HI Sophie
I’m an engineer and a science communicator, so you’d think I might be able to help with your problems. I’m afraid that there really aren’t any good solutions unless you’re prepared to spend some money, and even then the problems might not be solved in the way that you’d like them to be.
Taking your problems in order:
1. Moving Cars/Busses
The obvious solution here is to use electric power, and to make simple chassis for your models which have a battery holder, a motor and a suitable drive train. There are a number of commercially available things out there, sold by the likes of Rapid Education, TTS and Mindstorms that might be suitable. A more high-tech approach might be to produce a custom chassis kit which can be 3D printed. Maybe you’d have to change the workshop so that people get to assemble and decorate the bus body and then try it on a chassis, which either stays in the museum or can be bought separately. With a bit of imagination it might, jut might just be possible to make a chassis that could actually jump a gap, but this would need to be custom designed and made, and you’d need to pay for the R&D time to do this.
Another approach might be to make a ‘dumb’, unpowered chassis with very free running wheels - again these would need to be properly made components, bought in, and then to launch the busses down a ramp so they gained speed before running up a short ramp and then leaping a gap. When I were a lad, back in’t last century I has a race-track for toy cars that more or less did this, but the cars had very free running wheels indeed. Cardboard engineering, particularly done by kids, won’t do this at all.
Again there’d need to be significant R&D time expended to find a sensible way of making freewheeling busses.
2. Cogs
First, can I correct your language slightly? A wheel with a toothed rim designed to mesh with similar wheels to transmit power is NOT properly called a ‘cog’. The correct term is a gear, or a gearwheel. A cog is a lump of wood, and by extension is a separately made wooden tooth inserted into the rim of a large wooden or cast-iron gearwheel used in millwork and similar large-scale applications. If your machinery contains wheels with inserted wooden teeth these wheels, and these wheels alone, can be referred to as cog wheels, but the general term for toothed wheels that work together to transmit power is, as I note above, gearwheel or gear.
Wheels with teeth around the outside are called spur gears if the teeth run straight across the width of wheel rim. If the teeth slope then the wheels are correctly called helical or spiral gears, or sometimes chevron gears if the teeth make a vee shape on the rim - this is the origin of the Citroen car logo, because Andre Citroen developed an improved way of making this type of gear, which is very hard to produce.
Making spur gears by wrapping strips of cardboard around centres kind-of works, but it demands very great skill of the maker, or very precisely cut pre-made parts to work properly. The sides of teeth of proper spur gears are a very precisely defined mathematical shape, known as a hypocycloid. This shape is used because, in theory, it allows the teeth to make rolling contact so that there is no power loss. In practise this never quite happens and you will typically loose 1% of the input power for each pair of gears in mesh in a train.
Its difficult to think of an easy way to make something that looks more like a proper gear tooth as part of a craft-type activity, because there isn’t an easy way of making properly shaped gear teeth in a fully equipped engineering workshop. I know, I’ve cut a good few gears, often for models of millwork, in my time.
To maximise the chances of success with an activity like this I’d be tempted to spend some money on getting the centres of the gears made from MDF by laser cutting or some similar highly precise and repeatable process, because getting the diameter of the centre just right is critical to getting a strip of corrugated card to fit round the outside of the wheel so that a whole number of corrugations fit exactly round the circumference. Getting the centres machine made would also mean that the hole for the shaft was exactly in the middle of the wheel - another important criterion for success. Stiffening the teeth by treating them with something like shellac varnish (french polish) after fitting them to the centres will also help.
Hope this helps
Best wishes
Richard
Richard Ellam
L M Interactive
Science Shows and Hands-On Stuff
Hi Everyone!
I’m stuck with coming up with a solution to 2 craft activities I’m using for family activities, has anyone seen or run something similar before and so can offer advice?
- Moving bus/car models
I have been making model buses out of small boxes (c.10cm tall), cardboard wheels, straws and dowels which roll along fine but I’d really like to make them self-propelling. I’ve tried balloon powered and elastic band powered but neither give enough power to propel the buses up a slight slope (we’re recreating a time a bus jumped over the opening bascules of Tower Bridge!). Does anyone have another way of propelling vehicles other than pushing them?!
- Cogs
I would like to do a cog making activity but have got stuck on how to make them, other than by using corrugated card wrapped around something circular. I’m looking for something that looks more like a cog than that!
Many thanks in advance
Sophie
Sophie Jordan
Education and Marketing Assistant
Tower Bridge and The Monument
London SE1 2UP
Tel: 020 7332 1185
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