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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. Ser. No. 12/818,630 filed Jun. 18, 2010 now U.S. Pat. No. 7,975,821, which is a continuation application of U.S. patent application Ser. No. 11/814,934, filed Jul. 27, 2007 now U.S. Pat. No. 7,766,774, which is a U.S. National Stage Entry of International Application No. PCT/CA2006/000129 filed Feb. 3, 2006, which claims the benefits of U.S. Provisional Application Ser. No. 60/649,520, filed Feb. 3, 2005. The entire disclosure of each of the above applications are incorporated by reference as if fully set forth in detail herein. 
    
    
     FIELD 
     The present disclosure relates to a torque limited decoupler. 
     BACKGROUND 
     An automotive vehicle engine transfers a portion of the engine output to a plurality of belt driven accessories utilizing an endless serpentine belt. Typically, each component includes an input drive shaft and a pulley coupled to a distal end of the drive shaft for driving engagement with the belt. An example of such a belt driven accessory is an alternator. 
     A decoupler is operatively coupled between the pulley and the alternator to allow the alternator drive shaft to “overrun” or rotate at a faster speed than the pulley and to allow the speed of the pulley to oscillate with respect to the alternator drive shaft due to oscillations in the engine speed. Examples of decouplers are disclosed in U.S. Pat. No. 6,083,130, issued to Mevissen et al. on Jul. 4, 2000, U.S. Pat. No. 5,139,463, issued to Bytzek et al. on Aug. 18, 1992 and International Patent Application No. WO 2004/011818. 
     In PCT application no. WO 2004/011818, the decoupler reduces torsional fluctuations in the endless drive system. However, in certain applications in which the engine has an aggressive start profile or during conditions of rapid acceleration during a wide open throttle shift, the torques transmitted will over-stress the torsion spring reducing long term durability of the decoupler. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     According to one aspect of the present disclosure, a decoupler assembly is provided for transferring torque between a shaft and a drive belt. The decoupler assembly includes a hub configured to be fixedly secured to the shaft. A carrier is rotatably mounted on the hub. A torsion spring extends between the hub and the carrier for transferring torque therebetween. A pulley is rotatably coupled to the hub. The pulley has an inner surface formed therein. A clutch spring is secured to the carrier and has a plurality of helical coils frictionally engaging with the inner surface of the pulley to selectively couple the hub and pulley. The torsion spring and the clutch spring are mounted co-axially and wound in opposite senses enabling the clutch spring to expand into gripping engagement with the inner surface during acceleration of the pulley relative to the hub and to contract out of gripping engagement with the inner surface during deceleration of the pulley relative to the hub, while enabling the torsion spring to absorb minor torsional vibrations without decoupling the pulley from the hub. A torque limiter, in the form of a sleeve, is fitted about the torsion spring and is sized to limit expansion of the torsion spring enabling the torsion spring to fully couple the hub with the pulley at or above a predetermined torque. 
     According to another aspect of the present disclosure, the torque limiter is in the form of a wire coil, which is fitted about the torsion spring and is sized to limit expansion of the torsion spring enabling the torsion spring to fully couple the hub with the pulley at or above a predetermined torque. 
     According to a further aspect of the present disclosure, a decoupler assembly is provided with a hub, a drive member, a torsional damping system and a torque limiter. The hub that is configured to be coupled to a shaft for rotation therewith. The drive member is disposed concentrically about the hub and is configured to drivingly engage an endless power transmitting member to permit rotary power to be transferred between the drive member and the endless power transmitting member. The torsional damping system is disposed between the drive member and the hub and has a torsion spring that facilitates transfer of rotary power into the hub. The torque limiter is wrapped about the torsion spring and radially expands with the torsion spring as a magnitude of the rotary power transmitted through the torsion spring increases. Contact between the torque limiter and another structure in the decoupler assembly halts radial expansion of the torsion spring. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purpose only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a front view of an engine of an automotive vehicle incorporating a decoupler assembly according to one aspect of the invention; 
         FIG. 2  is an enlarged fragmentary sectional view of the decoupler assembly; 
         FIG. 3  is an exploded perspective view of a clutch spring in the decoupler assembly of  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of the clutch spring and carrier assembly in relation to the torque limiter and torsion spring of the decoupler assembly of  FIG. 2 ; 
         FIG. 5  a perspective view of the clutch spring of the decoupler assembly of  FIG. 2 ; 
         FIG. 6  is a perspective view of the carrier of the decoupler assembly of  FIG. 2 ; 
         FIG. 7  is a perspective view of the clutch spring and carrier assembly of  FIG. 2 ; 
         FIG. 8  is a perspective view of a second embodiment of the torque limiter of the decoupler assembly of  FIG. 2 ; 
         FIG. 9   a  is a perspective view of a third embodiment of the torque limiter of the decoupler assembly of  FIG. 2 ; 
         FIG. 9   b  is a perspective view of an alternate third embodiment of the torque limiter of the decoupler assembly of  FIG. 2 ; 
         FIG. 10  is an exploded perspective view of the decoupler assembly of a fourth embodiment of the decoupler assembly of the present invention; 
         FIG. 11  is an exploded perspective view of the clutch spring and carrier assembly in relation to a torque limiter and torsion spring of the decoupler assembly of  FIG. 10 ; 
         FIG. 12  a perspective view of the clutch spring of the decoupler assembly of  FIG. 10 ; 
         FIG. 13  is a perspective view of the carrier of the decoupler assembly of  FIG. 10 ; and 
         FIG. 14  is a perspective view of the clutch spring and carrier assembly of  FIG. 10 . 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     An engine for an automotive vehicle is generally indicated at  10  in  FIG. 1 . The engine  10  includes a crankshaft  12  driving an endless serpentine belt  14 , as commonly known by those having ordinary skill in the art. The engine  10  also includes a belt driven accessory  16  driven by the belt  14 . Described in greater detail below, a decoupler assembly  20  is operatively assembled between the belt  14  and the belt driven accessory  16  for automatically decoupling the belt driven accessory  16  from the belt  14  when the belt  14  decelerates relative to the belt driven accessory  16  and allowing the speed of the belt  14  to oscillate relative to the belt driven accessory  16 . Additionally, a detailed description of the structure and function of a decoupler assembly can be found in applicant&#39;s U.S. Pat. No. 6,083,130, which issued on Jul. 4, 2000 and PCT Application No. WO 2004/011818, the contents of which are incorporated herein by reference. 
     Referring to  FIGS. 2 and 3 , the decoupler assembly  20  generally includes a hub  22 , a pulley  50 , a clutch assembly  70 , a torsion spring  90  and a torque limiter  110 . In the first embodiment, the torque limiter  110  is preferably a sleeve. 
     Hub  22  has a generally cylindrical body  28  having an axially extending bore  24  and a flange  26  at one end thereof. Flange  26  has a generally helical first slot  46  on an inner face thereof. Since the slot  46  is helical, the slot  46  will have a step. The bore  24  is configured for fixedly securing the hub  22  to a drive shaft extending from the belt driven accessory  16 . 
     A pulley  50  is rotatably journaled to the hub  22 . A ball bearing assembly  57  is coupled between the pulley  50  and the hub  22  at a distal end while a bushing journal  102  mounts the pulley  50  on the circumferential face of flange  26 . The bearing assembly  57  is conventional comprising an inner race, an outer race and a plurality of ball bearings rollingly engaged therebetween. The pulley  50  typically includes a plurality of V-shaped grooves  66  formed on the outer periphery for engaging and guiding the belt  14 . Other belt or chain profiles may be utilized to facilitate other drive configurations, well known in the art. 
     A one-way clutch assembly  70  is operatively coupled between the hub  22  and the pulley  50 . The clutch assembly  70  includes a clutch spring  71  and a carrier  75 . The clutch spring  71  includes a plurality of helical coils  72 . Preferably, the clutch spring  71  is formed from an uncoated, spring steel material and has a non-circular cross-section to improve frictional contact. Most preferably, the cross-section of clutch spring  71  is rectangular or square. The clutch spring  71  is press fitted into frictional engagement with the inner surface  56  of the pulley  50 . Preferably, a lubricant similar or compatible with grease used in the ball bearing assembly  57  is applied to minimize wear between the clutch spring  71  and the inner surface  56  of the pulley  50 . 
     The carrier  75  is rotatably mounted on the hub  22 . The carrier  75  is generally ring shaped and has an inner face  78 , a bore  80  and an outer circumferential surface  82 . A slot  84  is formed on the inner face  78  and is configured to retain an end of the clutch spring  71 . A generally helical second slot  86  is also formed on the inner face  78  and inside of slot  84 , defining a second locating surface  88  and a step. 
     An annular thrust washer  39  is seated against the end of the carrier  75  and abuts against the inner bearing race of bearing assembly  57 . The outer periphery of the thrust washer  39  is circular with a step  41  to complementarily fit with a tab. Thrust washer  39  has one or more radial or circumferential serrations  43  to engage hub  22  and mechanically lock the thrust washer  39  to the hub  22  to prevent relative motion therebetween. 
     A helical torsion spring  90  is axially compressed between the hub  22  and the carrier  75 . The torsion spring  90  and the clutch spring  71  are co-axial and typically coiled in opposite directions. In certain applications, the torsion spring  90  and clutch spring  71  can be wound in the same sense to produce a desired decoupling action. One end of the torsion spring  90  is retained in the first slot  46  of the hub  22  and the other end is retained in the slot  86  of the carrier  75 . Axial forces due to the compression of the torsion spring  90  retain the carrier  75  in abutting engagement with the thrust washer  39 . 
     Typically, the shaft of the hub  22  has an area of reduced diameter  23  to provide clearance between the torsion spring  90  and the shaft  28  of hub  22  to prevent uncontrolled contact and friction wear at the interface between shaft  28  and torsion spring  90 . Thus, the torsion spring  90  allows relative movement between the carrier  75  and the hub  22  to accommodate minor variations in the speed of the pulley  50  due to oscillations in the operating speed of the engine. The oscillations are not sufficient to activate the clutch assembly  70 . 
     A torque limiter  110  is wrapped about the torsion spring  90  in a surrounding relation. Preferably, torque limiter  110  has a split or opening  112  and a circumferentially extending shoulder step  114 . Shoulder step  114  configures the torque limiter  110  to complementarily fit with bushing  102  mounted on the flange  26  of hub  22 . In a first preferred embodiment, torque limiter  110  is an organic resinous material, preferably a Nylon™ material, with or without reinforcement material such as glass fibres, etc. Torque limiter  110  has a thickness selected to take up the play between the torsion spring  90 , the clutch spring  71  and the inside diameter of the pulley  50 . As torque increases, the torsional spring  90  expands outwardly until physically constrained by the torque limiter  110  against the clutch spring  71  and the inside diameter of bore  56 . When the radial clearance between the torsion spring  90 , torque limiter  110 , the clutch spring  71  and the inside bore  56  of the pulley  50  is closed, the spring  90  is prevented from further expanding, locking the decoupler  10 , coupling the hub  22  with the pulley  50 . In other words, the torque limiter  110  limits the amount of outward expansion of the torsion spring  90 , preventing overloading of the torsion spring  90 . The amount of radial expansion of the torsion spring  90  can be calculated and the torque limiter  110  can be designed to ensure that the torque transferred through the torsion spring  90  is maintained below a predetermined torque value. 
     A second embodiment of the sleeve is illustrated in  FIG. 8 . Torque limiter  110 ′ is a closed metal ring. The metal ring would only expand to a relatively small degree, directly limiting outward expansion of the torsion spring  90 . 
     A third embodiment of the sleeve is illustrated in  FIG. 9   a . Torque limiter  110 ″ has a plurality of axially elongate openings  116  spaced circumferentially spaced about the torque limiter  110 ″. The openings  116  enable the grease lubricant to travel outwardly to the clutch spring  71 . 
     An alternative third embodiment of the sleeve is illustrated in  FIG. 9   b . The torque limiter  110 * has a series of circumferentially spaced openings  116 * and  117 . Preferably, openings  116 * are elongate and openings  117  are circular and spaced in a regular pattern, resembling dimples on a golf ball. Additionally, torque limiter  110 * has an integrally extending radial flange  119  that acts a thrust bearing. 
     A cap  100  is attached to the end of pulley  50  for preventing contaminants from entering the decoupler assembly  20  and for retaining the lubricant within the decoupler assembly  20 . 
     In operation, the engine  10  is started and the pulley  50  is accelerated and rotated in a driven direction by the belt  14  driven by the engine  10 . Acceleration and rotation of the pulley  50  in the driven direction relative to the hub  22  creates friction between the inner surface  56  of the pulley  50  and preferably all of the coils  72  of the clutch spring  71 . It should be appreciated that the clutch spring  71  will function even where at the onset at least one of the coils  72  of the clutch spring  71  is frictionally engaged with the inner surface  56  of the pulley  50 . The clutch spring  71  is helically coiled such that the friction between the inner surface  56  of the pulley  50  and at least one of the coils  72  would cause the clutch spring  71  to expand radially outwardly toward and grip the inner surface  56  of the pulley  50 . Continued rotation of the pulley  50  in the driven direction relative to the hub  22  would cause a generally exponential increase in the outwardly radial force applied by the coils  72  against the inner surface  56  until all of the coils  72  of the clutch spring  71  become fully brakingly engaged with the pulley  50 . When the clutch spring  71  is fully engaged with the inner surface  56 , the rotation of the pulley  50  is fully directed toward rotation of the drive shaft  15  ( FIG. 1 ) of the belt driven accessory  16 . Additionally, centrifugal forces help to retain the clutch spring  71  in braking engagement with the inner surface  56  of the pulley  50 . 
     The rotational movement of the carrier  75  in the driven direction is transferred to the hub  22  by the torsional spring  90  such that the carrier  75 , thrust washer  39 , hub  22 , and the drive shaft  15  ( FIG. 1 ) from the belt driven accessory  16  rotate together with the pulley  50 . Additionally, the torsional spring  90  resiliently allows relative movement between the carrier  75  and the hub  22  to accommodate oscillations in the speed of the pulley  50  due to corresponding oscillations in the operating speed of the engine  10 . 
     When the pulley  50  decelerates, the hub  22  driven by the inertia associated with the rotating drive shaft  15  ( FIG. 1 ) and the rotating mass within the belt driven accessory  16  will initially “overrun” or continue to rotate in the driven direction at a higher speed than the pulley  50 . More specifically, the higher rotational speed of the hub  22  relative to the pulley  50  causes the clutch spring  71  to contract radially relative to the inner surface  56  of the pulley  50 . The braking engagement between the clutch spring  71  and the pulley  50  is relieved, thereby allowing overrunning of the hub  22  and drive shaft  15  ( FIG. 1 ) from the belt driven accessory  16  relative to the pulley  50 . The coils  72  may remain frictionally engaged with the inner surface  56  while the pulley  50  decelerates relative to the clutch assembly  70  and the hub  22 . The coils  72  of the clutch spring  71  begin to brakingly reengage the inner surface  56  as the pulley  50  accelerates beyond the speed of the hub  22 . 
     In conditions of high loading, such as a fast engine start profile and/or rapid acceleration during a wide open throttle shift, the coils of the torsion spring  90  will be urged to expand outwardly, due to relative rotation between the hub  22  and the pulley  50 . The torsion spring  90  will expand, frictionally engaging the torque limiter  110  which will then engage the clutch spring  71 . Full frictionally engagement is selected to occur at a predetermined torque value by selecting the thickness of the torque limiter  110 . Once fully engaged, the hub  22  will be locked with the pulley  50  and torques above a predetermined torque value will be transmitted directly therebetween. Thus, the higher torques do not overstress the torsion spring  90  and ultimately improving durability of the decoupler assembly  10 . 
     Referring to  FIGS. 10 to 14 , a fourth embodiment of the torque limiter  110  is illustrated. Elements common with the embodiment of  FIGS. 2 and 3  retain the same reference number. 
     In this embodiment, the torque limiter  110 ′″ is in the form of a wire coil spring. Torque limiter  110 ′″ is positioned about the torsion spring  90 . Preferably, torque limiter  110 ″ is formed of a small gauge wire, compared to torque spring  90 , with a square or rectangular cross-section. The gauge and dimensions of torque limiter  110 ′″ are selected such that any play which would otherwise be present between torsion spring  90 , clutch spring  71  and the inside surface  56  of pulley  50  is substantially removed, while still allowing relative motion between torsion spring  90  and clutch spring  71 . Further, the coils of torque limiter  110 ′″ allow grease, or any other lubricant, to travel outwardly to the clutch spring  71 . 
     It is presently preferred that the coils of torque limiter  110 ′″ be wound in the same sense of the coils of clutch spring  71 , although this is not essential to proper operation of decoupler  20 . 
     As torque to pulley  50  increases, torsional spring  90  expands outwardly until physically constrained by torque limiter  110 ′″. When the radial clearance between torsion spring  90 , torque limiter  110 ′″, clutch spring  71  and the inside surface  56  of pulley  50  is closed, spring  90  is prevented from further expanding, locking decoupler  20 , coupling the hub  22  with the pulley  50 . In other words, torque limiter  110 ″ limits the amount of outward expansion of the torsion spring  90 , preventing overloading of torsion spring  90 . 
     The amount of radial expansion of torsion spring  90  can be pre-determined and torque limiter  110 ′″ can be designed to ensure that the torque transferred through torsion spring  90  is maintained below a preselected torque value. 
     Referring to  FIGS. 12 to 14 , a second variant of the clutch assembly  70  is illustrated. The clutch assembly  70  includes clutch spring  71 ′, comprising a helical coil, and a carrier  75 ′. Preferably, clutch spring  71 ′ is formed from an uncoated, spring-steel material and the material forming the helical windings  72  has a non-circular cross-section to improve frictional contact. Most preferably, the cross-section of the helical winding material is rectangular or square. Clutch spring  71 ′ is press-fitted into frictional engagement with the inner surface  56  of the pulley  50 . Preferably a lubricant, similar or compatible with the grease used in the ball bearing assembly  57 , is applied to minimize wear between the clutch spring  71 ′ and inner surface  56  of the pulley  50 . 
     Carrier  75 ′ is rotatably mounted on the hub  22  and carrier  75 ′ is generally ring shaped, with an inner face  78 , a bore  80  and an outer circumferential surface  82 . A slot  84 ′ is formed on inner face  78  and is configured to retain an end of the clutch spring  71 ′. A generally helical second slot  86  is also formed on the inner face  78  and inside of slot  84 , defining a second locating surface  88  and a step. 
     In this variant, the end of clutch spring  71 ′ is bent at  73  and  77 . Slot  84 ′ is complementarily configured to receive the end of the clutch spring  71 ′ and frictionally engage with the bends  73  and  77 . 
     The bore  80  of carrier  75 ′ has a keyway  81  and a series of axially extending dimples. 
     The decoupler illustrated in  FIGS. 10 to 14  operates in the same fashion as described with respect to the decoupler illustrated in  FIGS. 1 to 9 . 
     In conditions of high loading, such as a fast engine start profile and/or rapid acceleration during a wide open throttle shift, the coils of the torsion spring  90  will be urged to expand outwardly, due to relative rotation between hub  22  and pulley  50 . The torsion spring  90  will expand, expanding torque limiter  110 ′″ in turn, which will then frictionally engage the clutch spring  71 . Full frictional engagement is selected to occur at a predetermined toque value by selecting the thickness of the windings of torque limiter  110 . 
     Preferably, decoupler  20  further includes an adapter  104  which is press fit into the inner race of bearing  57  and which allows decoupler  20  to be fit to belt driven accessories with drive shafts of different sizes and/or to position decoupler  20  on the driven shaft to ensure correct alignment of grooves  66  with the serpentine belt. However, adapter  104  is not necessary and decoupler  20  can be installed directly onto the drive shaft of a belt driven accessory if the diameter of that drive shaft will properly engage the inner race surface of bearing  57  and/or if grooves  66  will be properly aligned with the serpentine belt. 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. Many modification and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.

Summary:
A decoupler assembly with a torsional damping system and a torque limiter. The torsional damping system has a torsion spring that facilitates transfer of rotary power into a hub. The torque limiter is wrapped about the torsion spring and radially expands with the torsion spring as a magnitude of the rotary power transmitted through the torsion spring increases. Contact between the torque limiter and another structure in the decoupler assembly halts radial expansion of the torsion spring.