Inspired by Jennifer Clark, who'd built slewring in her JCB JS220 Excavator
to drive the treads, while the motors were in the upper part, I thought this
slewring could be built smaller.
Clark uses 2 cluth-gears connected by a transmission driving ring (ldraw 
part 6539) and an extension ring (ldraw part 32187). This whole combination
is 4 studs long and spends therefore much usefull space.

I tried a combination of 3 studs long, but I couldn't make the gears fit well
without a 'large' construction. Thereafter I tried 2 clutch-gears hooked in
eachother. It fits perfect with all the gears, so this is my final design.

If you use any off my models, please refer to Sid Visser a.k.a. 0937