Sunday, 8 June 2014

More on the tramway...

Part 2 The auto-reverse mechanism 

Part one can be found HERE.

The trouble with having several projects on at the same time becomes evident when a few months go past and nothing has moved on. Well, today it was the turn of the automatic tramway to make a move for the limelight. In the first part of this story I got as far as setting up the chassis and getting it to run. At that point the electrical connections were just twisted in place and there was very little weight in the chassis. yes it ran but it was not that reliable as only one of the centre rail pick-ups was connected.

The chassis is now wired properly and has some temporary weight added
Temporary extra weight, attached to the chassis, has made a vast difference to performance. This will be removed once the body has been fitted... must get around to building that! The three-point suspension (described in part one) now keeps all the wheels firmly in contact with the track, providing a good electrical connection as well as keeping the tram stable.

Original mechanism
The reversing mechanism is a modern version of the unit we used on our transporter bridge we built a few years ago (right) We discovered after we built it that it needs to be assembled with the slotted dome-head bolts, otherwise it shorts out on the larger bolt heads.

The new unit uses the excellent geared motor found in the Evolution Helicopter set.

Note to Meccano: Please include this motor in one of the smaller sets!

The new motor also comes complete with a very neat reversing switch that can be operated in many ways. Here I have used the tri-axle pivot to operate it. The whole thing builds up onto a 5½ x 2½ inch flanged plate nicely.

New reversing switch is a vast improvement
You should be able to see how it goes together from the pictures. There is nothing complicated about it. I used plastic gears as they seem to be a bit quieter and there is no great load to cope with. Careful adjustment of the linkage is required to get the timing right. that can also be adjusted by changing the voltage supply to the motor. On my model I have set the supply to 6 Volts.

Another view of the switch may help if you are following this project
Power is supplied to the tram via a centre rail pick up and is returned back through the chassis and wheels to the track. The whole of the length of the centre rail is supported by plastic handrail supports. At each end of the centre rail, there is a section that has been isolated from the rest by inserting a short section of plastic rod, held in rod connectors. these insulated joints are bridged using diodes.  A diode will allow electricity to flow in one direction only, as the tram passes the isolation gap the tram will stop (if it doesn't, reverse the diode)  it will only start again if the reversing switch is thrown.

The insulated gap is filled with a section of plastic rod
The motor is hard-wired to the reversing switch and you will need to cut and strip the wires. the motor is connected directly to the power supply. The track switch is connected to the reversing switch and in-turn to the power supply. As the motor runs it causes the mechanism  to run throwing the switch from forward, through off, to reverse. This cycle takes longer than it takes the tram to travel from one end of the track to the other. When the tram crosses the isolated joint, it will stop until the mechanism has thrown the switch. at which point the diode will allow the electricity, to flow and the tram will set off again back down the track.

Wiring diagram
The diodes required for this project are 1N5401 These are 3A capacity and will be more than capable of handling the load. Smaller 1A diodes will probably do the job but there is nothing wrong with a bit of over-engineering! Electronic components are very cheap, these days and the recommended diodes sell for 10p of less if bought in bulk as low as ten at a time, they are even cheaper. We buy a lot of our components from a company called BitsBox and you can find them HERE. The diodes we recommend can be found HERE. If you would like to know more about diodes and how they work see HERE.

That all seems to work well. Testing over, all I need now is a body and to neaten up the wiring. I will show the finished thing soon...



  1. Those little grey motors can easily pull quite a bit more than 1A. I'd stick with the larger diodes.

    I do like the corner bracket worm cage in the top photograph. Very neat.

    - Pauli

  2. You are absolutely correct about those little motors they can pull huge current under load. It will work using the 1A diodes but they are at the top of their range and get a bit hot. Best to go for the 3A diodes even if they are twice the price - The 1A diodes are 4p and the 3A diodes are 8p!

    That small cage assembly was inspired by a similar item my mate, and fellow club member, Ivor Ellard came up with a year or so ago... just fine here!


  3. Ralph,
    looking at the changeover switch of the reversing mechanism, it seems to me that the action will be quite slow. I don't have one of those motors so I don't know but does the switch have a toggle action otherwise the slow changeover may cause sparking inside the switch. It is always best to have a quick "make" and "break" of contacts

    1. I do agree, but in this case I don't have a choice - seems to work aright at the moment... If it causes problems I will look at adding a 'snubber' capacitor across the contacts. It didn't need it with the old style (red) switch and the new switch has a similar contact arrangement inside (just smaller!)