On the right tracks

22nd September 2000 at 01:00
Just how do trains stay on the rails? John Stringer investigates

What keeps the train on the track when you travel InterCity? If you thought it was the flanges on the wheels, you'd be wrong. Model trains are kept on the track by flanges - rims that stick out of the wheels on the inside. But if the flanges of a real train were constantly rubbing on the track, they'd be worn out in no time. So how does the train keep going on the straight and around curves without falling off? The secret is in the shape of the wheels in contact with the track. Children can make the discovery for themselves - using recycled materials.

Modelling train wheels - a classroom activity

This is a pleasant alternative to running cars down ramps! Make a string "train track". You need a couple of pieces of scrap wood, some drawing pins or nails, and a few metres of thin string or knitting wool. Tap two nails into each wood block, 10cm apart. Loop the string round the nails in two blocks and stretch it to make two parallel railway lines.

Putting one block on a chair or low table gives a slope; you can secure the blocks with reusable adhesives or sticky tape if you like (see picture on page 24).

The rollers represent the train wheels. Collect a few cylindrical rollers, such as different sizes of kitchen roll tube, an empty can , even a section cut from a plastic bottle will do. Make two tapered rollers too by gluing pairs of paper or thin plastic cups together - one base to base, the other lip to lip - to form "dumb-bell" and "lozenge" shapes.

Ask the children to predict what will happen before they try the rollers. Which roller will go furthest on the track? Then ask them to try, one by one. It is important to give each roller a fair start - at right angles to the track, and from the same starting point (see over the page for answer).

Were they surprised? The winner is the shape that self-corrects as it rolls. If it moves left, a smaller circumference is on the track on the right, moving the roller back that way. This self-correcting is so effective that this shape will go around curves - which you can demonstrate with a track made from curved wire or curtain tracking.

Tappers and train wheels

When the first train wheels were cast, the moulds were tapered to make it easy to remove the finished wheel - like removing a cake from a cake tin. The tapered wheels had this wonderful self-correcting quality. Modern trains have wheels that stay on the track at speeds of 300km per hor. So what are the flanges for? They keep the wheel on the track if the train goes round a sharp bend.

Train wheels have tyres - metal tyres fitted to the train centre. Rail workers used to tap the tyres, listening for good tyres that rang like a bell. Cracked wheels sounded different, and had to be replaced. You can hear the "pure" sound of a metal object by tying it in the middle of a string and putting the string ends to - but not in - your ears, then swinging it to hit a table edge. Children will find that an old kitchen fork rings like a church bell. Try a cake cooling tray. Modern electronic techniques of testing wheels are far more accurate than testing "by ear", but it is still important to look for imperfections.

You can see examples of tapered rollers in action at: The Magician's Road, The National Railway Museum, Leeman Road, York, YO2 4XJ. Call 01904 621261. Also, Launch Pad, The Science Museum, Exhibition Road, London, SW7 2DD. Education Booking Office 020 7938 9774

STAYING ON TRACK

When the first trains were invented, engineers tried different ways of keeping them on the track. They tried:

* A pin sticking out of the bottom of the train, a bit like the ones on Scalectrix cars.

* Rails like gutters - so that the wheels ran in the channels.

* Flanges - like the ones on model trains.

But none of them worked very well. The flanges on the wheels kept wearing out!

MORE THINGS TO DO

Science

* Experiment with toy and model trains. There are good examples from Fisher Price and LEGO (, as well as in 00 gauge model railways. Play Craft trains run in grooves. Ask the children to investigate how the flanged wheels work. Will they take a curve? A tight bend? What happens if they take the corner too fast?

* Children could explore the use of wheels to make work easier. When do we move things on wheels? Children could look at home and school applications - televisions on trolleys, moving PE apparatus, moving large furniture.

Technology

* Children could try modelling a railway. The easiest thing would be to use a kit - like LEGO; but notice that LEGO wheels are not noticeably conical; and neither are the wheels on 00 gauge electric railways. Try making a vehicle with conical wheels that will negotiate a string track. How much will the vehicle carry? Try loads of pencils.

* Children could design vehicles with fixed wheels, or with fixed axles and moving wheels. Which work best on a straight track? On bends?


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