Originally Posted by
Eldon_Joh
The stiffness of the system the rails are bolted to are what define how straight the rails need to be relative to each other, simply because you can over stress the bearing blocks by twisting the rail as its clamped down to a non flat surface. but this isn't an exact science without a lot more information because you also have to factor in the stiffness of the bearings as well as the entire system.
so if you look up the data sheet for your bearings they will include moment loads. apply similar loads to whatever structure the moving bearing block is bolted to and measure how much it bends.
the distance between the linear rails is perhaps the easiest problem to fix, simply by bolting the moving bearings to a piece of metal, slide them along both rails and bolt the rails progressively as you slide the bearing. if you want to, you can intentionally make both rails a curve if you want.
the axial twist of the linear rail mounting surface is perhaps a much more difficult problem to fix because a low tensile 6mm bolt can provide 2000 pounds of axial force on the rail before the bolt stretches. if the rail is 20mm wide then it will take about 20 pound-meters of torque which is approximately 60 foot pounds of torque to stretch the bolt,
if i'm not mistaken a lot of 20mm wide, profile rails, even a short bearing block can handle 60 foot pounds of torque applied, so you're sort of safe there, but only if you use low grade 6mm bolts. you can also not torque them beyond 5 foot pounds.. anyhow its no surprise to me that you notice some changes in noise as you slide the bearings across the rail, but that change in sound is not evidence that you're actually at the limits of the bearings.