Abstract:
A hypoid gear set including a ring-shaped gear positioned about an axis and having a plurality of curved gear teeth arranged about the axis. Each tooth has a second side having a forward face with a shallow pressure angle relative to an axially extending line and defining the drive side of the gear. Each tooth has a first side having a rear face with a steep pressure angle relative to an axially extending line and defining the coast side. A pinion gear drives the rear faces of the ring-shaped gear such that the ring-shaped gear rotates in the direction of the forward faces of the teeth of the ring-shaped gear.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of gears for use in automobile differentials. In particular, this invention relates to a new design for a hypoid gear set mounted in a drive-on-coast side application for use in a power-take-off or rear axle of an automobile.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    It is common practice to use hypoid gear sets in automobile differentials. A hypoid gear set consists of a ring-shaped gear meshed with a pinion gear. The ring gear typically has a set of generally axially upstanding curved teeth arranged on a flat surface of a ring-shaped portion. The curved shape of the teeth results from their spiral arrangement, which cuts down on gear noise and increases the gear power. The pinion gear may contact the ring gear at a point either above or below the axis of the ring gear. The teeth of the pinion gear necessarily share the same pressure angles as the teeth of the ring gear. The pinion gear teeth are also curved in a spiral arrangement. This type of gear arrangement allows the gear set to change the direction of the power flow. In most cases, the direction of power flow is turned 90 degrees. For this reason, hypoid gear sets are used in the drive train of automobiles to transfer power to the wheels from the transmission drive shaft. Another benefit of hypoid gear sets is that they have high speed and strength capabilities.  
           [0003]    It is well known in the prior art that the sides of the gear teeth that are predominantly used in the driving direction should be cut with a steeper pressure angle than the coast, or reverse side. The steeper pressure angle provides increased strength and efficiency in the direction of rotation. If the drive side were not cut with this steeper pressure angle, there would be more noise from the gear and less power. It is not plausible to have a steep pressure angle on both sides of the teeth. This would result in a nearly square-shaped gear tooth, creating a weaker pinion and a noisier gear set.  
           [0004]    Recently, in some power-take-off applications, it has become necessary to mount some hypoid gear sets in a reversed direction. These are known as drive-on-coast side applications. This reverse mounting requirement results from the packaging constraints of these applications. In a power train system, there are two sets of these gears. The second set transfers some power to the rear wheels while the first set supplies power to the front wheels. This implements a four-wheel drive mechanism for the automobile. However, because of the small size of the power-take-off box and the numerous support rods along with other parts, there is insufficient space in which to mount the hypoid gears in the correct orientation.  
           [0005]    In many new power-take-off applications, the only way to fit the hypoid gear set in the package is to mount it such that the pinion is reversed from its normal position. By mounting the pinion in this position, the normal drive side of the hypoid gear set becomes the coast side, and vice versa. Therefore, to run the automobile in the forward direction, the drive force from the pinion is applied to the teeth with the shallow pressure angle. This results in a loss of efficiency and fatigue strength of the hypoid gear.  
           [0006]    In the past, the standard method to avoid this loss of efficiency and strength has been to mount the pinion such that it intersects the ring gear at a point above the ring gear axis instead of at a point below the axis. However, in new power-take-off applications, the supports obstruct the space around the gears. In order to keep the small size of the power-take-off box, the pinion must intersect the ring gear below the axis and must be run backwards.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In one embodiment of the present invention, a hypoid gear set includes a ring-shaped gear positioned about an axis and having a plurality of curved gear teeth arranged about the axis. Each tooth has a second side having a forward face with a shallow pressure angle relative to an axially extending line and defining the drive side of the gear. Each tooth has a first side having a rear face with a steep pressure angle relative to an axially extending line and defining the coast side. The embodiment also includes a pinion gear meshing with the ring-shaped gear. The pinion gear drives the rear faces of the ring-shaped gear such that the ring-shaped gear rotates in the direction of the forward faces of the teeth of the ring-shaped gear.  
           [0008]    The invention may also be embodied in a method of cutting a new hypoid ring gear based on a standard hypoid ring gear, the standard hypoid ring gear having a plurality of teeth each with a first side defining the coast side and having a first pressure angle, and second side defining the drive side and having a second pressure angle. The method includes the steps of determining the first and second pressure angles of the standard hypoid ring gear and then determining the inverse angles of the first and second pressure angles. Then, the inverse first pressure angle is cut into the drive side of the new hypoid ring gear and the inverse second pressure angle is cut into the coast side of the new hypoid ring gear.  
           [0009]    In another embodiment of the invention, a ring-shaped hypoid gear includes a base with a plurality of curved teeth each having a convex side facing in the direction of rotation of the ring-shaped hypoid gear. Each of the teeth also has a drive side on the concave face of each tooth that receives a driving force from a pinion gear and a coast side on a convex face opposite the concave face.  
           [0010]    The present embodiments provide a number of important advantages over the prior art. By locating the steep pressure angle on the coast side of the ring-shaped gear teeth, the hypoid gear set can provide more efficient power and greater fatigue strength when run in the reverse direction than can a standard hypoid gear set. This hypoid gear set can be mounted in a machine that requires that the gear set be run in the coast direction to drive the machine in a forward direction. Running this hypoid gear set in this manner does not result in a loss of power or fatigue strength, as did running a prior art hypoid gear set in the reverse direction.  
           [0011]    It is to be understood that both the preceding summary and the following detailed description are intended to be exemplary and are intended to provide a further explanation of the invention claimed. The invention will best be understood by reference to the following detailed description read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a prior art hypoid gear set consisting of a ring-shaped gear and a pinion;  
         [0013]    [0013]FIG. 2 is an enlarged perspective view of a prior art ring-shaped gear tooth;  
         [0014]    [0014]FIG. 3 is an enlarged perspective view of a ring-shaped gear tooth of the present invention;  
         [0015]    [0015]FIG. 4 is a perspective view of the ring-shaped gear and the pinion gear of present invention;  
         [0016]    [0016]FIG. 5 is an enlarged perspective view of the present invention along line  1 - 1  in FIG. 4, showing the interaction of the teeth of the ring-shaped gear and the pinion gear; and  
         [0017]    [0017]FIG. 6 shows an example of a hypoid gear set mounted within a rear axle carrier. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to the Figures, FIG. 1 shows a standard hypoid gear set known in the prior art. A pinion gear  10  intermeshes with a ring-shaped gear  12  at a point below the axis  14  of the ring-shaped gear  12 . The ring-shaped gear  12  has a first side  16  and a second side  18 . The first side  16  is flat and the second side  18  has a plurality of teeth  20  extending axially from the second side  18 , each of which has a drive side  22  and a coast side  24 . The drive side  22  is the side to which pressure is applied on the ring-shaped gear  12  by the pinion  10  in order to move the machine in a forward direction. The direction of rotation is called the drive direction as shown by the arrow  26 .  
         [0019]    [0019]FIG. 2 shows an enlarged view of one of the teeth  20  of a prior art ring-shaped gear  12 . Each tooth  20  has a first pressure angle  28  and a second pressure angle  30 . The first pressure angle  28  is on the drive side  22  and the second pressure angle  30  is on the coast side  24 . Again, the direction of rotation is shown in the Figure by the arrow  26 . In the prior art, the first pressure angle  28  is much steeper than the second pressure angle  30  in reference to an axially extending line  32 . This provides maximum power from the pinion  10  to the ring-shaped gear  12  and creates a more efficient hypoid gear set. It is not plausible to cut both the first pressure angle  28  and the second pressure angle  30  in a steep manner. Doing so would result in a nearly square-shaped tooth, which results in more noise, friction, heat and gear wear. Plus, the pinion gear  10  necessarily must match the teeth of the ring-shaped gear  12 , and a pinion gear  10  with square-shaped teeth is much weaker than a pinion gear  10  with angled teeth. However, the drawback to cutting a steep pressure angle in one side of the teeth  20  of the ring-shaped gear  12  is that the second pressure angle  30  must remain shallow. In certain situations, namely in new machines that utilize four-wheel-drive, it is necessary to run the hypoid gear set in a reversed, or coast direction  34  in order to get the machine to run forward. This results from the fact that it is impossible to mount the pinion  10  either above the axis  14  of the ring-shaped gear  12  or mount the pinion  10  such that it intersects the ring-shaped gear  12  from the opposite side  36 . The designer must therefore mount the pinion  10  in the manner shown in FIG. 1, and then run it in a coast direction  34  in order to drive the machine in a forward direction  26 . Since this method uses the coast side  24  of the teeth  20  to drive the machine, it is called a “drive-on-coast” mechanism. Unfortunately, due to the shallow second pressure angle  30 , the hypoid gear set loses much of its power when driven in the coast direction  34 . This leads to decreased efficiency and a loss of fatigue strength, causing the hypoid gear set to wear more rapidly than if it were run in the proper direction.  
         [0020]    An advantage to this invention is that it allows a hypoid gear set as shown in FIG. 4 to be mounted in the same position as the prior art hypoid gear set from FIG. 1. The new hypoid gear set can be run in a coast direction  34  without losing either its efficiency or its fatigue strength.  
         [0021]    [0021]FIG. 4 shows the preferred embodiment of the invention. The new hypoid gear set has a ring-shaped gear  38  and a pinion gear  40 . The pinion gear  40  intermeshes with the ring-shaped gear  38  at a point below the axis  42  of the ring-shaped gear  38 . The ring-shaped gear  38  has a base having a first side  44  and a second side  46 . The first side  44  is flat and the second side  46  preferably defines a plurality of teeth  48  which each have a drive side  50  and a coast side  52 . An arrow  26  shows the drive side direction. The drive side  50  receives driving force from the pinion gear  40 .  
         [0022]    [0022]FIG. 3 shows an enlarged view of a ring gear tooth  48  of the invention. Each tooth  48  has a first pressure angle  54  and a second pressure angle  56 . The first pressure angle  54  is on the coast side  52  and the second pressure angle  56  is on the drive side  50  which is on the concave face of the gear tooth  48 . In the preferred embodiment of the invention, the first pressure angle  54  is shallower than the second pressure angle  56  in reference to an axially extending line  58 . This puts the steeper second pressure angle  56  on the coast side  52  of the gear tooth  48 .  
         [0023]    In order to determine the proper measurements for this embodiment of the invention, the designer preferably first calculates the appropriate pressure angles for the machine. From these calculations, the designer then determines the inverse of each pressure angle. Preferably, subtracting the original pressure angle from the sum of the two pressure angles accomplishes this. Once the inverse of both the first and second pressure angles is determined, the designer preferably cuts the inverse of the original drive side pressure angle into the coast side, and cuts the inverse of the original coast side pressure angle into the drive side. This results in a hypoid gear set where the steeper pressure angle is actually on the coast side of the gear teeth. The hypoid gear set can be mounted in its usual position in the machine and can be run in a reverse direction without losing any efficiency or fatigue strength.  
         [0024]    [0024]FIG. 5 shows an enlarged view of the interaction between the pinion teeth  60  and the ring-shaped gear teeth  48  as viewed along line  1 - 1  in FIG. 4. A line  26  shows the drive direction and a line  34  shows the coast direction. The first pressure angle  54  and the second pressure angle  56  are displayed on the ring-shaped gear teeth  48  and the pinion teeth  60  are cut appropriately to match them.  
         [0025]    [0025]FIG. 6 shows a hypoid gear set in place in a rear axle carrier package  62 . This Figure illustrates the space constraints present in many of today&#39;s vehicles. This orientation shows the pinion gear  64  intersecting the ring-shaped gear  66  at a point below the axle  68 . This is the preferable orientation as it allows for a lower profile of the vehicle. Because of the shape of the package, it is impossible to either mount the pinion gear  64  above the axle  68  or on the opposite side  70  of the axle  68 . So, when these packages are mounted as part of a power-take-off or four-wheel drive mechanism, some of these packages will face the opposite direction. In order to have all the axles  68  spinning in the same direction, some of the packages must be run in reverse. These reversed packages become drive-on-coast. In other words, the drive direction is actually the original coast side. Utilizing the preferred embodiment of the invention, an identical configuration of hypoid gear set can be mounted in this reverse position and run in reverse without any loss of efficiency or fatigue strength. Because this hypoid gear set is relatively simple to construct, there is little redesign cost, and it is not necessary to perform many extra complex calculations.  
         [0026]    It should be noted that there could be a wide range of changes performed on the new hypoid gear set and it could be designed using any gear set as a basic template. Thus, it is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of the invention.