disposed in driving engagement with the sprocket formation on the shaft portion 52 of the hook drive member 50 therefore rotary motion imparted to the shaft 60 will be transmitted by way of the timing belt 64 to the drive member 50 and to the hook 22 because of the interconnection of the hook drive member and a hook member as described above referring in particular to fig2 and 3 an idler pulley 66 is provided for taking up any slack in the timing belt 64 and for maintaining tension thereon the idler pulley 66 is supported on an idler support plate 68 which is carried on one end of the hook support plate 36 and is secured to the hook support plate 36 by means of a screw 70 threaded into the hook support plate 36 through an elongated slot 72 in the idler support plate the idler support plate 68 is also formed with a locking finger 74 disposed in a slot 76 in the hook support plate 36 for holding the idler support plate at one end thereof during pivotal adjustment of the idler support plate 68 relative to the hook support plate 36 an elongated finger member 78 is disposed in abutting engagement with a rounded bump portion 80 on the hook support plate 36 the finger member 78 along with the.