Measuring Preload

Earlier I reduced the preload in my gearbox by adding a gasket. I didn't have time to properly measure and set the clearance so I did it "by ear". After assembly, I couldn't hear anything slopping about in the box, so I hoped my theory on how it came to be so tight was correct.

Unfortunately it didn't sound so good when driving. I took it apart again and found some bad indications. The oil left in the casing had some sparkly metal in it, and the bearing outer races looked "bruised". I'm not sure if this was caused by running the box too loose or too tight.

I measured the preload using some roofing lead. This is very soft (pure lead is about twice as soft as solder, I could dent the stuff I used with my fingernail) and crushed nicely under the bearings. The basic procedure is to replace the shims under the bearings with small lumps of lead, assemble the gearbox, crushing the lumps as you bolt the casing together. After turning the shafts a few times, you take it all apart and measure the lumps with a micrometer.

I went down to the gearbox factory and was shocked to find that they have a big box of mixed shims, all different diameters and thicknesses. It took half an hour with a micrometer to find shims that set the preload correctly. It would seem that they don't routinely set preloads -- if they did, they would surely have them sorted into some kind of size order.

While I was there, the guy behind me was stripping reconditioned gearboxes, and I recognised my Mirage box. They were making space by getting rid of old boxes that they would never sell, so I rescued one for a spare. This one only cost $150, down from $200 I paid for the first one.

I haven't yet measured the losses with the new preload.