spool 2 and has hole 4a extending through the central port of boss 4b annular groove 4c is provided on the peripheral surface of boss 4b flexible wire spring 10 fitted in spool 2 is removably engaged in groove 4c flexible wire spring 10 is partly shaped as a parallelogram and partly shaped as a circle as shown in fig2 flexible wire spring 10 is fitted in annular groove 2d of the internal circumferential surface of spool 2 as the diameter of flexible spring 10 is reduced against its resilient force acting in such a direction as to increase the diameter when wire spring 10 is fitted in annular groove 2d the diameter of the spring is increased by its resilient force so that arcuate portions 10a and 10b of the flexible spring on its major axis are resiliently engaged in groove 2d as a result flexible spring 10 does not drop off boss 4b of drag washer 4 is pushed in between straight portions 10c and 10d of flexible wire spring 10 on its minor axis so that straight portions 10c and 10d are pushed open outwardly in the radial direction of the flexible spring after that straight portions 10c and 10d are returned to their original positions and engaged in groove 4c as shown in fig2 the peripheral edge of the tip of boss 4b and its inner edge on groove 4c are chamfered into flat slopes 4d and 4e on round slopes 4f and 4g as shown in fig3 a or 3 b so that straight portions 10c and 10d of the flexible wire spring 10 can be easily.