Small Limited Slip Differential

I thought I would contribute to the unofficial small differential design competition, using an idea I have been toying with for a while, which is to use the 12-tooth tri-flat plastic pinions in a spur gear differential design, using Electrikit bush wheels as the carrier.

It worked out quite well by shortening tri-flat axles to 1” long and using small plastic mini collars to retain the axles within the bush wheels. To my knowledge, there is no useful wheel shaped part with holes at 5/8” pitch, so to mount this assembly to a driving gear would involve something non-standard.

The obvious solution was to remove the boss from a 50-tooth contrate gear, re-bore the central hole to clear the 3/8” diameter of a bush wheel boss and drill two bolt holes at the required 5/8” pitch circle diameter. This enabled a compact assembly, held together with two 1” bolts and hexagon nuts.

The nuts have to be positioned with their flats adjacent to the gears for clearance, and with nuts between the gear and bush wheel as spacers, to provide a good journal for the planet pinion axles.

This assembly also fitted inside a 1½” cube, forming the differential housing shown.

Initially I was disappointed that the plastic spacers just touched the pinion gear teeth, but on completion, the frictional resistance proved acceptable, enabling me to argue that this provided a limited slip function, just like the Torsen and Quaife production originals!

The half shafts carry plastic pinions at their inner ends, one retained axially with a plastic spacer between the pinions. The other half shaft is constrained axially with a plastic spacer behind the outer axle arm end plate (wheel disc).

The contrate gear is positioned at the open end of the cube, where extra clearance is achieved by opening it slightly, within the bolt hole clearance to the 1½” x 1½” flat plate.

The remaining construction depends upon the model application. The rigid axle arrangement shown, enabled me to test its traction performance.

I could claim that apart from cutting tri-flat axles, no Meccano was harmed in the creation of the model, because the mutilated contrate gear is Temsi!

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