An air gap between helicoil and mating part
An air gap between helicoil and mating part
(OP)
Hey everyone,
I have 2 mating parts, and the interface is a sphere surface.
In the lower part there are helicoils (marked with red pen), and for creating an axisymmetric 120 deg chamfer for installing the helicoil there is a need to create a small planar face as shown in the picture (a small planar surface above the helicoil).
As a result the helicoil isn't right below (and hence "supported" by) the upper part's mating surface, but it is a little bit deepened, and I am wondering if it may lead to a failure.
I haven't encountered such a design, and hence need your help, experience and opinion.

Thanks!
I have 2 mating parts, and the interface is a sphere surface.
In the lower part there are helicoils (marked with red pen), and for creating an axisymmetric 120 deg chamfer for installing the helicoil there is a need to create a small planar face as shown in the picture (a small planar surface above the helicoil).
As a result the helicoil isn't right below (and hence "supported" by) the upper part's mating surface, but it is a little bit deepened, and I am wondering if it may lead to a failure.
I haven't encountered such a design, and hence need your help, experience and opinion.

Thanks!
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
Cheers
Greg Locock
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RE: An air gap between helicoil and mating part
well, it depends. the gap causes a bending moment on the fastener under a shear load. so it all depends on the loading level on the parts, the resulting bending moment on the fastener, as to whether it will cause a failure under whatever loading you have.
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
Ted
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
I don’t think you will have a problem, the helicoils don’t sit flush with the surface they are inserted into, see link below.
They also incorporate a locking feature which prevents the insert from turning when screwing in or out. What you do need to pay attention to is the preload tension you can apply via the bolt on assembling.
https://eshop.boellhoff.fr/out/media/pdf/prospekte...
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: An air gap between helicoil and mating part
3D Dave - you wrote something I am not sure I understand. Why does the gap increase the amount of the extension of the screw? The extension depends on the preload, that is the result of the applied torque (and the friction coefficient that in most cases is unknown and evaluated).
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
Ted
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
couple of suggestions, spot face lightly, don't need to counterbore as deep as shown. so that the machinist has a square face to drill and tap.
make sure there is plenty of clearance at the bottom of the tap drill, and tap. might consider a keensert instead of a Heli coil. I use to have aluminum plate
tool plates for aircraft replace with keensert because the threads would tear out. aluminum is soft and does not take much torque to strip.
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
I also prefer using keensert when the insert is installed in aluminum part.
I read some articles which stated that using a keensert in Ultem 1000 may lead to a catastrophic failure of the thread's entire surroundings, while using a helicoil is a better option in which the failure will be predicted and limited to the thread itself. I can't tell what is the root cause (I think it was written that the cause is a stress concentration) of this difference, but only to say I read there ia a difference.
RE: An air gap between helicoil and mating part
quote: Key-locking inserts are threaded into a pre-tapped hole, and then the keys are hammered into grooves through the threads, permanently locking the insert in place. Their “keys”, which are driven down into the tapped threads, provide a mechanical lock against rotation, especially when the mating stud or bolt will be removed frequently.
Why use a Key-locking insert vs. a Helical wire insert?
There are three main differences between Key-locking inserts and Helical wire inserts:
1) Strength: Key-locking inserts are stronger than Helical wire inserts
2) Ease of installation: Key-locking inserts are easier to install than Helical wire inserts
3) Price: Key-locking inserts are more expensive than Helical wire inserts!
So here is some direction on how to select the correct threaded insert for your application: In heavy wear, high vibration and high heat situations where saving space is not a concern and hole depth is limited, Key-locking inserts are the best bet.
When reducing costs and minimizing space are priorities, Helical wire inserts are the way to go. They are particularly useful for creating permanent strong threads in softer materials such as aluminum, titanium and magnesium alloys, and are best suited to lower heat and lower torque environments.
RE: An air gap between helicoil and mating part
do you have white papers on the subject
supplier site
https://blog.mfsupply.com/2020/02/21/heli-coils-vs...
RE: An air gap between helicoil and mating part
If a Helicoil insert 1.5D long is used, along with a fastener with full thread engagement, the lower part's material's strength will not be much of an issue.
If any compromises are made on the depth of the hole tapped for the Helicoil, the deepest coil may be distorted and small, and create BIG problems when the bolt/screw is inserted at assembly and in the future.
RE: An air gap between helicoil and mating part
mfgenggear
Is using threaded insert in Titanium (6Al4v) a common practice? I designed custom T nuts for structural elements, and never used any threaded insert. My design was formally reviewed, checked and approved by several experienced engineers and no one suggested it.
Is using a threaded insert in titanium recommended as in the case of aluminium?
RE: An air gap between helicoil and mating part
One use for threaded inserts is they can incorporate self-locking features and can provide a more favorable dis-similar metal contact, particularly key-locked inserts, which can be installed with corrosion control materials.
RE: An air gap between helicoil and mating part
I just asked mfgenggear why he said he uses helicoil in titanium.
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
RE: An air gap between helicoil and mating part
By the way, I also want to use Nitronic 60 helicoil, so I will not be sensitive to which screw will be actually assembled and be afraid of cold welding. Unfortunately I found out that this type of helicoil isn't on stock and there is no even one manufacturer that agrees to buy them as long as he has to order less than 2000 pieces.
RE: An air gap between helicoil and mating part
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