If you are an artist working with airbrushes, woodworking in the garage, or own a small metalworking or auto repair business, then chances are you have an air compressor supplying pressurized air to your pneumatic tools. What do you do if the compressed air you rely upon so heavily is no longer available to power your equipment?
Is your air compressor not building pressure? Let’s fix it. Most problems are caused by leaks, faulty seals and gaskets, or broken valves, and the good news is that these are fixable. Many manufacturers even sell repair kits for these very issues.
Most pressure-related issues can be resolved with basic tools, a little elbow grease, and perhaps some parts from the manufacturer. The real challenge is knowing which part of the compressor is causing the problem, and we will guide you through the troubleshooting process. Half the battle is won when you correctly diagnose the issue at hand, so read on to learn more about fixing air compressor pressure problems.
Air Compressor Basics
To properly troubleshoot issues relating to your air compressor not building pressure, it is essential to familiarize yourself with its working parts. Today’s air compressors are more powerful and energy-efficient than ever before, and their affordability has enabled small shop owners and even do-it-yourself-ers and hobbyists to own them.
The overwhelming majority of air compressors found in garages and businesses are known as reciprocating compressors, and they produce compressed air via pistons inside cylinders. Since this type of compressor is so popular, we will focus on reciprocating compressors in this article as these are more likely to be serviced and maintained by their owners and operators.
(Other types of compressors such as rotary screw and centrifugal are designed for high-demand compressed air applications and are found in more extensive industrial operations. These types of air compressors are less prone to technical problems. They are also more complicated to service, and their repair is best left to the professionals.)
There are Four primary part groups to a reciprocating air compressor, all working in unison to produce compressed air:
- the motor (also referred to as the drive) assembly
- the pump assembly (which includes the cylinders and pistons)
- the compressor tank.
- Electrical system
Each of these consists of individual parts that we will be referring to throughout this article.Leaving out electrical System because your primary Question is Why its not building pressure, so for this article, we are going to assume the electrical is properly working.
The Air Compressor Motor
Air compressors need a power source to drive the mechanisms that compress air. This job is performed by the motor, which works in conjunction with the pump to produce compressed air. The overwhelming majority of compressor motors run off of electricity, although there are gas versions for work sites where electrical power is not readily available.
- Each motor has a shaft which rotates at an extremely high rate of speed (often measured by revolutions per minute or RPMs) when the motor runs.
- The motor shaft is attached to a pulley around which lies a drive belt.
- This belt also loops around a larger wheel attached to the pump, so when the motor runs and the shaft rotates, the pulley drives the belt which engages the pump where the actual work of compressing air occurs.
The Air Compressor Pump
The pump is the part of an air compressor that produces pressurized air. It consists of one or more cylinders that function much like the cylinders of a car engine.
- The drive belt, which is powered by the pulley on the motor, also loops around a flywheel, which is attached to the compressor pump.
- Thus, when the motor runs, its shaft is spinning the pulley, which drives the belt and turns the flywheel on the pump.
- The flywheel is attached to a crankshaft in the pump, which is connected to the pistons seated inside the cylinders.
- On the intake stroke, the piston draws in fresh ambient air.
- On the compressing stroke, the piston squeezes the air inside the cylinder, pressurizing it and sending it on to the tank for storage.
- It is the rapid-fire action of the piston taking in air and compressing it that builds pressure in the tank.
Along with the pump, there are various devices located on or near the pump assembly that ensure that the compressor operates efficiently and safely.
- A pressure switch that is responsible for cycling the motor off when a certain pressure is reached in the tank and turning the motor on when the tank pressure falls below a certain level and needs to be replenished.
- A safety release valve allows the operator to de-pressurize the tank to perform maintenance or repairs. The safety release valve also serves as an emergency safety feature allowing excess pressure to escape in the unlikely event that the pressure switch fails to regulate properly regulate tank pressure.
The Air Compressor Tank
The purpose of an air compressor tank is to safely store pressurized air for use by the pneumatic devices, tools, and equipment that are connected to the compressor. By storing a supply of compressed air, the tank enables the motor and pump to cycle on and off as needed. In other words, the tank allows the other components to idle for roughly 25% to 50% of the time to avoid overheating and excessive wear by running continuously for too long.
It should be noted that the air around us contains moisture in the form of water vapor. The amount of this moisture depends on temperature and humidity. When air is pressurized, this moisture condenses into liquid form, and water is the result. In an air compressor, the highly pressurized air will produce water with each cycle, and this moisture ends up in the tank.
Water is a corrosive substance, particularly to metals. Even though an air compressor tank has a protective coating on its inner lining, all compressor manufacturers strongly recommend daily draining of compressor tanks to avoid corrosion and possible corruption of tank integrity. Every compressor has a drain valve for this very purpose.
Common Causes of Air Compressors Not Building Pressure
Now that we have more familiarity with the layout of the compressor, we can examine specific causes of pressure loss or inability to build or hold air pressure. From a troubleshooting and diagnostic point of view, it is helpful to take a structured approach based on the major sections of the air compressor, namely the motor area, pump area, and the tank.
Motor-Related Issues
The following are potential causes of weak pressure build-up arising from motor-related issues.
PROBLEM: Electrical Issues
When it comes to air compressors, it all starts with the motor. It is the motor that provides the power necessary to run the pump, which in turn produces the compressed air. If the motor does not run correctly, there will be a noticeable and possibly significant reduction in air production.
There are a few possible scenarios where electrical issues can affect motor performance:
- First is that the electrical wiring connecting the motor to the site’s power is faulty, either with the wires connected to the wrong terminals, or one or more of the connections being loose.
- Another possibility is that the house voltage being supplied to the motor is less than what the motor requires. For instance, if a single-phase 230-volt compressor were to receive less than 200 volts, it would not be able to run at its standard horsepower, would fall short of reaching its peak RPM (revolutions per minute), and as a result, the pump’s performance would suffer significantly.
- In the case of a 2-stage compressor running on 3-phase power, if one of the power phases were deficient (in other words, a dropped leg), the motor would struggle to get up to proper speed, and the same lack of compressed air production would result.
SOLUTION: Verify Electrical Connections
The owner’s manuals that accompany new air compressors include detailed instructions on installation and first run-up procedures. Often, these manuals will have wiring diagrams that depict the exact wiring connections that need to be made. They also provide additional installation advice, for instance, recommending that the proper wire gauge is used (particularly concerning the distance between the compressor and the actual power source).
- The first step is to verify that the site’s available power matches the electrical requirements of the compressor in terms of voltage, phases, and amperage. All compressors have a metal tag affixed to the compressor body that clearly states its specifications.
- The next step is to confirm that the correct wire-to-terminal connections have been made and that the connections are tight.
A general rule of troubleshooting (regardless of equipment type) is to eliminate the most obvious and general conditions as possible causes before proceeding to specific components. In the case of an air compressor not producing adequate pressure, it is good practice to rule out basic things such as power and electrical connections before diving in and attempting to troubleshoot other components.
PROBLEM: Worn or Loose Drive Belt
The drive belt transfers energy from the motor to the pump and is one of the most critical components of a reciprocating air compressor. A faulty belt is a common cause of an air compressor failing to build pressure. Typically, the belt has stretched or deformed to the point that it is slipping and no longer able to turn the flywheel on the pump effectively. This would result in insufficient piston movement and inadequate production of compressed air.
Many manufacturers recommend regular inspection of the drive belt (at least monthly) and advise that if there is more than ½” inch of slack (when you move the belt up and down between the pulley and the flywheel), to tighten the tension between the pulley and flywheel. If there is visible wear on any portion of the belt, it must be replaced immediately.
SOLUTION: Tensioning or Replacing the Drive Belt
Because the motor pulley, drive belt, and pump flywheel move in unison at a remarkably high speed, this section of an air compressor is often enclosed in a protective cage to prevent injury. To access the belt, this cage will need to be opened or removed with the compressor powered off.
Recognizing that belts do wear out, many compressor manufacturers have implemented design features to make replacing them more manageable. One such feature is enabling the motor to slide toward and away from the pump to allow the drive belt to be disengaged and removed, and the new belt to be installed without much effort. This is done by loosening nuts that lock the motor in place during operation. Belts that are simply loose but in good condition can also be tightened in the same manner.
Before starting up the compressor, it is crucial to verify that the new drive belt is adequately seated in the grooves along with the pulley and the flywheel. The tension also needs to be adjusted so that there is no more than ½” of slack in the belt. There is a special type of spray called belt dressing that can be applied to a drive belt to make it tacky, thereby increasing its grip on both the pulley and flywheel.
Pump-Related Issues
The majority of issues relating to an air compressor not building up enough pressure will arise from the pump portion of the compressor. The simple reason for this is that the pump area has the most working components, many of which perform vital functions that directly affect the compressor’s ability to produce compressed air.
PROBLEM: Air Intake Filter
This is one of the first things to consider when dealing with a pressure build-up issue. On each cylinder of the compressor pump is an air intake through which the ambient air flows to be compressed by the piston. Within each air intake is an air filter that removes dust and debris. The compressor’s efficiency in producing pressurized air is dependent upon the cylinder drawing in the appropriate amount of ambient air to compress.
A clogged air filter will significantly reduce the volume of air that is drawn into each cylinder during the intake stroke, meaning that there is less air to pressurize and send forward to the tank for storage. If the volume of intake air is diminished to the point that negligible amounts of compressed air are being produced, there would not be any appreciable amount of built-up pressure in the tank.
There is also the risk that the motor and pump will overheat as they continuously work to build up to a pressure setting that simply cannot be met. At that point, both components can suffer damage from overheating or stress at which potentially resulting in very costly repairs and lengthy downtime.
SOLUTION: Clean or Replace the Air Filter
Air compressor manufacturers have made air filters easily accessible for service and replacement. Air intakes are located on each cylinder and can usually be identified by a dome-shaped cover which can be removed by loosening a screw in the center. With the air intake cover off, the air filter can be removed. Be careful to exercise caution as a significant amount of dust and particulate material can be caught in the filter material.
Most compressor air filters are designed to be re-usable (particularly those made from cloth or felt) and can, therefore, be cleaned and put back. The owner’s manual that came with your air compressor should specify whether any special instructions apply to clean the air filter, but most cloth filters can be washed with soapy water, and other types of filters can be vacuumed.
If the air filter is damaged in any way, then it must be replaced. Your operating manual should have a parts list for commonly replaced items like air filters, and many manufacturers have this information available on their websites.
Replacement air filters are a good thing to have on hand so that you do not experience any downtime while waiting for a part to be ordered and delivered.
PROBLEM: Valve Plate and Intake (Reed) Valve
This is a relatively common cause of reduced pressure build-up in reciprocating compressors, particularly those that are subjected to heavy use and maximized duty cycles. Within each cylinder is a piston that moves back and forth at a very high rate of speed. With each back stroke (intake stroke), the piston pulls fresh ambient air into the cylinder, and with each forward stroke (compressing stroke), the air is compressed and sent to the tank for storage.
There is a valve plate assembly that is responsible for allowing fresh air into the cylinder during the intake stroke and then preventing that same batch of air from escaping from the cylinder during the compressing stroke. If this valve plate were to fail, intake air would still be drawn into the cylinder, but it would then escape or leak from the cylinder through the intake valve during the compression stage. The intake valve (also known as a reed valve) is incorporated into the valve plate.
Diagnosing a problem associated with the intake valve on a compressor pump is a fairly straightforward process. In a previous troubleshooting section, we examined how to remove and replace the air filter, and for this step, you will need to perform this same procedure. With the air filter removed and the compressor cycling (running), carefully place your hand directly above the open port through which ambient air is usually drawn.
Under normal circumstances, you should feel the air being pulled or sucked into the cylinder; if you feel the air being pushed out of this intake port, then the cylinder is no longer sealing air during the compression stroke, and the valve plate is failing.
SOLUTION: Replace Valve Plate and Intake Valve
To access the valve plate, the head (top) of the affected cylinder will need to be removed. Be sure to fully de-pressurize the tank before performing these or any repairs. Also, the dispense tube that connects the cylinder to the tank will need to be disconnected before removing the cylinder head.
There are typically four bolts that attach the head to the cylinder body, and once these are removed, the head can be lifted off. The valve plate is rectangular, and within it is a series of thin wedges, which are the intake or reed valves. It is when one or more of the reed valves wear out that air will escape through the valve plate and prevent pressure build-up.
Replacement valve plates are usually sold as a complete kit with the reed valves pre-installed. There will be two or more rubber gaskets that must be replaced along with the valve plate. Here is a great video showing how this component is removed and replaced.
PROBLEM: Cylinder Gasket
It is inside the pump cylinders that the real work of producing compressed air is performed. Within each cylinder, a piston cycles back and forth, drawing in ambient air with each back stroke and compressing it with each forward stroke. For this to be done efficiently, the cylinder must be completely sealed, and there is a cylinder gasket that serves this very purpose.
If the cylinder gasket fails, then the air compression process will be compromised, either in the amount of ambient air that is taken in, the efficiency with which the air is compressed, or the amount of the pressurized air that is transferred to the tank for storage.
SOLUTION: Replace the Gasket
Fortunately, the procedures to access the cylinder gasket are the same as those for servicing the valve plate. As with drive belts and air filters, it would be a good idea to have a spare gasket on hand in your spare parts bin so that you can minimize downtime when it becomes necessary to replace this item.
Tank-Related Issues
The following are tank-related issues relating to a lack of pressure build-up.
PROBLEM: Drain Valve Allowing Compressed Air to Escape
Other than verifying proper electrical connections, this is one of the first diagnostic items that should be checked. Virtually every air compressor manufacturer recommends draining the tank daily. As such, it would come as no surprise if the drain valve was inadvertently left open after performing this task. Even if it were partially closed, the effect on air pressure would be the same.
An open drain valve, whether partially or entirely open, would allow compressed air to escape from the tank as it is being produced. No pressurized air would build up in the tank, and the motor and pump would run continuously. Since the pressure switch would only cycle the motor off when a specific tank pressure has been achieved, there is a real danger of the motor overheating.
SOLUTION: Close the Drain Valve
Although closing a drain valve that was inadvertently left open is about as simple as a solution can get, it can serve as a valuable reminder to make sure that the drain valve is fully shut after performing the daily draining of the tank.
With the frequency of that tanks are drained, it is possible that at some point the drain valve itself may become compromised and not fully seal or close. Drain valve assemblies are offered by most manufacturers as replacement parts, and this is one that you may want to have on hand in your spare parts bin.
PROBLEM: Moisture Accumulation in the Tank
The air around us contains moisture in the form of water vapor, and when a compressor pressurizes this air, the moisture condenses into water. This is the reason why air compressors have drain valves at the bottom of their tanks, and why manufacturers recommend draining tanks daily.
Neglecting to drain the tank for even a few days can lead to a significant accumulation of water inside the tank. Not only can the presence of this moisture cause reduced air pressure build-up in the tank, but also significant problems for pneumatic tools and equipment downstream. For instance, moisture in woodworking tools can cause warping in wood materials, and water in a spray painting equipment can cause dilution of paint and uneven finishes.
SOLUTION: Drain the Tank Regularly
Daily tank draining should become as routine as locking up the shop doors when you go home for the evening. Compressor manufacturers now offer automatic tank drains with higher-end models and aftermarket kits that can be installed on virtually any air compressor. Some work off of electronic timers while others are connected to sensors that can detect moisture levels in tanks, but they all perform this vital task automatically for you so that it is no longer a concern.
Tank Leak
Just about the worst-case scenario when it comes to an air compressor not building pressure is a tank leak. Because the amount of pressure in an air compressor tank is so high (typically 100 PSI for a single-stage and 175 PSI for a 2-stage compressor), it is nearly inconceivable that a compromised tank can be adequately repaired to restore its integrity to like-new condition.
For this reason, no repair of a tank leak should be attempted. This also highlights the importance of draining water from your compressor tank daily – accumulated moisture is a tank’s worst enemy.
Compressor Not Building Pressure? Not to Worry
Today’s air compressors are more user-friendly than ever before, and this includes performing your own maintenance and repairs. No longer do compressor problems automatically lead to expensive repair bills. With a willingness to learn a few basics, you can resolve even seemingly serious issues, including an air compressor not building pressure.