Patent Publication Number: US-5425345-A

Title: Mechanically driven centrifugal air compressor with hydrodynamic thrust load transfer

Description:
TECHNICAL FIELD 
     This invention relates to a mechanically driven centrifugal air compressor for an internal combustion engine wherein the compressor has an impeller shaft that is supported with plain journal bearings and includes a gear train having a high overdrive speed ratio that is driven from the crankshaft of the engine with a belt and pulley arrangement having a low overdrive speed ratio. 
     BACKGROUND OF THE INVENTION 
     The most common form of centrifugal compressor that is used with internal combustion engines is a turbocharger wherein the impeller is driven by a turbine powered by the engine exhaust gases. This works well for a 4-cycle engine where the nominal combustion air is drawn into the cylinders on the piston downstroke. But in an externally scavenged 2-cycle engine there is no separate intake and exhaust stroke and instead there is a combined stroke wherein the combustion residuals must be pushed out of the cylinders with the fresh air for the next combustion event. In such an application, the air compressor is the sole source of combustion/scavenge air and therefore at start up, there is insufficient exhaust gas flow to spin the turbine of a turbocharger with the result that there would be no combustion air for starting the engine. And thus a turbocharger is not suitable for such an application. Mechanically driving the compressor eliminates this start-up problem on a 2-cycle engine and can also be used in a 4-cycle engine. However, there is considerable difficulty in providing a suitable mechanical drive that will produce the necessary high compressor impeller speeds in a highly reliable manner and without objectionable noise levels. Moreover, it should be capable of being mass produced at a reasonable cost. 
     The mechanical drive between the engine crankshaft and the compressor impeller must provide a very high overdrive (output/input) speed ratio in order to drive the impeller at the extremely high speeds necessary to obtain acceptable operating efficiencies. For example, impeller speeds exceeding 100,000 RPM at peak engine speed are not uncommon and the overdrive speed ratio necessary to obtain same typically exceeds 15:1 and will of course vary with the relevant operating characteristics of both the engine and the compressor. Examples of mechanical drives that have been proposed for centrifugal air compressors are disclosed in U.S. Pat. Nos. 1,754,724; 1,852,569; 2,099,785; 2,294,743; and 5,063,904. As illustrated by these patents, such mechanical drives typically include various forms of gearing such as planetary gear drives and spur gear trains that may include an internal ring gear and pinion gear. Then there is also my U.S. Pat. No. 5,058,546 that discloses a mechanical drive employing a belt and pulley drive with a low overdrive speed ratio from the engine crankshaft in series with a friction drum and wheel providing a high overdrive speed ratio drive to the compressor impeller. The problems with an all gear drive include the noise of the gears and their attendant cost. And the gears, while providing a desirable positive type of drive, require a special mounting of the compressor gear train to include the engine crankshaft. On the other hand, a belt and pulley drive allows the centrifugal air compressor to be installed at various locations on the engine like other belt driven engine accessories. Moreover, the belt may be one that also drives one or more of the other engine accessories which is advantageous from both a cost standpoint and space allocation. But the belt and pulley drive must be limited to a low overdrive speed ratio of about 2:1 to prevent the belt from slipping on the smaller pulley at the high speed/load conditions encountered in such an application. As a result, a belt and pulley drive must be combined with a mechanical drive in some form that provides a high overdrive ratio that is highly reliable but yet does not produce objectionable noise levels and can be mass produced at a reasonable cost. In my above patent, such a high overdrive ratio is provided by a friction wheel and drum and while this has proven satisfactory and particularly with respect to noise, a geared overdrive has the potential for more load capacity and even greater reliability provided that the gear noise can be held to an acceptable level, the remainder of the compressor components are also highly reliable, and the cost is not prohibitive. 
     SUMMARY OF THE INVENTION 
     The present invention is an improvement over my above U.S. Pat. No. 5,058,546 and combines an internal helical ring gear and helical pinion gear drive providing a high overdrive speed ratio with a belt and pulley drive from the engine crankshaft that provides a low overdrive speed ratio through a pulley coupled to the internal helical ring gear. The helix angle of the ring gear and pinion gear is utilized in the compressor to reduce the thrust bearing loads imposed by air pressure from impeller operation acting on the compressor&#39;s impeller and a thrust ring on the impeller shaft is used to transfer these loads into a single thrust bearing mounted in the compressor housing. The reliability of this thrust bearing and the thrust ring is enhanced by the present invention beyond that provided by the helical gears by transferring the air pressure imposed loads by hydrodynamic action into the thrust bearing. The noise of the gears can be maintained at a low acceptable level by integrally forming the compressor pulley and helical ring gear of plastic and while such construction is preferred in practicing the present invention, it is not an essential part thereof and is included in my invention in my U.S. patent application Ser. No. 08/333,085 entitled &#34;MECHANICALLY DRIVEN CENTRIFUGAL AIR COMPRESSOR WITH INTEGRAL PLASTIC PULLEY AND INTERNAL HELICAL RING GEAR&#34; filed concurrently herewith and assigned to the same assignee. 
     In the compressor concerned with the present invention, it essentially has an impeller that is attached to one end of an impeller shaft that is supported in the compressor housing by two axially spaced plain journal bearings that are located adjacent the respective ends of the shaft. The thrust ring is mounted on the impeller shaft near the impeller and is sandwiched on one side by the thrust bearing mounted in the compressor housing and on an opposite side by both the compressor housing and the nearest journal bearing to axially locate the impeller shaft. The pulley and internal helical ring gear are coupled together and rotatably supported with anti-friction bearings on the compressor housing and the pulley is engaged by a belt that is driven by a pulley of larger diameter attached to an engine crankshaft. The ring gear meshes with the pinion gear which is formed integral with the other end of the impeller shaft and the gears provide a high overdrive speed ratio in series with the belt and pulley drive from the engine crankshaft which provides a low overdrive speed ratio. 
     In the above arrangement, the thrust bearing bears the axial load on the impeller shaft imposed by air pressure from impeller operation acting on the back of the impeller and the lead of the helical pinion gear is set in a direction so that the gears impose an axial force on the impeller shaft with their helix angle that is opposite the impeller imposed axial force to substantially reduce the impeller imposed axial load borne by the thrust bearing. Lubrication of the critical surfaces in the compressor is provided by engine oil which is delivered under pressure to a space surrounding the impeller shaft. The oil passes through the plain journal bearings to lubricate and cool the latter and with oil from the journal bearing near the pinion gear then directed to lubricate and cool the pinion gear and the ring gear. The anti-friction bearings supporting the pulley and internal helical ring gear are of the ball type and are permanently lubricated and sealed so that they do not require oil delivery. 
     Lubrication and cooling of the major thrust face of the thrust ring and the thrust bearing is provided by a passage formed in the housing that delivers pressurized oil to an annular channel extending about the thrust bearing. The annular channel is in turn connected by separate passages through the thrust bearing to a plurality of angularly spaced oil pockets formed in the face of the thrust bearing opposite the thrust ring on the impeller shaft. The oil pockets have an arcuate shape concentric with the impeller shaft axis and a depth that gradually decreases to zero at the thrust bearing face in the direction of impeller shaft rotation to form a ramp. The pressurized oil fed to the oil pockets or ramps is wiped from their deep end to their terminal end in a wedging lubricant film forming action to transfer the net axial load on the impeller shaft hydrodynamically into the thrust bearing face from the major thrust face of the thrust ring. This hydrodynamic action provides an assured constant flow of oil to these surfaces that experience a large differential in speed to thereby enhance their reliability in maintaining a precise axial position of the impeller shaft and thereby the axial clearances of the compressor vanes. The oil also acts to cool the thrust ring and thrust bearing and on exiting their thrust bearing surfaces drains to a sump in the housing along with the oil from the journal bearings and gears where it then returned to the to the engine lubrication system. 
     It is therefore an object of the present invention to provide a new and improved mechanically driven centrifugal air compressor for an internal combustion engine. 
     Another object is to provide a centrifugal air compressor having a mechanical overdrive that is reliable, does not produce objectionable noise levels and can be mass produced at reasonable cost. 
     Another object is to provide a centrifugal air compressor having a mechanical overdrive wherein a pulley driven internal helical ring gear meshes with a helical pinion gear that drives the compressor impeller, the lead of the pinion gear is set so that the meshing ring gear and pinion gear impose an axial load on the impeller shaft that is opposite to and reduces the axial load imposed thereon by air pressure acting on the impeller, only a single thrust bearing bears the net axial load imposed by the air pressure through a thrust ring on the impeller shaft, and the net axial load is transferred hydrodynamically from the thrust ring to the single thrust bearing to enhance their reliability. 
     These and other objects, advantages and features of the present invention will become more apparent from the following description with reference to the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an internal combustion engine including a preferred embodiment of the mechanically driven centrifugal air compressor according to the present invention; 
     FIG. 2 is an enlarged longitudinal sectional view of the centrifugal compressor in FIG. 1; 
     FIG. 3 is an enlarged view taken along the line 3--3 in FIG. 2 when looking in the direction of the arrows; 
     FIG. 4 is an enlarged taken along the line 4--4 in FIG. 2 when looking in the direction of the arrows; 
     FIG. 5 is an enlarged view taken along the line 5--5 in FIG. 4 when looking in the direction of the arrows; and 
     FIG. 6 is an exploded view of the centrifugal air compressor in the above Figures. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring to FIG. 1, there is illustrated an internal combustion engine 10 that includes a block 12, a cylinder head 14, an oil pan 16, and a crankshaft 18 that has an end extending from the front of the engine block. A pulley 20 is attached to this end of the crankshaft to drive various engine accessories that are mounted on the engine and include a water pump 22 having a pulley 24, an alternator 26 having a pulley 28, a power steering pump 30 having a pulley 32, an air conditioning compressor 34 having a pulley 36, and a centrifugal air compressor 38 having a pulley 40. 
     The above pulleys are part of an accessory drive system that further includes an idler pulley 44 and a tensioning pulley 46 which are also mounted on the engine. The tensioning pulley 46 is mounted on the engine with an adjustable mounting device 48 of conventional design that provides for adjusting the tension in a drive belt 50 that is received by the pulleys and transmits drive from the crankshaft pulley to the above accessories. In this accessory belt and pulley drive system, the engine crankshaft pulley 20 has a relatively large pitch diameter compared to all the other pulleys but particularly with respect to the accessory pulleys to allow the latter pulleys to have large enough pitch diameters to avoid belt slip at high speed/load conditions. The drive belt 50 is a polyvee type flat belt with a plurality of integral parallel V-shaped belt sections on its drive side and the accessory pulleys and the idler pulley have a corresponding number of parallel V-shaped grooves that receive these V-belt sections. The tensioning pulley 46 on the other hand has a cylindrical surface that is engaged by the plain back side of the belt to provide for belt tensioning in a conventional manner. 
     Referring to FIGS. 1-6, the centrifugal air compressor 38 forces fresh air to the intake ports in the cylinder head 14 and includes an aluminum housing 52 having a circular front end 54 and rear 56 that face in opposite directions and parallel each other. A cylindrical shaped end cover 58 made of aluminum is attached with bolts 60 to the front housing end 54 and an integral scroll section 62 and intake manifold section 64 and generally designated as 66 and also made of aluminum is attached with bolts 68 to the rear housing end 56. The scroll/intake manifold 66 and thereby the complete centrifugal air compressor 38 is attached at the exit end of the intake manifold 64 to the cylinder head 14 with bolts 69 at a location on the head where the manifold exit aligns with the intake ports in the head. 
     A steel impeller shaft 70 having an integral helical gear 72 formed at one end and axially spaced front and rear journals 74 and 76 of equal diameter is supported in a bore 78 in the housing 52 with front and rear plain journal bearings 80 and 82 that receive the respective journals. The journal bearings are made of bronze and are press fitted in the bore 78. The shaft journals extend radially from the main body of the shaft and together with their bearings are located at the opposite ends of the bore 78. The front journal 74 is located on the shaft close to the pinion gear 72 and the outboard end of its journal bearing 80 is mounted flush with a planar surface 84 on the front housing end 54 that is at right angles to the bore 78 and shaft 70. The outboard end of the rear journal bearing 82 is mounted flush with a flat machined surface 86 in the housing that is also at right angles to the bore 78 and shaft 70. Tight control of the radial runout between the pinion gear and the bearing journals is provided by grinding the pinion directly on the shaft rather than as separate part. 
     A hardened and ground steel thrust ring 88 is received on the impeller shaft outward of the rear journal 76 and contacts on one side with both the outboard end of the rear journal bearing 82 and an annular portion of the machined housing surface 86 about this end of the bore 78. A bronze thrust bearing 90 having a central opening 92 is received by the shaft 70 and is mounted at an outer cylindrical surface 94 thereof in a counterbore 96 in the rear housing end 56 that is co-axial with the bore 78. The thrust bearing 90 contacts on one side at a flat annular area 91 adjacent its center opening with the thrust ring 88. The other side of the thrust bearing has a central dish shaped cavity 98 and contacts outward thereof at a flat annular surface 100 with a metal seal retainer 102 having a central bore 103. The seal retainer 102 is received by the impeller shaft and supports an elastomeric lip seal 104 and is also mounted in the counterbore 96. The bearing retainer is forcibly retained therein together with the thrust bearing by a snap ring 105 wherein the thrust bearing is tightly held against the bottom of this counterbore and both it and the bearing retainer are thus prevented from rotating. Sealing is provided between the housing and the bearing retainer by an elastomeric O-ring 106. 
     A shaft spacer sleeve 108 made of steel and providing an impeller spacing function, an impeller and thrust ring attaching clamping function and dual oil sealing functions is received by the shaft, extends through the seal retainer 102 and the thrust bearing 90 and seats on the thrust ring 88. The sleeve 108 has a cylindrical outer surface 109 that is hardened and ground and is contacted by the lip seal 104. The sleeve also has a cylindrical land 110 of larger diameter than the cylindrical surface 109 at its impeller end that is closely received in the bore 103 of the seal retainer. An expansible split seal ring 112 made of cast iron or other suitable bearing grade resilient metal is mounted in a groove in the sleeve land 110 and expands outwardly to be held stationary by the retainer bore 103 to provide additional sealing between the sleeve and the seal retainer wherein the sleeve rotates relative to the seal ring 112. The seal ring 112 provides a tight outward gripping seal at its outer diameter at the retainer bore and sealing in a labyrinth manner at its stationary inner diameter and sides were it is received in the rotating sleeve groove. 
     An aluminum impeller 114 with alternating long and short helical vanes 116 and 117 of conventional design has a central bore by which it is received on the end of the impeller shaft outboard of the shaft spacer sleeve 108. This end of the shaft is threaded to take a nut 118 that is screwed thereon and tightened to securely pinch the impeller, shaft spacer sleeve, and the thrust ring against a shoulder 120 ground on the end of the rear shaft journal 76. By this pinching or clamping action, the impeller, the sleeve and the thrust ring are all effectively coupled or attached to rotate with the impeller shaft whose axial position is maintained by the thrust ring 88 operating between the thrust bearing 90 and the rear journal bearing 82 and machined surface 86. The control of the axial positioning of the impeller shaft in the compressor housing 52 is made relatively simple in the manufacture of the compressor by controlling to very close tolerances only the depth to which the surface 86 is machined and the thickness of the thrust ring 88. 
     The impeller 114 is received in the scroll section 62 which has a circular air intake opening 122 and a scroll cavity 124 of conventional design. The air intake opening opens to the impeller at the entrance to the vanes and the scroll cavity is open to the tips of the vanes and connects with the intake manifold section 64. The impeller operates in a conventional manner on high speed rotation to draw in air through the intake opening and deliver same by centrifugal force into the scroll cavity where it is then directed to the intake manifold section and thence by the latter to the engine intake ports. In such centrifugal pumping operation, air pressure is developed at the back of the impeller that imposes a tension load or pulling force on the impeller shaft that must be resisted by the thrust ring 88 to hold the impeller axially in place. 
     The thrust ring 88 is located nearest the end of the impeller shaft 70 closest to the impeller to minimize the effect of thermal growth of the shaft on the axial positioning thereof with the thrust ring. Thermal expansion of the shaft causes it to grow in length on both sides of the thrust ring because the thrust ring can not move axially within the housing. By keeping the thrust ring as close as possible to the impeller, the effect of thermal growth is limited and higher compressor efficiencies are achieved with minimum vane tip clearances. 
     The centrifugal air compressor pulley 40 is integrally formed with an internal helical ring gear 130 and are together as one piece referred to as an integral internal helical ring gear/pulley or one-piece input member 132. The input member 132 is supported for rotation on the compressor housing 52 as described in detail later with the ring gear 130 meshing with the pinion gear 72. The groove compressor pulley 40 has a pitch diameter not less than half that of the engine crankshaft pulley 20 to allow conventional pulley design such that the pitch diameter of the pulley 40 is large enough to avoid belt slip at high speed/load conditions in driving the centrifugal air compressor. In the embodiment illustrated, the pitch diameter of the pulley 40 is half that of the crankshaft pulley 20 and there is thus provided a low overdrive speed ratio (output/input) of 2:1 between the engine crankshaft 18 and the ring gear 130. In the particular engine/centrifugal air compressor combination illustrated, there is required an overdrive speed ratio of 16:1 between the engine crankshaft and the impeller 114. And accordingly, the ring gear 130 is provided with a pitch diameter that is eight times that of the pinion gear to produce a high overdrive speed ratio of 8:1 to factor the series arranged 2:1 overdrive ratio of the belt and pulley drive to the required high mechanical drive train overdrive ratio of 16:1 between the impeller and the engine crankshaft. 
     The noise resulting from the meshing teeth of the ring gear and pinion gear at the high speeds that they run is maintained at a low acceptable level by the integral formation of the helical ring gear 130 with the pulley 40 and by then making this dual function part of molded plastic and preferably of glass fiber reinforced phenolic in a compression molding process. In obtaining this low noise maintenance, both the plastic teeth of the ring gear 130 in their engagement with the steel teeth of the pinion gear 72 and the dampening resulting from the elasticity of the integrally joined plastic ring gear and pulley 40 contribute significantly to reduce the gear noise to an acceptable level not obtainable with metal ring and pinion gears and not obtainable with a plastic ring gear joined to a metal pulley. 
     The plastic ring gear/pulley 132 is supported on the compressor housing 52 by a steel support shaft 134 having a cylindrical nose 136 of larger diameter received in a blind bore 138 in the flat face 84 of the housing at the front end 54. The support shaft 134 is further formed with a radial flange 140 of relatively large diameter having a ground face 142 at the base of the nose 136 that seats against the flat front housing face 84. The flange is clamped to the housing end face by three bolts 144. This serves to fixedly locate the shaft axially as well as provide a large area or footprint at the interfaces of the flange and the compressor housing for transmitting support shaft loads to the compressor housing. 
     The plastic ring gear/pulley 132 is molded with a central bore 146 and with counterbores 148 and 150 at the opposite ends of the central bore. The ring gear/pulley 132 is received by and rotatably supported on the support shaft 134 by anti-friction front and rear bearings 152 and 154 of the ball type that are permanently lubricated and sealed. The bearings 152 and 154 are press fitted at their outer race in the respective counterbores 148 and 150 and have a slide fit at their inner race on the support shaft 134. The bearings are press fit into a bearing retainer shell and this assembly is then molded into the ring gear/pulley. The rear bearing 154 seats against a shoulder 156 ground on the shaft 134 at the base of flange 140 and the ring gear/pulley is held in place on the shaft with a preload wave spring 158 and snap ring 160. The bearings 152 and 154 are located so that they are radially aligned with the axial center of the respective pulley 40 and ring gear 130 to carry their load in a balanced manner. 
     The end cover 58 encloses both the helical ring gear 130 and the pinion gear 72 and has an opening for the pulley 40 in which is mounted an elastomeric lip seal 162 that provides sealing between the end cover and the pulley. The lip seal 162 is of slightly larger diameter than the pulley 40 and is received over same on mounting of the end cover. The lip seal 162 sealingly contacts the outer cylindrical surface of a metal seal ring 164 that is integrally molded into the ring gear/pulley 132 between the pulley and ring gear portions thereof. In the installed position of the end cover, the pulley 40 is located outward thereof to receive the drive belt 50. And sealing between the front housing end 54 and the front end cover 58 is provided by an elastomeric O-ring 165. 
     Super precision mounting of the ring gear 130 with respect to the pinion 72 is provided by making the diameter of the bore 138 that locates the support shaft 134 the same as that of the bore 78 in which the impeller shaft journal bearings are mounted, tightly tolerancing the bore 138 and grinding the support shaft nose 136 to closely fit this closely controlled bore. This allows for these two bores to be machined in the same operation and with the same tool to very closely control the center distance between these bores and thereby the center distance between the high speed ring gear and pinion gear. The center distance of these gears affects the gear lash and thus noise and by tightly controlling the center distance as described above, the gear lash (noise) is further lowered; i.e. lowered even further than the very significant amount of noise reduction obtained from the plastic ring gear/pulley 132. 
     The thrust ring has both a minor load carrying face 166 and a major load carrying face 168. During most of its operation, the minor face 166 transfers little or no load into the face 86 machined on the housing 52. The major face 168 carries the bulk of the axial load on the impeller shaft created by air pressure from impeller operation acting on the back of the impeller and which must be borne by the thrust bearing. It was found that this load can be reduced very significantly to enhance the life of this bearing and the thrust ring by generating an opposite force on the impeller shaft with the addition of the helical ring and pinion gear to provide mechanical overdrive to the impeller shaft. This is accomplished by setting the lead of the pinion gear 72 in the same direction as the helix of the impeller vanes 116, 117 so that the meshing teeth of the ring and pinion gears as a result of their helix angle produce a pulling or tension force on the impeller shaft in opposition to that produced by the air pressure acting on the back of the impeller. For example, in the compressor illustrated, the air pressure imposed load on the impeller shaft was reduced by about 50% when the helical ring gear and pinion gear were provided with a helix angle of 30° and the pinion gear was provided with a right hand lead the same as the impeller vanes. 
     Lubrication in the centrifugal air compressor is provided by oil that is delivered under pressure from the engine&#39;s lubrication system via an oil feed line 170 to a fitting 172 threadably connected to the compressor housing 52. A passage 174 connects the fitting 172 to a sump 176 defined by the bore 78 between the journal bearings 80 and 82. The pressurized oil delivered to the cavity 176 is forced out through the clearance in these bearings adjacent the ends of the impeller shaft providing lubrication and cooling of the journal bearings as it passes. The oil exiting the front journal bearing 80 impinges on the pinion gear 72 to provide lubrication and cooling of the high speed gears with the oil then draining to a sump 178 formed in the bottom of housing 52. The sump 178 has a drain hole 179 and is connected to return oil to the engine lubrication system by a fitting 180 threadably connected to the housing connecting the drain hole to an oil return line 182. The oil exiting the rear journal bearing 82 lubricates and cools the minor thrust face 166 of the thrust ring 88 and then drains to the sump 178 for return to the engine lubrication system. 
     Lubrication and cooling of the major thrust face 168 of the thrust ring 88 and the thrust bearing 90 is provided by a passage 184 formed in the housing 52 that connects the pressurized oil cavity 176 between the journal bearings with an annular cavity or channel 186 defined by a stepped shoulder 188 on a corner of the thrust face of the thrust bearing and by the bottom corner 189 of the counterbore 96. The annular cavity 186 is in turn connected to three equally angularly and radially spaced oil pockets 190 formed in the bearing face 91 of the thrust bearing. These connections are made by separate interconnected radial passages 192 and axial passages 194 that are also formed in the thrust bearing as illustrated in FIGS. 2, 4 and 5. The oil pockets 190 have an arcuate shape concentric with the impeller shaft axis and a depth that gradually decreases to zero at the thrust bearing face in the direction of impeller shaft rotation. The radial passages 192 intersect with the annular cavity 186 and the respective axial passages 194 which in turn intersect with the respective oil pockets 190 at their deep end. The pressurized oil fed to the oil pockets is wiped from their deep end to their terminal end in a wedging action that forms oil pads that transfer the net axial load on the impeller shaft hydrodynamically into the thrust bearing face 91 from the major thrust face 168 of the thrust ring. The oil also acts to cool the thrust ring and thrust bearing and on exiting their thrust bearing surfaces drains to the sump 178 along with the oil exiting the rear journal bearing 82 for return to the engine lubrication system. In addition, a plurality of equally angularly and radially spaced axial holes 196 are formed in the thrust bearing 90 to drain oil from the dished side of the thrust bearing and away from the lip seal 104 to the sump 178. 
     The invention has been described in an illustrative manner with respect to presently preferred embodiments, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than words of limitation. Obviously, many modifications and variations of the present invention in light of the above teachings may be made. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically shown and described.