Abstract:
A paint sprayer includes a motor-driven multi-stage turbine for compressing air, which subsequently flows through a venturi in a paint gun to draw in paint or some other liquid from a canister. To prevent the high-pressure air from expelling lubricant from the motor&#39;s inboard bearing, a pressure responsive annular seal axially deflects and sealingly engages an axial face of the bearing. To avoid subjecting the seal and bearing to excess air pressure and temperature created by the turbine, a bypass bleed line diverts some air to atmosphere when the paint gun is closed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending patent application Ser. No. 12/462,093 filed on Jul. 29, 2009, which in turn claims the benefit of provisional patent application Ser. No. 61/207,774 filed Feb. 17, 2009 by the present inventors. 
    
    
     FIELD OF THE INVENTION 
     The subject disclosure generally pertains to paint sprayers and more specifically to means for protecting a bearing from detrimentally high air pressure. 
     BACKGROUND 
     Many paint sprayers comprise an air compressor that supplies pressurized air to a portable paint gun. High-pressure air discharging from the compressor and flowing through the paint gun draws up liquid paint from a canister, and a nozzle on the spray gun then sprays the mixture of paint and air to a target surface. Although such paint sprayers are effective, there seems to be an ongoing need to improve their quality and longevity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows one example of a paint sprayer with its motor shown in cross-section and its paint gun shown schematically. 
         FIG. 2  is similar to  FIG. 1  but showing the paint sprayer&#39;s valve in an open position rather than the closed position of  FIG. 1 . 
         FIG. 3  is a cross-sectional view showing the paint sprayer&#39;s annular seal and  FIG. 3   a  is an enlarged detail of circled section  3   a  of  FIG. 3 . 
         FIG. 4  is a cross-sectional view similar to  FIG. 3  but showing another example seal arrangement. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3  and  FIG. 3   a  show one example of a paint sprayer  10 . In this particular example, paint sprayer  10  comprises an outboard motor bracket  12 , an inboard motor bracket  14  with a bearing bore  16 ; a stator  18  interposed between outboard motor bracket  12  and inboard motor bracket  14 ; an outboard rolling element bearing  20  supported by outboard motor bracket  12 ; an inboard rolling element bearing  22  that includes an outer race  24 , an inner race  26 , and a plurality of rollers  28  interposed between races  24  and  26 . The terms, “roller” and “rollers” refer to any shaped element meant for rolling between two bearing races. Examples of such rollers include, but are not limited to, spherical balls, cylinders, cones segments, etc. In some examples, a bearing retainer  30  holds inboard bearing  22  in place. In other examples, inboard bearing  22  is held in place by an adhesive and/or an interference fit between the bearing&#39;s outer race and the bore in which the bearing is installed. 
     Outer race  24  is supported by inboard motor bracket  14 . Outer race  24  and inner race  26  provide a first axial face  32  and a second axial face  34  that are substantially concentric, face away from outboard bearing  20 , and are radially spaced apart from each other. In some examples, axial faces  4  and/or  4  are the outermost axial surface faces that face away form outboard bearing  20 . For the illustrated example, first axial face  32  is on outer race  24 , and second axial face  34  is on inner race  26 ; however, the nomenclature of the terms, “first” and “second” could be reversed, i.e., first axial face  32  could be on inner race  26  and second axial face  34  could be on outer race  24 . 
     For this example, paint sprayer  10  further comprises a shaft  36  extending through bearing bore  16  of inboard motor bracket  14 . Shaft  36  is supported by inboard bearing  22  and outboard bearing  20 . This example of paint sprayer  10  also comprises a rotor  38  supported by shaft  36 ; a discharge volute  40  (sometimes known as a discharge horn) adjacent to inboard motor bracket  14  and defining a discharge opening  42 ; a turbine housing  44  adjacent to discharge volute  40  and defining a suction inlet  46 ; and a plurality of turbine wheels  48  attached to shaft  36  and disposed within turbine housing  44  such that shaft  36 , rotor  38 , and the plurality of turbine wheels  48  rotate as a unit at a rotational speed that may vary in response to changing discharge pressure. The present invention is particularly effective with rotor  38  and stator  18  being a universal motor with rotor  38  rotating at speeds in excess of 10,000 revolutions per minute. 
     This example of paint sprayer  10  also includes a paint canister  50  to hold a liquid  52  (e.g., paint, stain, etc.) and a paint gun  54  connected to paint canister  50 . Paint gun  54  has an air inlet  56 , a spray outlet  58 , and a paint gun valve  60 . A finger trigger  61  moves paint gun valve  60  between an open position ( FIG. 2 ) and a closed position ( FIG. 1 ) to respectively open or obstruct flow from air inlet  56  to spray outlet  58 . To convey pressurized air to paint gun  54 , a hose  62  connects discharge opening  42  of discharge volute  40  to air inlet  56  of paint gun  54 . 
     To prevent high pressure, high temperature air from flushing the lubricant out from within inboard bearing  22 , paint sprayer  10  includes an annular seal  64 . Seal  64  encircles shaft  36  and axially engages first axial face  32  and second axial face  34  of inboard bearing  28  with an upstream surface  66  of seal  64  facing away from inboard bearing  22 . Seal  64  is held against first axial face  32  (on race  26  as illustrated, or on race  24 ) and seal  64  is pneumatically urged against second axial face  34  with an axial pneumatic pressure  68  against upstream surface  66 . The axial pneumatic pressure  68  increases with the rotational speed of the plurality of turbine wheels  48 , wherein the rotational speed of the plurality of turbine wheels  48  increases in response to paint gun valve  60  moving from the open position to the closed position. 
     In some examples of paint sprayer  10 , seal  64  has a deflection coefficient of 0.0005 to 0.02. Such a deflection coefficient allows seal  64  to flexibly press in axial sealing contact against second axial face  34  of inboard bearing  22  yet provides seal  64  with sufficient stiffness to avoid excessive distortion of the seal. The deflection coefficient is a ratio defined as a numerator divided by a denominator, wherein the numerator is the axial pneumatic pressure  68  (against upstream surface  66 ) multiplied by a difference between the seal&#39;s outer diameter  70  and inner diameter  72 . The denominator of the deflection coefficient is an axial material thickness  74  of seal  64  multiplied by a Young&#39;s modulus of elasticity (i.e., specifically the tensile modulus of elasticity at 73° F.) of the seal&#39;s material. 
     The axial pneumatic pressure  68 , outer diameter  70 , inner diameter  72 , the axial material thickness  74 , and the Young&#39;s modulus of elasticity can be in any units that render the deflection coefficient a dimensionless ratio. For example, the axial pneumatic pressure can be units of psig (pounds per square-inch), diameters  70  and  72  can be in units of inches, the Young&#39;s modulus of elasticity can be in units of psi (pounds per square-inch), and the axial material thickness  74  can be in units of inches. 
     The axial pneumatic pressure is in terms of gage pressure rather than absolute pressure, the seal&#39;s axial material thickness  74  is taken at a radial midpoint between races  24  and  26 , and the Young&#39;s modulus of elasticity is with respect to tension rather than flexural. The Young&#39;s modulus of elasticity is with respect to the material being tested at 73° Fahrenheit, wherein the 73° F. is for testing purpose only, and that the actual temperature of seal  64  during operation can be dramatically higher than that. 
     In some examples, outer diameter  70  of seal  64  is about 1.0 inch, inner diameter  72  is about 0.5 inches, thickness  74  is about 0.032 inches, and seal  64  is comprised mostly or entirely of polytetrafluoroethylene. With a Young&#39;s modulus of elasticity of about 70,000 psi, this example provides a deflection coefficient of about 0.002 when pressure  68  is at 10 psig. In some examples, seal  64  is comprised of polytetrafluoroethylene impregnated with an additive such as molybdenum or graphite for lubricity. Other examples of seal  64  are made of other materials and/or different dimensions. 
     In some examples, the rotation of impellers  48  provides a discharge pressure  68  (axial pneumatic pressure against the seal&#39;s upstream surface  66 ) of about 10 psig. In other examples, the discharge pressure ranges from 5 to 15 psig. 
     Some examples of paint sprayer  10  includes a bypass bleed line  76  connecting discharge opening  42  of discharge volute  40  in restricted fluid communication with atmosphere to place upstream surface  66  of seal  64  in restricted fluid communication with atmosphere, thereby limiting pneumatic pressure buildup within discharge volute  40 , limiting the axial pneumatic pressure  68  against upstream surface  66  of seal  64 , and providing at least some airflow through discharge volute  40  when valve  60  is in the closed position. The point at which bypass bleed line  76  connects to paint sprayer  4  can be anywhere downstream of at least one impeller  48  and upstream of valve  60 . The expression, “restricted fluid communication” means that for a given pressure differential, the airflow through line  76  is less than the airflow through hose  62  when valve  60  is fully open. There are many ways of providing bleed line  76  with restricted airflow. Examples of such ways include, but are not limited to, line  76  having a smaller inner diameter than hose  62  or an orifice, capillary or some type of valve in series-flow relationship with line  76 . In some examples, bypass bleed line  76  includes a pressure relief valve that open in response to pressure  68  within discharge volute  40  reaching some predetermined limit. 
     To establish a predetermined axial position of the impeller nearest discharge volute  40 , some examples of paint sprayer  10  include a sleeve  78  on shaft  36 . Once installed, sleeve  78  is considered as being part of shaft  36 , i.e., shaft  36  includes sleeve  78 . Sleeve  78  axially engages inner race  26  of inboard bearing  22  and axially engages an axial surface of the nearest impeller  48 . In some examples, sleeve  78  radially engages an inner periphery  80  of seal  64 . In other examples, radial clearance  79  exists between sleeve  78  and the seal&#39;s inner periphery  80 . 
     It should be noted that the expression, “paint sprayer” and “paint gun” refer to any devices for spraying any liquid including, but not limited to, paint. The illustrated example of paint sprayer  10  has four impellers for four sequential stages of compression; however, paint sprayer  10  can have any number of impellers, more or less than four. Although bypass bleed line  76  limits pressure  68  in discharge volute  40 , the pressure is not limited to any particular value and may continue to increase with increasing rotational speed of impellers  48 . Outer race  24  of inboard bearing  22  can be a single piece as shown in the example, or outer race  24  can include additional pieces including, but not limited to, annular shims, rings, collars, spacers, sleeves, bushings, etc., wherein such additional pieces are fixed relative to outer race  24 . Inner race  26  of inboard bearing  22  can be a single piece as shown in the example, or inner race  26  can include additional pieces including, but not limited to, annular shims, rings, collars, spacers, sleeves, bushings, etc., wherein such additional pieces are fixed relative to inner race  26 . In some examples, as shown in  FIG. 3 , inboard bearing  22  includes its own integral seals or shields  81  that are considered generally non-removable from bearing  22 . 
     It should also be noted that various multiple component parts of paint sprayer  10  could be combined into single parts and vice versa. For the example shown in  FIGS. 1-3 , for instance, inboard motor bracket  14  and discharge volute  40  are a one-piece integral extension of each other. Canister  50  can be attached directly to paint gun  54  as shown, but in other examples of paint sprayer  10 , canister  50  is a separate piece with a long hose connecting the relatively remote canister  50  to paint gun  54 . In either case, a venturi  82  can be used to enable pressurized air flowing through paint gun  54  to draw in liquid  52  up from within canister  50 . 
     Additional information related to paint sprayer  10  is found in U.S. Pat. No. 6,952,062, which is specifically incorporated by reference herein. 
     In some examples of paint sprayer  10 , surprising and unexpected improvement in bearing protection and bearing life was achieved when seal  64  or  64 ′ was comprised mostly or entirely of metal instead of plastic with some radial clearance (e.g., 0.010 inches) at the metal seal&#39;s inner periphery ( FIG. 3   a ) or at the metal seal&#39;s outer periphery (seal  64 ′ of  FIG. 4 ). Although seal  64  can be made of various metals and alloys, seal  64  being made of steel or brass works particularly well. In some examples, seal  64  is comprised of steel with outer diameter  70  of seal  64  being about 1.0 inch, inner diameter  72  being about 0.5 inches, and thickness  74  being about 0.020 inches. In some examples, the outer diameter of the seal is a few thousandths of an inch less than the adjacent bearing&#39;s outer diameter, and/or the inner diameter of the seal is a few thousandths of an inch greater than the shaft&#39;s outer diameter (or a few thousandths of an inch less than the outer diameter of a shaft&#39;s sleeve or spacer). 
     Although certain example methods, apparatus, and articles of manufacture have been described herein, the scope of the coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.