Abstract:
A bearing for supporting a shaft submerged in molten zinc, has a bearing surface comprising a steel cylindrical liner having slots retaining ceramic shaft-engaging bearing elements.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
   Sink roll assembly bearings used for providing tension on a metal strip, being advanced in a bath of molten metal, have a very short life. The short life exists because of distortion and misalignment created between the bearing components, operating in high metal temperatures. In addition, chemical reactions occur between the molten metal and the bearings. Heavy surface unit loading combined with the low operating rotational speed experienced by the rolls, also shortens bearing life. 
   I have solved some of the problems related to these environmental conditions. An example, may be found in my U.S. Pat. No. 6,692,689 issued Feb. 17, 2004 for “Sink Roll Assembly with Forced Hydrodynamic Film Lubricated Bearings and Self-Aligning Holding Arms”. This invention substantially extended the life of the rolls by aligning the roll shaft with the bearing axis, and pumping molten metal under pressure to the bearing surfaces in the form of a hydrodynamic lubricating film, created by the reduction of the surface unit load acting on the bearing. 
   The invention disclosed in this application provides a longer operating life by providing ceramic elements in the bearing liner that supports the shaft. Ceramic has a greatly reduced wear pattern compared to the steel alloys normally used for sink rolls, because of its hardness, low coefficient of friction and resistance to chemical attack by the molten metal. 
   One approach for using ceramic bearing materials has been to use a ceramic liner in a steel alloy sleeve for supporting the shaft. This approach has worked in the laboratory, but is not suitable for an industrial application for several reasons. One reason is that the steel holding ring that carries the ceramic has a much larger coefficient of expansion than the ceramic. The result is that when the ceramic liner is heated to the operating temperature (900° F. to 1300° F. in most cases), the interface between the ceramic liner and the holding sleeve develops a substantial clearance which then causes the unsupported ceramic to shatter. An unsupported ceramic liner will shatter in an industrial application, because of the severe pounding it experiences, caused by the moving metal strip submerged in the molten metal bath. 
   I have found that employing a steel liner for supporting several ceramic elements in slots in a holding sleeve, prevents the ceramic elements from being separated from the liner and compensates for the differences in the coefficients of expansion of the steel and the ceramic, minimizing the high temperature running clearance. 
   In the preferred embodiment, the liner has an internal cylindrical surface, having elongated slots in a pattern that extends around the cylindrical surface. An elongated strip of ceramic material is inserted into each of the slots. The collective inner surface of the ceramic elements combines with the liner surface to form a shaft-supporting structure. This arrangement provides the advantage of using the long operating life of a ceramic component without the differential expansion problems. 
   The process for making such a bearing comprises forming a steel alloy liner having slots for receiving the ceramic elements. The slots, at room temperature, are slightly smaller than the ceramic elements. The liner is then heated to a suitable temperature, such as 1000° F. to expand the slots. The ceramic elements are inserted in the slots. The steel liner is then cooled so that the ceramic elements are tightly held in the slots (i.e. 0.0002±0.0001 inches interference fit). The outside and inside diameters of the steel liner-ceramic inserts assembly are then ground to appropriate dimensions, the outside diameter to receive the holding sleeve, the inside diameter to receive the sink roll shaft trunion. 
   To prevent the ceramic elements from moving radially outwardly from the liner when operating at high temperature, a holding sleeve is heated to expand it. The heated holding sleeve telescopically receives the liner and the ceramic elements to form a tight fitting connection between the holding sleeve and the steel liner. The holding sleeve and the liner are made of the same materials so that they have the same coefficient of expansion. Thus, as the bearing is heated to an operating temperature, the liner and the holding sleeve expand as a unit while retaining the ceramic elements in their respective slots. 
   The radial forces applied to the bearing by the shaft are directed toward a particular area of the bearing surface. Over a period of time the shaft will gradually wear the liner in the localized operating area of the bearing surface. The bearing can then be rotated (about 90°) to permit an unworn area of the bearing surface to support the shaft. This can be repeated several times by progressively rotating the bearing whenever required, thus improving by a factor of four the already extended life of the bearing, resulting in additional savings in bearing materials as well as the time in replacing a worn bearing. 
   Still further objects and advantages of the invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description. 

   
     DESCRIPTION OF THE DRAWINGS 
     The description refers to the accompanying drawings in which like reference characters refer to like parts throughout the several views, and in which: 
       FIG. 1  is a view illustrating the environment in which the inventive bearing is mounted; 
       FIG. 2  is a side view of the bearing of  FIG. 1 , but with the holding sleeve removed; 
       FIG. 3  is a sectional view as substantially seen along lines  3 - 3  of  FIG. 2 ; 
       FIG. 4  is a sectional view as seen along lines  4 - 4  of  FIG. 9 ; 
       FIG. 5  is a view of an alternative slot configuration; 
       FIG. 6  is a cylindrical view illustrating steps in the process for making a preferred bearing; 
       FIG. 7  illustrates further steps in making a preferred bearing; 
       FIG. 8  is a view illustrating the manner in which the bearing surface of the liner is finished; 
       FIG. 9  is an end view of a preferred bearing; and 
       FIG. 10  is a view illustrating the load placed on a typical bearing illustrating the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 4 , bearing  10  illustrates a preferred embodiment of the invention for supporting a rotatable shaft  12 . Structure  14  supports the bearing and the shaft below level  16  of a bath of molten metal. A bearing housing  18  supports a liner  20 , and a holding sleeve  22 . 
   The sleeve and the liner are cylindrical with the liner having a cylindrical inner liner surface  24 . 
   Referring to  FIGS. 2 and 4 , the liner has a plurality of openings or slots  26 . Each slot  26  is elongated and parallel to the axis  27  of rotation of a shaft. Other shapes and locations are possible. 
   A ceramic bearing element  28  is disposed in each slot  26 . Each ceramic bearing element could be straight or have a somewhat tapered configuration with an inner surface  32  curved to form a continuation of the cylindrical liner surface  24 , as shown in  FIG. 3 . 
   At room temperature, the width of each slot  26  is slightly smaller (0.0002″ to 0.001″) than the width of its respective ceramic bearing element. Referring to  FIG. 6 , a heater  34  is then used to heat liner  20  to a temperature of about 1000° F., to enlarge the individual slots sufficiently to receive their respective ceramic bearing elements. When the liner is cooled to room temperature, the slots shrink to tightly retain their respective ceramic bearing elements in position, so they can be mechanical (ground) together as an assembly, both the outside diameter and the inside diameter. 
   The next step in making the preferred bearing is illustrated in  FIG. 7 , in which holding sleeve  22 , formed of an alloy steel, is heated to enlarge its inner surface sufficiently to receive liner  20 . The holding sleeve is then cooled, to tightly retain the ceramic bearing elements in position and to prevent them from moving outwardly away from the shaft when operating at a high temperature in the metal bath. 
   Referring to  FIG. 8 , a finishing tool  36  is then employed to grind the entire inner bearing surface including both the liner and the ceramic bearing elements, to a smooth cylindrical surface with a diameter slightly larger than the intended shaft diameter. 
     FIG. 10  illustrates the manner in which the shaft is supported within the liner. The load is applied in the direction of arrow  38 . For that reason, over a period of time a portion of the liner bearing surface  24  will become worn. The bearing liner can be rotated in housing  18  in the direction of arrow  40 , 90°, for example, to engage the shaft with an unworn portion of the liner. This reduces downtime in replacing such a bearing because the liner can be rotated to a new position rather than being replaced. The ceramic inserts reduce friction; act as metal dross wipers; and allow very close bearing shaft running clearances (0.010/0.015) vs. (0.250/0.300) on standard bearings. 
   Referring to  FIG. 4 , liner  20  has a partially spherical convex bearing surface  42  which slidably engages a partially spherical concave bearing surface  44  of holding sleeve  22 . A pair of rings  46  and  48 , attached to the housing and the holding sleeve, retain the holding sleeve in position. 
     FIG. 5  illustrates an alternative embodiment of the invention in which those components similar to the components of  FIG. 4  are designated by a prime. 
   In this case, slots  26 ′ are parallel to one another, but inclined with respect to axis  27 ′.