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





RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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) ?
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
So I'd say the stiffness needs to be calculated accordingly.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
So I would use the frusrtum cone, angles, etc.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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 ??
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
+185F is not high temp for AL 6061
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
I found some information on helicoils see link below:-
http://www.noblefix.com/PDF/Helicoil/HeliCoil-Cata...
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
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
If Emhart does not have what you need regarding operating temperature you may need to perform your own test.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
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.
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
RE: Bolt design with heli-coil insert: stiffness of a joint with Heli-Coil type insert
http://www.vishaypg.com/micro-measurements/stress-...