Troubleshooting and Safety
Common faults and their causes, how to measure, contamination, injection injuries and stored energy.
A method for finding faults
Fluid power faults are found by thinking in terms of the three quantities: flow (speed), pressure (force) and direction. Is the flow reaching the actuator? Is the pressure right? Is the oil going where the schematic says? Work from the circuit diagram, put a gauge at the test points, change one thing at a time, and never adjust a relief valve upward to cure a slow machine: that hides the fault and overloads the parts.
- Look: level, leaks, oil color and smell (burnt, milky = water, foamy = air), temperature of the tank, pump and valves, filter indicator.
- Listen: whine or rattle at the pump (cavitation or aeration), a screech at a relief valve, hammer in the lines.
- Measure: pressure at the pump, at the valve and at the actuator; flow; temperature; the time of each movement.
- Isolate: block or divert a section to see where the fault changes.
Symptoms and causes
| Symptom | Likely causes | What to check |
|---|---|---|
| No movement | no flow (pump not turning, coupling failed, wrong rotation); relief valve stuck open; valve not shifting; load too heavy | pump flow and rotation; pressure at the relief; solenoid signal; manual override of the valve |
| Slow movement | worn pump (internal leakage); relief valve cracking too low; leaking cylinder seals; valve spool worn; oil too hot or too thin; blocked filter | delivered flow against rating; relief setting; temperature; filter indicator |
| Weak force | low relief setting; leaking seals or valves; pressure reducing valve set low | pressure at the cylinder; set pressures |
| Jerky or spongy motion | air in the oil; low oil level; cold oil; stick-slip of seals; compensator not working | bleed the air; check suction line and level; warm up |
| Noisy pump | cavitation (suction too restricted, oil too cold or thick, clogged strainer); aeration (leaking suction fitting, low level); loose coupling | suction vacuum; fittings; level; alignment |
| Overheating | relief valve passing flow; worn pump; undersized or dirty cooler; low oil level; the wrong oil; long throttling | where the heat is (infrared thermometer along the circuit); duty cycle |
| Load drifts down | leaking piston seals or valve spool; no counterbalance or pilot check; trapped air | drift test below; add a pilot-operated check |
| Oil leaks | worn seals; loose or over-tightened fittings; wrong seal material; pressure spikes; rod scoring | which joint; pressure peaks; the rod finish |
| Pressure spikes, hammering | quick-acting valves on high flow; no cushioning; accumulator pre-charge lost | valve closing time; add a snubber or accumulator; check pre-charge |
| Frequent failures | contamination; wrong oil; overheating; misalignment; cavitation | oil sample (particle count, water); filter history |
Simple tests that tell you a lot
- Pump delivery test: measure the flow to the tank (a flow meter or a timed bucket) at working pressure and compare it with the theoretical flow. A fall of more than 10 to 15 percent means a worn pump.
- Cylinder drift test: hold the cylinder against a load for a time and measure the movement; compare with the leakage rating.
- Relief valve test: block the line and raise the pressure slowly: the pressure at which the valve cracks and the pressure at full flow.
- Temperature check along the circuit with an infrared thermometer: a hot valve is passing oil.
- Oil sample from a running machine for particle count, water and viscosity: the best predictor of failure.
Worked example: a slow press
Worked example: the press has become slow and the oil is hot
- The pressing stroke now takes much longer than the design 4.0 s, and the tank has reached 70 °C. The speed is Q ÷ A, so the flow reaching the cylinder has fallen.
- Measure the pump delivery at 160 bar: it gives 26.0 L/min instead of the new 29.8 L/min: 13 percent lost (volumetric efficiency 82 percent against 94). The press speed is 86 mm/s against 100.
- The lost flow is internal leakage: it turns into heat in the pump, explaining the hot oil. The cause is wear from contamination or cavitation, so before fitting a new pump check the filter, the suction line and an oil sample.
- Cylinder drift test on the same press: if the oil leaks past the piston seals at 20 cm³/min, a load held at pressure creeps 4.0 mm every minute (20 cm³ ÷ 50.3 cm²).
- Fix the cause, replace the pump, fit a new filter element, drain and clean the tank if the oil is contaminated, and record the baseline readings for the next time.
Pneumatic faults
| Symptom | Likely causes | What to check |
|---|---|---|
| Cylinder slow or weak | low pressure at the cylinder (regulator, long or small lines, blocked filter); leaking seals; undersized valve or tubing | gauge at the cylinder with it moving; filter and lube-free condition |
| Stops half-way or shudders | stick-slip; wrong speed control; low pressure; excess friction from side loads | meter-out; alignment; bore finish |
| Water in the system | failed dryer or drain, aftercooler too small, damp climate | dew point; condensate drains; install a drain |
| Constant hiss | leaks; a valve that does not seal; exhaust never closing | soapy water or an ultrasonic leak detector |
| Compressor never stops | leaks, undersized, worn compressor, blocked inlet filter | measure the loaded fraction at night: a leak-tight plant should unload |
Safety: stored energy and injection injuries
- Injection injuries. Oil escaping from a pinhole leak at 160 bar leaves at 192 m/s (v = √(2 Δp ÷ ρ)), fast enough to penetrate skin from a pressure of only about 7 bar. The wound looks small and painless but oil spreads under the skin and can cause gangrene: a high-pressure injection injury is a medical emergency. Never feel for leaks with a hand: use cardboard or a gauge.
- Stored energy. A pressurized accumulator holds energy even with the pump stopped. The 14.7 L example accumulator releases 36 kJ, as much as a 100 kg weight dropping 37 m. Trapped load (a raised cylinder) and trapped pressure in lines are also hazards: dissipate all of it, and lock out and tag before working.
- Thermal pressure. Oil trapped between two closed valves rises about 10 bar per kelvin: a 10 K warm-up gives 98 bar, enough to burst a line. Fit a relief valve in every trapped volume.
- Hose bursts and whipping. A 5 m hose of 16 mm bore at 160 bar holds about 92 J in oil compression and moves violently when it fails: use guards, shields and restraints, and replace hoses on a schedule (typical life 6 years from manufacture).
- Fire. A spray of mineral oil on a hot surface (above about 300 °C) burns; use fire-resistant fluids near furnaces and shield hot parts.
- Pneumatics. Compressed air can inflate a body cavity and a jet at the skin is dangerous (never use it to blow clothing); a pressure above 2 bar at the eyes can blind. Exhaust silencers cut noise; pipes can whip if a coupling fails.
- Machine safety. Presses and clamps are guarded and interlocked to the standards for the machine (for example ISO 4413, ISO 4414, ISO 13849 for the control function and the relevant press standard).
This is general information and not a substitute for the machine’s manual, the safety standards and training.
Maintenance that prevents faults
- Keep the oil clean: change the filter on its indicator, sample the oil two to four times a year, store and fill oil only through filters.
- Keep it cool: clean the cooler fins, check the oil level and the thermostat, keep the temperature below about 55 to 60 °C.
- Check pre-charge on accumulators every six months, and hoses and fittings for leaks and abrasion on every shift.
- Record baseline pressures, flows, temperatures and cycle times when the machine is new; the first sign of wear is a change from the baseline.
- Train people: most faults and accidents come from a lack of understanding of how the circuit holds energy.
Related
- Previous: Designing a Fluid Power System. Back to the overview: Hydraulics and Pneumatics.
- Workshop Safety for the wider safety guide.
- Fluids, Filtration and Reservoirs for oil and filtration.
FAQ
What should I do after a high-pressure injection injury?
Go to hospital immediately; tell the doctors it is a high-pressure injection injury and what the fluid was. Do not wait for symptoms and do not squeeze the wound.
Why is my oil milky?
Water contamination (condensation, a leaking cooler or rain through a breather). Water ruins lubrication and corrodes parts: find the source and dry or change the oil.
Can I bleed air by loosening a fitting under pressure?
No. Never loosen a fitting on a pressurized circuit: use the bleed points with the system at low pressure, or cycle the actuator slowly with no load.
How can I tell cavitation from aeration?
Cavitation is a hard rattling noise like gravel at high flow or when the suction is restricted; aeration is a softer whine with foamy oil, from an air leak on the suction side.
References
- ISO 4413 (hydraulic safety) and ISO 4414 (pneumatic safety); ISO 13849-1 (safety-related parts of control systems); ISO 12100 (risk assessment).
- Esposito, A., Fluid Power with Applications, Pearson; Totten, G. E., Wills, D. K., Feldmann, D., Hydraulic Failure Analysis, ASTM.
- Fluid power societies’ and manufacturers’ safety bulletins on injection injuries and accumulator safety (NFPA, BFPA, VDMA).
Last reviewed: October 2026. Fault tables are typical; follow the machine maker’s procedures.