As far as I know, Bellville washers (named after the 19th century inventor) are intended to go onto the gearbox shafts between the gears, spacers and bearings to provide some preload, since the helical gears will develop a lot of end thrust and there's just a wee bit of slop in the bearings and so forth. They can also absorb shock, such as when going from forward to reverse. It could keep the stress off the gear teeth.
These are actually springs that can be stacked in "series" (like a tophat cymbal) or "parallel" (stacked in same orientation) to give various spring pressures or spring compression travel. I thought the MSC catalog had some info and specs on them, but now can't find it, other than a plain listing under the hardware section around page 1895. Check out
http://www.bellevillesprings.com/ for more than you want to know. A 5/8 inch inside diameter Bellville washer of 1 1/4 diameter washer can take something like 725 pounds to compress, although there are all different thicknesses and styles.
However, I can't figure out just what amount of preload to use (how much shorter to cut the spacers to allow for just the correct preload). If you compress them too much, the gearbox has too much friction. Also, they are too big for the small gear, which would require a machine washer or two to serve as a perch for the large end and a spacer at the bearing end to prevent rubbing the outer raceway.