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Hydraulic Cylinders

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Hydraulic cylinders are fluid-powered linear actuators. They deliver high force (thousands to millions of pounds) in a compact envelope and are the primary motion element in presses, heavy equipment, injection molders, and steel mills.

Overview

Typical industrial cylinders operate at 1,500–3,000 psi. Mobile and heavy-press cylinders run 3,000–5,000+ psi.

Construction

  • Tie-rod — bolted-up; the NFPA industrial standard. Repairable in the field.
  • Welded body — for higher pressures, mobile, & shock loads.
  • Mill / heavy-duty — large bore, heavy-wall, removable rod cartridges.
  • Telescoping — long stroke from short retracted length (dump bodies).
  • Position-feedback — built-in magnetostrictive or LVDT for servo-control.

Sizing & Force

  • Extend force: F = P × (π/4) × D²
  • Retract force: F = P × (π/4) × (D² − d²)
  • Apply 1.25–2.0 design factor.
  • Buckling: use Euler/Johnson with the rod’s effective length determined by mounting style.
  • Check minimum rod diameter chart from the cylinder OEM.
  • Calculate flow needed: Q = (Area × Speed); verify pump & valve sizing.

Mounting Styles

  • MF1 / MF2 — front / rear flange.
  • MS2 / MS4 — side lugs / side feet.
  • MP1 / MP2 / MP3 — clevis (fixed, detachable, eye).
  • MT4 — intermediate fixed trunnion.
  • Pivot mounts (clevis, trunnion) reduce side-loading.

Seals

  • Piston seals — single- or double-acting; PTFE-with-elastomer-energizer or polyurethane lip.
  • Rod seals — primary + secondary; polyurethane is the most common.
  • Wiper / scraper — keeps contamination out of the rod side.
  • Static seals — O-rings, often with backup rings.
  • Material compatibility: NBR (mineral oil), FKM (high temp / aggressive fluid), HNBR, PTFE.

Maintenance & Failures

  • Most failures come from contamination — keep fluid clean per ISO 4406.
  • Rod scoring → wiper failure → contamination ingress → seal failure (cycle).
  • External leakage usually means rod seal; internal leakage (drift) means piston seal.
  • Always reseal in a clean environment; check rod for nicks before reassembly.
  • Bleed air after a service to prevent stick-slip and shock.
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