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The accuracy class of a profile rail bearing is defined by three tolerances:
These are measured from the rail base to the center of the carriage top (H), and from the rail reference edge to the the carriage reference edge (A). See the figure below for "H" and "A" dimension locations.
describes the tolerance on H and A as a function of axial travel,
measured from one carriage down the length of the rail. This is
analogous to straightness of travel. As such, parallelism describes
attributes of the rail only.