Helicoil cycle lifespan and torque specs

broke · Apr 8, 2015

I am working on a design for one of our products and was looking for some guidance on helicoil usage.

While I am familiar with making parts with helicoil specs I am not entirely sure if this application best fits the bill.

The thread in question is a M5x.8 with an insert length of 1x dia. in a 6061-T6 pipe clamp.

How many tightening/loosening cycles can a helicoil be subjected to? The product in question could see up to 100 cycles.

The second question is related to torque limits. I am getting adequate function out of 35 in.lbs. How much can a helicoil of that size withstand before it strips? I would like to include a max limit with the instructions.

Thank you

quadman2007 · Apr 8, 2015

http://www.helicoil.in/pdf/helicoil catalogue.pdf

Page 9 has your torque specs

grbl · Apr 8, 2015

The locking torque (page 9) is just how hard the helicoil resists letting the screw vibrate loose. Max locking torque on the first cycle is shown as 26 in*lbf (1.6 N*m), but that won't strip it. The aluminum threads will strip first even though they're bigger because they're that much weaker than the steel helicoil. They'll strip at the same torque as a M6.25x0.8 screw with the same actual engagement length as the M5 screw you're installing.

Milland · Apr 8, 2015

Keep in mind that a major failure with stainless inserts is galling after repeated cycles, rather than wearing out. Helicoil offers an optional material:

http://www.stanleyengineeredfasteni...oads/Gall Resistant Data Sht HC1020 rev.5.pdf

that is inherently anti-gall, and will help prevent seizing of the fasteners, with possible unwinding of the insert when someone tries to remove the screw.

On a practical level, as long as the surrounding material is strong enough to bear the load of insert/fastener tightening, 100 cycles (non galling) will be no problem.

broke · Apr 8, 2015

Thanks for the reply.

Quadman, I saw that also and figured it was different information then I was looking for.

Speaking of galling, will dissimilar stainless types gall the way similar would. For instance, would a 316 bolt gall with a 18-8 nut?

Thanks for taking the time to answer.

grbl · Apr 8, 2015

18-8 is 300 series, so they're similar enough to gall readily. A difference in hardness helps (≥4 HRc apart, 20 recommended), but it's not a 100% solution. It's not a high-load application, but the friction from self-locking inserts promotes galling. You might get away with something like never-seez silver gray. Vacuum/medical/nuclear applications that can't have grease use platings like silver or TiN to lubricate the threads and prevent galling. Hi-temp applications like jet afterburners use dried milk of magnesia (no joke!) to prevent galling.

Depending on your application, the anti-gall inserts Milland suggested are probably your best bet.

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Helicoil cycle lifespan and torque specs

broke

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Milland

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broke

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grbl

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