We rebuilt a gear box that had helical gears and ball bearings on the input shaft. We were looking for info why these gear boxes were destroying themselves in very little time. We had one gear box last 3 or 4 days running with a positive and negative load on the dyne running it 24hrs a day for this test. The load was done with electronics and changed every 2 minutes it would load in one direction and ramp up the torque putting a load in that direction a little at a time till it reached the max torque. We were having problems in the field with this gear box and had to find out why.
I found that the input shaft ball bearing was seeing a side load thrust. The single row ball bearing isn't made for a heavy abuse that its seeing in this application. The bearings were destroying themselves.
WHY;
I finally figured it out that they were hammering themselves apart because they can be installed in two ways. If there is a half hole in both outer and inner races of the bearing thats used so it can be assembled when the balls are installed isn't good for a thrust side load. When this side of the bearing is seeing the thrust side load its hammer the balls past the assembly opening each time a ball goes by it. Even though there is a cage holding the balls in place one the centerline of the ball bearing the balls still tend to wander and strike this assembly opening when used in a thrust side load situation. Even though the balls and bearing races are hardened everything seems to get fretted and disaster is short comming once it starts.
The FIX:
You need to figure out the direction of the side thrust forces so you know which way to install the assembly opening on the ball bearing. You want the smooth side of the bearing to see the side thrust/load.
Example;
In our cubs tranny on the lower shaft the front ball bearing were the forces are going toward the clutch. The forces of the tapered drive pinion causes this to be towards the front of the shaft again towards the clutch. So the assembly openings need to be towards the rear of the tractor. It gets beat up a lot less in this direction and will last much longer. The straight cut gears in the tranny cause no problem its the tapered pinion gear that pushes the shaft forward.
Like i said it may apply or it may not apply but some ball bearings have this assembly opening and some don't. Its good to know which ball bearing were using so it gets installed in the correct direction so it has a long life. Even though were not loading that shaft beyond the max an incorrectly installed ball bearing will fail in time sooner or later. With our cubs time is a factor too. Its an assembly process that we have to follow.
This matters when putting new ball bearings in our generators too if they have these asembly openings. This applies to every ball bearing situation were a side thrust load is present.
Just something to think about, food for thought.......knowledge can be a good thing..
Always wondered why the notch 
