A review of a A/C bearing cross section clearly shows that the bearing can ONLY absorb thrust in 1 direction - ergo single direction. For bidirectionaly thrust, you have to mount them so that the thrust angles complement each other - ergo in DB or DF configuration.
If you fix both ends of a screw, the trick is to mount the thrust faces of the bearings so they face each other and put the screw into tension in the process. However, you just don't reef down on it. You have to put a calculated load onto the screw so as to maintain lead accuracy...
Thus, if you mount one A/C bearing on one end with the thrust side facing the ball screw threads, the one on the other side has to also face the screw threads. THus the one absorbs thrust in one direction and the other absorbs thrust in the other as they are effectively pulling against each other.
If you mount them in DB or DF, configuration, then all the thrust can be absorbed on one end of the screw. You need to let the other end of the screw float if you do this sort of thing.
You can mount bearings in duplex, triplex and quadraplex to fill up the space and gain huge amounts of axial thrust capacity. You do have have the same number of bearings with the thrust side mounted either back to back or face to face.  |