Chris,

Looking at what you've got, I'd suggest that you try for the 30:1 reduction from the worm & 30T 100dp gear (assuming it's a 'worm wheel' and correctly cut for a helical drive) and then use the 12T/28T combination as the second stage to the drive wheel. The diameter of the 28T 100dp should be about 7.62mm which is smaller than the 8.05mm of the 30T (there is a very useful file titled "GearWS.PDF" on the VAG) and will give better appearance behind the 8.5mm diameter drive wheel.

However, following your comment about the 30T looking a little worn, it may be better to go back to first principles and design the gear chain from scratch. Using the no-load motor speed and desired final top speed of the loco, you can calculate the reduction needed, and the select fresh gears to suit. The Scale Speed Calculator (here) is a good place to start.

A few comments - take them with a suitable pinch of salt as I have yet to build my first loco chassis:

1) If you've not already done so, read through the recent VAG thread where Steve's proposals to use alternative gears (due to the recommended ones being out of stock in the shop) drew comments from numerous quarters.

2) Comments there suggested that using a non-helical gear to connect to the worm would be a bad thing (though it wasn't very clear whether the proposed gears were helical or not).

3) I don't think Steve's suggestion of switching the 28T and 30T gears will work - different inside diameters, so the 28T will need opening out to fit over the "sleeve" on the driving wheel axle, and the 30T will probably need a similar "sleeve" adding onto its axle.

4) I suspect with the relatively small driving wheel, you will not want an overall gearing as high as the original design. I've seen a suggestion that about 50 to 1 will work fairly well in most cases - with smaller wheels it corresponds to a lower top speed, which makes sense. However, some of the Nigel Lawton motors have a very high speed, and higher gearing would be wise for them.

David

Steve, many thanks for the link. I did have a quick look on the VAG but got a bit confused, and then when I realised that some of the gears were out of stock anyway, I went to look elsewhere!!

This thread has helped a lot to restore my sanity on the subject of gear trains.

I reckon when I get around to building my own tenders for the locos which are presently driven by Union Mills, I will investigate the gearbox design facility further.

However, with this little 0pen Cab, the intention was to try to use up what I've already got in my boxes, and I reckon a little gearbox as described, with the 30T will just about work.

If it doesn't well, nothing is really lost, because the whole engine has been built from left overs at little cost.

I will be following your 57X with interest, even if it is the wrong colour.

I am enjoying your blog and finding it very useful.

Regards,

Chris

...3) I don't think Steve's suggestion of switching the 28T and 30T gears will work - different inside diameters, so the 28T will need opening out to fit over the "sleeve" on the driving wheel axle, and the 30T will probably need a similar "sleeve" adding onto its axle.

Agreed, looking back at the pictures & text, the 28T & 12T shown both appear have inside diameters of 1.5mm - I'd made the assumption that the 28T had a 3.2mm (1/8") inner diam to go with the association gear m*ff.

Chris,

Out of interest, what loco is this for? What is it's nominal top speed, and do you have a motor in mind?

Steve - the 12T had to be opened out from 1mm to 1.5 mm to match the 28T.

The motor is indeed a Lawton 8x16 midi.

The body shell is a half cab 1F, a picture of which is here..

http://oldtrent.webs.com/littlemidland.htm

( scroll down to the bottom)

I know nothing about its top speed, but it will be the branchlines main engine and will therefore have to assist in some shunting.

Don - thanks for the advice.

I did wonder about this point and will investigate further.

Regards,

Chris