Sunday, 30 September 2012

Gears - A Useful Tip


Recently I have been working on a couple of bogies for my diesel railcar. The chassis is based upon bits from a TOMIX chassis, unfortunately I have been having some problems getting them to run reliably after converting the wheels to 2mm finescale standards. Below are a couple of photos showing the conversion...

The conversion consisted of two pieces of 0.5mm thick PCB glued to the sides of the TOMIX bogie with picups and the cosmetic sideframes soldered to them. Sadly though the slots I filed for the wheels and intermediate gears had too much play in them which caused them to bind when rotated in one direction. I then made the decision to rework the whole geartrain...

How to Work Out What Gears Were Used...

I needed to figure out what gears were used in the bogie, firstly so when I made a new set of sideframes I could get the gear meshing distances correct. Secondly I could them order some replacement gears to fit some new axles. The information I had for the gears were the Outside Diameter and the Number of Teeth. From this I found out the following...

The wheel axle gears 1 and 5 where identical. The idler gears 2 and 4 were also identical. Therefore:

1 & 5 Outside Diameter = 3.9mm, Number of Teeth = 11
2 & 4 Outside Diamter = 4.2mm, Number of Teeth = 12
3 Outside Diameter = 5.1mm, Number of Teeth = 15

Firstly I needed to work out what the Diametrical Pitch (DP) of the gears were, this is basically the size of the teeth and could either be in MOD (metric) or DP (imperial). The equation I used based on the information I had was:

DP = (Number of Teeth N +2) / Outside Diameter OD


DP (11t gear) = (11 + 2) / 3.9 = 3.33
DP (12t gear) = (12 + 2) / 4.2 = 3.33
DP (15t gear) = (15 + 2) / 5.1 = 3.33

To convert this from Diametrical Pitch (DP) (imperial) to MOD (metric) I used another simple equation:

MOD = Diametrical Pitch DP / Number of Teeth N


MOD (11t gear) = 3.33 / 11 =  0.3 MOD
MOD (12t gear) = 3.33 / 12 = 0.28 MOD
MOD (15t gear) = 3.33 / 15 = 0.22 MOD

Taking the average of the three = 0.27 MOD

As metric MOD gears come in standard sizes of 0.2, 0.25, 0.3, and 0.4 MOD I decided 0.3 MOD gears would be the best match for these bogies.

Finally I needed to work out the Pitch Circle Diameter (PCD) of these gears so I could work out the meshing distances.

Before I jump into yet another equation the PCD of a gear as shown above is basically the meshing diameter of a gear. Technically if you put two gears together so the PCDs touch the gears should be perfectly meshed. Of course in real life its sensible to add a little extra to the dimension to allow for tolerances and such (I usually add 0.1 to 0.2mm). So...

Pitch Circle Diameter PCD = Number of Teeth N x MOD


PCD (11t gear) = 11 x 0.3 = 3.3mm
PCD (12t gear) = 12 x 0.3 = 3.6mm
PCD (15t gear) = 15 x 0.3 = 4.5mm

At last! I had all the measurements I needed to rework the bogies. I then placed an order with Mikroantriebe for enough gears for two bogies and them opened my copy of AutoCAD and come up with this...

I then used my compound table on my mill/drilling machine to accurately drill and machine some new sideframes out of 0.5mm thick PCB once again and now the bogie looks like this...

And yes, it works fine (so far!) I hope this will be useful to others, especially if you need to get hold of a replacement gear for something...

Julia :)