Noisy gears

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Phil.P
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Re: Noisy gears

Post by Phil.P » Sat Feb 04, 2023 11:14 am

Yes, it always seems strange that people talk about 'using 30:1 gear boxes' but no mention of the motor-rpm, or final wheel diameter..

I think many of our models are run very fast (compared to the prototype).

I suppose I need to design/build a 'speed-trap' as well?
:lol:

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Re: Noisy gears

Post by GTB » Fri Feb 17, 2023 3:07 am

Some more technical info to finish this topic, ie. an overview of what is needed to determine the gears needed and to lay them out........

The sort of gears we use in models are what is called involute gears, which refers to the shape of the teeth. The sides of the teeth follow a particular curve so that the tooth to tooth contact is a rolling motion, not a sliding one. Which reduces wear and noise if properly meshed.

When laying out the gears for a chassis or gearbox, the relevant dimensions needed are tooth number (sets the reduction ratio), PCD ( pitch circle diameter which determines the spacing of the gears) and either MOD (module) or DP (diametral pitch) which determines the tooth size and compatibility with other gears. These are the only dimensions that we need to be worried about when choosing commercial gears for a model, the other dimensions are only relevant to making the gears from scratch.

MOD is used to describe metric gears. The formula used is MOD = PCD x no. of teeth.

DP is used to describe gears made in english units and is still used in the US. The formula is DP = PCD/no. of teeth.

Standard commercial gears are usually made in a limited range of MOD/DP and tooth numbers, which keeps cost down.

Contrary to what you may read in the descriptions of cheap gears sold on Ebay, etc. metric unit gears and english unit gears are NOT compatible. As an example I recently bought some gears made for the gearbox of an r/c car that were claimed to be 42 DP. Being made in China I doubted that and when they arrived, as expected they were 0.6 MOD, which is equivalent to 42.333 DP. It didn't matter to me in this case, I wanted metric gears for my application anyway....... :roll:

If you do the calculations, for obvious reasons when converting from mm to inches there can be no exact equivalents between MOD and DP gears. It would be easy to make a gear with some multiple of 42.3 teeth by screwing up the indexing, but getting it to work would be interesting........

Since I much prefer to use reference books than online sources, my usual reference source when working with gears is the first half of 'Gears and Gearcutting' by Ivan Law. Aimed at model engineers, This is no. 17 in the Workshop Practice Series published by Special Interest Model Books in the UK and is still in print (and a lot cheaper than a copy of Machinery's Handbook :shock: ).

Yes, I have read the section on gear cutting in Ivan Law's book and no, I'm not planning on taking it up any time soon. :study:

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Re: Noisy gears

Post by GTB » Fri Feb 17, 2023 3:35 am

Phil.P wrote: Sat Feb 04, 2023 11:14 am I suppose I need to design/build a 'speed-trap' as well?
If you like fiddling with electronics go for it......

I just use a cheap timer and a calculator, or a mobile phone could do it. The mainline loop on my track is 120 ft long and if a model takes about 1 min 20 secs to travel back to the starting point it will be doing around 20 scale mph.

Which just happens to be the maximum speed the VR allowed on the ng lines.

Graeme

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Re: Noisy gears

Post by ge_rik » Fri Feb 17, 2023 7:50 am

That's a really interesting and helpful summary Graeme.

When I was trying to find replacements for the large diameter worms on my early Bachmann Lyn motor block, I discovered a website which calculated the MOD if I entered the worm wheel diameter and the number of teeth. Of course, even with a digital micrometer it was tricky trying to measure the diameter and a difference of even a few hundredths of a mm made quite a difference to the MOD. I sent off for replacement worms which were as near as I could find to the average of calculated MOD values and, inevitably, they didn't mesh. I see now that I should have been calculating DP, as Bachmann is a US company; though, probably, I still would have struggled to find an exact match.

In the end, I discovered that the worms from a different (and redundant) Bachmann loco were the same, as was the motor, and so used that.

Rik
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Re: Noisy gears

Post by GTB » Fri Feb 17, 2023 11:42 am

ge_rik wrote: Fri Feb 17, 2023 7:50 am I see now that I should have been calculating DP, as Bachmann is a US company; though, probably, I still would have struggled to find an exact match.
Bachmann used to be American, but for a long time has been a subsidiary of the Kader group, which is Chinese owned. More importantly the factories and design offices are in China, so their gears are more likely to be metric. I wouldn't be surprised though if that strange worm gear arrangement they used in the early Lyn was a unique MOD size.

I've avoided worm gearing in garden scale and know just enough about them to be dangerous. I think they use an involute tooth form, but a properly made worm wheel and a spur gear aren't interchangeable, even if they are the same MOD or DP.

One of the 'fun' things about using gears is that the critical dimension is the PCD and that can't actually be measured, it can only be calculated from the other parameters. Which is why I pay the extra and use gears where the manufacturer specs. are available.

Graeme

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