Abstract:
An airless paint sprayer has a pressure dampener in the paint supply line to the pump for providing consistent atomization at the spray gun orifice. During peak pressure times within the system and when an inlet check valve is open, paint within the pressure dampener is added to the paint in the supply line to overcome the tendency for the paint to cavitate and delivery an uneven spray pattern.

Description:
This application is a continuation of abandoned U.S. Ser. No. 08/734,901 filed Oct. 22, 1996, which in turn is a continuation of abandoned U.S. Ser. No. 08/370,377 filed Jan. 9, 1995, and assigned to the assignee of this invention, and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to airless paint sprayers, and more particularly, to a mechanism for providing a more consistent spray of paint without a loss of pressure over a range of operating parameters. 
     In a typical airless paint sprayer, a piston driven diaphragm pulls the paint from a supply line into a paint holding or diaphragm chamber. A spray gun has a trigger which, when depressed, opens a valve to allow the pressurized paint in the chamber to flow to a gun nozzle and atomize as it exits a paint orifice for spraying onto a surface to be coated. 
     Airless paint sprayers commonly include a suction tube inserted within a can of paint through which the paint is delivered to the diaphragm chamber. Suction is created in the suction tube by a deformable diaphragm which is secured around its perimeter. A central portion of the diaphragm is oscillated, by a piston-driven hydraulic system, for example, between a convex and a concave configuration to thereby pull the paint toward the diaphragm and hence force it outwardly to the spray gun. 
     In another format, a rotating eccentric cam drives a bearing which in turn drives a piston. The piston is coupled to the diaphragm and the rotation of the cam drives the piston to thereby move the diaphragm to and between the convex and concave configurations. The paint is drawn from the can through the suction tube and inlet valve toward the diaphragm and into the diaphragm chamber to be discharged through the spray gun. 
     Despite past efforts, the use of such systems for spraying paint, for example, have been subject to inconsistent results and unexplained, undesirable variations. For example, on a given day, a system may not work well with one paint, failing to fully atomize it and “spattering” it onto a surface while operating efficiently with the same paint at another time or in another location. 
     Other problems which are commonly identified in such airless paint sprayers include ineffective spraying of paint of a first type but efficient spraying of paint of a second type. Several possible causes of problems of this type have been proposed such as lack of consistent priming, paint buildup, clogged filters, paint viscosity, humidity, etc. However, these problems occur even when a problem paint is thinned to the general consistency of water, the filters are clean, or the flow path of the paint unclogged. These symptoms can even be apparent in using one paint while not in using another even though the paints have similar viscosities. 
     Accordingly, the effective and consistent use of an airless paint system appears to be a sometimes thing dependent on a variation of parameters, ever changing. 
     Therefore, it is apparent that there is a need for an airless paint sprayer which does not exhibit a loss of pressure while spraying and can reliably, efficiently and effectively spray all types of paint at a wide range of operating conditions without the above identified problems and inconsistencies. 
     It has thus been a primary objective of this invention to provide an improved airless paint sprayer which does not loose pressure while spraying. 
     It has been a further objective of this invention to provide such a paint sprayer which can be efficiently and effectively used with a variety of paint types without loosing pressure while spraying. 
     It has been a still further objective of this invention to provide such a paint sprayer which can be used with a variety of paints and paint viscosities to consistently atomize and spray the paint in a desired homogeneous pattern. 
     SUMMARY OF THE INVENTION 
     To these ends, a preferred embodiment of the invention contemplates the use of a dampener on the spray liquid or paint intake side of the paint sprayer. 
     One aspect of the invention is the realization of the basic problem which is responsible for inconsistent paint spraying performance. According to the invention, that problem is the inconsistency of the system by which paint is delivered from an open container to the pumping or diaphragm chamber of the spraying apparatus. 
     Typically the suction tube between the inlet check valve of the pumping chamber and the open paint container is vertically oriented and may be 1 to 2 feet long. Paint is sucked up from the container in this tube, through the inlet check valve and into the pumping chamber. In order to suck the paint past the inlet check valve the diaphragm must create a pressure drop in the chamber and it does so by virtue of its eccentric drive or by the piston-driven hydraulic drive. The nature of the diaphragm is cyclical; the diaphragm constantly accelerating and decelerating through each sucking and pumping direction. 
     For example, as the diaphragm is moved to enlarge the chamber for sucking paint up the supply tube, it accelerates due to the eccentric action of the piston. It decelerates as it reaches its maximum stroke and the check valve closes. During this time, the paint in the tube is subjected to a pressure drop which first accelerates then decelerates to near equilibrium when the inlet check valve closes. Thereafter, the diaphragm is accelerated into the chamber to pump out the paint therein. Once this stroke ends, the diaphragm accelerates in a reverse direction to again open the inlet check valve and suck paint up from the tube. Thus, the eccentric rotation of the cam drive and the acceleration/deceleration of the rod following the cam create acceleration spikes in the flow of the paint during each cycle. The acceleration spikes correspond to specific points or areas on the drive cam which result in significant acceleration/deceleration of the rod. These acceleration/deceleration forces are transferred from the rod to the diaphragm thereby resulting in acceleration spikes in the flow of the paint drawn into the diaphragm chamber through the inlet check valve and suction tube. The paint is thus being accelerated and decelerated with each stroke of the diaphragm. According to this invention, it is believed that the force required to accelerate the paint was in many instances greater than the paint itself could support without cavitation or boiling. 
     Accordingly, it has been discovered that the paint was cavitating or boiling in the diaphragm chamber in many instances due to the sum of the various forces to which the paint is subjected. Factors which contribute to paint cavitation in such paint sprayers are the ambient temperature and barometric atmospheric pressure (i.e., altitude) at which the sprayer was operated. Other factors which may contribute are the dimensions, configurations and tolerances of the suction tube, and the viscosity of the paint. Thus, under specific conditions, it has now been discovered that the force required to overcome the inertia of the paint and accelerate it through the system was greater than the paint could support. This resulted in the cavitation or boiling of some of the liquids in the paint, and the resulting interruption of full paint flow through the sprayer, a loss of pressure while spraying, and inconsistent spraying results, such as “spattering” and inconsistent atomization. 
     Accordingly, the dampener of one embodiment of the present invention comprises a generally T-shaped fitting connected to the suction tube leading to the inlet check valve of the pump or diaphragm chamber. The T-shaped fitting includes a first leg having a port through which paint is received from the suction tube inserted in the paint can or reservoir and a second leg perpendicular to the first leg through which paint is discharged via a second port to the inlet check valve of the pump. A third leg of the T-fitting comprises a closed chamber which is in line with the first leg and perpendicular to the second leg in a presently preferred embodiment of the invention. Other configurations of the dampener for different sprayer configurations are possible and within the scope of this invention. 
     The dampener of this invention solves a significant number of occurrences of the problem of pressure loss during the operation of the paint sprayer and the inability of the sprayer caused, in part, by acceleration spikes transmitted in the paint. This is initially accomplished with the T-shaped fitting positioned in-line on the suction tube on the intake side of the inlet check valve. The air trapped in one of the legs of the T-fitting dampens the acceleration spikes to thereby even the flow of the paint. While the paint in the discharge or second leg of the T-fitting is still subjected to some of the acceleration spikes and the acceleration/deceleration forces, the volume of paint which remains on the intake side or of the first leg of the T-fitting is isolated from the acceleration spikes. 
     More particularly, according to the invention, a dampening chamber such as the T-fitting described is operatively connected to the paint supply path upstream of the inlet check valve. On start up, the pump is primed normally, however it will be appreciated a slight negative pressure is created in the dampening chamber. On operation, when the diaphragm is pushed into the pumping chamber and the inlet check valve is closed, the pressure drop on the supply side of the inlet check valve is reduced. The slight negative pressure in the dampening chamber pulls an amount of paint therein. 
     When the diaphragm starts its reciprocal motion and begins to accelerate, the inlet check valve is open. As the pressure drop increases in magnitude, the suction on the paint supply increases to a peak. However, according to the invention, not only is the paint in the suction tube subjected to this drop, but the paint in the dampening chamber is sufficient to feed the increased paint demand. The intake paint is thus made up not only of paint from the supply can and in the tube above it, but also paint in the dampening chamber. 
     During operation, the pressure in the dampening chamber is greater than the pressure in the supply side of the fitting connected to the inlet valve. As a result, the paint at the higher pressure in the dampening chamber feeds the supply side of the fitting during extreme acceleration of the diaphragm. Thus, the acceleration spikes applied to the supply side paint are reduced and are not excessive enough to cause the paint to cavitate and incompletely fill the pumping chamber. 
     Thereafter, on a pressure stroke, the pumping chamber is full and design pressure drop at the spray orifice is maintained sufficiently to support consistent atomization and paint spray performance. At the same time, closure of the inlet check valve allowed the now slight negative pressure in the dampening chamber to suck up a small amount of make-up paint from the suction tube in readiness for another dampening cycle. 
     The dampener of this invention thus solves a significant number of the problems identified hereinabove with airless diaphragm paint sprayers. With the inclusion of the dampener of this invention, the even flow of the paint from the tube to the spray gun without cavitation, loss of pressure, or other problems associated with airless diaphragm paint sprayers is attained. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The objectives and features of this invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective view of an airless paint sprayer according to the invention; 
     FIG. 2 is a cross-sectional view along line  2 — 2  of FIG. 1 of the T-shaped dampener fitting according to this invention; and 
     FIG. 3 is an enlarged cross-sectional view of the dual spring inlet check valve assembly of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An airless paint sprayer  10  as shown in FIG. 1 includes a mobile hand cart  12  supported on the ground by wheels  14  mounted upon an axle  16  for rotation. The hand cart  12  includes a frame  18  to support a pump  20  and a motor  22  which draws paint from a can  24  or other receptacle mounted on a generally L-shaped carriage  26  secured to a lower portion of the frame  18 . The paint sprayer  10  can be moved about by grasping an upper generally U-shaped handle  28  and tilting the unit backwards to thereby raise the carriage  26  and paint  24  can supported thereon upwardly to balance the sprayer  10  upon the wheels  14 . Other structure for carrying the pump and motor  20 ,  22  and for supporting them over a paint container or spray liquid container can be used. 
     In operation, the paint is drawn from the can  24  through a generally cup-shaped intake  30  having a plurality of cut-outs  32  through which the paint enters the intake  30  supported on a bottom wall of the can  24 . The paint is drawn from the can  24  through the intake  30  and into a suction tube  34 . The paint flows through the suction tube  34  and into the pump  20  for pressurized delivery to a supply line  35  and spray gun  37  through which the pressurized paint is sprayed in the direction of a surface to be coated. The route of the paint from the can  24  through the pump  20  is identified as a paint path P in FIG.  2 . 
     Attached to the upper end of the suction tube  34  is a generally T-shaped fitting  36 . The T-shaped fitting  36  in one embodiment includes a first leg  38  which is inserted into the upper end of the suction tube  34  as shown in FIG. 2 and a first port  40  through which the paint is drawn from the suction tube  34 . A second leg  42  of the T-shaped fitting  36  is generally perpendicular to the first leg  38  and includes a second port  44  through which the paint exits the fitting  36 . Perpendicular to the second leg  42  and generally in line with the first leg  38  of the fitting  36  is a third leg  46  which extends upwardly and includes a third port  48 . The third port  48  is closed by a cap  50  which is secured on an upper end of the third leg  46  by inter-engaging threads on the cap  50  and an outer surface of the third leg  46  or another appropriate fastening mechanism. The cap  50  secured to the third leg  46  closes the third port  48  and defines a volume or dampening chamber  52  within the third leg  46 . 
     In a presently preferred embodiment of the T-shaped fitting  36 , the first leg  38  is approximately one inch in length and the first port  40  has an inner diameter of about 0.48 inches. The second leg  42  is approximately 2.1 inches in length as measured from the centerline of the first leg  38  and the second port  44  has an inner diameter of 0.78 inches. The third leg  46  is approximately 2.1 inches in length as measured from the centerline of the second leg  42  and the third port  48  has an inner diameter of approximately 0.9 inches. The T-shaped fitting  36  is preferably manufactured from 10% glass-filled nylon. 
     The second leg  42  of the T-shaped fitting  36  is connected to an inlet valve cartridge  54  by a coupling  56  or other appropriate mechanism as known in the art. The inlet valve cartridge  54  is mounted to a pump housing  58  of the pump  20 . The housing  58  is secured to the pump  20  as shown in FIG. 2 by bolts  60  or other mechanical fasteners. Seated within an end of the inlet valve cartridge  54  and mounted in the housing  58  is an inlet check valve assembly  62  which includes an elongated valve stem  64  projecting axially within the inlet valve cartridge  54 , and having a disk-shaped valve head  66  secured on one end opposite from another end  68  thereof. The inlet check valve assembly  62  translates between open and closed positions to permit the flow of paint through the inlet valve cartridge  54  to the hose  35  and spray gun  37  upon actuation by a trigger  39  or other appropriate mechanism as is well known by those of ordinary skill in the art. 
     The valve head  66  is positioned proximate a diaphragm chamber or pumping chamber  70  and is spaced from a deformable diaphragm  72 . The diaphragm  72  is secured around its perimeter so that a central portion of the diaphragm  72  can oscillate between convex and concave configurations. As it is pulled to the left as viewed in FIG. 2, it pulls the paint through the inlet valve cartridge  54  and the open inlet check valve assembly  62  toward the diaphragm  72 . As it moves to the right, it pressurizes chamber  70  and pumps paint through an outlet  73  having a check valve  75  and to the spray tube  35  and spray gun  37 . The deformable diaphragm  72  has a stem  74  secured to a central portion  76 . The stem  74  is driven indirectly from a piston and eccentric cam (not shown) as is well known in airless paint sprayers of the type described above. 
     As best seen in FIG. 3, the inlet check valve assembly  62  is biased to a closed position in which the valve head  66  is in sealing contact with a surface  78  of an annular seat  81 . The seat  81  is juxtaposed to a limiter  80 . The inlet check valve assembly  62  is shown in FIGS. 2 and 3 in the closed position with the valve head  66  in contact with the surface  78  of the seat  81 . The valve stem  64  projects through a hole  82  in the center of the limiter  80 . The valve  62  is biased toward the closed position by a pair of nested helical compression springs  84 ,  86  according to a presently preferred embodiment of this invention. The outer, primary spring  84  is mounted between the limiter  80  and an opposing retainer  88 . The end coils of the primary spring  84  are seated on flanges  92  on the retainer  88  and on the limiter  80  as shown in FIG.  3 . The retainer  88  is juxtaposed to an annular push-on retainer  94  proximate the end  68  of the valve stem  64 . The primary spring  84  is preloaded to a partially compressed configuration thereby urging the retainer  88  and the limiter  80  apart and biasing the valve stem  64  into a closed configuration with the valve head  66  in sealing contact with the surface  78  on the seat  80 . 
     The secondary spring  86  is nested within the primary spring  84  and around the valve stem  64 . The secondary spring  86  is seated within sockets  96  formed within the centers of the retainer  88  and the limiter  80  as shown in FIG.  3 . According to this invention, the secondary spring  86  may contribute to the preload of the valve  64  in the closed configuration or the secondary spring  86  may be offset within the sockets  96  from either or both of the retainer  88  and the limiter  80  so that it is not compressed while the valve stem  64  is in the closed configuration. 
     According to a presently preferred embodiment of this invention, the primary spring  84  has a relatively low spring rate and the secondary spring  86  has a significantly larger spring rate. In one embodiment of an airless paint sprayer  10  according to this invention, the primary spring  84  has a rate of approximately 1 lbf/in and the secondary spring  86  has a rate of 6 lbf/in. The primary spring  84  maintains engagement with both the retainer  88  and the limiter  80  and thereby remains in at least a partially compressed configuration. The relatively low spring rate of the primary spring  84  reduces sensitivity to valve wear and dimensional variation of the inlet check valve assembly  62  components. With the valve  64  in the closed position, the secondary spring  86 , depending on tolerance conditions, can range from being preloaded at approximately 0.01 inches of deflection to 0.20 inches of freedom in one particular embodiment of this invention. If the secondary spring  86  is engaged in the closed position, the combined preload of the primary and secondary springs  84 ,  86  should not exceed that of standard single spring inlet check valve assembly designs. As a result, the inlet check valve assembly  62  according to this invention can be used in many standard airless paint sprayers without detriment to the system, vacuum or priming operations. 
     During operation of the airless paint sprayer  10 , the deformable diaphragm  72  operates to draw paint into the diaphragm chamber  70  with the inlet check valve assembly  62  open and the head  66  spaced from the surface  78  of the limiter  80 . In the open configuration, the primary and secondary springs  84 ,  86  are compressed and the retainer  88  and the limiter  80  are drawn closer together as a result of the travel or movement of the valve stem  64  so that the valve head  66  is spaced from the surface  78 . The primary and secondary springs  84 ,  86  of the inlet check valve assembly  62  according to this invention enable the valve travel distance to be increased relative to known single spring inlet check valve assemblies. The increased travel of the valve head  66  enables greater fluid flow through the valve  62  without cavitation or boiling of the paint over a wide range of operating conditions, barometric pressures, ambient temperatures, and altitudes. 
     The T-shaped fitting  36  contributes to significantly reducing cavitation in the paint by dampening the energy spikes transmitted in the fluid from the deformable diaphragm  72 . After the paint sprayer  10  has been primed and during operation, the paint level in the third leg  46  of the T-shaped fitting  36  is indicated by reference numeral  98 . The chamber  52  in the third leg  46  contains a trapped volume of air, preferably at a partial vacuum of greater than about 1.0 in-Hg and approximately 3.0 in-Hg in one preferred embodiment. The air trapped within the chamber  52  in the third leg  46  of the T-shaped fitting  36  dampens the acceleration spikes being transmitted from the diaphragm  72  through the paint in the inlet tube  54  and second leg  42  of the T-shaped fitting  36  to thereby even the flow of the paint. While the paint in the discharge or second leg  42  of the T-shaped fitting  36  may be subjected to some of the acceleration spikes and acceleration/deceleration forces generated by the deformable diaphragm  72 , the volume of paint which remains on the intake side of the T-shaped fitting  36  or the first leg  38  is isolated from the acceleration spikes. The volume of paint within the chamber  52  in the third leg  46  is drawn into the second leg  42  along with paint from the suction tube  34  and first leg  38  while the inlet check valve  62  is open and drawing paint therethrough. The added supply of paint from the chamber  52  overcomes the acceleration spikes and inhibits cavitation in the paint path P. Therefore, the paint within the suction tube  34  does not cavitate, boil, or breakdown thereby avoiding a significant number of occurrences of pressure loss in the paint sprayer  10  and other problems previously associated with airless diaphragm paint sprayers. 
     As a result of the T-shaped fitting  36  which dampens acceleration spikes and acceleration/deceleration forces transmitted in the paint, the problems of cavitation and loss of pressure in airless paint sprayers  10  are corrected in a significant number of instances without major paint sprayer redesign or other system changes. 
     It will be appreciated that although the dampener and dual spring inlet check valve assemblies are shown and described herein, that either feature can be used alone to inhibit paint cavitation in the paint path of the airless sprayer. Each of these features and inventions independently solve the above described problems and should not be considered to be mutually dependant upon each other to attain the goals and objectives of this invention. 
     From the above disclosure of the general principles of the present invention and preceding detailed description of a preferred embodiment, those skilled in the art will readily comprehend the various modifications to which the present invention is susceptible. For example, the invention has been shown and described herein with reference to a paint sprayer, but could readily be used in other systems. Therefore, I desire to be limited only by the scope of the following claims and equivalents thereof.