fig1 illustrates a one - piece garage door 11 which is actuated by garage door operator 10 . the one - piece door 11 is supported by suitable pivoting and spring biasing kicker means 101 and 102 from opposite sides of the door frame . one end of the rail 16 of the garage door operator is connected by a bracket 18 to the header 21 above the door and the other end including the motor and driving components are connected to the rail 16 and are mounted in a housing 22 which is connected to the ceiling of the garage by frame members 23 and 23a . the operator drives a chain 26 which moves a trolley 14 which carries an l - shape pivotally supported arm 13 which is connected to a bracket 12 of the door 11 . fig2 illustrates the chain 26 which passes over a sprocket 24 rotatably supported near the bracket 18 with a suitable pin 25 which passes through an extension 17 that extends from the rail 16 to the bracket 18 and a pin 19 passes through the member 17 and connects it to the bracket 18 . as shown in fig3 and 5 , a motor 28 is mounted in the housing 22 and has a worm 29 mounted on its output shaft 31 . the worm 29 meshes with a worm gear 33 non - rotatably mounted on a shaft 32 which has a small end 36 which is received in a bearing 37 mounted in frame member 34 as shown in fig3 . a pin 30 extends through the end 36 of the shaft 32 to lock it in the bearing 37 . pins 42 and 43 pass through the shaft 32 and extend through slots 201 and 202 formed in collars 203 and 204 of the gear 33 so that the gear is non - rotatably mounted on the shaft 32 . an opening 44 is formed through the shaft 32 so as to allow the gear 33 to be mounted on the shaft 32 at a different vertical position relative to fig3 . the upper end relative to fig3 of shaft 32 is rotatably supported by an upper frame member 31 which has an aligned opening through which the shaft extends and a cup - shape bearing member 39 has an outer flange in which threaded openings 50 and 53 are formed and in a particular embodiment three equally spaced openings were formed in the flange of the support member 39 . the bearing member 39 is attached to the frame 31 by suitable bolts such as 51 and 52 shown in fig3 and 4 , for example . a bearing 41 is attached to the support 39 and a washer 45 is connected to the shaft 32 adjacent the bearing 41 . a first sprocket 46 is mounted on the shaft 32 above the bearing 47 and is welded or is otherwise suitably connected to the shaft 32 so it cannot rotate relative thereto . a second sprocket 47 having a different number of teeth than sprocket 46 is non - rotatably connected to the shaft 32 above the sprocket 46 as shown in fig3 for example . the sprockets 47 and 46 may be welded or braised to the shaft 32 , for example . the upper end 56 of the shaft 32 is reduced in cross - section as illustrated . a bearing washer 81 is connected to the shaft 32 and rotates therewith and engages the upper end of the bearing 41 . the frame member 31 has a shaft 60 which extends upwardly therefrom upon which is rotatably mounted an idler pulley 61 . the idler pulley 61 is formed with a hub 64 on one side thereof . the pulley portion 62 has side flanges 63 and 70 in which the chain 26 is received and a circular extension 72 is mounted in the groove between the portion 63 and 70 so as to guide the chain on the sprocket . a bolt 66 is threadedly received in the decreased diameter shaft portion 67 of the shaft 60 so as to lock the pulley on the shaft portion 67 such that it is free to rotate with the chain 26 . in operation , if it is desired to mount the chain 26 on the larger sprocket 46 which in a particular example had nine teeth , the idler pulley 61 is mounted as shown in fig3 such that the groove of the pulley between the portions 62 and 63 aligns with the sprocket 46 and the chain 26 is mounted over the sprocket 46 and fits in the groove of idler pulley 61 as shown in fig3 . an l - shaped cover plate 68 has one end receivable in an opening 69 of the frame member 31 and has a horizontal 71 which extends horizontally over the sprockets and 47 , 46 and the pulley 61 to form a guard . thus , with the chain engaged with the sprocket 46 which has nine teeth as shown in fig3 it will drive the door 11 upwardly and downwardly faster than when it is engaged with the sprocket 47 which has a smaller number of teeth as , for example , in a particular example 7 teeth . when it is desired to drive the garage slower , the idler pulley 61 is inverted to the position shown in fig4 by removing the cover plate 68 and the bolt 66 which is threadedly received in the smaller portion 67 of the shaft 60 and the pulley is turned over so that the hub 64 bears against the larger diameter portion of the shaft 60 as shown in fig4 and then the bolt 66 is again replaced to attach the pulley 61 to the shaft portion 67 but with an orientation so that if the pulley aligns the chain 26 with the sprocket 47 upon which it is mounted . fig5 is a top view of the sprockets 46 and 47 and the idler pulley 61 and illustrates the chain mounted on the larger sprocket 46 and the idler pulley mounted on the shaft 67 in the orientation illustrated in fig3 . it is seen that the present invention allows the motor of a garage door operator to be coupled to a chain through either a larger or smaller sprocket such that the speed of the garage door operator can be varied . although the invention has been described with respect to preferred embodiments , it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims .