Linear motion takes shape

Better manufacturing methods make square rails economical candidates for applications where only round rails once would do. Design engineers considering linear guides often struggle with which basic shape to choose: round or square. Square or profile rails once cost significantly more than round because the former need extensive grinding that is inherent to the design. But recent manufacturing techniques and economies of scale let square rails compete with round in a broader range of applications. Cost may still be a factor, though other metrics come into play, including load capacity, stiffness, accuracy, smoothness and geometry. Square or profile rails target such applications as machine tools needing high-load capacity (typically about 20 kN for a 25-mm size), good stiffness (on the order of 1 to 4 μm/kN at 13% preload, for example), and high accuracy (3 to 10-μm/m parallelism between the bearing guide and rail). Trading ball bearings for cylindrical rollers can roughly double load capacity in the samesized package. Profile rails get their performance, in part, through precise grinding of the rail ball tracks. The race on which the balls ride in the rail and carriage is only slightly larger in radius than the balls themselves. This geometry