To make a good system, linear actuator is one of the most important part in the system. There are many linear actuator out there. You can see this:
X, Y, Z axis are all using the linear actuator. This one: http://salecnc.com/catalog/02-Linear-Rail-TRH20-per-Meter
Inside the linear block has many ball inside. The good quality ball inside must have a good hardest and all the size are even. The ball inside are easily fall out of the linear block is off rail. So when to change the block, please change carefully.
The above is the data of the common use for linear actuator (linear square rail).