train wheel passing points?

[truckman060473]

when a train wheel passes through the frog of the points, does the wheel momentarily drop into the gap or does the tip of the flange run in contact with the base of the frog to support the train's weight during this time? I was wondering whether or not this happens in real life, as on my model railway the newer type wheels with a more life-like sized flange do drop into the hole in the frog, whereas older wheels with too big a flange run thru with the flange supporting the wagon weight

 

[PaulLothian]

I don't often quote Wikipedia, but...
"The frog is designed to ensure the wheel crosses the gap in the rail without "dropping" into the gap; the wheel and rail profile ensures that the wheel is always supported by at least one rail."

I believe that trams do partly roll on their flanges across points.

More here http://en.wikipedia.org/wiki/Railroad_switch#Frog_.28common_crossing.29

 

[DaveNewcastle]

This does leave a question unanswered in the rather unique case of the Newark flat crossing (which is at 90 deg.) and which provides a nice jolt (reminding everyone on board where they are).

 

[LE Greys]

This does leave a question unanswered in the rather unique case of the Newark flat crossing (which is at 90 deg.) and which provides a nice jolt (reminding everyone on board where they are).

It's more like 80-degree, which is why it produces a roll (eight asymmetric events alternating from either side).

 

[DaveNewcastle]

I didn't know that. Thanks for the info!

(I'm much less puzzled now, though I'm still slightly unclear how bogies traverse that crossing without incurring some potential for damage to wheels or track).

 

[Wyvern]

There is damage due to the slight gap in the track. Diamond crossings tend not be used on high speed sections. That, I believe, is why ladder junctions are used. Also some switches have movable frogs which close up the gap.

 

[Hydro]

"Frogs"? How American. "Crossing nose" please

 

[GB]

Is it not the case that swing nose crossings are installed on higher speed lines?

 

[Hydro]

Yes, where two routes diverge and both are high speed normally. Keeps it smooth and avoids potential striking of the crossing nose at high speed. They're expensive and maintenance intensive, so not very common.

 

[Old Timer]

The gap is only a problem if the diameter of the wheel is small enough to cause the wheel to "dip" into the crossing nose.

Here is an example of what happens when that is the case.

 

[daikilo]

when a train wheel passes through the frog of the points, does the wheel momentarily drop into the gap or does the tip of the flange run in contact with the base of the frog to support the train's weight during this time? I was wondering whether or not this happens in real life, as on my model railway the newer type wheels with a more life-like sized flange do drop into the hole in the frog, whereas older wheels with too big a flange run thru with the flange supporting the wagon weight

I think the simple answer to your question is yes, wheels run on their treads and not their flanges. And to your experience with older models is that the model was probably not representative.

There is one exception to "dropping into the gap", the swing-nose turnout or crossing, where there is no gap.

 

[Old Timer]

.....There is one exception to "dropping into the gap", the swing-nose turnout or crossing, where there is no gap.

The wheels do not drop into any gap.

 

[daikilo]

The wheels do not drop into any gap.

Well, in my days, wheels dipped into rail gaps of varying lengths and dropped into conventional point/switch frog gaps. This is why we all heard the dual clickety click on plain track and heard and felt the occasional one-sided clunk with minimal roll on switches and cross-overs. But in those days, I cycled to work as my parents were using their Ford Cortina mk1.

 

[O L Leigh]

Part of the problem with comparing what happens with your model trains to what happens on the real railway is the difference in geometry. Many model railway switches/points are of a far tighter radius than you would typically find on any real railway and the tolerances far more generous, therefore the vehicles will behave differently.

Part of the reason why the wheels on full size rail vehicles don't drop into the gaps in frogs is because of these differences in tolerance and geometry. What tends to happen is that as the inside of the tread reaches the end of the frog and becomes unsupported, the new rail on the outside of the frog has already come in and is supporting the outside of the tread. Therefore there is very little hole for the wheel to fall into saving both it and the frog from wear due to impact forces. There may be some locations where there is a gap (e.g. Newark) but the gap is modest when compared to the wheel diameter, so the drop is very small indeed and does not contribute to the overall wear rates of either frog or wheel, nor does it adversely affect the ride quality. It's certainly no worse than some wheel flats I've experienced.

O L Leigh

 

[Wyvern]

The wheels do not drop into any gap.

But surely there is a discontinuity in the rail which will wear? Otherwise there'd be no point in moveable frogs - sorry swinging noses. And it wouldn't matter if you had diamond crossings.

 

[Old Timer]

Firstly thanks to O L Leigh for a perfectly good explanation.

As he states, the nose of the crossing picks up onto the wheel tread and increases in size as the wheel passes into the crossing itself. There is no point where the wheel is unsupported.

Because however there is a "new" point of contact between the wheel tread and the crossing, inevitably there will be wear on the crossing nose which results in a slight dip over time. From this point onwards the wheel will start to impact the crossing nose with greater force, causing increasing damage to the nose itself, and damage to the wheel structure which will ultimately lead to the wheel fracturing.

The picture I enclosed earlier illustrates a well damaged crossing nose. At this stage the nose is developing fractures, and every wheel is being pounded as it hits the nose.

In due course the nose will fail catastrophically unless detected.

As for the wheels, they will develop fractures and they too will fail catastrophically at speed.

Because any discontinuity in the rail head is undesirable, it is becoming increasingly common on high speed railways, and especially heavy-haul freight railways, for the common crossing to be replaced by a swing nose crossing, thus removing the problem at source.

If I can find one, I shall post a photo of a wagon that derailed with a fractured wheel.

For the sake of clarity, the crossing is technically known as the "frog" something that is much used outside the UK, whereas we prefer to use the term "crossing nose"

 

[truckman060473]

Interesting replies thanks. Main reason I'd asked about it is after changing my model points to Peco which have a more accurate frog / checkrail to prevent derailing. They don't derail at all, BUT, the wagon's wheels DO drop into the frog making an uneasy sound that makes you think it is going to derail!