seals, special plating, and validated cleaning protocols, exactly the space where custom manufacturers like HETLOCK position their work [S1].
Tonnage is also a hard cutoff. General-purpose catalog cylinders top out near 100 t, and high-tonnage cylinders reach up to 1000 t, but going to the headline maximum without derating to 80% is a common engineering mistake. If your calculated load is 90 t, buy a 115–125 t cylinder, not a 100 t unit running flat-out [S2].
Selection rules and shortlist logic
Comparison of the three main construction styles against the four most common selection criteria: tie-rod cylinders win on field-serviceability and cost for general factory duty below 250 bar; welded cylinders win on compactness, pressure-spike tolerance, and mobile construction duty; telescopic cylinders win on collapsed-length to stroke ratio for dump trucks and reach-limited booms, at the cost of slower retraction, multi-stage seal wear, and higher unit price [S1][S2][S4].
Final shortlist logic: lock the six numbers (bore, rod, stroke, pressure, tonnage, duty cycle), choose single- or double-acting based on whether you need controlled retraction, pick tie-rod vs welded vs telescopic on envelope and field-service rules, then derate tonnage and stroke to 80% of catalog maximum. Match the hydraulic pump flow at the cylinder port to keep piston speed in the 0.1–0.5 m/s band, otherwise you trade seal life for cycle time. For deeper upstream sizing of the pump that feeds it, see the engineer's hydraulic pump sizing reference and the hydraulic pump selection spec map; for the hydraulic motor side of the same loop, the principle of derating catalog numbers to 80% applies identically.