From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: D/A spline cranks loosening
Date: 2 Jul 2001 02:28:50 GMT

Peter Chisholm writes:

> Read somewhere that spline cranks and BBs, that have force applied
> in both directions, like a track crank, can have frequent loosening
> w/o any solution....

You read that here when I introduced the subject.  If you heard it
elsewhere, I'm pleased that someone else recognized the problem.  I
brought this up a couple of years ago when I said I would not switch
to this type of crank until I was assured that there was a solution
for this problem that had not yet been reported.  Of course the flood
of responses insisted that there is no problem.

The problem was recognized in mechanical engineering many years ago.
That is why cams in automotive engines are not driven by gears.  Even
gears with preload have backlash under pulsating loads and make a
great racket.  This racket is the sound of gears being destroyed and
that's why we don't do that any more.  The Shimano spline may have no
backlash in the unloaded condition, but under load the gap opens
elastically and produces possibly as much as a degree of rotation,
which when repeated a hundred times can significantly back out a
retaining screw.

For MTB and road bicycles, the assumption that there is no reverse
torque due to a freewheel is grossly unrealistic.  I get the
impression that the engineering manager who approved this, neither
rides actively nor has a good test department.  Actually, I expect him
to have come upon this from machine design experience.  Bicycles are
my hobby, but it is his profession.

Jobst Brandt    <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: D/A spline cranks loosening
Date: 4 Jul 2001 17:55:00 GMT

John R Ellis writes:

>> The problem [dynamic backlash] was recognized in mechanical
>> engineering many years ago.  That is why cams in automotive engines
>> are not driven by gears.  Even gears with preload have backlash
>> under pulsating loads and make a great racket.  This racket is the
>> sound of gears being destroyed and that's why we don't do that any
>> more.  The Shimano spline may have no backlash in the unloaded
>> condition, but under load the gap opens elastically and produces
>> possibly as much as a degree of rotation, which when repeated a
>> hundred times can significantly back out a retaining screw.

> I can't say whether there are any currently in production, but some
> American V8 engines had gear-driven camshafts.  They seemed to last
> reasonably well in service at the time...and they were not preloaded
> to any significant degree.

Ford used them in their flat head V8's and they were made of a
Bakelite, a material that looked much like PC-board material without
the glass fill.  This was don to prevent the hammering noise that
metal gears that hot rodders used to avoid stripping the Bakelite gear
that easily lost teeth.  I recall our car needing a new cam gear on
two occasions.  They had backlash but with 16 valves to drive there
was little tendency for reverse torque, friction being high enough to
prevent oscillation except at idling speeds.

Other cars used timing chains and as a youth I often wondered why they
went to the trouble.  Later I found out.  Some racing motorcycles and
cars continued to use steel gears because chains also presented
problems and noise was not important, they thought, until computer
analyses and high speed photography revealed that valve float and
chatter was caused by the gear tooth impacts (noise).  Timing belts
also came to the rescue.

Jobst Brandt    <jbrandt@hpl.hp.com>