A question for the Tram minded folk about single point turnouts

[vsmith]

Since no known entity makes ready made turnouts for 32" diameter curved track I am loking to bash a single point turnout to my pizza layout.

Now every diagram and pic I see shows the movable point on the inner diameter rail with just a gap on the divergent point on the outer diameter rail, why is this?

This seams to me counter-intuitive to me. Given my uber-tight circle of track I want to build the switch with the point on the outside rail. If the movable point were on the outer rail, where the outer wheels flange would logically press more against the rail seams to be the better location for the point. IOWs if the points were straight, in the divergent position a train approaching the switch the outer wheel would come to the closed gap at the point blade and be diverted, or if it was open, the pressure of the flanges coming around the curve towards the point would automaticly be captured by the open gap at the point and the inner wheels would just follow rolling over the frog gap in either case. To place the movable point to the inner rail seams to invite more trouble, not so much in the closed straight position, but namely that in the divergent position it would be the inside edge of the inner wheels flange striking the face of the open point blade that would force the wheels onto the divergent track and would also risk the outer wheels getting wedged in the open gap of the frog of the outer rail, or at least clunking across it in loud protest.

Why were single points built that way? What am I missing?

 

[Tony]

I dont know the real reason but my quick thoughts would be either there is a danger that at speed a train could force the rails apart if all the force were just on the outside rail as in a standard point so you need frogs on each rail to guide both wheels at the same time
But as the speeds of a tram are 1: lower and 2: the rails are set in the road its far easyer build a mech that only works on one frog
other than that im as baffaled as you

 

[tramcar trev]

Discussion here:
http://www.gscalecentral.net/tm?m=175076&high=single+bladed

Usually the moveable blade does the steering directing the face of the flanges (wheels) in the desired direction..... though most single bladed points are sprung and hence "automatic" there were some that were moveable and could steer the flanges exerting their might on the inside of the flange. Trams are slightly diiferent than rail in as much as through points etc the wheel runs on the flange to provide greater passenger cumfort......
Yes well thats whats sposed to happen...

 

[trammayo]

Tramway points have always been a bone of contention - both for the engineer and the passenger. Early switches had an India Rubber block as the spring loading and the norm is that the points are set for the most used direction - on single track it was automatic (first switch loaded for straight ahead acting as a trailing point, and the second acting as a trailing point for the straight ahead and as a facing point diverging for the opposite direction).

As Trev said, its the wheel flanges on trams that matter with raised groove pointwork- a reasonable run-up to the cross-over lifts the wheel tread clear of the railhead.

Your two pics are interesting examples of single blade points - the first pic seems to defy all logic and is devoid of any tolerances (presumably the check rail has a tongue which operates in tandem with the switch blade - and there seems to be an external check rail on the straight for tubs joining the straight!) - whilst the second pic shows a manually operated tramway type single blade.

With the latter, the tram (or loco) has to come to a halt before someone sticks in the point iron to move the blade - so it wouldn't have any speed (or problems) when taking the diverging line. I see the blade is welded up here!

But as a direct answer, the only one I can think of is that if the blade was on the opposite rail, the wheel might climb out. As a further point (pun intended) some rail systems (horse drawn or maually pushed stock) didn't have any blades and just relied on being pushed or pulled over.

 

[tramcar trev]

The single blade point at the upper Snaefell terminus is interesting, just one very long moveable rail..

I agree; the picture in the mine I would say that the point work has been scavanged for elsewhere in the mine as there is no connectivity there at all...

 

[New Haven Neil 2]

Beat me to it Trev - I have a photo somewhere.....

 

[Gizzy]

I was wondering if the top picture somehow was a point incorporating a trap mechanism to de-rail a runaway wagon.

But on further examination, I reckon it's just broke and is awaiting repair....

 

[themole]

The point blade moves on ONE side only forcing the tram to follow through. Alyn

 

[a98087]

Firstly awesome bit of modelling with that tramway track, Its always good when people show off their hardwork

And Ive always wondered how they actually work, and appears that this thread may answer my questions,

Just to add a small piece of knowledge, all i know about single blade turnouts is there used at very slow speeds, and were sometimes found in yards/dock yards.

I also presume that they are quite cheap to make and also suffer from frequent derailments

Dan

 

[themole]

Dan, wrong, if they ( due to the points ) suffered from frequent derailments, then would the general public travel on the tramway? I think not. p.s. ask away with any question, I am sure somebody on the forum will have the info. Alyn

 

[MRail]

My own experience on 32mm with a point coming directly after a curve (bad practice) is that the leading wheel flange is pressing aginst the outer rail, and sometimes rides up over the end of the switch rail.

 

[vsmith]

Thanks that helps explain a few things, the return aspect for tram lines makes sense, but I'm still trying to figure how the inner rail setup would work for freight lines layed in the street where I've seen pics.
Now real world aside I still have to decide what would work best on my layout, This is the set up:

Nevermind the tie placement Thats just a side effect of the program. I havent shown the point blade as I'm still debating it, now for LGB size trains on code 332 rail Pizza layout using a 32" Diameter circle mainline with a spur as shown, I think you can see why I'm more disposed to putting the point blade on the outer rail as I think in real layout operation it would just work smoother. Anyone else think different?

 

[Walts-Playmobil]

Playmobil plastic train track has only one point blade, this is on the straight rail side.
This will cause the train to either go straight on or round the curve.
If the train approaches from the wrong direction it just pushes the blade to one side.
This is handy as you can set up passing loops and the trains will not derail.

 

[vsmith]

Thanks Walt, thats pretty much what I had in mind, anyone have any pics of one of these?
Nevermind, found one:

This makes alot of sense to me, I like the added guard rail.
I think I found my working prototype!

 

[tramcar trev]

The point blade moves on ONE side only forcing the tram to follow through. Alyn

Is that the HO gauge stick down system? Be nice if it was made in G.... prohibitivley expensive though, then the quality is long remembered once the cost is forgotten

 

[themole]

Spot on Trev, but it does show clearly a tramway point. Alyn

 

[MRail]

Now every diagram and pic I see shows the movable point on the inner diameter rail with just a gap on the divergent point on the outer diameter rail, why is this?
This seams to me counter-intuitive to me. Given my uber-tight circle of track I want to build the switch with the point on the outside rail. If the movable point were on the outer rail...

Finally located my shots of Merseyside's Museum Tramway at Birkenhead.
The moving blade here is on the outer rail.