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I think there must be something I don't understand about impact wrenches. After snapping a half-inch breaker bar (with a cheater) while trying to remove the axle nut, I decided that an impact wrench might be a good investment. Between positive reviews, a $99.99 price, and a 20% off coupon the Harbor Freight Earthquake seemed like a good bet.

Hooked up the wrench, set the compressor to 90 psi, grabbed a 30 mm impact socket and went after the axle nut. Sure enough, it came off. Good. Switched to a 17 mm socket and started in on the wheel lugs. It wouldn't touch them, not even close. The wheel lugs are torqued to 120 NM (90 ft/lbs) and came free easily with a 18" breaker bar (a 25" bar wouldn't touch the axle nut). I let the compressor come up to full pressure, I can't think of anything different between the two fasteners except for the size (and that one was initially much tighter).

Can anybody explain this? Is there a lesson about impact wrenches in this? I'm stumped.

dlu
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2 Answers2

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Would I be right an assuming your car is equipped with alloy wheels? Also, have the wheel bolts been fitted with a small smear of grease?

It sounds like the wheel bolts are reacting in a "springy" way to the forces of the impact wrench. It may be that they're butted up tightly against the softer metal of an alloy wheel or they're in the threads with a small amount of grease which is counteracting the effects of the wrench. Bear in mind that the hammer action of the wrench is applying a very large amount of torque to the bolt of a very small time, almost as if it's trying to "shake" the bolt loose. This works very well on large, rusted on bolts but not at all well for bolts which simply need a small but continually applied amount of torque to remove them.

Taken from the Wiki article on impact wrenches;

A wrench that is capable of freeing a rusted nut on a very large bolt may be incapable of turning a small screw mounted on a spring.

I personally only break out the impact wrench when I come to a nut / bolt that simply will not un-do. Otherwise I'll have a ratchet or spanner in my hand.

dlu
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Steve Matthews
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Each of the impacts is like a mini lever having a go at the fastener.

If there is a longer lever, ie larger socket, there will be more force applied for the same torque applied.

So, in some cases, it’s going to be easier to undo larger fasteners than smaller fasteners with the same impact gun.

Gary Goddard
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