Snapped drawbar - anyone ever seen this before?

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bedwards1966
Posts: 852
Joined: Sun Mar 07, 2010 10:36 pm
Location: North Yorkshire

Re: Snapped drawbar - anyone ever seen this before?

Post by bedwards1966 » Sun Dec 02, 2018 12:53 am

joeproctor wrote:
Sat Dec 01, 2018 9:09 pm
the fact that you run on the short setting would mean less springy and more ridgid plus the fact it had been welded!

i have just purchased a 1690t and the draw bar was snapped off, not been welded but i would guess it may have been abused!
I find it surprising that any amount of abuse will cause them to snap. I've seen seriously overloaded equipment many times and of all the things that fail never the drawbar itself. Given how dangerous a failure can be I expect them to be well and truly over engineered, but perhaps this isn't the case.

whesbrook
Posts: 19
Joined: Sat Mar 03, 2018 7:31 pm
Location: Co Down NI

Re: Snapped drawbar - anyone ever seen this before?

Post by whesbrook » Sun Dec 02, 2018 1:09 pm

This is classic brittle failure.
the drawbar is high tensile steel, quench and tempered to achieve probably 80+ tons per square inch tensile strength, so the elongation would be less than 15%.
In the short position the weld is directly above the rear support plate of the cradle on the tractor and produces the ideal 3 point bending scenario with with weld directly in the worst place, to achieve maximum bending moment and maximum shear loading.
the welding has damaged the materials grain structure in the weld heat affected zone, reducing its load carrying ability, hence failure.
if the drawbar had been in the extended position the loading would have been reduced and the break would have been delayed or avoided.
the best rule is never weld in anything around the drawbar, the support cradle or the linkage.

A point of interest is the practice of older tractors having welded drawbars, these were probably fabricated, welded and then heat treated,

Gard
Posts: 229
Joined: Sun Apr 06, 2008 2:25 pm
Location: Fairfax Vermont USA

Re: Snapped drawbar - anyone ever seen this before?

Post by Gard » Thu Dec 06, 2018 10:54 pm

I am curious how you can tell the difference between a brittle fracture and fatigue failure? I had assumed brittle failure for "modern" structural steels was not an issue at normal temperatures, that is what led me to think fatigue failure. Was it very cold when it broke? However some images I see on google of brittle fracture in steel look a lot like the posted photos of the drawbar so I think I stand corrected.
I use low hydrogen welding rods on structures that may be stressed at low temperature (like 7018), however I do not know how to heat treat after welding at home so I would not be welding a drawbar if I thought it was a high strength steel. In the end I think fatigue and brittle fracture both will start at a stress concentration such as a weld so the question is somewhat academic.

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cobbadog
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Location: beautiful Coopernook NSW. near Taree. NSW. Australia

Re: Snapped drawbar - anyone ever seen this before?

Post by cobbadog » Fri Dec 07, 2018 5:07 am

My understanding about the difference between brittle and stress fractures is by looking at the break closely.
In this case, the drawbar, it looks like a brittle fracture as it is a clear break all the way through with only small indications of where it started from, the welds.
A stress fracture can usually show a progressive fracture meaning that it is radiating out from an original source and shows small arcs rasiating across the surface.

whesbrook
Posts: 19
Joined: Sat Mar 03, 2018 7:31 pm
Location: Co Down NI

Re: Snapped drawbar - anyone ever seen this before?

Post by whesbrook » Fri Dec 07, 2018 7:19 pm

the difference in the fractured surfaces gives the indication of what probably happened. in brittle fracture the broken surfaces are very flat with little evidence of fibrous tearing.
on a fatigue fractured brea,k the beach lines as they are called, can usually be seen radiating out from the stress concentration point like ripples on a pond, there will also be some fibrous tearing of the surface as the remaining material resists tearing. sometimes the surfaces are hammer flat where the crack has been opening and closing as the load varies.
As to the welding, 7018 rods will not help here, 10018 still wont help, the problem comes from the welding heat cycle ruining the grain structure of the metal and the loading doing the rest over time.
back in the last century (pre 2000) but it sounds better, I saw a drawbar on a New Idea muck spreader which was made from high tensile steel (80 ton/in2) which had been welded and then broke when the air temp was 28C after showing no sign of failure for 6 months. it went off with a bang like a rifle shot. Scrap.
Steels basic strengths/types can be identified using a grinder and a book called a spark atlas, but this is getting metallurgically rather deep.

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