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# Torque friction coefficient - SAE grade 40 oil5

## Torque friction coefficient - SAE grade 40 oil

(OP)
I am trying to determine the friction coefficient to use for fastener torque calculations for grade 40 oil as lubricant. Google searches are not giving me any reliable information, some papers use values from 0.1 to 0.2. From where can I get this value or is there a standard way to calculate it without getting into experimental testing.

### RE: Torque friction coefficient - SAE grade 40 oil

3
Forget about calculating the friction. Either guess, or use a Skidmore-Wilhelm torque tension test. There are contractors who will conduct testing.

### RE: Torque friction coefficient - SAE grade 40 oil

What are thr grip lengths?
What are the conditions under the fastener (and nut?) heads?
Will you be using washers ?

"Google searches are not giving me any reliable information,"
I'd say the range of 0.1 to 0.2 IS reliable information.

https://issuu.com/arpbolts/docs/ut_details

"without getting into experimental testing."

A few hours on a Saturday morning with a torque wrench, several dozen fastener sets and a test set up simulating you bolted joint will be both eye opening and likely provide you with the answers you need. If you can design the test rig so bolt stretch can be measured it will be much better.

PLEASE also measure and record "break loose " torque every time a fastener set is disassembled.

My first tests would be to "torque cycle" a few lubricated sets to burnish and smooth all the sliding surfaces, carefully noting the final angle of the bolt head .
Those results will most likely make it abundantly clear why posts asking help "calculating" bolt torque inspire replies that sound kind of surly.

### RE: Torque friction coefficient - SAE grade 40 oil

Can measure bolt stretch with a micrometer over untensioned fastener and then on tensioned fastener. Or possibly with a test indicator. Micrometer needs fasteners close to edge of grip material. The compression of the portion under grip will affect the results - not as good as using the Skidmore-Wilhelm.

What dvd said!

### RE: Torque friction coefficient - SAE grade 40 oil

Unless this question concerns methods of unintended lubricant-free welding of components, this question is wholly inappropriate for this forum.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

### RE: Torque friction coefficient - SAE grade 40 oil

@ ironic metallurgist: This is a Welding, Bonding and Fastener Engineering forum. The question is fully appropriate.

The problem is that the question stems from a lack of understanding.

The "friction coefficient", aka "nut factor" aka other names is the mother of all fudge factors.

There is a large amount of variation and uncertainty to its value that stems from a wide range of factors involving the application, joint geometry, materials, plating or finish, surface condition, etc. etc.

Trying to establish "the value" and then using "the value" in calculations is foolish.

Rather, you need to look at the range of possible variations if "nut factor" and other factors involved in torque-controlled tightening and calculate the reasonably expected range of preload. Depending on the criticallity of the application you may take either a probabilistic (Monte Carlo) approach, or a worst-case approach.

From there, decide if your design can tolerate the probable preload variation.

### RE: Torque friction coefficient - SAE grade 40 oil

Check out..

MIL-HDBK-60 THREADED FASTENERS - TIGHTENING TO PROPER TENSION
TABLE II. Examples of coefficients of friction.

ALSO...

SAE AIR1471 TORQUE TIGHTENING THREADED FASTENERS
SAE AIR1551 TORQUE TIGHTENING METRIC SCREW THREADED FASTENERS
SAE J1701 TORQUE-TENSION TIGHTENING FOR INCH SERIES FASTENERS

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

### RE: Torque friction coefficient - SAE grade 40 oil

(OP)
All -

Thank you for the answers. I think the consensus for the "fudge factor" being 0.1-0.2 is reasonable backed by the MIL spec.

I will take up Tmoose and dvd's recommendation and replicate this exercise to determine the target torques. I can actually do this on the flanges themselves as we have them sitting around in our yard (may have to sacrifice some gaskets).

### RE: Torque friction coefficient - SAE grade 40 oil

DO... Sigh... One final thought...

Most discussions of nut torque-to-tension involves 'plain nuts'... torque ~= thrust/tension/friction at point where 'torque rise' occurs.

IF installing a self-locking** nut, the 'free-running torque' on the nut [with lubricant if present, nut fully engaged on the threads], has to be factored into the total torque to set a bolt at proper tension at torque-rise.

** self-locking feature may be a 'plastic' [nonmetallic] insert to drag on threads; or 'metallic'... where the nut barrel is distorted 'out-of-round' causing drag on threads. distortion can be present in 'many forms' based on general all-metal nut configuration.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

### RE: Torque friction coefficient - SAE grade 40 oil

Consider using Belleville washers instead of wasting time on torque. The Belleville washers would maintain tension during thermal cycles plus give indication of proper tensioning.

### RE: Torque friction coefficient - SAE grade 40 oil

MintJulep,
Apologies for writing first and reading the OP thoroughly later.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

### RE: Torque friction coefficient - SAE grade 40 oil

As I recall, the first few times I looked into belleville type washers it would have taken a big STACK of the typical commercially available size and thickness arranged <<<<< to create any where near the preload force provided by simply torquing a grade 5 or especially grade 8 fastener to the nominal values published all over.

So I pretty much stopped considering them for structural joints.
When my fear of preload loss due to embedment etc starts keeping me up at night I look at ways to get bolt grip length = 7 or 8 bolt diameters.

### RE: Torque friction coefficient - SAE grade 40 oil

As the OP has stated, this is a gasketed joint and not a structural connection. The paper cited above (and repeated here) has calculation methodology for flanged, gasketed joints, plus an example at the end of the paper.

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