Pourquoi un vérin hydraulique se déplace-t-il lentement ?

A hydraulic cylinder may move slowly because too little effective flow reaches the working chamber, the available hydraulic force is insufficient for the load, the control system limits motion, or mechanical resistance interferes with travel. Leakage and fluid conditions can contribute, but slow movement alone does not identify a failed pump or piston seal.

Begin by describing when the motion changed and which part of the movement is slow. A delayed start, uniform slow travel, intermittent stopping, and intentional end cushioning require different checks. This guide uses flow and force examples to organize the evidence, then connects it to control, temperature, leakage, and mechanical observations before any component is condemned or a correction is accepted.

Sommaire

1. Describe the slow motion precisely
2. Estimate the flow needed for the observed speed
3. Check chamber pressures against the actual load
4. Trace where the commanded flow goes
5. Compare directions, temperature, and other functions
6. Investigate leakage and mechanical resistance separately
7. Use an evidence record to avoid premature replacement
8. Verify the correction under matching conditions
9. Foire aux questions

Describe the slow motion precisely

Compare the present behavior with a valid baseline under the same load, operating mode, temperature, and command. A machine running a different recipe may deliberately use a lower speed. A replacement cylinder may also have a different effective area even if its stroke looks unchanged.

Record command time, first movement, position against time, and arrival at the required endpoint. Identify whether the problem is a long delay before motion, a lower steady speed, a pause under load, or deceleration near the end.

Do not include a long dwell or a programmed pause in the travel interval and interpret it as low cylinder velocity. Likewise, normal cushioning can reduce speed near an endpoint without indicating loss of supply flow throughout the stroke.

State which direction is affected and whether other functions have changed. Motion after the command has stopped belongs to a different diagnostic question, addressed in déplacement du vérin hydraulique.

Before investigating, use the machine's approved arrangements for load support, isolation, and stored-energy control. Do not search for pressure leaks with a hand or loosen a connection to see whether flow is present. HSE describes the serious injury potential of hydraulic injection. HSE guidance.

Estimate the flow needed for the observed speed

For steady movement with negligible leakage and compressibility effects, motion flow is effective area multiplied by velocity: Qmotion = A × v. This is a useful comparison, but it is not automatically the flow measured upstream of a valve or leaking cylinder.

Take an original example with a cap-end area of 50 cm² and a rod-end annular area of 30 cm². Suppose 15 L/min reaches the filling chamber. Ignore leakage, backpressure effects on achieved motion, acceleration, and any special regenerative circuit.

Fifteen liters per minute equals 250 cm³/s. Ideal extension speed is 250/50 = 5 cm/s, or 0.05 m/s. Ideal retraction speed at the same filling flow is 250/30 = 8.33 cm/s, or approximately 0.0833 m/s.

Illustrative condition	Filling area	Ideal speed	Time over a 400 mm steady-travel segment
Extension at 15 L/min	50 cm²	0.05 m/s	8 s
Retraction at 15 L/min	30 cm²	0.0833 m/s	4.8 s
Observed extension taking 16 s	50 cm²	0.025 m/s	Equivalent motion flow: 7.5 L/min

The last row does not prove that the pump delivers only 7.5 L/min. It shows the geometric flow associated with the observed motion. Supply diversion, leakage, compression, measurement error, or a different actual area may explain a difference from upstream flow.

Use a genuinely steady segment for this comparison, and keep all units consistent. The underlying areas are explained in pressure, force, and area.

Check chamber pressures against the actual load

Flow produces the expected speed only when the cylinder can develop the force needed to move the load. If available net force is inadequate, the cylinder may slow, pause, or fail to start, depending on the circuit and load behavior.

Using the same assumed areas, suppose the cap chamber is at 100 bar and the rod chamber at 15 bar during extension. Ignoring friction and acceleration:

Net hydraulic force = 10 × (100 × 50 − 15 × 30) = 45,500 N = 45.5 kN.

If the external resisting load is an assumed 50 kN, this state cannot sustain the intended motion under those assumptions. A higher upstream pump reading would not change the force calculation unless the actual chamber conditions changed.

This example is not a pressure-setting instruction. Check the load estimate, measurements, circuit control, pressure losses, and approved ratings before drawing a conclusion.

Measure or otherwise establish pressure at relevant locations under a reviewed test plan. Pump outlet pressure, valve inlet pressure, and cylinder chamber pressure answer different questions. The distinctions are developed in hydraulic system pressure loss.

Trace where the commanded flow goes

Start with the circuit and its current operating state. Determine which pump supplies the function, which valves route or meter its flow, and whether other branches share the available delivery. A commanded valve position does not guarantee that the intended opening has been achieved.

Possible restrictions include a partially opening valve, an incorrectly configured flow control, an unsuitable replacement connection, or another obstructed path. The return side also matters: its resistance can affect both motion control and the force available to the load.

A relief, unloading, bypass, or competing branch can divert flow away from the cylinder. Pressure-compensated or load-sensing controls may change pump delivery according to their state. Read the specific circuit instead of assuming a constant flow at every command.

Webtec's maintenance discussion shows why pressure alone cannot establish useful pump flow under load. The diagnostic requirement is to compare appropriate flow, pressure, and temperature information, not simply confirm that a gauge reaches a familiar value. Webtec flow and pressure discussion.

Any inserted instrument must suit the fluid, direction, pressure, flow range, and measurement objective. Its own pressure loss and connection location may alter the result. The Webtec tester manual illustrates these model-dependent considerations. Tester manual.

Utilisation pump, valve, and cylinder matching to review compatibility. Do not isolate or block an unknown branch merely to see whether the cylinder becomes faster.

Compare directions, temperature, and other functions

Patterns narrow the investigation but do not prove a cause.

Remarque	Useful next comparison	Limit of the clue
Only extension is slow	Area difference, cap supply path and rod return path	Different speeds can be normal
Both directions slow	Shared supply, command limits and mechanical load	The pump is only one possible cause
Several functions slow together	Common source, drive, fluid condition and shared controls	A common symptom still needs measurements
Slow only after warming	Flow under comparable load, temperature and control state	Heat-related behavior is not proof of wear
Slow only when cold	Fluid suitability, viscosity and inlet restrictions	Warming may hide the symptom without fixing the cause
Slow after maintenance	Changed parts, connections, settings and document revisions	Timing suggests a search area, not a verdict

Compare temperatures at identified points and conditions, not by touching a component. Confirm the fluid actually in service and whether a recent change matches the specification.

Changes in noise, foaming, or movement smoothness may support an inlet or air-related hypothesis. They also need their own evidence. Hydraulic power unit noise explains why a sound alone is not a reliable diagnosis.

For a hot-state problem, record how long it takes to appear and whether it follows load, elapsed time, or a particular sequence. That information can separate a thermal effect from a mode change that happens at roughly the same time.

Investigate leakage and mechanical resistance separately

Internal leakage can reduce the flow available for movement, but cylinder leakage is not the only possible path. Oil may bypass through control valves or other circuit elements. Comparing motion-derived flow with pump delivery does not isolate the piston seal.

A leakage test needs a defined boundary, pressure and temperature conditions, stabilization, instrumentation, and acceptance criteria. The method and interpretation are covered in contrôle d'étanchéité interne des cylindres. There is no universal allowable leakage inferred from a slow stroke.

Mechanical resistance deserves an independent assessment. Check the machine's guides, pivots, mounting alignment, rod condition, and any changed load or fixture. Binding at a particular position may point toward geometry or damage, while a control sequence can produce a similar pattern.

Parker's cylinder troubleshooting bulletin links misalignment and side loading with rod and bearing damage. This supports checking the load path rather than assuming every movement fault is caused by insufficient pump capacity. Its historical numeric tolerances are not applied as universal limits here. Parker troubleshooting bulletin.

Do not disconnect a load-bearing cylinder or force a mechanism through a bind without an engineered support and inspection plan. Separate mechanical inspection from any live hydraulic measurements.

Use an evidence record to avoid premature replacement

Consider a fictional machine that becomes slow in extension after a hose and fitting replacement. Retraction appears normal, but no pressure or flow measurements have yet been made.

The timing makes the changed connection worth checking. It does not prove that the fitting is restricted. A different command, altered load, trapped air, or another maintenance change may be involved.

Record field	Example entry
Observed fact	Extension time increased after a named maintenance event
Known condition	Same requested travel; load and temperature still need confirmation
Hypothesis	A changed flow path or configuration may limit extension
Missing evidence	Part specification, port orientation, chamber pressures and relevant flow
Approved next check	Review parts and circuit, then carry out the defined measurement plan
Conclusion	Remains open until evidence distinguishes the alternatives

Utilisation cylinder drawing review when a cylinder, port, or accessory configuration has changed. Compare complete part identities rather than outward appearance.

A sound repair record identifies the verified cause, the correction, and the evidence that the correction addressed it. Replacing several parts at once may restore motion while leaving the actual cause uncertain. If that happens, record the uncertainty honestly and retain the findings for follow-up.

Verify the correction under matching conditions

Repeat the relevant motion comparison with the same load, command, temperature range, travel segment, and instrumentation. Confirm both directions and any other functions affected by the work.

Record speed or travel time, pressures, relevant flow, temperature, and observed smoothness. Check that the correction has not created an unacceptable pressure peak, return restriction, thermal condition, or change in load control.

A single unloaded movement is not sufficient evidence that a fault occurring under hot loaded operation has been resolved. Conversely, a long list of repeated tests is not useful if the conditions are undocumented.

Follow the quality principles in inconsistent test results. Keep the original and corrected observations comparable, update the configuration record, and have the responsible technical person accept the result.

Foire aux questions

Why does the cylinder retract faster than it extends? At equal filling flow, a smaller annular area can produce a higher retraction speed. The actual circuit and load may change that relationship.

Can normal pressure coexist with slow motion? Yes. The system may reach a pressure while delivering insufficient useful flow, or the gauge may not represent the cylinder's actual conditions.

Does slow motion after seal replacement prove the seals are wrong? No. Review the seal specification, assembly, alignment, fluid, controls, and other work completed at the same time.

Should pressure be increased to make the cylinder faster? Do not use pressure adjustment as a general speed correction. Establish whether the problem is flow, force, control, leakage, or mechanical resistance within the approved system limits.

What information should accompany a service request? Include the circuit and cylinder identity, motion profile, load, command, temperature, maintenance history, and available measurements. The force-speed relationship is summarized in why cylinders generate high force.