Junction Mechanisms

It took much thought and trial and error to arrive at a suitable mechanism for the many junctions that would be required. The inspiration came from the design used by F-Track in Holland. After several false starts I managed to combine this with the same type of electric motor drive used by the Magnetic Stop Mechanisms so that the controls and wiring would be identical for both units.

The motor has a worm which drives a cut-down gearwheel. The rotation of the gearwheel is limited by the back of the unit. The rotation of the gearwheel is passed up the shaft to a disc set just below the top of the baseboard. The disc has grooves for the guide wires which rotate to line up with the two possible exits at each end of the travel.

Construction is from plastic card with motor, gears and shaft from Squires.

The motor is rated as 1.5-3.0V so can be operated from one or two standard batteries. You can either use a momentary contact switch to change the magnet‘s position or use a switch that keeps the power on. This will not hurt the motor but, believe it or not, the motor does draw more current when it is stalled (i.e. at the end of the crank‘s travel) than when it is turning. For the motors used on the Bolton‘s Bits units the stall current is in the order of 500mA so if you are running from batteries a momentary contact switch will make them last a lot longer.

A full tutorial on junction mechanisms can be found here.

Update: With the need to have automatic control I discovered that the electronics to interface this type of mechanism to computer or logic circuits was quite expensive and time consuming. Investigation of alternative mechanisms led me to the servos used in radio controlled models. These can be mounted under the baseboard with the output arm directly driving the junction’s disc. MERG provides a simple kit for a servo controller and the resulting mechanism scores over the home-brew design in a number of ways:

• When interfacing to computer/logic circuitry it is slightly cheaper
• The electronic controller is easier to build
• The junction mechanism is easier and quicker to put together
• The junction mechanism is easier to set up and aligns accurately each time with no possibility of the disc mvoing when the mechanism is not powered

As a result I have changed to servos for all the junction mechanisms.