Torquing prop bolts

[carlisle]

Was surfing around and found this little note on another forum. It applies to a Zenair but struck a note with me. Every time I go to retorque my GSC 66" adjustable pitch prop, the bolts are loose. The prop was just inspected and overhauled by GSC. My torque wrench is calibrated, etc, etc.

Anyway, here's the topic:

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Our CH601HD Zodiac with a Rotax 912S 100 HP engine has a 68-inch, 3-blade
GSC wooden prop. In our installation, there is a prop flange extension.
The prop is held on by 6 1/4-inch bolts (3 through the aluminum blade
roots), and by 6 5/16-inch bolts onto the flange extension. GSC Systems
says use 100 inch pounds of torque for the 1/4 inch bolts, and 140 for the
5/16; these values are critical. Normally, with a washer under both the
head and the nut, one holds the bolt head stationary, and torques the
nylon lock nut; these nyloks are the preferred nuts recommended by GSC.
Because of the design of the prop flange extension, it is not possible to
get the torque wrench onto the nuts. Thus, one holds the nut stationary,
and torques the bolt head. THIS IS NOT THE SAME THING. My son Peter
(co-builder of our aircraft, and a professional mechanical engineer) knows
from practical experience that putting the torque wrench on the bolt head
can undertorque the nut by possibly 20-30%, or even more. It stands to
reason: the bolt shaft (or grip) inevitably experiences some friction in
passing through the parts being bolted together on the way to the nut.
There is a little part called a "crow's foot" to overcome this difficulty,
which goes over the nut, extends radially outward for about an inch, and
has a square hole for attachment of the torque wrench. It happened that I
had always torqued the prop bolts on our aircraft. In setting out to
retorque the bolts yesterday because the weather is now turning cold,
Peter realized right away that there was a problem. I had never used a
crow's foot (I did not know of their existence!). I'm guessing that many
others using this Rotax/GSC prop installation were also unaware of this
problem. What have others done about this? Were they aware of the
problem?

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Anyone else run into this problem?

Chris
Model 2, 582
Sioux Falls, SD

[Av8r3400]

By using a crow-foot socket, you are extending the lever of the torque wrench and the torque applied is greater than the setting on the wrench.

(A good read on the subject.) (http://www.finishing.com/118/94.shtml)

Torque Calculator (http://www.belknaptools.com/extcalc.asp)

[wannafly]

I have always wondered about this. I have an IVO in flight adjustable and the bolts are about 4 inches long and go thru a number off pieces with the nut being held on the back side. Because the bolts go thru all the pieces there is definitly some resistance on the bolts. What I did is set the torque wrench to 0 and try each bolt without the nut on and slowly increase the torque setting till the wrench will not release. It seems to average around 3-5 foot pounds. I take the highest reading and added it to the IVO recommended of 30 foot pounds giving me 35 foot pounds.
Since this is a new prop with less than 10 hours I check it after every flight. Have had one bolt that needed about a 16th of a turn to bring it back to the 35 foot pounds. The silver tape is still intact and all appears good.

yours thoughts on my methods would be welcomed....

mic

[carlisle]

Mic...I think this is as valid a method as any, especially since it applies to your specific situation and doesn't rely on guessing of unknown factors.

My issue with the crow's foot is that it's imprecise. The reason you should torque on the nut instead of the bolt head is that there are no other resistances. If you're on the bolt head, one has no way of knowing how much resistance there is to torquing along the length of the shaft. Therefore, it's anyone's guess as to just how much you should compensate. I think that the 20-30% advocated in the original thread is an estimation and probably ok for non critical/structural applications. Holding the propeller on the airplane is not one of these however.

My A&P mechanic buddies say that a crow's foot is not designed for this. They also say that a little loosening of the prop flange bolts is normal and 'figured in' when determining the correct torque settings, especially with a wooden prop where expansion/contraction has to be taken into account.

Chris

[N218SM]

On torquing with a crowsfoot. You should allways turn the crowsfoot 90deg to the torque wrench. That keeps the torque the same as if you used a socket. If the crowsfoot is in line with the wrench there is a mathamatical formula to determine the correct torque. At least that's how it's done in the Boeing bolt installation spec. Actually used a crowsfoot to torque some jam nuts for an F-22 a couple hours ago.

And we do have a different torque requirement for torquing the head vs the nut. Torque from the head is only slightly higher.

Scott