Abstract:
A continuously variable drive ratio transmission includes a support, a first shaft rotatably mounted to the support so that shaft can rotate about a first axis, a helical rack supported by the first shaft so that the rack has a plurality of toothed convolutions facing away from and spaced apart along the first shaft, the diameters of said convolutions being such that together they define a conical envelope encircling the first shaft. A second shaft is rotatably mounted to the support so that the second shaft is spaced from and extends parallel to the envelope, and a gear member is mounted to the second shaft for rotation therewith, the gear member having teeth which mesh with those of the rack. The gear member is slidable along the second shaft so that the gear member may be positioned at any location along the rack.

Description:
This invention relates to a gear drive or transmission. It relates especially to such a gear drive or transmission with a continuously variable input/output drive ratio. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     A gear drive or transmission is often interposed between the motor and a driven device such as a rotor, shaft, wheel, etc. so that the device will rotate at a lower speed with higher torque than the motor shaft, or vice versa. A variable speed gear drive has a plurality of gears or gear sets which can be selectively interposed between the input and output shafts of the gear drive so as to change the gear or drive ratio of the drive. 
     2. Description of the Prior Art 
     There do exist in the prior art transmissions which have a continuously variable input/output drive ratio. Usually these have a first rotary member which is conical and a cylindrical rotary member, the two members being coupled together by an endless belt loop encircling the members. The torque applied to one member is coupled via the belt to the other member. The speed ratio may be changed by shifting the belt along the length of the conical member. In other words, if the conical member is rotated at a selected speed and the belt is located at the larger diameter end of the conical member, the other member will rotate at a relatively high speed. On the other hand, if the belt is located at the small diameter end of the conical member, the other member will rotate at a lower speed, the speed ratio being dependent upon the cone angle of the conical member. 
     Such transmissions employing belts are disadvantaged, however, in that there is slippage between the belt loop and the driving and driven members. Also the belt loop may stretch when under load. Consequently there is not a positive transmission of power between the driving and driven members. 
     In order to avoid the aforesaid problems, attempts have been made to construct transmissions or gear drives whose driving and driven members comprise gears whose teeth mesh to transmit power from the driving to the driven member. As shown in U.S. Pat. Nos. 5,608,390; 5,653,143 and 6,321,613, for example, the transmission may include a rotary conical gear member composed of a series of separate conical sections supported by rotary shaft and a second member in the form of a pinion slidably mounted to a second rotary shaft positioned alongside the conical gear member. The spur gear is rotatably coupled to the second shaft but slidable therealong so that the pinion can be positioned opposite any one of the conical sections making up the conical member so as to vary the input/output drive ratio of the transmission. 
     Such nominally continuously variable speed gear drives have a problem in that when the pinion is moved along its shaft to change the gear ratio of the transmission, it is momentarily disposed opposite two sections of the conical member at the same time. Since those sections have different diameters, they also have different numbers of teeth. Therefore, rather elaborate steps have to be taken to enable the pinion to mesh properly with the conical gear member at all positions of the pinion. Usually this involves providing a certain amount of rotary play between the various conical sections making up the conical gear member and coupling those sections to their common shaft by means of clutches. In other such drives, the conical gear sections making up the conical gear member are stepped along their diameters and provided with specially shaped teeth. Those attempted solutions devised to enable changing the drive ratio of such transmissions result in machines which are overly complex and costly. Furthermore, such transmissions do not really have a continuously variable drive ratio because the pinion cannot be left opposite two conical sections of the conical gear member at once for too long a time without causing excessive wear of the gear teeth and greatly increasing the likelihood that the transmission will freeze up or jam. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved variable ratio gear drive or transmission for transferring torque between an input shaft and an output shaft. 
     Another object of the invention is to provide a transmission of this type whose drive ratio is truly continuously variable over the entire operating range of the transmission. 
     Another object of the invention is to provide such a gear drive which is less complex than prior comparable variable ratio positive drive transmissions of this general type. 
     Other objects will, in part, be obvious and will, in part, appear hereinafter. 
     The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed description, and the scope of the invention will be indicated in the claims. 
     Briefly, my transmission comprises a first rotary shaft which carries a continuous coaxial helical toothed rack whose diameter progressively increases along the shaft. Positioned parallel to the toothed surface of the rack as a second rotary shaft which carries a pinion whose teeth mesh with those of the rack. The pinion is rotatably coupled to its shaft but slidable therealong so that its teeth can mesh with those of the rack at any location along the length of the rack. 
     The drive ratio of the transmission may be changed by sliding the pinion along its shaft by hand or by other suitable means such as a linear actuator, lead screw drive, piston, etc. Either one of the two shafts may function as the driving or input member, the other shaft then being the driven or output member. In either event, since the slidable pinion may remain in driving engagement with the rack at any point along the length of the rack, the transmission does have a drive ratio which is truly continuously variable over the entire operating range of the transmission. 
     Since the helical rack is a single continuous member, it may be connected directly to its shaft without the imposition of clutches and other such devices that are found in prior gear drives whose conical gear members are composed of a series of separate conical gear sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a sectional view with parts in elevation of a variable ratio transmission incorporating the invention; 
     FIG. 2 is a diagrammatic view illustrating the operation of the FIG. 1 transmission; 
     FIG. 3 is an elevational view with parts in section, on a larger scale, showing a portion of the FIG. 1 transmission, and 
     FIGS. 4A and 4B are sectional views taken along lines  4 A— 4 A and  4 B— 4 B, respectively, of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, my transmission comprises a housing or support  10  having opposite walls  10   a  and  10   b  containing bearing units  12  and  14 , respectively, for rotatably supporting the opposite ends of a shaft  16 . Shaft  16  is mounted to a continuous rack  18  which is wound about the shaft so that it has many spaced apart convolutions  18   a . The rack  18  has a conical profile in that the diameters of its convolutions  18   a  progressively increase along the length of shaft  16  as though the rack  18  were spirally wound about a conical envelope  20  as shown in phantom in FIG.  2 . Conical envelope  20  may be real, i.e. a conical segment of shaft  16  as shown in FIG. 1, or it may be in imaginary in which case the convolutions  18   a  may be connected to shaft  16  by a multiplicity of different-length spokes  22  extending out from shaft  16  to the convolutions  18   a  at spaced-apart locations along the rack. In both cases, the teeth of the rack convolutions  18   a  face away from shaft  16  and are more or less parallel to the axis of the shaft. On the other hand, they could face toward the shaft axis. 
     The transmission depicted in FIG. 1 also includes a second shaft  26  which is spaced parallel to the conical surface of envelope  20 . The opposite ends of shaft  26  are rotatably supported by bearing units  28  and  30  in the housing walls  10   a  and  10   b , respectively. Shaft  26  has one or more splines  26   a  and supports a gear member shown generally at  32  which is rotatably coupled to, but slidable along, shaft  26  so that the teeth of gear member  32  can mesh with those of rack convolutions  18   a  at any location along the rack. 
     Either one of the shafts  16  and  26  may be the input or output of the transmission, i.e. either the driving or driven shaft. Although not necessary, to enable the input and output shafts of the transmission to be co-linear, one end of shaft  26  may be provided with a cone gear  36  which meshes with a second cone gear  38  at the end of a third shaft  42  rotatably mounted by means of a bearing unit  44  in a housing or support wall  10   c  so that the shaft  42  rotates about the same axis as shaft  16 . Thus the transmission is completely reversible and either the shaft  16  or the shaft  42  may be the driving member of the transmission, the other shaft then being the driven member. 
     Still referring to FIG. 1, gear member  32  may be slid along its shaft  26  by means of a slider shown generally at  52 . In the illustrated embodiment of the transmission, slider  52  comprises a carriage  54  connected to the gear member  32  by an arm  54   a  and which travels along a lead screw  56 . Screw  56  has one end rotatably supported by a bearing unit  58  in housing or support wall  10   a  and its opposite end connected to the shaft  58   a  of a reversible step motor  58  mounted to housing or support wall  10   c . When the shaft  58   a  is rotated in one direction or the other, the gear member  32  is moved in one direction or the other along the shaft  26  and thus meshes with different convolutions of the rack  18 , to vary the drive ratio of the transmission. Since the rack  18  is a single continuous member, the gear member  32  can be positioned at any location along the rack so that the drive ratio of the transmission is truly continuously variable as the member  32  is moved between the larger diameter end of rack  18  and the smaller diameter end thereof. Of course, any other conventional actuator maybe used to move the gear member along its shaft. 
     Refer now to FIG. 3 which shows the gear member  32  in greater detail. It comprises a sleeve  62  whose inner surface is slotted to receive the spline(s)  26   a  of shaft  26 . Thus the sleeve  62  is rotatably fixed to, but slidable along, shaft  26 . Relatively loosely encircling sleeve  62  is at least one pinion,  64   a . The at least one pinion  64   a  is captured on the sleeve by sleeve end flanges  62   a  and  62   b . As best seen in FIG. 2, the teeth of pinion  64   a  mesh with those of the helical rack  18 . Depending upon the spacing X of the convolutions  18   a , the gear member  32  may have a single, pinion  64   a  rotatably fixed to sleeve  62  and thus to shaft  26  or member  32  may include an additional pinion  64   b  on sleeve  62  next to pinion  64   a.    
     More particularly, if the spacing X is small and rack  18  has a relatively small cone angle, the teeth of adjacent convolutions  18   a  are offset only slightly relative to one another. Therefore, a single pinion  64   a  may be used whose width Y is at least somewhat greater than X and whose teeth are formed so as to have a somewhat loose fit with those of rack  18 . 
     On the other hand, for larger convolution spacings X and larger rack cone angles, gear member  32  may include a second pinion  64   b  as shown wherein the combined widths Y and Z, respectively, of the two pinions should be greater then X, with both pinions having teeth which mesh normally with those of the rack. 
     If the gear member  32  does have two pinions  64   a , 64   b , the pinions are preferably formed so as to be rotatable on the sleeve  62  and relative to one another to an angular extent comparable to at least one gear tooth in either direction. For this, as shown in FIGS. 3,  4 A and  4 B, sleeve  62  is formed with a radially outwardly extending key  66  which projects into an arcuate recess  68  in the face of pinion  64   a  which is opposite pinion  64   b . That same key  66  also projects into a similar arcuate recess  72  in pinion  64   b  that recess being disposed directly opposite recess  68 . Preferably, springs  74  are provided in one of the recesses, say recess  72 , in order to bias the corresponding gear  64   a  to a neutral angular position on sleeve  62  (and shaft  26 ) when it is not engaged to the helical rack  18 , i.e. when it is positioned between convolutions  18   a.    
     Also as best seen in FIGS. 4A and 4B, one of the pinions, i.e. pinion  64   a , is provided with a spring-loaded ball  78  which projects into an arcuate groove  82  in the counterfacing surface of the other pinion  64   b . The bottom wall  82   a  of groove  82  is sloped so that it is deeper at the center of the groove than at the ends thereof. Thus, the wedging effect of the spring-loaded ball  78  in the groove  82  angularly biases pinion  64   b  to a home position wherein its teeth are in alignment with those of pinion  64   a.    
     During operation of the transmission, when one of the shafts  16  or  42  is rotated by suitable motive means (not shown), the other shaft  42  or  16  will rotate at a speed determined by the setting of the gear member  32  along the helical rack  18 . Normally for a given speed, gear member  32  is set so that it is centered on a selected rack convolution  18   a . When changing speed that member is moved to or over an adjacent convolution. During that change, one of the pinions  64   a ,  64   b , i.e. the leading pinion, will disengage from the selected convolution  18   a  and engage the adjacent convolution, while the other, trailing, pinion remains meshed momentarily with the selected convolution. The small misalignment of the corresponding teeth of the two adjacent rack convolutions will be accommodated by a small angular offsetting of the two pinions  64   a  and  64   b  until the trailing pinion disengages from the selected convolution, at which point the two pinions will return to their home position on sleeve  62  as the gear member  32  is centered on the adjacent convolution  18   a . Since the rack is rotating, the gear member can move easily from one convolution to the next with the pinions  64   a  and  64   b  meshing with two adjacent convolutions  18   a ,  18   a  at the same time while being biased to a home position related to shaft  26  and to each other. 
     Since the rack  18  is continuous and wound in a helix or spiral, the gear member  32  can be set at any location along the rack and remain there, even if it engages two convolutions  18  at once, without causing wear of the gear teeth or jamming of the transmission. In other words, there are no indeterminate positions of the gear member  32  as is the case with prior comparable transmissions employing a conical gear composed of individual gear sections. 
     It will thus be seen that the objects set forth above among those made apparent from the preceding description are efficiently attained. Also, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.