Rotary Screw Air Compressor Basics

How Does a Rotary Screw Air Compressor work?

Rotary Screw Air Compressor Basics: A rotary screw air compressor contains two interlocked helical rotors which rotate in opposite directions within its housing. Throughout the compressor, ambient air fills and circulates through the intent valve where the air becomes trapped between the two helical rotors. As the screws turn pressure begins to increase by reducing the airs volume.Lysholm screws found inside a rotary screw air compressor.

In many cases, air compressors will have the full power of two screws which are essential for the many large-scale productions. In rare instances, some rotary screw air compressors will only contain one screw but are only used during refrigeration.

Types of Rotary Screw Air Compressors

A rotary screw air compressor has two main types of oil: oil-free or oil-flooded. The main difference between these, is the presence of oil in the air end.

The Difference Between an Oil-free and Oil-flooded Screw Compressor

Lysholm screws found inside a rotary screw air compressor.

Animation of lysholm screw found inside a rotary screw air compressor.

Within an oil-flooded screw compressor, a thin film of oil is between the male rotor (rotated by the motor) and the female rotor (rotated by the male rotor). The oil acts as a hydraulic seal for the air end and transfers the mechanical energy in charge of rotating the screws. It also acts as the coolant for the compressed air when high temperatures occur.

In an oil-free screw compressor, there’s no seal present between the male and female screws. Instead, the timing gears are used to synchronize the screw rotation, where the motors will have no direct contact with each other. Since there’s no oil seal in the air end, some clearance between the rotors will be present.

What Are the Main Parts That Make Up a Rotary Screw Air Compressor?

To further understand the rotary screw air compressor basics, you need to understand the different components and how they work. Let’s dive into an air compressor’s parts and how each component functions.

Rotary Screw Compressor Air Compressor Diagram and parts identifier.

Rotors /Rollers

Rotors or rollers are the heart of the rotary screw compressor. They come in pairs and are inside the cylinders of the compressor. Rotors rotate at high, sweeping speeds creating a pipeline for suctioned air to move through, compress, and discharge out of the system.

Compression Cylinders

Rotary screw air compressors include the main compression cylinder housing the rollers. When air is collected, it runs through the cylinder’s chamber and into the interlocking, spinning rotors.

Air Filters

Air filters are one of many layers of filters within the rotary compressor. The air filter sits inside the compressor’s opening valve where it catches dust, particles, and moisture. This prevents them from damaging to the inside of the machine.

Oil Filters

For oil-lubricating air compressor screws, oil filters are placed both within the rotating chamber walls and near the discharge valves. These filters purpose is to sift oil from the compressed air. Oil lubrication is necessary as it offers cooling needed for compression machines, which tend to produce high heat.


Bearings are on both rotor ends to help rotors stay in place. This is due to rotor ends rotating constantly.

Suction Valve

This component sits on top of the compressing unit and is responsible for initial gas retrieval. During the unit’s stage control, the suction valves open to allow air to move inside.

Discharge Valve

The discharge valves sit on the opposite end of the suction valve and start the end of the compression cycle. Now-pressurized air is administered in the discharge valve and is released into either a holding, storage tank, or discharge pipe for immediate application.

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Motors power the rotations of the rollers automatically, helping to power the machine’s entire compression capabilities.

System Control

This component is necessary for when operators read and gauge the unit’s overall health and output. A system control displays and monitors different components of a compressor – operating, idling, and stop parameters.

Storage Tanks

Storage tanks receive now-condensed air from the discharge valve where it sits and maintains pressure until it needs to be used.


In oil injection compressors, separator tanks are used as another defense tactic against oil and gas mixtures. T can damage the purity of the compressed gas stream.

Gaskets & Seals

Gaskets and seals ensure a rotary screw compressor’s interior and exterior are locked, sealed, and leak-free.

Air Compressor Maintenance Checklist Download

Maintaining Your Rotary Screw Air Compressor

Now that you know the Rotary Screw Air Compressor Basics, let look at maintenance. Like any compressor, rotary screw air compressors require regular maintenance. The great part about maintaining a rotary air compressor is that it’s easy. By performing maintenance your unit will be more productive for a longer period of time. Not to mention the time and money you will save on emergency repairs.

To ensure you’re following the right maintenance program, we’ve provided the list below to make your maintenance procedure as simple as possible.

worker looking over a compressed air systemDaily

  • Monitor all gauges and indicators for normal operation
  • Check the oil level
  • Drain control line filter
  • Look for oil leaks
  • Listen and feel for any unusual noises or vibrations


  • Check safety valve operation
  • Drain any air receivers in the system
  • Drain the water from the oil
  • Check to make sure moisture separator is draining correctly


  • Service the air filter as needed (if dirt is buildup in the filters frequently, change them daily or weekly)
  • Clean aftercooler and oil cooler fins (air-cooled only)
  • Wipe the entire unit down

Every 3 Months

  • Obtain synthetic oil samples
  • Replace the compressor oil filter
  • Change the petroleum oil
  • Go over the unit and check all the bolts for tightness
  • Check the full load amps
  • Check all the pressure settings


  • Change oil separator every 2– 4 years with synthetic oil
  • Change the air filter
  • Lubricate the motors
  • Check the safety shutdown system
  • Contact a qualified service technician

Rotary Screw Air Compressor Logs

Use the log below to better track your maintenance.

Rotary Screw Compressor Air Log for tracking maintenance annually.

Summary Of The Rotary Screw Air Compressor Basics 

Two counter-rotating screws power rotary screw air compressors. They are often quieter & more efficient than their piston counterparts. Rotary air compressors are meant for continuous use—8+ hours per day and are used when large volumes of high-pressure air is needed. They also have prolonged lifespans, making rotary screw air compressors a great long-term investment. There is minimal wear due to no metal-to-metal contact. Finally, maintenance on these systems is relatively simple.

Establish a preventative maintenance plan for your long-term equipment reliability with Rasmussen Mechanical Services today! Call us at 1-800-237-3141, email sales@rasmech.comchat with a support agent, or contact us online.