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This invention relates to a motor and the component that it drives and, more particularly, to a coupling between the motor output shaft and the driven component that absorbs the misalignment between the two. 
   BACKGROUND OF THE INVENTION 
   In a common form of a motor drive system, a motor produces a mechanical output, usually through a motor output shaft. The motor output shaft is mechanically linked to the driven component to transmit the mechanical output to the driven component. In principle, the motor and the driven component are perfectly aligned. In practice, however, there may be small misalignments either in the motor drive system as first assembled, or that develop during the course of service. The small misalignments may not be sufficiently large to cause the motor drive system to be inoperable, but they impose stresses on the motor and on the driven component that may damage bearings and other elements of the two devices. 
   A flexible coupling may be used between the motor output shaft and the driven component to transmit the power therebetween while flexing to accommodate misalignments between the motor output shaft and the driven component. The universal joint of an automobile drive train is a common example of a flexible coupling that allows the transmission of power between the motor and the driven wheels as the drive train misaligns during driving. In other circumstances, other types of flexible couplings such as bellows and spirally machined couplings are used. 
   While operable for many applications, the available flexible couplings are difficult or impossible to utilize for very small motor drive systems. For example, “micro-motors” with overall diameters of less than about 12 millimeters and shaft output diameters of less than about 3 millimeters are used in applications where driven components must be powered and the required power is low, and where the available size envelope is small. Optical fiber systems that employ multiple optical filters that must be mechanically moved into and out of an optical path are one such application. Universal joints and bellows cannot be economically manufactured in proportionately small sizes. The smallest economical universal joints and bellows are typically larger than the motors, and are also expensive to manufacture. Spirally machined couplings are made in this size range only with difficulty, and are also relatively expensive to fabricate. 
   There is a need for a mechanical flexible coupling that is suitable for use in small mechanical systems. The present invention fulfills this need, and further provides related advantages. 
   SUMMARY OF THE INVENTION 
   The present approach provides a mechanical flexible coupling that is particularly suitable for use in small-size, low-power motor drive systems. The flexible coupling of the present approach is readily and inexpensively produced in small sizes on the order of 3 millimeters diameter or smaller, comparable in size with the diameter of the motor output shaft of a micro-motor. The flexible coupling is easily interconnected with such a small motor output shaft and with the power inputs of comparably sized driven components. 
   In accordance with the invention, a motor drive system comprises a motor having a rotary motor output shaft, a driven component, and a flexible coupling connecting the motor output shaft to the driven component. The flexible coupling comprises a helix comprising a first wound wire. The helix is bonded at one end to the rotary motor output shaft and at the other end to the driven component. The helix has a helix unbonded free length, defined as the portion of the length of the helix that is between and not bonded to the rotary motor output shaft and the driven component, that is less than about twice a helix outer diameter. 
   The flexible coupling is particularly suited for coupling between a small motor, such as one with a shaft diameter of less than about 3 millimeters and sometimes as small as about 0.5 millimeters, and a comparably sized rotary-driven shaft of the driven component. 
   The motor may be of any operable type, with examples being an electrical motor or a pneumatic motor. The driven component may be of any operable type, with an example being a leadscrew that receives the rotary output of the motor output shaft and transforms that rotary output into a linear movement. 
   In the flexible coupling, the first wound wire preferably has a round cross section. The shape of the first wound wire is an important economic consideration. Many flexible couplings use a manufacturing technique that produces a spiral-element shape that is square or rectangular, and the non-circular shape of the spiral element is required to stabilize the operation of the flexible coupling. On the other hand, most commercially available wire is round in cross section. By utilizing round wire rather than a non-standard shape, the present approach reduces the costs associated with the provision of non-standard shapes. 
   In the preferred embodiment, the flexible coupling has no lateral constraint for the helix. Lateral constraints such as flexible cores or external tubes are used in most types of flexible shafts to prevent the flexible shaft from “snaking up”, which is a mechanical buckling instability experienced when the helical shape is lost because too much power is transmitted through the flexible shaft. In the present approach, the small ratio of the helix unbonded free length-to-helix diameter of less than about 2 avoids such “snaking up” or buckling instability for moderate transmitted forces and torques, without the need for any lateral constraint. 
   The helix may be made of the first wound wire with a single lay to the wire. (The “lay” is the sense of the winding and advance of the wire that forms the helix, relative to the axis of the helix, and is usually expressed as a “right-hand lay” or a “left-hand lay”.) When only a single-lay helix is used, the winding tends to unwrap when the motor is driven in the opposite sense. To overcome this characteristic, the helix may have two layers of windings of opposite lay. Thus, for example, a bi-directional flexible coupling includes the helix having a first helical layer formed of the first wound wire having a first lay, and a second helical layer overlying the first helical layer, wherein the second helical layer is formed of a second wound wire having a second lay opposite to the first lay. The two helical layers are each bonded to the rotary motor output shaft at one end of the helix and to the driven component at the other end of the helix, leaving the unbonded free length between the ends of the rotary motor output shaft and the driven component. Whether one layer, two layers, or more layers of wire are used to form the helix, the turns of wire that form each layer are desirably wound tightly in a side-by-side manner, rather than with laterally spaced-apart turns. 
   It is preferred that a ratio of the diameter of the first wound wire to the inner diameter of the helix be in the range of from about 1:1.5 to about 1:3 for most applications, although the ratio may be as high as 1:5 for light duty applications. For a two-layer helix with wires of the same diameters, it is preferred that the diameter of the wire in each layer be about half the value indicated by these ratios. This ratio of the diameter of the wire to the inner diameter of the helix produces a mechanically stable flexible coupling. 
   A method for making a motor drive system comprises the steps of providing a motor having a rotary motor output shaft, providing a driven component, fabricating a flexible coupling by winding a first wound wire into a helix as a first helical layer, wherein the helix has a helix unbonded free length that is less than about two times a helix outer diameter, and thereafter affixing a first end of the helix to the output shaft and a second end of the helix to the driven component. To form a bidirectional coupling, the step of fabricating includes the additional steps of winding a second wound wire overlying the first helical layer to form a second helical layer, wherein the second wound wire has a second lay opposite to the first lay (and the helix outer diameter is measured for the helix with both layers). Other compatible features discussed herein may be used in relation to the method. 
   The present approach provides for the coupling of two rotating shafts while accommodating any misalignment between the motor output and the driven component. The wound wire, short—short helix configuration is particularly suitable for making small couplings for use with small motors and driven components. The small coupling aids in keeping the size and weight of the motor drive system small. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a motor drive system; 
       FIG. 2  is a sectional view of a first embodiment of the flexible coupling, taken on line  2 — 2  of  FIG. 1 ; 
       FIG. 3  is a sectional view of a second embodiment of the flexible coupling, taken on line  2 — 2  of  FIG. 1  and with a different arrangement of the shafts as compared with those of  FIG. 2 ; and 
       FIG. 4  is a block diagram of a preferred approach for making a motor drive system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  schematically depicts a motor drive system  20  having a motor  22  with a rotary motor output shaft  24  and a driven component  26 . The motor  22  has a motor diameter D M  and a shaft diameter D S . In a preferred application, D M  is less than about 12 millimeters, and D S  is less than about 3 millimeters and typically in the range of from about 0.5 to about 3 millimeters. In the pictured embodiment, the driven component  26  is a leadscrew drive  28  including a threaded leadscrew  30  which serves as a shaft of the driven component  26  that is driven and turned in a rotary manner by the rotary motor output shaft  24 , a leadscrew follower  32  that moves along the length of the leadscrew  30  responsive to the turning of the leadscrew  30 , and a driven article  34  that is affixed to the leadscrew follower  32 . The leadscrew drive  28  converts the rotary movement of the output shaft  24  to linear movement of the driven article  34 . 
   A flexible coupling  36  connects the motor output shaft  24  to the driven component  26 , in this case to the leadscrew  30 , so that the rotation of the motor output shaft  24  imparts rotary motion to the leadscrew  30 . The flexible coupling  36  is affixed, preferably with an adhesive bond, at one end to the end of the motor output shaft  24 , and is affixed, preferably with an adhesive bond, at the other end to the driven component  26  and specifically to the leadscrew  30 . 
   Two preferred embodiments of the flexible coupling  36  are illustrated in greater detail in  FIGS. 2–3 . Either of these embodiments, or any other operable embodiment, may be used as the flexible coupling  36  in the motor drive system  20  of  FIG. 1 . 
   In the embodiment of  FIG. 2 , the flexible coupling  36  includes a helix  38  formed of a first wound wire  40  cylindrically wound around a helical axis. The direction of winding of the first wound wire  40 , the “lay” of the first wound wire  40 , selected to be the same as the direction of rotation of the rotary motor output shaft  24  when power is transmitted through the rotary motor output shaft  24  to the driven component  26 . (The “lay” is the sense of the winding and advance of the wire that forms the helix, relative to the axis of the helix, and is usually expressed as a “right-hand lay” or a “left-hand lay”.) The helix  38  has a helix unbonded free length L UF  that is less than about two times a helix outer diameter D H . D H  is preferably less than about 4 millimeters, more preferably less than about 3 millimeters, most preferably less than about 2.5 millimeters, and typically in the range of from about 0.5 to about 3 millimeters. That is, the helix  38  is a “short” helix, whose purpose is not to serve as a flexible shaft to transmit the motor power over long distances and around obstacles. Instead, the flexible coupling  36  serves as a short coupling whose function is to negate the effect of misalignment between the rotary motor output shaft  24  and the driven component  26 , and to transmit the motor power over a relatively short distance. The misalignment is typically small, on the order of about 2 degrees or less. 
   The short helix configuration aids in mechanically stabilizing the flexible coupling  36  by avoiding any buckling instability of the helix  38  when mechanical power is transmitted through it from the rotary motor output shaft  24  to the driven component  26 . Consequently, no lateral constraint of the helix  38  is required, apart from the structural rigidity of the helix itself. In some prior flexible shafts whose unbonded free lengths are much longer than the present helix, in relation to the diameter of the helix, there is a flexible central core structure or a lateral tubular support around the flexible shaft, to prevent snaking mechanical instability of the flexible shaft. Such a lateral constraint is not needed or desired in the present approach, although it could be used in some cases. 
   The embodiment of  FIG. 2  is useful in those cases where the power is to be transmitted only when the rotary motor output shaft  24  rotates in a single direction having the same sense as the lay of the first wound wire  40 , which is useful in some applications but not, for example, in the case of the leadscrew drive  28  as the driven component  26 . In the embodiment of  FIG. 2 , if there is an attempt to reverse the movement of the leadscrew follower  32  by reversing the direction of the rotation of the rotary motor output shaft  24 , the first wound wire  40  of the helix  38  unwinds, because its adjacent turns are not bonded to each other. 
   The embodiment of  FIG. 3  is suited for applications in which power must be transmitted from the rotary motor output shaft  24  to the driven component  26 , whatever the direction of rotation of the rotary motor output shaft  24 . In the embodiment of  FIG. 3 , the first wound wire  40  forms a first helical layer  42  having a first lay. A second helical layer  44  overlies the first helical layer  42 . The second helical layer  44  is formed of a second wound wire  46  having a second lay opposite to the first lay. For example, if the first helical layer  42  has a left-hand lay, then the second helical layer  44  has a right-hand lay, or vice versa. 
   In either the embodiment of  FIG. 2  or the embodiment of  FIG. 3 , the motor output shaft  24  is mechanically connected to one end of the helix  38  with a first connection  48 , and the driven component is mechanically connected to the opposite end of the helix  38  with a second connection  50 . The mechanical connections  48  and  50  are preferably made with a permanent or temporary adhesive material. The mechanical connections  48  and  50  are preferably made in an axial manner, so that the motor output shaft  24  and the driven component  26  are axially aligned with the respective ends of the helix  38  of the flexible coupling  36 . The mechanical connections  48  and  50  may both be made with both of the shafts  24  and  26  affixed to the inside the helix  38  (as in  FIG. 2 ), both of the shafts  24  and  26  affixed to the outside of the helix  38 , or one of the shafts  24  affixed to the inside and the other of the shafts  26  affixed to the outside of the helix  38  (as in  FIG. 3 ). 
   The outer diameter D H  and the overall length L UF  of the helix  38  are selected according to the particular application, but within the constraints discussed above. The first wound wire  40  and the second wound wire  46  (where used) are preferably a high-strength steel material such as steel piano wire. The first wound wire  40  and the second wound wire  46  may be of the same or different materials, and the second wound wire  46  may simply be a further length of the first wound wire  40  that is wrapped with the opposite lay. The first helical layer  42  and the second helical layer  44  may be single layers of the respective wires, or multiple separated or interleaved layers. The first wound wire  40  and the second wound wire  46  are preferably each of a round cross section, as that is the form more readily available commercially and least expensive commercially. 
   The diameter of the first wound wire  40  and, if used, the second wound wire  46 , are related to an inner diameter D I  of the helix  38 . Generally, the larger the inner diameter D I , the larger the diameter of the wound wire  40 ,  46  to improve the buckling resistance of the helix  38  and thereby the ability of the flexible coupling  36  to carry higher loading torques. However, the wound wire  40 ,  46  cannot be of such a large diameter that it cannot be wound to the required diameter of the helix  38 . It is preferred that a ratio of the diameter of the first wound wire  40  to the inner diameter of the helix D I  be in the range of from about 1:1.5 to about 1:3 for most applications, although the ratio may be as high as 1:5 for light duty applications. For a two-layer helix with wires  40 ,  46  of the same diameters, it is preferred that the diameter of the wire  40 ,  46  in each layer  42 ,  44  be about half the value indicated by these proportions. This ratio of the diameter of the wire to the inner diameter of the helix produces a mechanically stable flexible coupling. In an example, for a helix inner diameter of about 1.2 millimeters, corresponding to a motor output shaft diameter of about 1.2 millimeters, the wound wire  40  is a high-strength round steel wire having a diameter of about 0.4–0.5 millimeters for a single layer helix  38 , or about 0.2–0.3 millimeters for each wire  40 ,  46  of the two-layer helix  38  of opposite lays. 
     FIG. 4  depicts a preferred method for making the motor drive system  20 . The flexible coupling  36  is first fabricated, step  60 , by winding the first wound wire  40  into the form of the helix  38 , typically on a winding mandrel, as the first helical layer  42  (or only helical layer in the event that no second helical layer is used), step  62 . If the second helical layer  44  is to be used as well, it is wound with the second wound wire  46  overlying the first helical layer  44  with the opposite lay, step  64 . The pitch of the second wound wire  46  may be the same as, or different than, the pitch of the first wound wire  38 . The helix  38  is wound so that the helix unbonded free length L UF  is less than about two times the helix outer diameter DH. The motor  22  having the rotary motor output shaft  24  is provided, step  66 , and the driven component  26  is provided, step  68 . The motor  22  and the driven component  26  are typically provided as completed subassemblies. A first end of the helix  38  is affixed to the rotary motor output shaft  24 , and a second end of the helix  38  is affixed to the driven component  26 , step  70 , to complete the motor drive system  20 . 
   The motor drive system  20  has been prepared and operated for each of the embodiments of the flexible coupling  36  of  FIGS. 2 and 3 , and found to operate satisfactorily. 
   Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Summary:
A motor drive system includes a motor having a rotary motor output shaft, a driven component, and a flexible coupling connecting the motor output shaft to the driven component. The flexible coupling is a helix formed of a first wound wire. The helix has a helix unbonded free length that is less than about two times a helix outer diameter. The approach is particularly suitable for miniature motors and their driven components, wherein the motor output shaft has a shaft diameter of less than about 3 millimeters and the helix outer diameter is less than about 3 millimeters.