Scaling Gears

KiwiPete · Jun 2, 2015

Probably a simple question but what is the accepted way of scaling a gear train from a prototype? Obviously he centre-to-centre distance (and hence the pitch circle diameters) scale according to the scale factor but what about the number of teeth. How do you go about selecting the number of teeth that both looks right and is achievable?
Pete

kvom · Jun 2, 2015

If you want the same gear ratio as well as the center-distance, then the number of teeth has to remain the same, and the diametric pitch has to have the same scale factor. This is doable if you can cut the tooth form with CNC or else find/make cutters with the needed DP

It's more likely to say you'll need the same ratio, so the numbers of teeth can vary, but then using standard cutters it's almost certain that the center distance will be off scale.

RichD · Jun 3, 2015

The scale should be as easy as

"the original DP"/"the proposed scale factor for the project"

thus keeping the number of teeth the same.

Example...
project scale 1/12th =(0.0833)
original DP=10DP

formula is
10/0.0833 or 10*12=120DP

Hope that helps...or at least is correct.

Rich

Swifty · Jun 3, 2015

RichD said:
The scale should be as easy as

"the original DP"/"the proposed scale factor for the project"

thus keeping the number of teeth the same.

Example...
project scale 1/12th =(0.0833)
original DP=10DP

formula is
10/0.0833 or 10*12=120DP

Hope that helps...or at least is correct.

Rich

Were getting into the scale of nano gears here, a strong microscope would be needed to see the teeth. You will find it difficult to keep the same number of teeth when scaling down. I feel that it will be a matter of how much you want to scale the size down, if you want to scale it down to lets say 1/10 size, you would have to pick a gear tooth size that looks about right for the diameter chosen. You could always Google "Spur Gears" and look for a catalog that shows gears about the diameter that you want, then check out what tooth size they use.

Paul.

Jasonb · Jun 3, 2015

It very much depends on the chosen scale of the model and the pitch of the original gears.

In a lot of cases the commonly available DP series gears work out nicelyfor the commonly used scales eg 1/4, 1/8, 1/12 particularly when the original had an even numbered DP for example

20T 4DP original has 5" PCD,

1/4scale = 20T 16DP (4x4) and has a PCD of 1.25"

1/12scale = 20T 48DP (12x4) and has a PCD of 0.416

So in all the above they scale exactly

Now if you picked an odd scale of say 1/5th and the original had 3DP gears to keep the ctrs and tooth count the same you would need a 15DP cutter (5x3) so not an off the shelf item but if you wanted to make your own cutters then not a problem as you just work it out for the chosen DP.
The other option is to fiddle it by going for 16Dp which is the nearest and adjusting teh tooth count and maybe ctr distance to suit.

The other thing to watch if you want "realistic gears" is that the commonly available 20pa gears and even the 14.5pa ones tend to have the wrong profile when compared to the older gears which tended to be longer in the tooth in more ways than one. So another case for making your own cutters

gbritnell · Jun 3, 2015

This is a the little Case traction engine that I made. When I scaled it down I scaled the size of the gears from my original model. I can't say whether the original had properly scaled gears or if they were built to some conveniently available pitch. The scale of this little engine is about 1/48th from full size. From full size I mean the 1:1 engine. As can be seen in the pictures the gears are, in my opinion, aesthetically pleasing. Unless you were going to make something really, really small you could cheat somewhat on the gear sizes and not have them look out of proportion.
gbritnell

KiwiPete · Jun 3, 2015

Thanks for all the helpful replies guys. I'm very grateful for the interest. I think Paul, JasonB and gbritnell all addressed the problem I was having, that of scaling the gears so that the approximate ratios and spacings were met, but the gears also looked 'sensible'. It seems that the suggested solution of playing around with the various parameters until a solution "looks right" is the way to go (as in gbritnell's impressive model). Back to the spreadsheet and drawing board! I hope to make my own cutters (another challenge yet to be met!) so that gives me an extra degree of flexibility.
Thanks again everyone
Pete

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