Troubleshooting Cylinder Drift and Creep in Hydraulic Systems: Internal Leakage, Seal Testing, Back Pressure Valve and Pilot Pressure

1. Problem description & scope of application

This guide covers diagnosing and correcting excessive cylinder drift and creep in hydraulic systems. Cylinder drift is the undesirable, slow movement (sinking or moving in one direction) of a cylinder under static load when it should actually remain in position. Creep, on the other hand, describes a jerky, uneven movement of the cylinder, especially at low speeds or under changing loads. Both phenomena lead to loss of precision, inefficient operation and can cause serious safety risks and production downtime.

Affected systems and cylinder types:

• Industrial presses (metal forming, plastics)
• Lifting devices and lifting tables
• Injection molding machines
• Test and test stands with hydraulic actuators
• Assembly and handling robots
• Construction machinery and mobile hydraulics (e.g. stabilizers, booms)

Classification of severity:

• Critical (danger to life / system failure): Uncontrolled, rapid drift of loads that can endanger people or lead to complete system failure. Immediate system stop and troubleshooting are essential.
• Major (Production Loss / Quality Issues): Drift or creep resulting in unacceptable quality tolerances, increased scrap, or significant downtime. Planning for immediate repair required.
• Minor (inefficiency / early warning signs): Minor, slow drift or slight creep that does not yet have a direct impact on production or safety, but indicates the beginning of wear. Planning of preventive measures and monitoring required.

2. Safety instructions

ATTENTION: Working on hydraulic systems poses significant risks due to high pressure, hot liquids and uncontrolled movements. Failure to follow the safety instructions can result in serious injury or death.

• Danger due to pressure: Hydraulic systems can have pressures of over 600 bar. Escaping oil under high pressure can penetrate the skin and cause serious internal injuries. Never look for leaks with your bare hands.
• Lockout/Tagout (LOTO): Before any diagnostic or maintenance work begins, the system must be completely de-energized in accordance with DIN EN 1037 and VDE 0105-100 and secured against being switched on again. All energy sources – electrical, hydraulic, pneumatic – must be isolated and labeled.
• Relieve residual energy: After switching off the system, remaining system pressures (especially in accumulators) must be safely relieved. Hydraulic cylinders must be secured mechanically to prevent uncontrolled lowering.
• Personal protective equipment (PPE): Always wear suitable PPE: safety glasses (DIN EN 166), cut-resistant and liquid-tight protective gloves (DIN EN 388, DIN EN 374), safety shoes with toe protection (DIN EN ISO 20345) and, if necessary, hearing protection.
• Danger due to heat: Hydraulic oil can reach high temperatures during operation. There is a risk of burns upon contact. Allow oil to cool before handling or wear appropriate protective gloves.
• Leaking oil: Leaking hydraulic oil poses a slipping hazard and can cause environmental contamination. Use suitable collection containers and collect spilled oil immediately.

3. Diagnostic tools required

The following tools are required for precise error analysis:

4. Initial assessment checklist

Before beginning the actual diagnosis, careful recording of the system information and symptoms is essential.

5. Systematic diagnostic flowchart

This flowchart guides the technician through a structured troubleshooting process to identify the cause of cylinder drift or creep.

1. Symptom: Cylinder drift or creep
   • Initial check: External leaks visible?
   • IF YES:
     1. SAFETY NOTE: Pressure relief before leak removal!
     2. Localization of the leak (hose, screw connection, rod seal).
     3. Eliminate leakage (replace seal, tighten screw connection).
     4. Fill and vent the system.
     5. Carry out a functional test. Problem solved?
     6. IF YES: Diagnosis completed.
     7. IF NO: Go to step 2 (internal leak).
   • IF NO (no external leakage):
     1. Diagnostic step 1: Check internal leakage of the cylinder.
        1. SAFETY NOTE: Secure the cylinder mechanically to prevent it from sinking. Perform LOTO.
        2. Extend or retract cylinder fully.
        3. Disconnect both cylinder connections from the hydraulic system (or close shut-off valves if equipped).
        4. Close a cylinder connection.
        5. Connect a hand pump with a pressure gauge to the other cylinder connection and pressurize the cylinder piston to the maximum operating pressure (e.g. 200 bar, if the operating pressure is 200 bar).
        6. Observe the pressure drop over 10 minutes.
        7. Rate results:
           • Acceptable: Pressure drop < 5 bar/10 min. (piston seal is probably intact).
           • Noticable: Pressure drop > 5 bar/10 min. (Probably internal leakage of the piston seal).
        8. Alternative test (if cylinder cannot be insulated):
           1. Move cylinder to center position under load.
           2. Bring the directional control valve into neutral position or lock it.
           3. Connect pressure gauges to the cylinder connections (pressure and return sides).
           4. Observe the pressure drop on both sides over 10 minutes.
           5. Rate results: Pressure drop on the loaded side while pressure builds up on the unloaded side (or return) indicates internal leakage.
     2. IF internal cylinder leakage confirmed: Go to 7.1 and 8.1 (seal replacement).
     3. IF no internal cylinder leakage:
        1. Diagnostic step 2: Check internal leakage of the directional control valve.
           1. SAFETY NOTE: Perform LOTO. Secure cylinder mechanically.
           2. Fully retract cylinder (if possible) and secure mechanically.
           3. Disconnect both cylinder connections on the directional control valve and connect them to the return line (tank) using a suitable flow meter.
           4. Move the directional control valve to a position that should keep the cylinder under load (e.g. "extend cylinder" when the cylinder is retracted).
           5. Observe the measured volume flow through the flow meter.
           6. Rate results:
              • Acceptable: Volume flow < 0.5 l/min (depending on valve size and manufacturer's information).
              • Noticable: Volume flow > 0.5 l/min (probably internal leakage of the directional control valve).
     4. IF internal directional control valve leakage confirmed: Go to 7.2 and 8.2 (valve revision/replacement).
     5. IF no internal directional valve leakage:
        1. Diagnostic step 3: Check back pressure valve.
           1. SAFETY NOTE: Perform LOTO. Secure cylinder mechanically.
           2. Attach the pressure gauge in front of and behind the counter-pressure valve (if access is available).
           3. Move the cylinder slowly and observe the pressures in front of and behind the valve.
           4. Check the setting value of the valve and, if necessary, compare it with the manufacturer's specifications.
           5. Rate results:
              • Acceptable: Pressure difference across the valve during flow corresponds to the manufacturer's specifications, no unnecessary pressure drop when the valve is open.
              • Noticable: Valve does not open at target pressure, opens too early, blocks or generates inconsistent back pressure (creep).
           6. Additional check (only for creep): Check the counter-pressure valve for contamination or mechanical blockage. A stuck slider can lead to uneven pressure build-up and therefore creep.
     6. IF counterpressure valve faulty: Go to 7.3 and 8.3 (repair/adjust valve).
     7. IF backpressure valve intact:
        1. Diagnostic step 4: Check pilot pressure (for proportional or servo-hydraulic systems).
           1. SAFETY NOTE: Perform LOTO. Secure cylinder mechanically.
           2. Connect the pressure gauge to the pilot pressure line of the directional control valve.
           3. Observe the pilot pressure during operation (or in simulated operating conditions).
           4. Rate results:
              • Acceptable: Constant pilot pressure according to manufacturer's specifications (often 5-20 bar).
              • Noticable: Fluctuating, too low or missing pilot control pressure.
           5. Additional check: Check the electrical control of the pilot control valve with a multimeter (setpoint current values ​​in mA).
     8. IF pilot pressure faulty: Go to 7.4 and 8.4 (check pilot pressure regulator/lines).
     9. IF all tests without a clear result: Possible system errors, air in the system, incorrect oil or more complex hydraulic interactions. A comprehensive system analysis or contacting UNITEC-D support is recommended.

6. Error-cause matrix

This matrix maps the most common symptoms to their likely causes, required diagnostic tests, and expected outcomes.

7. Root cause analysis for each malfunction

7.1 Internal leakage of the piston seal (cylinder)

Explanation: The piston seal separates the pressure chambers on both sides of the piston. In the event of wear, damage or aging, hydraulic oil can flow from the high-pressure side to the low-pressure side of the cylinder. This results in the cylinder no longer being able to maintain its position under load and drifting.

Confirmation: The cylinder insulation pressure test (see 5.1a) is the primary method. A rapid pressure drop (e.g. > 5 bar in 10 minutes) on the insulated cylinder when pressure is applied is a clear indicator of an internal leak in the piston seal. Further confirmation can be provided by dismantling the cylinder and visually inspecting the seal and the cylinder bore (scratches, grooves, traces of overheating).

Damage if not rectified: Persistent leaks lead to higher energy consumption (pump has to deliver more), increased oil temperature, accelerated oil degradation and ultimately to complete functional failure of the cylinder. The heat generated by leakage at the piston can further damage the seal and lead to a vicious circle.

7.2 Internal leakage of the directional control valve

Explanation: Directional valves control the flow direction of the hydraulic oil to the cylinder. Even when locked or neutral, they should block the flow of oil as much as possible. Wear on the mating surfaces of the spool and housing, contamination that prevents the spool from closing completely, or mechanical damage can cause oil to leak internally through the valve into the return or incorrect chamber, causing the cylinder to drift.

Confirmation: The valve leakage test with a flow meter (see 5.2a) is crucial here. A measurable volume flow (e.g. > 0.5 l/min) through the valve in a position assumed to be blocked confirms an internal leak. Measuring the valve housing temperature using an infrared thermometer can also provide information; Internal leaks create local heat (“hotspots”) that can be 10-20°C above the system temperature.

Damage if not rectified: As with cylinder leakage, this leads to loss of energy, heat generation and ultimately loss of function of the system control. A leaking directional control valve can also negatively affect the response time and precision of the cylinder movement.

7.3 Defective or incorrectly adjusted counter pressure valve

Explanation: Back pressure valves are often used in the return line of cylinders to generate a defined back pressure. This counterpressure is important to avoid the formation of voids (cavitation), to increase the rigidity of the system and to prevent or dampen uncontrolled sinking under vertical loads. A defect (e.g. broken spring, jamming due to dirt) or an incorrect setting (too low) can lead to insufficient counter pressure and thus to cylinder drift or to creep when opening and closing jerkily.

Confirmation: Testing using a pressure gauge in front of and behind the valve (see 5.3a) provides information about the pressure control characteristics. A deviation from the manufacturer's specifications for the opening pressure or an inconsistent pressure maintenance during flow are indicators. If creep occurs, it may be necessary to disassemble the valve for visual inspection for contamination or mechanical damage to the valve spool.

Damage if not rectified: In addition to drift, a faulty back pressure valve can lead to cavitation in the cylinder, which severely damages the piston seal and the cylinder bore. It reduces control over the load, increases wear on other components and can lead to unsafe operating conditions.

7.4 Insufficient or fluctuating input control pressure

Explanation: Many modern directional control valves (especially proportional and servo valves) are pilot-controlled hydraulically or electro-hydraulically. A constant and correct pilot pressure is crucial for the precise positioning of the main spool in the directional control valve. If the pilot pressure is too low, fluctuates or fails completely, the main spool cannot be positioned correctly or stably. This can lead to leaks within the valve, incomplete closing or jerky behavior (creeping) of the cylinder.

Confirmation: Measuring the pilot pressure directly on the pilot line (see 5.4a) is essential. A pressure value outside the manufacturer's specifications (usually 5-20 bar) or a fluctuating pressure is suspicious. Additional checks include the functionality of the pilot pressure control valve (pressure reducing valve), the associated filter and the control lines for blockages. For electro-hydraulic pilot control stages, the electrical control current (mA) should be checked using a multimeter and compared with the manufacturer's target values.

Damage if not rectified: Imprecise cylinder movements, reduced machine performance, increased wear on cylinders and valves due to uncontrolled movements or pressure peaks. The system can no longer fulfill its specified tasks.

8. Step-by-step repair procedure

Before any repair, the system must be secured in accordance with the safety instructions (Section 2)!

8.1 Repair: Changing the cylinder seals

1. Preparation:
   • SAFETY NOTE: Secure the LOTO system. Mechanically relieve and secure the cylinder. Relieve system pressure.
   • Remove the cylinder from the machine and position it on a clean workbench.
   • Drain hydraulic oil from the cylinder (collect it and dispose of it in an environmentally friendly manner).
2. Dismantling:
   • Dismantle the cylinder head (loosen the screws, use a puller if necessary).
   • Carefully pull the piston rod with piston out of the cylinder tube.
   • Remove old piston seals and rod seals. Thoroughly clean all seal grooves and seats. Check the cylinder bore and piston rod for damage (scoring, corrosion). Minor damage can be repaired by lapping or polishing; If there is major damage, replace the cylinder tube or piston rod.
3. Seal assembly:
   • Use new seals (piston and rod seals, wipers, O-rings) from the UNITEC-D spare parts catalog. Ensure material and size meet manufacturer specifications (e.g. NBR, FKM, PTFE; Shore A hardness).
   • Lightly moisten seals with clean hydraulic oil.
   • Carefully insert seals into the grooves, avoiding twisting or damaging them. Use special assembly tools if necessary.
4. Reassembly:
   • Carefully insert the piston rod with piston into the cylinder tube.
   • Install cylinder head. Tighten screws according to manufacturer's specifications using a torque wrench (e.g. 120 Nm for M16 screws, class 10.9).
   • Reinstall the cylinder in the system.
5. Commissioning and testing:
   • Reconnect the hydraulic lines.
   • Fill the system with clean hydraulic oil and bleed thoroughly (move the cylinder slowly over the entire stroke several times).
   • Carry out a functional test, place the cylinder under load and check drift behavior according to the initial assessment (Section 4). Acceptable drift < 2 mm/10 min under full load at system pressure.

8.2 Repair: Revision/replacement of the directional control valve

1. Preparation:
   • SAFETY NOTE: Secure the LOTO system. Relieve system pressure.
   • Clean valve block area.
2. Dismantling:
   • Disconnect and label electrical connections.
   • Disconnect the hydraulic lines at the directional control valve and immediately close them with blind plugs to avoid contamination.
   • Dismantle the directional control valve.
3. Revision (if possible and economical):
   • Check the valve body and slide for signs of wear, scoring or corrosion.
   • Thoroughly clean the slide and housing.
   • Replace seal set according to manufacturer's instructions.
   • If the slide or housing is heavily worn, the entire valve must be replaced.
4. Installation:
   • Assemble a new or revised valve.
   • Reconnect hydraulic lines and electrical connections correctly. Make sure the screw connections are tightened correctly.
5. Commissioning and testing:
   • Build up system pressure slowly and check for leaks.
   • Bleed the system.
   • Carry out a functional test, place the cylinder under load and check drift behavior according to the initial assessment (Section 4). Acceptable drift < 2 mm/10 min under full load at system pressure.
   • For proportional or servo valves: functional test and calibration according to the manufacturer's manual.

8.3 Repair: Repair/adjustment of the counter-pressure valve

1. Preparation:
   • SAFETY NOTE: Secure the LOTO system. Relieve system pressure.
   • Clean valve area.
2. Disassembly and inspection:
   • Attach pressure gauges to the test points on the valve (if not already done).
   • Dismantle the valve.
   • Check the valve slide, spring and seat for contamination, damage or spring breakage. Clean all components carefully.
3. Assembly and adjustment:
   • Assemble a revised or new valve.
   • Build up system pressure.
   • SAFETY NOTE: Only adjust the adjusting screw in small increments and observe the pressure change on the pressure gauge.
   • Set the counter pressure valve to the target pressure specified by the manufacturer (e.g. 10 bar for a defined system rigidity).
4. Test:
   • Move the cylinder with the usual working load and measure the back pressure. It must be stable and constant. If there are jerky movements or creeping, fine adjustment or further testing for internal blockages may be necessary.

8.4 Repair: Checking and repairing the pilot pressure circuit

1. Preparation:
   • SAFETY NOTE: Secure the LOTO system. Relieve system pressure.
2. Pressure measurement:
   • Connect the pressure gauge to the pilot pressure line of the directional control valve.
   • Put the system back into operation and measure the pilot pressure. Setpoint comparison with manufacturer information (e.g. 5-20 bar).
3. Research into the cause of deviations:
   • Pressure reducing valve: If the pilot pressure is too low, check the pressure reducing valve that provides the pilot pressure. This may be dirty, defective or incorrectly adjusted. If necessary, overhaul or replace the valve.
   • Filter: The filter in the pilot control circuit may be clogged. Change filter element.
   • Lines: Check pilot lines for kinks, blockages or damage.
   • Electro-hydraulic pilot control: For proportional or servo valves, check the electrical control (cable, plug) and the control current (mA) using a multimeter. Faulty electronics can lead to fluctuating pilot pressure.
4. Check:
   • After the cause has been eliminated, measure the pilot pressure again and check for stability.
   • Carry out a functional test of the system.

9. Preventive measures

Regular maintenance and monitoring are critical to prevent cylinder drift and creep.

10. Spare Parts & Components

High-quality spare parts are crucial for quick and reliable repairs. UNITEC-D offers an extensive range of hydraulic components.

Do you need further information or would you like to order spare parts? Visit our UNITEC-D e-catalog.

11. References

• DIN EN ISO 4413: Hydraulic fluid technology - general rules and safety requirements for systems and their components.
• DIN ISO 10762: Hydraulic fluid technology – cylinders – marking and rated pressures.
• VDI 2382 Sheet 1: Quality assurance in fluid technology – hydraulic cylinders.
• Manufacturer-specific documentation and maintenance manuals for the affected systems.
• Specialist literature on hydraulic diagnostics, e.g. “Fluid technology – basics, components, systems” by K.H. Grote & J. Feldhusen.