Abstract:
A CVVL system including a self-locking helical gear pair with, optionally, a transmission to increase output torque of the actuator. The self-locking helical gear pair provides high forward efficiency and a fully mechanically self-locking feature. The actuator therefore requires a smaller motor to perform the same actuation as a prior art worm-gear system. The CVVL system comprises two helical gears having a radial pressure angle {acute over (α)} between 45° and 75° and a helix angle β between 60° and 80°. An asymmetric tooth profile is presently preferred, reducing contact stress and permitting higher torque density. Preferably, the helical gears are discontinuous and comprise laminated spur gear slices and hence are less costly to produce than continuous helical gears. Configurations are possible within the scope of the present invention include a single stage gear system; a multiple stage gear system; a planetary gear system; and an internal or external gear system.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to variable valve lift systems for combustion valvetrains of internal combustion engines; more particularly, to such systems wherein the valve may be lifted to any height in a continuous range of heights, defined herein as “continuously variable valve lift” (CVVL); and most particularly, to such a system having a high-efficiency actuator employing a self-locking helical gear arrangement. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well known in the internal combustion engine arts to provide an engine with means for varying the lift of one or more combustion valves to improve engine efficiency and/or decrease emissions under certain engine operating conditions. Some known systems vary valve lift between two steps, for example, fully open (activated) and non-opening (deactivated); such systems can be thought of as switchable systems. Other systems, especially in the compression-ignited engine arts, benefit from mechanisms which are capable of lifting a valve to any desired height within a continuous range of heights; such systems may be thought of as continuously variable. 
         [0003]    An important consideration in variable valve lift systems is the mechanical rigidity of the valvetrain during a partial lift event. When the variable lift is actuated directly by an electric solenoid or a hydraulic system that may contain air bubbles, torque fluctuations imposed on a camshaft by the sequential and overlapping opening and closing of valves can cause variation in the desired lift and hence in the intended gas flow profile. 
         [0004]    To overcome these shortcomings, it is known in the art of CVVL systems to provide a mechanically self-locking actuation system employing a worm gear actuator. See, for example, U.S. Pat. No. 7,174,887 to Shuichi Ezaki. An inherent drawback of worm gear actuators is very low mechanical efficiency; that is, because of the high sliding velocity due to a small worm lead angle, a high percentage of the actuator torque, typically more than 50%, is consumed in friction with the pinion gear. Further, the mechanical self-locking feature becomes practical only at relatively high gear ratios, on the order of 80:1 and higher. For gear ratios lower than about 80:1, the backdrive efficiency remains low but the system is not completely self-locking. Because of the low mechanical efficiency, the actuator requires a relatively large and expensive motor to drive the worm gear. 
         [0005]    What is needed in the art is a self-locking continuously variable drive system for variable valve lift that provides higher forward efficiency, a smaller and less expensive motor, and is fully mechanically self-locking. 
         [0006]    It is a principal object of the present invention to improve the self-locking capability of a drive mechanism for a CVVL system. 
         [0007]    It is a further object of the present invention to increase the efficiency of the self-locking actuator of a CVVL system. 
         [0008]    It is a further object of the invention to reduce the size and cost of a CVVL system. 
       SUMMARY OF THE INVENTION 
       [0009]    Briefly described, a CVVL system in accordance with the present invention includes a self-locking helical gear pair with, optionally, an additional gear system to increase output torque of the actuator. The self-locking helical gear pair provides higher forward efficiency, being at least twice that of a conventional worm gear actuator, and a fully mechanically self-locking feature. The actuator therefore requires a smaller motor to perform the same actuation as a prior art worm-gear system, which reduces the cost and volume of the CVVL system. Thus, a CVVL system in accordance with the present invention is significantly less bulky and costly than a comparable prior art system and is fully mechanically self-locking over the entire range of action. 
         [0010]    The CVVL system includes a self-locking gear pair comprising two helical gears that have a high radial pressure angle, between about 45° and about 75°, and a high helix angle, between about 60° and about 80°. These gears require high addendum modification (positive and negative profile shift) such that the pitchpoint is beyond the active portion of the contact line, which creates a mechanically self-locking condition against backdrive. Tooth profiles may be symmetric, although an asymmetric profile is presently preferred to reduce contact stress and permit higher torque density. Preferably, the helical gears are discontinuous and comprise a plurality of spur gear slices which can produce smoother power transmission and lower transverse contact ratio. Such discontinuous helical gears are significantly less costly to manufacture. 
         [0011]    Various configurations are possible within the scope of the present invention, including but not limited to a single stage gear system; a multiple stage gear system; a planetary gear system; and an internal or external gear system. 
         [0012]    The principle of self-locking helical gears is disclosed in a Russian engineering paper from 1969 (“Self-locking in gear mechanism”, by T. G. Ishakov; T R. Kazanskova Aviation Institute of A. H. Typoleva, 1969 Ed. 105, pp 3-15 (in Russian)). However, this principle has not been applied previously to an automotive CVVL system such as is the subject of the present invention. Further, the use of asymmetric gear profiles in self-locking helical gears as disclosed herein is also novel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  is an elevational cross-sectional view of a prior art worm-gear driven CVVL system, substantially as disclosed in U.S. Pat. No. 7,174,887 to Shuichi Ezaki, and also showing substitution of a helical gear pair in accordance with the present invention; 
           [0015]      FIG. 2  is an isometric view of a driving helical gear and a driven helical gear in accordance with the present invention; 
           [0016]      FIG. 3  is a schematic view of a helical gear showing derivation of helical angle β; 
           [0017]      FIG. 4  is a schematic cross-sectional view of a driving helical gear and a driven helical gear showing the trigonometric derivation of the self-locking mechanism in accordance with the present invention; 
           [0018]      FIG. 5  is an isometric view in partial cutaway showing a discontinuous helical gear formed of a plurality of slices of spur gears; 
           [0019]      FIG. 6  is a schematic plan view showing assembly of a discontinuous 5-tooth helical gear from three 5-tooth spur gear slices; and 
           [0020]      FIG. 7  is an elevational cross-sectional view showing lamination of a plurality of spur gear slices to form a discontinuous helical gear in accordance with the present invention. 
       
    
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Self-locking helical gears are known in the prior art from relatively few publications that demonstrate the possibility of designing a helical gear pair to be self-locking and provide general guidelines on how to obtain the self-locking feature. Despite these early publications, however, self-locking helical gears have not been widely reduced to practice, especially in automotive applications, such as the present invention. Further, the CVVL system disclosed herein includes improvements that are novel in the art, including: laminated helical gears to reduce fabrication costs; a preferred range of pressure angle and helical angles appropriate for steel gears. 
         [0023]    Referring to  FIG. 1 , in a prior art CVVL system  10 , substantially as disclosed in U.S. Pat. No. 7,174,887 to Shuichi Ezaki, which is herein incorporated by reference, a motor actuator  12  causes rotation of a worm  14 , causing a worm gear segment  16 , also known as a worm wheel, to rotate a control shaft  18  on which the worm wheel is mounted for varying the lift of an engine combustion valve  20  in a manner described in the incorporated reference in response to rotation of an engine cam (not shown) in an internal combustion engine  22 . When control shaft  18  rotates clockwise in  FIG. 1 , the variable valve mechanism  24  decreases the operating angle and lift amount of valve  20 . 
         [0024]    Prior art CVVL system  10  typifies any CVVL system wherein changing the rotational position of a control shaft causes variation in the operating angle and lift amount of an associated engine valve. The shortcomings of such a prior art CVVL system including a worm gear drive are described above. The present invention overcomes these shortcomings. 
         [0025]    Still referring to  FIG. 1 , in accordance with the present invention, driven worm gear segment  16  is replaced by a driven helical gear segment  116 , and worm  14  and actuator  12  are replaced by a helical driving gear  114  driven by a rotary actuator  112  and meshed with helical driven gear segment  116 . Note that in the prior art, worm  14  rotates about an axis  26  contained in a plane orthogonal to a plane containing the axis  28  of worm gear segment  16  and control shaft  18 ; whereas in the present invention, the respective rotation axes  126 , 128  of helical gears  114 , 116  are contained in a common plane having parallel axes of rotation. If gears  114 , 116  were provided as ordinary spur gears having transverse teeth, the improved gearing arrangement shown would not be self-locking; that is, reverse torque of a driven spur gear  116  caused by alternating camshaft torque would cause a reverse torque in the driving spur gear  114 , allowing variation to occur in the rotary position of control shaft  18  and hence variation in the lift of valve  20 . However, as described in detail below, if the driving and driven gears are provided as specially-formed helical gears, reverse torque of a driven helical gear  116  caused by alternating camshaft torque causes a forward torque in the driving helical gear  114 , thereby self-locking the gear pair and preventing variation in the rotary position of driven helical gear  116  and control shaft  18 . Such an arrangement is defined herein as being “self-locking” of the driving and driven gears. 
         [0026]    Referring to  FIGS. 2 through 4 , an exemplary pair  100  of helical gears is shown for purposes of discussion that are representative of gears  114 , 116  shown in  FIG. 1 , gear  114  being the driving gear and gear  116  being the driven gear. Gear  114  is mounted to a shaft  130  for being driven by rotary actuator  112  ( FIG. 1 ). Gear  116  is mounted to shaft  18 . It will be seen that a conventional geared transmission (not shown) may be interposed conventionally as may be desired between gear  114  and actuator  112  to increase the torque available to driving gear  114  or between gear  116  and control shaft  18  to increase the torque to the control shaft. Gears  114 , 116  have opposite direction helical teeth  132 , 134 , respectively, which are defined by the helical angle β formed between the tangent  136  to any tooth  132 , 134  and a plane  138  containing the axis  126 , 128  of the gear, as shown in  FIG. 3 . Gears  114 , 116  in accordance with the present invention are characterized by having helix angles β between 60° and 80° and thus exhibit a relatively low number of teeth in a cross-sectional view taken orthogonal to the gear axes  126 , 128 , as shown in  FIGS. 2 and 4 . 
         [0027]    Helical gearing within the scope of the present invention may be either external or internal. “External” refers to gearing wherein the teeth are on the outside of both gears, and the centers of rotation are on opposite sides of the mesh point. “Internal” refers to gearing wherein the teeth are on the outside of one gear and on the inside of the other gear, as for example in a planetary gear system, and the centers of rotation are on the same side of the mesh point. 
         [0028]    Referring now to  FIG. 4 , the principles behind self-locking helical gears in accordance with the invention will now be discussed. 
         [0029]    Helical drive gear  114  having rotational center O 1  includes helical teeth  132 . Helical driven gear  116  having rotational center O 2  including helical teeth  134 . The teeth of both gears are modified involute teeth. Gears  114 , 116  are meshed along a center line  140  between O 1  and O 2 . Drive gear  114  rotates clockwise, exerting torque T 1 . Thus, the leading flanks  142  of teeth  132  are the driving flanks, and trailing flanks  144  are the coast flanks. Conversely, for driven gear  116 , the trailing flanks  146  of teeth  134  are the driven flanks, and leading flanks  148  are the coast flanks. The driving and driven teeth meet at a drive point  150 , and the coast flanks meet at a corresponding driven point  152 . The pressure exerted by driving gear  114  at drive point  150  is orthogonal to the contact tangents, defining a contact force direction  154  within driving gear  114  that in turn defines a driving torque arm  155 , at a radial pressure angle {acute over (α)}, that is the radius of a torque circle  156  for torque T 1 . In the absence of friction, contact force direction  154  creates a response force direction  158  in driven gear  116  again normal to the contact tangents. However, sliding friction between the helices turns the response force direction through a friction angle γ such that the effective response force direction is vector  160  from the driven torque arm  162 , creating a counterclockwise response forward torque T 2  in driven gear  116 . 
         [0030]    Considering now the geometry of back-drive in driven gear  116 , when gear  116  is urged in a clockwise direction as by a reversal of torque in an associated automotive cam as would pertain in a CVVL system  24  ( FIG. 1 ), a clockwise torque T′ 2  is produced in the opposite direction from forward torque T 2 . Because of friction angle γ, the force direction produced in driving gear  114  is vector  164  along line  166 , which is the direction of effective response force vector  160 . 
         [0031]    Of special interest is the fact that reverse force line  166  departs extensively from forward force line  154  to the extent that line  166  lies across the rotation center O 1  at response torque arm  168 , creating a response torque T′ 1  in the same direction as driving torque T 1  and counter to torque T′ 2 . Thus, back torque T′ 2  in gear  116  is effectively opposed by forward torque T′ 1  in gear  114 , preventing reverse rotation of gear  116 . Helical gear system  100  is thus self-locking, in accordance with the present invention. 
         [0032]    Important elements of the self-locking feature include the pressure angle α, and the helix angle β. Also of importance are material selection, surface finish selection and appropriate lubrication design to achieve higher gear efficiency. These latter elements lead to values of friction coefficients for the driving and driven wheel. In conventional gear designs, the angle α is typically in the range of about 14.5° to about 25°, and usually about 20°. Similarly, angle β is usually less than 45°. In order to achieve a self-locking condition according to the present invention, however, values of α and β outside these ranges must be considered. With these common upper limits removed, values of α, and β may be found and appropriate material and surface finish may be chosen, that will satisfy this self-locking condition and higher gear efficiency. Specifically, it was found that angles α between 45° and 75°, and angles β between 60° and 80°, are satisfactory. Preferable, the gears are made of steel having a grinded surface finish resulting in a coefficient of friction on the order of between 0.11 and 0.18. Together, angle values within these ranges and material selection to produce the desired coefficient of friction will generally satisfy the necessary self-locking condition according to the present invention. 
         [0033]    Referring still to  FIG. 4 , the drive and coast flank profiles as shown are symmetrical for each tooth. However, it is known in the art of gear manufacture to provide gears wherein the drive and coast flank profiles differ. Such gear teeth are said to be “asymmetric”. In a further aspect of the present invention, gears  114 , 116  are provided with asymmetric tooth profiles, resulting in different pressure angles for the coast contact point  152  from the driving contact point  150  (in  FIG. 4 , {acute over (α)}={acute over (α)}′ because the gears are symmetric). Asymmetric gears reduce contact stress, thus resulting in higher torque density. Such optimization can reduce operating noise and vibration. In a CVVL application using asymmetric gears, since self-locking is desirable in both the clockwise and counterclockwise directions of rotation, the values of the differing pressure angles are both between 45° and 75°. Within this range, pressure angles for the asymmetric gears may be optimized based on other design parameters such as gear efficiency, contact ratio, or ease of manufacturing. Thus, a symmetrical profile (angle α equal to angle α′) is a special case. In a presently preferred asymmetric embodiment, the pressure angle {acute over (α)} is 60° on the drive flank and 50° on the coast flank (drive and coast flank defined for clockwise rotation); helix angle β is 77°; the normal module (tooth-to-tooth distance in a direction normal to the helical angle) is 1.8 mm; center-center distance is 60 mm; and the friction coefficients are 0.118 on the drive flank; and 0.175 on the coast flank, based on the selected materials and lubrication. 
         [0034]    In another aspect of the invention, the helical gears can be formed from designed as discontinuous and comprise laminated spur gear slices, bolted or pinned together, to form a discontinuous tooth profile. Formed in this manner, the gears do not generate axial forces and allows lower transversal contact ratio on the slice for higher tooth strength. Manufacturing costs for helical gears in this form are substantially reduced as compared to cutting the individual gear teeth of a continuous helical gear. The number of slices used to form the laminated helical gear may be from two to as many as practical. More slices provide smoother transmission, creating more nearly continuous helix. Referring to  FIGS. 5 through 7 , a helical gear formed from spur gear slices, in accordance with the invention, is shown wherein each slice is rotationally offset from the adjacent slice or slices by a fixed rotation angle equal to helix angle β ( FIG. 3 ). 
         [0035]      FIG. 5  shows a discontinuous helical gear  200  formed of three 24-tooth spur gear slices  202   a , 202   b , 202   c,  each rotationally offset from the previous slice. The slices are bolted or otherwise fixed together as shown in  FIG. 7 . Such a discontinuous helical gear functions in the present invention identically to a continuous helical gear such as gears  114 , 116  shown in  FIG. 2 . 
         [0036]      FIG. 6  shows schematically a 5-tooth 3-slice discontinuous helical gear  300  wherein the second slice  304  is offset rotationally from the first slice  302  by 24°, and the third slice  306  from the second slice  304  by an additional 24°. Generally, the angle 24° is calculated by dividing 360° by the number of teeth (5 in this example) multiplied by the number of spur gear slices (3 in this example). Note that the apparent next 24° rotation represents the next tooth of first slice  302 . 
         [0037]    While the invention was described specifically for a CVVL mechanism, it should be understood to be applicable to any similar positioning mechanism where self-locking is desirable for internal combustion engine or any other application. 
         [0038]    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. Further, 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.