Welding on rectangular tube used for machine chassis

[najeral]

I'm looking for some referance information for welding on Rectangular tubes that are used to make a machine chassis. I've found some information on "C" channel used for the same purpose but nothing on rectangualr tubes. One specific question I have revolves around the effects of welding pads along the radius of the tubes rather then on the face. In "C" channel I know you should avoid it but I wonder if the same applies for tubbing.

 

[Ron]

If the chassis is subjected to vibration or cyclic loads, weld location should be chosen carefully. The only real issue I see with welding pads at the radiused corners is an odd weld profile that can cause stress raisers (risers).

 

[najeral]

On "C" channel chassis we've always avoided placeing long vertical welds along the face of the channel due to the effect the heat has on the channel. With a tube does this still apply or the fact that there is two faces change the effect. I understand that in extreme load applications this may be a contributing factor but is it allways the case.

 

[Compositepro]

I think you are asking about warping effects rather than structural effects. You should probably ask that question in the welding forum. I think it has more to do with welding technique than shape of the steel.

 

[Ron]

If your material is mild steel, don't worry about it. Weld heat affects are negligible. If aluminum, you might have an issue. Stainless steel structural effects are negligible but corrosion affect can apply.

 

[najeral]

Thanks, I appreciate the quick responses. The frame I'm writing in referance to had been tested & certified for a specific load and purpose. My original design was then hacked up and used on another machine with diffrent load arrangments. That machine had stabablity issues and large deffelctions throught the frame. The deffelections where attributed to the way I welded mounting pads to the beams. Hence the question on the vertical loads. From my point of view it seemed apperant that the original design was not intended to carry the loads that were applied to it. However, experiance seems to dominate test results and I'am now in a sittuation were I'm diffending my original design. My main concerin is that, other then physical test results, I don't seem to have any other means to back up my original design. If you guys have any suggested reading material on frame and chassis design. I would be more then greatful.

 

[dhengr]

najeral:

Your understanding of the whole machine chassis (machine base frame) design problem is somewhat suspect. And, the way you asked your original question and then respond to Ron and Cpro doesn’t add to the confidence level. You claim to have an original design which was tested and certified and apparently worked for its intended purpose. That is your defense, “the proof is in the pudding” so to speak. To call it a real design, you should have: analysis and design calculations, including weld sizing, secondary stress effects, etc., fab. drawings and the like. The only difference in the whole design process when using two different shapes for the members is that each shape may have a few of its own characteristics which must be accounted for in the design. Then you claim it was tested and certified for its specific load and purpose. What better position could you have to defend. Cobbled together and a real engineered design are two different things, and cobbled together is tough to defend as good engineering. Then, if someone else chopped up your design and turned it into something it was never designed to be, and it doesn’t work, you certainly don’t have to defend that. Let the guy with the ax defend his own work. Your’s supposedly worked.

As to your question about ‘welding pads along the radius of the tubes,’ are ‘pads’ the reinforcing plates that transfer the machine load to the frame and the frame load to the foundation? These pads must be thick enough so they are not a bending problem, either in themselves or as they transmit load to the shell of the tubing. They should be wide enough with respect to the width of the tube face they are attached to so as not to induce excessive bending in the tube face, and so as to transmit their loads directly into the tube webs. They should probably not be as wide as the tube, because the longitudinal weld btwn. the plate and the corner radius of the tube is very difficult to make. The root pass on the longitudinal weld is almost always of inferior quality, particularly when considering vibration and fatigue. Make the pads just slightly larger than the flat width of the tube so that you are assured the welding process you are using will provide proper penetration at the root of the weld. Then provide a fillet and a weld reinforcement out onto the radius to transmit the load into the webs. Do not weld around the corner at the end of the longitudinal weld, stop the weld short and back-weld to fill the termination. Welding around the corner will likely cause undercutting on the pad.

 

[Ron]

dhengr is correct...read his post again.

In addition, you are confusing parameters. As dhengr noted, you have both structural and thermal affects. The thermal affects are negligible. The structural effects can be, as long as you accommodate the machine vibration and cyclic affects.

 

[paintballJim]

I have a simular question. I am building a frame of 2x2 tubing and would like to place a vertical 2x4 tube between two horizontal 2x2 tubes. Some nagging memory in the back of my head tells me not to weld the face of the 2x4 to the corner of the 2x2 as it will cause the 2x2 tube to crack. Is this right or am I ok if the weld is treminated 1/2" from the ends and a 1" weld is centered on the ends of the 2x4?

 

[paddingtongreen]

paintballJim, There is no contact between the faces, the outside radius is normally twice the thickness of the tube, leaving a gap to bridge unless you are shaping the end of the 2x4.

It is not permitted to weld the crossing corners when one tube is in crossing contact with another, in that case, only the flats of one can be can be welded to the flats of the other.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paintballJim]

Thank you. I understand the gap issue and the 2x4 can be shaped to allow contact. I am more concerned about placing a weld on the corner of the tubing. I am not sure I understand your term "crossing corner". Is this where the vertical corner of the 2x4 contacts the 2x2 or does this include the cut face of the 2x4 to the radiused cornor of the 2x2? I could form a "C" channel with a lip extending up the side face of the horizontal 2x2s and weld to the face but would prefer a smooth surface between the vertical braces.

 

[paintballJim]

No edit option eh? I guess the same question would come into play if I formed a channel and extended a flange up the face of the 2x2. Do I run the flange up to the cornor and weld it with a flair bevel, or do I stop 3/8" short and fillet flat face to flat face?

 

[paddingtongreen]

I don't know of any restriction except where two flare bevels would meet if the 2x4 was laid alongside the 2x2 instead of butting into it.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paintballJim]

Thank you. That is what I wanted to hear.

 

[unclesyd]

The majority of these type questions would be readily answered by using the following book.

"Design of Weldments" by Omer W. Blodgett

This is a very inexpensive ($15.00) reference book that covers the gambit of welded fabrication, especially for machines.

https://ssl.lincolnelectric.com/lincoln/apdirect/store.asp?PID=16&cat=8

 

[paintballJim]

That is a good suggestion. I will check it out.

Thank you