Tension reducer for a power tool drive apparatus

#This invention relates to a drive belt tension control apparatus for a powered machine such as a power tool of the type wherein tension on one or more rotating parts is provided by pneumatic pressure through the operation of a pneumatic piston and cylinder assembly. The particular embodiment of a power tool incorporating the principles of the invention and disclosed in this application is an inverted router.

#An inverted router is a particular type of woodworking machine wherein the operator is protected from the router bit by positioning the workpiece on a table and making cut in the workpiece from the underside. The table has on opening through which the router bit protrudes. The router bit is mounted on a spindle. A foot-operated pedal controls movement of the router bit upwardly through the table opening into workpiece engagement and downwardly out of workpiece engagement. The router bit moves vertically on a precise axis in order that the cuts be precisely positioned. This axis is defined by a guide pin which is positioned on the same axis and is used to guide the workpiece on the table.

#In most types of inverted routers the spindle is driver by a motor through a pulley and belt arrangement. It is important to maintain the tension on this belt within a relatively narrow range. If the belt tension is too high the spindle may be deflected off of the proper rotational axis and the life of the bearings in the spindle assembly substantially shortened. If the tension is too low, slippage between the belt and one or both of the pulleys can reduce the speed of the spindle to below its optimum speed, especially when the spindle is under load while a cut is being made. Reduced spindle speed can also cause the router bit to chatter in the workpiece, stop, and/or cause excessive wear to the belt and pulley.

#Some of the objects of the invention have been set forth above. Other objects and advantages of the invention will appear as the invention proceeds when taken in conjunction with the following drawings, in which:

#FIG. 1 is a perspective view of an inverted router, with parts broken away, according to one embodiment of the invention;

#FIG. 2 is a fragmentary top plan view of the major drive components of the inverted router shown in FIG. 1;

#Referring now specifically to the drawings, an inverted router according to the present invention is illustrated in FIG. 1 and shown generally at reference numeral 10. Router 10 is constructed on a steel frame which includes a pair of hollow support skids 11 and 12 by which router 10 can be moved by a forklift. As is shown by simultaneous reference to FIGS. 1 and 2, the operating components of router 10 are housed within front and rear vertical wall panels 13 and 14, and side panels 15 and 16. A workpiece (not shown) is supported on a table 17 as it is moved over a router bit 18 which extends upwardly through an opening 20 in table 17. A guide pin 21 positioned on the end of a guide arm 22 is used to guide a template and thereby the workpiece.

#Router bit 18 is mounted on the upper end of a spindle 23 by means of a chuck (not shown). Spindle 23 and therefore the router bit 18 is moved up and down by a foot pedal 24. A spindle pulley 25 mounted on the lower end of spindle 23 carries a woven drive belt 26. Drive belt 26 also passes around a motor pulley 28 mounted for rotation on a drive motor 29. As is shown in FIG. 1, motor pulley 28 is much wider than belt 26. As spindle 23 is raised and lowered, belt 26 rides up and down motor pulley 28 so that it remains trained in driving relation to the vertically moving spindle 23.

#Proper tension is maintained on belt 26 by means of a pair of slide rails 30 and 31 mounted on side walls 16 and 15, respectively. Preferably, these slide rails 30 and 31 are mounted in vibration absorbing rubber mounts (not shown). Motor 29 is mounted on a steel plate 32, the opposing edges of which are mounted for sliding movement in slide rails 30 and 31. A pneumatic cylinder 33 is mounted in a fixed position in front of and in vertical alignment with the front edge of plate 32. A piston rod 34 is responsive to pneumatic pressure to urge plate 32 in a direction away from spindle pulley 25.