Abstract:
The invention relates to a refractory bearing and in particular to a bearing for use in contact with molten metal, such as zinc and aluminum. The bearing includes a housing with a working surface and at least one wear-resistant insert secured to the working surface. The insert is secured to the housing by mechanically interlocking with a retainer assemblage. In one embodiment, the retainer assemblage engages a relief in the insert to secure the insert to the housing despite dimensional changes caused by thermal expansion. The insert preferably comprises sialon.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to refractory bearings and is particularly concerned with refractory bearings for use in contact with molten metal.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    Bearings typically support a rotating or otherwise moving article. In high temperature applications, bearings often comprise a refractory metal, ceramic or composite. In such applications, bearings may even be in direct contact with molten metals, such as molten zinc or aluminum.  
           [0003]    For example, galvanization is the process of forming a protective, anti-oxidant zinc layer on a base metal. A continuous galvanizing apparatus comprises a bath of molten zinc, a sink roll at least partially immersed in the bath, a journal disposed along the longitudinal axis at each end of the sink roll around which the sink roll may rotate, and a set of roll arms with replaceable bearings for supporting the journals. The bath is maintained at a temperature sufficient to keep the zinc molten.  
           [0004]    The sink roll forces the base metal, which is often in the form of a sheet or wire, into the molten zinc. The roll rotates as the base metal passes into the molten zinc, under the sink roll, and finally out of the molten zinc. The journals cooperate and rotate with the sink roll. The roll arms support the journals, and the journals are often covered with a hard material, such as tungsten carbide, to resist wear from the bearing. The roll arm can be moved to adjust the depth of the sink roll within the bath.  
           [0005]    The requirements for bearings under these conditions are severe. Molten zinc is at least 420° C., typically around 460° C., and corrodes many common bearing materials. Mechanical abrasion is a ubiquitous complication. The bearings wear quickly and must be replaced frequently. Replacement requires the shutdown of the galvanizing operation while new bearings are inserted. Disruption of a continuous galvanizing operation results in significant operator costs and lost production.  
           [0006]    Prior art bearings include metal housings, often in the shape of a ring, that are fitted to the roll arm and cooperate with the journals. During operation of a galvanizing bath, the journals contact a working face of the bearing. The high temperature and corrosive enviromnent destroy metal bearings relatively quickly and cause the sink roll to rotate eccentrically, thereby reducing galvanizing efficiency. Worn bearings must be replaced, often at great cost. Prior art also includes metal housings having inserts comprising a refractory material selected for its erosion and corrosion resistance. The journals contact the inserts instead of the metal housing. The inserts are substantially more resistant to wear and corrosion than the metal housing alone and can extend the life of bearings many times.  
           [0007]    Sialon is particularly useful in this capacity and consists of a solid solution and/or dispersion of aluminum oxide and aluminum nitride in a silicon nitride matrix. One or more sialon inserts are embedded into the working face of the metal housing. Typically, the inserts are polygonal shapes and are embedded in a plurality of cavities along the working face of the metal housing.  
           [0008]    At room temperature, the inserts are secured tightly into the cavities. This may be accomplished using a retaining plate and one or more wedges to improve the tightness of fit. The retaining plate can be welded to the housing and may extend at least partially over the insert in the cavity. Still, inserts have a tendency to fall out at operating temperatures because the thermal expansion of the metal housing is greater than the ceramic inserts. Loss of an insert causes the journal to wobble or otherwise rotate eccentrically. Fortunately, the journals pressing against the inserts can hold the inserts in the cavities despite thermal expansion; however, pressure can be lost when the galvanizing operation is stopped or slowed. In such situations, the journal may separate from the insert by one-quarter inch or more. An insert can then slip from its cavity. The tendency of a ceramic insert to loosen and fall from a metal cavity increases with temperature and would be even more likely at higher temperatures, such as with molten aluminum baths which are typically at least around 700° C. and more commonly around 715° C.  
           [0009]    A need persists for a refractory bearing comprising a housing and a wear-resistant insert where the insert is more fixedly secured to the housing and does not depend on the journals to hold it in place. Advantageously, the bearing would be easily manufactured of substantially inexpensive materials and would be suitable for use with molten zinc and aluminum.  
         SUMMARY OF THE INVENTION  
         [0010]    The objective of the present invention is to provide a bearing for high temperature applications, particularly for contact with molten metals. The invention secures a wear-resistant insert to the working surface of the bearing despite disparate dimensional changes caused by thermal expansion.  
           [0011]    One aspect of the invention describes the bearing as comprising a housing having a wear-resistant insert at least partially embedded in a cavity of a working surface of the housing. A retainer is secured to the housing and simultaneously engages a relief in the insert, thereby locking the insert into the cavity.  
           [0012]    A further aspect of the invention describes the housing as a metal ring adapted to receive a journal. The inner surface of the ring comprises the working surface, which includes a plurality of inserts axially arranged along the working surface. One embodiment of the invention includes a metal ring having three inserts along the working surface.  
           [0013]    Another aspect of the invention teaches inserts comprising sialon. In a further aspect of the invention, the retainer may comprise one or more metal pieces and is conveniently welded to a metal housing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows a portion of a galvanizing apparatus, including sink roll and roll arms with bearings.  
         [0015]    [0015]FIG. 2 a  shows a metal ring bearing of the prior art.  
         [0016]    [0016]FIG. 2 b  is a cross-section of a prior art bearing.  
         [0017]    [0017]FIG. 2 c  is a cross-section detail of a prior art bearing.  
         [0018]    [0018]FIG. 3 a  shows a metal ring bearing of the present invention.  
         [0019]    [0019]FIG. 3 b  is a cross-section of the present invention.  
         [0020]    [0020]FIG. 3 c  is a cross-section detail of the present invention.  
         [0021]    [0021]FIG. 4 is a side view of a metal ring bearing.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Refractory bearings of the present invention can be used in many high-temperature applications, including continuous galvanizing operations in which a sheet or wire of a base metal is drawn through a bath of molten zinc to effect a zinc coating on the surface of the base metal. FIG. 1 shows a portion of a continuous galvanizing unit that uses refractory bearings.  
         [0023]    A sink roll  11  forces the base metal into the bath of molten zinc (not shown). The sink roll  11  is at least partially immersed in the molten zinc and is supported by journals  14  extending along the rolling axis of the sink roll  11 . Roll arms  12  engage the journals  14 . Manipulation of the roll arms  12  permits the sink roll  11  to be raised or lowered within the bath. Bearings  13  facilitate rotation of the journals  14  and the sink roll  11 . The journals  14  often has a sleeve  15  to reduce wear of the journal  14  by the bearings  13 . The sleeve  15  can be any wear-resistant, refractory material, and is often a tungsten carbide cermet overlaid on a stainless steel substrate.  
         [0024]    A refractory bearing often comprises a housing having a working surface with at least one cavity containing a wear-resistant insert. Depending on its intended use, a housing can be formed from a variety of refractory materials, including metal, ceramic and composites. Galvanizing bearings typically comprise metal housings. FIG. 2 a  shows a prior art bearing for a continuous galvanizing unit. The bearing  13  comprises an annular housing  26  adapted to cooperate with a journal along the bearing&#39;s inner surface  25 . A plurality of wear-resistant inserts  21  are secured within cavities along the inner surface  25 . The inserts  21  extend beyond the inner surface  25 . Inserts  21  are wear-resistant, refractory materials such as, for example, alumina, silica, magnesia, zirconia and combinations thereof. Sialon is particularly useful because of its resistance to corrosion, mechanical wear and thermal shock.  
         [0025]    [0025]FIG. 2 b  shows a cross-section of the bearing. The insert  21  sits in a cavity of the housing  26 . In this embodiment, the insert extends from a first annular surface  27  to a second annular surface  28 . The retainer assemblages  22  are fixedly secured to the housing  26  and prevent the insert  21  from leaving the cavity along a path perpendicular to the annular surfaces. FIG. 2 c  details one method of securing the retainer assemblage  22  to the housing  26  with a weld  23 . Other mechanical fasteners, such as bolts, rivets, screws, etc., or adhesive fasteners, such as a high temperature phenolic, could alternatively be used to secure the retainer to the housing. Obviously, the retainer assemblage could also be secured to the housing indirectly.  
         [0026]    Wedges (not shown) may be used to improve the tightness of fit between the cavity, insert and retainer assemblage. Improved tightness can reduce the tendency of inserts to fall out of the cavity. Wedges include any article that may be used to improve the tightness of fit and comprise, for example, wedges, shims, and the like.  
         [0027]    The present invention improves on the retention of the insert within the cavity. FIG. 3 a  shows a bearing  13  with a housing  26 . A cross-section of the bearing  13 , as seen in FIG. 3 b,  shows an insert  21  having a relief  35 . The insert  21  will commonly include more than one relief  35  depending on factors such as geometry, operating temperature and the particular materials in the bearing. FIG. 3 b  shows two reliefs  35 , one on each end of the insert  21 .  
         [0028]    The retainer assemblage  22  comprises two non-planar portions. An end plate  32   a  is fixedly secured to the housing  26 . A retainer  32   b  fits in a mating relationship with the relief  35  of the insert  21 , thereby locking the insert  21  in the cavity and reducing the likelihood that the insert  21  will fall out. Conveniently, for ease of manufacture, the relief  35  and the retainer  32   b  are mirror images.  
         [0029]    [0029]FIG. 3 c  shows detail of the retainer assemblage  22 , insert  21  and housing  26 . The retainer  32   b  mates with the relief  35  of the insert. The retainer  32   b  is attached to the end plate  32   a,  for example, with a weld  34 . The end plate  32   a  is fixedly secured to the housing  26 , such as with a weld  23 . End plates and retainers are preferably comprised of steel and are typically about one to two inches wide. The retainer should be at least about 0.12 inch thick in order to provide a sufficient relief that would secure the insert in the cavity. The required thickness will vary depending on the material used to make the retainer, the operating temperature, the size of the retainer, operating stresses, etc. Preferably, a gap will exist between the relief and retainer. The gap allows for manufacturing variations and thermal expansion. For example, when the retainer is steel, the insert is sialon, and the application is galvanization, a gap of 0.005 inch is normally sufficient.  
         [0030]    The relief on the insert is conveniently a rabbet to simplify manufacture. A matching rabbet on the retainer completes the joint. Alternatively, other joint constructions could be used including slanted or dovetail designs. Such joints are known to one skilled in the art. For example, the cavity could have inwardly sloping sides forming a dovetail groove. Such sides would comprise the retainer. The retainer may be an integral part of the housing, as in the dovetail design, or it may be separate, as is shown in the figures. The insert would have a relief comprising sloped sides that cooperate with the dovetail groove. The insert would be placed into the groove and an end plate would close the cavity opening, thereby preventing the insert from escaping. Importantly, the cavity and the retainer cooperate to product a mechanical interlock that cooperates with the relief of the insert to secure the insert in the cavity.  
         [0031]    Although the retainer assemblage should be a rigid unit, it may comprise one or more pieces. The retainer assemblage may be machined from a single piece of refractory material or comprise a plurality of pieces. FIG. 3 c  shows a retainer assemblage consisting of two pieces, an end plate  32   a  comprising a plate and a retainer  32   b  also comprising a plate. The two portions are secured together with a joining weld  34 .  
         [0032]    The bearing may comprise one or more wear-resistant inserts. Ring-shaped bearings for use with a journal will often have three bearings. FIG. 4 shows a ring-shaped bearing  13  adapted for use with a journal and an arrow  44  representing the direction of rotation of the journal. In such a configuration, a load insert  42  supports the force exerted by the journal on the bearing  13 . A lead insert  41  wipes dross from the journal so wear of the load insert  42  is reduced. Dross includes solidified metal, slag, dirt and other impurities found in the molten metal. A trailing insert  43  stabilizes the journal in the bearing  13 . Point loading on a single insert and a three-insert arrangement typically perform better than a single insert in the shape of an arc. The three inserts are frequently arranged in an arc of less than 180°, and preferably around 90°.  
         [0033]    Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described.