A bearing comprises an outer casing and an inner casing, wherein each of the outer casing and the inner casing are shaped to hold a plurality of ball bearings to facilitate rotational movement of the outer casing with respect to the inner casing, and wherein the inner casing is shaped to define a circular void that is substantially empty except for structure as needed to hold the plurality of ball bearings against the outer casing, and wherein the outer casing is shaped to form a groove within which to receive a rail. A mounting system is included on the inner casing capable of receiving a mounting bracket to be slid longitudinally with respect to the rail upon the rotational movement of the outer casing.