Cilindro idraulico
Cushioning Design
End-of-Stroke · Deceleration · Pressure Spike

WITHOUT
CUSHION

The piston strikes the end-cap mechanically. Depending on oil compressibility and response time of the pressure relief valve, the impact load can reach 3–10× working pressure for a 2–5 ms spike. This spike is transmitted through the end-cap welds, barrel, and all port connections simultaneously. On a high-cycle machine, each uncushioned impact is a fatigue event — weld crack propagation, port thread fatigue, and end-cap deformation accumulate with each cycle until visible failure occurs, often thousands of cycles before the seal or rod would otherwise need attention.

WITH
CUSHION

The cushion converts kinetic energy into a controlled hydraulic back-pressure over the cushion engagement length. The deceleration force acts in the direction opposing motion — decelerating the piston and load to near-zero velocity before mechanical contact with the end-cap. Peak cushion pressure is typically 1.5–3× working pressure, sustained over 20–80 mm of travel rather than delivered as a 2 ms spike. The cumulative fatigue damage to the lift cylinder structure is reduced by one to three orders of magnitude compared to uncushioned operation.

①

Kinetic energy at cushion entry: E = ½ × 1500 × 0.4² = 120 J

②

Allowable cushion peak pressure: 1.8 × 20 = 36 MPa

③

Cushion bore area (annular, spigot Ø 70 mm in Ø 100 mm bore): A = π/4 × (0.10² − 0.07²) = 4.00 × 10⁻³ m²

④

Deceleration force available: F = P × A = 36 × 10⁶ × 4.00 × 10⁻³ = 144 000 N

⑤

Required cushion length: L = E / F = 120 / 144 000 = 0.00083 m = 0.83 mm

⑥

Design cushion length with 30× safety factor: 0.83 × 30 = 25 mm — select standard 30 mm cushion. The large safety factor accounts for non-uniform pressure distribution during deceleration and the assumption of constant peak pressure which over-estimates efficiency. In practice, 25–40 mm is adequate for this duty.

HYDRAULIC
KNOCK

Symptom: Sharp metal-on-metal knock at end-of-stroke on a lift cylinder that previously operated silently. Cause: Cushion needle valve has vibrated open, cushion check valve has stuck open (bypassing the needle), cushion spigot has worn and clearance enlarged beyond specification, or contamination has blocked the check valve seat open. Action: Re-set the needle valve if accessible; if the knock persists with needle fully closed, the check valve or spigot requires inspection and likely replacement.

SLOW
START-OUT

Symptom: Lift cylinder accelerates very slowly from end position before reaching normal speed — particularly noticeable on the return stroke from cushioned end. Cause: Cushion check valve is not opening properly on reversed stroke — the oil filling the cushion pocket must pass through the restricted needle path rather than through the free-flow check valve path. Check valve may be stuck, seat debris, or spring broken. Action: Remove and inspect the check valve; replace the check valve cartridge.

VERY SLOW
END APPROACH

Symptom: Lift cylinder decelerates dramatically as it approaches end-of-stroke — cycle time is far longer than specified and the load appears to barely move in the last portion of travel. Cause: Adjustable cushion needle valve is over-closed — the flow restriction is so high that the oil can barely escape the cushion pocket. Common after maintenance by personnel who closed the needle “to be safe” without following the quarter-turn adjustment procedure. Action: Re-set the needle valve following the STEP 1–4 procedure in Section 03 of this guide.

END-CAP
WELD CRACK

Symptom: Oil seepage from the end-cap weld seam, particularly from the weld toe — the junction between the barrel tube and the end-cap face. May be accompanied by rust staining or external crack visible under the coating. Cause: Long-term fatigue crack growth driven by repeated uncushioned end-of-stroke pressure spikes. Each spike cycles the lift cylinder weld between high tension and return — classical high-cycle fatigue in the heat-affected zone of the weld. Action: The lift cylinder must be removed from service immediately. The weld crack will propagate to full barrel failure under continued operating pressure. A new lift cylinder with correctly specified end-of-stroke cushioning should replace the failed unit — the root cause (inadequate cushioning) must be corrected, not just the failed component.

Q 01

Can cushioning be added to an existing lift cylinder that was built without it?

Adding cushioning to an existing lift cylinder that was built without it is possible but requires full disassembly and machining of new end-caps with cushion bores, and a new piston or piston sleeve with the cushion spigot geometry. In most cases, this rebuild cost is comparable to purchasing a new lift cylinder with cushions built in from manufacture — particularly for small-bore cylinders where the machining cost is a high fraction of the total component cost. The exception is large-bore, long-stroke cylinders where the barrel and rod represent the majority of the value — in these cases, retrofitting cushion end-caps can be cost-effective if the bore and rod are in serviceable condition. A more practical approach for existing uncushioned lift cylinders is to reduce the operating speed via flow control valves so that the kinetic energy at end-of-stroke falls below the threshold that causes structural fatigue. This is a valid interim measure but reduces cycle time — the correct long-term solution is a replacement lift cylinder with cushions built in from new manufacture.

Q 02

Our lift cylinder has cushions on both ends but still makes a knock at the extension end-of-stroke only. Retraction arrives smoothly. Why only one end?

The asymmetry in cushioning performance between extension and retraction end-of-stroke almost always results from a difference in piston velocity or effective mass at the two ends. In a double-acting lift cylinder, the effective annular area on the rod end is smaller than the full bore cap end — so for the same pump flow rate, the rod-end retraction speed is higher than the cap-end extension speed. If the cushions are identically sized for both ends, the retraction cushion must absorb more kinetic energy per unit time than the extension cushion, yet the cushion area available is also smaller on the rod end. A lift cylinder that arrives smoothly at cap-end extension but knocks at rod-end retraction needs a longer cushion or smaller clearance on the rod end than the cap end. Review the lift cylinder cushion specification for each end independently, using the actual piston velocity and load at that stroke direction — do not assume symmetrical cushion sizing is correct when the areas and velocities differ.

Q 03

What is the difference between a cushion and an external deceleration valve — when should each be used?

An integral cushion built into the lift cylinder end-cap and an external deceleration valve (mounted in the hydraulic circuit) both achieve the same goal — decelerating the piston before end-of-stroke — but through different mechanisms and at different points in the circuit. An integral cushion acts on the oil trapped between the piston and the end-cap face — it is always correctly positioned relative to the piston’s actual location regardless of circuit configuration. An external deceleration valve acts on the oil flowing through the circuit to the lift cylinder and is triggered by a position cam or proximity switch — it can decelerate the piston from any point in the stroke, not just the final cushion length. External deceleration valves are preferred when the deceleration zone must be longer than a practical cushion geometry allows (for example, decelerating a fast-moving overhead crane trolley over 200–300 mm), when the cylinder cannot easily accommodate a cushion spigot (very short stroke cylinders), or when the deceleration point must be adjustable without opening the cylinder. Integral cushions are preferred for standard lift cylinder duty where the deceleration requirement is modest and the stroke geometry allows a standard spigot-and-bore cushion pocket to be machined into the end-cap. Many high-performance lift cylinder systems use both — the external valve reduces velocity and the integral cushion absorbs the final residual kinetic energy at full end-of-stroke position.

Q 04

Does low temperature affect cushion performance — should we adjust the needle valve setting in winter?

Yes — hydraulic oil viscosity increases significantly as temperature drops, and this directly affects cushion performance. A lift cylinder cushion needle valve set at warm operating temperature (50–60°C, where ISO 46 oil has a viscosity of approximately 10 cSt) will produce a much higher back-pressure at cold start temperatures (0–10°C, where the same oil may be 200–400 cSt). The increased viscosity means oil exits the cushion pocket far more slowly for the same needle opening — the effective cushion restriction is several times higher in cold conditions than at operating temperature. A hydraulic lift cylinder with an adjustable cushion whose needle is set correctly at operating temperature may arrive at end-of-stroke almost hydraulically locked at cold start, severely restricting the first few cycles of the day. For machines that must operate in cold conditions from cold start, the lift cylinder cushion needle valve should be set at cold temperature rather than hot, and a high-viscosity-index (HV-grade) oil used to minimise the viscosity difference between cold and operating temperatures. Alternatively, a warm-up cycle at reduced speed should be included in the machine’s start procedure before full-speed production operation begins.