Bill Freeman wrote:

This is a fairly common problem with wood props, esp where the humidity
changes a lot.  The wood expands in humid air enough to crush, since the
bolts won't give enough, then when it dries out, it shrinks back and the
preload is lost.  The bolts will very rapidly fatigue when the preload

is lost.

I suspect that you may have been more careful about checking bolt

torque,

and/or have less humidity variation, or in many cases a prop crushes a

certain

amount and then stops.  Thin layer laminate props do this less than 6-7
laminate props due to the high glue content and inability of moisture to

flow

as easily, plus higher compressive strength of the 1/16" laminate

"plywood".

This has nothing to do with the bolts, any bolt will fatigue with a

repetitive

load above about 50% of yield strength.

Bill Freeman

Lonnie Wood wrote:

Spring washers on the prop bolts of  wood props were discussed at one time
on this
list. Has anybody tried this approach? If so, what part number and spring
rates are you
using?
Lonnie Wood

Lonnie,
This might work.  To keep the bolt in tension to prevent fatigue you would need
to stack up bellville washers (same direction) until you can reach full
rated torque
on the bolt without flattening the stack (they add spring rate this way).
If there is
enough travel with a single (set) of washers this might be all you need.  If
you need
more travel to accomodate the wood expansion, you would make an identical
stack to the previous one but revers the whole stack. This doubles the
travel with
the same spring rate. You'll need a thick HARD washer to top off the stack.

  Imagine three (or whatever) washers cupped side down, then three cupped side
up and then a large hard washer and then the nut.  Getting a bit
complicated, but
should work.  You also want a hard surface (hardened steel washer?) on the prop
side.

Bill Freeman

 
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