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A drive unit comprising a guide channel that has a first pulley at one end that rotates about a fixed pulley axis. The drive unit also includes a flexible drive member having a plurality of spaced windows, an attachment assembly moveably attached to the guide channel, and an adjustable pulley assemb

A drive unit comprising a guide channel that has a first pulley at one end that rotates about a fixed pulley axis. The drive unit also includes a flexible drive member having a plurality of spaced windows, an attachment assembly moveably attached to the guide channel, and an adjustable pulley assembly coupled to the guide channel that has a second pulley that rotates around a moveable pulley axis that can be adjusted to change the distance between the fixed pulley axis and the moveable pulley axis to take up slack in the flexible drive member.

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1. A drive unit comprising: a guide channel;a first pulley positioned proximate one end of the guide channel, where the first pulley is rotatable about a fixed pulley axis;a flexible drive member having a plurality of spaced windows;an attachment assembly moveably attached to the guide channel, the

1. A drive unit comprising: a guide channel;a first pulley positioned proximate one end of the guide channel, where the first pulley is rotatable about a fixed pulley axis;a flexible drive member having a plurality of spaced windows;an attachment assembly moveably attached to the guide channel, the attachment assembly comprising, a yoke having laterally spaced side walls connected by a bridge wall, the yoke having a connector attached to the bridge wall,a coupler having a plurality of spaced teeth that are adapted to extend through the plurality of spaced windows of the flexible drive member, anda spring that has a central mounting portion and spring arms at each end, the central mounting portion being sandwiched between the bridge wall and the coupler; andan adjustable pulley assembly coupled to the guide channel substantially opposite the first pulley, the adjustable pulley assembly having a second pulley that rotates around a moveable pulley axis that can be adjusted with respect to the fixed pulley axis to adjust the slack in the flexible drive member. 2. The drive unit of claim 1, wherein the attachment assembly includes an outer shoe adjacent each side wall of the yoke, each outer shoe having a forward flange that engages the bridge wall and a rearward flange that engages a rearward edge of the associated side wall. 3. The drive unit of claim 1, wherein one of the first pulley and the second pulley is a drive pulley and the other of the first pulley and the second pulley is an idler pulley. 4. The drive unit of claim 1, further comprising a power unit attached to the guide channel to drive the first pulley or the second pulley. 5. The drive unit of claim 1, wherein the adjustable pulley assembly includes a housing having axially spaced journal boxes defining a fixed housing axis, and a cam shaft that rotates in the journal boxes about the fixed housing axis to define the moveable pulley axis. 6. The drive unit of claim 5, wherein the cam shaft has axially spaced bearing portions disposed in the axially spaced journal boxes and a circular cam portion positioned between the axially spaced bearing portions that defines the moveable pulley axis, wherein the cam shaft rotates about the fixed housing axis to adjust the distance between the fixed pulley axis and the moveable pulley axis. 7. The drive unit of claim 1, wherein the plurality of spaced teeth extend through holes in the bridge wall of the yoke. 8. A drive unit comprising: a guide channel;a first pulley positioned proximate one end of the guide channel, where the first pulley is rotatable about a fixed axis;an adjustable pulley assembly positioned opposite the first pulley, the adjustable pulley assembly including a second pulley rotatable about a moveable pulley axis and moveable into a plurality of positions to change the distance between the fixed pulley axis and the moveable pulley axis, and wherein the adjustable pulley assembly may be fixed in at least one of the plurality of positions;a flexible drive member trained solely around the first pulley and the second pulley to form the flexible drive member in a narrow loop, and wherein the adjustable pulley assembly takes up a set amount of slack in the flexible drive member in each of the plurality of positions;an attachment assembly moveable along the guide channel and coupled to the flexible drive member, the attachment assembly including a flexible drive member tensioning device, and wherein the flexible drive member tensioning device is configured to take up additional slack in the flexible drive member beyond the set amount of slack taken up by the adjustable pulley assembly; andwherein the adjustable pulley assembly includes a housing having axially spaced journal boxes defining a fixed housing axis and a cam shaft rotatable about the fixed housing axis to define the moveable pulley axis. 9. The drive unit of claim 8, wherein the cam shaft has axially spaced bearing portions disposed in the axially spaced journal boxes and a circular cam portion positioned between the axially spaced bearing portions that defines the moveable pulley axis. 10. The drive unit of claim 9, wherein the second pulley rotates about the circular cam portion. 11. The drive unit of claim 8, wherein the flexible drive member tensioning device includes a spring. 12. The drive unit of claim 8, wherein the first pulley is a drive pulley and the second pulley is an idler pulley. 13. A drive unit comprising: a guide channel having a first pulley rotatable about a fixed pulley axis and a second pulley positioned opposite the first pulley;an attachment assembly movable along the guide channel;a flexible drive member trained solely about the first and second pulleys to form the flexible drive member in a narrow loop, the flexible drive member extending a first length and being formed from inelastic material, the flexible drive member having a first end directly coupled to the attachment assembly and a second end opposite the first end, the second end directly coupled to the attachment assembly;a first flexible drive member tensioning device adjustable into one of a plurality of fixed positions, each position operable to take up a set amount of slack in the flexible drive member; anda second flexible drive member tensioning device disposed on the attachment assembly and having a spring to take up slack in addition to the set amount of slack taken up by the first flexible drive member tensioning device, wherein the second flexible drive member tensioning device operates independently of the first flexible drive member tensioning device; andwherein the first flexible drive member tensioning device and the second flexible drive member tensioning device create tension within the flexible drive member, and wherein the tension is constant along the entire length of the flexible drive member. 14. The drive unit of claim 13, wherein the second flexible drive member tensioning device includes a spring. 15. The drive unit of claim 13, wherein the first flexible drive member tensioning device includes an adjustable pulley assembly positioned opposite the first pulley, the adjustable pulley assembly including the second pulley rotatable about a moveable pulley axis that can be adjusted to change the distance between the fixed pulley axis and the moveable pulley axis.