Abstract:
According to at least one exemplary embodiment, a variable preload device is disclosed. The variable preload device may include at least two annular end members, each annular member adapted to receive a shaft therein, and a plurality of plates axially extending between the annular end members and hingedly coupled thereto. The plurality of plates may be adapted to bend inward so as to impart an hourglass configuration to the variable preload device when the variable preload device is at rest and to bend outward in proportion to increased rotational speed provided to the variable preload device. Furthermore, the axial distance between the annular end members may varies in proportion to the rotational speed provided to the variable preload device.

Description:
BACKGROUND 
       [0001]    Bearing devices, namely rolling-element bearings, are typically used to support a rotating shaft or spindle within a housing. Such bearings include an outer race coupled to the housing, an inner race coupled to the shaft, and a plurality of rolling elements disposed between and in contact with both races. The motion of the bearing components relative to each other allows the shaft to rotate within the housing while minimizing friction and resistance to rotation. Typically, bearings are assembled such that there exists a small internal clearance between the rolling elements and the inner and outer races during operation. 
         [0002]    In certain situations, it may be desirable to preload the bearing, i.e., to remove the internal clearance in the bearing and to introduce a negative internal clearance. Bearing preload is commonly introduced by the application of a permanent axial load to at least one a race of the bearing such that the two bearing races are displaced in the axial direction relative to each other. The result is a constant elastic compressive force being applied to the points of contact between the rolling elements and the two races. The internal stress thus generated in turn results in increased stiffness and rigidity, reduction in radial and axial free-play, reduction in vibration, and increase in rotating precision. 
         [0003]    The desired amount of preload may vary with the intended rotational speed range of the shaft to which the bearing is coupled. High bearing preloads are generally suitable for low rotational speeds; however, at high rotational speeds, high preloads result in the increase of frictional forces and the generation of excessive heat, which, in turn, reduces bearing life and may result in bearing failure. It is therefore desirable to lessen the preload for bearings that are expected to operate at high rotational speeds, and to increase the preload for bearings that are expected to operate at low rotational speeds. For machinery capable of operating at both high and low rotational speeds, it becomes desirable to dynamically and automatically alter the bearing preload during operation so that the preload is optimally correlated to the rotational speed. It is further desirable to dynamically and automatically alter bearing preload while minimizing the amount of and complexity of components and logic necessary therefor. 
       SUMMARY 
       [0004]    According to at least one exemplary embodiment, a variable preload device is disclosed. The variable preload device may include at least two annular end members, each annular member adapted to receive a shaft therein, and a plurality of plates axially extending between the annular end members and hingedly coupled thereto. The plurality of plates may be adapted to bend inward so as to impart an hourglass configuration to the variable preload device when the variable preload device is at rest and to bend outward in proportion to increased rotational speed provided to the variable preload device. Furthermore, the axial distance between the annular end members may vary in proportion to the rotational speed provided to the variable preload device. 
         [0005]    According to another exemplary embodiment, a rolling-element bearing assembly is disclosed. The assembly may include a housing, a spindle arbor, at least two bearings each having an inner race keyed to the spindle arbor and an outer race keyed to the housing, at least two end plates keyed to the spindle arbor, each end plate operatively coupled to an inner race of one of the at least two bearings, a variable preload device substantially concentric with, surrounding, and keyed to the spindle arbor, and extending between the at least two end plates and operatively coupled thereto, and an opposing spring force acting on an inner race of at least one of the at least two bearings. The variable preload device of the rolling-element bearing assembly may exert an axial force on an inner race of a bearing, the axial force acting in opposition to the opposing spring force and having a magnitude proportional to the rotational speed of the spindle arbor, so as to increase the internal clearance of the bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]      FIG. 1   a  is a side view of an exemplary embodiment of a centrifugal variable preload device. 
           [0007]      FIG. 1   b  is an end view of an exemplary embodiment of a centrifugal variable preload device. 
           [0008]      FIG. 2   a  shows an exemplary embodiment of a centrifugal variable preload device in a low-speed configuration. 
           [0009]      FIG. 2   b  shows an exemplary embodiment of a centrifugal variable preload device in a high-speed configuration. 
           [0010]      FIG. 3   a  is a cross-section of exemplary embodiment of a centrifugal variable preload device in a typical operative environment under low-speed conditions. 
           [0011]      FIG. 3   b  is a detail of  FIG. 3   a  showing a high-preload configuration. 
           [0012]      FIG. 4   a  is a cross-section of exemplary embodiment of a centrifugal variable preload device in a typical operative environment under high-speed conditions. 
           [0013]      FIG. 4   b  is a detail of  FIG. 4   a  showing a low-preload configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Alternate embodiments may be devised without departing from the spirit or the scope of the invention. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. Further, to facilitate an understanding of the description discussion of several terms used herein follows. 
         [0015]    As used herein, the word “exemplary” means “serving as an example, instance or illustration.” The embodiments described herein are not limiting, but rather are exemplary only. It should be understood that the described embodiment are not necessarily to be construed as preferred or advantageous over other embodiments. Moreover, the terms “embodiments of the invention”, “embodiments” or “invention” do not require that all embodiments of the invention include the discussed feature, advantage or mode of operation. 
         [0016]      FIGS. 1   a - 1   b  show an exemplary embodiment of a centrifugal variable preload device  100 . Variable preload device  100  may include a first annular end member  102 , a second annular end member  104 , and a plurality of plates  106  extending therebetween. Each plate  106  may have a first end coupled to first annular end member  102  and a second end coupled to second annular end member  104 , thereby defining a cylindrical inner cavity  108  which may be capable of receiving a spindle arbor. 
         [0017]    Each plate  106  may include a central groove  110  defined therein. Central groove  110  may be disposed at the midpoint of the longitudinal axis of plate  106  and may be situated perpendicular to said longitudinal axis. Central groove  110  may have a depth and profile that facilitate the bending of plate  106  between a linear configuration and an angled configuration, wherein central groove  110  may be the vertex of the resultant angle α. Variable preload device  100  may further include a first circumferential groove  112  and a second circumferential groove  114 . First circumferential groove  112  may be disposed proximate to first annular end member  102 . Second circumferential groove  114  may be disposed proximate to second annular end member  104 . Each of circumferential grooves  112 ,  114  may have a depth and profile that facilitate changing the angle of a plate  106  relative to the corresponding annular end member  102 ,  104  by allowing for flexing at the grooves. It should be appreciated that grooves  110 ,  112 ,  114  facilitate transforming the shape of variable preload device  100  between an hourglass configuration, as shown in  FIG. 2   a , and a cylindrical configuration, as shown in  FIG. 2   b , and thereby altering the length and inner diameter D of variable preload device  100 . 
         [0018]    In some embodiments, variable preload device  100  may be formed from a unitary piece of a material that allows device  100  to function as described herein. Such a material may be any material known in the art that is capable of resisting fatigue at flex joints such as grooves  110 ,  112 ,  114 . For example, variable preload device  100  may be formed from a high-stiffness, low-friction and fatigue-resistant material, such as polyoxymethylene. In other embodiments, variable preload device may be formed from nylon, spring steel, or other materials having similar properties. In other embodiments, variable preload device may be formed from a combination of materials selected so as to impart device  100  with the desired characteristics. In such embodiments, the flex joints of device  100 , i.e. parts of device  100  proximate to grooves  110 ,  112 ,  114 , may be formed from a different material than the other components of device  100 , and coupled with the other components of device  100  via adhesion, fusion, or any other desired coupling that enables device  100  to function as described herein. 
         [0019]    Further referring to  FIGS. 2   a - 2   b , the length L of centrifugal variable preload device  100  may vary in relation to the configuration of variable preload device  100 . When variable preload device  100  is in an hourglass configuration, angle α may be equal to a minimum angle α h . Minimum angle α min  may be any desired value that enables variable preload device  100  to function as described herein, and may be varied as desired. For example, minimum angle α min . may be varied by altering the profile of central groove  110  such that contact between the side walls of central groove  110  impedes bending of a plate  106  beyond the desired angle. In other embodiments, such bending of plate  106  may be impeded by contact between the inner surface of a plate  106  and the surface of a spindle arbor over which variable preload device  100  may be fitted. When α=α min , the length L of variable preload device  100  may likewise be equal to minimum length L min  and inner diameter D of variable preload device  100  may likewise be equal to minimum inner diameter D min . Thus, a desired angle α min  may be chosen based on a desired minimum length L min , on a desired minimum inner diameter D min  (which can be based on the diameter of the spindle arbor passing through inner cavity  108 ) or any other desired considerations or combination thereof. 
         [0020]    Imparting rotational motion to variable preload device  100  can generate a resultant centrifugal force that increase in relation with increasing rotational speed. The application of such centripetal force to variable preload device  100  may then result in the reduction in the bending of plates  106  as the vertices thereof, which are disposed along central groove  110 , may be induced to move outward from the longitudinal axis of variable preload device  100 . That is, angle α increases proportionally with the rotational speed of variable preload device  100 , and, due to the configuration of variable preload device  100 , length L increases proportionally with the increase of angle α. Thus, ΔL=kω=k′F, where ΔL is the change in length, ω is the rotational speed imparted to variable preload device  100 , F is the resultant centrifugal force action on variable preload device  100 , and k, k′ are proportionality constants that are representative of the physical properties of variable preload device  100 . The values of proportionality constants k, k′ may be varied by altering the physical properties of variable preload device  100 . 
         [0021]    Varying the physical properties of variable preload device  100  allows for the tuning of variable preload device  100  such that it is optimized for a desired application or a desired range of operating speeds and such that it achieves the desired amount of preload. Such physical properties may include, but not limited to, the density of the material from which variable preload device  100  is formed, the weight of variable preload device  100  and the distribution of mass therethrough, the length and diameter of variable preload device  100 , and the resistance to bending of grooves  110 ,  112 ,  114 . Such resistance to bending may further be varied as desired by altering the lengths of grooves  110 ,  112 ,  114 . In some embodiments, variable preload device  100  may include a plurality of apertures  116  and cutouts  118 . Apertures  116  and cutouts  118  may be defined such that they are disposed along grooves  110 ,  112 ,  114 , thereby effectively lessening the length of grooves  110 ,  112 ,  114  and consequently reducing the rigidity and resistance to bending thereof. The dimensions of apertures  116  and cutouts  118  may be altered during or after manufacture of variable preload device  100 , thereby allowing variable preload device  100  to be easily optimized for a desired operating range. 
         [0022]    Variable preload device  100  may also be tuned by the addition of mass to device  100 . For example, inserts or weights (not shown) may be added to plates  106  to increase the mass of plates  106 . Such weights or inserts may be constructed of materials having various densities, such that the weights or inserts may have a substantially similar size or configuration while having varying mass. For example, the materials for the weights or inserts may be formed from plastics, polymers, metals, and so forth. The weights or inserts may then be coupled to plates  106  by mechanical coupling such as pins or rivets, or by adhesion. In some embodiments, the weights or inserts may be formed as pins, rivets, or other mechanical coupling structures. 
         [0023]    As rotational speed of variable preload device  100  increases, angle α can approach a maximum value α max  of about 180°, and may result in the halves of each plate  106  being substantially coplanar. When α=α max , the length L of variable preload device  100  may likewise be equal to maximum length L max  and variable preload device  100  may have a generally cylindrical configuration. As variable preload device  100  approaches the cylindrical configuration, it furthermore becomes more resistant to axial counterforces exerted thereon when variable preload device  100  is operatively coupled to a bearing or bearings, as described below. 
         [0024]      FIGS. 3   a - 4   b  illustrate an exemplary embodiment of a shaft-supporting apparatus  200 , wherein centrifugal variable preload device  100  may be fitted, keyed, or otherwise coupled to an exemplary spindle arbor  202  such that the rotational speed of spindle arbor  202  is imparted to variable preload device  100 . Variable preload device  100  may further be operatively coupled to exemplary first and second rolling-element bearings  210 ,  220 . First rolling-element bearing  210  may include an inner race  212 , an outer race  214 , and rolling elements  216 . Similarly, second rolling-element bearing  220  may include an inner race  222 , an outer race  224 , and rolling elements  226 . Variable preload device  100  may further include first and second end plates  230 ,  232  and be disposed therebetween. In turn, first end plate  230  may be coupled to first rolling-element bearing  210  and second end plate  232  may be coupled to second rolling-element bearing  220 . 
         [0025]    Apparatus  200  may further include at least one opposing spring force  234  applied to an inner race  212 ,  222  of at least one of bearings  210 ,  220 , thereby providing a constant preload to the at least one bearing. The preload force applied to the inner race of one bearing may be transferred via end plates  230 ,  232  and via variable preload device  100  to the inner race of the complementary bearing. Apparatus  200  may further include a conventional bearing spacer  236  disposed between the outer races  214 ,  224  of bearings  210 ,  220 , thereby maintaining a desired spacing therebetween. 
         [0026]    In some embodiments, variable preload device  100  may further include an outer cylinder  238  disposed between end plates  230 ,  232 . Cylinder  238  may have a diameter greater than the diameter of variable preload device  100  and may have a cavity therein for receiving variable preload device  100 . The diameter of cylinder  238  may further be configured such that it provides a contact surface for transverse plates  100  when α=α max , thereby reducing the likelihood of variable preload device  100  adopting a convex configuration at high rotational speeds. Furthermore, in some embodiments, cylinder  238  may have a length that is greater than minimum length L min  of variable preload device  100 . In such embodiments, cylinder  238  impedes end plates  230 ,  232  from contracting past the length of cylinder  238 . Consequently, the length of cylinder  238  may define an effective minimum length L min  for variable preload device  100 . It should thus be appreciated that the length of cylinder  238  may be adjusted to obtain a desired preload value when variable preload device is at rest or operating at low speeds. 
         [0027]      FIGS. 3   a - 3   b  show apparatus  200  at low rotational speed ranges, where high bearing preload is generally desirable. At such speeds, variable preload device  100  may be an hourglass configuration with plates  106  in contact with spindle arbor  202  and length L approaching minimum length L min . Bearings  210 ,  220  may be preloaded by opposing spring force  234 , which may be applied, for example, to the inner race  212  of first bearing  210 . The preload force may be distributed to the inner race  222  of second bearing  220  via endplates  230 ,  232 , cylinder  238 , and variable preload device  100 ; thus, the preloading force may be substantially evenly split between bearings  210 ,  220 . 
         [0028]      FIG. 4   a - 4   b  show apparatus  200  at high rotational speed ranges, where low bearing preload is generally desirable. At such speeds, variable preload device  100  may be in a cylindrical configuration with plates  106  in contact with outer cylinder  238  and length L approaching maximum length L max . The increase in length L of variable preload device  100  results in an application and even distribution of axial force via end plates  230 ,  232  to the inner races  212 ,  222  of bearings  210 ,  220 . The resultant axial force may be applied counter to opposing spring force  238 . Consequently, the inner races  212 ,  222  of bearings  210 ,  230  may be axially displaced, as shown in detail in  FIG. 4   b , thereby lessening the preload of bearings  210 ,  220 . As the change in length L is proportional to the rotational speed of spindle arbor  202 , the use of variable bearing device  100  may facilitate dynamically and progressively reducing the preload of bearings  210 ,  220  with increased rotational speed, thereby providing an optimal bearing preload force across a wide range of rotational speeds. 
         [0029]    It should be noted that at high rotational speed ranges, as length L increases beyond the length of outer cylinder  238 , a gap  239  may result between cylinder  238  and end plates  230 ,  232 . In certain situations, the presence of gap  239  may result in outer cylinder  238  floating axially between plates  230 ,  232 , which in turn may result in rattling and vibration. Therefore, in some embodiments, dampeners, for example, o-rings, may be disposed between the edges of cylinder  238  and end plates  230 ,  232 , thereby providing axial centering to cylinder  238  and reducing gap  239  such that rattling and vibration is minimized. The dampeners may by formed of a resiliently compressible material that compresses sufficiently at low rotational speed ranges to allow variable preload device  100  to function as described herein. 
         [0030]    In some embodiments, first and second annular end members  102 ,  104  may interface with end plates  230 ,  232  via a tongue-and-groove interface. First and second annular end members  102 ,  104  may have a convex profile, and may be received within a concave groove defined in end plates  230 ,  232 . Such a configuration may serve to minimize wear on the components at the interface points. Furthermore, end plates  230 ,  232 , variable preload device  100 , and spindle arbor  202  may incorporate anti-slip features to facilitate maintaining synchronous rotation of such components over the range of rotational speeds. Such anti-slip features may include, but are not limited to, O-rings, pins, and keying of the components to each other. 
         [0031]    The foregoing description and accompanying figures illustrate the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. Additional variations of the embodiments discussed above will be appreciated by those skilled in the art. 
         [0032]    Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive. Accordingly, it should be appreciated that variations to those embodiments can be made by those skilled in the art without departing from the scope of the invention as defined by the following claims.