WHAT A BOLT IS, AND HOW IT WORKS
- From: "TeGGeR®" <tegger@xxxxxxxxxx>
- Date: 4 Nov 2005 14:33:12 GMT
WHAT A BOLT IS, AND HOW IT WORKS
--------------------------------
1) A bolt's sole function on earth is to serve as a clamp. Its job is to
keep two or more things from moving relative to each other. That's it.
Nothing else.
2) A bolt works /only/ because one thing happens: Its shaft rides up an
incline formed by helical threads, and the shaft thereby gets "axially
loaded" (stretched) in the process.
3) The one-and-only purpose of helical threads is to impart that "axial
loading" (stretching). Axial loading IS what gives the torque (clamping
pressure) necessary to hold the bolt's clamped parts in place, and the only
thing that keeps it from eventually backing off again. And the only way the
bolt can ride up the threads is if it rotates.
To sum up: Rotation + stretch = torque.
If no incline, then no rotation, no stretching, and therefore no torque.
The sentences above are as fundamental to the concept of a bolt (or screw)
as air is to life on earth. It is why the idea of a bolt can exist in the
first place. If the sentences above are /not/ true, then the very concept
of a bolt cannot exist.
If, as some have contended, the bolt stretches/distorts with heat but does
NOT rotate, then it has not ridden up the incline, cannot impart additional
stretch to itself, and thus cannot apply additional torque. In order to
permanently stretch, it would have to skip threads, and jump up to the next
rotation of the helix. I think we all would agree this does not happen.
If the bolt DOES rotate, then this should be readily apparent by placing a
paint mark that crosses the bolt head and the pulley, or the pulley and the
crankshaft. I GUARANTEE to you that bolt will not have rotated, and neither
will the pulley.
If the bolt has somehow rotated only on its shank, and not at its head,
then you have torsional loading (twisting), which is something that is
death to fasteners and is never allowed to happen, because a bolt that is
allowed to twist will eventually snap.
(In some rare, low stress cases, such as a bicycle brake, the bolt can be
constructed so as to allow a small degree of bending. Also, some bolts can
be specially constructed to allow shear forces, such as in a movable clevis
joint. A clevis joint is loose by design though, so not applicable to this
discussion.)
A bolt, especially one used in a high-stress automotive application, cannot
be allowed to deal with anything more than stretch. It cannot undergo
torsion, shear, or bending. The parts it is clamping are supposed to deal
with that load. It is critical, imperative, fundamental to the function of
any bolt, that it clamp with enough force to prevent its clamped parts from
moving relative to each other. If the clamped parts should start moving
relative to each other or to the bolt, the joint has failed, and soon the
bolt will also. Or it will come out.
In the case of a Honda crankshaft pulley, the pulley itself is located by a
Woodruff key. This key resists most of the torsional forces imparted by the
crankshaft and the engine's accessories. The rest of the resistance is
donated by the crankshaft pulley bolt. If the crank pulley bolt is not
tight to the point where no relative movement is possible, the Woodruff key
will get hammered flat from constant shock loading, and/or the bolt will
eventually come out. The bolt will not work as a clamp if it is
insufficiently tightened, even if it "looks" like it's tight enough.
So why are crankshaft pulley bolts so hard to remove when they've been
tightened properly?
1) Corrosion around the perimeter of the bolt head and washer. When a
clutch disc seizes to its flywheel, it's not held by very much pressure,
but it is held enough that you won't be able to free it without some
effort. Same thing happens with that ring of rust. It's not holding by
much, but getting the seal to break takes effort. The more rust, the more
effort needed.
2) Breakdown of the friction stabilizer coatings on the bolt. High-stress
bolts are coated with materials ranging from cadmium to Teflon. These
coatings make actual friction more predictable, so the engineers have
better control over actual bolt stretch in the real world. When these
coatings break down, as they can with time and heat, friction will tend to
increase.
3) "Embedment", which is when the thread mating surfaces deform with time
and vibration, and mesh more closely together at a microscopic level.
Embedment, believe it or not, actually results in BOTH a slight /decrease/
in actual bolt tension as well as an /increase/ in removal torque.
Much of the information above comes from the Web site
www.boltscience.com , which is run by Bolt Science Limited, a consulting
firm in Great Britain.
The rest comes from a series of emails between me and an individual at Bolt
Science Ltd., and some conversations with my mechanic, who has owned his
own shop since the early '80s, and has some thirty years of experience as a
licensed mechanic specializing in Japanese cars.
Finally, there is no such thing as a "one way" thread in a bolt that is
intended to be removed and reinstalled.
.
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