impact wrenches and air compressors

how do the spec-s of an air impact wrench relate to the air
compressor?
i need to loosen a crankshaft pulley bolt that is torqued at 134 ft-
lb. on a 1998 honda civic LX. i have a sears craftsman 1.5hp 4-
gallon [email protected]@90psi air compressor. i don't yet
have an air impact wrench. what should i look for in terms of psi
when i go to buy one, such that i can use that air compressor to drive
the impact wrench to loosen the bolt? tia.

 

I wouldn't buy an impact wrench just to break that bolt loose. 134 lb-ft
is not that big of a deal for a 1/2" drive breaker bar.

A little compressor probably won't drive a cheap impact wrench. I have a
Porter Cable 135 psi, 6 gal, 2.6 SCFM@90 psi compressor, and a Husky 260
ft-lbs 1/2" impact wrench. With 25 feet of 3/8" air hose, it won't break
lug nuts loose. The manual says it needs 1/2" hose, and 5.1 SCFM@90PSI for
intermittent usage.

 

by the time you have corrosion and/or the self-tightening that hondas
are, er, "known" for on this bolt, you'll need a lot more than 134
ft.lbs to get that thing off.

check out removal hints at tegger.com

if you're lucky, a decent impact wrench may undo that bolt, but with
that tiny capacity, you'll only get a second or two of full torque
before you have to wait on the tank refilling. in my experience, you'll
need to look at 300ft.lbs, maybe more, to guarantee removal success.

 

The bolt is neither corroded nor "self-tightened". It's simple embedment
that makes it hard to get off again.

See this Honda document:
http://www.tegger.com/hondafaq/cranktool/A930200.pdf

 

so what's the mechanism for "embedment" then? and why doesn't it happen
on cranks that rotate in the opposite direction?

 

It's simply the surface textures of the threads settling and "meshing"
together.

It certainly does.

Since our original thread on this subject, I have talked to a few engineers
who work for auto industry OEM suppliers. All of them tell me bolts never
"self-tighten". If movement between the parts is present, fasteners will
loosen, not tighten.

 

so they move relative to one another then.

well, i've been around the block on this stuff too. and if there is
relative movement, fasteners /do/ usually loosen. however, that is not
always the case. the fact that some /do/ tighten is exploited in
bicycles in no less than two separate instances.

1. pedal axles.

2. the bottom bracket bearing cups.

in both cases, left hand threads are used on the left pedal and the
right hand bb bearing cup to take advantage of what's called
"precession" and keep things tight. right hand threads in those
applications always loosen. and you can prove it too. tandems usually
use left hand drive on the sync chain between the two cranks. if you
try redeploying one of those cranks in a right hand drive, you'll
discover this effect for yourself in just a few minutes riding when both
pedals fall off. [legend has it that the wright brothers solved this
problem - r/h thread on the r/h pedal, l/h on the left.]

getting back to car applications, and the honda crank bolt, we have
three facts:

1. there is indeed relative movement in this case - i've posted picture
evidence of that before.

2. unlike other cars, the momentum impulse of the honda crankshaft
rotation is to tighten, not loosen - crank rotation is counter clockwise.

3. unlike other vehicles that either loosen in service or stay the same,
this /one/ honda bolt is, without exception, an unholy mfsob to get off.

if you lay [incorrect] aside dogma about "but it can't tighten", those
three facts only point in one direction.

and there's more: this "tightening" effect is there within moments too.
my crx, doing the timing belt, i tightened, drove around the block,
then went back in. literally, within a mile, that bolt, carefully
tightened to 165Nm, took my full body weight at the end of a 24" breaker
bar to loosen again, just like before. that's not corrosion. that's
not "embedment".

bottom line, the fact that there's a rotation impulse is recognized by
honda. in my version of the d-series engine, there's just a single
woodruff key for the crank pulley. in later versions of that engine,
there's a key, but the crank/pulley are also splined, and the fit is a
much closer tolerance meaning much lower rotational lash. if that's
/not/ to control rotation and therefore bolt tightening, it's one heck
of an expensive finishing operation just to hold a crank pulley in place!

 

Super fine threads with thermal and high load cycling.

The pitch on the 91 Civic's 14 mm pulley bolt is 1.25 mm.
The normal fine thread pitch for a 14 mm bolt is 1.5 mm. I
think we all know from experience that, the finer the
thread, the more difficult a bolt may be to loosen. This is
the purpose of fine threads, after all. More surface area;
more adhesion; greater chance for "stickiness" in general.

But I do not reject the hypothesis that the pulley bolt and
the shaft into which it inserts also tend to move relative
to each other during driving, and so the bolt tightens.

 

Embedment is an axial-motion phenomenon (in and out), not a rotational one.

There is a fundamental difference between axial movement and rotational
movement.

 

Just to elaborate a bit for everyone's benefit...

Ever seen a metal surface under a microscope? It's a smeared
topographical jumble of mountain peaks and ridges, canyons, plateaus,
pits, craters, cliffs, furrows, canals, and other mess. Even a mirror-
finished surface is ANYTHING but smooth. Screw threads -- whether rolled
or cut -- are no different.

Now, take two of those surfaces and put them together, as you would end
up doing if you screwed a bolt into a threaded hole. Those topographical
features thus get to meet up head-to-head. Some of those features will
sit on top of each other (peak to plateau), others will mesh (ridge in
canyon). Still others will be just on the verge of meshing, just hanging
on the edge.

Remember, the bolt is under tension, so these uneven surfaces are being
squeezed together with some force.

With heat-cycling, vibration and flex over time and use, the "just on
the verge" features wiggle into mesh. The already-meshed features wiggle
into more positive mesh, often displacing metal to do so. And the next
time you try and remove that bolt, it takes a lot more force to remove
it than it did to install it.

The movement necessary to effect embedment is on the microscopic level.
It is so tiny that it does not even materially affect the axial tension
to which the bolt is subject. Plus there is no rotational movement
involved.

The concept is that simple; it's well-known and understood by engineers.
Bolts do not tighten themselves.

 

http://en.wikipedia.org/wiki/Embedment conflicts with your
post.

 

Different concepts at work here. Do not confuse them.

 

i respect your sincerity tegger, but i think you've had a bunch of smoke
blown up your skirt by non-experts on this one.

bolts that loosen in service are a real problem that gets big-time
attention. and that bolts that tighten don't because they're generally
not failure items. in addition to that, i also know that there are a
whole bunch of "engineers" out there that can't tell you why there are
l/h threads on the left side wheel studs of a big rig.

getting back to the problem, we know this is a rotational issue simply
from observing the bolt interfaces on disassembly.

http://www.flickr.com/photos/38636024@N00/sets/72157607709293562/

here are two junkyard honda crank bolts. one had been loctited - the 92
- [don't know why, it's not in the spec, but the evidence is clear], the
91 not. the loctited one shows no evidence of rotational damage, the 91
shows extensive rotational damage.

having looked at these, and if you have yourself experienced a bolt
suddenly going crazy tight in the course of a couple of minutes
operation, you can't just dismiss the evidence.

 

quite.

 

but we have a rotational loading [momentum] problem here, not an axial
loading problem.

indeed. see above.

 

you were fed the wrong word dude.

 

You are misusing the term "embedment."