Troubleshooting Hydraulic Cylinders: Drift and Creep Diagnostics

1. Description of the Problem and Scope of Application

This manual is intended for the systematic diagnosis and elimination of malfunctions associated with the uncontrolled movement of hydraulic cylinders, namely drift (uncontrolled movement in a static position) and creep (uneven, jerky movement during operation). These problems can occur in a wide range of industrial equipment that uses hydraulic actuators, including presses, hoists, clamping devices, actuators and other production equipment.

Types of Equipment:

• Machining machines from ChPK
• Hydraulic presses
• Lifting platforms and jacks
• Foundry machines
• Loading and unloading equipment

Classification of Severity:

• Critical: Loss of load control, direct threat to personnel safety, significant production downtime. Requires immediate intervention.
• Significant: Reduction of positioning accuracy, increase in cycle time, deterioration of product quality. Affects production efficiency.
• Minor: Intermittent or barely noticeable symptoms that may indicate the initial stage of a malfunction. Requires monitoring and scheduled diagnostics.

2. Security measures

CAUTION: SAFETY! Before starting any diagnostic or repair work on hydraulic systems, it is necessary to strictly follow the safety rules. Failure to do so could result in serious injury or death.

• Lockout/Tagout (LOTO): Always switch off the equipment power supply and apply lockout/tagout procedures according to UNITEC internal instructions and the DSTU EN 1037:2018 (Machine Safety. Prevention of Unexpected Start) standard.
• Pressure Relief: Make sure all hydraulic pressure in the system is relieved before disconnecting any lines or disassembling components. Use pressure gauges to check zero pressure.
• Energy Storage: A load held by a hydraulic cylinder can have significant potential energy. Always mechanically secure or support the load before working with the cylinder.
• Personal Protective Equipment (PPE): Be sure to use safety glasses (DSTU EN 166), gloves (DSTU EN 388), protective clothing and protective shoes (DSTU EN ISO 20345).
• Hot Surfaces and Fluids: Hydraulic fluids and components can be hot. Always allow the system to cool before operating. Jets of hydraulic fluid under high pressure can penetrate the skin, causing serious injury.

3. Necessary Diagnostic Tools

The following list of tools is required for effective diagnosis:

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, it is necessary to collect and record the following information:

5. Systematic Diagnostics (Diagnostic Algorithm)

SYMPTOM: Hydraulic cylinder drift (uncontrolled movement in a static position)

1. STEP 1: Check for external leaks.
   • Action: Thorough visual inspection of the rod, cuffs, ports, hydraulic lines, and cylinder connections.
   • IF leaks are detected (visible traces of liquid, dripping) → PROBABLE CAUSE: Damaged external seals (rod, cap) or loose/damaged hydroline connections. → Go to Section 7: Root Cause Analysis (External Leakage of Seals/Couplings).
   • OTHER (no external leaks) → Go to STEP 2.
2. STEP 2: Check the cylinder piston seals for internal leakage.
   • Action:
     1. Move the cylinder rod to the middle position.
     2. Disconnect hydraulic lines from both cylinder cavities (piston and rod) or use special diagnostic adapters.
     3. Shut off both cylinder ports or connect a flow meter to one of them and a pressure gauge to the other.
     4. Apply operating pressure (for example, 100 bar) to one of the cylinder cavities by blocking the output from the other, or by measuring the flow with a flow meter.
     5. Observe stem movement (if ports are plugged) or flow meter readings (if flow is measured).
   • IF the rod moves or the flowmeter shows significant flow (eg > 0.1 l/min at 100 bar) → PROBABLE CAUSE: Internal leakage of piston seals. → Go to Section 7: Root Cause Analysis (Piston Seal Internal Leakage).
   • OTHER (flow is normal or absent, stem does not move) → Go to STEP 3.
3. STEP 3: Checking shut-off valves (check, check, balancing).
   • Action:
     1. Lock the cylinder in a certain position under load.
     2. Isolate the cylinder from the rest of the system with shut-off ball valves or disconnect the hydraulic lines and silence them.
     3. Apply operating pressure to the hydraulic line holding the cylinder and monitor it with a pressure gauge.
     4. Measure the pressure drop over a period of time (e.g. 5 minutes).
   • IF cylinder drifts or pressure gauge shows pressure drop (eg > 5 bar in 1 minute) → PROBABLE CAUSE: Failure of check valves (check valve, pilot controlled check valve, balancing valve). → Go to Section 7: Root Cause Analysis (Faulty Check Valves).
   • OTHERWISE (valves hold pressure) → Go to STEP 4.
4. STEP 4: Check pilot pressure (for pilot operated valves).
   • Action: Connect a pressure gauge to the valve pilot line (if applicable). Measure the pressure while trying to hold the cylinder.
   • IF control pressure is absent or unstable (eg < 20 bar for typical systems) → PROBABLE CAUSE: Control line or pilot pressure source problem (eg blockage, pilot pressure relief valve failure). → Go to Section 7: Root Cause Analysis (Low/Unstable Pilot Pressure).
   • OTHER (pilot pressure normal) → Check other possible causes or consider combined faults.

SYMPTOM: Hydraulic cylinder creep (uneven, jerky movement during operation)

1. STEP 1: Check system pressure.
   • Action: Connect a pressure gauge to the pressure supply line to the cylinder and to the return line. Observe the readings as the cylinder moves. Measure the relief valve opening pressure.
   • IF pressure is unstable (fluctuations > 10% of nominal) or below normal → PROBABLE CAUSE: Pump failure, clogged filter, defective relief valve. → Go to Section 7: Root Cause Analysis (Unstable Pump Pressure).
   • OTHERWISE (pressure is normal) → Go to STEP 2.
2. STEP 2: Check for air/gas in the system (cavitation).
   • Action: Visually observe the hydraulic fluid in the tank during operation (presence of foam, bubbles). Listen to the pump for characteristic cavitation noises. Check the fluid level in the tank.
   • IF air detected (foam in tank, bubbles, pump cavitation noise) → PROBABLE CAUSE: Pump cavitation, air intake due to leaky suction line, or low fluid level. → Go to Section 7: Root Cause Analysis (Air/Gas in System).
   • OTHER (no air) → Go to STEP 3.
3. STEP 3: Check for friction in the cylinder or mechanism.
   • Action:
     1. Disconnect the cylinder from the working load (if possible).
     2. Move the cylinder rod by hand along the entire length of the stroke.
   • IF the movement of the rod is uneven, there is resistance, jamming, or noticeable beating of the rod (>0.05 mm) → PROBABLE CAUSE: Mechanical jamming, distortion of the rod/cylinder sleeve, wear of the guide elements, incorrect centering of the cylinder. → Go to Chapter 7: Root Cause Analysis (Mechanical Friction/Seize).
   • OTHERWISE (rod movement is smooth) → Go to STEP 4.
4. STEP 4: Check hydraulic fluid quality.
   • Action:
     1. Take a sample of hydraulic fluid from the tank and from the work line.
     2. Visually assess pollution, color, smell.
     3. Use the kit for rapid testing (water, acidity).
     4. If necessary, send the sample for laboratory analysis (purity according to ISO 4406, viscosity).
   • IF fluid is contaminated (visible particles, emulsion, discoloration/odor) or lab analysis shows degradation (e.g. purity class worse than ISO 4406 18/16/13) → PROBABLE CAUSE: Fluid contamination, additive degradation. → Go to Section 7: Root Cause Analysis (Contaminated Fluid).
   • OTHER (Fluid OK) → Go to STEP 5.
5. STEP 5: Check balance valve adjustment (if installed).
   • Action: Check balance valve adjustment according to manufacturer's specification or design data. Using a manometer, measure the valve opening pressure.
   • IF settings are not normal (for example, pressure differs from specification by more than 10%) → PROBABLE CAUSE: Incorrect balancing valve setting. → Go to Section 7: Root Cause Analysis (Balancing Valve Adjustment Incorrect).
   • OTHER (settings are normal) → Check other possible causes or consider combined faults.

6. Matrix of Malfunctions and Causes

This table provides a quick overview of common symptoms, likely causes, and diagnostic methods:

7. Root Cause Analysis for Each Malfunction

Internal leakage of piston seals:

• WHY IT HAPPENS:
  • Wear and aging: Natural process of degradation of seal material over time and under cyclic loads.
  • Mechanical damage: Scratches or gouges on the stem/sleeve surface caused by contamination in the fluid or improper installation.
  • Thermal degradation: Overheating of the hydraulic fluid leads to hardening and brittleness of the seal material.
  • Chemical incompatibility: Using a liquid incompatible with the material of the seals, causing them to swell or decompose.
  • Incorrect installation: Damage to seals during installation.
• HOW TO CONFIRM: Test for flow through the piston with a flowmeter (Chapter 5, Drift, STEP 2).
• WHAT DAMAGE WILL BE CAUSED: Loss of positioning accuracy, reduction in cylinder speed, increase in hydraulic fluid temperature (due to the conversion of flow energy into heat), increased load on the hydraulic pump, potential destruction of the cylinder itself due to wear of internal surfaces.

Malfunction of holding valves:

• WHY IT HAPPENS:
  • Contamination: Dirt particles can get under the valve seat, preventing it from closing completely.
  • Seat/spool wear: Repeated actuation cycles and high pressures can wear the valve working surfaces.
  • Spring damage: Weakening or breakage of the spring that keeps the valve closed.
  • Malfunction of electrical control: For solenoid valves – break of the solenoid winding, jamming of the spool.
• HOW TO CONFIRM: Isolated valve pressure hold test with pressure gauge (Chapter 5, Drift, STEP 3).
• WHAT DAMAGE WILL BE CAUSED: Uncontrolled movement of the load, which poses a direct threat to the safety of personnel and equipment, damage to equipment due to falling or incorrect positioning, loss of production.

External leakage of seals/connections:

• WHY IT HAPPENS:
  • Wear and aging of rod seals: Similarly to pistons, but also the influence of external factors (dust, abrasive).
  • Stem surface damage: Scratches, corrosion, or dents on the chrome surface of the stem that damage the seal.
  • Loose Connections: Vibration or cyclic loads can cause hydraulic line threaded connections to loosen.
  • Incorrect installation: Insufficient or excessive tightening torque, distortions during assembly.
• HOW TO CONFIRM: Visual inspection, pressure leak test (Section 5, Drift, STEP 1).
• WHAT WILL BE DAMAGED: Loss of hydraulic fluid resulting in a drop in tank level and potential air entrainment, environmental pollution, increased risk of slipping, damage to nearby electrical components.

Low/unstable pilot pressure:

• WHY IT HAPPENS:
  • Clogged control line: Contaminant particles can block or restrict fluid flow to the valve pilot port.
  • Pilot pressure relief valve failure: Worn, contaminated, or improperly adjusted pressure relief valve that supplies pressure to the pilot lines.
  • Internal leakage in controls: Loss of pressure in the pilot line due to internal leaks in other valves or lines.
• HOW TO CONFIRM: Measure pilot line pressure with gauge (Chapter 5, Drift, STEP 4).
• WHAT DAMAGE WILL BE CAUSED: Hydraulic valve control failure resulting in uncontrolled cylinder movement or drift.

Air/gas in the system (cavitation):

• WHY IT HAPPENS:
  • Low liquid level in the tank: The pump starts to suck in air along with the liquid.
  • Damage to the pump suction line: Cracks, leaking connections on the suction side allowing air to enter the system.
  • Pump failure: Wear of internal pump components causing cavitation.
  • Insufficient air removal: The system has not been completely air purged after replacing components or fluids.
• HOW TO CONFIRM: Visual inspection of tank (foam, bubbles), typical pump cavitation noise, unstable pressure (Chapter 5, Creep, STEP 2).
• WHAT DAMAGE WILL BE CAUSED: Intensive wear of hydraulic components (pump, valves, cylinders) due to micro-hydraulic shocks, fluid degradation, loss of system efficiency, uneven movement (creep).

Mechanical friction/seizing in the cylinder:

• WHY IT HAPPENS:
  • Cylinder rod or sleeve warping: Mechanical impacts or overloading can deform components.
  • Guide wear: Stem guide bushings or rings can wear, causing misalignment.
  • Damage to the inner surfaces of the cylinder: Corrosion, erosion or abrasive wear on the inner surface of the sleeve.
  • Misalignment: Misalignment of cylinder axis and load axis causing lateral loads on the rod.
• HOW TO CONFIRM: Rod movement without load (manual), measuring rod runout with watch type indicator (>0.05mm), visual inspection of cylinder after disassembly (Chapter 5, Creep, STEP 3).
• WHAT DAMAGE WILL BE CAUSED: Irregular motion, creep, increased seal wear, cylinder overheating, potential cylinder or rod failure.

Contaminated or degraded hydraulic fluid:

• WHY IT HAPPENS:
  • Non-observance of replacement intervals: Fluid loses its properties with time and use.
  • Inefficient filtration: Clogged or missing filters do not remove pollution particles.
  • Ingress of external pollution: Dust, water, metal shavings from the environment.
  • Thermal degradation: Overheating of liquid accelerates oxidation and decomposition of additives.
• HOW TO CONFIRM: Fluid sample analysis (visual, rapid test, laboratory analysis per ISO 4406) (Chapter 5, Creep, STEP 4).
• WHAT DAMAGE WILL BE CAUSED: Abrasive wear of seals, valves, pump, reduction of system efficiency, increase in operating temperature, damage to internal surfaces of components, seizure of moving parts.

Incorrect adjustment of the balancing valve:

• WHY IT HAPPENS:
  • Incorrect initial setting: Errors on first start or after valve replacement.
  • Unqualified intervention: Adjusting the valve without understanding its function and effect on the system.
  • Change in operating parameters: Change in load mass or pressure in the system, which requires readjustment of the valve.
• HOW TO CONFIRM: Check valve setting pressure with gauge, compare to specification (Section 5, Creep, STEP 5).
• WHAT DAMAGE IS CAUSED: Irregular cylinder movement, creep, load control instability.

Unstable pump pressure:

• WHY IT HAPPENS:
  • Pump wear: Internal leaks in the pump due to wear of moving parts (gears, pistons).
  • Failure of the pump pressure regulator: Jamming, clogging or incorrect setting of the regulating element.
  • Air in the system (cavitation): The pump sucks in air, which causes pressure pulsations.
  • Clogged suction filter: Restriction of fluid flow to the pump, resulting in cavitation and unstable pressure.
• HOW TO CONFIRM: Measure the pump outlet pressure during operation with a pressure gauge (Chapter 5, Creep, STEP 1).
• WHAT DAMAGE WILL BE CAUSED: Loss of hydraulic system performance, cylinder creep, accelerated wear of other system components due to pressure pulsations.

8. Step-by-Step Troubleshooting Procedures

8.1. Replacement of internal piston seals of the hydraulic cylinder

1. CAUTION: SAFETY! Turn off equipment power, release all hydraulic pressure, perform lockout/tagout (LOTO) procedures. Mechanically secure or support any load held by the cylinder.
2. Disconnect the hydraulic lines from both cylinder ports. Cap the ends of lines and ports to prevent contamination.
3. Remove the hydraulic cylinder from the equipment. Make sure the cylinder is securely clamped to the workbench or vice.
4. Disassemble the cylinder according to the manufacturer's instructions. Carefully remove the stem so as not to damage the surface.
5. Thoroughly clean all cylinder components (sleeve, rod, piston, covers) from dirt and liquid residues. Use recommended cleaners.
6. Inspect the cylinder liner and rod for scratches, burrs, corrosion, or ovality. Minor defects on the rod can be polished with abrasive pastes. In case of significant damage, replace the component.
7. Replace the old piston seals with new ones that meet the standard (for example, DSTU ISO 5597, DSTU EN 813). Use seals made of materials compatible with the fluid and temperature of your system (eg NBR, FKM, PTFE).
8. Assemble the cylinder, following the manufacturer's instructions and recommended tightening torques (for example, for rod nuts 80-120 Nm, for fixing caps 200-300 Nm - check with the specification). Lubricate the seal with clean hydraulic fluid before installation.
9. Install the cylinder on the equipment, connect the hydraulic lines. Check for proper alignment and tension.
10. Start the system at low rpm, repeatedly move the cylinder to full stroke without load, using special sequences to remove air from the system.
11. Check the cylinder for drift and external leaks under operating pressure and load.

8.2. Repair/replacement of check valves

1. CAUTION: SECURITY! LOTO, release the pressure.
2. Remove the faulty valve from the system.
3. Disassemble the valve, clean all internal components.
4. Inspect the spool, valve seat, and springs for wear, damage, or clogging.
5. Replace worn parts using the original repair kit, or replace the valve completely with a similar specification (e.g. check valve DN 15, PN 320 bar).
6. Assemble the valve, install it in the system.
7. Test for tightness and functionality under pressure.

8.3. Elimination of external sources

1. CAUTION: SECURITY! LOTO, release the pressure.
2. Determine the exact source of the leak (rod seal, boot, fitting, port).
3. Replace damaged seals (rod sleeves, O-rings, gaskets) with new ones that meet the cylinder specification and standards (eg DIN 3760 for rotary seals).
4. If there is a leak through the connection - check the fitting, tube/hose. Replace damaged components. Tighten the connection to the recommended torque (for example, for G1/2" fittings - 60 Nm).
5. Check the stem surface for damage (scratches, corrosion) that could have caused the leak. Minor damage can be sanded off.
6. Start the system, check for leaks under operating pressure.

8.4. Recovery of pilot pressure

1. CAUTION: SECURITY! LOTO, release the pressure.
2. Check pilot lines for clogging or kinks. Clean or replace damaged lines.
3. If there is a pilot pressure reducing valve, check it for operability, clean it. Adjust the outlet pressure according to the scheme (for example, 25 bar).
4. Check pilot pressure control valves for internal leaks.
5. Check the electrical control of the solenoids (if any) with a multimeter: the winding resistance should correspond to the nominal value (for example, 12-24 ohms).

8.5. Removing air from the hydraulic system

1. Check the fluid level in the tank, top up to the required level recommended by the manufacturer.
2. Check all connections on the suction line of the pump for leaks.
3. Start the hydraulic system at low pump speed.
4. Repeatedly move the cylinder to full stroke without load. Usually, 5-10 cycles are enough. Some systems have special pumping procedures.
5. If there are air valves on the cylinder or manifold, open them for a short time until the liquid comes out without bubbles.
6. Make sure there is no foam in the hydraulic tank.

8.6. Elimination of mechanical friction/clogging

1. CAUTION: SAFETY! LOTO, release the pressure, lock the load mechanically.
2. Disassemble the cylinder.
3. Disassemble the cylinder. Inspect the rod, sleeve, piston, guide bushings for deformation, wear, burrs.
4. Replace damaged parts (eg stem, sleeve, guide rings).
5. Ensure correct centering of the cylinder when mounting to avoid side loads.
6. Use the lubricant recommended by the manufacturer for the guide elements.

8.7. Replacement/filtration of hydraulic fluid

1. CAUTION: SECURITY! LOTO, release the pressure.
2. Drain contaminated hydraulic fluid from tank and system.
3. Clean the hydraulic tank, flush the system if necessary.
4. Replace the hydraulic filters (pressure, return, suction) with new ones with the recommended filter fineness (for example, 10 µm).
5. Fill with fresh hydraulic fluid that meets the equipment's specifications (eg ISO VG 46, purity grade as of ISO 4406 17/15/12).
6. Remove air from the system (see 8.5).

8.8. Adjusting the balancing valve

1. CAUTION: SAFETY! LOTO, lock the load mechanically.
2. According to the equipment manufacturer's specification or the hydraulic circuit, adjust the balancing valve's operating pressure. Usually, it is set 20-30% higher than the maximum pressure arising from the load.
3. Use a pressure gauge to accurately monitor the setting pressure.
4. Check the operation of the cylinder under load after adjustment.

8.9. Hydraulic pump repair/replacement

1. CAUTION: SECURITY! LOTO, release the pressure.
2. Perform detailed pump diagnostics using a flow meter and pressure gauge to identify internal leaks or performance issues.
3. Repair the pump by replacing worn components (eg gears, bearings, seals) or replace the pump completely.
4. Check pump outlet pressure and flow after repair/replacement.

9. Precautions

10. Spare Parts and Components

For quick and efficient repairs, it is important to have the necessary spare parts on hand. UNITEC-D GmbH offers a wide range of high-quality components.

To order quality components and spare parts, visit our e-catalog UNITEC.

11. Links

• DSTU ISO 5597:2018 (EN ISO 5597:2018, IDT) "Hydraulics. Cylinders. Dimensions of piston and rod seal housings".
• DSTU EN 982:2009 "Safety of machines. Safety requirements for hydraulic and pneumatic systems and their components".
• DSTU EN 1037:2018 "Machine safety. Prevention of unexpected start-up".
• ISO 4406:1999 "Hydraulic fluid - Method for determining the level of contamination by solid particles".
• Equipment manufacturer (OEM) hydraulic cylinder operation manual.
• UNITEC. Repair and maintenance of hydraulic systems. (Available at www.unitecd.com/maintenance-guides/)