Heli-Coil Is either side ok?

[LuckyDuc]

I know Heli-Coil inserts are usually installed flush with the mating surface. For strength considerations, I want to keep as much unthreaded bolt shank as I can through the joint. Is it acceptable to install the Heli-Coil on the far side of the joint, basically like a nut? I do not see anything in the Heli-Coil literature and specs that forbids this practice.

Thanks!
LD

 

[CoryPad]

Are you joining one piece with a through hole to another piece with an internally threaded hole? Are you proposing that the internally threaded hole will have some unthreaded region towards the top so that a screw/bolt with full shank diameter will not interfere? And that a screw thread insert will reside at the bottom of the hole? That sounds fine.

 

[hydtools]

Install the Helicoil as deep as you like and still maintain enough screw thread engagement in the Helicoil. Installation of a Helicoil is usually below the surface so that the edge of the Helicoil will bite and not come out when the screw is removed.

Ted

 

[LuckyDuc]

Thanks guys, that is exactly what I propose.

 

[Metaljon]

I would be very careful with installing the helicoil as you have proposed. I seem a number of accidents where the helicoils pulled out as result of insufficient thread engagement. The design you are using would be very weak. One case I was involved where the transmission casing separated from the engine as a result of helicoils being used resulting in a fatality. Helicoils are a last resort and should not be used where significant loads exist. Instead of helicoils a better fix is to install a bolt, cut the head off and then re-drill the hole.

 

[racookpe1978]

Metaljon _ Not sure I understand your recommendation; I don't think the original poster intends on "refilling" the drilled hole and re-drilling it, but does want a bolt going through both pieces of metal. That assembly bolt, however, is to penetrate the lower piece with a section of un-threaded shaft, then the threaded end of the bolt is to engage in the helicoil.

Could the lower part be tapped only only the bottom, rather than deliberately require a helicoil be fitted and threaded all the way deep into a part? If (during fabrication) any one of many helicoils being threaded into these deep hole gets jammed or breaks off or gets cross threaded, you've lost the whole part. .

 

[MikeHalloran]

Helicoils are typically installed in a blind threaded hole, and trapped axially by whatever part is being retained.

In the case of installing a helicoil at the far end of a through- threaded hole, there is some risk that the helicoil will screw itself out of the far end as the mating bolt is inserted and screwed in, so the helicoil probably should be installed just a little shy of the far end, and retained by staking the thread so the helicoil is trapped.

Or tap the hole not quite all the way through.



Mike Halloran
Pembroke Pines, FL, USA

 

[LuckyDuc]

This application is for new design, aircraft. We are using the coil for its self locking feature. You can't tap a self locking thread and we do not have room for a nut. I finally got in touch with the folks at Heli-Coil and they said that this application is perfectly acceptable.

Thanks!
L

 

[hydtools]

Have you considered Spiralok threads?

http://www.spiralock.com/

Ted

 

[mfgenggear]

luckyduc

you may want to try a few test, then make up your mind.
I am with Mike on this one. I believe this can be a problem.
I have had helicoil's physically unscrew.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.