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
Bill, thanks for taking the time to help me with this.
My prop is 4 inches thick, 5 laminations. 2 of ash, 2 of cherry and 1 of
red oak. Checking the charts on wood movement for these species,
tangential and radial, figuring an equilibrium moisture content fluctuation
of 7%, I come up with wood movement of about 1/16" (.063). Pretty
close to your .070" but then you were figuring for 7" of wood.
Your idea of putting the prop in a box and changing the moisture is
a good one, but with a sealed prop that thick I think it would take months
for the moisture content to change.
Cam 18 shows prop torque to be 15 to 24 ft./lbs. Bingelis says this is
probably too much and recommends tightening just until the face plate
barely embosses the wood. I did a quick test with a very thick large area
washer on ash and found that it started to emboss at about 14 ft.lbs.
The formula you gave for bolt tension comes up with a way different
result than my calculation. Here they are again:
1" divided by
24tpi gives a nut travel of .0417" for one complete turn of the nut. If a
1 foot
arm is on that nut then the end of that arm travels 75.384" (pie x 2' x
12") to
the nuts .0417". This gives a mechanical advantage of 1808 to 1. Assuming a
torque on the prop bolt of 25 ft.lbs. this is a bolt tension of 45200 lbs.
I
understand that only about 20% of the torque goes into tension though from
thread friction and head friction, so that would leave 9040 lbs.
Using my way of figuring 160 in lbs. this gives me a tension of 4820 lbs.
that is over double the tension that the F = Torque/(0.2*d) formula
comes up with. Did I mess up somewhere in my calculations? Maybe
not enough friction figured in?
Lonnie
Lonnie,
It is pretty late and my brain is a bit fuzzy so I'm not quite following your
calculation. In any case, Shigley is a std Mech Eng text, and in the section
on tensioning bolts he refers to a bunch of test data for both lubricated and
unlubricated bolts, and comes up with the empirical formula above. I would
trust it. I'll get back and try to puzzle thru your method when I am more
awake, but seems like you found some washers that were rated at 1200
lbs at full compression, so a pair (doubled) would give the spring rate you
need (~2400) and you'll need to use the right number to get the stroke
that you need.
Gotta sleep, big pistol match tomorrow. It's 1 am.
Bill
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