YEA Perch LWRT wagon - qu for the track workers/engineers amongst you

[tomstaf]

Hi,

Quick qu regarding the shape of the Long Welded Rail Train YEA Perch wagons - Is the top deck slightly bow shaped?

Looking at the pic you can see the last 3 rail supports are increasingly built up towards the end of each wagon. I presume that the side rails need to be as flat as possible for the crane to easily be able to move along the train, so why would the top deck be bowed? Or is it actually flat and it's an optical illusion or something to do with the huge length of the wagon?

Cheers

Tom

 

[mickoo]

No It'll probably be bowed, it's a common trait of items that need to carry large weights, they pre stress them in the opposite direction to the weight so they deform to flat when loaded, normally seen in steel bridges or in my case our new traverser which is 3" higher in the middle but almost flat when a class 70 or 66 is on it. It's more common on items that do not have deep or large vertical load bearing beams.

Hard to tell from this shot but the bow can just be made out by looking at the rail head.



Here's some photos of YKA's, these are loaded but are still humped in the middle, they'd be more bent if empty.




And a couple of GPO's


I've also seen it on some Freightliner wagons, but no photos from the right angle to show it.

 

[tomstaf]

Thanks Mick,

I can't quite understand where would the 'flex' go on the Perch seeing as the sides are filled unlike those YKA/Salmons, and the support under the rails keep the wagon in the bowed position all the time. Am I missing the physics point here?

Cheers

Tom

 

[mickoo]

Tom,

That support beam under the Perch will bend when loaded, if it's loaded with rail and the travelling crane passes over that wagon 'will' bend in the middle

What's throwing you is the gap between the travelling crane rail and the wagon, that's probably because they are bent differently, the wagon having a large pre stressed bow than the rail, the gap doesn't matter so long as it all bows together and that bit at the end near the head stocks won't bow very much at all, most of the deflection will be in the middle of the wagon where the load is centred.

In reality they could of made the gap the same all the way along but I suspect the rail hump has been reduced to account for the travelling crane having to move over the wagon fully loaded or fully empty.

When fully empty the wagon will have lets say a negative deflection (upward) of say 3" in the centre, the rail will be 1.5" due to the difference in the gap between rail and wagon along its length, when fully loaded the wagon will be 0" and the rail will be -1.5", this means that the maximum deflection the crane will ever face is 1.5"

I'll whizz up a quick drawing

Here you go, should explain it a bit better.

 

[flexible_coupling]

Most flat wagons (container flats etc) have a tiny bit of 'pre-stress' to compensate but it's nigh-on impossible to see/replicate in the smaller scales, I reckon! The eye would lead you to think you've damaged the wagon if it's not clinically flat!!

 

[tomstaf]

Thanks Mick,

That diagram helps So the rail bows movement is far less - more like a quivering knife whereas the wagon is more like a bouncy castle? Would this difference also be something to do with how rail and wagon metals expand with heat perhaps?

For modelling, the rail supports at the ends are fairly noticeable but if you modelled them you'd end up bowing the model the wrong way so you'd have to do both I guess. I'm sure you could leave out pre-stressing though as the forces don't scale down hehe.

Cheers

Tom

 

[mickoo]

Tom, not quite, the rail moves just as much as the wagon, it's just the gradient is an average of the max deflection of the 'whole' vehicle.

The whole deflection of wagon and rail will be 3", that's physics and because both are joined then both must move 3"

Put another way, when empty the crane will climb a small hill of +1.5" up toward the centre of the wagon, when fully loaded there will be a small dip of -1.5" in the middle of the wagon. The rail movement is still the same, from +1.5" to -1.5" = a total movement of 3", the same amount as the wagon underneath, however in the wagons case it's +3" to 0", thus, the maximum hill or dip the crane will ever encounter is 1.5".

If the wagon was half loaded then the wagon would have a +1.5" deflection but the rail would be flat.

If you did not have this offset then when empty the crane would have to travel up a gradient twice as steep as the difference between head stocks and middle would be 3" however, when the wagon was fully loaded then the crane would have no gradient as the wagon would be perfectly flat.

 

[Dog Star]

GWR drawings for carriage underframes provide a measurement, I recall that the frames are constructed with 0.75" rise in the centre over (say) 60'0". Most of the rise is "lost" when the carriage body is put onto the underframe...

 

[Heather Kay]

GWR drawings for carriage underframes provide a measurement, I recall that the frames are constructed with 0.75" rise in the centre over (say) 60'0". Most of the rise is "lost" when the carriage body is put onto the underframe...

I think all carriages were built that way? It kind of prestresses the underframe, and when the body is constructed on top it flattens out and makes things stronger.

 

[AndyB]

Even quite short goods wagons were built with camber.
The subject of this thread, with a design from 1913, had 1/2" camber explicitly shown in the side frame drawings.
The 18T version (same length / wheelbase but heavier section frames) shows 7/8" camber for the frame member on its own, reducing to 3/4" when built (i.e. 1/8" deflection purely from the wagon's own weight).

Andy

 

[tomstaf]

Thanks Mick,

Right, I get it now.

Cheers

Tom