Patent Publication Number: US-2018031041-A1

Title: Low creep bearing and method for installing in supercharger

Description:
FIELD 
     This application relates to damping techniques in a supercharger and provides a bearing installation for damping rotor shaft motion in a supercharger. 
     BACKGROUND 
     Roots style, or twin rotor, superchargers are subject to chatter as rotating lobes mesh. The chatter causes the rotor shaft to shift in the supercharger housing. Prior art bearings creep in the housing due to chatter and shifting. 
     Tolerance stack-up can contribute to this chatter, causing vibrations during operation because space exists between parts, for example, in a bearing, between the bearing and the shaft, and between the bearing and the housing of the supercharger. These spaces can expand and contract due to thermal expansion caused by both changes in operating temperature and changes in ambient temperature. 
     Vibrations can also cause the rotors in a Roots style supercharger to “walk,” that is, move in an axial direction. Walking is undesirable as it can decrease the performance of the supercharger and damage coatings, surfaces, and other parts. 
     Shear forces created by tolerance stack-up and normal operating conditions can cause parts to deform over time, often referred to as creep. 
     Conventional superchargers use low viscosity lubricants to lubricate parts, including bearing parts. These low viscosity lubricants are often applied using a pressurized lubricant feed. These low viscosity lubricants can fill some of the spaces, but they do not provide effective damping capability or resistance to shear forces. 
     SUMMARY 
     The disclosure overcomes the above disadvantages and improves the art by way of using a high viscosity damping grease in a supercharger. The supercharger can be of the Roots style, parallel lobe or twin screw lobe, among other styles. 
     A supercharger assembly comprises a housing, a shaft bore in the housing, a shaft extending into the shaft bore, wherein the shaft comprises an axis, and a bearing assembly located between the shaft bore and the shaft. The bearing assembly comprises an outer ring. The outer ring comprises an outer face. A grease is located between the outer face of the outer ring and the shaft bore. The grease has a viscosity greater than International Standards Organization Viscosity Grade 100. 
     A method of assembling a supercharger comprises the steps of installing a shaft into a shaft bore and press-fitting a bearing onto a shaft, wherein the bearing comprises an outer ring. The outer ring comprises an outer face. The method further comprises injecting a layer of grease between the outer face of the outer ring and the shaft bore, wherein the grease has a viscosity greater than International Standards Organization Viscosity Grade 100. 
     A supercharger assembly comprises a housing, a shaft bore in the housing and a shaft extending into the shaft bore. The shaft comprises an axis. The supercharger assembly comprises a bearing assembly located between the shaft bore and the shaft. The bearing assembly comprises an outer ring. The outer ring comprises an outer face. The bearing assembly further comprises a grease located between the outer face of the outer ring and the shaft bore. The grease has a base oil viscosity greater than 1,000 centistokes at 25 degrees centigrade. 
     Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure. The objects and advantages will also be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section view of a supercharger. 
         FIG. 2A  is a cross-section view of a bearing installation. 
         FIG. 2B  is another cross-section view of a bearing installation. 
         FIG. 3  is another cross-section view of the supercharger. 
         FIG. 4A  is another cross-section view of the supercharger. 
         FIG. 4B  is an enlarged view of area X of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the examples which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Directional references such as “left” and “right” are for ease of reference to the figures. 
       FIG. 1  shows an example of a supercharger assembly  10  comprising rotors  30 ,  31  in housing  20 . Shafts  40 ,  41  are positioned in the center of rotors  30 ,  31  along axes A, B. Shafts  40 ,  41  fit into shaft bores  22 ,  23  at one end of axes A, B and into transfer case  50  at the other end of axes A, B. 
     Shafts  40 ,  41  fit into bearings  60 ,  61 , all of which are located in shaft bores  22 ,  23 . One can fit shafts  40 ,  41  into bearings  60 ,  61  before or after placing shafts  40 ,  41  into shaft bores  22 ,  23 . For example, one can first press-fit first bearing  60  onto first shaft  40 , then slip-fit first shaft  40  with first bearing  60  into first shaft bore  22 . Or one can first slip-fit first bearing  60  into first shaft bore  22 , then press-fit first shaft  40  into first bearing  60 , which is already positioned in first shaft bore  22 . 
     Shaft bores  22 ,  23  can also include compression springs  70 ,  71 , which can serve to apply a spring force to bearings  60 ,  61  along axes A, B. Compression springs  70 ,  71  can rest against a steps  220 ,  221  on one end of compression springs  70 ,  71  and against bearings  60 ,  61  on the other end, creating pressure against bearings  60 ,  61 . Compression springs  70 ,  71  can be preloaded. This arrangement can reduce the axial movement of rotors  30 ,  31  during operation of the supercharger assembly. Compression springs  70 ,  71  can also damp axial vibrations, reducing the overall chatter of the supercharger assembly. Because chatter is reduced, bearings  60 ,  61  can be smaller than prior art bearings. 
       FIG. 2A  shows a cross-section of bearing assembly  60  as it fits in housing  20 . First shaft  40  can have a step  42  that abuts inner ring  618 . One can press-fit the end  43  of first shaft  40  into bearing  60  so that inner ring  618  and end  43  contact each other at the outer face  619  of inner ring  618 , fastened together by friction created by the interference where the outer face  619  of inner ring  618  contacts end  43 . 
     Superchargers often include a gap G between the housing  20  and bearing  60 . This gap G might not have any material separating bearing  60  from housing  20 , thereby creating an open space. The open space increases the vibration of bearing  60 . While vibrating, bearing  60  repeatedly contacts housing  20 , creating unwanted noise during operation of the supercharger. 
     In some superchargers, gap G is filled with a bore lubricant  613  often used to lubricate parts of the supercharger. This lubricant can be the same lubricant as bearing lubricant  612  found in bearing  60 . The lubricant, however, does not have the damping capability of grease  602 . 
     The gap G can increase during the operation of the supercharger when housing  20  is made of a different material than bearing  60 . Housing  20  is often made from aluminum while bearing  60  is often made from steel. Aluminum has a higher rate of thermal expansion than steel. A supercharger can heat up due to many factors, including an increase in the engine operating temperature or an increase in the ambient temperature. If heated, the aluminum housing will expand more than the steel bearing. Thus any gap between the housing and the bearing will increase. 
     A bearing assembly can be fit into a supercharger housing at a shaft bore such that there is an interference fit between the shaft bore and the bearing. With such a fit the gap G would equal zero. This gap, however, can increase with the change in temperature, thus, losing the interference fit and creating open space where vibration occurs. 
     To reduce the maximum gap G experienced during thermal expansion, one can create more interference in the interference fit by increasing the diameter of outer ring  604  of the bearing, reducing the diameter of shaft bore  22 , or both increasing diameter of the outer ring of the bearing and decreasing the diameter of the shaft bore. This approach, however, can decrease the performance of the supercharger assembly. The interference fit can create unwanted loads on roller elements (e.g., roller elements  608 ) and on shafts (e.g., shaft  40 ). These loads can deform the rollers, internal bearing components, and shafts. 
     Filling gap G with grease  602  alone or in combination with o-rings  606 ,  607  can reduce these negative affects. For example, grease  602  can damp vibrations and noise not otherwise damped. Using grease  602  to damp vibrations can reduce the radial and axial movement of shafts  40 ,  41 , thus, reducing walking between the supercharger housing  20  and bearing  60 . 
     It can also prevent outer ring  604  from contacting shaft bore  22  during operation. One can also use grease  602  to provide better shear resistance at the interface between shaft bore  22  and outer face  605  of outer ring  604 . 
     Grease  602  can also have a high resistance to creep. Creep, or deformation over time, can occur on parts, especially metal parts such as steel or aluminum, when those parts are exposed to loads over a long period of time. This deformation can increase the noise and vibration during operation and even cause the supercharger assembly to fail. 
     Having a viscosity greater than International Standards Organization Viscosity Grade (ISO VG) 100 allows a grease  602  to have exceptional damping capability. Greases such as damping grease by Nye Lubricants, Inc. can perform well. High-viscosity damping greases ranging from 1,000 centistokes (cSt) at 25 degrees centigrade to 50,000 cSt or more at 25 degrees centigrade provide excellent damping capability. Some high-viscosity damping greases can withstand temperatures ranging from −40 degrees centigrade to 120 degrees centigrade without immiscibly separating. 
     Having grease  602  located between bearing  60  and shaft bore  22  can reduce the tolerance between bearing  60  and shaft bore  22 . Allowing for more expansion of bearing  60  and housing  20  during operation, grease  602  can reduce other tolerances required, for example, the tolerance distance between shaft end  42  and inner ring  618  of bearing  60 . Thus, grease  602  can reduce the overall stack up of tolerances in a supercharger assembly. 
     Grease  602  does not need a pressurized feed or sump mechanism to maintain its location in the shaft bore. The viscosity of the grease  602  is such that it is placed in the shaft bore and plugged in place. It does not require continual replacement like prior art squeeze-film dampers. It is also more effective at eliminating squeal and other NVH conditions. 
     The bearing assembly need not have o-rings. Grease  602  can be an unbounded layer between bearing assembly  60  and outer ring  604 . O-rings  606 ,  607 , however, can prevent grease  602  from traveling beyond the space in between first o-ring  606  and second o-ring  607 . This allows grease  602  to dampen vibrations and help protect the bearing  60  at areas most likely to contact housing  22 . 
     O-rings  606 ,  607  can also damp vibrations. The material and size of o-rings  606 ,  607  can be selected so that o-rings  606 ,  607  damp vibrations and noise of a different frequency than those damped by grease  602 . This gives the supercharger assembly the ability to dampen a wider range of frequencies. 
     It&#39;s advantageous to use o-rings  606 ,  607  that have high resistance to creep so that o-rings  606 ,  607  prevent grease from leaking beyond o-rings  606 ,  607  during the useful lifespan of grease  602 . 
     O-rings  606 ,  607  can also form an interference fit with shaft bore  22  and outer ring  604 . This fit not only helps seal grease  602 , but it also helps keep bearing  60  from moving axially along axes A. 
     O-rings  606 ,  607  can include a radial spring that provides further damping capability. The radial spring can also center bearing  60  and also provide compression force to keep o-rings  606 ,  607  from moving in the axial direction along axes A. 
     O-rings  606 ,  607  might fit into circumferential groves (not shown) along outer ring  604 . In a similar way, shaft bore  22  might have circumferential groves (not shown) to accommodate o-rings  606 ,  607  to provide better sealing capability. 
     When sizing o-rings  606 ,  607 , one can consider the maximum and minimum distance of gap G experienced during the operation of the supercharger. For example, gap G might be significantly larger when housing  20  heats up, whether due to engine operation or changes in ambient temperature. One can also consider the desired amount of grease  602  when determining the distance of gap G. 
     Bearing lubricant  612  has a lower viscosity than grease  602 . It can be more advantageous for bearing lubricant  612  to provide lubrication rather than damp vibrations. 
     As shown in  FIG. 2B , bore lubricant  613  can serve to lubricate the bearing assembly as the bearing assembly is installed in the shaft bore, or as the bearing assembly is installed on the shaft. The bore lubricant  613  can have a viscosity similar to bearing lubricant  612 . Or, bore lubricant can differ from bearing viscosity by being higher or lower in relative viscosity. 
     Bearing lubricant  612  surrounds roller elements  608 , which are illustrated as ball bearings. The viscosity of bearing lubricant  612  can be governed by bearing speed, load, and temperature. One can select bearing lubricant  612  to facilitate high rotation rates of rolling elements  608 , but such a lubricant would likely have a very low shear resistance. 
     Bearing lubricant can be located around raceways (e.g., inner raceway  616  and outer raceway  614 ) and roller elements. In addition to ball bearings, one can use needle bearings, roller bearings, or taper bearings. The bearing assembly structure can be race-less with no inner race or it can be cage type having rollers retained in and dropped through a cage to contact the rotor shaft. 
     The bearing  60  can be slip-fit or press-fit to shaft  40  and first shaft bore  22 . Lubricant can facilitate the process. Lubricant can facilitate installation of the bearing on the shaft as by being placed on the shaft or as by being placed on outer face  619  of inner ring  618 . Lubricant could additionally or alternatively be placed on the outer face  605  of outer race, or on the shaft bore. 
     A different lubricant than the lubricant used around roller elements  608  can be used to facilitate bearing assembly installation. For example, bearing lubricant  612  used around roller elements  608  can be different than the bore lubricant applied to outer face  619  of inner ring  618  and to outer face  605  of outer ring  604 . 
     However, because grease  602  can be much more viscous than conventional greases and lubricants (e.g., bearing lubricant  612 , bore lubricant  613  and other lubricants used to facilitate installation), one can use an unconventional assembly method. Instead of lubricating the exterior of the bearing  60  before slip-fitting or press-fitting the bearing into first shaft bore  22 , one can omit the exterior bore lubricant  613  altogether. The bearing  60  can sized with a slightly smaller outer diameter and can be inserted into first shaft bore  22  with o-rings  606 ,  607 . One can then inject grease  602  into gap G. 
     The distance of gap G can be selected to facilitate installing bearing  60 . Using grease  602  in gap G allows one to relax manufacturing tolerances and increase the space between parts because grease  602  can damp and resist motion previously addressed by tightly fitting bearing  60  into first shaft bore  22 . Inner diameter of shaft bore  22  can be larger, or outer bearing diameter can be smaller. 
       FIG. 3  is a cross-sectional view of a supercharger assembly  300 . This arrangement includes a first channel  360  and a second channel  362  allowing fluid communication from outside housing  320  to second shaft bore  323 . One can inject grease (e.g., grease  602  of  FIG. 1 ) into shaft bore  323  through channels  360 ,  362 . One can then contain the grease using plugs  365 ,  366 . 
     One can inject grease into one channel, for example, first channel  360  and monitor second channel  362  for the presence of grease. The presence of grease in second channel  362  indicates that the grease traveled from first channel  360  to second channel  362 . After detecting grease in second channel  362 , one can plug both first channel  360  and second channel  362  so that grease remains in the shaft bore  323  during operation of the supercharger assembly  300 . 
     The grease can be located between bearing  361  and shaft bore  323  in a manner similar to the arrangement in  FIG. 2A , where grease  602  is positioned between first shaft bore  22  and outer ring  604  and retained by o-rings  606 ,  607 . 
       FIG. 4A  shows a cross-section view of a supercharger assembly  400 . 
     A circumferential groove  472  can be located in shaft bore  423 . Channels  460 ,  462  can allow one to inject a grease into shaft bore  423 . One can inject grease into one channel, for example, first channel  460  in a manner that forces the grease to travel along circumferential groove  472  to second channel  462 . 
     Subsection X identifies the area of supercharger assembly  400  where grease is used to dampen vibrations during operation. This area can include a bearing assembly, for example, any one of the bearing assemblies shown in  FIGS. 1-3 . 
       FIG. 4B  is a more detailed view of the area identified by subsection X in  FIG. 4A .  FIG. 4B  shows a shaft  440  located in shaft bore  423 . 
       FIG. 4B  shows a compression spring  471  abutting step  442  of housing  420 . A bearing assembly is not shown, but the compression spring  471  can abut a bearing, thus, exerting an axial force on the bearing assembly. 
     First channel  460  is in fluid communication with second channel  462  such that a high-viscosity grease can be injected into one channel and travel to the other channel along circumferential groove  472 . 
     The ends of first channel  460  can be defined by first hole  481  and second hole  482 . The ends of second channel  462  can be defined by fourth hole  484  and third hole  483 . Reference to the ends as first, second, third, or fourth hole is only for convenience to the reader. Any one of these holes can be plugged. Grease can be injected into the holes by many different ways. For example, one can inject grease into first hole  481  or fourth hole  484 . Likewise, one can monitor any one or all of these holes for the presence of grease after injecting the grease. 
     Supercharger assembly  400  need not have both channels  460 ,  462 . Supercharger assembly  400  can be arranged such that a high-viscosity grease (e.g., grease  602  of  FIG. 2A ) is injected into one channel, for example, first channel  460 . After being injected into first channel  460 , the high-viscosity grease can travel along circumferential groove  472  until it fills circumferential groove  472 . After filling circumferential groove  472  with high-viscosity grease, one can plug first channel  460 . 
     Other implementations will be apparent to those skilled in the art from consideration of the specification and practice of the examples disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.