Tack Welding Bolt or Nut in confined space (Fully grut)

[bmw318be]

Hi,

Refer to attached photo

https://www.dropbox.com/s/g6bwtbapkxn8mjr/Bolt and Nut.png

I need to decide whether to tack weld a hex bolt head or hex nut ? . The reason being is the design of mounting bolt is meant to be drop down with hex head at the bottom, however it seems there is accessibility issue for the maintenance to access the spanner from confined space from bottom. Furthermore, this skid is going to be fully grouted with the foundation.


I am considering the following option:

1. Tack weld the head from the bottom
2. Tack weld the Nut from the bottom

Option 1: It was preferable to just do a tack welding, I am intending to do 3 point tack weld.

1 where should be the location of tack weld on hex head to prevent snapping during torquing?

2 Do you think the tack weld could alter the structure of the Mild steel ?


Option 2: Tack weld the nut from the bottom

Issue: risk of splatter ruin the thread, If this has to be done, we are going to tighten bolt and nut, after that maybe use 1 more nut to cover the threads.


In this case, if we are fully grout, We are going to have issue of breaking the thread too, as bottom of the C channel would be fully grout and the thread could have come into contact with the grouts.Upon grouting, if there threads ruined, there is difficulties in replacing the nuts. What is your view on this.

if nut at the bottom, should we full weld or tack weld few points ?

Option 3 : No weld at all just fully grout the Hex head ( 1 inch thickness to the fully grouted), however I do not think, there is sufficient strength to hold tight the bolt preventing from rotating upon tightening the equipment

What are the considerations for welding 2 materials, My skid is mild steel and the bolt and nut is galvanized steel.

 

[connectegr]

What is the bolt grade? It is not recommended to weld quenched and tempered material.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[bmw318be]

Hi,

The bolt is Steel grade High Tensile 8.8.

I wonder, if I have to tack weld 3 points, would there be enough strength when tightening the head from top. The concern is if the Nut at the bottom would be buried by grouting.

 

[connectegr]

Depending on the weld quality, they should be more than sufficient to tighten the bolts. However, if the nuts are quenched and tempered, the resulting heat input can result in a brittle material. If the bolt is loaded in tension or vibration the result may be a crack. If the forces are shear only, this may not be an issue.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[bmw318be]

Actually I would prefer Tag weld the hex head from botom
Instead of weld the nut.

My objective to weld the nut so it would not rotate as we tighten the bolt, i believe the torquing of the bolt to tie down the equipment would not break the nut or snap ?

By the way is there a cap nut of M 20, M22 M24 in the market so I would probably bolted the nut and then weld it to prevent splattering to the threads

 

[Tmoose]

How much of the equipment is going to be installed and aligned on the skid before the gearbox hold-down bolt nuts or bolts are welded?
I would worry a lot that When it comes time to align the components to each other some components will be found "bolt bound." So I would provide bolting details that allow extra motion well beyond the assumed true position.

Are there going to be any web stiffeners near the equipment feet?

http://www.steelconstruction.info/File:R19_Fig4.png

If not the hold down bolts can peel the top flange of the channel upward fairly easily.
Similarly It appears the gearbox manufacturer may be expecting a full length flat surface under each edge of the gearbox. As shown it will be supported essentially on knife edges (the channel webs).
Creating a flat surface for each component often requires machining the top of the skid.

Protecting the anchor bolt threads and providing room for future tightening is on often overlooked detail.
"Cap nuts" ( acorn nuts ) limit the length of the bolt that can stick into the nut. The acceptable range of bolt length may be smaller than the increments of commerically available bolts, so extra washers may be needed under the bolt head. Even worse, the bolt may bottom in a rugged nut and feel "tight" before it is actually clamping the component to the skid. The same thing can happen when the anchor bolts end in grout.

 

[racookpe1978]

My experience tack welding matches the above: Expect the entire hex head (nut OR bolt and part of the threads) to turn red-hot and then cool off quickly. Its thoroughly annealed, you just don't have any control over how fast it got heated, how hot it got, and how fast it cooled off. 8<) You've also lost the "stainless steel" rust resistance you may have had.

But, if you tack weld the nut, then the bolt (the thin threaded stem) threads are not also annealed and will be less likely to break off under load and later vibration. On the other hand, you have to position accurately! the nut around the drilled hole and hold it square and flush to the underside of the surface while you tack weld it in place. Also, you need to protect the nut hole from sparks and weld debris so you can later assemble the parts.

My recommednation: If this were built US ANSI threads, get a longer heavier nut (probably also available in metric) like a heavy hex nut so you have more strength after your heat mis-treatment. Stick a threaded rod through the hole, into the heavy hex nut and pull up firmly. This positions the nut against the mating surface. Tack weld twice - not three times - but DON'T use a "ball" or "bubble" weld: go just a little longer on each side 8mm long x 5 mm leg lengths for example. The weakest part of a tack weld is the start and stop, not the middle.

Then unscrew your temporary rod from the nut. Paint thoroughly after assembly.

 

[racookpe1978]

Buy the way ....

Your expenses will be labor and operations: aligning and drilling the holes, positioning the nut (or the bolt) and holding it firmly in the right place while you tack weld; the tack welding itself twice for each nut (or bolt); body movement and welder repositioning time; threading the temporary rod, etc.

The price of a slightly larger nut and bolt is very, very small!

 

[gtaw]

Why not make a small bracket that captures the nut when the bracket is positioned and tack welded? No damage to the fastener and some movement permitted for proper alignment. Once the fastener is snug tight, it is not likely to rotate freely as more torque is applied to fully tighten the fastener.

Best regards - Al

 

[bmw318be]

Tmoose,

Yes, you got the points, the equipments gearbox, pump and motors are needed to be aligned horizontally and vertically, When the design of my equipment are the equipment holes is bigger than the bolts assuming

Gearbox Bolt hole M 22, I would use M20 Hex Bolt

The Channel drilled hole is also bigger M 22, in the first place, I would have more horizontal adjustment for alignment. If I weld the Nut or hex head from bottom Channel, I would probably lose the horizonal adjustment for my gearbox. However I would still rely on equipment bolt hole which would be bigger than the holding hown bolt.


My concern if I have to weld the Nut at the bottom, in the case of equipment gearbox need to be shifted, it would have bolt bound and I would not be able to force the bolt, if I have too, the hex bolt would be bend toward the required side and this I believe would ruin the thread in the long run


However, If having tack weld the hex head from the bottom of C Channel, I would have a more secure in any case of the equipment would be shifted to one side, I Do not think It would ruin the Bolt which would already become anchor bolt after fully grouting.

is having a nut on top of equipment is a better option here ?

I would also having another concerns, since the equipment has been aligned, the gearbox hole may be shifted further from center of drilled hole, as a result if I have to weld the nut or hex bolt, I would not be able to weld center now


I have seen some a good welded nut where it has a flat and parallel

https://www.dropbox.com/s/yzi3isjx6i3k3po/Photo 7-8-13 4 17 01 PM.png

https://www.dropbox.com/s/1pkzik5498vb1om/Photo 7-8-13 4 44 11 PM.png

 

[bmw318be]

gtaw,


You make a good point and idea, are you suggesting using this kind of design where the nut is at the bottom

https://www.dropbox.com/s/4fr1wh2nf8bonea/Nut would be housed inside the brackets.docx

I would have love this design, but this equipment are going to be outdoor, so they going to fully grout under the equipments to prevent water going in , All hollow area under the Skid would be grouted and I would not be able to access anything under, That is why now I have to weld it

 

[Tmoose]

I think I'd drill and tap a mild steel bar that was 2X as thick as the bolt diameter. 2X thick First so the pull out strength exceeded the bolt strength by a lot. Second to buy some tolerance for protecting from grout any extra bolt length that might be needed.
The length would need to be sufficient to ensure it bumps up against the channel web and resist torque
I assume the channel is a commercial section with tapered flanges so a tapered washer is needed to make the hole perpendicular to the underside of the flange, and so the block will touch the web not a radius.

http://ecx.images-amazon.com/images/I/41BctLwfxqL.jpg

I'd tack weld the tapered washer while clamped tight to the under side of the bar.

Or if there are no web stiffeners (which I think are probably needed) I'd make a "nut plate" for each pair of mounting feet.
Kind of like this, but longer.

http://www.currieenterprises.com/cestore/images/product//currie/ce9900xjp.jpg

If it needed to be simplified I'd use a small channel (maybe 3 or 4 inch / 75 or 100 mm ) and 100 mm longer than equipment mounting hole spacing center to center, and drill oversized holes at the foot center to center, and weld a short chunk of tapped bar to each end of the channel. Tapered washers would still be required to provide a flat seat when tightening.

 

[gtaw]

There have been several good comments made with regards to the welding of a hardened nut or bolt. I have to imagine you have selected the high strength fasteners for a reason, i.e., strength. Welding the nut or the bolt will compromise the strength of the component welder. The hardened fastener components more than likely tempered Martensite. This material is sensitive to hydrogen cracking if not welded with a low hydrogen welding process/procedure and allowed to cool slowly. However, the steps taken to prevent hydrogen cracking are going to reduce the strength of the component that is welded (tack welded or continuous weld makes little difference).

As noted in another post, if you are constructing the subframe using standard channels, the flanges will have a taper that must be accommodated by bevel washers. The washers are cast material that may not take to welding willingly. Again, there is a possibility of cracking immediately after welding or while tightening the bolt.

Welding either the nut or the bolt compromises the material properties of the fastener assembly and provides no adjustment for alignment purposes.

A small metal bracket would capture the nut or bolt head. The bracket can be welded to he frame without affecting the properties of the fastener and it would permit some limited movement to align the gear box. You will still need a bevel washer to accommodate the tapered channel flanges.

I can e-mail you a sketch, but I don't know how to attach a sketch to a post.

Best regards - Al