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Feel for Air-Impact Torque
restoring a 1955 M38A1, and currently am still in the disassembly mode. I’ve pored over your two books to help
with teardown and to plan for the rebuild. You mention the use of impact wrenches with known or definable torque
settings to finish an assembly requiring high torque settings (150 to 250 ft-lbs). Do you know of a source for
such air impact wrenches? Are the prices reasonable enough for a home garage “handyman” shop? I use air tools
quite often, but if the final torque value is important then I have to go back and check it with a torque
wrench. I searched the internet every way I could think of and the closest tools I could find were the “torque
sticks” that seem to be designed for tire work. Being able to confidently set torque with an air impact wrench
would be a handy, time-saving thing! If not, is there a source for a 250 ft-lbs torque stick?—Warren
from any impact gun can vary. The energy source is compressed air and the application of impact hammers.
Therefore, the air supply pressure, the line sizing, coupler types, tank size, line pressure range and length of
hose each affect the impact force at the tool’s drive head.
Overall, if you
need to regulate the line pressure with a quality regulator and maintain tank pressure well above that setting.
The air storage should be ample, allowing for constant pressures for a while before the compressor kicks on.
Given all of this, you can be reasonably assured of uniform air flow volume and pressure. (For a note of humor,
read my article on the big air compressor: Downsizing and Air
Since you can
establish a given air volume and flow rate (CFM, SCFM, however you want to rate the flow), the remaining issue
is the actual driving force at the impact tool’s head. With impact force,another factor is the air motor of the
impact gun and the constancy of hammer force. A new gun, for example, may have more force available than a worn
or aged gun. A gun operated for a while will heat up, and that affects both the impact force and
More to the
point, the impact guns that actually have controlled, precisely regulated torque are industrial designs used in
manufacturing processes. My references in the books refer to knowing your automotive impact gun’s
character. Over time, we learn how an impact tool behaves, including its approximate torque output. The book
comments reflect the use of an air impact gun on an axle flange or pinion nut with a wide torque allowance.
I worked in full-service gas stations, a tire store, as a fleet truck mechanic and as a
professional truck dealership tech. I mounted a lot of wheels and tires! I have spent the last four decades
around air tools.
Never relying upon air guns for the final torque on an exact, close-tolerance fastener or soft
alloy threads, I can say that years of operating particular classes of tools develop an instinct for the output
of an air tool. You learn, from feel and sound, how long to hold the trigger or apply bursts of air and
experience, the only current way to produce precise torque with common air impact tools is the torque stick.
Torque sticks have become popular for tire stores and environments that require quick, repetitive torque
settings on nuts or bolts.
There are two
limitations with torque sticks: a finite torque range for each stick and the stick length when performing tight
access work. Beyond this, sticks are actually a good approach for those unsure of their impact gun skills or for
fast, repetitive use—like tire store work.
The sticks look
much like socket extensions. A good example of these products is at the website http://www.torquestick.com/cart/home.php?cat=17.)
full-set of torque sticks offer popular wheel nut torques from 65-140 ft-lbs or higher. This presumes the use of
a gun that consistently puts out 375-400 ft-lbs torque.
By design, the
stick reaches the torque setting and begins flexing, enough to dissipate or “bleed off” torque beyond the
setting. Applied torque holds at the pre-set limit and does not exceed that force. A quality set of sticks is
accurate to +/- three percent, which is not bad. A quality torque wrench may have +/- two percent
In my books, I
talk about becoming familiar enough with your air gun to come close to an expected torque setting. In
particular, I am talking about high torque applications like the flange nut on shim-style Dana/Spicer pinion
shafts. This setting may be a range of, let’s say, 180-250 ft-lbs. You can develop the skill to replicate such a
range of torque repeatedly. I’ll explain how.
Let’s say you
want to accurately install wheel nuts to 90-100 ft-lbs. Hold the gun steadily. “Feel” and listen to the gun and
impact noise as you blip the trigger. When you have the wheel nut reasonably tight, verify the torque with a
torque wrench and socket. Note the amount or degrees of socket rotation necessary to reach the desired torque.
Loosen and tighten the nut to a slightly tighter point. Again, note how much you need to rotate the nut to reach
the torque setting.
Repeat the air
gun process on a fresh wheel nut. Consciously allow the socket to rotate to the same point as the last time.
Then rotate the nut further, to the position near the final torque setting. Verify torque with the torque
wrench. If this is at or close to the actual torque desired, repeat the process on another nut. Verify torque
with a torque wrench. Repeat this process until you can use the impact gun to set torque at or just slightly
under the desired torque. Confirm with a torque wrench.
This may sound
complicated, but professionals who use an air gun and sockets on wheel nuts all day long can reach within a few
foot-pounds of desired torque—every time. Why? Because the “feel” of the gun and repetitive practice equals a
clear sense for the torque applied.
You can get to
the point where the final check with a torque wrench involves a rotation of 1/12th turn (30-degrees) or less.
With practice, you can get the turns down to zero nearly all of the time.
manufacturers, like ARP, actually rely on degrees of bolt or nut rotation to reach desired torque. This is
because alloy hardware has a predictable stretch when tightened. This stretch is measureable with a dial
For many of its
fasteners, ARP prefers stretch to actual torque figures to ensure proper torque settings. They also consider
torque angle-degrees in many cases. After setting the fastener to a lighter torque, the specification calls for
an additional turn of the nut or bolt by so many degrees. A torque angle gauge is used on the ratchet or breaker
bar. See the ARP comments at its website:
alternative ARP guidelines apply to the hardware at hand, you should verify torque with a torque wrench. This
applies to all parts that are either safety-related or set to close-tolerance specifications—or when working
with softer metals or easily distorted, precision parts.
When I taught
adult education level mechanics courses, I was able to demonstrate the use of an air impact gun under various
conditions. Keep in mind, when exploring torque values on your own, that impact force on high-powered guns is
quickly reached. The slightest turn of a nut or bolt beyond a high torque setting is very high
is to take out the slack in the nut and threads, not to hammer away at a nut. When you cannot produce enough torque while
hammering with a gun that has a high torque rating, either the air volume available is too low or the gun is not
Caution: Always lubricate your air tools according to the manufacturer’s
recommendations! This affects torque output and tool life. I use inline oilers or filter/oilers with my air
tools. Make sure you do not use that same line for airing tires or spray painting. Oil can destroy rubber and
ruin paint finishes.
Ironically, the best way to learn air gun torque is with a high-powered gun and a high flow
air system. This will
produce predictable, repetitive torque readings. Set the air gun flow very low to start
with, then work your way up to high torque settings.
a smaller compressor with a light CFM rating, the gun can act erratically or not repeat the same torque values.
You will have difficulty meeting torque readings when using a light-duty compressor or an inexpensive, poorly
engineered air impact tool.
One dramatic example of under-capacity air flow is attempting to operate
a bead-blasting cabinet with a small compressor and air tank. (Such was the case when I wrote the blog, “A Lot
of Hot Air!”) Using the right air gun and air supply system, you can learn torque values and how much trigger to
apply. It just takes practice!
To reach the torque settings I talk about in the book (a wide, 150-250
ft-lbs range for pinion nuts and such), I use an I-R professional gun. An ample air supply is crucial for
pinion nuts have fine threads and take up quickly once the nut seats, it is easy to achieve range settings in
excess of 150 ft-lbs without over-tightening beyond the 250 ft-lbs point. In this case, if you have to verify torque
with a hand wrench, you will need a lot of counter-force with a holding bar to keep the pinion yoke from
turning. This is where
achieving the right torque with an air gun is very valuable and time saving.
hardware is a learned art. Many years ago, there were no torque wrenches, and various hardware sizes required a
certain “pull” with a ratchet or breaker bar. To this day, I can tighten a 3/8ths, USS-threaded bolt to exactly
35 ft-lbs in iron threads—without the use of a torque wrench. (I can do this with a hand wrench of various
lengths, a ratchet or a breaker bar.) This ability came from years of experience with hand tools, including a
torque wrench. You can do
contemporary alternative is the torque stick. If you choose this approach, use the extension style that requires
the use of detachable sockets. And yes, there are torque sticks for high torque. Here is a source for a 1”
drive, 250 ft-lbs torque stick:
this is a 1” drive tool. 1”
drive would be a large truck application air tool. This means that the gun driving such a
stick must handle torque in the 650 ft-lbs range—or higher. I’m not sure you want to invest in such an
air gun or a compressor with the air supply/volume to operate one.
unclear whether you can use this stick with a reducer/adapter from 1” drive down to ¾-inch or
½-inch. Consider contacting
the tool manufacturer to see if that is possible. If your ½-inch or ¾-inch gun has enough torque (some have very
high ratings these days), perhaps it could drive that torque stick.
I trust this helps, Warren…It’s much easier to teach or demonstrate
these techniques in a hands-on setting. Please feel free to continue this dialogue.
More Air Impact Tool and Compressor
From: Drew M.
Saturday, February 05, 2011 1:32 PM
To: 4WD Q & A
Subject: RE: 231 TC, '94 G Cherokee Laredo
cannot get the transfer case front yoke nut loose with my air gun and 4-gallon, 135 PSI compressor. PB
Penetrating "Blaster" and even a lot of heat, to the point of that big nut is glowing, and still not
loosening! I am stubborn and dont want to pay the high cost of
labor by towing it to a shop when I can do all of it, except the nut. I think I will hit up
some guys in my area and offer them a case of beer for their help and
expertise. I looked closely and it doesnt appear that the last person to work on the TC (when they took off the old chain) welded the nut on the threads.
Sunday, December 19, 2010, 4:26 PM
this is frustrating, and I’ve been there! Assuming this in not an oil-less compressor, and at least a twin
stage if not the even better two-stage type, one would think the
four-gallons could do it if you set the discharge air at full 135 psi setting. Yes, given the low volume
supply from the tank, I would crank up the input pressure to the air
tool. Try at least 110 psi. You have a small, likely quarter-inch supply line. Given the reduced volume of the line and minimal air supply available, you can
try to compensate with more line pressure and short bursts with the
air impact tool.
I have fought the high-output air tools you describe, with home/garage air
compressors and even some two-stage, 80-gallon types with 175 psi max output. The secret is a larger tank plus higher pressure with greater volume, using
adequate (3/8th inch) hose. High output air impact tools require commercial compressors to achieve rated torque output.
Having been over this path with bead blasting cabinets, I finally got the
message. The goal for a blasting cabinet is an 80-plus gallon (minimum) tank with
a compressor that puts out 12-19 cfm or better at 90 psi. I
finally found such a compressor—fortunately used. It has a horizontal, 120 gallon tank
with a Champion R15 two-stage iron compressor head capable of 23
cfm output @ 125 psi (spinning only 750 rpm at the compressor crankshaft!). Where do
you find such a beast? I shopped diligently, finally finding
a failing body shop that had treated its 5 hp single phase 220V unit with TLC…The
magnetic start (big plus) Baldor replacement motor was only one
year old…Price: An incredible $500! It would have been a bargain at $1000. My nearly new,
7 gallon I-R two-stage compressor has been since relegated to
inflating tires, HPLV painting and light torque air tool use.
You have encountered a nut that someone put on (maybe even cross-threaded?) with a
lot of force and a big air supply system. Try hammering your air tool back-and-forth, tightening then loosening. If you must, take the transfer case to a tire
store/garage with strong impact tools and a large air supply. They should
be able to remove the nut in two seconds. I could with my Champion unit and your new air impact
Let me know when you get that nut loose!