Abstract:
The invention relates to a liquid paint pumping apparatus comprising a piston reciprocable in a cylinder to pump liquid paint during a substantially horizontal pumping stroke of the piston and to charge the cylinder with paint during a reverse stroke of the piston. The apparatus further comprises a check valve disposed within the piston, the check valve comprising: a ball member, a seat, a spring urging the ball member into engagement with the seat, and a plastics cage guiding movement of the ball towards and away from the seat. The seat receives the ball member so as to form a seal to prevent flow of liquid paint between the ball and the seat during the pumping stroke of the piston, and the ball member moves away from the seat during the reverse stroke of the piston.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus for pumping liquid paint, and particularly, but not exclusively to a check valve for use in a liquid paint pumping apparatus. 
       BACKGROUND 
       [0002]    Reciprocating paint pumps are widely used for providing a pressurised flow of liquid paint to spray booths or the like, particularly in automotive finishing plant. These pumps employ pistons that reciprocate inside cylinders. In a first stroke of a piston paint is drawn into a cylinder chamber through an inlet port. In a second, reverse stroke of the piston paint is pumped out of the chamber through an outlet port. To prevent paint being pumped out through the inlet port, or drawn back in from the outlet port, automatic check valves are provided. Thus, for example, a check valve in the outlet port is configured to open automatically under the influence of paint pressure from the pump during the second stroke of the piston to allow the paint to be pumped out, but to close when the paint pressure drops as the piston draws in paint from the inlet port during the first stroke. 
         [0003]    Double-action pumps employ the above principle using two cylinder chambers so that paint is drawn into one chamber while being pumped out of the other chamber, thereby maintaining a substantially continuous flow of paint. 
         [0004]    Check valves are known that use a ball member to engage a seat for closing the valve. The ball and seat are retained by a valve housing, and the valve is configured such that the ball lifts clear of the seat under the influence of paint pressure. The paint flows through the housing past the ball in passages defined by a cage structure. The cage structure guides the ball member within the housing while allowing the paint to flow past. Conveniently, the cage structure is integrally formed with the housing (for example as a metal casting). In many pumping arrangements the valves are vertically aligned so that the paint pressure lifts the ball clear of the seat, and when the pressure is reduced on the reverse stroke of the pump, the ball falls under gravity back into engagement with the seat. 
         [0005]    The movement of the ball away from the seat is guided by the cage structure such that the surface of the ball contacts the surfaces of the cage structure. Prolonged use results in the problem of wear or abrasion of the ball surface, which can eventually result in valve leakage. A further problem is that these check valves can be very noisy due the impacting contact between the ball and the cage surfaces. 
         [0006]    In reciprocating paint pumps, it is often preferred that the pistons are arranged to move horizontally. Also, to maintain a more uniform flow of paint it is frequently preferred to employ two or more pistons, reciprocating out of phase with one another in corresponding cylinders. In such cases, to ensure that the balls correctly engage their seats during the reverse strokes of the pistons, it is either necessary to mount inlet and outlet check valves in a vertical orientation, or if the check valves are oriented horizontally, to provide springs that urge the balls towards their seats. However, the use of springs contacting the balls adds further to the problems of ball wear and leakage. 
         [0007]    It is an object of the present invention to alleviate these problems. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect of the present invention there is provided a liquid paint pumping apparatus comprising a piston reciprocable in a cylinder to pump liquid paint during a substantially horizontal pumping stroke of the piston and to charge the cylinder with paint during a reverse stroke of the piston. The apparatus further comprises a check valve disposed within the piston, the check valve comprising: a ball member, a seat, a spring urging the ball member into engagement with the seat, and a plastics cage guiding movement of the ball towards and away from the seat. The seat receives the ball member so as to form a seal to prevent flow of liquid paint between the ball and the seat during the pumping stroke of the piston, and the ball member moves away from the seat during the reverse stroke of the piston. 
         [0009]    It is an advantage that the mounting of the check valve inside the piston reduces the size of the pumping apparatus. In addition, the use of the plastics cage helps to reduce the wear of the ball, while the check valve is mounted horizontally within the piston. 
         [0010]    According to a second aspect of the present invention there is provided a liquid paint pumping apparatus comprising a piston reciprocable in a cylinder and configured to pump liquid paint during a substantially horizontal stroke of the piston. The apparatus further comprises a substantially horizontally mounted check valve. The check valve comprises: a ball member; a seat for receiving the ball member so as to form a seal to prevent flow of liquid paint between the ball and the seat, the check valve being configured for the ball to move away from the seat under the influence of paint pressure; 
         [0000]    a spring providing a biasing force urging the ball member into engagement with the seat;
 
a plastics cage configured to guide movement of the ball towards and away from the seat; and a spring keep interposing the spring and the ball member.
 
         [0011]    In embodiments of the first and second aspects, the pump comprises a plurality of substantially horizontally mounted pistons reciprocable in corresponding cylinders. The pump may be configured for the pistons to reciprocate out of phase with one another. 
         [0012]    According to a third aspect of the present invention there is provided check valve for use in a liquid paint pumping apparatus, the check valve comprising a housing for retaining:
       a ball member;   a seat for receiving the ball member so as to form a seal to prevent flow of liquid paint between the ball and the seat, the check valve being configured for the ball to move away from the seat under the influence of paint pressure;
           a plastics cage configured to guide movement of the ball away from the seat and to provide a passage for flow of liquid paint through the housing, a spring providing a biasing force urging the ball member into engagement with the seat; and   
           a spring keep interposing the spring and the ball member.       
 
         [0017]    Preferably, the cage is separable and removable from the housing. 
         [0018]    The spring keep may be of a plastics material. The plastics material of the cage and/or of the spring keep may be an acetal resin engineering plastic such as polyoxymethylene (POM), polytrioxane or polyformaldehyde. 
         [0019]    It is an advantage that the use of a plastics material for the cage reduces the effects of abrasion and wear on the ball. Instead it is the cage that suffers the effects of the contact from the ball. Wear to the cage will not result in valve leakage. Moreover, where the cage is separable and removable from the housing, it can readily be replaced. Furthermore, the use of a plastics spring keep helps to protect the ball member from becoming scratched and worn resulting from contact with the spring. A further advantage with the use of plastics for the cage is that the noise caused by the contacting surfaces is greatly reduced when compared with, say, a metallic cage. 
         [0020]    In embodiments of the invention, the check valve is configured for mounting in a vertical orientation such that the ball member lifts away from the seat under the influence of paint pressure. 
         [0021]    The spring may be a helically coiled spring. The spring may be a compression spring. 
         [0022]    In a preferred embodiment, the helically-coiled spring is shaped to prevent contact with adjacent surfaces. Preferably, the helically coiled spring has a frusto-conical shape. It is an advantage that the frusto-conical shape means that the moveable end of the spring, which engages the ball member, can be kept away from surfaces of the housing that might otherwise rub against the spring and cause wear. 
         [0023]    In embodiments of the invention, the check valve is configured for operation with the ball member moveable in a substantially horizontal direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a sectional elevation of a single-piston pump with a vertically-mounted check-valve; and 
           [0025]      FIG. 2  is a sectional elevation of a twin-piston double-acting pump with horizontally-mounted check valves. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring to  FIG. 1 , a paint pumping arrangement  10 , includes a cylinder casing  12 , within which is a piston  14  mounted for reciprocating movement along an axis X-X. An annular sealing ring  16  forms a seal between the cylinder casing  12  and the piston  14 . The cylinder casing is provided with an inlet port  18  for coupling to a supply of liquid paint. The piston  14  is coupled to a shaft  20 , which is driven in reciprocating manner by a drive arrangement (not shown). A bellows seal arrangement  22  provides a seal around the shaft  20  with the cylinder  12 . 
         [0027]    Within the structure of the piston  14  is an inlet check-valve arrangement, which comprises a ball member  26 , which is biased towards contact with a seat  28  by a helical compression spring  29 . The inlet check valve arrangement (in the position shown in  FIG. 1 ) provides a seal between an inlet-side passage  24  and an outlet-side passage  30 , the inlet-side passage is in fluid communication with the inlet port  18  via the space  23  to the right of the piston  14  in  FIG. 1 . The outlet passage  30  opens into a space  32  to the left of the piston  14  in  FIG. 1 , which is in fluid communication with an outlet port  34 , to prevent flow of paint through the piston  14 . 
         [0028]    Mounted above the outlet port  34  is an outlet check valve  40 , which includes a housing  42 . The housing  42  retains a ball member  44 , which forms a sealing contact with a seat  46 . Surrounding the ball member  44  is a plastics material cage  47 . The cage  47  has a structure that includes a plurality of vertically oriented splines  48  defining a plurality of flow passages  49  disposed around the ball member  44 . In the embodiment shown in  FIG. 1 , the ball member  44  is biased towards contact with the seat  46  by means of a helical compression spring  50 . Interposing the compression spring  50  and the ball member  44  is a spring keep  52 , formed of a plastics material. At its top end, the spring  50  engages a central abutment  54  that forms part of the housing  42 . 
         [0029]    A particularly suitable plastics material for the cage and/or of the spring keep is an acetal resin engineering plastic such as polyoxymethylene (POM), polytrioxane or polyformaldehyde. An exemplary material is marketed under the name Delrin®. This material is a lightweight, low-friction, and wear-resistant plastic capable of operating in temperatures in excess of 90 degrees celsius and often marketed and used as a metal substitute. 
         [0030]    In use, the piston  14  is driven to reciprocate within the cylinder  12 . In a first stroke, the piston is moved to the right, as shown in  FIG. 1 , so that paint in the space  23  enters the inlet-side passage  24  and causes the ball member  26  to lift clear of the seat  28  so that the paint flows through the outlet-side passage  30  into the space  32 . Accordingly, the space  32  becomes charged with paint. 
         [0031]    When the piston  14  reaches the end of the first stroke and reverses to commence a second stroke, the ball member  26  is urged back into contact with the seat  28 , so that paint cannot pass back from the space  32  to the inlet side. Instead the paint in the space  32  is pumped out through the outlet port  34 . The pressure of the paint (which is being pushed by the piston  24 ) acts against the underside of the ball member  44  of the outlet check valve  40 , causing it to lift away from the seat  46 . The movement of the ball member  44  is guided by the splines  48  of the cage  47  so that the paint can flow through the flow passages  49  in the cage  47 . When the piston  14  reaches the end of the second stroke and commences a new first stroke, the pressure of the paint acting against the ball member  44  reduces such that the ball member drops back into contact with the seat  46 . 
         [0032]    In the embodiment shown in  FIG. 1 , the ball member is urged back into contact with the seat with the aid of the compression spring  50  acting through the spring keep  52 . However, in some embodiments it may not be necessary to include the spring  50 , but to simply allow the ball member to drop back into contact with the seat  46  under its own weight. Once in contact with the seat, as the piston moves during another first stroke (i.e. to the right as shown in  FIG. 1 ) the ball member  44  will be urged into sealing contact with the seat  46  by the differential pressure of the paint above and below the ball member  44 . 
         [0033]    As stated above, the cage  47  is formed of a plastics material. In the life of a pump the check valves may be expected to undergo many thousands of openings and closings. It is important to maintain integrity of the sealing surfaces of the ball member  44  and the seat  46 . For this reason, the ball member is preferably a hard-wearing, precision formed metal ball. However, there will be many thousands of contacts between the ball member  44  and the cage  47 , particularly the splines  48 . These multiple, repeated contacts inevitably cause wear or even abrasion of the surfaces. For cage surfaces formed of a hard (for example metallic) material, then wear/abrasion will result on the surface of the ball member  44 . Eventually the surface of the ball may become worn to an extent that leakage can occur between the ball member  44  and the seat  46 . 
         [0034]    However, with a plastics material cage, although wear and abrasion may still occur, this will be predominantly on the softer plastics material. Wear/abrasion to the cage can be tolerated to a much greater extent than wear to the ball member because the cage does not have to form a paint seal. Furthermore, the plastics cage  47  is a much simpler and less costly component to manufacture and replace than the ball member  44 . For this reason the check valve  40  is configured so that the cage  47  is a separate component that is removable from the valve to allow it to be removed and replaced when it becomes worn. 
         [0035]    Another way that the ball member  44  may become worn is through contact with the compression spring  50 . If the compression spring  50  were to directly contact the ball member  44 , then, any relative movement of the surfaces would be a potential cause of wear or abrasion. For this reason the spring keep  52  is used to interpose the spring  50  and the surface of the ball member  44 . Again, the spring keep is a plastics material, which protects the ball surface. The spring keep  52  can readily be replaced if it becomes worn. 
         [0036]    A further advantage with the use of plastics for the cage is that the contact between the ball surface and the plastics surface of the cage is much less noisy when compared with, say, a metal to metal contact between the ball and a metallic cage. 
         [0037]    Referring to  FIG. 2 , there is shown a twin-piston pump, shown generally as  100 , and with corresponding features identified by the same reference numerals as  FIG. 1 . The pump  100  consists of two pistons  41   a ,  14   b , each of which reciprocates within a corresponding cylinder within a common casing  12 , the casing  12  has a single inlet  18  from which paint can flow into inlet spaces  23   a ,  23   b  on the right of each corresponding piston  14   a ,  14   b . Associated with each piston  14   a ,  14   b  is a corresponding outlet check valve  40   a ,  40   b , similar to the check valve  40  of  FIG. 1 , except that the check valves  40   a ,  40   b  are mounted horizontally. The check valves  40   a ,  40   b  are interconnected by a conduit  56  and have a common outlet  58  for connection to a paint delivery system (not shown) 
         [0038]    In this arrangement, each piston operates in the same manner as described above for the single piston pump of  FIG. 1 , except that the pistons are driven out of phase with each other. Thus, when the first piston  14   a  is on its pumping stroke to pump paint out through its outlet  34   a  and check valve  40   a , the second piston  14   b  is moving in the opposite direction to draw in a charge of paint to be pumped out at the next stoke. In this way paint can be pumped substantially continuously. 
         [0039]    The check valves  40   a ,  40   b  are each mounted horizontally, which means that the valves cannot make use of the weight of the ball members  44   a ,  44   b  to aid closure. Therefore, in this embodiment the springs  50   a ,  50   b  are required. Also shown in  FIG. 2  are check valve cover plates  60   a ,  60   b , which are secured to the respective valve housings by way of releasable fasteners  62   a ,  62   b , such as screws. The cover plates are easily removable to allow the check valve components to be removed. It is particularly convenient that the plastics cages  47   a ,  47   b  and/or the spring keeps  52   a ,  52   b  can be removed in this way to be inspected and replaced if they become excessively worn. 
         [0040]    As can be seen in  FIG. 1 , the compression spring  29 , which biases the ball member  26  in the piston  14 , has a frusto-conical shape. Similarly the compression springs  50   a ,  50   b  in the check valves  40   a ,  40   b  in  FIG. 2  also have a frusto-conical shape. For each of the springs, one end of the spring is at a fixed position relative to the surrounding surfaces, while the other end moves to increase or decrease the amount of compression as the ball member moves. The frusto-conical shape means that the surfaces adjacent the springs (which will generally be metallic surfaces of the associated pump or valve components) are kept away from the moving parts of the springs so as to prevent contact with those surfaces, which would otherwise lead to wear of the springs.