Patent Publication Number: US-2019186550-A1

Title: Toothed disk coupling

Description:
FIELD OF THE INVENTION 
     The present invention relates to coupling. More particularly, the present invention relates to a toothed disk coupling. 
     BACKGROUND OF THE INVENTION 
     Mechanical devices often require a transmission between a driver, which may include a motor or other device for generating a torque, and a driven component, such as a wheel of a vehicle. A coupling may connect a driving shaft of the driver with a driven shaft of the driven component. The coupling is designed to transmit the torque from the driver shaft to the driven shaft. 
     A coupling may be designed to accommodate misalignment between the driver shaft and the driven shaft. In some cases, the coupling may be designed to enable the driver shaft and the driven shaft to move with respect to one another when in operation. 
     For example, angular misalignment may occur when one of the shafts changes its orientation relative to the other. A universal joint, sometimes referred to as a Cardan joint, may be designed to accommodate such angular misalignment or motion. The universal joint typically includes a pair of axes that enable bending of one shaft with respect to the other in different directions. 
     Parallel misalignment may occur in which one shaft is displaced laterally with respect to the other so that the shafts remain parallel with one another but not collinear. An Oldham coupling may accommodate such parallel misalignment. An Oldham coupling typically includes a pair of grooves that enable lateral sliding of one shaft relative to the other in different lateral directions. 
     Axial misalignment may occur in which one shaft may be displaced relative to the other along their common axis (thus remaining both parallel and collinear). A spline may accommodate such axial misalignment. A spline typically includes elongated grooves that enable axial movement of one shaft relative to the other. 
     In some cases, a coupling may include a clutch mechanism to enable the driving shaft to disengage from the driven shaft, or to engage the driven shaft in order to apply torque and rotate the driven shaft. 
     SUMMARY OF THE INVENTION 
     There is thus provided, in accordance with an embodiment of the present invention, a coupling including: a driving disk connected to a driving shaft and rotatable about a driving axis, an engagement face of the driving disk including a predetermined number of at least three driving teeth that are distributed azimuthally on the engagement face of the driving disk along a circle concentric to the driving axis; and a driven disk connected to a driven shaft and rotatable about a driven axis, an engagement face of the driven disk including the predetermined number of driven teeth azimuthally distributed on the engagement face of the driven disk along a circle concentric to the driven axis, wherein at least one of the driving shaft and the driven shaft possesses translational freedom or angular freedom of motion with respect to the other so that when the driving teeth of the driving disk engage the driven teeth, the driving axis and the driven axis are self-aligned. 
     Furthermore, in accordance with an embodiment of the present invention, the driving teeth are uniformly distributed along the circle concentric to the driving axis and the driven teeth are uniformly distributed on the circle concentric to the driven axis. 
     Furthermore, in accordance with an embodiment of the present invention, each driving tooth includes a face that is slanted at an acute angle with respect to the engagement face of the driving disk and toward a direction of rotation of the driving shaft and each driven tooth includes a face that is slanted at substantially the acute angle with respect to the engagement face of the driven disk toward a direction opposite to a direction of rotation of the driven shaft. 
     Furthermore, in accordance with an embodiment of the present invention, a face of each driving tooth that is opposite the face of that driving tooth that is slanted toward the direction of rotation of the driving shaft is slanted at an acute angle to the engagement face of the driving disk, and a face of each driven tooth that is opposite the face of that driven tooth that is slanted opposite the direction of rotation of the driven shaft is slanted at an acute angle to the engagement face of the driven disk. 
     Furthermore, in accordance with an embodiment of the present invention, an outer face of each driving tooth that is distal to the driving disk is substantially flat, and an outer face of each driven tooth that is distal to the driven disk is substantially flat. 
     Furthermore, in accordance with an embodiment of the present invention, the outer faces of at least three of the driving teeth are substantially coplanar and parallel to the engagement face of the driving disk or the outer faces of at least three of the driven teeth are substantially coplanar and parallel to the engagement face of the driven disk. 
     Furthermore, in accordance with an embodiment of the present invention, each tooth of the plurality of driving teeth and of the plurality of driven teeth is laterally rotated. 
     Furthermore, in accordance with an embodiment of the present invention, the driving disk is incorporated into a detachable propulsion unit. 
     Furthermore, in accordance with an embodiment of the present invention, the portable propulsion unit is configured to be mounted onto a chassis of a vehicle. 
     Furthermore, in accordance with an embodiment of the present invention, the chassis includes the driven disk, the driven disk being coupled to a propulsion wheel of the vehicle. 
     Furthermore, in accordance with an embodiment of the present invention, the vehicle includes a bicycle. 
     Furthermore, in accordance with an embodiment of the present invention, the driven disk is coupled to a chain sprocket of the bicycle. 
     Furthermore, in accordance with an embodiment of the present invention, the portable propulsion unit includes a motor for rotating the driving shaft. 
     Furthermore, in accordance with an embodiment of the present invention, the portable propulsion unit includes a transmission for transmitting torque from the motor to the driving shaft. 
     Furthermore, in accordance with an embodiment of the present invention, the transmission includes a belt. 
     There is further provided, in accordance with an embodiment of the present invention, a portable propulsion unit including: a motor; and a driving disk connected to a driving shaft that is coupled to the motor and that is rotatable about a driving axis, an engagement face of the driving disk including a predetermined number of at least three driving teeth that are distributed azimuthally on the engagement face of the driving disk along a circle concentric to the driving axis, wherein when the portable propulsion unit is mounted to a chassis of a vehicle, the chassis including a driven disk connected to a driven shaft and rotatable about a driven axis, an engagement face of the driven disk including the predetermined number of driven teeth azimuthally distributed on the engagement face of the driven disk along a circle concentric to the driven axis, the driven axis being coupled to a propulsion wheel of the vehicle, at least one of the driving shaft and the driven shaft possessing translational freedom or angular freedom of motion with respect to the other, operation of the motor causes the driving teeth of the driving disk to engage the driven teeth and causes the driving axis and the driven axis to self-align. 
     Furthermore, in accordance with an embodiment of the present invention, the driving teeth are uniformly distributed along the circle concentric to the driving axis. 
     Furthermore, in accordance with an embodiment of the present invention, each driving tooth includes a face that is slanted at an acute angle with respect to the engagement face of the driving disk and toward a direction of rotation of the driving shaft. 
     Furthermore, in accordance with an embodiment of the present invention, the portable propulsion unit includes a transmission for transmitting torque from the motor to the driving shaft. 
     Furthermore, in accordance with an embodiment of the present invention, the transmission includes a belt. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order for the present invention to be better understood and for its practical applications to be appreciated, the following figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals. 
         FIG. 1  schematically illustrates a coupling with toothed coupling disks, in accordance with an embodiment of the present invention. 
         FIG. 2  schematically illustrates the coupling shown in  FIG. 1 , with the teeth of the disks engaged. 
         FIG. 3  schematically illustrates structure of a tooth of a toothed coupling disk of the coupling shown in  FIG. 1 . 
         FIG. 4A  schematically illustrates forces that are exerted by a driving tooth of the coupling shown in  FIG. 1  on a driven tooth of the coupling when in partial contact. 
         FIG. 4B  schematically illustrates forces that are exerted by a driving tooth of the coupling shown in  FIG. 1  on a driven tooth of the coupling when one of the disks is angularly misaligned relative to the other. 
         FIG. 5  schematically illustrates engagement of parallelly misaligned toothed coupling disks of the coupling shown in  FIG. 1 . 
         FIG. 6  schematically illustrates a variation of the coupling shown in  FIG. 5  in which the teeth are laterally rotated. 
         FIG. 7  schematically illustrates teeth of toothed coupling disks that are configured to apply torque in either direction of rotation. 
         FIG. 8  schematically illustrates a propulsion unit that includes a driving disk of the coupling shown in  FIG. 1 . 
         FIG. 9  schematically illustrates mounting the propulsion unit shown in  FIG. 8  on a chassis. 
         FIG. 10  schematically illustrates the propulsion unit and chassis shown in  FIG. 9  configured to drive a wheel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, units and/or circuits have not been described in detail so as not to obscure the invention. 
     Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same point in time, or concurrently. Unless otherwise indicated, the conjunction “or” as used herein is to be understood as inclusive (any or all of the stated options). 
     In accordance with an embodiment of the present invention, a coupling mechanism includes a pair of toothed disks. Each disk may be connected to a distal (e.g., from a motor or driving mechanism that drives the driving shaft, or from a mechanism that is to be operated by rotation of the driven shaft) end of a driving shaft or a driven shaft. In some cases, each shaft may be constructed as a single unit together with its toothed disk. For example, in some cases, a pulley wheel or gear on which the toothed disk is mounted, or that incorporates the toothed disk, may be configured so as to function as a driving shaft or driven shaft. 
     As used herein, the disk that is connected to the driving shaft is referred to as the driving disk, and teeth of the driving disk are referred to as driving teeth. The disk that is connected to the driven shaft is herein referred to as the driven disk, and teeth of the driven disk are referred to as driven teeth. When operated in a typical manner, the driving teeth of the driving disk engage the driven teeth of the driven disk. When so engaged, a torque that is applied to the driving disk may be applied via the engaged teeth to the driven disk. Furthermore, the teeth are configured such that when the driving disk and the driven disk are initially misaligned (e.g., angularly, parallelly, or axially), engagement of the teeth together with application of the torque may force the driving shaft and the driven shift into alignment. 
     The teeth are azimuthally distributed on the disk about a circle that is concentric to an axis of rotation of the disk. Typically, the teeth are uniformly distributed about the circumference of the disk, such that the azimuthal distance between each pair of adjacent teeth (at a given radius from the axis of the disk, e.g., on a circle that is concentric with the disk) is substantially identical for all such pairs of adjacent teeth. The number (at least three) and distribution of teeth on both disks is typically identical. In some cases, one of the disks may have more teeth than the other (e.g., where the number of uniformly distributed teeth on one of the disks is an integral multiple of the number of teeth on the other disk). 
     The teeth on each of the toothed disks may have at least one face that is inclined with respect to the axis of rotation of the attached shaft. For example, on the driving disk, at least a forward-facing face of each driving tooth may be inclined forward and outward. As used herein, the terms “forward” and “forward facing” refer to the direction toward which the driving disk is rotated when the driving disk is to apply torque to the driven disk. As used herein, “outward” refers to the side of one of the disks that faces toward the other disk that is to be engaged. 
     In some cases, the forward-facing face of a driving tooth on the driving disk may be positioned on the disk such that the forward-facing face is laterally rotated, inclined, or slanted (e.g., rotated about an axis that is parallel to the common axis of rotation of the disk and the connected shaft) relatively to a radius of the disk through the driving tooth. Thus, the face may be oriented diagonally with respect to the local radius of the disk (e.g., a radius of the disk that intersects a midpoint of the face). The lateral rotation may be such that the forward-facing face is oriented forward and proximally (e.g., toward the axis of rotation). 
     Typically, the driving disk and the driven disk have substantially identical disk faces and teeth. Thus, a driving shaft with a connected driving disk may be interchangeable (e.g., at least as far as the coupling between the shafts and disks, possibly not with regard to structure on the shafts that is not adjacent to the connected disk) with a driven shaft and its connected driven disk. When configured to couple between the driving disk and the driven disk, the driven disk faces in a substantially (e.g., except for any misalignment) opposite direction to the driving disk. Thus, the inclined face of each driven tooth of the driven disk may be oriented so as to be engaged by the inclined face of a driving tooth of the driving disk. 
     When the driving shaft is to drive the driven shaft, a torque may be applied to the driving shaft, and to the connecting driving disk, in a forward direction. For example, a motor, crank, pedal, or other mechanism may be operated to rotate the driving shaft. The forward direction may correspond to a direction in which the driven shaft is to be rotated, e.g., in order to propel a vehicle or other self-propelling device in a particular direction, in order to cause a machine or mechanism to execute a particular operation, to generate electrical power, or for another purpose. 
     Prior to, concurrently with, or subsequent to the beginning of the application of the torque, one or both of the driving disk and the driven disk may be moved toward the other (e.g., by releasing a clutch mechanism or activating another engagement mechanism). Typically, a disk that is moved toward the other may be moved substantially along its own axis of rotation, or along another direction. Thus, the teeth of the driving disk and of the driven disk may be brought into proximity of one another. For example, when brought into proximity, the driving teeth of the driving disk may be located within spaces between the driven teeth of the driven disk. Thus, the rotation of the driving disk may bring one or more driving teeth of the driving disk into contact with one or more driven teeth of the driven disk. 
     Typically, when a driving tooth of the driving disk comes into contact with a driven tooth of the driven disk, the inclined face of the driving tooth may meet or press against the inclined face of the driven tooth. A force that is applied by the inclined face of the driving tooth to the inclined face of the driven tooth may include one or more components that tend to align the driving shaft with the driven shaft. 
     For example, the applied force may have an axial component that pulls the driving disk and the driven disk toward one another. Thus, this component of the force may tend to correct an axial misalignment. The axial pulling force may also act to correct an angular or parallel misalignment. For example, when the driving shaft and the driven shaft are axially or parallelly misaligned, driving teeth of the driving disk may engage one or more driven teeth on one side of the driven disk before they engage driven teeth on an opposite side (e.g., an approximately diametrically opposite side) of the driven disk. The resulting imbalance in forces may tend to pull the driving disk and the driven disk, and thus the driving shaft and the driven shaft, into angular and parallel alignment. In some cases, a lateral slant of the teeth may further facilitate correction a misalignment (e.g., parallel misalignment). 
     Thus, the toothed coupling mechanism may enable relative motion between the driving shaft and the driven shaft. One or both of the driving shaft and the driven shaft may possess translational or angular (rotational) freedom to enable self alignment of the shafts in response to forces that are exerted by the driving teeth and the driven teeth when engaging one another. For example, the shafts and coupling mechanism may accommodate five degrees of freedom (three translational, and two rotational). The degrees of freedom may enable self-corrections of any misalignment, whether angular, parallel, or axial. Some or all of degrees of freedom may be provided by an assembly that includes one of the shafts (e.g., a motor assembly for mounting on a bicycle). 
     Once the driving teeth have engaged the driven teeth, forces that are applied by the inclined faces of the teeth may act to counteract any forces (e.g., caused by a vehicle encountering a bump or depression in a road, or otherwise created) that may temporarily disturb the alignment and that could otherwise cause the teeth to disengage from one another. 
       FIG. 1  schematically illustrates a coupling with toothed coupling disks, in accordance with an embodiment of the present invention. 
     Toothed coupling  10  is configured to enable application of a torque from driving shaft  12  to driven shaft  14 . In the example shown, toothed coupling  10  is disengaged such that a torque that is applied to driving shaft  12  is not applied to driven shaft  14 . In the example of toothed coupling  10  that is shown, toothed coupling  10  is configured to be engaged to enable transmission of a torque from driving shaft  12  to driven shaft  14  when the direction of the torque is as indicated by rotation direction  20 . When the torque is in the direction opposite to the direction indicated by rotation direction  20 , toothed coupling  10  may not transmit the torque. 
     Driving shaft  12  may be connected to a motor, crank, pedal, turbine, or other source of torque, either directly or via a transmission. Driving shaft  12  is connected to driving disk  22  of toothed coupling  10  such that driving disk  22  rotates together with driving shaft  12 . Thus, a torque that is applied to driving shaft  12  is also applied to driving disk  22 . 
     Engagement face  22   a  of driving disk  22  faces outward (e.g., distally toward driven disk  26 ) and includes a plurality of driving teeth  24 . Forward face  24   a  of each driving tooth  24  is on a leading side of driving tooth  24  when driving disk  22  rotates in rotation direction  20 . Forward face  24   a  is substantially flat and sloped forward and outward to form an acute angle with engagement face  22   a  of driving disk  22 . 
     Thus, when a torque is applied to driving shaft  12  and driving disk  22  in the direction of rotation direction  20 , forward face  24   a  may apply a pushing force to an object or surface that comes into contact with forward face  24   a . The net applied force may be directed forward (e.g., toward rotation direction  20 ) and inward (toward the base of driving tooth  24 , e.g., toward engagement face  22   a ). 
     In the example shown, the outermost (distal to driving disk  22 ) end of driving tooth  24  terminates in outer face  24   b , which is substantially flat. In other examples, the outermost end of driving tooth  24  may form a sharp edge, may be slanted, blunted, curved, or otherwise shaped. In other examples, outer faces  24   b  of at least three driving teeth  24 , e.g., of those driving teeth  24  whose outer faces  24   b  extend furthermost from engagement face  22   a  of driving disk  22 , may be substantially coplanar in a plane that is substantially parallel to engagement face  22   a  Similarly, outer faces  28   b  of at least three of teeth  28 , e.g., of those driven teeth  28  whose outer faces  28   b  extend furthermost from engagement face  26   a  of driven disk  26 , may be substantially coplanar in a plane that is substantially parallel to engagement face  26   a.    
     Driven shaft  14  may be connected to a drive wheel, to a generator, a propeller, or to another mechanism that may be operated by a torque, either directly or via a transmission. Driven shaft  14  is connected to driven disk  26  of toothed coupling  10  such that driven disk  26  rotates together with driven shaft  14 . Thus, a torque that is applied to driven disk  26  is also applied to driven shaft  14 . 
     Engagement face  26   a  of driven disk  26  faces outward (e.g., distally toward driving disk  22 ) and includes a plurality of driven teeth  28 . Rearward face  28   a  of each driven tooth  28  is on a trailing side of driven tooth  28  when driven disk  26  is being rotated in rotation direction  20 . Rearward face  28   a  is sloped rearward and outward to form an acute angle with engagement face  26   a  of driven disk  26 . 
     Thus, when a pushing force is applied to rearward face  28   a  of one or more driven teeth  28  on driven disk  26  (e.g., approximately normal to rearward face  28   a  or approximately parallel to engagement face  26   a  of driven disk  26 ), a torque may be applied to driven disk  26  in the direction of rotation direction  20 . The applied force may be decomposed into a component that is directed forward (e.g., toward rotation direction  20 ), and a component that is directed inward (toward the base of driven tooth  28 , e.g., toward engagement face  26   a ). 
     In the example shown, the outermost (distal to driven disk  26 ) end of driven tooth  28  terminates in outer face  28   b , which is substantially flat and parallel to engagement face  26   a  of driven disk  26 . Outer faces  28   b  of all driven teeth  28  are substantially coplanar. In other examples, the outermost end of driven tooth  28  may form a sharp edge, may be slanted, blunted, or curved, or may be otherwise shaped. 
     Toothed coupling  10  may be engaged, when torque is applied to driving shaft  12 , to rotate driving disk  22  in rotation direction  20 . For example, driving disk  22  may be moved toward driven disk  26  with axial motion  30 , driven disk  26  may be moved toward driving disk  22  with axial motion  32 , or both driving disk  22  and driven disk  26  may be moved toward one another. When driving disk  22  and driven disk  26  are sufficiently close to one another, one or more of rotating driving teeth  24  may engage one or more of driven teeth  28 . When one or more driving teeth  24  engage one or more driven teeth  28 , the torque may be transmitted to driven disk  26  and to driven shaft  14 . 
     In addition, the engagement of driven teeth  28  by driving teeth  24  may apply a force to correct one or more initial misalignments (e.g., angular, parallel, or axial) between driving shaft  12  and driven shaft  14 . For example, the engagement may apply axial forces to pull driving disk  22  and driven disk  26  toward one another to eliminate axial misalignment. Initial asymmetric engagement of driven teeth  28  by driving teeth  24  may apply lateral forces (e.g., perpendicular to the axial forces) to eliminate or reduce or parallel misalignment. 
       FIG. 2  schematically illustrates the coupling shown in  FIG. 1 , with the teeth of the disks engaged. 
     When toothed coupling  10  is fully engaged, driving disk  22  and driven disk  26  have moved maximally toward one another. For example, the movement of driving disk  22  and driven disk  26  toward one another may be checked by contact of outer faces  28   b  of one or more driven teeth  28  with engagement face  22   a  of driving disk  22 , by contact of outer faces  24   b  of one or more driving teeth  24  with engagement face  26   a  of driven disk  26 , or both (as in the example shown). 
     When toothed coupling  10  is fully engaged, the forces that corrected any initial misalignment may continue to hold driving disk  22  and driven disk  26  together. Thus, driving disk  22  and driven shaft  14 , as well as driving shaft  12  and driven shaft  14 , may rotate together with a common angular velocity in rotation direction  20 . 
       FIG. 3  schematically illustrates structure of a tooth of a toothed coupling disk of the coupling shown in  FIG. 1 . 
     In the example shown, the tooth and the coupling disk are indicated to be a driving tooth  24  and a driving disk  22 , respectively. However,  FIG. 3  could equally apply to a driven tooth  28  and a driven disk  26 , respectively. Typically, driven teeth  28  are identical to driving teeth  24 , and driven disk  26  is identical to driving disk  22 . 
     Outer face  24   b  of each driving tooth  24  extends a distance h outward from the base of driving tooth  24  at engagement face  22   a  of driving disk  22 . In the example shown, outer face  24   b  of driving tooth  24  is parallel to engagement face  22   a . When driven disk  26  and an identical driving disk  22  are moved toward one another, one or more driving disk  22  may engage one or more driven teeth  28  when the separation distance between engagement face  22   a  of driving disk  22  and engagement face  26   a  of driven disk  26  is less than 2 h. When toothed coupling  10  is fully engaged (as in  FIG. 2 ), the separation distance between engagement face  22   a  of driving disk  22  and engagement face  26   a  of driven disk  26  may be equal to h. 
     Forward face  24   a  of driving tooth  24  is sloped forward, such that the angle formed between forward face  24   a  and engagement face  22   a  of driving disk  22  is an acute angle. The slope of forward face  24   a  may be characterized by slope angle α between forward face  24   a  and normal  34  to engagement face  22   a  of driving disk  22  (and complementary to the acute angle between forward face  24   a  and engagement face  22   a ). Slope angle α and height h may be selected in accordance with an anticipated application of toothed coupling  10 . 
     Typically, all driving teeth  24  are identical to one another (e.g., at least with regard to height h and slope angle α) Similarly, all driven teeth  28  are identical to one another and to driving teeth  24 . 
       FIG. 4A  schematically illustrates forces that are exerted by a driving tooth of the coupling shown in  FIG. 1  on a driven tooth of the coupling when in partial contact. 
     In the example shown, driving disk  22  is rotating in rotation direction  20  such that driving tooth  24  is moving (locally) in forward direction  21 . An outer region of forward face  24   a  of driving tooth  24  is in contact with a similar region of rearward face  28   a  of driven tooth  28 . In the example shown, rearward face  28   a  of driven tooth  28  is parallel to forward face  24   a  of driving tooth  24 . Therefore, at the region of contact, forward face  24   a  exerts a normal force F on rearward face  28   a  Similarly (as a consequence of Newton&#39;s third law of motion), rearward face  28   a  exerts a normal force that is equal and opposite to normal force F on forward face  24   a.    
     Normal force F may be decomposed into a parallel component F 1  that is parallel to both engagement face  26   a  of driven tooth  28  and engagement face  22   a  of driving disk  22 , and a perpendicular component F 2  that is perpendicular to outward faces  22   a  and  26   a  (and parallel to normal  34 ). In the example shown, F 1 =F·cos (α) and F 2 =F·sin (α). 
     Parallel component F 1  may impel driven tooth  28  to move together with driving tooth  24  in forward direction  21 . Thus, parallel component F 1  may drive driven disk  26  and driven shaft  14  in rotation direction  20 . 
     Perpendicular component F 2  impels driven tooth  28  and driven disk  26  toward engagement face  22   a  of driving disk  22 . Thus, perpendicular component F 2  may drive driving disk  22  and driven disk  26  toward one another, e.g., to correct or prevent axial misalignment. Driving disk  22  and driven disk  26  may continue to move toward one another until contact between outer face  24   b  of driving tooth  24  and engagement face  26   a  of driven disk  26  (as well as between outer face  28   b  and engagement face  22   a ) applies a force that is equal and opposite to perpendicular component F 2 . At that point, toothed coupling  10  may be fully engaged. 
     When toothed coupling  10  is fully engaged, continued exertion of perpendicular component F 2  may maintain the full engagement. 
       FIG. 4B  schematically illustrates forces that are exerted by a driving tooth of the coupling shown in  FIG. 1  on a driven tooth of the coupling when one of the disks is angularly misaligned relative to the other. 
     In the example shown, driven disk  26  is tilted relative to driving disk  22 , e.g., due to an initial angular misalignment between driving shaft  12  and driven shaft  14 . The region of driving disk  22  and driven disk  26  shown in  FIG. 4B  is an end where, as a result of the angular misalignment, driving disk  22  and driven disk  26  are closest to one another. Thus, at that region, a driving tooth  24  may contact a driven tooth  28  before such contact is made at another region of driving disk  22  or of driven disk  26 . 
     Thus, when driving tooth  24  is impelled in forward direction  21 , forward face  24   a  may exert a force  25  on driven tooth  28  at contact region  29  (e.g., a line of contact in three dimensions). Exertion of force  25  at one or more contact regions  29  may impel driven teeth  28  toward driving disk  22  (by perpendicular component F 2 , as described above) and apply a net torque  27  to driven disk  26  and to driven shaft  14 . Impelling driven teeth  28  toward driving disk  22  may result in contact between outer faces  28   b  of driven teeth  28  and engagement face  22   a  of driving disk  22  (and between outer faces  24   b  of driving teeth  24  and engagement face  26   a  of driven disk  26 ), forcing driven shaft  14  into alignment with driving shaft  12 . Thus, toothed coupling  10  may operate to self-correct an angular misalignment. 
     Toothed coupling  10  may enable self correction of parallel misalignment. 
       FIG. 5  schematically illustrates engagement of parallelly misaligned toothed coupling disks of the coupling shown in  FIG. 1 . 
     In the example shown, driven shaft  14  is parallel to, but laterally displaced from, driving shaft  12 . Driving shaft  12  and, thus, driving disk  22  are being rotated in rotation direction  20 . As driving disk  22  and driven disk  26  are moved closer to one another along the axes (perpendicular to the plane of  FIG. 5 ) of driving shaft  12  and driven shaft  14 , respectively, one of driving teeth  24 , designated driving tooth  24 ′, may contact one of driven teeth  28 , designated driven tooth  28 ′, before other driving teeth  24  contact other driven teeth  28 . In the initial contact between driving tooth  24 ′ and driven tooth  28 ′, driving tooth  24 ′ may apply a contact force  31  to driven tooth  28 ′. Contact force  31  may be transmitted to the remainder to driven disk  26  and to driven shaft  14  to laterally push driven shaft  14  toward parallel alignment with driving shaft  12 . 
     In some cases, teeth on each disk of a toothed coupling may be laterally rotated or slanted relative to a radius of the disk. The lateral slanting may assist in correction of misalignment. 
       FIG. 6  schematically illustrates a variation of the coupling shown in  FIG. 5  in which the teeth are laterally rotated. 
     In toothed coupling  50 , driving teeth  54  on driving disk  52  are laterally slanted (each driving tooth  54  represented schematically by a line indicating the forward face of the driving tooth  54 ). The lateral slant may be characterized by nonzero slant angle ( 3  (e.g., a rotation angle) relative to local radius  60  of driving disk  52 . The lateral slant of each driving tooth  54  the may be forward and inward (e.g., toward the axis of rotation of driving disk  52 ) from a part of driving tooth that is closest to the circumference of driving disk  52  (e.g., when driving disk  52  is rotated in rotation direction  20 ). Driven teeth  58  on driven disk  56  are similarly laterally rotated (with the lateral rotation being rearward and inward, each driven tooth  58  being represented schematically by a line indicating the rearward face of the driven tooth  58 ). 
     In the example shown, driving shaft  12  and, thus, driving disk  52  are being rotated in rotation direction  20 . As driving disk  52  and driven disk  56  are moved closer to one another along the axes (perpendicular to the plane of  FIG. 6 ) of driving shaft  12  and driven shaft  14 , respectively, one of driving teeth  54 , designated driving tooth  54   a , may contact one of driven teeth  58 , designated driven tooth  58   a , before other driving teeth  54  contact other driven teeth  58 . In the initial contact between driving tooth  54   a  and driven tooth  58   a , driving tooth  54   a  may apply a contact force  51  to driven tooth  58   a . Contact force  51  may be transmitted to the remainder to driven disk  56  and to driven shaft  14  as a parallel alignment force in order to laterally push driven shaft  14  toward parallel alignment with driving shaft  15 . The lateral slant of driving tooth  54   a  and of driven tooth  58   a  may enable contact force  51  to have larger parallel alignment component than it would have in the absence of the lateral slant (e.g., relative to the situation shown in  FIG. 5 ). 
     Use of toothed coupling  10  (or of toothed coupling  50 ) may be advantageous in a situation where a driving shaft (e.g., of a motor) is to couple to a driven device (e.g., a wheel) where initial alignment may be expected to be imperfect, or where alignment may be disturbed. 
     For example, toothed coupling  10  may be applied to a removable motor of a motorized bicycle. In this case, the motor may be removed from the bicycle when the bicycle is parked, e.g., to prevent the motor from being stolen. In another scenario, the motor may be privately owned by each user of a bicycle (e.g., of a pool of bicycles) while the bicycle is provided for public use. In these situations, the motor may be replaced onto the bicycle by a user with minimal technical training. In addition, as the bicycle is being powered by the motor, various forced (e.g., centrifugal during turning, or bumps or ruts that are traversed by the bicycle) may tend to knock the motor out of alignment with the bicycle drive system. Therefore, if a drive shaft of the bicycle (e.g., a shaft replacing the usual pedal-operated crank of the bicycle) and the motor are provided with toothed coupling  10 , the motor shaft may remain aligned with the drive shaft. 
     In many applications, such as in the case of a motorized bicycle, a driving shaft  12  may be required to apply torque to a driven shaft  14  in only one direction of rotation. For example, a motor may be expected to drive a motorized bicycle in a forward direction only, and not in reverse. In other applications, a driving shaft  12  may be required to reversible drive a driven shaft  14 . 
       FIG. 7  schematically illustrates teeth of toothed coupling disks that are configured to apply torque in either direction of rotation. 
     In toothed coupling  40 , driving disk  22  is provided with doubly-sloped driving teeth  42  (only one is shown). Similarly, driven disk  26  is provided with doubly-sloped driven teeth  44 . In the example shown, the sloped faces on opposite sides of each doubly-sloped driving tooth  42  are identical. Also in the example shown, the sloped faces on opposite sides of each doubly-sloped driven tooth  44  are identical to one another and to the sloped faces of each doubly-sloped driving tooth  42 . Thus, each doubly-sloped driving tooth  42  and doubly-sloped driven tooth  44  may have an azimuthal profile (e.g., as viewed along a radius of driving disk  22  or of driven disk  26  through the tooth) in the form of an inverted wedge or trapezoid. In some cases, the slopes on opposite faces of the doubly-sloped teeth may be different from one another (e.g., as designed for application where coupling in one direction of rotation is expected to behave differently from coupling in the other direction). 
     Thus, when driving disk  22  is rotated to move doubly sloped driving tooth  42  in either direction indicated by double-headed arrow  46 , driving tooth  42  may contact, and apply aligning forces to, one of doubly sloped driven teeth  44  on driven disk  26 . Thus, driving disk  22  may engage and self-align with driven disk  26  regardless of the direction of rotation of driving disk  22 . 
     When the teeth are provided with a lateral slant, the teeth may have a lateral profile, e.g., when viewed along an axial direction, that is similarly wedge-shaped or trapezoidal. 
     A method for using toothed coupling  10  may include placing driving shaft  12  and driven shaft  14  in approximate alignment, e.g., as shown in  FIG. 1 . The end of driving shaft  12  that is provided with driving disk  22  faces driven shaft  14 . Similarly, the end of driven shaft  14  that is provided with driven disk  26  faces driving shaft  12 . Thus, when in the approximate alignment, driving teeth  24  on driving disk  22  face driven teeth  28  on driven disk  26 . 
     Driving disk  22  and driven disk  26  may be brought toward one another, e.g., by moving driving disk  22  with axial motion  30 , driven disk  26  with axial motion  32 , or by moving both with both axial motions. 
     Prior to, subsequent to, or concurrently with the movement of driving disk  22  and driven disk  26  toward one another, driving shaft  12  and driving disk  22  may be rotated in rotation direction  20 . 
     The rotation of driving disk  22  together with motion toward one another of driving disk  22  and driven disk  26  may cause one or more driving teeth  24  to contact one or more driven teeth  28 . As a result of continued rotation of driving disk  22 , mutual forces that are applied by the contacting driving teeth  24  and driven teeth  28  may continue to pull driving disk  22  and driven disk  26  toward one another, correcting any initial axial misalignment. Concurrently, the mutually applied forces may cause driving shaft  12  and driven shaft  14  to align with one another, thus correcting any initial angular or parallel misalignment. 
     When all initial misalignments are corrected, driving disk  22  and driven disk  26  may be fully engaged. When fully engaged, torque and rotational power that are applied to driving shaft  12  may be transmitted to driven shaft  14 . 
     Continued application of torque to driving shaft  12  may, via application of the mutual forces between driving teeth  24  and driven teeth  28 , maintain engagement and alignment between driving disk  22  and driven disk  26 , and thus between driving shaft  12  and driven disk  14 . 
     A toothed coupling as described above may be incorporated into a propulsion system, e.g., of a bicycle or other vehicle. For example, the propulsion system may include a portable propulsion unit that may be attached to or detached from a chassis of the vehicle. The detachable propulsion unit may include a motor, and a driving shaft and driving disk of the coupling. In some cases, the propulsion unit may include a transmission for transmitting a torque from the motor to the driving shaft. The chassis may include the driven disk of the coupling. When attached to the chassis and operating, the motor of the propulsion unit may drive a wheel of the vehicle. 
       FIG. 8  schematically illustrates a propulsion unit that includes a driving disk of the coupling shown in  FIG. 1 .  FIG. 9  schematically illustrates mounting the propulsion unit shown in  FIG. 8  on a chassis. 
     Propulsion unit  70  may be provided with unit handle  71  to facilitate portability of propulsion unit  70 . In the example shown, driving disk  22  is mounted on driving disk pulley  78 , both rotating about driving shaft  12 . Motor pulley  72  may be rotated by motor  73  to which driving disk pulley  78  is coupled. For example, motor pulley  72  may be mounted to a rotatable shaft of motor  73 , or may be rotated by torque that is applied by motor  73  via a transmission. In the example shown, rotation of motor pulley  72  may drive rotation of driving disk pulley  78  via a transmission  74  in the form of a pulley belt or other driving belt. In the example shown, the diameter of driving disk pulley  78  is larger than that of motor pulley  72 . In other examples, another transmission mechanism may be provided to enable motor  73  to drive driving disk pulley  78 . 
     Propulsion unit  70  may be mounted onto chassis  80 . For example, chassis  80  may represent a bracket or other structure that is mounted onto, or incorporated into, a chassis or body of a vehicle. In the example shown, grooves  86  on propulsion unit  70  may engage pins  84  on chassis  80 . Rotation of propulsion unit  70  toward chassis  80  may cause lock bar  76  of propulsion unit  70  to engage lock mechanism  82  on chassis  80 . Thus, propulsion unit  70  may be secured to chassis  80 . Alternatively, or in addition, other securing or locking mechanisms may be provided (e.g., one or more latches, bolts, screws, or other securing mechanisms). 
     When propulsion unit  70  is mounted onto chassis  80 , driven disk  26  on chassis  80  may be initially misaligned with driving disk  22  on propulsion unit  70 . Operation of the motor to rotate driving disk  22  may cause driving disk  22  to engage driven disk  26 , causing driving disk  22  and driven disk  26  to self-align, as described above. For example, one or both of driving disk  22  and driven disk  26  may include an axle or shaft with a section that is at least partially flexible. Alternatively, or in addition, other mechanisms may be provided to enable limited translational or rotational freedom of movement. One or more of grooves  86 , pins  84 , lock mechanism  82 , and lock bar  76  may be configured to enable at least minimal freedom of movement such that driving disk  22  and driven disk  26  may self-align. 
       FIG. 10  schematically illustrates the propulsion unit and chassis shown in  FIG. 9  configured to drive a wheel. 
     For example, vehicle  90  may represent a bicycle or other type of vehicle. Driven shaft  14  may be coupled to propulsion wheel  96  of the vehicle such that when torque is applied to driven shaft  14  via driving disk  22  and driven disk  26 , torque is applied to propulsion wheel  96 , e.g., to propel the vehicle. Alternatively, or in addition, torque that is applied to driven shaft  14  may rotate or drive another component of the vehicle, or of another type of machine, device, or mechanism. 
     In the example shown, driven shaft  14  functions as a drive pulley or drive sprocket of vehicle  90 . Driven shaft  14  is configured to operate transmission  92  when rotated. For example, transmission  92  may represent a bicycle chain or a pulley belt. Operation of transmission  92  may cause drive wheel  94  (e.g., a pulley wheel or a chain sprocket) to rotate, which in turn may cause propulsion wheel  96  to rotate, thus propelling a vehicle on which propulsion wheel  96  is mounted. 
     Different embodiments are disclosed herein. Features of certain embodiments may be combined with features of other embodiments; thus, certain embodiments may be combinations of features of multiple embodiments. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be appreciated by persons skilled in the art that many modifications, variations, substitutions, changes, and equivalents are possible in light of the above teaching. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.