Overhauling a rotary screw compressor: A step-by-step guide

Follow this step-by-step approach to determine if an existing rotary screw compressor is worth the time to overhaul.

By Dan Kent, technical service manager, Kaeser Compressors, Inc.

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Compressed air, commonly referred to as the fourth utility, can be found in use almost everywhere. New and old facilities grow and often require additional compressed air capacity. Opportunities may arise to meet these compressed air needs using existing or used rotary screw compressors. While purchasing a new compressor has its advantages, do not dismiss the existing equipment as "not worth the time to overhaul." Since most rotary screw compressors use oil to seal the airend (the component containing the compression chambers), very little wear occurs in the compressor itself. This lack of internal wear means that frequently, the compressor can be returned to like-new condition for a fraction of the cost of a new compressor. The following information is offered to help a mechanic or engineer determine if an existing rotary screw compressor is worth the time to overhaul.

How to test

Safety is paramount. Disconnect and lock out all electrical supplies before attempting any analysis or repairs. Vent any compressed air trapped in the equipment.

Do a literature search. Get the Service and Parts Manual for the compressor. If you are unable to locate this manual, contact your local manufacturer's representative for a copy. Usually, compressor manufacturers provide the manual for a reasonable fee.

Check the feasibility of the project. The preliminary inspection steps below provide the information to determine if a used rotary screw compressor is worth the cost and time to overhaul. Performing the inspection should take a knowledgeable mechanic approximately four hours for a V-belt driven compressor. Gear driven or direct coupled compressors take somewhat longer.


The best w ay to determine the condition of the airend is to run the compressor. Before doing so, find out if running the airend is safe or if it will cause damage. Disconnect the motor from the airend. If the airend is connected to the motor, simply remove the V-belts. If the airend is directly coupled to the motor or is gear-driven, the task is more difficult. Once the airend is disconnected, slowly rotate the airend's input shaft by hand.

If the airend rotates, continue rotating it for several full revolutions. The resistance to turning should be even — no tight spots. Try to move the airend shaft up and down and from side to side. You should detect only the slightest movement. If you find any tight spots in the rotation or considerable lateral movement, have your local manufacturer's representative inspect and evaluate the airend for possible bearing damage.

If the airend rotates freely and lateral shaft movement is minimal, the airend should be safe to run. If the airend does not rotate, then overhauling the compressor will likely require replacing the airend. Factory rebuilt airends offer an affordable alternative to a new airend by providing a quality replacement at a reduced price. Consult your local manufacturer's representative for more information. Also, returning the old airend under an exchange program may further reduce the cost of the replacement.

Electric motors

Disconnect the airend and motor as stated in the previous step, remove any electrical wires and jumpers and turn the motor by hand.

If the motor rotates, continue rotating it several full revolutions. Listen and feel for any cluncking or tight spots that indicate bearing problems. Typically, replacing the motor bearings is a relatively inexpensive repair.

Check the condition of the windings using a Mega-Ohm meter or High Pot Tester. If you are not familiar with these tests, ask your local motor repair facility for assistance. There are three electrical tests to perform on the motor.

  • Measure the resistance of each winding. Open windings have considerably more resistance than the others. A short within the winding is identified by a lower reading when compared to a good winding.
  • Resistance between windings should be extremely high — infinity on some meters — since windings should not be interconnected. A lower resistance in one or two windings may indicate a short between the two windings.
  • Resistance to ground should be extremely high since the windings should not be connected to a ground. A lower resistance in a winding may indicate a short to ground.

If the motor does not rotate, the motor bearings are likely damaged. More extensive damage is likely. Have a qualified electrical motor repair facility determine if the motor is salvageable.

Control panel

Inspect the control circuits. Be sure to inspect starters, relay timers, terminals, wires and other components for signs of high heat. Look for melted or heated-discolored plastic and burnt contacts. If the components look fine, use a Mega-Ohm meter or High Pot Tester to verify whether the circuit is safe to operate. If you notice damage, replace the components.

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