At McMaster.com I found 18-8 stainless spring washers, 10.5 mm I.D.,

23mm

O.D.
and 2.54 thick. They do not give a load rating but they are the same
thickness
as the heavy duty Belleville at about 5000 lbs. They are reasonably

priced

at $11 for
25. Does anybody know if McMaster has a minimum order?

At this point I am thinking of using 4 washers, 2 in parallel and 2

in

series. I will put
them together on some scrap wood to test the compression of the

springs in

relation
to the compression of the wood at various torque settings.

McMaster does not have hardened washers in large enough O.D. to put

under

these.
Found some at Earnestmachine.com, 100 for $6.48.
Lonnie Wood.

 Lonnie,

 What size (thread) bolts are you using on your prop?

Bill Freeman

They are 3/8 by 24 tpi. I will have to get longer bolts to hold all
of these washers. The Belleville washers need about .5 mm play
on the bolt, seems they neck down when compressed.
I carved the prop about 3 years ago, still need to get set up for
drilling the bolt holes.
Lonnie Wood

 Lonnie,
 AC43.13-1A (the old edition) shows a 3/8-24 AN3 thru 20 bolt
with a steel tension nut having a min torque of 160 in-lb and a max
torque of 190 in-lb. Shigley reports that  Torque = 0.2 * F*d,
where F is the preload (bolt tension) and d is the bolt nominal
diameter (0.375 in this case).  Rearranging terms we have

     F = Torque/(0.2*d)

For the 3/8-24 bolt at 160 in-lb torque, the tension is about 2133 lbs,
and for 190 in-lb torque, the tension is about 2533 lbs.  So, you need
either a single belleville washer with a rating of about 2300 lbs, or a
pair stacked (nested) each rated at about 1150 lbs.   My guess is that
your prop is about 7" thick, and (WAG) might vary in thickness by
up to 1% or less.   1% of 7" is .070", so you would want your stack
travel to be more than .070".   Since I actually have no idea what the
variation in size of a wood prop is, it might be good to store the prop
in an intentionally wet environment, like in a sealed box with a wet rag
for a couple days, then very accurately measure the thickness of the
hub, with a dial caliper to 0.001" resolution.  Next, dry the prop out
well, perhaps in a plastic bag with a substantial amount of silica gel,
dried and changed regularly, and remeasure the prop hub thickness.
Your total washer travel should be significantly more than the change
seen in the hub, like 1.5X or 2X to ensure good tension.

Hope this helps a bit, altho I just retorque mine if I spend much time
in extremely humid areas.  Have left the Long EZ at Tamiami (south
Fla) outside for a week and then flown directly back to KC area,
which is moderately dry, at least compared to Fla.  Have also left
the LE at Gilespie in San Diego (pretty dry) for a week with no ill
effects.  BUT - I have one of the old Great American props with
1/16" maple laminations, which I am told is more resistant to this
problem than conventional 6-8 laminate props.

Bill Freeman

 
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