of the transmission mechanism 104 shown in fig9 will easily be understood by comparison with that of the preceding figures and that therefore no further description need be provided herein it will of course be appreciated that while the embodiment of fig9 is illustrated in association with a cylindrical shaft 76 and acircular recess 114 having a rib 116 this is by way merely an example this embodiment of the invention will equally be applicable to a shaft having a hexagonal or other cross section and the recess 114 would then of course be modified to suit a preferred form of bearing assembly 18 will now be described in more detail in connection with fig1 as already explained in relation to fig1 and 1a the bearing assembly 18 is of integral one piece thermoplastic construction the two walls 18a and 18b are joined by a junction wall 130 having side channel walls 132 on either side therefore for greater strength in the embodiment of fig1 the u shaped recess in wall 18a is preferably provided with inturned abutment portions 134 on either side so as to enclose slightly more than a 180 deg of arc the through opening in wall 18b is formed with a bridge formation already described extending completely thereacross and thereby providing a entirely enclosed through opening for retention of the shaft therein in order to provide for passage of the cords and tapes and the like openings 136 and 138 are formed in walls 18a and 18b in order for downward guidance of those.