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Torque/Preload Calculation for tension vs shear joint Questions

Torque/Preload Calculation for tension vs shear joint Questions

Torque/Preload Calculation for tension vs shear joint Questions

(OP)
I am calculating the required installation torque/pre-load for a series of fasteners. I am using the basic formula of:
T=KFd
where K is the nut factor, F is the bolt preload, and d is the bolt nominal diameter.

Using the Machinery's Handbook 30th edition as a reference I calculate the preload as .75% of the proof load which I take as 85% of yield of the bolt.

I've used my values to compare against AC 43.13-1B


I understand that ideally experimentation should be done to find the appropriate preload and that I've made quite a number of assumptions, but that aside I have some questions.

My values are in the upper half or slightly higher than recommended installation torque values for tension type nuts. Looking at my hardware I only use tension type bolts and nuts. However some of my joints are definitely more shear joints than tension. I noticed that the fasteners with shear type nuts have considerably lower torque. I am not sure if I am misunderstanding correctly but if I use tension type nuts with the higher torque values on my shear type connections, is my pre-load too high? I am concerned I am over-loading a shear bolt connection.

Some other related questions I have:
-Are the torque values given in AC 43.13-1B independent of the actual type of loading (dependent solely on the hardware used)? this is what I thought but am having second guesses.
-For my calculations I use yield values I find in the bolt/screw milspec or do a very rough estimate of yield based off ultimate strength (about 85%) but for shear type connections, should I be using shear allowable to calculate torque (seems out of the ball park because torque related to tension not shear). I got this idea from NASA_MSFC-STD-486-B_standard_threaded_fastener_torques.

-My concern is that my estimated torque is too high for my shear connections. I was wondering, is there was a clever, acceptable way to factor down my calculated torque? Again from looking at NASA_MSFC-STD-486-B_standard_threaded_fastener_torques, I read that the torque for shear applications should not exceed 60% of the torque for a tensile application


Any advice would be great. Reading some other threads on this forum has clarified some things but also made me question my approach. If you have any advice that would be great. I know this is a bit of a can of worms but any insight would be much appreciated.

Thanks!

RE: Torque/Preload Calculation for tension vs shear joint Questions

the one is that bolts reacting shear don't need a lot of preload. In fact excessive preload reduces their shear capability (consider combined tension and shear loads in the bolt). The other point is (for tension loaded bolts) …
1) do you care about gapping the bolt ? at ultimate load this is normally accepted, usually frowned on for limit load (but not necessarily unacceptable)
2) do you care about fatigue of the bolt ?
preload in a bolt is used to control the load at which the joint gaps and also to improve fatigue performance.

If analyzing the bolt …
1) with the applied tension and shear loads, then you're saying the bolt has gapped and the load is applied directly to the bolt, or
2) with the applied shear and a tension load that includes a component of preload then you're saying the bolt hasn't gapped.

So next you'll ask "component of preload" ? This is a whole field of study on it's own. A simple typical assumption is that the bolt load is equal to preload for loads less than the preload, and equal to the applied load for loads that are higher.

another day in paradise, or is paradise one day closer ?

RE: Torque/Preload Calculation for tension vs shear joint Questions

One problem with that chart is that the fasteners listed on it are hardly in common use any more. You would do better to look at the SRM for the aircraft you're working on (if it should have one).

My take is that you select fasteners designed for tension to fasten joints in tension, and vice versa for shear. So by that simple definition, all tension-type nuts are torqued to the higher torque range specified in that table, and same goes for shear-type nuts. If our designs were always that simple... Often, just choosing a cheap fastener gets the job done with plenty of margin, but there are often cases where we care a great deal and need something specific to do it right.

I could turn the question around, and ask why you are installing nuts sized for tension on a joint that functions in shear?

I would defer to the guidance provided in Mil-Hdbk-60 for this. It covers bolted joints in both tension and shear, and the distinctions to be made in the torque needed for each.
The central question when the joint in shear is "do you want the members to slide?" It has a lot more useful stuff than that, and you can download it from ASSIST.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Torque/Preload Calculation for tension vs shear joint Questions

(OP)
Thanks rb1957, SparWeb--> reading more into this and definitely combing through Mil-Hdbk-60 now. I was wondering if there was a rule of thumb for pre-load as a % of yield for a tension joint vs a shear joint but that doesn't seem to exist. General guidelines suggest a set pre-load independent of joint type. I definitely know that a tension joint will have greater torque values but wanted to get a ball park sanity check as to what % of yield that was. It looks like some testing is in my future.

I got Mil-Hdbk-60 from the everyspec website but I was unfamiliar with ASSIST --it looks neat, thanks for the info!

RE: Torque/Preload Calculation for tension vs shear joint Questions

I recently read NASA TM-2012-217454:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa....

Which states:

"The results of this test program confirm that the application of an initial preload does not affect the ultimate strength capability of a bolt in shear, tension, or combined shear and tension loading when the joint separates before the bolt ruptures under pure tension loading."

So with that stated-do the old recommendations to torque shear joints to a lower value to preserve their shear capacity still stand? If the "one-time" ultimate strength is unaffected by the preload, are there any fatigue/joint life reasons to use more/less preload? For nut retention I'd think the highest preload that the joint substrate and bolt choice (shear vs tension head etc) can take would be preferable.

The paper also references another relatively recent (1995) paper relating to preload vs shear capacity, but it's not publicly available like the NASA tech memo:

Olson, S.M.: “The Effect of a Tensile Load on the Ultimate Shear Capacity of a Fastener Shank,” Structural Integrity of Fasteners, ASTM STP, Vol. 1236, May 1995.

RE: Torque/Preload Calculation for tension vs shear joint Questions

yes, when the jinot has gapped preload plays no further part in internal loads,
but if a high preload is applied and the joint doesn't gap (like a predominately shear loaded joint but with the bolts preloaded to 50% Ftu) then preload is a factor.

another day in paradise, or is paradise one day closer ?

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