Knuckle Coupler/Drawbar/Carframe tonnage limits?

[Isambard]

The knuckle coupler/drawbar/car frame engineering design strength of a locomotive or freight or passenger car meeting AAR standards obviously must take into account the trailing tonnage to be pulled, maximum shock resulting when slack is pulled out, aging effects and other factors.
Does the maximum strength of such designs ever become a limitation on the trailing tonnage in practice, rather than other limiting factors on train tonnage e.g. available tractive effort, staging and siding track length, grade etc?
Is knuckle coupler/drawbar/frame failure an occurence of any significance today?

[Anonymous]

Yes to both questions. Drawbar strength is limiting, on how much power you can put on the head end and expect knuckles not to break off (the "broken drawbar" is almost always a broken knuckle). Couplers come in two varieties, standard and high-strength. Almost all Western roads use high-strength, at least on bulk cars (hoppers, covered hoppers), as do most private owners of unit-train cars. Eastern roads often use standard strength even on hoppers.

By comparison of why this matters, on the 2.0% climb from Taberna***o the West Portal of Moffat Tunnel, a train is allowed 8000 trailing tons with high-strength couplers, but only 5000 tons with standard couplers. The difference for a unit coal train might be three or four power sets versus two or three.

Knuckles break off with some regularity. Bad train handling gets most of them, but also DPU failures, undesired emergency brake applications (from many causes), and mechanical failures of various types.

[Modelcar]

....That makes me wonder how the drawbars handle such power as I witnessed in Kingman, Az. some years ago [east bound], with 9 engines in front and 3 mid train....Now that would be a bit difficult to keep pushing at a rate to not let the front engines lean into the loads too hard hence tearing the coupling apart....That is, if the power has to be limited because of the coupler.

[M636C]

I was riding a Hamersley Iron ore train in 1978 (in a test car at the head of the train). My recollection is that the operator in the yard tower changed his mind about what track we should take, and while he changed the switches, reversed the signal from green to red. Unfortunately, the train, led by GE C36-7 5057, had to go into emergency to stop clear of the signal. The train weighed about 15000 (long) tons, consisting of 150 cars in drawbar connected pairs. It broke in five places just from the emergency stop, on a falling grade, and one of the breaks was a solid drawbar which pulled the pin out of the yoke. The rest were failed knuckles. The tests were instruments on cars 99 and 100, and we had 1000 metres of coaxial cable strung down the side of the train, attached by four plastic cable ties to each car. About five metres of the cable total was found, at the test car. We never found the rest (it cost $2 per metre then). I don't think we even found many cable ties.

These were AAR Type F high strength couplers in fixed to rotary pairs.

It hadn't been a good day. We had lost the slip rings that took the data from a strain gauged axle (and I mean lost, we never found them either) earlier in the day. This delayed the train more than 30 minutes while one of the engineers very slowly realised that we couldn't fix that outside a major workshop. In that time I got some nice photos of the train while the train controller was trying to get us banned from his railroad.

The thought behind this is that coupler strength is only marginal with very heavy trains and careless handling could easily result in a broken knuckle in most big coal or ore trains.

Peter

[PNWRMNM]

Modelcar

Your train at Kingman probably did not have all of the units on the head end on line. There is an isolation switch in the cab that is used to take units off line, or "isolate" them so they do not respond to MU control signals. They just stay in idle.

Mac

[tree68]

I recall a TRAINS article about AC traction that cited a coal train running in notch 8 at a fractional MPH (ie, < 1) on a grade. Until a coupler parted.

[Modelcar]

....I have no way of knowing of the Kingman train now, that is how many were under full power as this occured back about 1970 and I do remember I was sitting in a restaurant and at a location I could watch the activity on the main route east / west on the Sante Fe and as I mentioned this train was traveling east and I believe I remember the grade up through there is close to 2%. I remember thinking at the time...Wow, to the quanity of engines on front and then the amount of cars following...seemed like it wouldn't end...and then surprised to see even 3 more back in the train....it was awesome.

[mvlandsw]

Here's a site with a good explanation of the physics involved. http://www.vcn.com/~alkrug/rrfacts/rrfacts.htm

[heavyd]

I know the knuckle is designed to break first. Obviously it is the easiest and quickest to change. I can't find the draw-bar rating for a knuckle failing point, anyone else know?

[Anonymous]

heavyd: The drawbar rating is based on the weakest thing, which is the knuckle -- it's the thinnest piece. There's no separate rating for the drawbar itself, not that exists outside of an engineering analysis.

I've ridden a 17,000-ton iron ore train up 1.0% at less than 1.0 mph. Took us well over an hour to cover the last mile, too. But nothing failed. And I've seen them break right in front of me, too, on trains starting on a grade.

[Modelcar]

...mvlandsw: That site you provided is really great reading...so fluently written and easy to understand...

[Anonymous]

mvlandsw, thanks for site. I learned a lot. I highly recomend it to anyone that wants to learn about braking, tractive effort/HP, or signals.

[Allen Jenkins]

Thanks for the info, I needed to find the Krug Pages, I lost the link recovering after a config file recovery the other day.

[TH&B]

Knuckles can break seemingly easy from if there was a previous crack to weeken it and/or coupler heights are off by a large margen like when the drawbar hangs too low.

The railroad is not always working with perfect equipement.

[Allen Jenkins]

The record is now set, about Hamersly Iron. M636 is a Heavy on this. The work their doing is positively doing the best they can do with the transportation medium their doing! Let us know the work your doing, an M636, Stay Safe! ACJ!

[Anonymous]

Case an point....The CNW.
I still remember the day a westbound loaded ore train came through Blair with only 3 CNW EMD SD40-2's on the point......As the power went past me doing less than 20mph..As it hit the 1.0% grade it's speed draged to a very slow 10mph.
As it got half way up the grade the coupler nuckle between the 3rd locomotive and the hopper car gave way.
Of course back in the CNW days thay had to cut the train and take one half of the train to kennard put it in the siding then the crew went back to pick up the other half and drag the rest off to kennard to double up again and hoping to make the arlington hill.
ILL tell you what.....I realy did love the CNW back then....but there were day's I just stood there and shook my head in disbelief alot of times too.