Torque wrench or impact gun on large bolts

Question

For very large bolts (example: axle nut) I often see mechanics just use an impact wrench, typically they let it "rattle" a certain time and then they consider the bolt tightened. Is this also common/considered acceptable practice in other shops/locations? For smaller bolts or applications like head bolts, alloy rims and other delicate stuff I am 100% sure that you need a torque wrench and taking shortcuts leads to disasters, but on real massive bolts I am inclined to consider the mentioned practice to be alright (within certain limits).

Why I am asking this question:

The axle nut on my Astra H demands 250NM, my larger torque wrench has a range up to 200NM, so I cannot use it for this job. I do not want to buy a wrench that goes slightly over 250NM, because then I would have bought two wrenches with mostly the same range. An expensive alternative would be a 3/4" wrench, so I would be prepared for future tasks, but it seems to be overkill, also financially.

I am quite reluctant to spend a lot of money on tools just for the axle nut, also I do not think that I will be using it often. The axle is a M22 fine thread, the nut is self locking, the bearing is in an integrated unit and already pre-loaded. Should I buy a matching torque wrench or just bash the nut on with the impact wrench ?

Answer 1

To answer your question directly, this is a "standard practice" but a VERY poor one. Getting the correct torque is by far more important than getting it done quickly.

With the axle nut it is imperative you tighten it to the correct spec. If you tighten it too tight, the wheel bearing will self destruct within a few hundred miles. Using an impact gun can surely over tighten it. You'll not know what torque it really has gone to. I tightened up the axle nut on my 06 Silverado with an impact gun, then went back to torque it to spec, only to figure out I had over tightened it. I had to go back, loosen, then torque it. Like I said, an impact can easily go over your torque threshold. If left unchecked, can cause catastrophic damage. It is well worth your time to get the tool or borrow one which will allow you to get where you want to be.

Answer 2

Neither.

Just get a 1m long bar (by extending a breaker or similar), stick it on the nut so that it is horizontal and simply hang 250N of mass at the end of it: (*)

250N = 56.2 lbs = 25.5 Kg

250N * 1m = 250Nm

Rinse and repeat until it hangs and doesn't tighten the nut anymore. The trick is to keep the bar horizontal, you'll likely need a 12-point socket to achieve this.

Doesn't need to be 1m, just do the math for whatever length you've got there:

Kg = 25.5/length

lbs = 56.2/length

Water in a bucket could work well for mass.

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^{(*) this ignores the integrated mass of the breaker bar itself, which will mean that there will be slightly more torque than calculated. - this effect will be 'worse' the longer the bar is. But then we're also not accounting for static vs. dynamic friction anyway, which isn't usually accounted for with standard torque wrenches... :)}

Answer 3

I've worked on axle nuts holding tapered roller bearings together where the correct torque was way lower than a windy gun (common UK name for an air powered impact driver) would produce... using an air or electric driver would take the torque up to the point where you'd risk putting flats onto the rollers. Axle nut torques can be pretty low which is why you see staked nuts or castellated nuts with pins to stop them adjusting in use. So using a high torque tool to do up such nuts or bolts to some random high torque is a recipe for future failures. Of course, it's not the mechanic who will have to pay for a replacement bearing in a few hundred miles...

My personal peeve is tyre places that use windy guns to do up the wheel bolts (lug nuts...) to ridiculously high torques... add a bit of corrosion and you end up with bolts you can't undo with typical roadside tools if you get a flat.