Servicing Press Motors: Installation, Monitoring and Maintenance

Before the Motor Starts: Site and Supply Checks

Most press motor faults are installed rather than manufactured. Before the motor is mounted, confirm that supply voltage and frequency match the nameplate, that the cable is rated for full load current plus starting current, and that the cabinet protection is chosen for the motor. Check phase sequence with a rotation meter before the first start, because a reversed phase sequence on a pump drive can destroy the pump within seconds.

The environment also sets limits the nameplate does not show. Confirm the plate stays readable, that hot oil lines and heater elements do not radiate onto the motor, and that the ventilation path from the fan cover is not blocked by the frame or by stored stock. Record the ambient temperature at the worst point of the shift. Ambient above 40 degrees centigrade requires derating, roughly one percent of rated output per degree above the reference.

Alignment and Coupling: The First Two Hours Matter

Alignment should be checked with the machine in its running configuration, ideally with a laser or dial indicator, and rechecked after a few hours at operating temperature. Parallel and angular misalignment at the pump coupling should be held below 0.05 mm for a rigid coupling and below 0.1 mm for a flexible coupling. Excess angular offset loads the coupling element and the pump shaft bearings, and that appears later as a motor bearing failure blamed on the motor.

Inspect the coupling element itself. Jaw couplings have elastomer spiders that crack and crumble; gear couplings need lubrication checks and backlash measurement; grid couplings need intact grid packs free of oil contamination. Verify the coupling guard allows access for inspection, rotate the shaft by hand before energising, and record alignment values in the press file.

Electrical Supply, Phase Balance and Starting Current

Phase imbalance is the quietest killer of press motors. A three percent imbalance between phases raises motor temperature by several degrees and shortens winding life. Measure the voltage between each pair of phases at the motor terminals under rated load, and calculate the imbalance as the maximum deviation from the average divided by the average. Anything above one percent deserves investigation, and above three percent the motor should be taken out of service.

Starting current is why so many presses trip on commission. A direct on line start draws six to eight times rated current for a fraction of a second, so a 30 kW motor of say 55 A can pull close to 400 A at the instant of start. Soft starters and star delta starters reduce that peak, while a frequency converter reduces it to a fraction and removes the shock to gearbox and pump. If the supply cannot carry the inrush, fix the starting method, not the cable.

Cooling, Ventilation and Ambient Derating

Motors lose heat through the frame and the ventilation system, and both are easily compromised on a press. The cooling fan needs clearance and a clear air path; a motor with its fan cover close to a steel frame or a wall recirculates hot air and eventually cooks itself. Check that the fan cover is present and undamaged, and that the frame fins are free of oil mist, swarf and dust. Oily fins are a common cause of overheating in forming shops because the oil film insulates.

Where the motor sits inside an enclosure, measure the temperature rise inside during a full cycle and compare it with the nameplate rise. A rise more than 15 K above the nameplate value indicates inadequate ventilation, and the drive should be derated accordingly.

For oil heated presses, a hydraulic oil temperature of 60 degrees centigrade near the motor adds to the ambient load. The nameplate assumes 40 degrees ambient and an 80 K rise for insulation class F, so the practical ceiling in a hot bay is lower than the label suggests. Record oil and motor temperature together, because a widening gap points to airflow restriction.

Vibration, Bearings and Routine Condition Monitoring

Vibration is the earliest signal a press motor gives. Establish a baseline at commissioning with a simple vibration pen or a small collector, and compare against it during routine checks. Acceptable running vibration on a small to medium motor is generally below about 4.5 mm per second RMS as an overall reading. A reading that rises gradually over weeks points to bearing wear or misalignment; a sudden rise points to a mechanical event such as a loose coupling element.

Listen as well as measure. Grinding that changes with load suggests bearing damage, rhythmic knocking at twice shaft speed suggests a bent shaft or a loose foundation bolt, and a noise that tracks pump frequency rather than shaft frequency points to the pump or coupling. A loose foot bolt produces a high axial reading and damages bearing housings.

Bearing lubrication is the item most often missed. Grease at the manufacturer interval, more often in dusty or hot conditions, since overgreased and undergreased bearings both raise temperature. On sealed units with lifetime lubrication, watch noise and vibration instead, and keep spare bearings of the correct size on the shelf.

Motor Temperature and Winding Insulation Checks

Temperature is the most useful continuous indication of motor health. Check the load current against the nameplate first, because an overloaded motor runs hot by definition and the cause is on the hydraulic side. Compare the frame temperature with a reference motor of the same type, or use an infrared thermometer at a fixed point on the frame and keep the reading in the maintenance log for trend comparison.

Insulation condition is checked with an insulation resistance tester at 500 volts for motors rated up to 500 volts. Readings should be in the megohm range and, more importantly, stable. A falling trend between tests is more meaningful than any single number, and a reading near zero indicates a failure to earth, so the motor must not be energised. Polarisation index above two indicates healthy insulation.

Check the terminal box for heat discolouration, loose terminations and moisture ingress. A loose supply connection produces local heating that never reaches the motor frame, so a cool frame with a hot terminal box is a serious warning. On older motors, ask a specialist to test the windings.

Servo Drives, Commissioning and Fault Diagnosis

On a servo press the motor and drive are a single system, and commissioning errors show up as faults rather than symptoms. Verify the motor and drive model pairing, encoder cable routing in a shielded conduit, and earth continuity between drive and motor. Set the motor direction against the tool movement before enabling the position loop, otherwise the controller chases an error that looks like a runaway.

Auto tuning should run with the ram free of the workpiece and at the lowest speed setting. If tuning reports abnormal noise or vibration, check the coupling and brake release before touching parameters.

Gearbox Oil, Bearing Service and Rewind versus Replacement

If the press drive has a gear reducer, the gearbox has its own service schedule that is easy to forget because nothing about it moves. Check oil level and condition at the specified interval, usually every 500 to 1000 running hours depending on duty, and look for metal particles, water contamination or a burnt smell. Keep the level within the marked band, and confirm the breather is clean, because a blocked breather pressurises the housing and forces oil past the seals.

Renew with the specified grade and volume, then run the gearbox unloaded briefly so oil distributes and leaks become visible. A whine that changes with load is normal for a helical unit, a rasp is not, and a knock at a fixed frequency points to a bearing defect.

When a motor fails, rewinding versus replacement is a genuine decision. A rewound motor keeps the frame, shaft and mounting, so the press runs with no engineering changes. A new motor of standard size improves efficiency and carries a warranty, but may need a new coupling and base plate.

Scheduled Maintenance and Troubleshooting Press Motors

A simple calendar prevents most failures. Inspect monthly: fan cover and cooling fins, terminal box condition, vibration reading and load current against the nameplate. Inspect quarterly: coupling condition, alignment check, gearbox oil level and appearance, brake operation if fitted. Inspect annually: insulation resistance test, gearbox oil change and bearing lubrication.

Four complaints cover most press motor service calls, and the table below sets out the sequence to follow. Check simple causes first, because a mechanical obstruction on the hydraulic side presents as an electrical fault more often than operators expect.

Two habits are worth building into the routine. Record the load current of a healthy cycle, because that number turns an argument about overload into a comparison. And keep a spare motor of the correct frame on the shelf.

Frequently Asked Questions

How often should a hydraulic press motor be serviced?

A light inspection every month, a thorough inspection every three months and an annual overhaul of oil, bearings and insulation is a workable schedule for two shift production. The interval should shorten in dusty, hot or humid bays. The most valuable routine item is comparing running load current with the nameplate figure, because a slow rise in current often appears weeks before a failure.

What causes a press motor to overheat?

Three causes account for most cases: sustained overload from the hydraulic side, blocked cooling from dusty or oily cooling fins and a restricted fan cover, and poor ventilation because the motor sits in a hot enclosure. On a motor fed by a frequency converter, overheating is also caused by the drive being set above the motor nameplate frequency. Measure load current and frame temperature, then check airflow before changing drive parameters.

Is it better to rewind a motor or replace it?

Rewinding keeps frame, shaft, coupling and mounting and cuts downtime, which matters when the frame size is not standard. Replacing with an equivalent frame gives a modern efficiency class and warranty, while replacing with a different frame needs a new base plate, coupling and alignment work but makes future spares easy. Compare efficiency gains against downtime cost before ordering either route.

How do I tell whether a noise comes from the motor or the pump?

Disconnect the coupling with the drive locked out and rotate the motor shaft by hand. If the noise follows pump rotation and the motor is silent, the pump or coupling is the source. If noise is present at the motor bearings with the pump removed, the motor is the source. In service, note the frequency of the noise against motor rpm and against pump displacement times rpm. A seized pump can stall a motor within seconds, so isolate the drive before removing any coupling.