Patent Publication Number: US-8967004-B2

Title: Armature with torque limiter for engine starter

Description:
FIELD 
     The present disclosure relates generally engine starters for an internal combustion engines and particularly to engine starters including torque limiters. 
     BACKGROUND 
     Engine starters, which are also commonly referred to as “starter motors” or simply “starters”, are used to crank vehicle engines. Most engine starters include an electric motor that is coupled to an internal gear train or other gear assembly. The gear assembly transfers rotation of the electric motor to a pinion gear of the engine starter. Exemplary gear assemblies include planetary gear arrangements connected to an output shaft of the electric motor. An overrun clutch is typically connected between the gear assembly and the pinion gear. A solenoid arrangement is configured to move the pinion gear between an engaged position where the pinion is meshed with the engine ring gear and a disengaged position where the pinion is removed from the engine ring gear. 
     To start an engine with the typical engine starter, the pinion gear is moved to the engaged position, in which the pinion gear becomes engaged with the engine flywheel via the ring gear. Next, the electric motor is fully energized, causing the pinion gear and the flywheel to rotate. The rotating flywheel puts the engine pistons into motion, which typically causes the engine to start. When the engine does start, the flywheel begins to rotate at a rate that is greater than that of the pinion gear, and the overrun clutch decouples the pinion gear from the output of the gear train. This prevents damage to the gear train, which may occur as a result of the rapidly rotating flywheel. The pinion gear is moved to the disengaged position after the engine is started. 
     When the pinion gear is engaged with the flywheel and is rotating the flywheel, the gear assembly and pinion gear of the engine starter experiences a pulsating torque resulting from moving engine parts, including piston movement within the engine cylinders. This pulsating torque is typically less than the stall torque (i.e., a magnitude of torque that causes the output shaft of the electric motor to stop rotating). However, the gear assembly may be loaded with a torque that is much greater in magnitude than the stall torque during certain engine events. These engine events may include engine backfire, hydraulic lock-up, a jammed pinion, or attempted engagement of the pinion gear with the flywheel after the engine is already started. The high torque is primarily caused by kinetic energy stored in the output shaft of the electric motor, which is then converted to strain energy upon rapid deceleration of the output shaft. 
     Vehicle manufacturers require that the engine starter should not fail or cause failure of other engine components as a result of the high-torque engine events such as those mentioned above. To meet this requirement, engine starter manufacturers design engine starter components to withstand a torque in excess of the stall torque. This often results in engine starter components being larger, heavier, or made from more robust and expensive materials than if the components were only required to withstand the torque encountered during normal engine operation. Additionally, many engine starters include torque limiters coupled to the gear assembly. These torque limiters are configured provide relief from excessive torque events preventing the pinion from being driven by the electric motor when a threshold torque is exceeded. Unfortunately, these torque limiters add unwanted additional size to the engine starter. Moreover, some torque limiters that have added only limited additional size to the engine starter have typically failed to accommodate sufficient torque capacity while also providing sufficient durability. 
     In view of the foregoing, it would be desirable to provide a torque limiter for an engine starter that is durable and accommodates large torque capacity. It would also be desirable for such torque limiter to add little or no additional size to the engine starter. Furthermore, it would be desirable for such torque limiter to be relatively easy and inexpensive to manufacture. 
     SUMMARY 
     According to one embodiment of the present disclosure, an engine starter comprises a gear assembly including a pinion gear configured to engage an engine ring gear. An electric motor is coupled to the gear assembly and is configured to drive the gear assembly and the pinion gear. The electric motor includes an armature (referred to herein as the “armature”) configured to rotate within a stator. The armature includes a core member, an armature shaft positioned within the core member, and a clutch arrangement positioned between the core member and the armature shaft. 
     According to at least one embodiment of the present disclosure, an engine starter comprises a gear assembly including a pinion gear. The engine starter further comprises an electric motor including an armature coupled to the gear assembly and configured to drive the gear assembly and the pinion gear. The armature includes a core member defining a central cavity extending in an axial direction within the core member. An armature shaft extends from the central cavity. A clutch arrangement is positioned in the central cavity. The clutch arrangement is configured to releasably couple the core member and the armature shaft. 
     According to another embodiment of the present disclosure, an engine starter comprises a gear assembly including a pinion gear. The engine starter further comprises an electric motor including an armature coupled to the gear assembly. The armature is configured to drive the gear assembly and the pinion gear. The armature includes a core member, an armature shaft positioned within the core member, and a clutch arrangement configured to couple the core member to the armature shaft when a torque on the armature shaft is less than a threshold torque. The clutch arrangement is further configured to de-couple the core member from the armature shaft when the torque on the armature shaft is greater than the threshold torque. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above-described features and advantages, as well as others, should become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying figures in which: 
         FIG. 1  is a perspective view of an engine starter including an armature with a torque limiter according to one embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view of the armature with torque limiter for the engine starter of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the armature along line III-III of  FIG. 2 ; 
         FIG. 4  is a perspective view of the armature shaft of the armature of  FIG. 3 ; 
         FIG. 5  is a perspective view of a first clutch disc provided on the armature shaft of  FIG. 4 ; 
         FIG. 6  is a perspective view of a second clutch disc provided on the armature shaft of  FIG. 4 ; 
         FIG. 7  is a perspective view of a core bushing for the armature of  FIG. 2 ; and 
         FIG. 8  is a perspective view of a plurality of the first clutch discs and second clutch discs assembled on the armature shaft of  FIG. 4 . 
     
    
    
     DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
     With reference to  FIG. 1 , an engine starter  10  includes a solenoid a gear assembly  12  positioned within a housing  14 . The gear assembly  12  is configured to drive a pinion gear  16  that is configured to engage the ring gear of a vehicle engine (not shown). A solenoid  18  is also provided within the housing  14  and is configured to move the pinion gear  16  between a first position where the pinion gear  16  is disengaged from the ring gear and a second position where the pinion gear  16  engages the ring gear. An electric motor  20  is coupled to the gear assembly and is configured to drive the gear assembly. As explained in further detail below, a torque limiter is provided within the electric motor and is configured to limit the torque output that may be provided from the electric motor. 
     With continued reference to  FIG. 1 , the gear assembly  12  includes a planetary gear arrangement  22 , as are known to those of ordinary skill in the art. The output of the planetary gear arrangement  22  is connected to an output shaft  24  such that rotation of the planetary gear arrangement  22  results in rotation of the output shaft  24 . A spline gear (not shown) is provided on the output shaft of the gear arrangement  22 . 
     The pinion gear  16  is configured to slide along the spline gear in the axial direction  15  between the engaged position and the disengaged position. The pinion gear  16  includes teeth that are configured to mesh with the ring gear of the vehicle engine when the pinion is in the engaged position. With reference to  FIG. 1 , the engaged position is an outermost position on the output shaft  24  where the pinion gear  16  is furthest away from the electric motor  20  and is in position to mesh with the engine ring gear. Conversely, the disengaged position is a more inward position on the output shaft  24  where the pinion gear  16  is closer to the electric motor  20 . 
     The solenoid  18  is configured to move the pinion gear  16  between the engaged position and the disengaged position using a shift lever  26 . The shift lever  26  extends between the solenoid  18  and an overrun clutch  28  that is slideably positioned on the drive shaft  24  along with the pinion gear  16 . An output bearing  34  is also provided on the output shaft  24  between the overrun clutch  28  and the pinion gear  16 . One end of the shift lever engages the plunger rod  30  of the solenoid  18  and the opposite end engages the slideable overrun clutch  28 . The shift lever  26  is configured to pivot about a pivot point  32 . When the solenoid  18  is activated, the plunger rod  30  on the solenoid  18  is drawn in the axial direction toward the solenoid  18 . This causes the shift lever  26  to pivot about the pivot point  32  and move the overrun clutch  28  and the pinion gear  16  in the axial direction away from the electric motor  20  and toward engagement with the ring gear. In many starter motor embodiments, full power is provided to the electric motor after engagement of the pinion gear  16  with the ring gear, thus allowing the starter to crank the vehicle engine. 
     As is known in the art, the overrun clutch  28  is configured to decouple the pinion from the gear assembly  12  after the engine fires and the speed of the engine flywheel and associated ring gear is such that the ring gear actually drives the pinion gear  16 . In this situation, the overrun clutch  28  prevents the pinion gear  16  from driving the gear assembly  12  at an excess speed before the pinion gear  16  is moved to the disengaged position. 
     With continued reference to  FIG. 1 , the electric motor  20  is configured to drive the gear assembly  12 , which in turn drives the pinion gear  16  during engine cranking. The electric motor  20  may be any of various types of electric motors as will be recognized by those of skill in the art. In the embodiment of  FIG. 1 , the electric motor  20  is a direct current motor including a stator with permanent magnets or other means for developing a field flux. The electric motor also includes an armature (not shown in  FIG. 1 ; see  FIG. 2 ) that serves as the armature and includes armature windings. The armature also includes an armature shaft with a gear on the end of the armature shaft. The armature is configured to rotate within the stator, thus resulting in rotation of the armature shaft and the gear on the end of the armature shaft. The gear on the end of the armature shaft engages the gear assembly  12  and acts as the sun gear of the planetary gear arrangement  22 . 
     With reference now to  FIGS. 2 and 3 , an armature  40  is shown that serves as the armature for the electric motor  20  of  FIG. 1 . The armature  40  includes a core member  42 , conductors  44 , a commutator  46 , a shaft coupler  48 , an armature shaft  50 , a clutch arrangement  70 , and a core bushing  90 . 
     The core member  42  of the armature  40  is provided as a stack of laminated steel plates. The core member  42  includes a substantially cylindrical outer wall  52  and a central cavity  54 . The central cavity  54  extends in an axial direction from one end to another end of the core member  42 . The core bushing  90  is fixed to the core member  42  within the central cavity  54  of the core member. A plurality of axial slots are also formed in the core member  42  between the central cavity  54  and the outer wall  52 . These axial slots are configured to receive the conductors  44  that provide the armature winding. The slots of the core member  42  may be open, closed, or semi-closed slots, as will be recognized by those of skill in the art. 
     The conductors  44  in the slots may have any of various cross-sectional shapes including round, oval, square, rectangular, etc. Each conductor  44  extends through two different slots in the core member with a U-turn portion extending between the slots at one end of the core member  42 . At the opposite end of the core member  42 , the ends of the conductors  44  are connected to the commutator  46 . To this end, the commutator  46  includes a plurality of segments configured to receive the conductors  44 . Accordingly, the commutator  46  is fixed in relation to the core member  42  and rotates with the core member within the electric motor  20 . 
     The shaft coupler  48  is positioned within the commutator and extends the length of the commutator. The shaft coupler  48  is a shaft-shaped member that includes a cup-like mouth  56  at an end closest to the central cavity  54 . The opposite end of the shaft coupler is rotatably retained within a bearing  57 . The shaft coupler  48  is fixed in relation to the commutator  46  and rotates along with the commutator and the core member  42 . 
     The armature shaft  50  extends through the central cavity  54  of the core member  42 . One end  58  of the armature shaft  50  is positioned in the mouth  56  of the shaft coupler  48 . The end  58  is smooth and cylindrical in shape and is rotatably supported by a shaft bushing  60 . Accordingly, the armature shaft  50  is rotatable with respect to the shaft coupler  48  and the core member  42  within the armature  40 . An opposite end  62  of the armature shaft  50  includes an output gear  64 . This end  62  of the armature shaft  50  extends from the end of the core member  42  where the conductor U-turns are located. As best shown in  FIG. 4 , a middle portion  66  of the armature shaft  50  is positioned between the two ends  58  and  62  of the armature shaft  50 . The middle portion  66  of the armature shaft includes a plurality of axial splines  68 . The axial splines  68  are formed by ribs that extend axially along the middle portion of the shaft with axial grooves formed between the ribs. 
     As best shown in  FIGS. 2 and 8 , the clutch arrangement  70  for the armature  40  includes the armature shaft  50 , a plurality of clutch discs  72 ,  82 , positioned on the armature shaft  50 , a plurality of springs  80 , and the core bushing  90 . The plurality of clutch discs include first clutch discs  72  and second clutch discs  82  positioned on the middle portion  66  of the armature shaft  50 . In the disclosed embodiment, eleven first clutch discs  72  and twelve second clutch discs  82  are positioned on the armature shaft  50 . However, it will be recognized that any number of different clutch discs may be used to provide the desired threshold torque. For example, in at least one embodiment, the clutch arrangement  70  includes at least five first clutch discs  72  and at least five second clutch discs  82 . These clutch discs  72  and  82  act as the friction plates for a multi-plate clutch arrangement, as will be described in further detail below. 
     With particular reference to  FIG. 5 , each first clutch disc  72  includes an outer perimeter  74  that is configured to fit within the central cavity  54  of the core member  42  and, more specifically, within the core bushing  90  within the central cavity  54 . The outer perimeter  74  of the first clutch disc  72  is substantially smooth and circular in shape, allowing the first clutch disc  72  to rotate within the core bushing. The first clutch disc  72  further includes a central hole defined by an inner perimeter  76  that is configured to pass the armature shaft  50 . The inner perimeter  76  includes a plurality of teeth  78  configured to mesh with the splines  68  on the armature shaft  50 . Accordingly, the engagement between the teeth  78  and the splines  68  allows the first clutch discs  72  to slide in the axial direction  15  along the armature shaft  50 , but prevents the first clutch discs  72  from rotating with respect to the armature shaft  52 . 
     Each side of the first clutch disc  72  includes a face  79  that is configured to engage a face  89  of one of the second clutch discs  82 . The faces  79  and  89  may be somewhat textured to provide a desired amount of friction between the discs  72  and  82 . Friction between the discs  72  and  82  is also dependent upon the material discs  72  and  82  are comprised of. The discs  72  and  82  may be comprised of various materials, including, for example, metal, graphite, polymer, or composite materials. 
     With reference now to  FIG. 6 , each second clutch disc  82  includes a central hole defined by an inner perimeter  86  that is configured to pass the armature shaft  50 . The inner perimeter  86  is substantially smooth and circular in shape. Thus, the inner perimeter  86  of the second clutch disc  82  rides on top of the splines  68  on the armature shaft  50 , and the second clutch disc  82  is allowed to slide in the axial direction  15  and also rotate relative to the armature shaft  50 . The second clutch disc  82  further includes an outer perimeter  84  that is configured to fit within the central cavity  54  of the core member  42  and, more specifically, within the core bushing  90  within the central cavity  54 . A plurality of teeth  88  are provided on the outer perimeter  84 . The plurality of teeth  88  are configured mesh with splines  98  on the core bushing  90 . Accordingly, the engagement between the teeth  88  and the splines  98  allow the second clutch discs  82  to slide in the axial direction  15  within the core bushing  90 , but prevent the second clutch discs  82  from rotating with respect to the core bushing  90 . 
     With reference now to  FIG. 7 , the core bushing  90  includes a first end  92 , a second end  94 , and a middle portion  96 . As shown in  FIG. 2 , the first end  92  extends from the end of the core member  42  where the U-turn portions of the conductors  44  are located. The second end  94  is fixedly connected to the mouth  56  of the shaft coupler  48 . The outer surface of the middle portion  96  is fixedly connected to the core member  42 . Accordingly, the core bushing  90  is fixed relative to the core member  42  and the shaft coupler  48 . The inner surface of the middle portion  96  includes a plurality of axial splines  98  comprised of axial ribs with axial grooves between the ribs. As mentioned previously, these axial splines  98  are configured to mesh with the teeth  88  on the outer perimeters  84  of the second discs  82 , preventing the second discs  82  from rotating relative to the core bushing  90  and the connected core member  42 . However, the engagement of the teeth  88  with the splines  98  does allow the second discs to slide in the axial direction. Additionally, because the outer perimeters  74  of the first discs  72  are smooth and only engage the tips of the splines  98  on the core bushing  90 , the first discs  72  are allowed to rotate relative to the core bushing  90  and slide within the core bushing  90  in the axial direction. 
     With reference now to  FIGS. 2 and 8 , a biasing member in the form of at least one spring  80  is positioned about the armature shaft  50  and is configured to urge the first clutch discs  72  into engagement with the second clutch discs  82 . In the embodiment shown in  FIGS. 2 and 8 , the spring  80  is retained between stationary disc  36  and axially slideable disc  37 . The first and second discs  72  and  82  are retained between axially slideable disc  37  and stationary disc  38 . The spring  80  urges the axially slideable disc  37  toward the first and second discs  72 ,  82 , causing the first and second discs  72  to slide in the axial direction toward the stationary disc  38 , thus forcing the faces of the discs  72 ,  82  to press against one another. As a result of this close engagement between the faces of the discs  72 ,  82 , friction exists between the discs, and the discs tend to rotate together. The amount of friction between the discs is dependent on the material the discs  72 ,  82  are made of and any surface texturing that may provide some interlocking effect between the discs. The torque that the clutch arrangement  70  can transfer is a function of the friction coefficient of the discs  72 ,  82 , the clamping force of the spring  80  on the discs  72 ,  82 , the total number of clutch surfaces (i.e., the number of discs×2), and the area of contact of the clutch surfaces of the discs as determined by the difference in the outer radius of the first disc  72  minus the inner radius of the second disc. In other words, τ=ƒ(μ, F, N, D), where τ=the maximum torque that the clutch arrangement can transfer, μ=the coefficient of friction of the discs, P=the clamping force of the spring, N=the number of clutch surfaces, and D=the difference in the outer radius and inner radius of the clutch discs. 
     In operation, electro-magnetic force causes the core member  42  of the armature  40  to rotate about axis  15 . When the core member  42  rotates, the core bushing  90  also rotates. The engagement between the axial splines  98  on the core bushing  90  and the teeth  88  on the second discs  82  causes the second discs  82  to rotate along with the core member. The friction between the faces of the second discs  82  and the faces of the first discs  72  results in rotation of the first discs. The engagement between the teeth  78  of the first discs  72  and the axial splines  68  of the armature shaft  50  causes the armature shaft  50  to rotate along with the first discs. The output gear  64  then drives the planetary gear arrangement of the engine starter  10 , resulting in rotation of the pinion gear  16 . 
     When the engine starter experiences a high-torque engine event, such as those described above, the clutch arrangement  70  provides a torque limiter for the engine starter  10 . In particular, during a high torque-engine event, the torque experienced by the planetary gear arrangement  22  and other drive train components is limited to the maximum torque that the clutch arrangement  70  can provide. Accordingly, consider an event where the pinion gear  16  suddenly jams and stops rotating. In this situation, the maximum torque that the electric motor can deliver to the pinion and other drive train components is limited by the maximum torque that the clutch arrangement  70  can transfer. When the drive train including the armature shaft  50  and output gear  64  suddenly cease rotation, the torque experienced between the first discs  72  and the second discs  82  of the clutch arrangement will be such that the first discs  72  slip relative to the second discs  82 . Accordingly, the core member  42 , shaft coupler  48 , core bushing  90 , and second discs  82  will continue to rotate even though the first discs  72  and armature shaft  50  have completely stopped rotation. Moreover, the torque transferred through the drive train will be limited to a threshold torque of the clutch arrangement  70 . 
     As described above, the clutch arrangement  70  is configured to release the armature shaft  50  from the core member  42 , allowing the core member  42  to rotate relative to the armature shaft  50  when a torque on the armature shaft is greater than a threshold torque. Advantageously, this arrangement limits the damage to the drive train components of the engine starter  10  in the event of a high-torque engine event. Moreover, because the clutch arrangement  70  is positioned completely within the armature  40  of the electric motor  20 , no additional space within the engine starter  10  is required, and design of the engine starter may remain compact. Indeed, in the embodiment described herein, the entire clutch arrangement  70  is provided within the boundaries of the armature as defined on a first end by the U-turns of the conductor, and as defined on the second end by the commutator. More particularly, in the disclosed embodiment, the entire clutch arrangement is positioned within the core member  42  at the first end without extending to the conductor U-turns, and just past the core member  42  at the second end without extending to the commutator  46 . 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.