Impact driver usage (+ impact wrench q's)


wondering if I should get an impact driver. I keep looking at the Sears driver, looks pretty good.

so this is the hand-held tool that is hit on the end with various masses of hammers.

is this only useful for drive types where the driver sits inside the fastener? like phillips head screws, hex bolts (“allen” type)? I figure socket drives are absolutely not for use with an impact driver. My understanding is that there is an amplification of the force that is converted both into in-line force and rotational force against the fastener walls. I figure also that with enough blows on a stuck fastener, the inside of the fastener can very well strip, and then would need perhaps drilling out.

is there an air-tool version?

UPDATE 1: see below for question about how fast the tool works
UPDATE 2: the thread derailed into impact wrenches and air compressors. the impact wrench discussion is helpful. however, I didn’t want to encourage launching yet another “air compressor and impact wrench” discussion because there is already tons of that to read on the internet. I think we can have a better discussion on this forum at some point - but again, I’m not encouraging bloating up this thread for it.


I have one of these but use it very little. It is good for loosening stuck Japanese motorcycle engine case Phillips screws. As you said, you hit it with a hammer and two things happen: The tool bit is forced more securely into the Phillips head screw and the tool bit is turned by an internal taper. The heavy body of the tool resists turning due to its inertia but does turn backwards a little. The tool can be set for right or left hand threads.

Once the screw is loose, don’t try again as you said or the threads in the aluminum engine case might be stripped.

A much better tool for everything else is an inexpensive 1/2" drive air impact wrench or an electric one if you can find it. For home use, the quality is quite adequate.


this tool is a manual version of an impact wrench/tool. you hit it, and it turns slightly. works well on philips and flat heads that are rusted. worth a try as you want to preserve the fastener and not strip the contact points

but often times, you can go up and just use an air impact wrench instead. both have their own needs and uses.


oh another thing I forgot to mention was that I think another aspect of the impact driver is the high rate at which the rotational force gets transferred to the hardware - that is, a breaker bar is for applying slow rotational force, but the impact driver is really also about applying the force as fast as possible.

how does this rate of force application work compared to a breaker bar? does it avoid the problem of stripping fasteners?

… also what should I do here, if I want to loosen tough fasteners with penentrating oil - buy a manual impact driver and a good hammer, or get a good impact wrench (if I understand) to use with the 12 gallon air compressor I already have - or one of those options with a torch (but not oil!) - and keep under budget?

and there’s another job I have been reading about - front strut mounts - where (skipping the details) one of the nuts requires spinning so fast that an air-powered tool - presumably and impact wrench - is required. this is related to my question of how fast the tool spins. how does that work compared to spinning slowly?


The impact drivers are very useful If properly used. And they can loosen frozen bolts using impact sockets.


I have an impact driver but seldom use it. I found a 1/2 inch Chicago Electric impact wrench (electric) at Harbor Freight for about $50. I expected it to break years ago but so far it still functions. I used it to drive about 50 or so 6 inch lag screws when I built my back deck. I’ve also used the impact wrench to disassemble a few parts engines. A can of PB Blaster and an impact wrench with the proper impact socket has never let me down when it comes to loosening rusted or stuck bolts.


OK it seems impact wrench VS. impact driver is a well-traveled topic on the internet. for the purposes of this thread, I would request some generous user to check my understanding as laid out in this writeup :

an impact wrench, for example on a hex bolt, functions as if there were six hammers pounding on the six faces of the hex head. however, there is no downward force. sockets must be rated specifically for impact wrench work - not the typical shiny ones. If a phillips bit is actually used with an impact wrench, the bit is likely to jump out of the drive slots, just as if a drill were used to do the work.

the impact driver does the above but also pushes the bit into the fastener. the effect is two-fold : (1), to keep the bit in the fastener exactly when rotational force is exerted, thus maximizing force delivery, and (2), loosening the interface between the threads of the fastener and the hole.

it appears that impact wrenches will not in practice work on slotted screws, and impact drivers will not in practice work on hex head bolts.

I think the only question I have remaining is, if a hex head bolt is stuck, is an impact wrench going to give an advantage over using a breaker bar? I can see an IW snapping the head off a bolt, while a breaker bar will round the hex head off.


Impact devices take advantage of the laws of physics, you know about inertial forces and f=m*a. The “impact” represents a rapid change of velocity. Changing the velocity of something quickly is the same as acceleration, and the more mass it has, and the faster the velocity is changed, the more inertial force will develop.

I have one of those impact gadgets (I bought it at Sears), the kind you hit with a hammer. And it has saved my DIY bacon a few times. I don’t use it very often, but when I do I’m very happy to have it on hand. And this tool isn’t very expensive, so it is definitely worth it to have on your team as a DIY’er.

A pro would probably be more inclined just to use an impact driver for this I’d guess. Plus sometimes clearance issues won’t allow you to whack something with a hammer in a tight space.

On my prior car, a 70’s VW Rabbit, to remove the brake disc there was a Phillips screw you had to remove. And that screw would get rusted solid to the disc, impossible to turn with a regular screwdriver without damaging the screw head. But whack it with the phillip’s equipped impact gadget and it would loosen the screw sure as the day is long. Worked every time, never damaged the screw head. The force of the whack places the tool surface secure against the screw head surface so there is no slipping and damaging of the screw head, and the impact of the hammer also, through the gearing, transforms the direction of the hammer impact so it turns the screw CCW with a very large, brief force.

I’ve never tried it with a socket, so don’t know about that.


@GeorgeSanJose - yes I follow - now what is the difference in forces at the hex head between an impact wrench (the air tool) and using a hammer on a combination wrench (given that it fits in the space and all that)?


I’d guess an professional air driven impact wrench can probably generate a quicker velocity change and therefore greater inertial force compared to whacking a combination wrench with a hammer. I’ve used the hammer-on-the-wrench technique though and it often has worked for me. When I’ve had problems, it is because the wrench slips due to the force of impact and damages the bolt head.



If you use an air impact wrench to try to remove that phillips head rotor hold down screw, you WILL bugger it up

Speaking from experience here


@db4690 I can see it.

this is making sense. so I have a related q about IW and AC’s - don’t really want to start a new thread, and I have already seen a lot of discussion all over the internet. but if I should start a new one, say so.

question is about matching the wrench to the compressor. I have a 3.7/2.4 scfm at 40/90 psi, how many scfm’s of a wrench above that can I get away with, how many is too much - e.g. how much of a performance hit will a 4.7 or 5.2 scfm wrench show with my compressor? these are Sears items but HF has some and I am not clear on how each manufacturer’s numbers translates to the others (yes of course 4.7 is 4.7, but the conditions of that measurement - not a hard number, I think).



In my opinion, you need a much bigger compressor . . . bigger tank and more scfm

2.4scfm at 90psi won’t get you very far

A bit of advice . . . do NOT get an oilless air compressor


@db4690 great - so this oil-based compressor I have is useless, but what’s worse is the higher models are oil-free…

so even a low-end impact wrench with that compressor, like … well, on this one 3-pc. kit, they say average 4.4 scfm, 300 fp torque whatever that means… it is simply not going to work?.. maybe I should just get an electric.


you want to get an oil-based compressor, but a fairly big one


Curious. Wouldn’t the problem with having too small of a compressor be that you’d have to wait a certain amount of time between uses? Wouldn’t work for a pro, but that might be ok for a DIY’er. Or is there a more fundamental problem with having too small of a compressor @dB4690?


@db4690 I edited it - what about going for an electric IW, I found one with 350 fp torque…


@GeorgeSanJose yes that’s what I’m wondering - of course if the wait isn’t too bad! I’d prefer to get a good wrench than a weak one… that’s the problem with getting the AC well before the IW.



A 1/2 cordless impact would serve you well, with a few conditions . . .

It had better have ball-busting torque . . . or it won’t do what needs to be done

this is the one I have. It’s older, but it’s got a pretty mean bite

It needs to have a quick charger . . . less than one hour

Lithium-ion batteries are the way to go nowadays


175 psi in a 60 gallon tank is needed to operate a good 1/2 inch impact properly. Such a compressor is 2 stage with multiple belts to drive it and such equipment is somewhat expensive and large. Even with that capacity removing truck wheels often requires waiting for the compressor to get caught up. Most direct drive compressors, oilless or oiled are for minimal DIYers,