at right angle by a central lower inverted u shaped groove in the guide arm a the opposite ends of a pressing coil spring s 39 are provided with a top cap 32 and a bottom cap 31 to form parallel surfaces at right angles with the central axis of the spring s 39 extending along said opposite end surfaces the upper surface of the guide arm is formed with a race for the ball k usually in the race for the roller arrangement the center of the roller k is located on the datum axis xx 39 of the weighting arm construction so that the roller will not displace forwardly or rearwardly of the guide arm a while a particular limit range is provided in the direction of swing thus the arrangement is similar to those shown in fig1 a 21b more than the upper half of the ball k is received in the bottom cap 31 and its top is in contact with the ceiling surface thereof under pressure the bottom cap 31 and top cap 32 are fitted in the inner diameter of the pressing spring s 39 with a minimum clearance therebetween and their outer surfaces are fitted likewise in the guide arm case b with a minimum clearance therebetween so that they are slidable only in the pressing direction the upper assembly 36 of the guide arm case b forms an attaching base part for selectively fixing the weighting arm assembly at a predetermined position on the weighting arm main body 42 and also serves as a base for adjusting a screw 35 which adjusts the pressure.