Abstract:
A thrust bearing pad includes a relatively low wear and low friction contact layer disposed on a metallic substrate. The metallic substrate allows a manufacturer to couple the thrust bearing pad to a corresponding metallic thrust bearing in a relatively secure manner while the contact layer extends the operating life of the thrust bearing and minimizes maintenance.

Description:
RELATED APPLICATIONS 
     This patent application is a continuation of U.S. Utility application Ser. No. 14/102,657, filed on Dec. 11, 2013, entitled “Thrust Bearing Pad Having Metallic Substrate” which claims the benefit of U.S. Provisional Application No. 61/735,767, filed on Dec. 11, 2012, entitled “Thrust Bearing Pad Having Metallic Substrate,” the contents and teachings of each of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     In conventional drilling systems, such as indicated in  FIG. 1A , a thrust bearing  10  is installed at the end face  12  of a rotating shaft  14  to substantially maintain the shaft in a given position within a housing  13  and relative to a longitudinal axis  15  of the drilling system. For example, the thrust bearing  10  opposes an axial load  16  generated by the shaft  14  during operation to maintain the longitudinal positioning of the shaft. The axial or thrust load  16  can be relatively high for mud pumps, such as used in drilling for the oil and gas industry, and for other rotating equipment, such as large gas and steam turbines as well as blowers, for example. 
     SUMMARY 
     As the face of the shaft  14  rotates against the thrust bearing  10 , a typical thrust bearing  10  includes a set of thrust pads  20  mounted to a disk  22 , as illustrated in  FIG. 1B . Certain conventional thrust pads  20  are manufactured from a special polymeric or metallic material, such as polyether ether ketone (PEEK) or bronze. However, during operation under extreme loading conditions, such as under millions of pounds of axial load, the thrust pads  20  can wear away from the disk  22  relatively quickly, thereby limiting the operating life of the thrust bearing  10  and requiring frequent maintenance. Other conventional thrust pads  20  are manufactured from polycrystalline diamond material and are typically utilized in extreme loading applications. However, polycrystalline diamond thrust pads are relatively expensive and impractical for conventional applications. 
     By contrast to conventional thrust pads, embodiments of the present innovation relate to a thrust bearing pad having a relatively low wear and low friction contact layer disposed on a metallic substrate. The metallic substrate allows a manufacturer to couple the thrust bearing pad to a corresponding metallic thrust bearing in a relatively secure manner while the contact layer extends the operating life of the thrust bearing and minimizes maintenance. 
     In one arrangement, the contact layer is manufactured from a ceramic pad that is brazed to a metallic substrate. In one arrangement, the contact layer is configured as a monolithic ceramic material brazed to the metallic substrate. In one arrangement, the contact layer is configured as a cermet material applied to the metallic substrate. In one arrangement, the contact layer is configured as a relatively hard metallic layer. The ceramic, cermet, or relatively hard metallic layers can each include a hard diamond-like carbon (DLC) type coating disposed thereon. In one arrangement, the contact layer is a plastic material having an interlocking structure, such as a dovetail channel, configured to mate with a corresponding interlocking structure of the metallic substrate. 
     The resulting thrust bearing pads are cost effective yet provide high performance relative to conventional thrust pads. In addition to low wear characteristics, the thrust bearing pads are configured with a relatively low coefficient of friction. Accordingly, during operation when running against a rotating, metallic shaft, the thrust bearing pads can reduce heat generation to minimize damage to the shaft and thrust bearing. 
     In one arrangement, a thrust bearing pad includes a metallic substrate configured to be coupled to a carrier element and a ceramic pad brazed to the metallic substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the innovation, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the innovation. 
         FIG. 1A  illustrates a schematic representation of a prior art shaft having a thrust bearing. 
         FIG. 1B  illustrates a schematic representation of a prior art set of thrust pads of the thrust bearing of  FIG. 1A . 
         FIG. 2A  illustrates a side view of a schematic representation of a thrust bearing having a set of thrust bearing pads, according to one arrangement. 
         FIG. 2B  illustrates a side view of a contact layer of a thrust bearing pad having a monolithic ceramic layer and a DLC coating layer, according to one arrangement. 
         FIG. 2C  illustrates a top view of a schematic representation of a thrust bearing having a set of thrust bearing pads, according to one arrangement. 
         FIG. 3A  illustrates a brazed joint configuration between a monolithic ceramic layer and a metallic base configured as a metallic cup, according to one arrangement. 
         FIG. 3B  illustrates an exploded view of a brazed joint configuration between a monolithic ceramic layer and a metallic base using a substantially continuous butt joint with a soft metal interlayer, according to one arrangement. 
         FIG. 3C  illustrates an assembled brazed joint configuration between a monolithic ceramic layer and a metallic base using a substantially continuous butt joint with a soft metal interlayer, according to one arrangement. 
         FIG. 3D  illustrates a brazed joint configuration between a monolithic ceramic layer and a metallic base using an interrupted butt joint with a soft metal interlayer, according to one arrangement. 
         FIG. 4  illustrates a schematic representation illustration of a thrust bearing pad having a cermet layer and a metallic base layer, according to one arrangement. 
         FIG. 5A  illustrates a prior art molded plastic pad over a metallic base layer. 
         FIG. 5B  illustrates a thrust bearing pad having a plastic layer configured with an interlocking structure that mates with a metallic base layer, according to one arrangement. 
         FIG. 6A  illustrates a set of thrust bearing pads having alternating shallow concave and convex geometries configured to define a relatively wavy surface, according to one arrangement. 
         FIG. 6B  illustrates a trapezoidal thrust bearing pad defining a relatively shallow radial groove, according to one arrangement. 
         FIG. 7  is a schematic illustration showing hydrodynamic pressure build up in the shallow radial groove of  FIG. 6B , according to one arrangement. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2A  illustrates a schematic representation of a thrust bearing  40  having a set of thrust bearing pads  42  disposed on a carrier element or carrier disc  44 , according to one arrangement. Each thrust bearing pad  42  includes a contact layer  46  configured to contact a rotating shaft and a metallic substrate  48  configured to secure the thrust bearing pad  42  to the carrier disc  44 . While the metallic substrate  48  can be manufactured from a variety of materials, in one arrangement, the metallic substrate  48  is manufactured from a corrosion resistant material, such as stainless steel materials. While the thrust bearing pads  42  can be secured to the carrier disc  44  in a variety of ways, in one arrangement, the metallic substrates  48  of the thrust bearing pads  42  are bolted to the carrier disc  44 . Alternately, the metallic substrates  48  of thrust bearing pads  42  can be secured to the carrier disc  44  to allow tilting of the pads  42  in the direction of rotation during operation. 
     As indicated above, the contact layer  46  is disposed on the metallic substrate  48  and is manufactured from a relatively low wear and low friction material. As described below, the contact layer  46 , metallic substrate  48 , and resulting thrust bearing pad  42  can be configured in a variety of ways. 
     In one arrangement, with reference to  FIGS. 2B through 3C , the thrust bearing pad  142  is configured as a ceramic disc or pad  146  brazed to a metal substrate  148 . For example, the ceramic pad  146 , as illustrated in  FIG. 2B , can be a monolithic ceramic material, such as silicon carbide (e.g., both sintered and reaction bonded or a composite of silicon carbide and other ceramics such as aluminum oxide), silicon nitride (e.g., both sintered and reaction bonded or a composite of silicon nitride and other ceramics such as aluminum oxide), aluminum oxide mixed with zirconium oxide, or transformation toughened zirconium oxide. Additionally, the ceramic pad  146  can be manufactured from other monolithic carbides, nitrides, and oxide ceramics having superior low friction and low wear characteristics relative to conventional pads. For example, a coefficient of friction in the range of between about 0.1 and 0.3 is considered low. Conventionally, the coefficient of friction of plastic to metal is within this range. However, the corresponding wear rate of plastic to metal is high. Typically, plastic thrust bearing pads utilized in a mud pump wear off within 8 and 24 hrs of operation. By contrast, interaction between the ceramic layer  146  and a metallic rotating shaft simultaneously provides a relatively low coefficient of friction (e.g., between about 0.1 and 0.3) and a relatively low wear rate. 
     In one arrangement, to further enhance the relatively low wear and low friction characteristics of the ceramic pad  146 , these monolithic ceramics can be coated with a diamond like carbon (DLC) coating  150 , such as on a bearing or contact surface of the ceramic pad  146 . Typically, DLC coatings are formed of a carbon material having an amorphous, non-crystalline carbon structure, such as produced through a chemical vapor deposition or sputter deposition process using a graphite target. DLC coatings have relatively high hardness values, in a range of about 3400 and about 4800 Knoop hardness (HK) Additionally, DLC coatings have relatively low coefficient of friction values, in a range of about 0.09 to about 0.15 running against hard metallic surfaces, such as a high strength steel rotor. 
     In one arrangement, the DLC coating  150  is applied using a Physical Vapor Deposition (PVD) or sputtering process which improves the effectiveness of the DLC coating  150 . If DLC is applied directly on the metallic substrate  148 , which can deform under a relatively high contact load, the thin hard and brittle DLC coating layer  150  will also deform with the metallic substrate  148  and can fracture. Such fracture can create fragments that become lodged between the thrust bearing and the end face of the rotating shaft, thereby resulting in three body wear of the shaft and cause serious damage. Accordingly, application of the DLC coating  150  over the ceramic pad  146  minimizes such cracking and generation of fragments. 
     As indicated in  FIGS. 3A through 3D , the ceramic layer  146  is secured to the metal substrate  148 . Monolithic ceramic materials have a relatively lower coefficient of thermal expansion compared to that of the corresponding metallic substrates. For example, thermal expansion coefficients for ceramic materials are between about 3×10 −6  and 5×10 −6  per ° C. whereas thermal expansion coefficients for metallic alloys are between about 10×10 −6  and 15×10 −6  per ° C. In one arrangement, to account for the difference in thermal expansion coefficients, to provide a relatively high joint strength, and to limit or prevent cracking of the relatively brittle ceramic layer  146 , a manufacturer brazes the ceramic layer  146  to the metal substrate  148 . A variety of brazing processes can be used to attach the ceramic layer or pad  146  to the metallic substrate  148 , as will be described in detail below. 
     One brazing process, such as illustrated in  FIG. 3A , couples a circumferential surface of the ceramic disc or pad  146  disposed within a metallic base or cup  149 . During the brazing process, a circumferential surface of the ceramic pad  146  is first metalized  152 . Conventional metalization processes use a Mo—Mn slurry, which is applied on the ceramic part and fired in a reducing atmosphere. The process forms a Mo—Mn layer chemically bonded to ceramics. As an alternate to the conventional Mo—Mn process, a manufacturer can apply a relatively thin nickel-base, copper-silver base, silver-base and/or similar braze alloy paste with active metals such as titanium around the circumference of the ceramic pad  146  and run the ceramic pad  146  through a braze cycle. The braze paste reacts with the ceramic pad  146  and forms a thin metalized braze alloy layer  152  chemically bonded to the ceramic pad  146 . Because the metallic braze alloy layer  152  is relatively thin, it does not cause damage to the ceramic face during cooling, which stems from stresses generated by a thermal expansion coefficient mismatch between the braze alloy layer  152  and the pad  146 . 
     Following the metallization process, the metalized braze alloy layer  152  of the ceramic pad  146  is brazed directly to the metallic base  149 . In one arrangement, the first metalizing braze alloy  152  should have a higher brazing temperature compared to the second braze alloy  158  used for joining the metalized ceramic pad  146  and the metallic base  149 . For example, a SiC ceramic pad  146  can be metalized with a nickel base brazing alloy, such as BNi-2, at a brazing temperature of about 1000° C. The metallization process is followed by brazing the braze alloy layer  152  of the SiC ceramic pad  146  to an inner surface of the metallic cup  149  with a soft Ag—Cu braze alloy  158  at a brazing temperature of about 850° C. The two step brazing process is known as step brazing. 
     In one arrangement, a radial clearance or joint gap (G) between the ceramic pad  146  and the inner surface the metallic cup  149  is between about 0.0005 inches and 0.01 inches and can be more specifically between about 0.001 inches and 0.004 inches. For example, the radial clearance G is configured as the total thickness of a braze joint  158 , such as a Ag—Cu braze joint, disposed within the annular space between the outer diameter of the braze alloy layer  152  and the inner diameter of the metallic cup  149 . The size of the radial clearance G is selected based upon the physical properties of the ceramic pad  146 , the braze joint  158 , and the metalized layer or braze alloy layer  152 . A relatively larger radial clearance G can cause the ceramic pad  146  to crack because of a relatively high compressive stress generated by a thicker circumferential braze alloy ring. By contrast, a relatively smaller gap is difficult to maintain. 
     In one arrangement, a rim of the metallic cup  149  is beveled to hold additional braze paste to fill the larger gap created by higher thermal expansion of the metallic cup  149  at the brazing temperature. If the joint between the ceramic pad  146  and an inner diameter the metallic cup  149  is too thick, the annular braze joint  158  can impart enough compressive force during solidification of the braze alloy to crack the ceramic pad  146  even if the braze alloy is relatively soft, such as Ag—Cu. 
     In one arrangement, the area of metallization  152  is also important. For example, the metalized area  152  extends substantially up to the cup rim  154  of the metallic cup  149  to minimize spreading of the braze alloy could spread beyond the joint (i.e., beyond the cup rim  154 ) and to maintain the strength of the bond between the ceramic pad  146  and the metallic cup  149  at the cup rim  154 . By contrast, if the braze alloy layer  152  were to extend beyond the partial length PL, such as along a length L of the ceramic pad  146 , during the brazing process, the braze alloy could spread beyond the joint between the ceramic pad  146  and the metallic cup  149 , leaving the joint porous and weak. 
     In one arrangement, the metallic cup  149  defines a clearance  156  at the bottom corner relative to the ceramic pad  146 . For example, the clearance  156  extends about an inner periphery of the metallic cup  149 . The clearance  156  is configured to minimize or limit any contact between the ceramic pad corners and the metallic cup  149  to limit or eliminate localized stress raisers at the ceramic pad corners. This joint design is configured to impart compressive stress on the ceramic pad  146  which is beneficial as ceramics typically cannot withstand tensile stresses. 
     In another brazing process, as illustrated in  FIGS. 3B and 3C , a manufacturer forms a butt joint  160  with a substantially continuous interface between the ceramic pad  146  and the metallic substrate  148 . Use of the butt joint  160  maximizes the contact area of the thrust bearing pads  142  and the rotating shaft by eliminating the relatively lager foot print of the metallic cup  148 , illustrated in  FIG. 3A . During the brazing process, as indicated in  FIG. 3B , a brazing interface  162  of the ceramic pad  146  is metalized first. For example, a metallic braze alloy layer  152  is formed about the outer periphery and the base of the ceramic layer  146 . A thin foil of a soft metal  164 , such as Cu, is then inserted between the base of the ceramic pad  146  and a support surface of a disc-shaped metallic base  168 . A first layer of soft braze alloy paste  165 , such as Ag—Cu, is applied on the support surface of the metallic base  168 , followed by placement of the Cu foil  164  over the first paste layer  165 , and placement of a second layer of soft braze alloy paste  167  (e.g., the same braze paste used for the first layer  165 ) on top of the Cu foil  164 . The ceramic pad  146  is then disposed on the paste-coated foil  164 . The assembly is brazed with a dead weight to hold all the components together during brazing to create the final thrust bearing pad  142 , such as illustrated in  FIG. 3C . 
     In another brazing process, as illustrated in  FIG. 3D , a manufacturer forms a butt joint  170  between ceramic pad  146  and the metallic substrate  178  with an interrupted interface prior to brazing the ceramic pad  146  and the metallic substrate  148 . For example, the metallic substrate  178  is interrupted by a set of channels  172  disposed between the metallic substrate and the ceramic pad  146 . In one arrangement, the metallic substrate  178  is configured with relatively thin vertical fins  175  disposed on either side of each channel  172  and which can minimize joint stress by making the interface more compliant. For example, with the vertical fins  175  configured as relatively thin structures, the vertical fins  175  can bend to accommodate stresses resulting from a thermal coefficient mismatch between the ceramic pad  146  and the metallic substrate  178 . The interrupted butt joint generates less stress between the ceramic pad  146  and the metallic substrate  178 , such as caused by a mismatch in the thermal expansion coefficients. Additionally, the vertical fins  175  include braze joints  167  disposed substantially at a top surface  177  of each fin  175 . Accordingly, the braze joints  167  are separated by the channels  172  thereby making the braze joints  167  discontinuous to reduce stress at the interface between the ceramic pad  146  and the metallic substrate  178 . 
     For the brazing methods described above, in one arrangement, the braze alloy is configured as a relatively soft and ductile material, such as Ag—Cu and/or pure Ag based braze alloys. With such a configuration, following the brazing process and during cooling of the thrust bearing pad  142  from the braze temperature, the relatively soft braze alloy is configured to plastically deform which diffuses stress between the ceramic pad  146  and the metallic substrate  148 . In addition, the relatively soft transition layer, such as the Cu foil described with respect to  FIGS. 3B and 3D , can undergo plastic deformation during cooling to diffuse stresses generated due thermal expansion mismatch. 
     In another arrangement, in order to minimize stresses within the ceramic pad  146 , multiple layers of materials can be deposited between the ceramic pad  146  and the metallic substrate  148  where the layers provide a gradual change in the thermal expansion coefficient. While the materials can be applied in a variety of way, in one arrangement, a manufacturer utilizes a PVD process to deposit various layers on the ceramic pad. For example, the ceramic pad  146  can be coated with a metallic sputter deposited layer, such as tungsten, having a low thermal expansion coefficient such as between about 4×10 −6  per ° C. and 5×10 −6  per ° C. This layer, in turn, can be coated with materials having subsequent layers of increasingly higher expansion coefficients, such as materials having a thermal expansion coefficient between about 6×10 −6  per ° C. and 18×10 −6  per ° C., until the thermal expansion coefficient of the last layer matches the expansion coefficient of the metallic substrate  148 . 
     In one arrangement, with reference to  FIG. 4 , a thrust bearing pad  242  includes a composite ceramic and metal binder material, or cermet, layer  246  brazed to a metal substrate  248 . For example, the cermet layer  246  can be manufactured from WC—Co, WC—Cr—Co, WC—Cr—Ni, Cr2C3-Cr—Ni, Al2O3-binder, Ni and Co base Tribaloys, and other cermets. The cermet layer  246  is configured to increase the fracture toughness of the thrust bearing pad  242 , as well as reduce the coefficient of friction associated with the metal substrate  248 . For example, fracture toughness of monolithic ceramics is between about 3 and 10 MPa-m −0.5  whereas fracture toughness of high strength steels is between about 30 and 90 MPa-m −0.5 . For cermets, fracture toughness values will be in between these two ranges based on the relative volume fractions of the ceramic and the metallic binder. 
     The cermet layer  246  can be applied to the metallic substrate  248  using a variety of techniques, such as by thermal spray, sintering of ceramic and metal powder, or by a ceramic/metal injection molding (MIM) process. For example, WC—Cr—Ni cermet can be applied on a metallic substrate  248  by a thermal spray process such as High Velocity Oxy Fuel (HVOF). During application, a mixture of WC and Ni—Cr alloy powder particles are injected into a supersonic oxygen and fuel gas stream. Fuel is ignited to create melted and semi-melted ceramic and metal powder droplets which impinge on the metallic substrate  248  creating a cermet layer  246 , as shown in  FIG. 4 . The cermet layer  246  can also be formed by electro deposition of a metal ceramic composite coating onto the metallic substrate  248 . The cermet layer  246  can also be deposited by Ni, Co, Fe base and similar hardfacing alloys. For example, the cermet layer  24  can be applied with a plasma transfer arc process. In another example, the cermet layer  246  can be created by applying a hardfacing alloy powder with an organic binder to the metallic substrate  248  and by heating the coated metallic substrate  248  up to the melting point of the hardfacing alloy. 
     In one arrangement, the cermet layer  246  includes a DLC coating layer  250  to reduce both the coefficient of friction and wear associated with the cermet layer  246 . 
     In one arrangement, a thrust bearing pad is configured as a relatively hard metallic layer disposed over a metallic substrate (not shown). With such a configuration, the thrust bearing pad has an increased toughness relative to conventional thrust pads to better withstand shock, load, and vibration during operation. The relatively hard metallic layer can be an electroplated hard chrome layer, electroplated Ni, Co and W alloys with ceramic particles, an electro composite layer, or thermal sprayed Co—Mo—Cr and Ni—Mo—Cr Tribaloys. While the relatively hard metallic layer can have a variety of hardness values, in one arrangement, the hard metallic layer can have a hardness range of between about 600 and 1000 Vicker&#39;s Hardness Number (VHN). By comparison, high strength steels have a hardness range of between about 400 and 600 VHN. During the application process, thermal spray is utilized to attach Tribaloy to the metallic substrate and an electrolytic process is utilized to attach an electrocomposite (e.g., Ni—Co—P—SiC) to the metallic substrate. In one arrangement, the relatively hard metallic layer has a thickness of between about 0.005 inches and 0.01 inches. In one arrangement, a DLC layer can be applied to the relatively hard metallic layer to reduce both the coefficient of friction and wear associated with the hard metallic layer. 
     In one arrangement, plastic materials can also be used with a metallic substrate to produce a thrust pad. 
     With reference to  FIG. 5A , a conventional plastic/metal composite thrust pad  500  is shown. During the assembly process, small bronze balls  502  are first brazed to a stainless steel base  504  to produce a rough surface. Various grades of engineered plastics  506  are then molded on the steel base  504  to create a mechanically interlocking structure which provides good mechanical bond between the plastic pad  506  and the metallic base  504 . The bronze balls  502  also provide a conductive path of heat transfer to remove frictional heat generated at the thrust pad and rotating shaft end interface. To promote heat transfer, plastics are generally filled with copper powder. 
     By contrast, in one arrangement and with reference to  FIG. 5B , a thrust bearing pad  342  includes a plastic layer  344  having a first interlocking structure  346 , such as a dovetail channel, configured to mate with a corresponding second interlocking structure  347  of the metallic substrate  348 . The thrust bearing pad  342  eliminates the brazing step described with respect to  FIG. 5A . The thrust bearing pad  342  simultaneously enhances heat transfer by replacing the above-referenced stainless steel base  504  and brazed bronze spheres  502  with a bronze base  348  having machined dovetail cross section channels. The thrust bearing pad  342  is more cost effective, provides superior heat transfer, and provides superior joint strength between the plastic pad  344  and the metallic base  348 , relative to the conventional thrust pad  500 . 
     With reference to  FIG. 2A , each of the thrust bearing pads  42  can be configured with a variety of shapes to maximize the load bearing area between the thrust bearing  40  and a rotating shaft and to reduce contact pressure on each thrust bearing pad  42 . For example, each of the thrust bearing pads  42  can be configured in a generally trapezoidal shape, as illustrated in  FIG. 6A , or in a generally circular shape, as illustrated in  FIG. 2C . 
     In one arrangement, to mitigate the relatively high contact pressure found between the thrust bearing pads  42  and a conventional rotating shaft, the thrust bearing pads  42  are configured with a hydrodynamic lift-off mechanism. For example, with continued reference to  FIG. 6A , the hydrodynamic lift-off mechanism  600  is defined by the set of thrust bearing pads  42 . As illustrated, first and third thrust bearing pads  42 - 1  and  42 - 3  define relatively shallow concave surfaces and the second thrust bearing pad  42 - 2  defines a relatively shallow convex surface. The alternating concave and convex surfaces defines, as the hydrodynamic lift-off mechanism  600 , a relatively wavy contact face for the thrust bearing  40 . In another example, and with reference to  FIG. 6B , the thrust bearing pad  42  defines, as the hydrodynamic lift-off mechanism  600 , a relatively shallow radial groove  604 . 
     During operation, the hydrodynamic lift-off mechanism  600 , as shown in either  FIG. 6A or 6B , forms a converging wedge of fluid (e.g., bearing oil) between the rubbing surfaces of the thrust bearing (e.g., the bearing surface of the contact layer) and the rotating shaft to decrease the fluid flow cross sectional area and to build up pressure, as shown in  FIG. 7 , which tries to separate the two rubbing faces. For both the designs in  FIG. 6A or 6B , the moving shaft surface drags the viscous fluid into a converging gap between the shaft end face and thrust bearing pad  42  with the shallow groove creating an opening pressure profile. The pressure mitigates contact pressure between the shaft end face and the thrust pad. 
     While various embodiments of the innovation have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the innovation as defined by the appended claims.