What is preload?  

When there is clearance in between the components, the rigidity of linear guide is not enough.It is possible to eliminate the clearance by enlarging the rolling elements to preliminarily apply an internal load to enhance the rigidity.

Preload grade

Increase the preload will eliminate the vibration and the inertia impact in a reciprocating movement. However, increase of preload will increase the internal load and increase the assembly difficulty. Therefore, selection of linear guide must bring into account the balance between the impact of vibration and the lifetime decrease by preload.

Comparison of interchangeable and non-interchangeable