At 10:00 AM 10/18/2005 -0400, you wrote:
At 09:13 PM 10/15/2005, Alan Brooks wrote:
...Coning lowers the risk of a sharp edge that will cause a stress
concentration on the shaft (it's a 45* edge instead of a 90* edge with
the risk of a burr). Filling the coning with epoxy will likely
strengthen the tip very, very little - it's too soft compared to the
graphite and steel (the same is true for a plastic ferrule) but it may
help 'grade' the stress concentration from the hosel edge somewhat, and I
can't imagine a material that you could pack into the tip that would have
enough strength to effect the tip strength at all.
I have to disagree with most of this.
(1) Disagree: 45* edge instead of 90*. Most coning is with a 20* bit or
burr. The angle is much "softer" than 45*.
Thank you, I stand corrected.
(2) Agree: The risk of a burr is the major problem with 90*.
(3) Disagree: 20* angle reduces the stress concentration compared with a
90* angle. It doesn't, or not so the shaft would notice anyway. Until the
shaft bent enough to touch the 20* side, it makes no difference whether
it's 20* or 90*. And by then, the shaft would be broken anyway.
(4) Agree: Filling the coning with epoxy will grade the stress concentration.
(5) Disagree: Filling the coning with epoxy will likely strengthen the tip
very, very little. Actually, I'm pretty sure it is the major value of
coning. A couple of ways to look at this:
If it's that important why don't they do a better job of radiusing the
edge? In my mind, this argues that it isn't significant, except for
minimizing the risk of a burr and that it's a 'reasonable' thing to do -
that won't hurt.
- The coning is a much lower-angle gradient than the 45* you are
assuming. True, epoxy is much more compressible than steel. But there is
very little of it near the edge. It gets thicker as you move away from
the edge, and thus the pad becomes "softer". But this is exactly what
"grading" is about. It reduces the stress concentration at the point, by
gradually changing the compression on the shaft from that of steel below
the cone to epoxy maybe 1/16" above the cone. Instead of a sudden
transition from cone to air (or even epoxy, which is almost
indistinguishable from air when you compare it to steel), the transition
occurs over a small fraction of an inch. MUCH less stress concentration.
- The compressibility of the shaft wall itself is much closer to
the epoxy fill than it is to the steel, so the compression grading is
rather well matched to the shaft.
We certainly agree on what grading does. The disagreement seems to come on
how MUCH good it does.
Filling the shaft tip with epoxy would likely strengthen it, not weaken it.
I've always been confused by that as well. But there's not much doubt that
the empirical evidence from repair shops says that an epoxy-filled shaft
is encouraged to break.
Does anyone examine the tips of shafts that don't break to see how much
epoxy has been squeezed up into them and that the presence of epoxy in
shafts that fail is not unique? Is it possible that a lot of epoxy up the
bore of the shaft reflects poor workmanship and there are other reasons
that the shaft fails due to that? I can think of no reason that would
explain epoxy in the bore of the shaft increasing the risk of failure. I
haven't seen that many failed shafts. Is there a common failure mode? Is
it tensile? Torsional shear? Inner fiber buckling that precipitates
failure? Does this failure mode reflect a stress concentration at the
bottom of the taper in the hosel? Does it appear to be related in some way
to epoxy in the bore of the shaft?
Have we got anybody in the composite shaft design business, or even the
composite design business lurking here? I haven't been involved with
composite design projects for awhile, but when I was failure modeling at
that scale (millimeter, or the scale of the size of the tapes) was pretty
much still an art form.
DaveT
Thanks for the comments, Dave, always a pleasure.
Alan Brooks
