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An air gap between helicoil and mating part 5

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elinah34

elinah34

Mechanical
Joined
Aug 19, 2014
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149
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IL
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.
20230115_215357_bex32b.jpg

Thanks!
 
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Helicoils are not supposed to be installed flush with the surface. I don't have a standard to quote but in practice I try to install helicoils at least one full turn below the surface. This helps prevent the helicoil from walking out when the fastener is removed. It ensures the helicoil does not protrude above the surface which prevents proper clamping of the joint.
 
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Spotfacing a helicoil. Problem is that unscrewing the bolt MAY unscrew the helicoil, there's nothing restraining it vertically. Appropriate use of antiseize and loctite will sort that out. I suppose in theory the helicoil could walk out even under load. Does the big official book of designing helicoils go into this? (late edit, Tug's got it)

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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elinah34 said:
I am wondering if it may lead to a failure
Click to expand...

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.
 
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The gap also increases the amount of extension in the screw, making it more resistant to losing tension due to small surface embedment and vibration effects. There is already a gap in the clearance hole of the mating part allowing bending if the relative position shifts, so an additional slight gap isn't a main contributor.
 
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The helicoil has an edge to prevent its unscrewing and that edge must be below the surface in order to 'bite' to prevent unscrewing.

Ted
 
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This applies to both ends. Too often I see helicoils that too long and installed so they nearly/do protrude in be back side. Helicoils cut easily with a good set cutters.
 
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Hi Elinah 34

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.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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Thank you all for replying, now I am more comfortable with this design, though I would still try to avoid it if possible.

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).
 
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Imagine the gap is 0.00 inches or the gap is 1000 inches. Which will have more extension of the screw for the same tension?

 
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The screw could be longer, therefore more length to stretch.

Ted
 
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elinah34

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.
 
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Helicoils resist galling better than keenserts. They each have their advantages.
 
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Can you put a spacer (standoff) in the counterbore?
 
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mfgenggear, I didn't mention that the part with the helicoil is made of Ultem 1000.
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.
 
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tugboat
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.

 
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As drawn the tap drilled hole is not deep enough for the helicoil tap, or even a simple tapped hole.
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.
 
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