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Checking prop nut torques during annual

Tacco

Well Known Member
Does the prop have to be removed ( I.e. the pitch setting disturbed) to check the torque on the mounting bolts during the condition inspection?
 
No, I just set the TQ on my wrench to spec ( I believe 18-20 LBS )and check for tightness,no reason to loosen them up if your not resetting the pitch.
 
But aren’t the mounting bolt heads capture inside the hub, I.e..you can get to the nuts but not the bolt heads.
 
RTFM

RTFM = Read The Freaking Manual
What does the propeller manufacturer say?
Almost certainly the manual says how and how often to do that.
 
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Chill man. I have the manuals sitting right in front of me. This is a lot more nuanced question than you’d think. The Sensenich manual only says to check the clamping bolt torque (6). That’s easy. The Vans manual says to check the hub fastener torque. I assume those are the flange mounting bolts. The problem is the head of those bolts is covered and not accessible without removing the hub front half. You can get to the nuts on the back face of the spinner plate. So…is it acceptable just to check the torque of those nuts with no wrench on the bolt half?
 
In the Vans MM, page 11-3 and the service schedule checklist only mention hub clamping bolts, so I think "hub fastener" on page 3-7 is referring to those bolts.

The Sensenich manual does mention checking the mounting bolt torque if there is "looseness of the bulkhead" on preflight or "evidence of roughness on operation" but not on the annual as you noted.
 
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You could put a torque wrench on the nuts with the required torque and check, if it passes then add a torque stripe...
 
From my experience you need a spanner on the bolt to ensure it doesn’t slip when you put a torque wrench on the nut. Unfortunately that means removing the blades.
 
Me too. But that would mean repitching the propeller at every annual. Is that what we’re saying?
 
Usually I can check the nut torque without slippage. When the bolt slips I take the hub apart, put a wrench on the bolt head and retorque the nuts on the rear half of the hub. Needless to say I keep my fingers crossed that the bolt doesn’t slip, especially if I have hit the sweet spot on prop pitch.
 
Has anyone found prop hub bolts to be loose / under torque spec? Why not just mark assembled bolt/nut on the back of the crankshaft flange using Torque Seal Lacquer? If lacquer remains intact then bolt/nut remain at torque setting. This would become a visual inspection. Am I missing something here?
 
Why not just mark assembled bolt/nut on the back of the crankshaft flange using Torque Seal Lacquer? If lacquer remains intact then bolt/nut remain at torque setting. This would become a visual inspection. Am I missing something here?

Yes, bolts can lengthen slightly due to the stretching and the compressed aluminum can deform, both creating an under torqued bolt. Wood is constantly changing density and retorques are required often. It doesn't take much change to affect torque by 25% on fine thread bolts. The risk isn't very high and why most bolts don't have re-torque requirements. The prop, however, sees a LOT of mechanical stresses and forces. Therefore, the risk is higher and the costs much higher. Remember that you have a 4" chunk of aluminum. a Compression/deformation of just .5% is .020" That is just shy of 180* of bolt rotation on a 20 TPI thread (don;t know what the 12 uses here). A set of prop bolts that moves just one halft of a flat (30*) will leave the hub interface loose enough to heat up and fret, according to some research that Catto did when discussing the need for TIGHT safety wire on wood prop bolts.

CS props don't have the requirement, because the aluminum under the bolt is only 1/2" thick, unlike an FP.

Larry
 
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Has anyone found prop hub bolts to be loose / under torque spec? Why not just mark assembled bolt/nut on the back of the crankshaft flange using Torque Seal Lacquer? If lacquer remains intact then bolt/nut remain at torque setting. This would become a visual inspection. Am I missing something here?

Yes. I found 3 bolts under torqued (bolts rotated) while checking mine.
 
Think about this… You don’t check torque on engine connecting rod cap bolts every year. If connecting rod bolts are properly torqued and secured, they remain unchanged for complete engine run. The design torque stretches the bolt so that all forces applied during operation remain below the stretched force and the bolt then sees no cyclic loads.

Same design philosophy applies for cylinder head bolts and bolts that hold crankcase halves together. All are pre-stretched above cyclic loads.

Prop flange bolts are same thing. It would be disastrous if prop bolts were subjected to cyclic loads. The design torque puts the bolts in sufficient tension that cyclic loads are always less than the force applied and held by the torque. Wood props are a deferent story. Alloy and geometry used for aluminum hub, aluminum spinner back plate, steel crankshaft flange, bolt grade and fit, are all accounted for by the designer when specifying torque. Computer models and real-world testing verify safety margin for the torque value.
 
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