[Public-List] Chain plate spacers - role of friction

David Tessier 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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