Patent Publication Number: US-6209698-B1

Title: Speed control wrap spring clutch

Description:
This application is a divisional application of U.S. application Ser. No. 09/023,525, filed Feb. 13, 1998 and now U.S. Pat. No. 6,138,808. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wrap spring clutch and more particularly to an electromagnetically actuated wrap spring clutch that is capable of generating a high torque output using a low power input. 
     2. Disclosure of Related Art 
     A conventional, electromagnetically actuated, wrap spring clutch includes an output shaft, a shaft hub disposed about the shaft and connected for rotation therewith, and an input hub. The input hub is also disposed about the shaft, but may rotate independently of the shaft when the wrap spring clutch is deenergized. The clutch further includes a coil substantially disposed about the shaft hub and an annular wrap spring disposed about a portion of the input hub and a portion of the shaft hub. A first end of the wrap spring is connected to the input hub so that the spring rotates with the input hub. Energizing the coil establishes magnetic flux circuits or closed loops in the magnetically permeable portions of the clutch. Attractive forces arising from the flux draw a second end of the spring into contact with the shaft hub (which may be non-rotating at this point). Frictional forces restrain the second end of the spring from rotating. The difference in relative rotation between the first and second ends of the spring causes the spring to wrap down upon the shaft hub, thereby transmitting torque from the input hub to the shaft hub and output shaft. 
     Conventional clutches have long suffered from poor efficiency. The geometry of the magnetically permeable portions of a conventional clutch generally does not permit generation of a sufficient level of magnetic flux in response to relatively low power inputs. A decreased magnetic flux provides less attractive forces internal to the clutch (i.e., between the input and output components of the clutch) which, in turn, results in a diminished torque capacity. The inability to transmit a sufficient level of torque to the output shaft in response to low power inputs renders conventional clutches unsuitable for certain applications, such as automotive applications, where available power is limited as an initial matter, and in which large variations in voltage must be provided for (e.g., as a result of a reduction in the voltage output of a chemical battery during extremely cold weather). 
     There is thus a need for a wrap spring clutch that minimizes or eliminates one or more of the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a relatively high efficiency electromagnetically actuated wrap spring clutch. An object of the present invention is to provide a clutch that is capable of generating a high torque output using a low power input to the clutch coil. 
     An electromagnetically actuated wrap spring clutch in accordance with the present invention includes a shaft assembly, a control assembly and a coil assembly. The shaft assembly includes a shaft extending along a longitudinal axis and a shaft hub disposed radially outwardly of the shaft and mounted to the shaft for rotation therewith. The control assembly includes an input hub that is disposed radially outwardly of the shaft and that is rotatable relative to the shaft. The control assembly also includes an annular control collar having a collar flange portion and a wrap spring that is connected at one end to the control collar and at a second end to the input hub. The input hub, spring, and control collar rotate as a unit about the shaft. The coil assembly includes a housing disposed radially outwardly of the shaft hub and a coil within the housing. The housing has a housing flange portion that is axially adjacent the collar flange portion of the control collar. 
     In a preferred embodiment, both the collar flange portion and the housing flange portion are perpendicular to the longitudinal axis extending through the shaft. The collar flange portion extends away from the axis while the housing flange portion extends towards the axis. The geometry of the housing flange portion and the collar flange portion insures that a relatively large portion of the surface area of the housing is adjacent a relatively large portion of the control collar. This geometry promotes a high level of magnetic flux transfer between the housing and the control collar—two of the magnetically permeable components in a magnetic flux circuit created when a current is generated in the coil. Moreover, the geometry promotes a high level of magnetic flux transfer between the control collar and the shaft hub. Therefore, even with a low power input, there is a strong attraction between the input components of the clutch, including the control collar—and the input hub it is connected to—and the output components of the clutch, including the shaft hub. A clutch in accordance with the present invention is therefore able to transmit a high level of torque from the input hub to the shaft hub using a low power input. 
     An advantage of the present invention is that it can be used in applications, such as automotive applications, where available power is limited as an initial matter, and in which large variations in power must be provided for (e.g., as a result of a reduction in the voltage output of a chemical battery during extremely cold weather). 
     In a preferred application, the inventive clutch may be utilized in a speed control device of a vehicle. Conventional speed control devices employ means, such as a tooth clutch, for selectively transmitting torque from a motor driven input shaft to an output shaft in order to selectively actuate a throttle control. Because return springs bias the throttle control to a predetermined position, the torque transmitted to the output shaft must be sufficient to overcome the biasing force of the return springs. As a result, conventional speed control devices have employed large and/or expensive torque transmitting means to transmit a high level of torque from the input shaft to the output shaft. The present invention, however, is able to transmit a high level of torque despite its relatively small size and weight—thereby representing a significant improvement in the art. In addition, the present invention is able to transmit a high level of torque despite using significantly less electrical power than conventional torque transmitting means. 
     These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an electromagnetically actuated wrap spring clutch in accordance with the present invention. 
     FIGS. 2 and 3 are sectional views of the clutch of FIG. 1 viewed in the direction of lines  2 — 2  and  3 — 3 , respectively. 
     FIG. 4 is an enlarged view of one portion of the sectional view shown in FIG.  1 . 
     FIG. 5 is an exploded, perspective view of a speed control device incorporating an electromagnetically actuated wrap spring clutch in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, FIG. 1 shows a sectional view of an electromagnetically actuated wrap spring clutch  10  in accordance with the present invention. Clutch  10  includes an end plate  12 , an output shaft assembly  14 , a control assembly  16 , and a coil assembly  18 . 
     End plate  12  is provided to maintain the physical location and relationship of the component parts of clutch  10 . End plate  12  may be annular and has an aperture  20 . End plate  12  may be made from a powdered metal such as bronze. 
     Output shaft assembly  14  is provided to selectively rotate an object affixed to shaft assembly  14 . Shaft assembly  14  includes a shaft  22  and a shaft hub  24 . Shaft assembly  14  may also include a flux transfer hub  26 . 
     Shaft  22  is provided as a means for mounting an object for selective rotation with shaft  22 . The object may be mounted at end  28  of shaft  22 . Shaft  22  is preferably made of a non-ferromagnetic material, but may be composed of other materials. Shaft  22  extends through aperture  20  of end plate  12  and is centered about, and extends longitudinally along, a longitudinal axis  30 . Shaft  22  may include a knurled portion  32  extending longitudinally along axis  30  for mounting shaft hub  24  to shaft  22 . 
     Shaft hub  24  is provided as a means for selective engagement of output assembly  14  with control assembly  16 . Hub  24  is preferably made from a material having a relatively high magnetic permeability, such as powdered iron or another ferromagnetic material. Alternatively, in another preferred construction, a first part  24 A of hub  24  may be made of a magnetic material while a second part  24 B is made of a non-magnetic material so as to better channel magnetic flux from control collar  42  to hub  24  as described in greater detail hereinbelow. As shown in FIG. 2, hub  24  is annular and disposed radially outwardly of axis  30  and shaft  22 . Referring again to FIG. 1, hub  24  includes a first portion  34  having a first diameter and a second portion  36  having a second diameter that is less than the first diameter. First and second portions  34 ,  36 , define a shoulder  38  therebetween (best shown in FIG.  4 ). As illustrated, hub  24  is unitary in construction. However, hub  24  could be divided into a plurality of sections press fit together. Hub  24  is mounted to shaft  22  for rotation therewith by sliding hub  24  onto knurled portion  32  of shaft  22 . 
     Flux transfer hub  26  is provided as part of a magnetic flux circuit for the transfer of magnetic flux from shaft hub  24  to coil assembly  18  as discussed more fully hereinbelow. Hub  26  may be made of a ferromagnetic material such as powdered iron and is press fit onto shaft hub  24  between shaft hub  24  and coil assembly  18 . Although shaft hub  24  and flux transfer hub  26  are shown as separate components of clutch  10 , it should be understood that they could be combined into a single hub. As illustrated in FIG. 2, flux transfer hub  26  is annular and is centered about axis  30 . Hub  26  may be disposed radially outwardly of shaft hub  24  and radially inwardly of coil assembly  18 . 
     Control assembly  16  is provided to selectively transmit a torque to output shaft assembly  14 . Control assembly  16  may include an input hub  40 , a control collar  42 , and a wrap spring  44 . 
     Input hub  40  is preferably composed of a non-ferromagnetic material such as glass filled nylon and may be disposed radially outwardly of shaft  22 . Hub  40  is rotatable relative to shaft  22  and axis  30 . A portion  46  of hub  40  may be axially adjacent portion  36  of shaft hub  24  and may have the same diameter as portion  36 . It should be understood, however, that the diameters of shaft hub  24  and input hub  40 , and their respective portions  34 ,  36 , and  46 , may vary depending upon a particular application. The outer periphery of input hub  40  may comprise a gear  48 . Input hub  40  may be integral with gear  48  as shown in FIG. 1 or may comprise a separate component mounted within gear  48  for rotation with gear  48 . Input hub  40  may be held in place adjacent shaft hub  24  by an annular sleeve  50  disposed about shaft  22 . 
     Control collar  42  is provided to selectively, frictionally engage shaft hub  24  of shaft assembly  14 . Collar  42  may be made from a ferromagnetic material such as powdered iron. As illustrated in FIG. 3, collar  42  is annular and is centered about axis  30 . Collar  42  is disposed radially outwardly of shaft  22 . Referring again to FIG. 1, collar  42  is generally L-shaped in cross-section, having an annular collar flange portion  52  extending away from, and in a direction perpendicular to, axis  30 . Collar  42  also includes a friction surface  43  (best shown in FIG.  4 ). Collar  42  is coupled to one end of spring  44 , the other end of which is connected to input hub  40 . Like input hub  40 , collar  42  is rotatable relative to shaft  22  and axis  30 . Collar  42  may be disposed radially outwardly of a portion of input hub  40 , such as portion  46 , and a portion of shaft hub  24 , such as portion  36 . Collar  42  may also be disposed radially outwardly of spring  44 . 
     Wrap spring  44  is provided to securely engage input hub  40  and shaft hub  24  upon the frictional engagement of collar  42  and shaft hub  24  as described hereinbelow. Spring  44  is conventional in the art and may be made from known materials such as music wire. As best shown in FIG. 4, spring  44  is connected at one end to input hub  40  and at a second end to collar  42  by a first tang  54  and a second tang  56 , respectively, that may be inserted within corresponding notches  58 ,  60 , cut within input hub  40  and control collar  42 . Spring  44  may be disposed radially outwardly of a portion of input hub  40 , such as portion  46  and portion of shaft hub  24 , such as portion  36 . 
     Coil assembly  18  is provided to generate a magnetic field to cause control assembly  16  to selectively engage shaft assembly  14 , and, consequently, to cause the selective rotation of shaft  22  and an object affixed to shaft  22 . Coil assembly  18  includes an annular housing  62  and a coil  64  disposed within housing  62 . 
     Housing  62  is provided to house coil  64  and to form part of a magnetic circuit for the transfer of magnetic flux within clutch  10 . Housing  62  may be made of a ferromagnetic material, such as powdered iron. As shown in FIG. 2, housing  62  is annular and is centered about axis  30 . Housing  62  is disposed radially outwardly of shaft hub  24 . Referring again to FIG. 1, housing  62  is substantially L-shaped in cross-section and includes an annular housing flange portion  66  that extends towards, and in a direction perpendicular to, axis  30 . Housing flange portion  66  is axially adjacent collar flange portion  52 . 
     Coil  64  is provided to generate a magnetic field, and create a magnetic flux circuit among the magnetically permeable components of clutch  10 , when current is supplied to coil  64 . Coil  64  is conventional in the art and can be made from known materials such as copper. Coil  64  is disposed within housing  62  and current is supplied to coil  64  through housing  62  by a power source (not shown). 
     A power input, or control signal, to clutch  10  causes current to flow within coil  64 . Generation of the current in coil  64  produces a magnetic field and creates a magnetic flux circuit comprised of housing  62 , collar  42 , shaft hub  24 , and flux transfer hub  26 . The magnetic circuit provides a path for the transfer of magnetic flux resulting from the magnetic field. The particular geometry of collar flange portion  52  and housing flange portion  66  insures a high level of magnetic flux transfer between housing  62  and collar  42  because large portions of the surface of housing  62  and collar  42  are adjacent. As collar flange portion  52  is drawn closer to housing flange portion  66  due to the magnetic attraction between the two, friction surface  43  of control collar  42  is brought into engagement with shoulder  38  of shaft hub  24 . Frictional forces then restrain collar  42  and the end of spring  44  that is connected to collar  42  from rotating. The other end of spring  44  continues to rotate with input hub  40 . The difference in relative rotation between the ends of spring  44  causes spring  44  to wrap down upon shaft hub  24  thereby connecting input hub  40  of control assembly  16  to shaft hub  24  of shaft assembly  14 . In this manner, torque is selectively generated in shaft assembly  14 . 
     The axial alignment of large portions of the surfaces of housing  62  and control collar  42  results in a high level of flux transfer and, therefore, a strong magnetic attraction, between housing  62  and control collar  42 . The strong magnetic attraction between housing  62  and collar  42  results in a tight frictional engagement between friction surface  43  of control collar  42  and shoulder  38  of shaft hub  24 —even during a low power input to clutch  10 . In this manner, a clutch in accordance with the present invention is able to generate higher torque outputs than conventional clutches receiving the same input power. This ability makes the inventive clutch particularly suitable for applications, such as automotive applications, where the available power is limited as an initial matter, and in which wide variations in power must be provided for (e.g., as a result of a reduction in the voltage output of a chemical battery during extremely cold weather). 
     Referring now to FIG. 5, a vehicle speed control device  70  is illustrated incorporating an electromagnetic spring clutch, such as clutch  10 . Besides clutch  10 , device  70  includes a housing  72 , means, such as motor  74 , for rotating an input shaft  76 , a transfer gear  78 , and means, such as cable assembly  80 , for controlling a throttle assembly (not shown) of the vehicle. 
     Housing  72  is provided to protect the internal components of device  70  from external elements. Housing  72  may be made from a plurality of conventional materials including various metals and plastics. Housing  72  includes a module  82  through which electrical connections may be made to clutch  10  and motor  74  and control signals provided to clutch  10  and motor  74 . Module  82  may be secured to the rest of housing  72  by screws  84 , bolts, or other fastening means. 
     Motor  74  is provided to cause input shaft  76  to rotate. Motor  74  is conventional in the art and may take on any of a plurality of forms well-known in the art. Input shaft  76  extends from motor  74  and is also conventional in the art. 
     Transfer gear  78  is provided to impart rotation to input hub  40  of clutch  10  responsive to rotation of input shaft  76 . As mentioned hereinabove, the peripheral portion of input hub  40  may comprise a gear  48 . The teeth of gear  78  mesh with the teeth of gear  48 . Device  70  may also include an anti-backlash gear  86  connected to transfer gear  78  by anti-backlash springs  88 . The teeth of gear  86  also mesh with the teeth of gear  48 . Gear  86  and springs  88  function in a conventional manner to reduce mechanical noise and increase response time in response to a change in rotational direction of gear  48 . Transfer gear  78 , anti-backlash gear  86 , and springs  88  are all conventional in the art. 
     Clutch  10  is utilized within device  70  to selectively couple input shaft  76  of device  70  with output shaft  22  extending from clutch  10 . As described in greater detail hereinabove with reference to FIG. 1, wrap spring  44 —which is connected to input hub  40 —is selectively, electromagnetically actuated through a power input, or control signal, to wrap down upon shaft hub  24  of clutch  10 . Because shaft hub  24  is connected to output shaft  22 , the torque generated by input shaft  76  and transmitted to input hub  40  via transfer gear  78 , is transmitted to output shaft  22 . Clutch  10  may be positionally secured by a plurality of conventional retainers  89  and bearings  91 . It should be understood that a speed control device in accordance with the present invention is not limited by the particular structure of the inventive electromagnetic wrap-spring clutch described herein as clutch  10 . A speed control device in accordance with the present invention may alternatively include any of a plurality of conventional electromagnetic spring clutches. 
     Cable assembly  80  is provided to control the throttle assembly (not shown) of the vehicle according to a rotation of output shaft  22 . Assembly  80  includes a plurality of conventional components including cover assembly  90 , cable  92 , spool  94 , seal  96 , and return spring  98 . Cover assembly  90  is provided to house cable  92  and spool  94 . Cable  92  is provided to actuate the throttle assembly and is wound upon spool  94  which is connected to output shaft  22 . Seal  96  is provided to prevent the loss of lubricants from cover assembly  90  and the introduction of foreign objects into cover assembly  90 . Finally, return spring  98  is provided to bias spool  94 , and therefore, cable  92  and the throttle assembly, to a predetermined position. 
     The use of an electromagnetic spring clutch within speed control device  70  represents a significant improvement over conventional speed control devices. Because of the biasing force of return spring  98 , a high level of torque must be transmitted from input shaft  76  to output shaft  22  in order to rotate shaft  22 . Conventional speed control devices have relied on large and/or expensive torque transmitting means, such as a tooth clutch, to transmit the necessary amount of torque. An electromagnetic spring clutch, however, is much smaller and less expensive. Moreover, an electromagnetic spring clutch can produce a high torque output despite a low power input. Furthermore, the use of the inventive electromagnetic wrap-spring clutch described and illustrated herein (i.e. clutch  10 ) will result in an even higher torque output than conventional electromagnetic spring clutches using the same low power input. 
     The use of an electromagnetic spring clutch in speed control device  70 —and, in particular, the use of an electromagnetic wrap-spring clutch in accordance with the present invention—has several advantages over conventional devices. First, because an electromagnetic wrap-spring clutch in accordance with the present invention utilizes only a few parts it can be assembled much easier than conventional torque transmitting means—and without machining. Second, the inventive clutch is more tolerant of vibrations and the harsh automotive environment. Third, there is less inertia associated with the moving parts of the inventive clutch. Fourth, an electromagnetic wrap-spring clutch in accordance with the present invention has an unlimited stroke length whereas, for example, a conventional tooth clutch is limited to 180 degrees of rotation. Finally, the inventive clutch produces less noise than conventional torque transmitting means. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention.