Sanity Check -- AN6 Torque

[vernon smith]

If you want to get really anal retentive about it you measure the amount of bolt elongation. Not practical on any level but that is how the Double A Fuel dragster guys torque their connection rod bolts. Apparently it removes all the variables associated with metallurgy, lubrication and alignment. I'm sure others will have a different idea

Since they are getting in excess of six grand of horse power from five hundred cubic inches, the stress levels they are dealing with are astronomical. Remember, con-rod bolts are in tension.

[johnbright]

Originally Posted by vernon smith

... .measure the amount of bolt elongation... not practical on any level but that is how the Double A Fuel dragster guys torque their connection rod bolts...

It's off topic. Short answer is torque to Vans / AC 43.13 values for AN bolt in tension.

Important thing about connecting rod bolts is, properly torqued, bolt load does not vary as the crank rotates! An under-torqued bolt will fail in fatigue. Bolt tension is the goal and elongation is a better measure because of friction variation in threads and nut face.

[EXflyer]

Originally Posted by vernon smith

If you want to get really anal retentive about it you measure the amount of bolt elongation. Not practical on any level but that is how the Double A Fuel dragster guys torque their connection rod bolts. Apparently it removes all the variables associated with metallurgy, lubrication and alignment. I'm sure others will have a different idea

Since they are getting in excess of six grand of horse power from five hundred cubic inches, the stress levels they are dealing with are astronomical. Remember, con-rod bolts are in tension.

Going to A&E school way back we had to torque the main rod connecting bolt on a radial engine, 2800?, it required you use a mic to check it with in other words length. Side note, many truck drivers think that there wheel studs fail due to over torque of them, its more likely under torque I have found. Just something from a grey hair mechanic.

[az_gila]

Originally Posted by vernon smith

If you want to get really anal retentive about it you measure the amount of bolt elongation. Not practical on any level but that is how the Double A Fuel dragster guys torque their connection rod bolts. Apparently it removes all the variables associated with metallurgy, lubrication and alignment. I'm sure others will have a different idea

Since they are getting in excess of six grand of horse power from five hundred cubic inches, the stress levels they are dealing with are astronomical. Remember, con-rod bolts are in tension.

Some Lycoming connecting rod bolts are specified that way - labeled as "stretch" rather than "torque" bolts.

https://www.lycoming.com/sites/defau...%20%281%29.pdf

[MED]

[quote=control;1248315] Got my bolt hole-castle nut lined up on all four attachments with a torque of between 51 and 52Nm.

My RV-14A does not use castle nuts for the engine mount bolts. Does the RV-14? I know other RVs (7 and 8) also use castle nuts. Anyone know why the difference?

[az_gila]

[quote=MED;1248344]

Originally Posted by control

Got my bolt hole-castle nut lined up on all four attachments with a torque of between 51 and 52Nm.

My RV-14A does not use castle nuts for the engine mount bolts. Does the RV-14? I know other RVs (7 and 8) also use castle nuts. Anyone know why the difference?

Interesting change on the latest kit. I used metal self-locking nuts on my -6A since that is what called for on my certified Tiger's O-360.

Much easier to install and they haven't moved yet on either plane,.

[n982sx]

I think this thread may be getting into the weeds.

People seems to be addressing two different sets of bolts

The 14 uses AN6 bolts - and locking nuts - to mount the engine mount to the fuselage.

It uses AN7 bolts - and castle nuts - to mount the engine onto the engine mount.

Then again, maybe I was the only one confused.

[az_gila]

Originally Posted by n982sx

I think this thread may be getting into the weeds.

People seems to be addressing two different sets of bolts

The 14 uses AN6 bolts - and locking nuts - to mount the engine mount to the fuselage.

It uses AN7 bolts - and castle nuts - to mount the engine onto the engine mount.

Then again, maybe I was the only one confused.

Yes, two similar locations but previously treated identically... and the interesting point is that the -14 differs from the previous RVs in the engine mount to engine bolts.

My -6 plans, and the other similar RVs called for castle nuts at that location too.

The factory apparently has had a change in standards from previous designs.

[Aluminum]

Originally Posted by az_gila

Yes, two similar locations but previously treated identically... and the interesting point is that the -14 differs from the previous RVs in the engine mount to engine bolts.

My -6 plans, and the other similar RVs called for castle nuts at that location too.

The factory apparently has had a change in standards from previous designs.

The build manual for the -14 shows castellated nuts here too, torqued to spec and safetied with a cotter pin. Perhaps you meant "metal locknut" which also appears castellated but is not safetied? Metal locknuts are inferior to non-locking castellated nuts in a controlled preload situation because the extra thread friction introduces random uncertainty to the resulting preload value--it gets worse with every reuse/inspection too. That said, this is not a particularly critical location for super-accurate preload.

Van's dry torque spec for the non-locking AN7 is 37.5-41.5 foot-pounds.

Curious what prompted the change from AN6 to AN7 at Lycoming, the AN7 bolts weigh probably a pound more than AN6. Mount-to-fuse bolts are AN6 on the -14, with plastic locknuts on the cabin side.

[scsmith]

I've always been surprised by how low the torque specs for AN bolts are compared to similar strength and size bolts in general engineering application. It is true they are predominantly used in shear applications, so it isn't very important. But some applications are tension, or combined, and deserve higher torques.

General engineering practice calls for torques that produce preloads between 60-80% of yield strength of the bolt, and as others have said, this gives the best fatigue resistance and strength and prevents joint gapping for general-purpose loads that have some varying content. Bolts holding the engine mount to the fuselage would qualify under this practice. You don't know or analyze the varying loading in detail, but want the longest service life.

People think that over torquing uses up available strength margin, but that is incorrect. A pre-loaded bolt sees (almost)* no change in bolt stress or elongation until the applied load exceeds the torque preload. For applied loads below the torque pre-load, the combination of the applied load and the clamping pressure from the parts maintains a (nearly)* constant total load in the bolt. At the point where the preload is exceeded, the bolt is now carrying only the applied load and elongates farther, causing minute gapping of the joint. Gapping and motion in the joint are undesireable. Higher preloads create stronger joints. An exception to this is in predominantly shear applications, where the tensile preload reduces slightly the shear strength (see: Mohr's circle)

If you want to compare the AN specs to general engineering practice, look up a torque table for SAE grade 5 bolts of the same thread. SAE grade 5 is 125ksi, same as AN. (for bolts 1" dia. or less) For a 3/8-24 bolt, the dry torque spec is 420 in-lbs, the lubricated-thread torque spec is 300 in-lbs. But an AN6 (3/8-24) spec is 160-190 in-lbs (dry). If you add to that the fact that we are usually torquing an elastic lock nut or a metal lock nut, which takes torque to turn, I think we often have significantly undertorqued bolts.

* the 'almost' caveat here is because the parts clamped by the bolt do compress very slightly, and as the load varies, some of the compression in the parts relaxes as the applied load supplies the tension rather than the clamping of the parts. The joint becomes a system kind of like a suspension, where the deflection in the spring (bolt) matches the deflection in the parts. The more rigid the parts being clamped, the less variation in bolt stress for loads less than the torque preload. But nothing is infinitely stiff, there is always a minute amount of compression in the clamped parts.