Compressed Air System

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.

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.

Inspect the oil

Much of the wear materials and dirt the compressor has been subjected to collects on the bottom of the separator tank. Drain the tank from the very bottom and analyze the contents. Local labs can identify what types of contaminants are present in the oil at a reasonable price (often below $ 25 at 1997 prices). You should expect to find considerable dirt. If wear metals like copper, iron and the like are also present in some quantity, there may be other unidentified damage inside the unit.

Decision time — overhaul or not

Performing these test procedures should make it possible to estimate the cost of overhauling the compressor. Keep in mind that a compressor that requires the replacement of only one major component such as an airend or motor may still be worth overhauling. However, a unit that requires some combination of a new airend, motor, starters and has a considerably contaminated oil circuit may require more work than it is worth.

A point often overlooked is the aspect of post-rebuild efficiency. While the compressor may be sound enough mechanically to warrant a rebuild, it is prudent to investigate how the compressor holds up to a power cost analysis (consult your local manufacturer's representative for assistance). Since over 70% of the total cost of compressed air is in energy input, you need a rebuilt compressor that is not only reliable but also efficient by current standards. Compare full-load power consumption versus compressed air output at rated pressure. Also, compare them at various levels of partial load.

How to rebuild

If you decide to overhaul the compressor, a knowledgeable mechanic will need 16 to 24 hours to perform the following tasks.

Remove the airend, valves, separator tank, piping, hoses and other mechanical components. Except for the airend, carefully disassemble and clean each component. Note: In order to maintain proper clearances, disassembling the airend is not advisable. It should be possible to clean most of the airend without disassembly.

Inspect each part. Replace O-rings, gaskets, V-belts, hoses and other worn parts. Lightly lubricate moving parts as specified by the equipment manufacturer. Pay particular attention to solenoid operated valves and valves in the compressor's oil circuit. Typically, valves can be bench tested for proper operation.

Reassemble the mechanical components. Remember to replace the filters.

Change the lubricant. Fill the compressor with the proper compressor lubricant as specified in the Operation & Maintenance Manual.

Test the control circuit. Occasionally it is possible to dry run the control circuit. Simply put, this step allows you to operate the compressor's control circuit without operating the motor and other higher voltage circuits. Dry running the controls identifies malfunctioning electric components that may have been overlooked in the previous inspection.

Most of the time dry running the compressor's control circuit can be done easily. However, if you are not familiar with how the circuit operates, it is safer to contact your local manufacturer's representative for assistance.

Initial start-up

Starting the compressor for the first time after an extensive overhaul should be done carefully. To ensure the compressor is not started without lubricant, pour some compressor lubricant directly into the airend intake and turn the airend by hand. Also, make sure the airend and motor are aligned properly.

IMPORTANT! Verify that the motor rotates in the correct direction before running the compressor. This can usually be done by "bumping" or "jogging" the compressor. This means starting and immediately stopping the motor to see in which direction it rotates. The safest way is to start the motor without the V-belts connected. Also, remember to check any fan motor(s). If a compressor has a separate fan motor, it is possible that one motor rotates in the correct direction and the other rotates backwards.

Start the compressor by following the manufacturer's recommended start-up procedure usually found in the Service and Parts Manual. Once the compressor is up and running, keep an eye on operating pressure and temperature. If the compressor exceeds its rated pressure, be prepared to shut down the compressor. Usually a simple adjustment to the air pressure switch solves the problem. If the compressor starts to run hot, note the rate of temperature rise. If the airend temperature rises very quickly (within seconds) above 200° F, immediately shut down the compressor. This rapid rise indicates a problem with the oil circuit. It may be simple to fix, but continuing to run or re-start the compressor at this point could ruin your hard work. Instead, call your local manufacturer's representative for assistance. Typically, you will find that your hard work has paid off because the compressor runs smoothly and reliably. One last task makes your overhaul complete.

Flush the oil circuit. Now it's time to dispose of the new compressor oil and some of the new filters you've installed. Don't let this stop you from performing this last step. While you've done a great job cleaning the parts, there is still dirt and varnish in oil journals, piping, fittings and other nooks and crannies. Use an appropriate compressor flush to clean the oil circuit completely before these contaminants cause unnecessary component failure. There are many flushing products on the market. One example is Thermasolv, a Pennzoil product. When using these products, always follow the manufacturer's instructions. It may be good to repeat the flush two, three or even four times, changing the oil between flushes until are no signs of varnish and dirt remaining. Once flushing is complete, replace the oil filter and oil separator element. This eliminates recontaminating your oil circuit.


If you have an efficient, quality compressor that is old or broken down, it is worthwhile to consider rebuilding it into a reliable and efficient part of your compressed air system. Since rotary screw compressors have few major components, the overhaul procedure is fairly straightforward requiring only a reasonable amount of time. As a rule, do not spend more than 60% of replacement cost for the first overhaul and not more than 30% of replacement cost for the consecutive overhaul.