Abstract:
An eVCP camshaft phaser comprising a harmonic gear drive unit having a circular spline and a dynamic spline linked by a common flexspline, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of a crankshaft-connectable sprocket or an engine camshaft, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an eMotor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. A coaxial coil spring is connected to the sprocket and to the phaser hub and is positioned and tensioned to bias the phaser and camshaft to a stop position wherein the engine can run or be restarted after the eMotor is de-energized. Preferably, the spring comprises a spring cassette for easy assembly into the eVCP.

Description:
TECHNICAL FIELD 
     The present invention relates to camshaft phasers for varying the timing of combustion valves in internal combustion engines by varying the phase relationship between an engine&#39;s crankshaft and camshaft; more particularly, to oil-less camshaft phasers wherein a harmonic gear drive unit is controlled by an electric motor (eMotor) to vary the phase relationship, also referred to herein as an “electric variable cam phaser” (eVCP); and most particularly, to an eVCP including a bias spring to return the eVCP to a predetermined default phase position. In one aspect of the invention, the bias spring may be provided in a housing, in cassette form. 
     BACKGROUND OF THE INVENTION 
     Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in an internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine&#39;s crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine&#39;s camshaft. 
     U.S. Pat. No. 7,421,990 B2, herein incorporated by reference, discloses an eVCP comprising first and second harmonic gear drive units facing each other along a  25  common axis of the camshaft and the phaser and connected by a common flexible spline (flexspline). The first, or input, harmonic drive unit is driven by an engine sprocket, and the second, or output, harmonic drive unit is connected to an engine camshaft. 
     A first drawback of this arrangement is that the overall phaser package is undesirably bulky in an axial direction and thus consumptive of precious space in an engine&#39;s allotted envelope in a vehicle. 
     A second drawback is that two complete wave generator units are required, resulting in complexity of design and cost of fabrication. 
     A third drawback is that the phaser has no means to move the driven unit and attached camshaft to a phase position with respect to the crankshaft that would allow the engine to start and/or run in case of drive motor power malfunction. eVCP have been put into production by two Japanese car manufacturers; interestingly, these devices have been limited to very low phase shift authority despite the trend in hydraulic variable cam phasers (hVCP) to have greater shift authority. Unlike hVCP, the prior art eVCP has no default seeking or locking mechanism. Thus, phase authority in production eVCPs to date has been undesirably limited to a low phase angle to avoid a stall or no-restart condition if the rotational position of the eVCP is far from an engine-operable position when it experiences eMotor or controller malfunction. 
     What is needed in the art is an eVCP including means for the eVCP to mechanically return to a default engine-operable position in the event of eMotor malfunction. 
     It is a principal object of the present invention to return an eVCP to a predetermined ‘default’ position in the event of eMotor malfunction. 
     SUMMARY OF THE INVENTION 
     Briefly described, an eVCP camshaft phaser comprises a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine crankshaft sprocket or an engine camshaft, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an eMotor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. The eMotor may be equipped with an electromagnetic brake. At least one coaxial coil spring is connected to the sprocket and to the phaser hub and is positioned and tensioned to bias the phaser and camshaft to a default position wherein the engine can run or be restarted should control of the eMotor be lost resulting in the eMotor being unintentionally de-energized or held in an unintended energized position. In one aspect of the invention, the spring is contained in a spring cassette for easy assembly into the eVCP. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded isometric view of a eVCP in accordance with the present invention; 
         FIG. 2  is an elevational cross-sectional view of a the eVCP shown in  FIG. 1 ; 
         FIG. 3  is a perspective view in cross-section of the eVCP shown in  FIGS. 1 and 2 , with the eMotor, coupling, and bias spring omitted for clarity; 
         FIG. 4  is a perspective view of the eVCP hub showing detents for engaging the inner tang of the bias spring; 
         FIG. 5  is a schematic drawing showing a first gearing relationship in an eVCP, referred to herein as the baseline splines arrangement, wherein the dynamic spline drives the camshaft and the circular spline is driven by the sprocket; 
         FIG. 6  is a schematic drawing showing a second gearing relationship in an eVCP, referred to herein as the inverted splines arrangement, wherein the circular spline drives the camshaft and the dynamic spline is driven by the sprocket; 
         FIG. 7  is a first table showing advance and retard times for exemplary baseline and inverted eVCPs when the harmonic drive unit is provided with a mechanical biasing spring in accordance with the present invention and the eMotor is provided with an electromagnetic brake; and 
         FIG. 8  is a second table showing advance and retard times for exemplary baseline and inverted eVCPs when the harmonic drive unit is provided with a mechanical biasing spring in accordance with the present invention and the eMotor has no electromagnetic brake. 
     
    
    
     The exemplifications set out herein illustrate currently preferred embodiments of the invention. Such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 through 4 , an eVCP  10  in accordance with the present invention comprises a flat harmonic gear drive unit  12 ; a rotational actuator  14  that may be a hydraulic motor but is preferably a DC electric motor (eMotor), operationally connected to harmonic gear drive unit  12 ; an input sprocket  16  operationally connected to harmonic gear drive unit  12  and drivable by a crankshaft of engine  18 ; an output hub  20  attached to harmonic gear drive unit  12  and mountable to an end of an engine camshaft  22 ; and a bias spring  24  operationally disposed between output hub  20  and input sprocket  16 . Spring  24  may be a component of a spring cassette  26 . eMotor  14  may be an axial-flux DC motor. 
     Harmonic gear drive unit  12  comprises an outer first spline  28  which may be either a circular spline or a dynamic spline as described below; an outer second spline  30  which is the opposite (dynamic or circular) of first spline  28  and is coaxially positioned adjacent first spline  28 ; a flexspline  32  disposed radially inwards of both first and second splines  28 , 30  and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on both first and second splines  28 , 30 ; and a wave generator  34  disposed radially inwards of and engaging flexspline  32 . 
     Flexspline  32  is a non-rigid ring with external teeth on a slightly smaller pitch diameter than the circular spline. It is fitted over and elastically deflected by wave generator  34 . 
     The circular spline is a rigid ring with internal teeth engaging the teeth of flexspline  32  across the major axis of wave generator  34 . 
     The dynamic spline is a rigid ring having internal teeth of the same number as flexspline  32 . It rotates together with flexspline  32  and serves as the output member. Either the dynamic spline or the circular spline may be identified by a chamfered corner  33  at its outside diameter to distinguish one spline from the other. 
     As is disclosed in the prior art, wave generator  34  is an assembly of an elliptical steel disc supporting an elliptical bearing, the combination defining a wave generator plug. A flexible bearing retainer surrounds the elliptical bearing and engages flexspline  32 . Rotation of the wave generator plug causes a rotational wave to be generated in flexspline  32  (actually two waves 180° apart, corresponding to opposite ends of the major ellipse axis of the disc). 
     During assembly of a harmonic gear drive unit  12 , flexspline teeth engage both circular spline teeth and dynamic spline teeth along and near the major elliptical axis of the wave generator. The dynamic spline has the same number of teeth as the flexspline, so rotation of the wave generator causes no net rotation per revolution therebetween. However, the circular spline has slightly fewer gear teeth than does the dynamic spline, and therefore the circular spline rotates past the dynamic spline during rotation of the wave generator plug, defining a gear ratio therebetween (for example, a gear ratio of 50:1 would mean that 1 rotation of the circular spline past the dynamic spline corresponds to 50 rotations of the wave generator). Harmonic gear drive unit  12  is thus a high-ratio gear transmission; that is, the angular phase relationship between first spline  28  and second spline  30  changes by 2% for every revolution of wave generator  34 . 
     Of course, as will be obvious to those skilled in the art, the circular spline rather may have slightly more teeth than the dynamic spline has, in which case the rotational relationships described below are reversed. 
     Still referring to  FIG. 1 and 2 , sprocket  16  is supported by a generally cup-shaped sprocket housing  36  that is fastened by bolts  38  to first spline  28 . A coupling adaptor  40  is mounted to wave generator  34  and extends through sprocket housing  36 , being supported by bearing  42  mounted in sprocket housing  36 . A coupling  44  mounted to the motor shaft of eMotor  14  and pinned thereto by pin  46  engages coupling adaptor  40 , permitting wave generator  34  to be rotationally driven by eMotor  14 , as may be desired to alter the phase relationship between first spline  28  and second spline  30 . 
     Hub  20  is fastened to second spline  30  by bolts  48  and may be secured to camshaft  22  by a central through-bolt  50  extending through an axial bore  51  in hub  20 , and capturing a stepped thrust washer  52  and a filter  54  recessed in hub  20 . In an eVCP, it is necessary to limit radial run-out between the input hub and output hub. In the prior art, this has been done by providing multiple roller bearings to maintain concentricity between the input and output hubs. Referring to  FIG. 2 , in one aspect of the invention, radial run-out is limited by a singular journal bearing interface  35  between housing  36  (input hub) and output hub  20 , thereby reducing the overall axial length of eVCP  10  and its cost to manufacture over a prior art eVCP having multiple roller bearings. 
     Spring cassette  26  includes a bottom plate  56  and a top plate  58  disposed on opposite sides of spring  24 . Shouldered spring spacers  60  extending between bottom and top plates  58  create an operating space for spring  24  and also provide an anchor for outer tang  62  on spring  24 . Spring spacers  60  pass through top plate  58  and are secured by nuts  64 . First and second retainer plates  66  may be used to secure cassette  26  to housing  36 . For example, first and second retainer plates  66  may be positioned on top plate  58  by studs  68  and secured to bottom plate  56  by bolts  70 . Retainer plates  66  may extend radially beyond the edges of top plate  58  to engage an annular groove or slots formed in sprocket housing  36 , thereby axially positioning and locking cassette  26  in place on hub  20  such that the inner tang  72  of spring  24  engages one of two alternate detents  74  formed in hub  20 . Retainer plates  66  exemplarily demonstrate only one arrangement for attaching cassette  26  to eVCP  10 ; obviously, all other alternative attaching arrangements are fully comprehended by the invention. 
     In the event of an eMotor malfunction, spring  24  is biased to back-drive harmonic gear drive unit  12  without help from eMotor  14  to a rotational position of second spline  30  wherein engine  18  will start or run, which position may be at one of the extreme ends of the range of authority or, in one aspect of the invention, intermediate of the phaser&#39;s extreme ends of its rotational range of authority. For example, the rotational range of travel in which spring  24  biases harmonic gear drive unit  12  may be limited to something short of the end stop position of the phaser&#39;s range of authority. Such an arrangement would be useful for engines requiring an intermediate park position for idle or restart. 
     Referring now to  FIGS. 5 and 6 , an advantage of a flat harmonic gear drive unit such as unit  12 , as opposed to a cup-type unit such as is disclosed in the incorporated reference, is that unit  12  may be installed in either of two orientations within sprocket housing  36 . In the baseline splines arrangement ( FIG. 5 ), first or input spline  28  is the circular spline and is connected to sprocket housing  36 , and second spline  30  is the dynamic spline and is connected to hub  20 . In the inverted splines arrangement ( FIG. 6 ), first spline  28  is the dynamic spline and is connected to sprocket housing  36 , and second spline  30  is the circular spline and is connected to hub  20 . 
     Fail-safe performance of the harmonic gear drive unit in eVCP  10  is not identical in the two orientations. Thus, a desired orientation may be selected during installation to minimize the response time for eVCP  10  to return to a preferred default position when eMotor  14  is de-energized when the engine is shut down or as a fail-safe response when eMotor experiences a failure (unintentionally energized or de-energized). In both orientations, the output gear, which is second spline  30  rotates with respect to first spline  28 . When the circular spline is first spline  28  and the dynamic spline is the second spline  30 , as shown in  FIG. 5  (baseline arrangement), the dynamic spline rotates in a direction opposite from the input direction of the wave generator; however, when the dynamic spline is first spline  28  and the circular spline is the second spline  30 , as shown in  FIGS. 2 and 6  (inverted arrangement), the circular spline is the output gear and rotates in the same direction as the input direction of the wave generator. 
     Referring to  FIG. 7 , it is seen that if an exemplary eVCP is equipped with both a bias spring  24  and also a fail-safe electromagnetic brake (not shown but known in the art) on eMotor  14 , the baseline spline arrangement shown in  FIG. 5  is preferred because the failsafe advance time upon loss of power is minimized. 
     Referring to  FIG. 8 , it is seen that if an exemplary eVCP is equipped with a bias spring  24  but without a fail-safe electromagnetic brake on eMotor  14 , the inverted spline arrangement shown in  FIG. 6  is preferred because the fail-safe advance time upon loss of power is minimized. 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.