Episode 7: Realistic Switching of Turnouts

Turnouts in Reality

Turnouts in 1 scale are too large and too visible to operate them without realistic switching. Not only should you not be able to see drives: Nothing disturbs the realistic impression on a system more than when the turnout bangs into the end positions at lightning speed and with a loud ‘CLICK!’ And all the more so when several turnouts have to be switched in a road and this clack occurs several times. If we take a quick look at the original, we see slow movements with switching times of 2 and more seconds:

So what could be more obvious than to recreate this on the model railway? For this post a 1 scale turnout from Märklin (→e.g. 5977) for manual switching serves as a visual object. Since only the pure control is to be shown here at first, elements such as counterweight or the turning lantern are dispensed with here. The turnout slider (not the track tips) can be operated with a wire coming from below:

Modified turnout with steel wire for switching

This has the advantage that the spring between slider and tips works normally and the tips are always pressed cleanly against the rails. Furthermore, it is then also possible to ‘cut open’ the tips if the entry direction is ‘wrong’.

Slow movement

How do you drive something slowly but powerfully? The very common older magnetic drives described above are definitely not suitable for this purpose. Gear or servo motors are now available. If a geared motor were to be chosen, the following aspects would have to be taken into account:

  • large torque
  • long travel (unlimited)
  • mechanical conversion motor shaft (rotary) to crossover (translatory)
  • relatively high price from 10 – 15 €, →e.g. Pololu
  • Limit switches are required
  • a suitable electrical control (H-bridge) in connection with a digital decoder

The following things are important when using →a model builder servo:

  • limited torque depending on size
  • limited travel; 180° is usually no problem
  • mechanical conversion motor shaft (rotary) to crossover (translatory)
  • manageable prices from approx. 4 €
  • no limit switches required
  • Digital decoders with servo control are available

Due to the price and the simpler design without mechanical limit switches, the servo version should be chosen.

For the optics of the system, it may even be advisable to extend the operating time a little bit so that the observer can also perceive it correctly.

Servo and control

A possible digital decoder for controlling servos via DCC is the →ESU Servoswitch. With 35 € it is not quite cheap, but up to four servos can be connected and of course individually controlled via DCC address. Both end positions as well as the traversing speed can be programmed individually via CV variable or hardware button.

→The video clearly shows the setting process. During the adjustment process, only the quiet transmission noise of the servo can be heard. In my opinion, this is still acceptable.

Mechanics (Kinematics)

Now the translation of the rotary movement of the servo into a linear movement of the switch lever has to be solved. The above aspects should be taken into account:

  • High holding forces: We solve this requirement by allowing the servo to move in each direction to the 90° position. If a lateral force is applied (for example by cutting), the servo is only loaded radially in the direction of the pivot point and not in the direction of rotation (tangential). The current consumption of the servo does not increase, which has a positive effect on noise and service life.
  • Cost-effective: Apart from the servo, some wire and wood as well as a screw plus washer screw, no parts are required. It should all be in your tinkering box. Depending on the position of the servo switch, an extension of the servo cable may have to be planned.
  • Reliability: Because there are few moving parts and they are very robust, the reliability for model making purposes is more than given.
  • Maintainability: Nothing is more annoying than when a switch fails, collisions or derailments occur and you don’t reach the culprit or have to rip open the entire track bed for repair. Therefore, this mechanism is designed in such a way that it can be completely removed downwards and serviced at any time after loosening a single fastening screw. Only when reinserting the threading of the wire into the hole of the turnout slider is a little tricky. But everything has its price 🙂

Optionally, the switch end position could also be detected by push-buttons and be integrated into the automatic system by means of a feedback device. However, since the drives are very reliable, this has been dispensed.

Above-ground version

Of course, the kinematics shown are only possible if sufficient space is available below the surface. But various variants are conceivable, for example for staging yards:

Above-ground servo position

With a little skill, this installation variant can also be used for the visible part. Then the servo could perhaps be ‘camouflaged’ by the yellow cover of a DB electrical distribution box and some bushes.

Conclusion:

  • Very realistic and quiet setting process
  • Finely adjustable positioning speed
  • High holding forces
  • Comparatively cheap
  • Functional turnout spring: Forcing open the points is possible

Indeed there must be lost of other solutions. But this one impresses with its simple design and ease of maintenance.



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