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Pneumatic impact wrench usage

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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

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.

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.

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.


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