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Re: welding nuts follow-up

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On 6/20/06, chuck utzman <chuckuc(--nospam--at)pacbell.net> wrote:
Why not use Loctite?
Chuck Utzman, P.E.


Daryl Richardson wrote:
Fellow engineers,
 
        I can see two reasons to tack weld the nut in place: to prevent the nut from turning while you turn the bolt from the other side; and to prevent the nut from falling off or out of alignment while you try to install the nut from the other side.
 
        Thinking outside the box, there are a couple of solutions to both problems.
 
1.)  Put a spot weld on the base metal beside the nut.  This would not affect the nut but would prevent it from turning.
 
2.) Tack weld a small tab of steel to the main member beside the nut to prevent the nut from turning.
 
3.) Stick the nut in place on the back of the main member with silicone calking.  This will keep the nut in place while you thread the bolt through from the other side.  It may not prevent the nut from turning and it certainly will not resist any serious shear load applied to the bolt.
 
4.) You could probably stick on (with silicone calking) both the nut and a small bar of scrap metal jammed between the nut and, say a nearby beam flange, that would even resist serious bolt torque.
 
        Just a couple of thoughts.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
From: Jim Getaz
Sent: Tuesday, June 20, 2006 6:06 AM
Subject: Re: welding nuts follow-up

            Steve Gordin said in part, "My feeling is that the heat of such tack welding should not be able to affect the properties of a relatively massive nut."

 

            Steve,

                        Even a tack weld melts the base metal. Just because the rest of the nut is does not become molten is immaterial. The properties of the nut have changed. The metallurgical notch that may have been created can propagate into the rest of the nut.

                        A tack weld is the worst kind of heat input into a piece of metal. It is more akin to an arc strike than a weld. It heats a small section of the metal then that volume cools as rapidly as it can, because it is the only part of the metal that was heated. This rapid heating and cooling can cause cracking. The reason for preweld and postweld heat treating is that they help the metal maintain a consistent crystal lattice, with less interruption by the molten weld or even the changed properties of the heat-affected zone. A tack weld has the opposite effect. And that is the reason that tack welds are to be incorporated into a qualified weld.

                        I have not gone back through the entire thread to see the main thrust of your posts, but the statement above got my attention this morning.

            Jim Getaz

            Precast Concrete Engineer

            Winchester, Virginia



Chuck-

I was going to suggest one of the Loctites but I don't have any experience with it on large very fasteners in a less than very  clean shop environments. 

Loctites are typically anaerobic adhesives that stay liquid in the bottle but harden is the nut/bolt spaces.  I do know from experience that unclean threads can prevent Looctites from curing.

Loctite primers are available & they really improve cure time & stength but their use make it a 2 step process; primer takes about a very minutes.

I did a quick search & they have a semi soid product that might be better for this app.

http://www.loctite.us/int_henkel/loctite_us/index.cfm?pageid=449&layout=2

I'm glad you brought up possible alternative to tack welding since the consensus seems to have been "tack welding not a great choice"......

How do you feel about mechanical staking with a punch & hammer?
I have typically used either staking or Loctites but again in a more controlled environment.

cheers
Bob