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In depth idiot
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Subjectively *, the very lossy nature of worm drives is a function of the power transfer by a sliding contact of the working surfaces. Spur gears can be made pretty efficient with tooth profiles optimised so that the working surfaces have a line contact which rolls across the tooth face, there is prac tically no sliding action at all. Which is fine when the drive axis stays parallel, but when there is a requirement to turn the axis of a drive line, it is no longer possible to maintain a rolling line contact. It is a shame that bevel gearing is more expensive than worm drive, otherwise this more efficient option for our frequent need to turn the drive in railway modelling might be more widely available.

* I once had the great privilege of working with a team of people who engineer serious geared transmissions. (Think as big as your house, for electrical power generation.) When they started into the arcana of their speciality, my head very quickly began to hurt!
 

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In depth idiot
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A toothed belt or 'bicycle' style linked chain is pretty efficient (you would die on a bicycle were it not so!), and the losses in using such a drive to turn the drive axis are small. However, wear on the belt or chain is significant.

When the basics of the motor car were still being developed there was much experimentation to find the best trade off between efficiency losses and manufacturing cost in the gear arrangement to turn the propshaft drive onto the back axle. Forms of worm drives were tried, but it quickly became apparent that the relatively high cost of machining curved face bevel gears paid off in getting a greater proportion of engine output to the road wheels. I suspect for model railway products the increment for a more powerful motor to offset the losses caused by simply machined but inefficient gearing, is smaller than the increment for gears which present a more exacting machining task.

A 'fat' worm doubles up as a flywheel, Bachmann still make some use of this in their models. To be effective the flywheel should be as large a diameter as possible, to maximise kinetic energy storage, which increases as the square of the velocity. However friction and wear increases as velocity rises, so it is more effective to use a separate flywheel (of as large a diamneter as possible) and a significantly smaller diameter worm.
 

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In depth idiot
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QUOTE (Richard Johnson @ 11 Mar 2009, 03:44) <{POST_SNAPBACK}>.. but how many other modellers really care ..
I care enough to be pleased that most of the better UK RTR now has a two stage reduction, by a fairly coarse worm, followed by a spur gear reduction stage. The reason why the older Bachmann 'fat worm' drives works as well as they do, is that the reduction ratio on the worm stage is fairly small. But the newer designs use smaller diameter worms that are still fairly coarse, which as you observe is better yet.

When building a chassis or installing a replacement drive in a chassis I would like to do it that way too, and always use a two stage box if it can be accomodated. But old habits die hard, and the usual worm/pinion combos economically available in the UK are higher reductions than is desireable, dating from the days of motor worm directly driving the pinion on the axle. Offer a fold up gear box kit with a fairly coarse worm, and adjustable reductions (and axle positions too ideally) via a spur gear set; all made in a low noise materials combination for the gear set, and I would be interested. This would be ideal for the typical small boilered 1/2/3 F 0-6-0 that is unavailable RTR, but so necessary for any UK steam era layout...
 
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