Subject: 8f.11   Installing Cranks
From:  Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Tue, 2 May 2006 10:25:45 -0700

> My cranks get loose, quite quickly too; over about 10 miles or so 
> from being solid to flopping about in the breeze.  Any suggestions? 

One or both of the cranks are ruined!  Once ridden in the "floppy"
mode, the tapered square bore of the crank has been deformed and can
no longer be secured on a spindle.  Install and properly tighten new
cranks on the spindle after lubricating the facets.  Proper tightness
should be achieved with a torque wrench or by a skilled hand.

The admonition to not lubricate the facets of the spindle finds life
mainly on bicycles.  Having discussed the "dry assembly" rule with
crank manufacturers, I discovered that they had warranty claims from
customers who split cranks.  However, that cranks cannot be split by
over-tightening can be shown by attempting to do so.  An M8x1 screw is
not strong enough to split a major brand crank.

Failure from "over-tightening" is caused by repeated re-tightening of
properly installed cranks.  In use, an aluminum crank squirms on its
taper and, because the retaining bolt prevents it from moving off the
taper, it elbows itself away from the bolt and up the taper ever so
slightly.  The resulting loss of preload, after hard riding, can be
detected by how easily the bolt can be turned.

Loss of crank bolt preload is greater on left than the right cranks,
because left cranks transmit torque and bending simultaneously while
right cranks transmit these forces separately.  The left crank
transmits driving torque through the spindle to the right crank and
chainwheel while the right crank drives the chainwheel directly.
Besides that, the right crank transmits torque to the spindle only
when standing on both pedals.  With the right foot forward (goofy
footed) this causes the only reverse torque on the spindle.

Mechanics, unaware of why crank bolts lose preload (and commensurate
crank tightening), have re-tightened bolts until cranks split.  No
warnings against re-tightening properly installed cranks are evident
although it is here where the warning should be directed rather than
at lubrication.
 
Because friction plays no role in torque transmission, preload in the
press fit must be great enough to prevent elastic separation between
the crank and spindle under torque and bending.  This means that no
gap should open between crank and spindle facets under forceful
pedaling.  Crank bore failure occurs when the press fit is loose
enough that a gap opens between spindle and crank.  Torque is
transmitted by both leading and trailing half of each facet, contact
pressure increasing and decreasing respectively.  In the event of
lift-off, the entire force bears only on the leading edge of facets
and causes plastic deformation, causing the bore takes on a "pin
cushion" shape (loose crank syndrome).  Subsequent tightening of the
retaining screw cannot correct this because neither the retaining bolt
nor crank are strong enough to re-establish the square bore.

The claim that a greased spindle will enlarge the bore of a crank and
ultimately reduce chainwheel clearance is also specious, because the
crank cannot operate in a plastic stress level that would soon split
the crank in use.  However, increased engagement depth (hole
enlargement) may occur without lubricant, because installation
friction could ream the hole.
 
With or without lubricant, in use, cranks will make metal-to-metal
contact with the spindle, causing fretting erosion of the steel
spindle for all but the lightest riders.  Lubricating the spindle for
assembly assures a predictable press fit for a given torque.  Without
lubrication the press is unknown and galling (aluminum transfer to the
steel spindle) may occur during assembly.  After substantial use,
spindle facets may show rouge and erosion from aluminum oxide from the
crank, showing that lubricant was displaced.
 
Crank "dust caps" have the additional duty to retain loose crank
bolts.  Because crank bolts lose preload in use, they can become loose
enough to subsequently unscrew and fall out if there is no cap.  If
this occurs, loss of the screw will not be noticed until the crank
comes off, after the screw is gone.