Improvising symmetrical Y-turnouts and split-deflection switches

a step-by-step tutorial sequence

 October 2008: There are some update notes available for this tutorial for use with Templot versions 082d and later. Many of the operations detailed below are simplified. Click Y-turnout update notes.

 This is a mini-tutorial. Please refer to the general notes about using these tutorial pages at the beginning of the track plan tutorial.

At present (July 2001), Templot's switches and turnouts are fully-handed, 100% Left or 100% Right. I'm intending to introduce split-deflection switches of variable handing (which includes those for symmetrical Y-turnouts with 50% Left + 50% Right deflection) in a future upgrade, but we are not there yet. Click split-deflection switches for more prototype information and diagrams explaining this subject.

In the meantime if we want a symmetrical Y-turnout we have two choices:

  1. Approximate to a symmetrical turnout by setting a negative curving line radius in the main road of a handed turnout which has a curved type of crossing, such that the resulting external geometrical radius for the turnout road is approximately the same.

  2. Improvise a true symmetrical split-deflection turnout by superimposing partial templates.

Below are some notes about using both of these methods in Templot. The example screens are for S4/P4 gauge.

1. Approximation method:

The screen above shows the first method.

This is a turnout with a left-hand REA 'A' switch (track > switch... menu item) and 1:6 curved crossing. The timbering is equalized (geometry > timbering > equalized-incremental menu item), and this is important in order to have the check rails symmetrically placed. 

The curving-line radius (arrowed 1) has been adjusted (using the F6 curving mouse action) until the dimension showing in the information panel for "external geometrical radius =" (arrowed 2) is approximately equal to the curving radius.

 The geometrical radii dimensions are not available prior to version 0.71. Use the "resultant turnout radius =" dimension instead.

The main road curves to the right in the opposite direction to the hand of the turnout, so the curving-line radius shows negative (arrowed 1). The hand of the turnout is easily seen by looking at the track centre-lines.

In the majority of cases with negative curving it is better to use a curved type of crossing like this, so that the turnout road curve continues through the crossing (arrowed 3), and this is always the case if a symmetrical result is wanted (track > crossing... menu item, then click curved crossing in the list).

Also when the curving is negative it is usually possible to save some space by using a shorter switch without infringing your minimum radius setting, and I have done so here with an A switch used in place of the more usual B in conjunction with a 1:6 crossing.

After using the shove timber functions to square up the timbers, and removing the misleading track centre-lines, the turnout looks like this:

which for many purposes is an acceptably close approximation to a symmetrical Y-turnout. As you can see the result is not quite perfectly symmetrical, because the switch deflection remains 100% Left Hand.

 Note however, that if the main road (here to the right) is a running line, and the turnout road (here to the left) leads to a crossover, siding or loop, this is a correct result, avoiding a switch deflection (and hence a speed restriction) in the running line. (In the case of a crossover or loop, a regular type or generic type of crossing would normally be used instead, and the turnout would then be non-symmetrical.) In most prototype circumstances the running line radius is of course much greater than shown here - the non-symmetrical switch geometry becomes much more noticeable when we use tight model curves like this.

On close examination the switch looks like this:

and the discrepancies become apparent.

Because the curving-line radius is constant, the switch front section is also curved, and we can see this at the track centre-line (arrowed 1) and at the rails (arrowed 2) which can both be seen to to be curving slightly downwards in this view, whereas for a symmetrical turnout they should of course be dead straight.

And the 100% left-hand deflection in the stock rail (arrowed 3) means that it is shifted too far to the left (upwards in this view), and is also curving downwards, leading to an unwanted reverse curve effect at the switch heel (arrowed 4).

The switch front problem could be cured by putting a zero-length transition from straight to the curving radius at the switch toe (blade tips), but the result of this is to shift the stock rail further to the left, making the effects at (3) and (4) worse. Also, when a turnout is on a transition curve it is no longer possible for Templot to calculate the geometrical radii, so the adjustment to achieve symmetry becomes less straightforward.

Many modellers will feel that these discrepancies are not sufficient to warrant further work, and that the switch can be easily corrected by eye during construction to give a straight switch front and equal deflection in each road.

But when we want greater precision we can improvise a true symmetrical turnout by superimposing partial templates. This also gives us the option of making one or both roads straight through the crossing. Here are the methods:

2. Improvisation method:

Assuming we want to attach a perfectly symmetrical Y-turnout to the track approaching from the left, our first task is to peg on a straight centre-line only plain track template to act as a guide centre-line (geometry > track centre-lines only menu item, then control > store and background menu item).

Then peg over it our starting turnout. This is initially an A-6 turnout again (track > quick set... menu item) and the timbering is again equalized (geometry > timbering > equalized-incremental menu item) to give symmetrically placed check rails.

Again I have set a curved type of crossing (arrowed), (track > crossing... menu item, then click curved crossing in the list).

This improvisation method is more straightforward when using straight pattern switches, so I have changed to a 9ft straight switch (arrowed), (track > switch... menu item, and click the ? help info button for more information about straight switches). Using your modeller's licence, you can of course construct them with flexible blades instead of loose heels if you wish.

 Note that the common REA semi-curved pattern switches (e.g. A,B,C, etc.) require an additional superimposed template to achieve symmetry. The single-curved pattern switches (e.g. GWR) are easier, but need some calculations. Further notes later.

Click the geometry > curved (transition) > easement from straight menu item. The transition start marker (arrowed 1) and transition end marker (arrowed 2) appear. Our intention now is to change the transition settings to set a zero-length (instant) transition at the switch heel position (arrowed 3, and marked across between the rails with a pale yellow line).

In other words, we want both markers to be coincident at this position, so that the radius changes abruptly from straight (for the switch) to curved (for the remainder of the turnout).

Click the geometry > curving data... menu item. The Templot data-entry form appears. The initial transition radius is already straight to suit this straight pattern switch, so simply press ENTER. For the final radius enter any convenient trial negative figure. I entered -6000 mm.

For the length along the straight (initial radius) to the transition start, I entered the magic number p173,

which Templot duly converted to 57.6667 mm as you can see (arrowed 1).

The "p" prefix letter is an input conversion factor which allows us to enter prototype dimensions in full-size inches when model dimensions in mm are being requested. Templot then automatically converts the dimension to the current model scale.

For more information about other input conversion factors, click the what ? button on the form.

The 173 inches figure is arrived at as 65 inches of switch front length between the stock rail joint and the switch toe (blade tips), plus the length of the switch to the switch heel (here a 9ft switch = 108 inches). So 65+108=173. In 4mm scale here, 173 inches = 57.6667 mm.

 The 65 inch dimension is standard for all straight and REA semi-curved switches. (For GWR switches the dimension is 64 inches.)

 For this setting to work this way there can be no approach track on this template. (If there is a length of approach track, the 173 inch dimension must be converted to model mm manually and added to the approach length.)

In the next Templot upgrade there will be a simpler way of setting the transition start at the switch heel and similar fixed positions.  

We want a zero-length (instant) transition, so enter zero for the transition zone length (arrowed 2).

Here is the result. The transition start and end markers (arrowed 1 and 2) are both positioned at the switch heel position, giving an instant change from straight to a curved main road at this position.

Now press CTRL-2 or click the adjust > set peg options > peg on TP menu item to put the fixing peg at the switch Toe Position (blade tips) (arrowed 3).

To adjust the switch deflections between left and right, we need to rotate the switch slightly at the toe. Click the adjust > shift / rotate current > rotate current by... menu item. The data-entry form appears as above.

We want a 50-50 split instead of the existing 100% Left setting, so we need to rotate the switch clockwise to the right by half of the total deflection angle. The total deflection angle for a 9ft switch is 1:24, so half of that is an angle of 1:48. The form is asking for the angle in degrees. To avoid converting this on the calculator ( ArcTan(1/48) ), we can again use an input conversion factor. The n letter prefix converts RAM unit angles to degrees, so we can simply enter n-48. The angle is negative [-1:48] because we want a clockwise rotation. Click the help flag for more information about rotation angles.

For a table of total deflection (planing) angles for different switch sizes, click split-deflection switches .

If a non-symmetrical split switch is wanted (e.g. 75% Left + 25% Right), the rotation angle should be reduced accordingly. Further notes later.

Here is the result of the rotation. We can see the original centre-line (arrowed 1) and the new centre-line shifted from it to the right (arrowed 2), so that the switch stock rails (arrowed 3 and 4) are now symmetrical about the original centre-line. But the rotation has of course misaligned the original rail joint (arrowed 5), so our next task is to blank off this front part of the switch.

Click the track > lengths (mm) ... menu item. On the data-entry form we don't want to change the first two items, but for the blanking length we can again use the p conversion factor to blank off, say, the first 50 full-size inches of the template.

Here's the result of the blanking. Because of the way Templot works it is not possible to blank a precise amount. Our 50 inch guess has left just the right amount of stock rail (arrowed 1), but also an unwanted partial sleeper (arrowed 2). It's easier to remove this using the shove timber functions.

We shall later fill the gap with a short length of straight plain track.

(An alternative approach would be to create a custom switch with this switch front length set to zero. An exercise for the reader!).

Now zoom in on the nose of the crossing. Using the F6 curving mouse action, adjust the 2nd transition radius (arrowed 1). Make sure that you are not inadvertently adjusting the 1st radius, which must remain straight as shown. (To quickly toggle between these two radius adjustments click the mouse action symbol (arrowed 2).)

Adjust the 2nd radius until the fine-point marker (arrowed 3) is exactly aligned over the centre-line (arrowed 4).

 If the mouse action response is too course to get a good result, zoom in closer or click the adjust > modify mouse response > set response factors... menu item and set a higher response factor for the F6 mouse action.

For this 9ft-1:6 turnout in S4/P4 I arrived at a radius of [-2703] mm in the main road for a symmetrical turnout. Your figure may differ slightly.

Zooming out we can see that the resultant radius in the turnout road is 2701 mm, which is a discrepancy from pure symmetry of only 2/2703 or 0.07%. (This is mostly due to the very slight trigonometrical error in setting half of 1:24 as 1:48).

In this next view I am using the shove timber functions (adjust > shove timbers menu item) to square up the timbering. The along mouse action is used (up and down the screen) to twist each timber until it is square to the centre-line (arrowed). The unwanted partial sleeper J1 has now been omitted.

 This process is somewhat tedious. In the next Templot upgrade I am introducing some changes to the shove timber functions to make this task easier.

Then it's store and background for this template.

Now we need a short length of plain track to replace the blanked off switch front. track > quick set... menu item to get a length of straight plain track, then track > lengths (mm)... menu item to set its length to 65 full-size inches.

Then CTRL-1 or a click on the peg indicator (arrowed) to put the peg at the rail-joint end (arrowed).

Peg it in place on the notch and a little more shoving omits the unwanted sleeper.

Finally we have our symmetrical Y-turnout. Remember that this consists of two partial templates, so to shift it into a different position the shift group functions should be used. (Don't inadvertently peg the turnout part onto the switch front part when doing this, otherwise you will of course lose the essential rotation between the two.)

Further notes for curved switches, straight crossings and non-symmetrical split switches coming here soon.


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© 22-July-01