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Can fatigue be estimated with accuracy

Can fatigue be estimated with accuracy

Can fatigue be estimated with accuracy

(OP)
Hi all,

I'll post this thread in the Materials forum as I am convinced that my problem is not structural in nature.

We have an overhead monorail conveyor which uses S4 x 7.7 C-1045 rail.  The rail is supported every 9'-0" with a vertical hanger and clip such that the clip is welded all round to the top of the S4 giving 9" of 3/16" fillet weld in tension. The vertical leg of the clip is welded to the hanger with a total of 5" of 3/16" fillet in shear and across the top for 2" of 3/16" fillet in tension.

Hanger is L 3 x3 x 1/4", clip is L 3 x3 x 3/8 A36.

The C-1045 rail splice is a 3/16" butt weld all round , ground flush and has a L 2 x 2 1/2 x1/4" strongback, long leg vertical, stradling the joint on the top flange. Also welded all round.

Occasionaly we are having weld failures at either the rail splice or at the hanger clip.

When the construction work is done during plant down time, the welders (all certified) are monitored fairly closely and all welds are inspected by an independant inspection firm. The only thing I can't say for sure is if the welders are taking the time to preheat the C-1045.

This conveyor system is run on a three shift basis and load of approx 1000lbs cycles across the rail splices and past the hangers at a rate of approx 1200 times a day. No impact loads are present.

So after this long winded description, my question is: Can fatigue at these types of joints using the methods and materials mentioned, be accurately estimated so that preventative maintenance can be done or would a crystal ball be a good investment.  Maintenance do conduct periodic checks but when a crack is discovered, failure is too close for comfort. Inspection is better than nothing but a crack can develop 2 minutes after.

Thanks for any help or opinions in advance.

RE: Can fatigue be estimated with accuracy

Welding anything over 1035 CS is problematic under the best of circumstances.  I would never welded 1045 CS under a cyclic load such yours. The consequence of failure as you are seeing is too great.  1045 CS can be welded, but has to be under very controlled conditions, preheat, interpass temperatures, cooling, PWHT, and luck.  
We had quite a few miles of overhead conveyers and the prime directive was no welding on the rail itself. I fact some of the rail comes in with a don't weld sticker attached.  
It doesn't take much to cause an impact load on rail, the slightest of surface irregularities will do this.  Monorails take all types of loads from the away of the trolley  and load.

As you are using US units I assume that you are in the US and I would inform you that OSHA frowns on conveyer accidents.    

I would be hesitant to recommend any procedure for this due to consequences of a failure.  Saying this I would get with the rail supplier and see if he has a welding procedure for the rail.

RE: Can fatigue be estimated with accuracy

(OP)
Unclesyd

Thanks for the reply. I knew of the potential problems with not following the welding procedures for 1045 but this is becoming quite a regular occurance. The equipment has been up for a few years and maybe its time for a revamp. You say that the directive you had was no welding to the rail. Can you suggest the use of or know of any good suspension hardware for S shapes that would eliminate the welding.

This would be cheap in comparison to someone being maimed or killed.

Haggis

RE: Can fatigue be estimated with accuracy

Here two people that we delt with.  I haven’t had time to check out the sites but I think if you look at the American Monorail distributers one of them at one time had a lot of ancillary equipment.
The other site had a little bit of everything.  Like I stated before it always pays to make contact with a body.  They may not have what you are looking for, but will probably steer you in the right direction.  They were several others that I maybe able to come up with a lead.

www.tcamerican.com

http://www.jervisbwebb.com/jbw/jervis_webb_homepage_def.htm

Lets stay on the safe side and work around the cracking
problem.
As I like to say: WE HAD SOME NEAR HITS.

RE: Can fatigue be estimated with accuracy

Fatigue can be well predicted in lab samples, but not too well in the real world.  The natural variations in geometry, weld profiles, material conditions and such will create so much variability that forcasting will not work.

I would rather see these rails hung from mechanical fasteners than welds.  Bolts are easier to control and inspect.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Can fatigue be estimated with accuracy

(OP)
Gentlemen,

Thanks for the replies. Done quite an extensive search for bolted rail splices and only found one company that used to manufacture them but are now discontinued. I think I will work on one which is adaptable to both splice and hanger locations.

Haggis

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