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Checking bolt torque

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Gilles.Thesee(at)ac-greno
Guest

Posted: Mon Nov 27, 2006 3:59 pm Post subject: Checking bolt torque

Hi all,



There has been considerable discussion on the Jabiru Yahoo list as to 

how to check torque on critical flywheel bolts.

The owner/mechanic is suppose to regularly check those bolts tension, 

but they are bonded with Loctite 620.

How would you experts proceed ?



- Just apply torque wrench and check for movement ? (But remember there 

is Loctite)

- Unscrew, extract, clean Loctite, reapply Loctite, reinsert and torque 

to specified value ?

- Put red paint on bolt heads and flywheel and just check if nothing 

moved ? (but what if the preload relaxed, the bolts stretched, the joint 

fretted ?)

- Etc..



I posted some info on bolt torquing and the Jabiru SB which started it all :



http://contrails.free.fr/engine_jab_bolts.php



Any qualified input appreciated



Regards,

Gilles Thesee

Grenoble, France

http://contrails.free.fr

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Bob H

Joined: 01 Dec 2006
Posts: 43

Posted: Sat Dec 02, 2006 9:46 am Post subject: Re: Checking bolt torque

I would use your second procedure of removing bolts, clean threads of any residue and reinstall to correct torque, using suggested Locktite. 

It is more common practice in US to install bolts dry as torquing dry provides a more uniform/repeating loading and bolts used a cadmium plating to provide lubricity and corrosion protection. However, many fastener suppliers have been forced away from cad plating because of toxicity issues in the plating process.



When you add a Locktite to threads, friction is reduced and bolt receives higher loading for a given applied torque. Jabiru must have accounted for this higher loading when making that recommendation. 



Taking a tensile bolt to 90% of Yield is a risky business in that the variations in thread geometry/hole tolerance and friction and torque wrench errors can easily cause overtorquing. We usually select a torque value around 60% of Yield to provide sufficient margin for errors. If a situation requires higher clamping forces, a larger dia fastener is chosen. With hardware already in service, installing shear pins is a way to takeout that loading and reduce bolt torque to a more reasonable level.



It would be interesting for Jabiru to indicate what the mode of failure has been on those bolts; a shear overload or tensile fatigue. And the bolt pic showed failure in threads which can indicate that threads are overloaded and geometric stress concentrations from thead form contribute to failure. You don't want threads in bearing and the bolt grip length is sized to carry shear loads on smooth shank, never in threaded portion.

Bob H

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