Pneumatic impact wrench usage

[Zeyneb]

Hi there,

Not so much engineering in this topic. More like standard car mechanics wrenching.

I’m a member of a DIY car repair shop where I can wrench on my car. That is nice. They have shop air including these gun style air impact wrenches which I can use. I don’t have much experience with them but they work quite well for loosening fasteners that have been seized up with rust.

I’ve been looking around the internet on some safety topics regarding the use of this. But I haven’t found an answer on a topic I’m wondering about right now. It is a very standard situation where a nut and a bolt is used to squeeze two steel plates together. So if there is sufficient clearance is it is preferred to put the impact wrench on the nut right? And if you don’t hold the bolt in place the whole fastener assembly can rotate while the nut won’t come loose. What I did was to put a standard box end wrench on the bolt head while putting the impact gun on the nut. It worked quite well but I’m not so sure how safe this is. Could the box end wrench turn violently and hurt my hand? And also should this box end wrench also be rated for impact use? I don’t know if they even exist.

Regards, Zeyneb

[bob91343]

Perhaps you should dial down the force of the impact wrench so that if something breaks loose you won't lose a body part.  And wear gloves.

After a few years of working on stuff like this you will be able to show some scars.

[jpanhalt]

Often an impact wrench is used for rusted/stuck fasteners or on something that rotates, and you cannot easily stop the rotation (e.g., a nut attaching the drive shaft to the differential).  As you suggest, putting it on the nut is standard practice, as with wheel lug nuts.  If there is a bolt head, I usually use a hand held wrench the head.  Impact wrenches are not that fast until the nut loosens, then you have to release the pressure.  Some people spin the nuts off, I do not like doing that nor is it a good practice in my opinion.  Similarly, I do not think they should be used routinely on spark plugs, particularly plugs that are made of 2 or 3 parts (as some Ford plugs are) or when the cylinder head is aluminum (quite often).  There are other considerations with spark plugs, so unless you are sure of what you are doing, don't use an impact wrench on them.  Another proscription is using them on bleeder screws, such as on brakes, and other small screws.  You will likely break the screw. 

On rare occasion, such as with bolts that hold the cutting edge on a snow plow, I will use a Vise-Grip or C-clamp to keep the head from turning (they are "plow bolts" and do not have a proper head).  Be sure to use proper impact rated sockets on the impact wrench.

I rarely use an impact wrench for assembly.  Lug nuts are the one exception, then I use a torque wrench for the final tightening.  I have never considered impact wrenches a safety issue.  Mine has 3 settings, I, II, and III for removal.  I use the lowest setting that works.

On frozen bolts you can help loosen them by adding a penetrating oil and letting it sit for at least 15 minutes or even overnight.  Do not do that on lug nuts, they are assembled dry. 

[Stray Electron]

  It's always good practice, that whenever possible, to position the "holding" wrench so that it is up against some obstruction so that it can't rotate any further. It will make getting the other half of the fastener off easier and will prevent the wrench from suddenly turning and pinching your hand. Given that most nuts and bolts are six sided and most sockets are six or twelve sided, you may not get the wrench tight against the stop but you can get it close enough that it can't move far before hitting the obstruction.

[james_s]

I use an impact wrench frequently, they're great for loosening up stuck fasteners, just don't use them to tighten stuff. If you have to use a backing wrench and have to hold it without wedging it against something, make sure you hold it such that it cannot pinch a part of your body if the wrench spins.

[BradC]

For small stuff I use the torque limiter on an old battery operated drill as an "impact wrench".

For the pneumatic, I concur with everyone else about not using one to "do stuff up" as in a couple of moments of idiocy I've snapped a couple of fasteners. One instance I took the device back to have a new fastener spot welded on and the guy asked me "What were you thinking?" to my reply of "I wasn't, charge me full price for being an idiot".

My neighbour has a 3/4" drive impact wrench for "the big stuff" that I'd be careful around, but you'd have to be pretty lax *and* unlucky to get hurt with the 1/2" drive unit. Mine has a variable air valve on the bottom, so you can always start weak and turn it up as required.

I'll re-iterate the battery operated drill though. Thinks like the side cover bolts on an old Yamaha 2 stroke bike, where the bolts are known to be made out of cheese. A couple of sessions with "gentle vibration" from the drill and they backed right out with no head damage, whereas the usual routine was to drill them out.

[CatalinaWOW]

It is always good to think about safety.  In my personal experience I have damage fingers and knuckles numerous times with a manual wrench when a joint suddenly frees or the wrench slips off of a nut or bolt.  And I have had no injuries with an impact wrench.  But the potential is there, and good practice will minimize the possibility of damage.  A sizeable number of my mishaps with manual wrenches were preventable with good practice and as I have slowly learned over the years they have become less frequent.  There is much good advice on this subject in prior responses.  I will add the obvious (that isn't apparently always obvious to me).  You know which way you are applying force, so you know which way things will turn when they let go.  Make sure that your fingers are not between the wrench and any solid object, or the massive force that either you or the torque wrench are applying will soon be smashing the wrench handle into your fingers.

Torque wrenches come in a wide range of powers, from the ones sold in inexpensive kits in auto parts stores and other venues which can barely turn normally torqued and non-corroded small bolts up to behemoths that can break the strongest bolts made.  All in the same nominal size.  Be sure to understand each wrench you use, as well as the air supply pressure, and adjust your behavior appropriately.

Finally, for much assembly and at least some disassembly I have fallen in love with the battery electric impact drivers that drive the standard Apex hex drive accessories.  They act like a drill motor until resistance occurs and then apply rotary hammer shocks.  The shock and drive provide impressive turning power, but it is relatively easy to control max applied torque by letting of the trigger as soon as the hammer shocks are felt.   With some practice and touch you can get pretty consistent results.

[Nusa]

Impact tools aren't as dangerous as grinders, but beware of shrapnel when bits break. Even impact-rated bits eventually break. Eye protection and keep your head out of the plane of rotation.

[Benta]

Impact wrenches are only useful for loosening bolts/nuts, nothing else. This applies to wheel lugs or to car butchering in a wrecking yard.

If a bolt/nut in, eg, an exhaust coupling is so rusted in that it won't budge at all, I prefer cutting it off with an angle grinder (if the overnight penetration oil didn't work).

Impact wrenches have no business tightening things.

[Stray Electron]

Impact wrenches have no business tightening things.

  I wish that you could convince the tire stores in the US of that! Those seem to be the only kind of "wrench" that the monkeys there own.

[tautech]

Impact wrenches have no business tightening things.

Strongly disagree.
In the hands of an experienced user there is nothing wrong with them and for the inexperienced torque limiting must be used and maybe a pressure regulator too.
Much production assembly has been done for decades with properly calibrated air tools.

Brought my first a pistol grip 3/8" dive rattle gun nearly 45 years ago when on small motor repairs however a butterfly type may have been better I needed it for auto repairs at home also. 

[james_s]

Much production assembly has been done for decades with properly calibrated air tools.

Which is exactly not what most people have. It's one of those things that an experienced person with proper tools that are properly set up can do safely, but it's something I would never recommend to anyone who is asking basic beginner questions about using an impact wrench. Unless you are an expert and know what you are doing, never use an impact wrench for tightening.

[jpanhalt]

Here's a discussion of pulse tools v. impact tools: https://blog.mountztorque.com/power-assembly-tools/impact-drivers-vs-pulse-tools-how-to-improve-process-efficiency/  One also has to consider tools with slip clutches (air or electric).  However, the subject of this thread seemed to be the proper use of a relatively uncontrolled impact wrench.

I am on the side that when assembling, one needs torque control.  When disassembling, an impact wrench can be helpful, but is not always advised.  As an example of that, take loosening a bleeder valve on car brakes.  They are often rusted and stuck.  The valve is usually soft steel.  Applying brute force can easily break them.  Applying force with a tapping motion (not a 1/2" impact wrench) will work better,  A piece of hardwood or small mallet is a great tapping tool.

As for assembly, slip clutches and pulsing are often quite useful.  Take building a steel shed (frame and covering both steel).  Hundreds, even thousands of self tapping sheet metal screws are used.  Those are best installed using a pulse tool with torque and depth limiting.   It is probably a rare building that is put up professionally without such a tool.

[BrokenYugo]

Don't forget the ear protection, air tools are that loud.

I'm also firmly in the hand assemble everything camp. The pros (barely) get away with it, but it's fairly easy to make dangerous mistakes if you don't know how things are supposed to look and feel going together, nevermind with a impact gun forcing things with little feedback to the operator. Know the torque specs and at least try to approximate them (this comes with experience, super cheap import torque wrenches are a good learning aid).

I have personally seen an impact tightened wheel come loose, and that wasn't done by an amateur, it was done by a seasoned licensed mechanic on his personal vehicle.

[mzzj]

I use an impact wrench frequently, they're great for loosening up stuck fasteners, just don't use them to tighten stuff. If you have to use a backing wrench and have to hold it without wedging it against something, make sure you hold it such that it cannot pinch a part of your body if the wrench spins.

Normal 1/2" impacts are quite harmless even if you attempt to hold the bolt with wrench as long as you keep a firm grip.
Mass of the wrench and your hand absorbs the impact and average torque is really small (just like the impact gun doesn't spin in your hands)
Probably would exceed some OSHA vibration limit but if you don't keep doing that for 8 hours per day its fine.

Maybe not good idea with some monstrous 1½" drive impact guns but those never leave you empty-handed anyways..

[james_s]

I use an impact wrench frequently, they're great for loosening up stuck fasteners, just don't use them to tighten stuff. If you have to use a backing wrench and have to hold it without wedging it against something, make sure you hold it such that it cannot pinch a part of your body if the wrench spins.

Normal 1/2" impacts are quite harmless even if you attempt to hold the bolt with wrench as long as you keep a firm grip.
Mass of the wrench and your hand absorbs the impact and average torque is really small (just like the impact gun doesn't spin in your hands)
Probably would exceed some OSHA vibration limit but if you don't keep doing that for 8 hours per day its fine.

Maybe not good idea with some monstrous 1½" drive impact guns but those never leave you empty-handed anyways..

I was referring to the backing wrench spinning, I know from experience that this can have painful consequences even with a 1/2" impact wrench if it's a good one and the nut is rusted onto the bolt.