Are You Getting Enough Air?

Courtesy of TechShop by Eric Garbe

One of the biggest mistakes I’ve seen in years started with an auto technician who wasn’t happy with the performance of his air impact wrench. To make matters worse, there is always a certain aura surrounding this tool, and whoever has the most powerful one in the shop has “bragging” rights over everyone else.

Engineers are not only looking for the kind of power that can unscrew almost any bolt, but admit it or not, there is a competitive spirit in most of us that wants to exert the most power in the workshop.

Then one of the local tool dealers walks in with a new impact wrench featuring all kinds of earth-shattering torque and before you know it, technicians are willing to make payments over the next six months in the hopes that their new tool will step down. top dog and breaking loose bolts without breaking a sweat. The unfortunate reality is that in most cases their new tools fall short of expectations. Even if it works better than the old one, it may still not be at full capacity, a fact that often goes unnoticed by the owner.

All air tools are designed to work with a certain amount of air pressure, and most of the ones we use for automotive repairs range from 90-100 pounds per square inch (psi). Just as an internal combustion engine produces maximum power and torque over a certain RPM range, your air tool is designed to operate at a certain psi and airflow to provide the speed and torque for which it was designed.

This is where the wheels usually start to come off the cart. We are often warned by tool dealers not to operate air tools at pressures higher than specified as this will cause accelerated wear, and possibly sudden tool failure. Are we listening? Usually not. If the tool works and gets an extra boost from the higher psi in the shop, all is well, right? We have work to do and no time to fiddle with the little details!

We also know that the lower the pressure, the lower the power output of the air tool. We’ve all waited countless times for the air compressor to start running at its upper pressure limit before we tried to remove that crank bolt. And we know what the pressure gauge on shop compressors reads, with most being between 120-150 psi, so we think we’re under a lot of pressure. But unfortunately, we were disappointed because the crank bolt still wouldn’t budge.

The reality is that your air tool is most likely not getting the correct pressure and volume, and regardless of the pressure in the compressor, it will rarely experience higher pressures than needed. The key lies in static versus dynamic air pressure.

Static pressure is the amount available in the line when nothing is operating. When you read the gauge on the compressor, it’s easy to think this is where your air tool is operating, and if you check the pressure at the end of the air hose, you’ll see the same reading. But this is only static pressure.

Dynamic pressure, on the other hand, is the pressure available in your air hose while the air tool is connected and running. Air tools require large volumes of air to operate properly, and it is not uncommon to see static pressure drop to 30 psi or more when taking dynamic readings. The reason for this is volume. If your airline cannot deliver sufficient volume, you will never be able to maintain the proper pressure for proper air tool operation.

There’s usually a lot of pressure and volume in the air compressor tank in your shop, but how many bays fill it? And what is the diameter of the duct or pipe that runs throughout the shop? How many corners does it turn and how many bays does it cross before it reaches you? And finally, what is the diameter of your air hose, and are your fittings rated to circulate the right amount of air?

All of these questions must be answered when a dynamic air pressure test proves that you are not getting enough to make your air tool work as it should. To do this, you will first need to build a test gauge (Figure 1) using pipe tees, gauges, and air fittings. Connect it to the end of your air hose and see the pressure. This is your static pressure. Then connect your air tool and hold the trigger wide open. This is your dynamic pressure. If it is too high, a regulator must be installed to limit the pressure. If it is too low, then you should investigate the answers to the questions above to determine the cause of the problem.

I’ve been in a lot of shops over the years, most of them don’t have pressure gauges on the supply lines in each bay, and I’ve never seen anyone do a dynamic pressure test. But how real is it? I decided to find out and I had the perfect opportunity. My air tools haven’t been doing well in the house lately, and it seems obvious that they aren’t getting enough air, but I wanted to know how bad it was and why.

My compressor is only small but rated to provide 6 cubic feet per minute at 90 psi. my 1/2-in. impact recommends a 3/8-in. hose size, operating psi of 90 and has an average air consumption of 3.5 cfm (cubic feet per minute). According to the specifications, the compressor should provide adequate output, and all of my hoses are 3/8-in.
I connected a test gauge to the end of the air hose. I adjusted the regulator on the compressor until I had 90 psi of static pressure available in the hose (Picture 2) I connected my impact and pulled the trigger. I’m sure I’m going for a sizable loss but I’m amazed to see the dynamic pressure drop to 20 psi (Picture 3).

Next, I adjusted the regulator on the compressor until the static pressure in the hose reached 120 psi. I pulled the trigger on impact one more time, and the dynamic pressure immediately dropped to 40 psi, less than half of the applied impact value. The available air volume is insufficient to maintain pressure. Obviously I had a limit somewhere so I went straight to the compressor and connected a gauge directly to the regulator outlet. I adjusted the regulator to its maximum setting and the gauge read around 135 psi.

With my impact connected, I pulled the trigger and the dynamic pressure immediately dropped to less than 60 psi. Looks like I didn’t have to look very far to find the problem. I averaged about a 70-80 psi drop from static to dynamic pressure at all points.

I removed the regulator and screwed the gauge in place, got a reading of about 140 psi pulled the trigger and ended up having a dynamic 100 psi slightly higher than needed, but proof that the compressor output was adequate. In this case, the regulator is the culprit.

This is an extreme example, but perfect for demonstration. I bought the parts to build my test meter at a local hardware store for less than $20. It’s money well spent. TS