seat and means defining an angular groove in approximately the middle of the bearing seat defining a lubrication channel application_number US 13150480 A description according to the drawings in particular fig1 and 2 a castor 1 is mounted in fork 2 which is mounted beneath a mobile device 5 via a rolling bearing arrangement 3 and connected by screws 4 balls 6 of the rolling bearing are locked between inner and outer bearing races 7 and 8 the inner race 7 is produced by rolling the tubular upper end 9 of fork 2 or in general by forcing without removal of metal an aperture which is closed by a plug 10 made e g of plastics is located in inner race 7 and serves for inserting balls 6 of rolling bearing 3 the outer race 8 is made by first splitting the edge of the aperture of a generally rectangular plate provided with a central aperture and then forming to the desired configuration of the race the part of the plate located outside race 8 is then flanged so that race 8 is radially spaced apart from an edge flange 12 with holes for the passage of screws 4 shown schematically the castor structure of fig1 and 2 is completed by a plastics closure 13 which is held in place by snap action a cover 14 and a dust seal 15 for.