Weld Analysis for support hitch

[bricklayer]

As shown in the attachment I have a hitch which will be welded to a 1/2” plate. The max force the hitch would see is 1500Lbf. The position of the force will induce a bending moment. I have been going though my Shigley’s book and the attached calculation is what I have come up with. I am really unsure if it is even close to being correct. So I was looking for some feed back as to if I am on the right path or not. The hitch is to have a 1/4” fillet weld around the base. So I calculated the moment of inertia of the weld using the line method as described in Shigley’s. My first question is do I need to rotate my bending stress element using a Mohr’s circle like I did prior to adding it to my stress caused by the tension force or do I just add the two and do a Mohr’s circle only once? My next question pertains to the base material. I know the allowable is 0.4 * the yield strength of the material but what area do I use, can I use the full leg of the weld or do I use the throat? And then what force do I use, do I do a P/A+MC/I? I have asked these questions to some of the guys I work with but they always use some rule of thumb fillet size and never question it, where as I want to fully understand it. Any help is greatly appreciated. Thanks in advanced.

 

[desertfox]

Hi bricklayer

I used the "treat weld as a line" method from Machine Design by Schaum's Outline series.
I am not sure why you are doing Moh'r stress circles?
Anyway I have attached a file of how I would do your problem.

regards

desertfox

 

[bricklayer]

desertfox,

Thanks for the response; I like your methodology it’s simple and easy to understand. When I get into the office tomorrow I will give it a shot.

The use of the Mohr’s circle came from my Shigley’s book. From what I understand it assumes that the weld should fail in shear, along the throat plane. Therefore you calculate your stresses and put them into a Mohr’s circle to get your max shear stress. Then you compare that to 0.3 * Su (Ultimate Strength) of the filler material. I got the allowable (0.3*Su) from some AWS standard.

Have you or anyone ever seen a weld calculation done this way?

Thanks

Bricklayer

 

[racookpe1978]

I'd check (double-check ?) your assumptions of a load.

You're using 1500 lbs as a load on a "hitch" - as if it were a trailer or towing (pulling) point.

Are you comfortable with that (maximum) load - a steady pull on a known load (like a come-along or chain fall) is (somewhat) predictable - as long as the mechanic obeys your posted load limit sign at least.

(A library shelf would be predictable too - until somebody climbs the shelf to put a book away on the top.....)

But a trailer hitch on the road or shop conditions should allow for "g" forces from shock and bounce - even unexpected ones like hitting a curb or hole that might easily triple your "normal" tongue and pulling weight.

 

[bricklayer]

racookpe1978,

That 1500 lbs load is the load considering "g" forces and shock.

Thanks

Bricklayer

 

[Tmoose]

Not to muddy the water, but I spend half my time thinking about what I'm welding to.
If its a 2.5 inch square tube then the ~1.25 inch long welds inline with the top and bottom sides of the tube are the only ones capable of transferring much of the load.

I suspect the gusset that appears here above the receiver resulted partly from that concern.

http://www.etrailer.com/mm5/graphics/00000001/ct/31367_2_500.jpg