Thrust Bearing

The idea behind using a thrust bearing is to take the longitudinal load off of the motors bearings. The motors bearings are not designed to support a load parallel to the shaft (thrust).  What they are designed to support is the loading perpendicular (radially) to the motor shaft.

When running a flex cable you need to leave a gap between the drive dog and the drive strut to allow for flexcable shorting due to the torsional load. This necessary gap in turns means that more of the propellers thrust is transferred up the flex cable to the motor shaft because its not directly bearing on the strut.  Although at moderate levels this is OK it will wear out the motors bearings faster. So this is where the thrust bearing comes into play.
 

There are several types of thrust bearings, the ones I use include 3 separate parts. These parts include two hardened washers and a bearing ring (nylon or metal) with pressed in ball bearings (see picture). To use these you sandwich the bearing ring between the two washers which allows for free spinning while taking up the load.

You install a thrust bearing between the motor and the flex coupling.  With it in place the prop thrust will be transferred to the motors case and not to its bearings. You can see the installed thrust bearing in the picture.

Why not just have the drive dog push/rest on the strut?  Remember when running a flex cable it shortens under the torsional (twist) load we put on them.  So in this setup the thrust would be on the strut via the drive dog which a first sounds like a good thing. However remember  that the flex shaft is getting shorter which significantly increases the driveline friction and increases the load on the flex which makes it want to shorten even more! This may even snap the flex or may it may just pull apart between the flex and the coupler.

Flex Couplings

The fit of the flex hex coupler to the motor shaft is area that is often overlooked but very important.  If the motor shaft is smaller than the flex coupling (or any style coupling for that matter), then it won't be aligned after you tighten the grub screws.  Now spin that off center load at 15K RPM and you will have a vibration problem and be unnecessary loading the motors bearings.

So what to do if the fit is not hand in glove is you make it that way.  You need to either find a better fitting coupling or add a sleeve to the motor shaft to increase its diameter.  When I do this I open an area on the sleeve for the grub screw to pass through such that the grub screw is clamping on the motor shaft and not the sleeve.