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  #21  
Old 03-24-2018, 10:16 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by n982sx View Post
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.
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  #22  
Old 03-25-2018, 11:28 AM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by az_gila View Post
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.

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  #23  
Old 03-25-2018, 01:57 PM
scsmith scsmith is offline
 
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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.
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  #24  
Old 03-25-2018, 02:29 PM
jrs14855 jrs14855 is offline
 
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Default 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.
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  #25  
Old 03-25-2018, 10:45 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by scsmith View Post
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.
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  #26  
Old 04-13-2018, 11:40 AM
Jake14 Jake14 is offline
 
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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

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