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Friday, 23 December 2016

Project 17 - Swanage Turntable

I scratch built this motorised turntable before I built the layout module shown in Part 1 of this series. The turntable is the pre-1967 version that was probably made by Cowans Sheldon and Comp. The other notable manufacturer being Ransomes and Rapier.

At the outset I had no idea on the best way to make a motorised turntable from scratch. Years ago I had made a manual operation turntable from an Airfix kit but that experience was no help in making this one. So, it was a case of developing it step by step.

This turntable called upon all the technical and craft skills used in railway modelling from paper modelling to mechanical and electronics engineering with a bit of carpentry thrown in as well.

The research phase, mostly on the web,  found many photographs and two engineering drawings of Cowans turntables so that was a good start. What I ended up with is very close but sadly not identical to the Swanage prototype due to mistakes I made in my design by not following closely enough the research data.

Deck

The turntable Deck was made from Grey Board with decorative paper overlays. The rail is Code 100 flat bottom (because I'm using my spare stock of code 100 throughout) with 3D printed base plate fittings. The handrail is copper wire and the brake/locking mechanism (dummy) 3D printed.

The wheel trucks at each end are 3D printed and the metal wheels (2 at each end) are coach disk wheels with the flanges ground off. Phosphor bronze wires pick up track power from the wheels and these are wired to the deck rails.

Well 

The Well has a half inch chipboard base and is cambered as the prototype using wood filler. I believe the base is concrete in practice so this was easily painted as such. The walls are Grey Board with decorative paper overlays. the Well track is code 100 mounted in base plates cut from proprietary flexi-track which are themselves mounted in a ring of foam board with the top paper layer removed. This gives a bit of flex to the rail but is probably not a necessary feature.

Track Power Feed

The track in the well is two pieces wired to the train controller and are isolated from each other with insulated fish plates. Ideally only one wheel at each end of the deck should pick up power but I wired both in case one wheel lifts off the track. Two live wheels would result in electrical shorting when the two wheels straddle an insulated fish plate during operation so, the power is fed via a switch. Track power is turned off during rotation.

Electronics and Mechanics

I used a 1.5V-6V high torque motor found in my spares box. The gear reduction ratio of 12:1 was taken from that specified in a proprietary turntable motor and gear set. But that still spins the turntable too fast without further measures being used to control rotation.

The gears are 3D printed and don't need to be physically that big. They are that size due to printing limitations for the teeth profile.

The most important motor parameter is torque because the motor has to overcome at low speed friction, gravity and the weight of a locomotive. Torque is related to motor current and at low speed, i.e. low voltage, the current is reduced proportionally leading to motor stalling. Simple potentiometer voltage control is not therefore recommended. The solution is pulse width modulation. The motor is pulsed at full voltage all the time but the pulse width is varied by a potentiometer and this changes the average voltage thus controlling speed but since the amplitude of the motor voltage remains constant  the motor is always at full strength. The result is that the motor can be rotated much more slowly without it stalling. Here is a web link to one such electronics circuit: http://www.circuitstoday.com/dc-motor-controller that I used. One undesirable side effect is a screaming motor in operation due to the pulse width frequency. I suppose it could be likened to a rusty prototype in need of an oiling.

The motor body is held in the base by an interference fit. The big gear has a built-in 15mm spigot that passes through a roller bearing fixed in the base (interference fit). The Deck is screwed to the top of the spigot.

The whole turntable is clamped to the layout baseboard with a ring of cable tie wraps. These can be easily cut so that it can be removed without damage if necessary.

Control Panel

Pretty self explanatory from the photo. I have not included a track alignment locking mechanism. The tracks are lined up by switching the Rotate Switch, back and forth as necessary. The Rotate Switch is sprung, centre off.



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