we are maufacturer of propeller shafts.we have to test our propeller shaft(Carden Shaft) under dust(cement ,talcum power etc).we made a chamber & run the shaft @ 500 rpm. In the mean time mixture of dust & air were allowed to enter in to the chamber with high velocity. our ultimate aim is dust should no enter into the splines(even over the grease in spline shaft).what i want to know how much can be allowable and also how to calculate it?