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Gear Advice - shopping list help?


garygfletcher

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If I was doing it, I would go with a spur reduction from the motor shaft in the tender to drop the drive shaft below the footplate and another in the loco firebox to lift the drive back up to the height needed for the worm gear on the driven axle. 

 

Incidentally, I realise it is probably just a representation at this stage so you are probably aware of it, but if you make your universal joints as shown in post No. 10 the shaft will fall out. They need a sleeve on the outside to keep it in place.

 

This arrangement tends to result in a noisy mechanism, especially with metal gears, as the spur gears are then rotating at high speed. Alan Smith has a loco like this, which he regrets having built that way.

 

Chris 

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If I was doing it, I would go with a spur reduction from the motor shaft in the tender to drop the drive shaft below the footplate and another in the loco firebox to lift the drive back up to the height needed for the worm gear on the driven axle. 

 

In my view this would only add unnecessary complication, a loss of efficiency and lots of noise. The thin, typically 10thou, shaft is all but invisible, particularly when running. Don't take the log that Dapol use on their tender drives as an example.

 

Jerry

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In my view this would only add unnecessary complication, a loss of efficiency and lots of noise. The thin, typically 10thou, shaft is all but invisible, particularly when running. Don't take the log that Dapol use on their tender drives as an example.

 

Jerry

 

I seem to recall there was an article in MRJ by Guy Williams regarding a 4mm model (a ROD?) which came to similar conclusions.

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Like I said:

If I was doing it, 

 

  :D

I don't think I have ever seen a Dapol tender drive so I have no idea what they use but as far as possible I like to have everything that is not a part of the reproduction of the loco out of sight, so, the drive shaft goes under the footplate as a principal of designing the mechanism.

My understanding has always been that worm gears are more efficient (or should that be least inefficient) at lower speed and lower ratio as well as reducing wear on the worm wheel. My preference is to put the worm gear as the last stage of the drive. (Ideally there would not be worm gear at all but bevel or helical gears small enough to be used in a 2mm loco mechanism and still get a reasonable reduction don't seem to exist.)

Jim's 0-4-2 on the other thread, which I use an example as some of you may possibly have seen it, has spur gears from the motor shaft to take the drive below footplate level and while I have only seen that loco running in the environment of an exhibition hall I can't say it struck me as particularly noisy.

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I personally think the OP's original posted design in #10 is fine, it keeps the drive shaft at a nice level, there was just some confusion over the second/spur gear stage, the particular gear sizes, the desired total ratio, and the actual arrangement. Still think it offers the best, easiest, most flexible combination for simple construction.

 

Izzy

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I personally think the OP's original posted design in #10 is fine, it keeps the drive shaft at a nice level, there was just some confusion over the second/spur gear stage, the particular gear sizes, the desired total ratio, and the actual arrangement. Still think it offers the best, easiest, most flexible combination for simple construction.

 

Izzy

 

There are many cases where a two stage reduction is more flexible, it allows you to adjust the point at which the drive rises above footplate level so it is concealed inside the firebox, tanks or whatever. Whereas in a single stage mechanism this always has to be above the driving wheel. Also with most smaller wheeled locos, a single stage implies inclining the worm drive so it can connect to a motor.

 

But there are other cases where the opposite is true. A two stage mechanism implies a gear stage offset from the centre line. That is sometimes impossible to conceal from view, whereas a wormwheel on the centre line going up into the boiler is less obtrusive. Dapol used this technique on the Terrier to good effect. The designs drawn here have not really shown this effect, as the gears have been drawn much thinner than our gears really are (2mm width). In reality, our two stage gear trains end up with the second stage pretty much over by the frames. 

 

I am not sure how one could ever describe a two stage mechanism as 'easier' than a one stage - all other things being equal, surely having only one set of meshing to worry about will be a benefit. 

 

All of my loco chassis etches include a two stage reduction, partially because most of them have been small wheeled locos - although anyone who wants just to put in a single stage can do that if they prefer. And a number of the big names in 2mm loco modelling have done just that.

 

Chris

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There are many cases where a two stage reduction is more flexible, it allows you to adjust the point at which the drive rises above footplate level so it is concealed inside the firebox, tanks or whatever. Whereas in a single stage mechanism this always has to be above the driving wheel. Also with most smaller wheeled locos, a single stage implies inclining the worm drive so it can connect to a motor.

 

But there are other cases where the opposite is true. A two stage mechanism implies a gear stage offset from the centre line. That is sometimes impossible to conceal from view, whereas a wormwheel on the centre line going up into the boiler is less obtrusive. Dapol used this technique on the Terrier to good effect. The designs drawn here have not really shown this effect, as the gears have been drawn much thinner than our gears really are (2mm width). In reality, our two stage gear trains end up with the second stage pretty much over by the frames. 

 

I am not sure how one could ever describe a two stage mechanism as 'easier' than a one stage - all other things being equal, surely having only one set of meshing to worry about will be a benefit. 

 

All of my loco chassis etches include a two stage reduction, partially because most of them have been small wheeled locos - although anyone who wants just to put in a single stage can do that if they prefer. And a number of the big names in 2mm loco modelling have done just that.

 

Chris

 

 

Yes, I was only really thinking in terms of this particular locos design requirements, and that placing the gearbox/worm reduction behind the driven wheel means that more space might be available over the driving wheel for situating as much weight as possible for traction, while the spur gears won't show due to the size of the splashers. It would also give the option to vary the gear reduction between 30-1 to 47-1 using M0.4 14/14 to 14/22 or 30-1 to 68-1 using M0.3 14/14 to 14/32.

 

I always prefer to be able to drop wheels out of a chassis wherever possible, and having the drive via spur gears also makes this slightly easier, as meshing 64dp/M0.4 or 80dp/M0.3 spurs is more forgiving than meshing 100dp/M0.25 worm gear sets. That's often the problem though in trying to give advice isn't it, we all have our own preferred way of doing things and tend to think in those terms, which others might not want to use.

 

Izzy

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Not mentioned so far (I think) in the discussion is the gear mesh calculator http://2mm.org.uk/products/gear_calc.htmas well as the info on gears in the Yearbook. Both very importnat to loco builders.

 

There is a sort of clever trick with the gears that for certain gear ratios, it possible to use a gear centre for the spur train that allows either M0.4 and M0.3 to be used. One example is 14:18 using M0.4 and 18:25 using M0.3, which have pretty much the same meshing centres.You can also, provided the gears don't foul anything, vary the gear teeth. For example, 18:25 and 19:24 in M0.3 have the same meshing centre.

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Incidentally, I realise it is probably just a representation at this stage so you are probably aware of it, but if you make your universal joints as shown in post No. 10 the shaft will fall out. They need a sleeve on the outside to keep it in place.

Another way of doing it, which I use, is to cut the slot in the shaft itself and end the shaft flush with the outer face of the bearing.  That way the bearing acts as the sleeve.

 

Jim

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Where can I buy some shaft rod this size?

 

thanks

 

g

 

Piano wire, or guitar strings? Eileen's Emporium sell piano wire.

 

https://www.eileensemporium.com/index.php?option=com_virtuemart&page=shop.browse&category_id=277&Itemid=9

 

Although I tend to buy mine from the more obvious sources of supplies for pianos and guitars!

 

http://www.amazon.co.uk/Ernie-Ball-Single-Guitar-strings/dp/B005JC9NLG/ref=sr_1_2?ie=UTF8&qid=1429428217&sr=8-2&keywords=guitar+strings+high+e

 

Also a good material for handrails.

 

Chris

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