Standard HeliCoils Untreading Resistance

[paulcook]

We are putting standard stainless steel HeliCoils into aluminum castings. Thread is 5/16-18.

We specify that the installer use medium strength (blue/242) Loctite, torquing to 125in-lbs.

If the installer re-adjusts the product after the Loctite cures, is there a chance the HeliCoil comes out with the bolt?

I did a 72 hours test and it seemed fine (using both primer + Loctite 242) and it wasn't an issue. I applied the Loctite quite liberally, but not sure how much a contractor would use.

In your expereince, is this an issue?

 

[Tmoose]

Are you expecting the Loctite to function as a thread locker after the "installer re-adjusts the product " ?

Helicoil locking inserts might be useful for fasteners to be tightened.

https://www.stanleyengineeredfasten...-Inserts/Helicoil-Tanged-Screw-Thread-Inserts

However, if the screw's function is maintaining an adjustment ( like idle speed adjustment on a carburetor) wicking Loctite 290 has always worked slick for me.

 

[drawoh]

paulcook,

I have been specifying helical thread inserts for around forty years now. I don't recall one coming out with the screw.

--
JHG

 

[paulcook]

Thanks @drawoh - I guess the concern is the bolt will have threadlocker on it, but the Helicoil will not. If ever the contract wants to back out the crews, we want to make sure the Loctite won't take the Helicoil with it.

I did contact Stanley and they said it should not be an issue / They can treat the HeliCoil like threads in the base material. They're designed to expand and grip to resist loosening (or something).

 

[3DDave]

The pitch radius of the helicoil contact is larger with the bulk material than it is with the screw, so there is an area and leverage advantage. The disadvantage is the cure between the helicoil and the aluminum might not be as good as between the screw and the helicoil, but you could install the helicoil with the good stuff and some accelerator/primer and let that cure first, but unless this happens enough to be annoying then it's not worth the extra effort.

 

[TugboatEng]

Helicoils are great. If you're experiencing negative results you're doing something incorrectly.

Firstly, the helicoils needs to be installed below the surface of the hole. Give 1 full thread below surface at a minimum. This will help prevent the coil from backing out with the bolt.

Helicoils leave lots of room for air. This is not compatible with anaerobic thread lockers.

Why do you think you need a thread lockers in the first place? Consider better design of the joint first.

There are helicoils available with locking style threads if you still think additional locking is necessary.

 

[paulcook]

Thanks for all the feedback guys. The test we did (primer + 242) locked the parts effectively. Removal did not take the Helicoil with it.

Think we're all set to remediate and ship.

FWIW, did some torque tests and both the Helicoil and properly tapped threads in our ADC12 castings did not strip at 440 in-lbs (max of our torque wrench).

The improperly tapped holes (allowed for Class 2B no-go entry, and was still loose) stripped out at 120 in-lbs.

 

[mfgenggear]

Paul Cook

Like every one else said , helicoil provides a procedure and sells the correct tooling for installation. The lower strength loctite works very well from preventing screws , nuts or bolts vibrating loose.
The issue is aluminum threads can easily be damaged if over torqued. So i would recommend to to do several test and inspect and record.
Then set a procedure for installation in addition to helicoils procedure. It is very very easy to damage aluminum threads. And after a few disassemble and reassemble then may be some failures. It's the nature of t he beast.
As a part time mechanic I have had helicoils back out. I just added new ones.

 

[paulcook]

As a part time mechanic I have had helicoils back out. I just added new ones.

frequently? Not really something we want to put on the contractor.

 

[mfgenggear]

If the contractor over toeques the bolts it will fail in the field. Then it becomes a problem.
Just run torque test an eval and set a torque value.

 

[hydtools]

I've used Helicoils with screws in cast A356 and wrought 2024-T4 aluminum with no problem. The tang on the trailing end of the Helicoil does its job of biting into its tapped thread to keep it in place. Can you use the thread-locking Helicoil instead of Loctite?

Ted

 

[mfgenggear]

Hydtools and Paul Cook
It is correct helicoils are way better performing threads. I repaired many tool cast fixtures With stripped out bare threads.
Just make sure the contractor is using the special go and no go gages for the required helicoil size and the correct torque values.