Analysis of underspeed problem of hydraulic power unit
When a hydraulic power unit (HPU) runs below its normal speed, actuators may slow down, flow may drop, and pressure can become unstable. The unit may also overheat or trip its motor or drive repeatedly. Before looking for the cause, determine whether the motor and pump shaft are actually running too slowly or the shaft speed is normal but hydraulic output is low.
Measure RPM, pressure, flow, temperature, and electrical load in a controlled sequence. Those readings provide better evidence than replacing pumps, motors, or valves based on symptoms alone.
The distinction matters because a hydraulic system uses the pump to produce flow, while pressure develops as that flow meets resistance. Flow mainly determines actuator speed, and pressure determines the force or torque available at the load. A slow cylinder therefore does not prove that the pump shaft is turning too slowly.
Before servicing an HPU, control all electrical, hydraulic, pneumatic, gravitational, and accumulator energy. OSHA 29 CFR 1910.147 requires hazardous energy to be isolated and stored or residual energy to be rendered safe before covered servicing or maintenance.
The hydraulic power unit is a system that provides power to the hydraulic press. It cannot underspeed. If underspeed, it will cause insufficient power and affect normal work. Let's look at the problem of hydraulic power single underspeed:
One, hydraulic power unit underspeed elimination method
1) Eliminate the failure of insufficient output flow and low output pressure of the oil pump:
2) Eliminate the pressure failure caused by pressure valves such as overflow valves:
3) Find out the crossbow that produces internal leakage and external leakage, eliminate internal and external leakage: replace severely worn parts to eliminate internal leakage;
4) Control oil temperature;
5) Clean parts such as flow valves, and change the oil in time when oil pollution is serious;
6) Find out the reason for the air intake of the hydraulic power unit and remove the air in the hydraulic power unit.
Two, reasons for underspeed of hydraulic power unit
1. During work feed, the work feed speed is significantly reduced under load, even if the speed control valve (throttle valve, etc.) is opened.
1) There is air in the hydraulic power unit.
2) Impurities mixed in the oil block the orifice of the flow control valve, causing the working speed to decrease; when the oil is blocked, the speed is unstable;
3) The design of the hydraulic power unit is unreasonable. When the load changes, the flow into the actuator of the hydraulic equipment also changes, causing a change in speed:
4) The system oil temperature increases, the oil viscosity decreases, the leakage increases, and the effective flow decreases;
5) When the system is under load, the working pressure increases and the leakage increases, and the adjusted speed decreases due to the increase of internal and external leakage:
2. Reasons for insufficient speed of fast movement
1) Large resistance during fast forward: For example, the guide rail is lubricated and the oil is cut off, the guide rail's insert pressure plate is adjusted too tightly, and the installation accuracy and assembly accuracy of the oil cylinder are poor, causing the friction resistance to increase during fast forwarding.
2) The internal and external leakage of the system is serious: the working pressure is generally lower during fast forward, but it is much higher than the oil return pressure. When the piston seal of the oil cylinder is damaged, the two cavities of the oil cylinder leak large due to the series of cavities (there is a pressure difference), which makes the fast movement speed of the oil cylinder insufficient, and the internal and external leakage of other parts will also cause this phenomenon;
3) The overflow valve is deformed or installed as a weak spring due to spring deformation or wrong installation, the main spool orifice is partially blocked, and the main spool is stuck in the small opening position. As a result, part of the pressure oil output by the oil pump overflows back to the tank, making it open The effective flow of the system to the actuator is greatly reduced, making the rapid movement speed insufficient
4) The output flow of the oil pump is not enough and the output pressure is not increased;
Third, the adverse effects of hydraulic power unit underspeed
The underspeed of hydraulic equipment's executive components (oil cylinder and oil motor) includes two situations: one is that the speed is not fast enough during fast movement (fast forward), and it will not reach the design value and the specified value of the new equipment; the other is that it works under load The speed (work advance) decreases significantly as the load increases.
Especially for large-scale liquid equipment and equipment with heavy load, this phenomenon is particularly significant, and the speed is generally related to the flow rate.
Under-speed first affects production efficiency and increases the cycle time of hydraulic equipment; under-speed phenomenon often stops moving under heavy loads, which will affect whether the equipment can work normally. For equipment that requires rapid movement, such as a surface grinder, the speed is not enough to affect the surface roughness of the grinding.
Inspect the unit before startup
Apply the appropriate energy-control procedure whenever the inspection involves servicing. Check:
- Reservoir oil level
- Loose suction connections
- Cracked hoses
- Coupling condition
- Belt tension
- Filter restriction indicator
- Electrical terminals and grounding
- Signs of leakage or aerated oil
Never loosen a high-pressure fitting to find out whether the line is pressurized.
Check oil temperature and condition
Cold, viscous oil can cause excessive suction resistance and cavitation. Oil that is extremely hot and thin increases internal leakage.
Stop the unit if the fluid is milky, heavily foamed, smells burnt, or is visibly contaminated. Find the cause before running it for an extended period.
Verify the electrical supply
For a three-phase motor, measure all three line-to-line voltages. Calculate voltage unbalance as follows:
**Voltage unbalance (%) = Maximum deviation from average voltage / Average voltage x 100**
DOE motor guidance recommends corrective action when voltage unbalance exceeds approximately 1%. An unbalanced supply increases motor heating and can reduce performance.
Verify rotation and unloaded operation
Check pump rotation against the arrow on the pump or the OEM documentation.
Run the unit unloaded only if the hydraulic circuit is designed for it. Record RPM, current, pump outlet pressure, noise, and vibration, then introduce the load gradually.
If the unit reaches normal RPM without a load but slows sharply as pressure rises, possible causes include excessive mechanical or hydraulic torque demand, a weak motor, current limiting, or incorrect drive settings.
Stop the unit immediately if it has repeated drive trips, severe cavitation, smoke, a rapid temperature rise, damaged couplings, major leaks, metal debris, or unstable pressure. Never keep it running by bypassing a safety device or raising relief pressure above an approved setting.
Read RPM and hydraulic output together
Use shaft speed and measured hydraulic output to decide which part of the system to test next.
Test result | Most useful next checks
RPM is low and flow is low | Electrical supply, VFD command, motor condition, coupling or belt slip, and excessive torque demand
RPM is normal but flow falls as pressure rises | Internal pump leakage and case-drain flow where applicable
RPM and pump flow are normal but the actuator is slow | Downstream restrictions, valve bypass, cylinder or motor leakage, and flow-control settings |
Flow fluctuates and the pump is noisy | Oil level, suction restriction, inlet air leakage, oil viscosity, and aeration
Separate slow work feed from inadequate fast travel
The point in the machine cycle at which speed drops can narrow the search.
If work-feed speed falls noticeably as load increases, even with the speed-control or throttle valve open, check for:
- Air in the hydraulic system
- Contamination blocking the flow-control valve orifice, especially when speed is also unstable
- A circuit design that allows flow to the actuator to change with load
- High oil temperature and lower viscosity, which increase leakage and reduce effective flow
- Internal or external leakage that rises with working pressure
If fast travel does not reach its specified speed, check for:
- Excessive mechanical resistance caused by poor guide-rail lubrication, an overly tight guide-rail insert or pressure plate, or poor cylinder installation and alignment
- Leakage across a damaged cylinder piston seal, as well as internal or external leakage elsewhere in the circuit. Fast-travel pressure is often lower than work-feed pressure but still higher than return pressure, so a damaged piston seal can allow oil to pass between the cylinder chambers
- A relief valve that bypasses oil because of a deformed or incorrect spring, a partly blocked main-spool orifice, or a spool stuck slightly open
- Inadequate pump flow or an inability to build the required pressure
Either condition increases machine cycle time and may cause motion to stop under a heavy load. On equipment such as a surface grinder, inadequate travel speed can also affect the finished surface roughness.
Mechanical causes: pump leakage, couplings, belts, and load torque
Mechanical testing needs to distinguish among three conditions: internal pump bypass, inadequate oil supply to the pump, and overload of the prime mover.
Test for internal pump leakage
For a fixed-displacement pump:
**Theoretical flow = Pump displacement x RPM**
Actual flow should be reasonably close to the manufacturer's expected flow at the measured speed and pressure.
Run a controlled flow test at three load points:
1. Low pressure
2. Intermediate working pressure
3. Normal operating pressure
If RPM remains almost constant but delivered flow falls excessively as pressure rises, internal pump leakage is a likely cause.
For an axial piston pump, measure case-drain flow if the manufacturer provides a case-drain specification. These pumps require a case-drain path for internal leakage, so an abnormal rise in case flow can indicate wear.
Do not apply a generic case-drain limit. Compare the reading with the manual for that pump or with an established baseline.
A suction problem usually produces a different set of symptoms. Noise increases, inlet conditions deteriorate, flow may fluctuate, and cold, viscous oil can make the problem worse. Air leaks or excessive inlet restriction may also aerate the oil and shorten pump life.
Isolate the pump from the load
Use a properly rated hydraulic test circuit to determine whether the fault follows the pump or the equipment downstream.
Measure pump flow while controlling load pressure independently. If the pump performs correctly on the test circuit, inspect the downstream valves, motors, cylinders, and possible bypass paths.
Check couplings and belts
Misalignment raises bearing loads, vibration, temperature, and torque demand.
Coupling tolerances vary considerably. Some flexible jaw couplings, for example, may accommodate about 1° of angular misalignment and roughly 0.015 in of parallel offset. These figures are maximum accommodation values, not installation targets. Lovejoy recommends installing some coupling types at only a fraction of their maximum allowable misalignment.
Close-coupled HPUs can have much tighter alignment. One commercial hydraulic pump and motor mount specifies approximately 0.003 in concentricity and 0.002 in face parallelism.
Use the installation tolerance from the actual coupling manufacturer.
On a belt-driven unit, inspect belt tension, glazing, pulley alignment, and belt dust. If slippage increases with hydraulic pressure, the pump may appear to be running below speed.
Compare torque with load
For a rotating shaft:
**Torque (ft-lb) = HP x 5,252 / RPM**
Approximate hydraulic horsepower is:
**Hydraulic HP = Pressure (psi) x Flow (GPM) / 1,714**
If motor torque and current rise sharply while RPM falls, check for excessive hydraulic load, mechanical binding, or an incorrectly adjusted relief or control circuit.
Electrical and control faults: motors, VFDs, sensors, and error codes
Electrical faults can slow the unit even when the hydraulic circuit is working correctly.
If testing exposes workers to energized equipment, OSHA generally requires live parts to be deenergized unless doing so creates additional hazards or the test cannot otherwise be performed. Only qualified personnel using appropriate safe work practices may work on exposed energized parts.
Check:
- Incoming three-phase voltage
- Voltage balance among phases
- Motor current on all phases
- Starter and contactor condition
- Motor winding resistance
- Ground insulation condition
- VFD output frequency and current
- Commanded speed against actual speed
Before using a megohmmeter to test insulation resistance, isolate the motor from the VFD and sensitive electronics, then follow the motor and drive manufacturers' procedures. Do not megger through a VFD.
How VFD faults appear at the HPU
| VFD alarm category | Possible HPU symptom | First checks |
| Undervoltage or power loss | Slow acceleration, trip, or low speed | Incoming voltage, fuses, and contactors |
| Overcurrent | RPM falls as load rises | Pump binding, excessive pressure, or short circuit |
| Motor overload or stall | Motor cannot reach commanded RPM | Hydraulic torque, motor sizing, and acceleration settings |
| Drive overtemperature | Derating or shutdown | Cooling fan and enclosure temperature |
| Analog input loss | Incorrect speed command | 4 to 20 mA or 0 to 10 V wiring |
| Speed-feedback loss | Unstable or incorrect RPM | Encoder, sensor wiring, and feedback configuration |
Fault codes are specific to the drive manufacturer and model. ABB ACS580 documentation, for example, uses separate codes for overcurrent, DC overvoltage, DC undervoltage, and overtemperature. Rockwell PowerFlex products use a different F-code structure. Decode each alarm with the manual that matches the drive model and firmware.
Verify sensors and transducers
A failed pressure or speed sensor may cause the controller to reduce the pump command intentionally.
For a 4 to 20 mA pressure transducer:
**Measured value = (mA - 4) / 16 x Full-scale range**
For a 0 to 3,000 psi sensor producing 12 mA:
**(12 - 4) / 16 x 3,000 = 1,500 psi**
Compare the PLC reading with a calibrated mechanical gauge. On a CANbus-controlled pump, check commanded speed or displacement, actual feedback, supply voltage, communication faults, and diagnostic status before concluding that the pump has failed.
Hydraulic settings and fluid-related causes
A pump can run at the correct speed yet provide too little useful flow if oil is bypassing through the circuit.
Check the relief valve
Install a pressure gauge at the pump outlet, preferably upstream of major downstream restrictions.
Then follow this sequence:
1. Verify the specified system pressure.
2. Confirm that the gauge has the correct range and calibration.
3. Warm the fluid to the normal test condition.
4. Apply load through an approved test procedure.
5. Note the pressure at which the relief valve begins to control.
6. Make adjustments only within the limits set by the machine and component manufacturers.
7. Lock the adjustment and test the complete cycle again.
A relief valve set too low may bypass oil before the actuator develops the required load. If the valve is stuck partly open, the system may run slowly and heat up quickly.Do not raise a relief setting simply to compensate for slow operation.
Account for temperature and viscosity
An ISO VG number describes nominal viscosity at about 40°C. At that reference temperature, typical ISO VG 32, 46, and 68 hydraulic oils have viscosities of approximately 32, 46, and 68 cSt. Viscosity rises rapidly as the oil gets colder.
Allowable ranges depend on the pump. One Parker mobile piston pump, for example, lists a continuous viscosity range of 7 to 160 cSt and permits much higher viscosity only during a cold start. This is a product-specific example, not a general hydraulic specification.
| Condition | Likely effect |
| Oil is too cold or thick | High suction loss, sluggish response, and cavitation risk |
| Oil is within the OEM viscosity range | Best efficiency and lubrication |
| Oil is too hot or thin | Greater internal leakage and lower volumetric efficiency |
Choose the oil with the fluid manufacturer's viscosity-temperature curve, not ambient temperature alone.
Check contamination
ISO 4406 expresses contamination with particle-count codes for particles at 4 µm(c), 6 µm(c), and 14 µm(c).
No single cleanliness code suits every HPU. Set the target according to the most contamination-sensitive component. An individual Parker hydraulic valve, for example, may specify ISO 4406 18/16/13, while a servo system may require a different level of cleanliness.
Whenever possible, take the sample through a dedicated port in a representative, turbulent operating line. Sludge collected at a reservoir drain is not a suitable routine sample of overall system condition. Good sampling practice is only one part of [preventing hydraulic fluid contamination](https://www.dallasthydraulics.com/info/avoid-contamination-of-hydraulic-fluid-57069159.html).
Used hydraulic oil from U.S. industrial operations is generally subject to the EPA used-oil management requirements in 40 CFR Part 279. State requirements may also apply.
Testing, monitoring, and preventive maintenance
After locating the immediate fault, record operating data to confirm the diagnosis.
At minimum, log:
- Motor RPM
- VFD command or frequency
- Motor current
- Pump outlet pressure
- Pump flow
- Case-drain flow where applicable
- Filter differential pressure
- Reservoir temperature
- Hydraulic return pressure
- Shaft torque where it can be measured
Fast cycles may require pressure and RPM sampling at 10 to 50 Hz or faster to capture brief events. Slower logging is usually enough for gradual changes in temperature and contamination.Record a baseline while the HPU is operating correctly. Future test results can then be compared with known healthy data.
A practical preventive maintenance schedule
| Interval | Recommended checks |
| Every shift | Oil level, leaks, noise, temperature, and alarms |
| Every 250 to 500 operating hours | Filter indicators, coupling or belt condition, and electrical connections |
| Every 500 to 1,000 operating hours | Oil sample, pressure and flow trends, and hose condition |
| Annual shutdown | Alignment, sensor calibration, motor testing, relief verification, and reservoir inspection |
Use these intervals for planning only. Equipment in severe, dusty, hot, wet, or continuous service may need more frequent attention. OEM instructions take precedence.
Replace filters according to differential-pressure indicators, contamination trends, and manufacturer requirements. A single calendar interval does not suit every filter or operating condition.
When sourcing replacement parts in the United States, compare OEM-authorized distributors, regional hydraulic repair centers, national MRO suppliers, and qualified remanufacturers. Evaluate each supplier on:
- Confirmed part compatibility
- Same-day or next-day availability
- Test documentation
- Core-exchange terms
- Warranty length
- Technical support
- Emergency shipping
- Traceability of rebuilt components
A remanufactured pump should ideally come with test documentation showing pressure, flow, leakage, and operating condition, not only a statement that it was "rebuilt."
Risk levels for temporary operation
Low risk: Performance has declined slightly, but temperature is stable, noise is normal, pressure and flow remain acceptable, and no safety alarms are active. Continue only with documented monitoring.
Elevated risk: Temperature is rising, the filter shows restriction, RPM loss is moderate and unexplained, or leakage is getting worse. Reduce the duty cycle if permitted and arrange an immediate repair.
Stop: Shut down for severe cavitation, repeated electrical trips, unstable pressure, damaged rotating parts, high-pressure leakage, smoke, metal contamination, rapidly rising temperature, or any condition outside OEM safety limits.
Temporary measures must not defeat interlocks, bypass guards, block a relief valve, or allow operation above rated pressure.
HPU underspeed troubleshooting checklist
A downloadable HPU diagnostic checklist can turn the measurements in this guide into a repeatable maintenance process. It should include startup checks, fields for RPM, pressure, and flow tests, a 4 to 20 mA conversion sheet, oil-sampling records, and a trend-log template.
Before carrying out high-pressure hydraulic work or electrical servicing, follow the facility's hazardous-energy procedure and applicable OSHA lockout and tagout practices.
Useful website actions include:
- Download the PDF troubleshooting checklist
- Request a remote diagnostic review
- Request a U.S. repair and fast-ship parts source
FAQ
Which startup checks should I perform first when an HPU is running below speed?
Measure the actual shaft RPM first. Then inspect the oil level and temperature, filter condition, supply voltage, phase balance, pump rotation, VFD commands, unloaded RPM, and loaded pressure and flow. This sequence separates an electrical speed problem from a hydraulic flow problem.
How can I distinguish internal pump leakage from suction loss?
Measure pump RPM and flow at several pressures. An excessive drop in flow as pressure rises, especially when piston-pump case drain also increases, suggests internal leakage. Cavitation noise, aerated oil, sensitivity to cold weather, and abnormal inlet conditions point more strongly to suction restriction or air ingress.
Which electrical faults commonly cause HPU underspeed?
Common causes include low or unbalanced supply voltage, damaged contactors, motor winding faults, incorrect VFD parameters, current limiting, motor overload, lost analog commands, and faulty speed feedback. Measure the relevant values instead of repeatedly resetting VFD faults without diagnosing them.
How can an incorrect relief valve cause slow operation?
A relief valve that opens below the required working pressure sends some or all of the pump flow back to the reservoir. The actuator then loses available flow or force, while the bypassed hydraulic energy turns into heat. Check the pressure with a calibrated gauge before adjusting the valve.
How do contamination and cold oil affect pump output?
Contamination accelerates wear and may make valves stick. Cold oil is more viscous and raises pump-inlet losses. Both conditions can reduce useful flow. If the fluid condition is uncertain, perform ISO 4406 particle-count testing, viscosity analysis, water testing, and a filter inspection.
Which temporary measures are acceptable while waiting for a repair?
Use only measures allowed by the equipment manufacturer and the site's risk controls. Depending on those instructions, this may include reducing the duty cycle or warming an approved hydraulic fluid before applying load. Do not raise pressure above specification, bypass interlocks, defeat guards, or keep operating with severe cavitation, repeated trips, high-pressure leakage, or a rapidly rising temperature.