Why Single Direction Thrust Ball Bearings Can Only Be Used in One Direction

[gas_guzzler]

This isn't a homework question, but something I'm interested to know that my engineering corespondance doesn't mention.

I'm aware there is both single direction and double direction thrust ball bearings but can anyone tell me why can a single direction thrust ball bearing only be used in one direction?

 

[Q_Goest]

Some bearings can take thrust in one direction only, such as http://commons.wikimedia.org/wiki/File:Tapered-roller-bearing_din720_120-ex.png" . Click on the links above to see pictures.