Patent Publication Number: US-2009229391-A1

Title: Mechanical power transfer device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the present invention is mechanical systems for power transfer, such as clutches and transmissions. 
     2. Background 
     The “worm gear”, commonly referred to simply as a “worm”, has a very long history. The name comes from the spiral, worm-like grove traversing almost the entire functional surface. Over the centuries, the design and use of the worm has evolved and improved. Today, the worm enjoys many varied applications-from power transmissions to manufacturing. However, for all the applications in which the worm is employed, the worm is generally found rotationally engaging a spur gear for purposes of high ratio gear reduction or for transferring power between shafts at right angles. In such applications, the worm is capable of driving high loads, and it may be designed to be back-drivable or to resist back-drive through locking engagement with the spur gear. A lesser used secondary function of the worm is to use it and the spur gear in combination as a rack and pinion. Such a use is described in U.S. Pat. No. 1,940,101, in which the worm has two modes, it does work while rotating through its engagement with the spur gear, and it does work while translating, performing as a rack and pinion with the spur gear. What follows builds upon this lesser used mode of operation. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a device for transferring mechanical power. An input drive is coupled to an output drive through a worm and a spur gear, the worm being coupled to one of the input drive or the output drive, and the spur gear being coupled to the other. Power from the input drive is transferred to the output drive through the worm and the spur gear. The worm engages the spur gear such that when the worm is rotated, no power is transferred from the worm to the spur gear, and when the worm is translated without rotation, power is transferred from the worm to the spur gear. In this manner, the input drive may be used to intermittently power the output drive using a single worm and spur gear. Preferably, when the worm is translated without rotation, the worm and spur gear perform as a rack and pinion, and the worm moves the spur gear in the forward direction. Also, when the worm rotates, the worm preferably rotates in the reverse direction and simultaneously translates in the reverse direction. Additional worms may be added, with or without additional spur gears, to constantly power the drive output. 
     When the worm is rotated in the reverse direction, it is rotated at a rate which permits the spur gear to remain stationary or to continue rotating in the forward direction. A coupling may link the worm to one of the drive input or the drive output to rotate the worm in the reverse direction. The coupling may comprise a coil spring adapted to be tensioned and rotate the worm upon release, or the coupling may comprise two gears and a cam adapted to periodically engage one gear with the other such that when the gears engage, the worm is rotated. 
     Accordingly, an improved device for transferring mechanical power is disclosed. Advantages of the improvements will appear from the drawings and the description of the preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, wherein like reference numerals refer to similar components: 
         FIG. 1  illustrates a first embodiment of a mechanical power transfer device; and 
         FIGS. 2A-D  illustrate a second embodiment of a mechanical power transfer device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning in detail to the drawings,  FIG. 1  illustrates the basic design concept of a power transfer device  11 . In this device, the cam shaft  13  serves as the drive input and is fitted with a cam lobe  15  which engages a first roller tappet  17  mounted on a rocker arm  19 . The rocker arm  19  is mounted to a wheel  21  at a pivot point  23 , and the wheel  21  is in turn mounted on and rotationally isolated from the cam shaft  13 . The wheel  21  is rotationally isolated from the cam shaft  13  and is rotatable to adjust the relative position of the pivot point  23  with respect to the cam shaft  13 . Once the wheel  21  is set at a desired position, it is preferably locked from further unwanted or accidental rotation. This arrangement allows the amount of displacement of the rocker arm  19  to be changed regardless of whether the cam shaft  13  is rotating or stationary. 
     As the cam lobe  15  rotates and displaces the rocker arm  19 , the rocker arm engages a second roller tappet  25 , this one being affixed to the support  27  at one end of a splined shaft  29 . A worm  31  is fitted to the splined shaft  29 . A thrust bearing  33  at one end of the splined shaft  29  cradles the splined shaft  29  and the worm  31 , providing a bearing surface between them and the support  27  so that the worm  31  and splined shaft  29  may rotate while the support  27  does not. The end of the splined shaft  29  opposite the thrust bearing  33  is coupled to a coil spring  35 . The coil spring  35  is coupled to one of the drive input or the drive output through an appropriate linkage (not shown) which tensions the coil spring  35  during operation. 
     The worm  31  engages a spur gear  37  which is mounted on the drive output shaft  39 . During operation, the worm  31  does not transfer power to the spur gear  37  through rotation. Rather, as the cam lobe  15  rotates on the cam shaft  13 , the worm  31  translates in forward and reverse directions, with respect to the spur gear, on the splined shaft  29 . As the cam lobe  15  causes forward displacement of the second tappet  25 , the worm  31  is translated in the forward direction and engages the spur gear  37 . Translation of the worm  31 , without rotation, causes the worm  31  and the spur gear  37  to perform as a rack and pinion. As the cam lobe  15  continues to rotate and the tappet displacement rate in the forward direction decreases, the driving pressure between the worm  31  and the spur gear  37  decreases sufficiently to allow the worm  31  to rotate freely of the spur gear  37 . When this happens, the tension built up in the coil spring  35  causes the worm  31  to rotate in the reverse direction and translate along the splined shaft  29  to follow the thrust bearing as the second tappet  25  reverses direction and completes a full cycle. 
     In the above device, for every cycle of the cam lobe  15 , the spur gear  37  is rotated by an amount determined by the linear displacement of and the length of the worm  31 . During each cycle, the relative translational position of the worm  31 , with respect to the spur gear  37 , is reset by rotating the worm  31  in the reverse direction. Further, rotation of the worm  31  in the reverse direction is capable of resetting the position of the worm  31  regardless of whether the spur gear  37  remains stationary or continues to rotate in the forward direction during the reset phase of the cycle. In designs where the spur gear continues to rotate in the forward direction, rotation of the worm needs to be performed at a higher rate as compared to designs where the spur gear remains stationary. 
     Those skilled in the art will appreciate that the transfer ratio of this basic design may be easily decreased by driving the output shaft using multiple worms, two of which may easily interact with a single spur gear, or multiple worms, multiple spur gears, and multiple cam lobes. In fact, a great many possibilities exist for modifications of this basic design to suit different needs. For example, where multiple worms are used, instead of using a coil spring to reset each worm, planetary gears or other similar mechanical dividers might be employed. As another example, a single cam shaft with one or more cams could be used in combination with rocker arms to displace two worms driving the same spur gear. Another aspect that will be appreciated is that the drive ratio, even when a single worm is employed, may be continuously varied, through adjustment of the position of the rocker arm, without the use of any frictional gear interactions within the drive chain. 
       FIGS. 2A-C  illustrate an alternative embodiment which uses the same design concept of interaction between the worm and the spur gear. As shown in  FIG. 2A , the worm  51  is mounted on a splined shaft  53 . The splined shaft  53  is directly coupled to a pinion gear  55 , which is in turn rotationally coupled to a large spur gear  57 , which is mounted on the driven shaft  59  (the output drive), through a bevel gear  61  and two other coupling gears  63 ,  65 . As shown in  FIG. 2B , the worm  51  is coupled to a small spur gear  67 , which is also mounted on the driven shaft  59 . The gear ratio of the worm  51  is the same as the combined ratio between the pinion gear  55  and the bevel gear  61  and between the large spur gear  57  and the coupling gear  63 . By equalizing the gear ratios in this manner, continuous rotation is allowed despite the presence of two separate, but linked, gear arrangements. 
       FIG. 2C  shows additional support structure for the splined shaft  53 , along with the mechanism that drives the worm  51 . The splined shaft  53  is mounted within a support arm  69 . One end of the splined shaft  53  includes a thrust bearing  71 , which cradles both the shaft  53  and the worm  51  and provides a bearing surface between them and the support arm  69 . The support arm  69  is mounted to and rotates independently of the driven shaft  59 . When the support arm  69  is rotated in a clockwise direction the worm  51  abuts against the thrust bearing  71  and causes the small spur gear  67  to rotate in the forward direction. The biasing spring  75  connected to the distal end of the support arm  69  is anchored and biases the support arm  69  toward a neutral position. The support arm  69  is cyclically driven by the cam shaft  77 , cam lobe  79 , roller tappet  81 , and rocker arm  83 , which all operate to displace the second roller tappet  85  mounted on the second rocker arm  87 . As shown in  FIG. 2D , which is a view of the same embodiment from the opposite side of  FIG. 2C , this second rocker arm  87  is split, so that it straddles the support, and is connected to a pivot point  89  that is coaxial with the coupling gear  63 . The second rocker arm  87  also supports the second coupling gear  65  and extends to engage a driving spring  91 , which in turn engages an extension  93  of the support arm  69 . The driving spring  91  also serves to ensure that the roller tappet  85  does not lose contact with the first rocker arm  83  as the cam lobe  79  rotates. Through this arrangement, the support arm  69  is driven in reciprocating motion, with the biasing spring  75  opposing the action of the driving spring  91 . 
     During operation, as the cam lobe  79  rotates, the first rocker arm  83  causes the second rocker arm  87  to rotate in the clockwise direction (as seen in  FIG. 2C ). During this action, the second coupling gear  65  introduces resistance to movement of the second rocker arm  87  as the second coupling gear  65  is lifted between the first coupling gear  63  and the large spur gear  57 . Resistance is introduced by the large spur gear  57 , which is coupled to the small spur gear  67 , which is in turn driven by the worm  51  as a rack and pinion. Similarly, resistance is introduced by the coupling gear  63  that is mechanically coupled to the pinion gear  55 , and therefore to the worm  51 . This resistance, in combination with the pressure placed on the worm  51  by the thrust bearing  71  as the support arm  69  rotates in the clockwise direction, generates positive torque between the worm  51  and the small spur gear  67  to prevent back drive by the worm  51 . 
     The action of the second rocker arm  87  causes the support arm  69  to rotate in the clockwise direction through the linkage with the spring  91 . This causes the pinion gear  55  to rotate about the output shaft  59  at the same rate of rotation as the bevel gear  61 . At this point in the cycle, the pinion gear  55  and the bevel gear  61  are rotationally disengaged, i.e., rotation of one does not directly affect rotation of the other, although neither is able to rotate freely of the other as their respective teeth remain interlocked. As the cam lobe  79  continues to rotate, the biasing spring  75  causes the support arm  69  to rotate in the counter clockwise direction and back to the neutral position, thus completing a full cycle. When the pinion gear  55  and the bevel gear  61  rotationally disengage, the worm  51  stops rotating, and rotation of the support arm  69  causes the worm  51  to abut against the thrust bearing  71 , thereby causing the worm  51  and the small spur gear  67  to act as a rack and pinion, driving the small spur gear  67  in the forward direction. As the support arm  69  is biased back to the neutral position, the worm  51  slides up the splined shaft  53  away from the thrust bearing  71 . At the end of the cycle, the pinion gear  55  and the bevel gear  61  rotationally reengage, thereby rotationally driving the worm  51  in the reverse direction and causing it to return to a position abutting the thrust bearing  71 . As with the previous embodiment, many alterations are possible without changing the functional interaction between the worm and the spur gear as described herein. 
     Thus, a device for transferring mechanical power is disclosed. While embodiments of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. For example, the skilled artisan will recognize that the gear ratios and timing of a power transfer device based upon the concepts described herein will vary based upon the particular design specifications. The invention, therefore, is not to be restricted except in the spirit of the following claims.