Patent Abstract:
A tapered roller bearing for use in transmission cases made from aluminum alloy or other lightweight materials where the transmission contains a steel shaft which is supported in the case on two directly mounted tapered roller bearings, so that the two bearings confine the shaft both radially and axially. To compensate for the differences in expansion and contraction between the aluminum case and the steel shaft as the transmission or transaxle experiences variations in temperature, a race of at least one of the bearings is fitted with a compensating ring having a coefficient of thermal expansion greater than that of the case or shaft. As a consequence, the bearings operate at a generally uniform setting over a wide range of temperature variations.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     This invention relates in general to bearings and more particularly to a bearing having the ability to compensate for differential thermal expansion and contraction between a structure in which the bearing is mounted and a shaft located within the bearing.  
         [0004]     In an effort to reduce weight, the cases for various mechanical transmission devices are being constructed from lightweight material such as aluminum alloys. However, the shafts which turn in these cases and carry the gears that transmit the torque remain of steel, obviously because steel has great strength and resists wear.  
         [0005]     A variety of bearing arrangements exist for mounting shafts in transmission and transaxle cases, but the most compact and durable utilize tapered roller bearings. In a typical in-line transmission device, the input and output shafts are axially aligned and are confined at opposite ends of the case in two single row tapered roller bearings which, with respect to each other, are directly mounted, that is the large ends of the rollers for each bearing are presented inwardly toward the interior of the case and toward each other. Moreover, the input shaft has a pocket which receives the end of the output shaft, and here the output shaft is provided with another single row tapered roller bearing, known as a pocket bearing, which is also mounted directly with respect to the bearing for the output shaft. The tapered roller bearings in these applications carry extremely heavy loads for their size. Furthermore, they take axial or thrust loads as well as radial loads, and thus, a minimum number of bearings accommodate all of the loads to which the shafts are subjected.  
         [0006]     Ideally, opposed tapered roller bearings should operate within an optimum setting range dictated by application requirements. Generally speaking, the objective is to minimize axial and radial free motion in the shafts, for this maximizes the bearing life, reduces noise, and improves gear mesh. The directly mounted bearings which support the aligned input and output shafts in effect capture those shafts axially.  
         [0007]     If the transmission device case were made from steel, like the shafts and bearings, the case and shafts and the bearings would expand similarly with temperature variations, and the settings of the bearings for each shaft would not change drastically over a wide range of temperatures. However, the aluminum alloys from which many cases for the transmission devices are currently manufactured, have coefficients of thermal expansion greater than that of the steel from which the shafts and bearings are made. Assuming such a transmission device is assembled at room temperature with its directly mounted bearings in a condition of zero end play, the bearings will experience preload when the temperature drops, because the case contracts more than the shafts. By the same token, the bearings will experience end play as the temperature rises above room temperature, since the case expands more than the shafts. While the expansion and contraction of the tapered roller bearings, due to the geometry of the bearings, tends to offset some of the effects of the differential expansion and contraction between the case and shafts, it is not enough to maintain bearing settings generally constant over a wide range of temperature.  
         [0008]     Excessive preload into the bearings at assembly can compensate for some of the looseness caused by the case expansion, however, this increases the amount of friction in the internal components and subsequently increase the amount of wear on the internal components. Additionally, the effort rotate gears on the shafts will increase during cold start up of the transmission device. when On the other hand, when the transmission device is heated, excessive end play decreases the size of the zones through which loads are transmitted in the bearings resulting in spaces and gaps between the components of the bearings thereby reducing the life of the bearings. Since end play allows some radial and axial displacement of the shafts, it may also change the positions in which the gears of a transmission device mesh.  
         [0009]     U.S. Pat. No. 5,028,152 discloses a machine with thermally compensating bearings and is incorporated herein by reference. In that device, the bearings require a specially machined bearing cup that includes creating a rabbet in the face of the bearing. An elastometric compensating ring is placed within the rabbet and acts to compensate for differences in thermal expansion of the bearing having steel components and a machine having lightweight aluminum casing. However, the rabbet design of the bearing in that patent requires the bearing cup to be specially machined thereby adding increased cost to the bearing and increases the difficulty of assembling the bearing.  
         [0010]     The bearing of present invention is a tapered roller bearing that requires little if any machining of the bearing cup. Instead, the thermal compensation components can be positioned against the back face of the bearing in the manner of an add-on accessory to the bearing. This results in a lower cost bearing that is less complex to assemble and which allows for the possibility of adding thermal compensating components to existing bearings or incorporating thermal compensating components to a bearing with less effort.  
         [0011]     Additionally, the present invention may be mounted at the ends of the shafts of a transmission having a case made from an aluminum alloy or other light weight material having a thermal coefficient of expansion greater than the steel used to manufacture the bearings and the shafts. The unique design of the bearing has the capability to compensate for differential thermal expansion and contraction between the case, and the bearings and shaft within the case. As a result, the bearings remain at a more uniform setting over a wider range of operating temperatures.  
         [0012]     Additional features of the present invention will be in part apparent and in part pointed out hereinafter.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]     In the accompanying drawings which form part of the specification and wherein like numerals and letters refer to like parts wherever they occur:  
         [0014]      FIG. 1  is a sectional view of one embodiment of the present invention as mounted would be mounted in a transmission device.  
         [0015]      FIG. 2  is a sectional view of one embodiment of the bearing of the present invention showing a tapered roller bearing having thermally compensating components.  
         [0016]      FIG. 3  is a sectional view of a second embodiment of the bearing of the present invention showing a tapered roller bearing having thermally compensating components.  
         [0017]      FIG. 4  is a sectional view of a third embodiment of the bearing of the present invention showing a tapered roller bearing having thermally compensating components. 
     
    
       [0018]     While three embodiments of the present invention are illustrated in the above referenced drawings and in the following description, it is understood that the embodiments shown are merely for purpose of illustration and that various changes in construction may be resorted to in the course of manufacture in order that the present invention may be utilized to the best advantage according to circumstances which may arise, without in any way departing from the spirit and intention of the present invention, which is to be limited only in accordance with the claims contained herein.  
       DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings, one embodiment of the present invention is shown that includes a transmission device A ( FIG. 1 ) having a case  1  that is cast from a lightweight metal such as aluminum alloy. The transmission device A also has an input shaft  2  and an output shaft  3 , with each of the two shafts  2  and  3  having an end  4  and  5  respectively. The shafts  2  and  3  carry gears  6  and  7 , which mesh in different combinations to produce different speed ratios between the input shaft  2  and the output shaft  3 . The shafts  2  and  3  are machined from steel, as are the gears  6  and  7  on them.  
         [0020]     The input shaft  2  rotates in two single row tapered roller bearings  8  and  9  that fit around it and within a bore  10  in the wall  11  at each end of the case  1 , the bearings  8  and  9  being located between abutments: that is, a shoulder  12  at the end of the bore  10  and another shoulder  13  on the shaft  2 . The end  4  of the input shaft  2  rotates on another single row tapered roller bearing  9  located in a bore  14  at the opposite end wall of the case  1 . It is also located between abutments in the form of a shoulder  15  at the end of the bore  14  and a backing face  16  on the shaft  2 . The output shaft  3  rotates in a similar manner between bearings  17  and  18  in the walls of the case  1 .  
         [0021]     Each of the bearings  8 ,  9 ,  17 , and  18  has an axis X of rotation which lies coincident with the axis X of the shaft  2  or  3  which it supports, and being a single row tapered roller bearing, it includes ( FIG. 1 ) a cone  19  which fits around one of the shafts  2  or  3 , a cup  20  which fits into one of the bores  10  or  14  and around the cone  19 , tapered rollers  21  which are arranged in a single row between the cone  19  and cup  20 , and a cage  22  for maintaining the proper spacing between the rollers  21 . The cup  20  remains essentially stationary in the case  1 , while the cone  19  rotates within the case  1  as its particular shaft  2  or  3  turns about its axis X of rotation.  
         [0022]     The cone  19  has a bore  23  ( FIG. 2 ), which is slightly smaller than the shaft  2  or  3  over which the cone  19  fits, so that an interference fit exists between the cone  19  and its shaft. It also has a tapered raceway  24 , which is presented, outwardly toward the cup  20 . The raceway  24  lies between a thrust rib  25  and a retaining rib  26 , both of which project outwardly beyond the raceway  24 . The two ends of the cone  19  are squared off with respect to the axis X of rotation, the end at the thrust rib  25  forming a cone back face  27 .  
         [0023]     The cup  20  has an outwardly presented cylindrical surface  28  which may be slightly smaller or slightly larger than the bore  10  or  14  into which it fits, depending on whether an interference or loose fit is desired. In addition, the cup  20  has a tapered raceway  29 , which is presented inwardly toward the tapered raceway  24  of the cone  19 . The ends of the cup  20  are squared off with respect to the axis X, the larger of the end faces, which is at the small end of the tapered raceway  29 , forming a cup back face  30 .  
         [0024]     The tapered rollers  21  lie in a single circumferential row between the raceways  24  and  29  of the cone  19  and cup  20  with their large end face presented toward the thrust rib  25  of the cone  19 . The thrust rib  25  prevents the rollers  21  from being expelled from the space between the two raceways  24  and  29  when a radial load is transmitted through the rollers  21 . Moreover, the rollers  21  are on apex, meaning that if the side faces of the rollers  21  were extended to their respective apexes, those apexes would lie at a common point along the axis X, and the same holds true with regard to the two raceways  24  and  29 .  
         [0025]     The taper of the cone raceway  24  and the cup raceway  29 , together with the taper of the rollers  21  which fit between them, enables the bearings  8 ,  9 ,  17 , and  18  ( FIG. 1 ) to transmit radial loads and axial loads, with the latter being resisted by shoulders  12  and  15  at the ends of the bores  10  and  14  and by the shoulders  13  and  16  on the shafts  2  and  3 . In this regard, the cone  19  ( FIG. 2 ) of the bearing  8  fits tightly around the input shaft  3  with its back face  27  against the shoulder  13 . The cup  20  of that bearing fits snugly in the bore  10 . While the cone  19  for the other bearing  9  ( FIG. 2 ) fits snugly around the input shaft  2 , the cup  19  ( FIG. 1 ) of that bearing fits loosely in the bore  14  with its back face presented toward, but not contacting, the shoulder  15  at the end of the bore  14 . Similarly, the cones of the two bearings  17  and  18  for the output shaft  3  fit snugly around the output shaft  3  with their back faces against the shoulders  31  on the output shaft  3 . The cup of the bearing  17  fits snugly within its bore  32  where its back face is against the shoulder  33  at the end of the bore  32 . On the other hand, the cup of the bearing  18  fits loosely into the bore  41 , and while its back face is presented toward the shoulder  35  at the end of that bore, it does not actually contact the shoulder  35 . The cage  22  of each bearing  8 ,  9 ,  17 , and  18  maintains a slight separation between adjacent rollers  21 . It further holds the rollers  21  around the cone raceway  24  when the cone  19  is removed from the cup  20 . The bearings  8  and  9  of the input shaft  2 , and the bearings  17 , and  18  of the output shaft  3  are located along the common axis X of the input and output shafts  2  and  3 , and operate at a common setting. That common setting depends on the location of the cups  19  ( FIG. 2 ) for the two bearings  8  and  9 , or  17  and  18 , that are in the transmission device case  1 —or at least is controlled by the location of those cups  20 . For example, if the cups  20  are spread too far apart, the shafts  2  and  3  will be loose between the cups  20 , or in other words, will be in a condition of end play. On the other hand, if the cups  20  are too close together, the bearings  8  and  9 , or  17  and  18 , and those portions of the shafts  2  and  3  that are between them, will be in a state of compression, or in other words, in a condition of preload.  
         [0026]     When subjected to temperature variations, the case  1 , being formed from an aluminum alloy having a high thermal coefficient of expansion, undergoes greater dimensional changes than the shafts  2  and  3 , which are formed from steel. In fact, aluminum alloy has about twice the coefficient of thermal expansion as does steel. Thus, an elevation in temperature of the entire transmission device A will cause the end walls of the case  1  to spread farther apart and they of course will carry the shoulders  12  &amp;  15 , and  33  &amp;  35 , that locate the cups  20  of the bearings  8 ,  9 ,  17 , and  18 , outwardly with them. The shafts  2  and  3  will also grow and this spreads the backing shoulders  13 ,  16 , and  31  on the aligned shafts  2  and  3  farther apart. But, due to the substantial difference in the thermal coefficient of expansion between aluminum alloy and steel, the increase in distance between the shoulders  12  and  15  of the bores  10  and  14  can be about twice as great as the increase in the distance between the backing shoulders  13 ,  16 , and  31  on the shafts  2  and  3 . This differential expansion could significantly alter the setting of the bearings  8 ,  9 ,  17 , and  18  were it not for a compensating ring  34  ( FIG. 2 ) of the present invention.  
         [0027]     More specifically, in the present embodiment, an annular U-shaped support ring  35  is located at the front face  30  of the cup  20 , such that the bottom surface  37  of the annular U-shaped support ring  35  is against the back face  38  of the cup  20 . In the present embodiment, the annular U-shaped support ring  35  is made from steel and is annular with respect to the axis X. The compensating ring  34  is generally rectangular in shape and is positioned inside the annular U-shaped support ring  35 , the top surface  39  of the compensating ring  34  being between the flanges  40  of the annular U-shaped support ring  35 . The compensating ring  34  of the current embodiment is made from a resilient material. Some polymers are suitable for this purpose including some polymers, which are elastomers. One such elastomer is sold by E. I. duPont de Nemours under the trademark VITON. This elastomer has a coefficient of thermal expansion of about  
         [0028]      120 × 10   −6  in/in/degree F. Other resilient materials may be used as long as the coefficient of thermal expansion is greater than the coefficient of thermal expansion of the material used to manufacture the case  1  of the transmission device A.  
         [0029]     A backing ring  36  is positioned between the compensating ring  34  and the shoulder  12  of the case  1 . In the present embodiment, the backing ring  36  is made of steel. The backing ring  36  is sized to fit between the two flanges  40  of the annular U-shaped support ring  35  with the fit between the two flanges  40  being tight enough to allow the backing ring to remain between the two flanges to hold the compensating ring  34  in position, but loose enough to allow the backing ring to be pushed away from the annular U-shaped support ring  35  when the compensating ring  34  expands after being warmed to a higher temperature.  
         [0030]     The compensating ring  34  maintains all of the bearings  8 ,  9 ,  17 , and  18  that are along the two shafts  2  and  3  at a generally uniform setting over a wide range of temperature variations. Should the transmission device A experience an increase in temperature, its case  1  will expand more than the two shafts  2  and  3 . However, because the coefficient of thermal expansion of the compensating ring  34  is greater than that of the case  1 , the compensating ring  34  will maintain the spread between the two bearings  8  and  9 , or  17  and  18 , consistent with that of the expansion of the two axially aligned shafts  2  and  3 . To this end, as the case  1  expands, thus moving apart the shoulders  12  and  15 , or  33  and  35 , which confine the cups  20  of the bearings  8  and  9 , or  17  and  18 , the compensating ring  34  likewise expands axially and forces the cup  20  for the bearings  8  and  17  farther from the shoulders  12  and  33 . The distance that cup  20  for the bearing is displaced corresponds roughly to the difference in expansion between the case  1  and the two shafts  2  and  3  measured in the region between the two bearings  8  and  9 , or  17  and  18 , less any axial offset caused by axial expansion in the bearings.  
         [0031]     Of course, when the transmission A experiences a decrease in its operating temperature, the opposite occurs. The compensating ring  34  will axially contract about the same as the difference between the contraction of the case  1  and two shafts  3  and  4 , less the axial offset caused by contraction of the bearings so that the setting for the bearings remains essentially the same. Thus, the compensating ring  34  compensates for differential thermal expansions and contractions between the case  1  and the axially aligned shafts  2  and  3  that are within the case  1 .  
         [0032]     As a result of the thermal compensation provided by the compensating rings  34  of the two bearings  8  and  17 , the bearings along the aligned shafts  3  and  4  do not experience excessive preload at cold temperatures. Additionally, the compensating rings  34  eliminate excessive end play in the bearings  8 ,  9 ,  17 , and  18  at higher operating temperatures, and this causes a better distribution of loads within those bearings, extends their lives, and improves machine reliability. Also, the compensating rings  34  expand radially, although slightly, and this tends to prevent the cups  20  in which they are located from rotating in the bores  10 ,  14 ,  32 , and  41  for the cups  20 . The compensating rings  34  may also serve to dampen vibrations in the shafts  2  and  3 , and this together with the reduction in end play may reduce the noise generated by the transmission device A.  
         [0033]     It is understood that while the present embodiment of the invention shows only bearings  8  and  17  as having thermal compensating rings, in other embodiments the bearings for the input shaft  2  and the output shaft  3  may also have thermal compensating rings depending upon the specific application.  
         [0034]     The length “l” of the compensating ring  34  depends on a number of factors including the distance (d c ) between the case shoulders  12  and  15 , or  33  and  35 , the distance (d s ) between shaft shoulders  13 ,  16 , or  31  and backing face  27 , the coefficient (C Al ) of the thermal expansion for the aluminum alloy of the case  1 , the coefficient (C St ) of thermal expansion for the steel of the shafts  2  or  3 , the coefficient (C p ) of thermal expansion for the compensating ring  34 , the temperature differential (AT), and the geometry of the bearings. To determine the length l, one first calculates the maximum setting change (MSC) that results from the maximum change in temperature from ambient. This calculation not only considers the differences between the expansion of the case  1  and the shafts  2  and  3 , but also the offsetting difference in the stands of the bearings  8  and  9 , or  17  and  18 , which occur primarily as a result of radial and axial expansions within the bearings themselves. In this regard, the geometry of a single row tapered roller bearing is such that the radial and axial expansion resulting from an increase in temperature will enlarge the stand of the bearing, that is to say the bearing will experience an increase (b) in the distance between the back face  27  of its cone  19  and the back face  37  of its cup  20 . Formulas familiar to bearing engineers exist for calculating the increase (b) in the stand of a tapered roller bearing.  
         [0035]     The maximum setting change (MSC) is calculated using the following formula: 
 
 MSC=[d   c ( C   Al )− d   s ( C   st )](Δ T−εΔb )
 
         [0036]     where: 
        i. εΔb is the sum of the changes in the stands for the bearings  8  and     ii.  9 , or  17  and  18  in the case  1  of the shafts  2  and  3 .        
 
         [0039]     The length l of the insert is derived from the following formula:  
       L   =     MSC       (     C   p     )     ⁢     (     Δ   ⁢           ⁢   T     )             
 
         [0040]     As an example, assume the bearings  8  and  9  on the steel input shaft  2  have the cup back faces  37  set 13.00 inches apart; that the distance between cone back faces  27  is 10.00 inches; that the ambient temperature is 70 degrees F.; that the normal operating temperature is 220 degrees F., and that the coefficient (C p ) of expansion for the compensating ring  34  is 120×10 −6  in/in/degree F. Aluminum has a coefficient (C Al ) of expansion of 13×10 −6  in/in/degree F., while the coefficient (C St ) for steel is 6.5×10 −6  in/in/degree F. Also assume the sum of the changes (εΔb) in the stands of the two bearings  8  and  9  amounts to 0.005 inches. The maximum setting change (MSC) as the temperature of the transmission device A rises from 70 degrees F. to 220 degrees F. amounts to: 
 
 MSC└ 13   (13×10 −6 )−10(6.5×10 −6 )┘(220−70)−0.005=0.011 in.
 
         [0041]     The compensating ring  34  must have a length l of:  
       l   =       0.011       (     120   ×     10     -   6         )     ⁢   150       =     0.611   ⁢           ⁢     in   .             
 
         [0042]     It is appreciated that because the compensating ring  34  is confined radially as well as axially, and indeed retained in a state of axial compression when the transmission A is at ambient temperature, the volumetric expansion of the material in the compensating ring  34  is in effect converted into a linear expansion. In other words, the compensating ring  34 , being confined both radially and circumferentially, experiences only axial expansion from an increase in temperature, and what may have otherwise occurred as radial and circumferential expansion, manifests itself as linear expansion. In short, the radial confinement produces a volumetric condition in which the coefficient of linear expansion is increased. In order to utilize the volumetric principle of compensation, the material of the insert should be somewhat flexible, and for this reason elastomers, such as the elastomer sold under the trademark VITON, are generally better suited than more rigid polymers.  
         [0043]     Thus, when the length I of the compensating ring  34  for the forgoing example is calculated on a volumetric basis, it becomes:  
       l   =         0.011       (     120   ×     10     -   6         )     ⁢   150       -     1   3       =       0.611   3     =     0.204   ⁢           ⁢     in   .               
 
         [0044]      FIG. 3  shows another embodiment of the present invention A wherein the compensating ring  50  fits between and is captivated by an L-shaped support ring  51 , a backing ring  52 , and the case  1  of the transmission device A. The materials used and the operation of the compensating ring  50 , L-shaped support ring  51 , and the backing ring  52  are the same as described in the first embodiment above. Additionally, the above formulae may be used for this second embodiment.  
         [0045]      FIG. 4  shows yet another embodiment of the present invention wherein the compensating ring  60  is held in place by a cylindrical support ring  61  and the backing ring  62 . Again the materials used, the operation, and the necessary calculations for this third embodiment are the same as for the first embodiment.  
         [0046]     It will be appreciated that the compensating ring in each of the above embodiments may have one surface attached to either the bearing cup, the backing ring, or the support ring. This attachment retains the compensating ring within the assembly to prevent repositioning of the compensating ring and to reduce the possibility of any wedging of the compensating ring between any of the bearing components. It is understood that the method by which the compensating ring is retained can be accomplished by using adhesives, chemical welding, threaded or non-threaded fasteners, or any other mechanical methods that would prevent the compensating ring from moving from its preferred position.  
         [0047]     While the above description describes various embodiments of the present invention, it will be clear that the present invention may be otherwise easily adapted to fit any configuration where a bearing having thermal compensating capability is required.  
         [0048]     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Technology Classification (CPC): 5