This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
> My cranks get loose, quite quickly too; over about 10 miles or so
> from being solid to flopping about in the breeze. Any suggestions?
Your cranks are ruined! Once ridden in the "floppy" mode, the square
taper in the crank can no longer be secured on the spindle. Get some
new cranks and properly tighten them after lubricating the tapers.
Proper tightness can be guaranteed only by torque wrench or a skilled
mechanic. The second of these is less expensive and you might be able
to get a demonstration of what is tight enough.
The admonition to not lubricate the tapers of the crank spindle seems
to find life only on bicycle cranks, of all the machines I have seen.
I have pursued the "dry assembly" instruction by talking to crank
manufacturers and discovered that they apparently had warranty claims
from customers who split their cranks open. It is easy to prove that
cranks cannot split by over-tightening simply by attempting to do so.
It is not possible to split a major brand crank this way, the bolt
will fail first.
Crank failure from "over-tightening" is caused by the re-tightening of
previously properly installed cranks. Once installed, a crank always
squirms on its taper, and because the retaining bolt prevents it from
coming off, it elbows itself away from the bolt and up the taper ever
so slightly. This can be detected by the looseness of the retaining
bolt after the bicycle has been ridden hard.
Grease in this interface does not affect performance, because only the
press fit, not friction, transmits load from crank to spindle. As any
bicycle mechanic can tell you, crank bolts are often appreciably
looser after use, the left one more so than the right. This occurs
because the left crank transmits torque and bending simultaneously
while the right crank transmits these forces one at a time. The right
crank puts no significant torque into the spindle. Either way, the
looseness occurs because loads make the crank squirm on the spindle
and the only direction it can move is up the taper, the retaining bolt
blocking motion in the other direction.
Regardless, whether grease or no grease is used, in use the spindle
and crank will make metal to metal contact and cause fretting
corrosion for all but the lightest riders. The purpose of the
lubricant is to give a predictable press fit for a known torque. If
the spindle is completely dry this cannot be said, and even with
marginal lubrication, some galling may occur on installation.
Lubrication is only used to guarantee a proper press because the
lubricant is displaced from the interface in use. Taper faces of
spindles show erosion and rouge after substantial use, evidence that
the lubricant was displaced.
"Dust caps" aren't just dust caps but retention for loose bolts. It
is not that the bolt unscrews but that the crank moves up the taper.
However, once the screw is unloaded it can subsequently unscrew and
fall out if there is no cap.
Because cranks squirm farther up the taper when stressed highly, the
unwitting mechanic believes the screw got loose, rather than that the
crank got tighter. By pursuing the crank with its every move up the
spindle, ultimately the crank will split. It is this splitting that
has been incorrectly diagnosed as being caused by lubrication. I have
never seen a warning against re-tightening cranks after having been
installed with a proper press fit. It is here where the warning
belongs, not with lubrication.
For the press fit to work properly, the pressure must be great enough
to prevent elastic separation between the crank and spindle under
torque, bending, and shear loads. This means that no gap between
crank and spindle should open when pedaling forcefully. Friction
has no effect on the transmission of torque because the crank creeps
into a position of equilibrium on the spindle in a few hard strokes.
Failure of this interface occurs when the press fit is too loose
allowing a gap open between spindle and crank. Torque is transmitted
by the entire face of the press fit, both the leading edge whose
contact pressure increases and the trailing edge whose contact
pressure decreases. If lift-off occurs, the entire force bears only
on the leading edge and plastic failure ensues (loose crank syndrome).
Tightening the retaining screw afterward cannot re-establish a square
hole in the crank because the retaining screw will break before the
spindle can exert sufficient stress to reshape the bore. Beyond that,
the crank would split before any plastic deformation could occur even
if the screw were sufficiently strong.
Because retaining screws could become entirely lose from squirming
action, especially if the press is relatively light, "dust caps"
should be used to prevent screws from subsequently unscrewing and
causing crank bore failure. Besides, the loss of the screw won't be
noticed until the crank comes off, long after the screw fell out.
The argument that the greased spindle will enlarge the hole of the
crank and ultimately reduce chainwheel clearance is also specious,
because the crank does not operate in the plastic stress level. At
the elastic limit it would break at the attachment knuckle in a short
time from metal fatigue, that occurs rapidly at the yield stress. In
fact, the depth of engagement (hole enlargement) can increase with an
unlubricated fit faster than with a lubricated one, because
installation friction is the only mechanism that reams the hole.
Jobst Brandt <firstname.lastname@example.org>