[Public-List] Chain plate spacers - role of friction
dfjtessier at hotmail.com
Mon Mar 23 18:53:49 PDT 2009
Thanks all for the advice.
I think that friction likely has a role to play. This is considered
explicitly in descriptions of the pre-load which one applies when
bolting two metal plates together (and the resulting shear strength of
the resulting assembly).
Fibreglassed knees, and especially 40 year old ones, will
be more compressible than the steel chainplates. I think this
indicates the need for backing plates, and SUBSTANTIAL ones,
rather than washers. How SUBSTANTIAL? I suspect that aside
from having an area equal to that of the chainplates, the backing plate's
thickness should be chosen to be sufficiently strong so as to remain
essentially planar as the bolts are tightened, thereby transferring
the above bolt pre-load force over a wide area of the knee. Less pressure
(i.e., force per unit area) but a more sure friction set owing to the
relative deformable nature of the fibreglassed knee.
In terms of dimensions rather than forces,
for example, if a standard flat washer were to deform the knee
by 1.0 mm upon tightening the bolt/nut, then perhaps the alternate
backing plate should have sufficient strength to deflect outward less
than 5% or 1% of this value at the midpoint between the bolts.
I plan to use very substantial backing plates, at least
as wide and long as the chainplates, and plenty thick.
The spacers that are present on my starboard cap and aft-lower shrouds
are not washers, but pads of sorts of the same area as the chainplates they
support -- better than washers but not quite as solid as they could be if
mated directly to the knee or bulkhead.
I wonder whether I could post a photo of the spacer arrangement for discussion?
Per the suggestion, I will certainly measure to see whether the positions of the
corresponding opposite chainplates are equidistant from the stem
(before I remove the chainplates). If they are equdistant, then somehow,
the starboard side of the main bulkhead and the aft starboard knee would
seem to be hav ebeen installed about 3/4" too far forward...
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