Starting work on the locomotive
Entry posted by whart57 in The locomotive
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The Brede tramway only had one locomotive, a pony substituted for it when the locomotive was out of service for maintenance or other reasons. So I envisaged only having one locomotive in this project too. There were two obvious choices. One was to build a small Bagnall loco like the one actually used on the Brede tramway, the other was to build a model of the small Hunslets used at Deptford by the War Department and later sold to the Sand Hutton line in Yorkshire. I had better documentation in the form of drawings and photos of the Hunslets but the Bagnall design won out because I had a kit of one, albeit for a 2' gauge line and to 7mm scale.
This kit was designed and produced by Roy Link in the 1980s, and was, I believe, still available from Narrow Planet until fairly recently
I really wanted to model a 18" gauge line though, and I also wanted to model in a scale larger than 7mm scale. I had made a start on the kit but idly thinking about things and making measurements and comparing them to drawings I came to the conclusion that the kit could be modified to suit both 1:32 scale and an 18" gauge prototype. How well it managed was not apparent until a day or two ago when another rmweb contributor posted a drawing of a Bagnall Sipat class in response to the opening entry in this blog. A little jiggery-pokery in GIMP and I was able to produce a drawing where the outline of the Link kit was superimposed onto the Sipat drawing.
The chassis is pretty much spot on and while some bits of the superstructure need remaking to a larger scale, a lot is re-usable.
I also wanted to try radio control. One of the issues I have with models of narrow gauge and light railway prototypes is the shiny rails needed for good electrical contact. Track also has to be well made because electrical continuity is the first thing to suffer with uneven track. Compensated chassis aren't quite so badly affected but a rigid four wheel chassis will stutter as a result of poor continuity long before anything falls off the rails. I thought that having a battery powered loco controlled by radio would mean rusty and uneven rails would be a possibility.
The technology to do that is now available and not that expensive. Lithium-Polymer (Li-Po) batteries pack a lot of punch for their size - rather a lot of punch if you see some of the YouTube videos of these exploding. However in those horror cases it would appear the disaster factors were the fact they were big power-packs to drive aerial drones and they were being abused by fast charging procedures. Small 3v Li-Pos didn't seem to be so scary.
A 3v battery implies a 3 volt motor. These are now easily available in the form of the N20 motors used in all sorts of devices. These Chinese motors have the reduction gearing in the form of tiny spur gears rather than a worm which means they have a good bit of torque despite the low voltage. A fellow 3mm Society member, Geoff Helliwell, has done a lot of work on developing motor bogies and locomotive mechanisms in 3mm scale using this motor type, albeit the 12v version, and I was able to draw on his work. The motor itself fitted where a standard 12v motor was meant to go, thus pointing itself up into the firebox
Obviously a worm wheel was now out of the question - unless I wanted ultraslow running - but the Helliwell concept uses a crown and pinion arrangement. An idler gear was needed to offset the motor, else it wouldn't go into the firebox of the superstructure.
The arrangement was tested using a couple of AA batteries and found to give an acceptable top speed and enough oomph to pull some wagons.
I'm now at the stage where I really need to commit to finishing the valve gear in order to finish the chassis, and I have to admit, that is a little bit of a scary thing given the motion doesn't have a lot of waggle room.
Edited by whart57
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