pic: Experimental VP drive gearbox

[Chris is me]

Originally Posted by pilleya

The reason the VP load rating charts are very different is that the 775pro has a free speed of around 18730rpm and produces 0.71 N.m of torque at stall. The CIM on the over hand has a free speed of around 5330rpm and can produce 2.41 N.m of torque at stall.

The 775pro needs a greater reduction (3 or 4:1) in order to match the speed and torque of the CIM. This is the reason why with a CIM 64:1 is the maximum ratio for safe operation, and with a 775pro a 210:1 is the max.

That's his point - the max safe ratio for a box with 2 CIMs would probably be something in the ballpark of 32:1.

While you get some flexibility in gearing in a setup like this, I think in reality you'd find the ratios you get to choose would be a bit too spread out to actually be practical for this application. You can do 3:1, 4:1, 5:1, and 7:1 stages. So you'd have to jump quite a large gap in your reduction to change things out. Generally when you want to change a drivetrain gearbox ratio you don't want to go from 16 FPS to 8 FPS, you want to raise or lower the ratio just a small amount. This kind of gearing flexibility can be designed into custom spur gearboxes fairly easily - ie a 20:54 reduction could be replaced with a 24:50 or even a 18:56 reduction in order to fine tune the precise reduction.