Abstract:
A pulverizer mounted on a foundation and comprising a housing enclosing a pulverizing zone and a base plate attached to the housing. The pulverizing zone includes a horizontally disposed rotatable grinding ring, rolling grinding balls positioned on the grinding ring, and means for rotating the grinding ring including an upright main shaft. An annular yoke is attached to the main shaft, and has an outer peripheral portion which is disposed superjacent to the base plate and supports the grinding ring. A load carrying member or thrust bearing is sandwiched between the base plate and the outer peripheral portion of the yoke to minimize the impact, shock, and eccentric loads that are transmitted from the coal grinding process to the main shaft. The load carrying member or thrust bearing achieves this by transmitting the vertical loads directly through the base plate to the housing and therethrough to the pulverizer foundation.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates, in general, to the field of coal pulverizers, and more particularly, to stress reduction modifications that provide longer shaft life and reduced incidences of shaft failure in ball-and-race type coal pulverizers. 
         [0002]    Coal pulverizers are used to grind, dry and classify raw chunks of coal into fine solids which can be fluidized and fed, for example, to burners used in conjunction with utility and/or industrial boilers or furnaces. As is known to those skilled in the art, several different types of coal pulverizers or coal mills exist today, including one known as the EL pulverizer that was first produced by The Babcock &amp; Wilcox Company in the early 1950&#39;s, and is a ball-and-race type pulverizer which employs the ball thrust bearing principle to grind the coal. 
         [0003]    In prior art  FIGS. 1 and 2 , there is shown an EL pulverizer  10  which includes an upper housing section  32  and a lower housing section  34 . The lower housing section  34  encloses a gear box  35  mounted on a foundation  37 . The upper housing section  32  encloses the pulverizing zone  36  which includes two vertical axis horizontal grinding rings  12  and  14 , and a set of balls  16  placed between the grinding rings. 
         [0004]    In the EL pulverizer  10  of  FIGS. 1 and 2 , the lower or bottom grinding ring  12  rests in and is secured (via a key or the like) to a ring seat  44 . The ring seat  44  rests upon and is secured to the pulverizer yoke  18 . The ring seat  44  is typically made of a conventional steel material, instead of the high hardness materials used in the grinding rings  12  or  14 . The ring seat  44  deflects hot incoming primary air from directly impacting the lower grinding ring  12 , thereby acting as a heat shield for the lower grinding ring  12 . As is known to those skilled in the art, the hot, incoming primary air is used to dry the coal being ground in the pulverizer  10  and to transport the ground coal particles out of the pulverizer  10 . 
         [0005]    The pulverizer yoke  18  rotates through connection to a rotating, vertical main or drive shaft  20 , while the upper or top grinding  14  remains stationary and is spring loaded to provide the pressure for grinding the coal. The pressure required for efficient grinding is obtained from externally adjustable dual purpose springs  22  which are referred to as such, because in addition to providing the loading forces required to efficiently grind the coal, the dual purpose springs  22  also supply the forces required to keep the upper grinding ring  14  from experiencing excessive radial movement, circumferential twisting, and eccentric rotation with respect to the bottom grinding ring  12 . The coal is ground by contact with the upper and lower grinding rings  14  and  12 , and the balls  16 . The upper and lower grinding rings  14  and  12  are each provided with a race having a predefined, matching track contour that engages the balls  16 . The force from the upper grinding ring  14  pushes the balls  16  against the coal layer on the lower grinding ring  12 . Ground coal is swept from the pulverizing zone  36 , defined by the grinding rings  12  and  14  and the balls  16 , by air for final particle size classification and subsequent pneumatic transport to one or more coal burners. Oversized coal particles are returned to the pulverizing zone  36 . The pulverizer  10  is provided with a gear drive assembly  24  which includes bevel gears  26  and  28  positioned on a horizontal pinion shaft  30  and at the base of the vertical main shaft  20 , respectively. 
         [0006]      FIG. 2A  illustrates another known EL pulverizer  10  which differs from the EL pulverizer  10  of  FIGS. 1 and 2  by providing a one piece lower or bottom grinding ring, here designated  12 ′, which rests upon and is secured to the pulverizer yoke  18 . The separate ring seat  44  of  FIGS. 1 and 2  is thus omitted. 
         [0007]    Typically, the grinding balls  16  operate within a predetermined range of acceptable resistance force exerted on the grinding balls by the coal engaged between the upper and lower grinding rings  14  and  12  (or  12 ′) and the grinding balls  16 . If the acceptable resistance force is exceeded due to, for example, an encounter with coal particles of relatively high hardness and of greater than acceptable size, this may result in impact and shock loads. It can also be appreciated that foreign matter such as tramp iron may be engaged between the upper and lower grinding rings and the grinding balls, and this occurrence may cause the grinding balls to partially leave their original track and result in eccentric loads. The main shaft has a closely fitted mechanical joint with the lower grinding ring and, in addition to driving forces, this joint is subjected, at some frequency, to impact, shock, and eccentric loads from the coal grinding operation which all contributes to severe stresses on the shaft. 
         [0008]    Currently, there is some concern as to main shaft failure. It is believed that the shaft failures are initiated by the deterioration of the finely machined outside surface of the tapered portion of the main shaft  20  and the tapered bore surface of the yoke bushing  21 . The deterioration is caused by cyclic movement between the contacting surfaces or mutually facing side  23  of the main shaft  20  and the yoke bushing  21 . This movement is a consequence of the cyclic or alternating type bending loads experienced at the top end of the main shaft. Because the loads are cyclic, there occurs a progressive form of damage known as fretting. Fretting damage, sometimes referred to as fretting corrosion, is a condition of surface deterioration brought on by very small relative movements between bodies in contact. The fit between the main shaft  20  and the yoke bushing  21  is an interference type fit. This type fit generates a stress concentration or stress multiplier. Also of concern is fatigue failure when stress concentration, cyclic loading and fretting corrosion are combined. Like fretting, fatigue has a definite set of characteristics which combine to identify this failure phenomenon. Pulverizer vibration usually results in high shaft stress levels, and may have a role in main shaft failures. Vibration may be caused by abnormal grinding element wear such as out-of-round wear of balls or rings and, also by failure to maintain a proper air/fuel ratio to the pulverizer  10 . 
         [0009]    There have been many attempts at correcting main shaft failure frequency such as applying a dry lubricant or a ceramic coating between the yoke end of the main shaft and the yoke bushing bore area, providing the yoke bushing with circumferential grooves, providing the main shaft with a reduced diameter portion below the yoke bushing, employing an anti-seize compound at the main shaft-to-yoke bushing joint, using a full contact yoke bushing or one with an undercut center portion, shot peening, and nitriding as a surface hardening process. Remedial efforts notwithstanding, even carefully fitted taper joints, when subjected to cyclic bending forces often exhibit vulnerability to fatigue failure of main shafts because of fretting and stress produced at the joint between the main shaft and the yoke bushing. 
         [0010]    In accordance with the present invention, longer pulverizer shaft wear life and reduced incidences of shaft failure are achieved by minimizing the impact, shock and eccentric loads from the grinding process that are transmitted to the shaft, and thus provide reliable ball-and-race type coal pulverizer performance. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed at a ball-and-race type pulverizer mounted on a foundation and comprising a housing enclosing a pulverizing zone and a gear box, a top bearing plate or base plate attached to the housing, the pulverizing zone including a horizontally disposed rotatable grinding ring, rolling grinding elements positioned on the grinding ring, and means for rotating the grinding ring including an upright drive shaft or main shaft. An annular yoke is connected to the main shaft, and the yoke has an outer peripheral portion which is disposed superjacent to the base plate and supports the grinding ring. In accordance with the invention, a thrust bearing or stress reduction member is sandwiched between the outer peripheral portion of the yoke and the base plate to minimize the impact, shock, and eccentric loads that are transmitted from the coal grinding process to the main shaft. The thrust bearing achieves this by transmitting the vertical loads directly through the base plate to the housing and therethrough to the pulverizer foundation. A seal assembly is mounted on the outboard side of the thrust bearing to isolate the bearing from coal dust escaping the pulverizing zone. 
         [0012]    In one embodiment of the invention, the thrust bearing is a cylindrical roller thrust bearing and includes an upper raceway and a lower raceway, and a plurality of cylindrical rollers respectively interposed between the upper and lower raceway. 
         [0013]    In another embodiment of the invention, the thrust bearing is a ball thrust bearing and includes an upper raceway and a lower raceway, and a plurality of balls, respectively interposed between the upper raceway and the lower raceway. 
         [0014]    In still another embodiment of the invention, the thrust bearing is a tapered thrust roller bearing and includes a tapered upper raceway and a tapered lower raceway, and a plurality of tapered rollers respectively interposed between the upper raceway and the lower raceway. While both of the raceways can be tapered, alternatively only one of the raceways (either an upper or a lower raceway) may be tapered, with the other raceway (either an upper or a lower raceway) being flat, with the plurality of tapered rollers being located accordingly. 
         [0015]    In a further embodiment of the invention, the thrust bearing is a friction bearing and includes an upper raceway and a lower raceway spaced from each other and having a sliding surface on a mutually facing side. 
         [0016]    In a still further embodiment of the invention, the friction bearing includes a lubricating fluid between the mutually facing side of the upper raceway and the lower raceway. 
         [0017]    In another embodiment of the invention, the thrust bearing includes a lower raceway, and a plurality of load bearing idler rollers respectively interposed between the lower raceway and the yoke, the idler rollers being mounted on the yoke. 
         [0018]    In still another embodiment of the invention, the thrust bearing includes an upper raceway, and a plurality of load bearing idler rollers respectively interposed between the upper raceway and the yoke, the idler rollers being mounted on the base plate. 
         [0019]    These and other features and advantages of the present invention will be better understood and its advantages will be more readily appreciated from the detailed description of the preferred embodiment, especially when read with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a sectional side view of a prior art type EL ball and race pulverizer, wherein the lower or bottom grinding ring rests in and is secured to a ring seat which rests upon and is secured to the pulverizer yoke; 
           [0021]      FIG. 2  is a detail cutaway view of a portion of the prior art EL pulverizer shown in  FIG. 1 ; 
           [0022]      FIG. 2A  is a detail cutaway view of a portion of another prior art EL pulverizer similar to shown in  FIGS. 1 and 2 , wherein the lower or bottom grinding ring is a one-piece grinding ring; 
           [0023]      FIG. 3  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a cylindrical roller type thrust bearing is interposed between the yoke and the base plate; 
           [0024]      FIG. 4  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a ball type thrust bearing is interposed between the yoke and the base plate; 
           [0025]      FIG. 5  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a tapered roller type thrust bearing is interposed between the yoke and the base plate; 
           [0026]      FIG. 6  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a slide type thrust bearing is interposed between the yoke and the base plate; 
           [0027]      FIG. 7  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a thrust bearing is interposed between the yoke and a base plate, and includes a plurality of load bearing idler rollers mounted on the yoke; and 
           [0028]      FIG. 8  is a detail cutaway view of a portion of an EL pulverizer wherein, according to the present invention, a thrust bearing is interposed between the yoke and a base plate, and includes a plurality of load bearing idler rollers mounted on the base plate. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    Reference will hereinafter be made to the accompanying drawings wherein like numerals designate the same or functionally similar elements throughout the various figures. 
         [0030]    The present invention resides in providing stress reduction for the pulverizer main shaft by placing a load carrying member or thrust bearing between the shaft-driven yoke and a housing supported to minimize the impact, shock, and eccentric loads from the grinding process to the pulverizer main shaft. This is accomplished by the thrust bearing which transmits the vertical loads directly through the base plate to the housing and therethrough to the pulverizer foundation. 
         [0031]    It will be appreciated by those skilled in the art that some of the following embodiments of the present invention employ traditional thrust bearing concepts. A thrust bearing is a bearing which is designed to handle axial loads, and traditionally those axial loads are handled by means of intercooperating raceways and rolling elements, or by intercooperating smooth, sliding surfaces. In the case of rolling elements, the rolling elements can be in the form of cylindrical rollers, tapered rollers, or rolling balls. The present invention not only utilizes such traditional thrust bearing concepts and methods, but also includes apparatus and methods of handling axial loads which include use of plural, individual idler rollers arranged in such a fashion so as to handle the axial loads. Accordingly, for the purpose of the present invention, the term thrust bearing or load carrying member embraces any structures as disclosed herein as well as equivalents thereof which are intended to carry or accommodate such axial loads. Further, in the setting of the present invention, the axial loads which have to be dealt with are those axial loads which are aligned substantially parallel to the vertical axis of the pulverizer main shaft, regardless of how these axial loads may be directed or transmitted through the thrust bearing or load carrying members used in the present invention. Finally, while the majority of the FIGS. in the present disclosure illustrate ball-and-race pulverizers  10  which employ a lower or bottom grinding ring  12  which rests in and is secured (via a key or the like) to a ring seat  44 , and wherein the ring seat  44  rests upon and is secured to the pulverizer yoke  18 , the concepts of the present invention are not limited only to pulverizers  10  provided with such types of lower or bottom grinding rings  12 . The concepts of the present invention also apply to ball-and-race pulverizers  10  which employ a one piece lower or bottom grinding ring  12 ′, which rests upon and is secured to the pulverizer yoke  18 , such as illustrated in  FIG. 2A , for example. 
         [0032]    Referring again to prior art  FIGS. 1 and 2 , and  2 A, there is shown the ball-and-race pulverizer  10  including the upper housing  32  and the lower housing  34 . The upper housing  32  encloses the yoke  18  which rotates through connection to the rotating vertical main shaft  20 . A tapered top end portion of main shaft  20  lies within the bore of yoke  18  and is surrounded by the yoke bushing  21 . The fretting damage occurring as the result of cyclic or alternate type bending loads generated by the pulverizer operation and the tight fit required between the mating surfaces at the top end of the main shaft  20  and the yoke bushing  21  cause bending fatigue stress and develop a stress concentration which may lead to failures of the main shaft  20 . 
         [0033]    A means for preventing the damage due to fretting is to reduce the cyclic movement between the main shaft  20  and the yoke bushing  21 . One approach to reducing this cyclic movement is to minimize the impact, shock, and eccentric loads from the grinding process that are transmitted to the main shaft  20 . 
         [0034]    In order to accommodate the retrofitting of the present invention to the prior art EL pulverizer shown in  FIGS. 1 and 2 , and  2 A, the connection between the upper end of the main shaft  20  and the yoke bushing  21  is modified from that of the prior art EL pulverizer by eliminating the respective taper on a mutually facing side  23  of the upper end of the main shaft  20  and the yoke bushing  21 , and by eliminating the hump  27 , shown in prior art  FIGS. 1 and 2 , and  2 A, and leveling the lower face or underside of the outer peripheral portion of the yoke  18 , as seen in  FIGS. 3 through 8 . 
         [0035]    Referring to  FIGS. 3 through 8 , there is shown a detail cutaway view of a portion of an EL pulverizer  110  which includes a cylindrical upper housing section  132  and a lower housing section  134 . The lower housing section  134  encloses a gear box, not shown, and is mounted on a foundation, not shown. The upper housing section  132  encloses the pulverizing zone  136  that includes the grinding parts of the pulverizer  110  which comprise a rotatable yoke  118  connected to the upper end of a rotating main shaft  120 . A yoke bushing  121  is interposed between the yoke  118  and the shaft  120  and, in accordance with the present invention, the mutual face  123  of the yoke bushing  121  and the shaft  120  are straight. The yoke  118  has an outer peripheral portion  138  with leveled upper and lower faces  140  and  142 , respectively. A lower or bottom grinding ring  112  rests in and is secured (via a key or the like) to a ring seat  144  and rests upon and is secured to the outer peripheral portion  138  of yoke  118 . The upper face of the lower grinding ring  112  is shaped to form a track for a circular row of rolling grinding balls  116 . The lower face of the upper grinding ring  114  is shaped to form a track for the rolling balls  116 . A resilient grinding pressure is exerted downwardly on the grinding parts by coil springs  122 . An annular base plate or bearing plate  150  has a level or flat upper surface  152 , and is attached to the upper and lower housings  132  and  134 , respectively. A thrust bearing or load carrying member is interposed between the upper surface  152  of bearing plate  150  and the lower face  142  of the yoke  118 . The thrust bearing or load carrying member includes an upper raceway  154  and a lower raceway  156 . The upper raceway  154  is connected to the level underside or lower face  142  of the yoke  118 . The lower raceway  156  is connected to the level upper surface  152  of the base plate  150 . A seal assembly  158  isolates the stress reduction member from the pulverizing zone  136 . The seal assembly  158  may be any of a number of known seals practiced in the art such as, for example, labyrinth-air seals, labyrinth-brush seals, and packing-brush seals. The seal assembly  158 , as schematically shown, includes a pair of laterally spaced skirt plates  160  and  162 , and a brush seal  164  and packing  166  interposed between the skirt plates  160  and  162 . The skirt plate  160  is attached to the distal end of the yoke  118 , and the skirt plate  162  is attached to the base plate  150 . 
         [0036]    As shown in  FIG. 3 , the thrust bearing or load carrying member is a thrust bearing  168  which includes one of a plurality of cylindrical rollers  170 , respectively interposed between the upper raceway  154  and the lower raceway  156 . 
         [0037]    As shown in  FIG. 4 , the thrust bearing or load carrying member is a thrust bearing  172  which includes one of a plurality of balls  174 , respectively interposed between the upper raceway  154  and the lower raceway  156 . 
         [0038]    As shown in  FIG. 5 , the thrust bearing or load carrying member is a thrust bearing  176  which includes one of a plurality of tapered rollers  178 , respectively interposed between the upper raceway  154  and the lower raceway  156 , with each of the raceways  154  and  156  being tapered or conically-shaped to match the taper of the tapered rollers  178 . While both of the raceways can be tapered, alternatively only one of the raceways (either an upper  154  or a lower  156  raceway) may be tapered, with the other raceway (either an upper  154  or a lower  156  raceway) being flat, with the plurality of tapered rollers  178  being located accordingly. 
         [0039]    As shown in  FIG. 6 , the thrust bearing or load carrying member is a fluid lubricated friction type thrust bearing  180  which includes the upper raceway  154  and lower raceway  156  separated by a narrow space  182 . A lubricating fluid, either gas or liquid, is introduced in the space  182  and acts as the actual bearing surface between the stationary base plate  150  and the rotating yoke  118 . For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. 
         [0040]    As shown in  FIG. 7 , the thrust bearing or load carrying member is a thrust bearing  184  which includes one of a plurality of load bearing idler rollers  186  interposed between the lower raceway  156  and the yoke  118 . The load bearing idler rollers  186  are mounted on the yoke  118 . 
         [0041]    As shown in  FIG. 8 , the thrust bearing or load carrying member is a thrust bearing  188  which includes one of a plurality of load bearing idler rollers  190  interposed between the upper raceway  154  and the yoke  118 . The load bearing idler rollers  190  are mounted on the base plate  150 . 
         [0042]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. For example, the present invention may be applied in new construction involving type EL pulverizers, or to the repair, replacement, and modification or retrofitting of existing type EL pulverizers. Thus, while the present invention has been described above with reference to particular means, materials, and embodiments, it is to be understood that this invention may be varied in many ways without departing from the spirit and scope thereof, and therefore is not limited to these disclosed particulars but extends instead to all equivalents within the scope of the following claims.