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Sanity Check -- AN6 Torque

bkervaski

Hellloooooooo!
Testing
For clarification, AN6's get torqued 160-190 inch pounds, right? 180 inch pounds just didn't feel that tight .. for an engine mount .. if someone could verify, just a sanity check :p :confused: :( :eek:
 
Yep, that's where I got my numbers .. I just expected to have to really muscle these bolts .. guess I'm underestimating how strong these parts are.

Thanks!
 
What you and many people are doing is over-estimating the amount of pre-load a bolt needs, in order to have that maximum strength available when really needed.
 
my seat of the pants observation is that people tend to over torque small bolts and under torque large bolts when relying on feel rather than a torque wrench

erich
 
For clarification, AN6's get torqued 160-190 inch pounds, right? 180 inch pounds just didn't feel that tight .. for an engine mount .. if someone could verify, just a sanity check :p :confused: :( :eek:

Depending on the actual friction coefficient present, that torque will result in about 3,000 pounds of clamping force on that size/pitch bolt, which is about 25% of their tensile strength. Lots of margin in bolts holding aircraft together is a good thing! Don't squander the margin by honking down bolts by "feel".

A separate but related fun-fact: Critical bolts subjected to cyclic loads (think about the studs holding the cylinders on) are often specified to be torqued to a value which maximizes their fatigue life. Torqued below or above the spec and their fatigue life will likely be lower.
 
It's the leftover military mentality for me .. give me a hammer and a direction and by God it will get done .. so if the bolt still turns then keep on turnin' :D

Of course, I'm following the torque specs to the letter (just so it's clear to readers). I just expected to have to enlist the large torque wrench and some muscle and it's simply not the case.

Good information, thanks all!
 
What is torque . . . really ??

It's the leftover military mentality for me .. give me a hammer and a direction and by God it will get done .. so if the bolt still turns then keep on turnin' :D

Of course, I'm following the torque specs to the letter (just so it's clear to readers). I just expected to have to enlist the large torque wrench and some muscle and it's simply not the case.

Good information, thanks all!

Like Alex said. There is a sleeve in the mounts that can be crushed, so too tight is not good. Did you get an instruction with the mounts?

And - 99% of the aircraft applications for fasteners are loaded for shear, not tension, that is why careful attention is paid to the hole sizes and why threads are not allowed in the joints. Other terrestrial things we are familiar with are primarily tension applications. That is where the bolt tension holds something and takes additional load, or the bolt tension clamps parts together and the tension/force prevents the parts from sliding. Engine parts are one example, all heavy duty machinery, off highway machines all are built to those standards. This allows hole sizes to be larger for assembly. Sliding causes fretting and won't last very long. Another difference is dry vs oiled threads for torquing. Dry threads result in a wide variation in actual tension of the bolt. Like plus/minus 20%. All (nearly?) the aviation fasteners are dry torqued. So NEVER build an engine without oiling the threads before torque.

[ok so the engine mount frame is tension loaded to the firewall and appears to be an exception, but it isn't.]
 
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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.
 
... .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.
 
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.
 
Just like the dragster...

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/default/files/SI1458G Connecting Rod Bolts (1).pdf
 
:) 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?
 
:) 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,.
 
Confusing thread

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.
 
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. :)
 
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.

nn7138.png
 
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.
 
AN Nuts

A nut for a drilled shank blot using a cotter pin is a CASTLE NUT.
Certificated aircraft have a long history of successfully using all metal lock nuts on engine mount to engine bolts on dynafocal mounts.
 
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.
...
General engineering practice calls for torques that produce preloads between 60-80% of yield strength of the bolt,

I'll venture a guess that this is not out of concern for the bolt itself but for the materials joined by it, which in aerospace applications will typically be of much lower tensile strength than the bolt. Wouldn't an overtorqued bolt concentrate stress just outside the compressed area of the joint?

For instance, my 1/4" prop bolts have a torque spec of 140 in*lbs, compared to 50 in*lbs for the same AN4 bolt joining two aluminum extrusions. Prop bolts see very little variation in tensile load and feel no shear if torqued properly, because all the engine power is transferred to the prop through friction with the flange. Higher preload here increases the power that can be transferred to the prop without slipping, while still maintaining a safe margin of bolt fatigue.
 
Quote: "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"

Me too.

Here's a page from the Lycoming manual: looks like about double the Van's torques

2a5hslv.jpg
 
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