Prop Bolt Torque

[mike.smith]

Looking through the recent issue of Shop Talk, on page 6 I see a photo of the torque wrench being used on the bolt head instead of the nut. I would suggest this is not the best procedure.

http://www.boltscience.com/pages/nutorb ... tening.htm
"One of the disadvantages of using torque control is that there can be a significant variation in the bolt preload achieved for a given torque value. There are several reasons for this e.g. inaccuracy in applying the torque, dimensional variations of the thread and hole size variation amoungst others. However, the dominant factor is usually due to the frictional variation that is present between the contact surfaces that are being rotated."

In the ST photo there is a substantial amount of friction on the socket that is within the prop hub, which will affect the torque. In other words, the prop hub hole is providing extra friction on the socket, that will result in a bolt pre-load that is too low when the torque value is reached.

In this instance I believe torquing the nut is more appropriate and will more likely give the correct value.

[lgsievila]

Looking through the recent issue of Shop Talk, on page 6 I see a photo of the torque wrench being used on the bolt head instead of the nut. I would suggest this is not the best procedure.

Generally I agree with Mike but in this instance it doesn’t matter. Mikes comment, “However, the dominant factor is usually due to the frictional variation that is present between the contact surfaces that are being rotated." says it all.

The tension created in a particular joint is the same whether the tightening torque is applied to the nut or the bolt, provided the element that is driven (nut or bolt) is seated on a similar surface. In other words since you have the same washer under head of the prop bolt and prop nut, it doesn't matter. Some of the studies I have looked at indicate less than ½ of 1 percent difference in torque values.

If you have so much friction in your prop bolts that they will not turn with little effort before tightening, you have a bigger issue that needs to be addressed. The whole torque thing can get confusing and very technical, wet torque or dry torque. Is your torque wrench calibrated?

All that said, I will tighten the nut first if I can.

[gammaxy]

I haven't found the article yet. Has it been posted on the Sonex Foundation site yet?

My experience with the prop bolts agrees with what Mike said. There's a significant difference required to twist the entire bolt through the crush plate, prop hub, and wooden propeller than there is to just twist the nut. Slight misalignments between all the parts probably plays a role along with the imperfections inherent in a wooden propeller that slightly changes shape and dimensions with humidity. Lonnie Prince gave a talk last year where he mentioned the occasional need to ream or otherwise sand the bolt holes through his propeller--perhaps this would help.

[mike.smith]

And part of my point was that the prop Hub/Spinner recesses, since they are so deep, also create friction against the socket, disturbing the torque. I've done it both ways and I have found a HUGE difference between the torque value I get at the prop bolt head, versus the nut. Like 40-50 inch pounds.

"It depends" is the mantra for where you torque from. Feel free to try it yourselves and see what you get. For me, torquing at the prop bolt is quite different than at the nut. I just wanted to bring my observations to everyone's attention in case it's useful.

[hickej]

Thought I'd chime in since it's my arm in the newsletter picture. I torque from the front on the Jabiru because it's easier clearance for the wrenches. I followed the prop manufacture's instructions which includes adding measured friction torque to the specified value for the AN6 bolt. In my case they called for 17.5 ft lb and I measured the bolt friction at 2 ft lb for a total of 19.5 ft lb. Make sure to use the value from your prop manufacturer. Here is a video of the whole prop changing experience.

https://m.youtube.com/watch?v=S17yS22Z93s

Jim - Waiex 510 hours and climbing

[mike.smith]

Thought I'd chime in since it's my arm in the newsletter picture.

Nice :-) Ever thought of being an "arm model?" (if you know Seinfeld, you'll probably know the humorous reference...)