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Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert

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tomcrooks

Aerospace
Oct 12, 2012
8
Hey all, first time posting here...

I've been wondering how a Heli-Coil insert (or similiar type threaded insert) would affect the stiffness calculations in a blind tapped hole.

We are working with #6-32 A286 strain hardened fasteners with 180 ksi yield strength, with 6061-T6 aluminum clamped material. Initially I am thinking that you would just assume that the Heli-Coil and the fastener are "one" in the area that the Heli-Coil and the fastener are in contact (threaded region of bolt)...Then you would have a series stiffness calculation for the fastener something like:

k_fastener_total = [(1/k_head) + (1/k_shank) + (1/k_fastener-helicoil)]^-1

Does that sounds right? Anyone else do this differently? Appreciate the comments.

Thanks,

T.J. Crooks
 
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is this a tension stiffness or a shear stiffness ?

surprised you're using 180ksi bolts in a heli-coil in 6061 ... i'd've thought the limiting strength would have been the 6061 thread (for the heli-coli) ?
 
I have never done this particular calculation, but I do not think your proposal is correct. The screw thread insert will deform elastically in addition to the elastic deflections of the screw threads and the tapped hole threads. This is where a single test would be very helpful. Have you conacted Emhart?
 
Thanks rb1957 and CoryPad,

I did pullout calculations based:

pullout = (pi*pitch_diameter*E_shear_aluminum*L_thread)/3

and am not too worried about that based on the pre-load I want to put on the #6, but I am most concerned in calculating the stiffnesses for our thermal environments, -185F to + 185F

CoryPad,

Are you suggesting that the fastener stiffness and insert stiffness need to be calculated seperately, and THEN summed in series?

Forgetting about how each stiffness is calculated for now then the joint stiffness might be like:

k = (1/k_fastener) + (1/k_helicoil) + (1/k_clamped_material)

Thanks,

T.J.
 
Edit:
Forgetting about how each stiffness is calculated for now then the joint stiffness might be like:

k = [(1/k_fastener) + (1/k_helicoil) + (1/k_clamped_material)]^-1
 
the thread AND the clamped part are t6061 ?

Are your concerns about the preload applied to the joint?

What is grip length of bolt? If the bolt is proprely long, the bolt body is normally the flexible part, and I don't recall even MIL STDs making adjustments for heli-coil repaired threads, but I too would be interested in what Heli-coil Emhart has to say.
 
Thanks Tmoose,

The fastener is A286 steel, right now the clamped material is 6061-T6 (although I'm pushing to go with 7075-T6) and the helicoil will likely be some austentitic nickle steel.

The grip length the bolt is approximately 0.28125", and I'm not sure if I can ask Emhart to make helicoils exactly that length, so I think the helicoil might be a little shorter than 0.28125" The major diameter on a #6 is 0.138" so I think I am OK on thread engagement based on a 2*d rule of thumb.

Also, the heli-coils are not being put in to repair any threads, they are going to be part of original design, due to fear of galling the female aluminum threads with these hard A286 screws.

I guess I should cold call Emhart...

Thanks,

T.J.
 
If you consider the load path the fastener and the helicoil are in series - load must pass through thru the fastener, through the helicoil and then into the base material.

So I'd say the stiffness needs to be calculated accordingly.
 
I agree with MintJulep unless someone else can convince me otherwise...For calculating the stiffness of the helicoil (simply, without using frustum cone angles etc...), do you think it is appropriate to assume that it is the stiffness of a hollow cylinder, with effective area:

A_eff = (pi/4)*(OD_pitch^2-ID_pitch^2)?
then
k_helicoil = (A_eff*E)/L_heli

Honestly I don't think that this is the best way to calculate k for the helicoil and it will drastically over estimate the stiffness of the heli-coil... Maybe it is simply the spring constant before it is placed in the joint?

-T.J.

 
From the perspective of the base metal the helicoil is exactly the same as a threaded fastener.

So I would use the frusrtum cone, angles, etc.
 
you're mostly worried about the thermal stresses, yes? the AL fttg will expand more than the Steel bolt, causing tension in the bolt. As a thermal problem, I think the impact of the helicoil might be very small ... maybe consider the problem as an Al flange with a Steel fastener the size of the heli-coil ?

presumably it's a cyclic temperature variation ... i wonder what this does to the heli-coil ? as the AL flange heats up does the heli-coil sit deeper into the thread ? and then as it cools down again, does it trap the heli-coil straining the thread around it ??
 
Yes, I am mostly concerned about loss of pre-load when you go from ambient to the max cold operating temp. I think you are correct that the impact of the helicoil on this effect would be very small. The temp does cycle at some rate from -185F to +185 F...

At cold temp the aluminum will contract the coil and fastener, and at hot temp the aluminum will expand, making for a more loose fit... The amounts the materials expand and contract is basically negligible for tolerancing I think...The temp changes do affect the pre-load changes. My initial calcs interestingly show that the pre-load loss in cold, and pre-load gain in hot are independent of the actual initial pre-load.

How many times can one use the term pre-load in a forum? lol.

-T.J.
 
ohhh that takes me back ... my 1st job was calculating bolt preloads for the ESA Spacelab pallet (cargo carriers for the Shuttle). Ti bolts in Al, +120 to -140, +1/3 to -1/3 preload scatter (from torque wrench).

my 2c ... either ignore the heli-coil, or include as the steel fastener.

if you are cycling the temp, i'd expect the heli-coil would loosen over time.
 
Start out by just running it as a simple A286 bolt and do the thermal calcs. You will probably find little or no residual clamp load at that max cold temp. You may have to add a belleville to compensate for the differential expansion. Also watch out for the contact bearing stress at high temp, it is easy to yield the bearing surface at high temps against alum.
 
Hi tomcrooks
I found some information on helicoils see link below:-


It appears to me that, provided you select the right helicoil for the parent material shear stress at the appropriate service temperature I think you can ignore the helicoil
material in the stiffness calculations.
There is a temperature range given that each helicoil material can work within so I can't see a problem.

desertfox
 
In ambient thermal conditions, it's safe to say that if the Heli-Coil was not rigid enough, that the bolt would never develop full torque. But they do develop some torque, and generally it's a more gradual buildup of torque vs. turns of the bolt compared to plain threads. That's a good thing.

If Emhart does not have what you need regarding operating temperature you may need to perform your own test.
 
Geesamand,

I think I will be heading down the road of running my own test....Has anybody ever modified bolts with strain gauges before? Or has anyone ever bought quality commercial strain gauge bolts?

I looked on-line very briefly and only found one commercial company, Strainsert or something selling aluminum 2000 series T4 strain gauge bolts...yuck...'

The idea will be to make a little test cell out of the materials and joint design, then give it a nitrogen bath and record pre-load loss from the strain gauge, calibrated of course.

Yeah, has anyone made their own strain gauge bolts before?

-T.J.
 
First , before you go to the trouble of just running the numbers of the effect of the differencial expansion and ignore the helicoil. The liekly problem that you will run into is that at the warmer temp, you will exceed your underhead bearing stress limit and get underhead yielding of the clamped material. With the limited stretch that you will have in the short screw, you are probably going to see that at the low temp that you have almost zero retained clamp load because the alum clamped component will contract so much.
Trying to strain gage a #6 screw is going to be about like trying to glue postage stamps onto toothpicks. You might want to think about getting a longer screw and putting a force washer into the joint, it will be easier and probably more accurate.
 
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