Forces in couplings

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YorkshireBear

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In terms of how much damage it does to a coupling, (talking modern DMUS an EMUS here) is it better for the first set to power up first are the second set?

So is more damage done in compression or tension? I wondered if there was a way round which was better and that when units are joined this is automatically what happens?

Now im sure its better they power up together, but i have noticed recently it seems like the seperate units power up at different times.
Not only that is a 158 and 150 are connected they will have different accelerations surely?
 
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eastwestdivide

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I'd have thought no damage at all. Surely the couplings would be designed for much higher forces than a little bit of imbalance from which unit's providing the power. And the designers must take into account the possibility of the train being hauled dead by a loco or shunted or being rescued dead by another similar unit.
In terms of passenger comfort, maybe that's another story - has anyone experienced travelling in the coach the far end from the loco on the push-pull Norwich services? There would be all the slack in each coupling to be taken up as the train accelerates/decelerates.
 

ash39

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Im not sure I understand the question, surely its just the same as couplings in a loco hauled/propelled train with standard coaching stock? Or even couplings within a multiple unit,as even though its one unit the engines still may not be perfectly synchronized.
 

YorkshireBear

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Im not sure I understand the question, surely its just the same as couplings in a loco hauled/propelled train with standard coaching stock? Or even couplings within a multiple unit,as even though its one unit the engines still may not be perfectly synchronized.
I cant think how to word it properly.

Basically what i want to know is which does more damage, putting the coupling under higher than usual compression or higher than usual tension... and this can include couplings between coachs in a MU.
 

Cherry_Picker

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As both compression and tension would both have to be well inside the failure point of the coupler it seems to me the question is purely academic. You should probably be asking a scientist the question rather than a railwayman. I found this link on the internet, which offers no conclusive answer.
 

YorkshireBear

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As both compression and tension would both have to be well inside the failure point of the coupler it seems to me the question is purely academic. You should probably be asking a scientist the question rather than a railwayman. I found this link on the internet, which offers no conclusive answer.
I thought there were some rolling stock engineers on here, and thought some of them may know. And it isnt just railwaymen on this forum either...
 

Mike C

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Oh don't ask a scientist. What you want is an engineer with experience of couplers.

As luck would have it, I fit that description. Most modern autocouplers will be able to take greater loads in buffing than tension. However as someone already said, they are designed to cope with far greater loads than typically seen in service anyway.

A coupler will typically consist of a coupler head which has the links for mechanically joining the vehicles. Alongside or above/beneath will be the electrical auto couplers. The brake pipe and main res are normally housed within the head. Behind the head is a spring and damper assembly that takes the shock loadings and differences in loads between the vehicles. Then there is the mounting to the headstock which usually has a large pivot or spherical bearing to let the whole thing move. It is all suported on a large spring to keep it level when not coupled up.
 

WatcherZero

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If you wanted to simply know which put the most strain/wore the coupler more it would depend on the material. Some have greater strengths in tension rather than compression and other vice versa. Steel is generally thought to have equal compressive and tension strength, Aluminium has better compressive than tensile strength, things like bricks are 10 or 100 times better at compression than tension.

However while the material itself has a failure point its shape would be distorted beyond use far earlier, for example a bar of aluminum could be flattened like a pancake before it actually cracked.
 

Mike C

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I can see what you are trying to get at, but that won't tell you if the load is greater in tension than compression.

You have to consider geometry as much as anything else, but in this case, the shock loading of two sets being brought together is almost always greater (and by quite some way) than the tractive forces from acceleration.
 

YorkshireBear

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Thanks Mike youve answered my question.
And watcher i wasnt just talking about the physical connection but if in tension or compression damage is done to any electrical or brake systems.
Thanks Mike.
 

Yew

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Thanks Mike youve answered my question.
And watcher i wasnt just talking about the physical connection but if in tension or compression damage is done to any electrical or brake systems.
Thanks Mike.
So for example if a 158 tried to pull a 150 at 90mph?
 

Crossover

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So for example if a 158 tried to pull a 150 at 90mph?
Then you probably wouldn't have much left of one of the gearboxes :)

(I believe something similar happened with a 153 being forced to run at the top speed of the leading unit which had the capability of a greater top speed)
 

Crossover

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I guess worst case is they could separate, but the through connection of the brakes should dump the air and cause an EBA in such a case.
 

dubscottie

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The Irish 2700's had a nasty habit of separating when they worked the Dublin - Rosslare route years ago.. Now banned south of Bray on that section.

Cant remember the if the reason was track or unit related.

I would say that couplers are more likely to go in compression than tension. For example on buckeye fitted units/locos a rubbing plate is needed as the buckeye cant take compression loads.

I would say it is worse with modern stock as the couplers are designed to collapse in the event of a collision so the coach anti-override "ridges" engage. What force this takes I don't know, but I would guess about the 200 ton mark..

(for example a Mk1 underframe has a max compression load of 220t. If they had modern couplers, You would want the coupler to collapse before that so the over-ride cups lock and the underframe/body does not bend)

Hope this makes sense!

There is a video of a 24(?) car 375 on youtube somewhere.. The first 12 were being pushed by the back 12..
 

CarltonA

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I read that couplers in the uk are meant to be capable of a 660,000lb load. Maybe this only applies to freight locos and wagons. The US/Canada max is apparently 1,000,000lb on the couplings.
 
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