a number of balls inserted in the circulation path and rolling during the movement of the slider to thereby smoothen the movement of the slider application_number US 79258885 A description as shown in fig1 and 2 a rail 10 has a rectangular cross section and extends long and the opposite side surfaces thereof are each formed with a ball rolling grooves 12 extending lengthwisely of the rail a bearing 15 comprises a body 20 and a pair of end caps 30 and is formed with a recess 22 in the central portion of the underside thereof and thus it has a saddle like shape in the inner side surfaces of a pair of opposite sleeve portions 24 of the bearing ball rolling grooves 26 only one of which is shown extending lengthwisely of the bearing are formed opposed relationship with said ball rolling grooves 12 the outer side surfaces of the body 20 are stepped in the intermediate portions thereof in the direction of height and upper halves 23 project more sideways than lower halves 25 in the upper portion of the lower halves 25 open concave grooves 28 of semicircular cross section only one of which is shown are formed lengthwisely of and over the full length of the lower halves threaded holes 23 for bolts for mounting the bearing 15 on a moving table not shown are formed in the upper surface of the body 20 and threaded holes 25 for bolts for mounting the end caps 30 are formed.