Standard torque 2

[foglights]

I recently encountered a situation where an AN4 bolt was used in conjunction with an MS21043 nylon insert self-locking nut (I think I have that PN correct). The torque chart I was using had the bolt in the 160,000 psi column and the nut in the 125,000 psi column and therefore had very different torques - approx a 200 inlb difference.

In this situation, which torque value is used?

 

[thruthefence]

How is the fastener loaded? 200" lbs in itself seems like a heckava lot of torque for a 1/4-28 fiberlock nut.

 

[HANDCALC]

Have a look at "FAA Advisory Circular 43.13-1B, Aircraft Inspection Repair and Alterations", Table 7-1 recommended torque values for fasteners loaded in shear or tension.

http://rgl.faa.gov/Regulatory_and_G...99C827DB9BAAC81B86256B4500596C4E?OpenDocument

I agree with "thruthefence", 200 in-lbs seems inappropriate for general installation of an AN4 fastener. What torque chart were you using?

Compare it with torque values found in:

Aircraft Maintenance or Structural Repair Manuals
MIL-HDBK-60 Threaded Fasteners - Tightening to Proper Tension
MSFC-STD-486B - Standard, Threaded fasteners, Torque Limits For,

 

[foglights]

Ok yes. I mis-spoke. The fastener size is actually 7/16 inches. The bolt is an MS20007 and the nut is the MS21043. But I don't believe the sizes are significant here. My question is:
In the torque chart I am using (SS9300) bolts and nuts are paired together based on their tensile strength or shear application. In my situation the bolt fell in a tensile strength category (160,000psi) different from that of the nut (125,000psi). When this is the case, does one use the chart according to the bolt specs or the nut specs? That said I can believe that application is paramount and when the above situation occurs it would make sense that the manufacturer would explicitly specify the torque. But is there a general rule of thumb for these situations?

 

[johnSchwaner]

Your concern is valid since it agrees with Bickford when he states: "Knowing the strength of the bolt alone is not sufficient, since it's never used alone." "An Introduction to the Design and Behavior of Bolted Joints"

Preload should be based upon calculated joint requirements and not a general usage chart, therefore, what you have is incomplete maintenance instructions. I would ask for clarification, not from this forum, but from your supervisor (off-load the responsibility).

Without proper and complete maintenance data, the mechanic question becomes which error (too tight or too loose) is, in YOUR opinion, least likely to cause a problem? Use your best judgement and make note of your request for proper data.

 

[tbuelna]

foglights,

The MS21043 locknut is an all-metal, deformed thread design made from A286 cres. I believe the material is 140-160ksi. However, the MS21043 is not available in 7/16" thread size:

http://www.everyspec.com/MS+Specs/MS2/MS21000-MS21999/download.php?spec=MS21043E.007568.PDF

I could not find any data on an MS20007 spec bolt.

As johnSchwaner points out, torque of tension fasteners should be based on calculated preload requirements. Relying on generic tables is probably not a good idea if the fastener installation is critical.

Your fastener installation procedure should also include a check of the locknut prevailing torque at each installation, to ensure that the nut's locking feature is not worn. Knowing the actual locknut prevailing torque value is important, since it must be accounted for in the final installation torque value. And all metal, deformed thread locknuts, like MS21043, tend to have prevailing torque values that vary widely.

Here's a recent example of how not having (or not following) a proper fastener installation procedure can cause big problems:

http://www.flightglobal.com/article...r-loss-on-qantas-a380-caused-by-oil-leak.html

Regards,
Terry

 

[thruthefence]

Have you considered an NAS 1804 or NAS 1805 nut? They are similar to the MS21043 but are available in the larger thread sizes.

here's the spec sheet on the MS20007 bolt.

http://tinyurl.com/42rq97b

 

[thruthefence]

Going back & reading your post again, I missed the part where the bolt is in tension; I feel the MS21043 (even if it WAS available in the "-7" size) would not be appropriate. These things are designed for a confined area, and are of a reduced "footprint" for that reason. Not very friendly to being snugged up too tight.

If you need tension, consider an internal wrenching "barrel nut", I think the p/n is HBV-F2-14-0089-XX. I need to pull up the ref at work to be sure.

 

[foglights]

The nut was definitely a fiberlock nut making the correct P/N therefore the MS21044. I apologize for my inaccuracies in this thread. We did assemble with the hardward callouts in the maintenance manual. I'm going off memory for the part numbers. The bolt was a 7/16" diameter internal wrenching hex pattern steel alloy bolt.

As for the application, it was actually used in shear for a tail landing gear mount on an S-61N helicopter.

 

[AviatorJim]

I'm reading the question a little differently. The FAA expects a bolt to be torqued to "standard torque" in the absense of a specified torque. It assumes the joint is designed with the right size hardware to be torqued to "standard".

Since the MS21043 and 44 nuts are described as tension nuts, AC43.13-1B, Table 7-1 says "standard" torque is 450 to 500 inch lbs, and with no other guidance I would torque to 475 plus the running torque required to override the lock. If other circumstances appied, I would go as high as 840 plus running torque (to align a locking device, reach an index mark, etc. . .) if needed. The higher strength bolt (161KPSI) is stronger than the nut.

 

[foglights]

I understand how to read a torque chart. I understand that joints, specifically designed, should have specific torque callouts for their fasteners.

I guess my real question is stated:
Since the FAA requires the bolt to be torqued, why would a torque vale for a nut take precedence? When the pieces of hardware used for joining a joint fall into different tensile strength categories on a torque chart and the bolt is the piece getting the preload, does it not make sense, therefore, to use the value associated with the bolt?

 

[thruthefence]

"and with no other guidance"

This thing is a part 29, transport category helicopter. I find it hard to accept that there is "no guidance" for torquing the fastener that attaches the aft gear to the airframe.

and I would avoid using AC43.13-1B as my reference, as it is "accepted data" and NOT "approved data".

As for torque for a bolt vs a nut; here's what I was taught:

When installing a bolt into a fixed component, (imagine a head bolt on your chevy.)the "Bolt" torque is referenced (assuming you're using a standard torque chart.)

When clamping two components together, like your landing gear forging to helicopter structure, you reference the "Nut" torque.

"does it not make sense, therefore, to use the value associated with the bolt"

 

[tbuelna]

foglights,

thruthefence brings up an excellent point in FAR 29.

FAR 29.607: "(a) Each removable bolt, screw, nut, pin, or other fastener whose loss could jeopardize the safe operation of the rotorcraft must incorporate two separate locking devices. The fastener and its locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(b) No self-locking nut may be used on any bolt subject to rotation in operation unless a non-friction locking device is used in addition to the self-locking device."

Per FAR 29.607, if this joint would fail with the loss of a single fastener,then your fastener installation would appear to require dual locking features for proper fault tolerance. One locking feature would be provided by the nut's locking element. The other locking function would be provided by the thread frictions due to axial preload. This means that sufficient axial preload must be maintained at the nut under all conditions (ie. thermal, vibration, service loads, etc.). If your bolt is only required to take loads in shear (no clamping friction), then the nut installation torque would only need to be sufficient to ensure a constant locking preload.

Regarding your question about relative nut and bolt material strengths versus preload, there are a few things to consider. With a shear fastener, like your example, the nut is not really subject to operating loads. So the nut can usually have much lower strength than the bolt. With a tension fastener, the bolt is subject to axial loads, plus torsion loads due to torquing, shear in the threads, and bearing under the head. The nut only sees shear in the threads and bearing at its face. Most tension nuts are designed with thread lengths that ensure the thread root shear capability is somewhat greater than the tension capability at the bolt minimum cross section. So even though the nut may have slightly lower material strength, the bolt would likely fail in tension/torsion before the nut threads failed in shear.

Hope that helps.
Terry

 

[thruthefence]

"though the nut may have slightly lower material strength, the bolt would likely fail in tension/torsion before the nut threads failed in shear"

Exactly so...

As a young man, I observed a fellow attempting to complete the installation of a tailboom on a Bell 206L. Four 3/8" d bolts (as I recall) in tension, using what we referred to as "bathtub" fittings in the structure. Suddenly, it sounded like a shotgun blast as the bolt failed under tension. Throwing the perp & his FOOT POUND torque wrench to the hangar floor. A big beefy guy, (but lacking both in supervision, and critical thinking skills, but persistent). Luckily the bearing surface of the fittings were not damaged, but the MS20002-XX chamfered washer under the bolt head had actually extruded a bit.