Patent Publication Number: US-6212997-B1

Title: Reciprocating fluid pumps with chromium nitride coated components in contact with non-metallic packing and gasket materials for increased seal life

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to thin film coating of materials including metal or steel machine components and, more particularly, to coating of metal or steel parts which are intended for assembly and moving contact with parts made of non-metallic or non-ferrous materials. 
     BACKGROUND OF THE INVENTION 
     Steel and metal products and machine components are commonly plated or coated for corrosion protection, hardness, friction reduction and appearance. Platings such as chromium, nickel, copper, gold, etc. are commonly applied by an electroplating dipping process. For close tolerance and wear surface applications, these processes are difficult to tightly control to achieve uniform coatings which have the desired physical properties such as hardness, durability and resistance to cracking. 
     Modern metal coating processes such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) provide improved coating uniformity, strength and hardness. These types of metal coating processes are widely applied to cutting tools and machine components which bear heavy mechanical loads and are in moving contact with other steel or metal components. Metal coatings are applied to steel or metal parts to withstand contact with or cutting of other steel or metal parts. These types of coatings, having extreme hardness and strength, have to the inventors&#39; knowledge not been used in applications where mechnical contact is made with softer non-metallic materials. 
     Machine components which are in moving contact with components or parts made of non-ferrous/non-metallic materials have also been plated, such as chrome plating of plunger and housing components of reciprocating pumps, which slide against packings made of leather, plastic, rubber or other materials. A common failure of this type of arrangement occurs when cracks form in the chromium plating layer. The cracks form due to volume contraction which occurs when the as-deposited chromium hydrides decompose to molecular hydrogen and chromium metal during post-plating bake-out. Post plating grinding can also produce foreign chrome particles which can damage a seal. Eventually small particles of the chromium layer imbed in the packings. The packings then act as a tool holder of the particles which cut into and score the plated component as it continues to slide against the packings, forming abrasions in the plated surface. The scored abrasions on the plated surface in turn damage the packings, ultimately causing the seal between the packings and the plated surface to fail. Also, as the packings are damaged, they are more likely to collect oxide particles, such as titanium dioxide, from fluid material such as paint being transferred through the pump. These particles can have hardness comparable to chrome and further contribute to scoring of the plunger. Failure of the seals in a reciprocating style pump causes loss of output pressure and loss of containment of the pumped fluid and contamination of other pump components. 
     In analyzing these type of seal failures, it is most intuitive to suspect the relatively softer non-metallic material, of which the packings are made, as the failing component. However, the inventors have discovered that the above described failure process starts with the failure of the plating or coating of the steel or metallic parts which bear against the packings. FIGS. 3-6 are micrographs of a chrome plated surface of a reciprocating pump plunger component designed for sliding/sealing contact with a non-metallic packing such as V-rings made of Teflon. In FIGS. 3 and 4, chrome particles are indicated at CP, and linear scores S are clearly seen running in the direction of reciprocation of the part past a seal. FIG. 5 shows an unused surface of a pump plunger chrome plated to prior art design specifications. Cracks CC are shown in the coating, along with chrome particles CP. FIG. 6 shows a plunger surface after 500 cycles of operation in sliding contact with a packing, exhibiting cracks CC, chrome particles CP within the cracks, and two score lines S. 
     FIG. 7 shows a mixture of particles P, including chromium and titanium dioxide, imbedded in a Teflon V-ring used as a packing in a reciprocating pump. Linear score marks S are visible in the glass fibers F embedded in the Teflon ring. FIGS. 8 and 9 show chromium particles CP embedded in a Teflon packing ring. And FIG. 10 shows a titanium dioxide particle P from paint trapped in a score S in a glass fiber in a V-ring of a pump packing. These micrographs support the inventive discovery that a source of seal failure in combined machine components of metallic and non-metallic materials is the metallic component such as the plating layer on a reciprocating plunger. 
     In testing, a reciprocating pump in which the plunger is chrome plated to design specifications and in accordance with quality controls, seal failures have occurred at as few as 100,000 pump cycles. The thickness of chrome plating, on the order of approximately 0.004-0.008 inches, requires substantial pre-grinding of the plunger stock to arrive at post-plating tolerances. A post plating grind is also required. Both grinding operations add significantly to manufacturing costs. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention overcomes these and other disadvantages of the prior art by providing chromium nitride coated machine components which are placed in moving contact with non-metallic components, for increased lifespan of the non-metallic components. In accordance with one particular application of the invention, there is provided a reciprocating pump which has a plunger which is actuated to linearly reciprocate within a housing and to bear against one or more non-metallic packing seals. 
     In accordance with one aspect of the invention, there is provided a reciprocating type fluid pump having a hydraulic section including a housing and a plunger driven to reciprocate at least partially within the housing and to pass through an opening in the housing, the housing having a fluid intake port and a fluid exit port, a ball check operatively associated with the fluid intake port, and a packing seal positioned about a periphery of the opening in the housing through which the plunger passes, the packing seal being made of a non-metallic material and dimensioned to extend into the opening in the housing through which the plunger passes so as to make contact with the plunger as the plunger passes through the opening as it reciprocates at least partially within the housing, and at least one additional seal between the plunger and the housing spaced from the opening in the housing, the plunger having an external surface area configured to bear against and make sliding contact with the packing seal about the periphery of the opening through which the plunger passes as it reciprocates at least partially within the housing, an internal passageway within the plunger which extends generally axially from one end of the plunger to an exit point spaced from the one end of the plunger, and a ball check operatively associated with the internal passageway of the plunger, the external surface of the plunger which contacts the packing seal being coated with chromium nitride, whereby the chromium nitride coating is in continuous contact and sliding with the packing seal, whereby a hydraulic seal is formed between the plunger and the packing seal. 
     These and other aspects of the invention are herein described in particular detail with reference to the accompanying Figures. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     In the accompanying Figures: 
     FIG. 1A is a cross-sectional view of one type of reciprocating pump which can be manufactured in accordance with the present invention, shown near the end of a siphon stroke of the pump; 
     FIG. 1B is a cross-sectional view of the pump of FIG. 1A shown near the end of a pressure stroke, and 
     FIG. 2 is a cross-sectional view of another type of reciprocating pump which can be manufactured in accordance with the present invention. 
     FIGS. 3-6 are micrographs of a chrome plated surface of a reciprocating fluid pump plunger component, and 
     FIGS. 7-10 are micrographs of Teflon V-rings used as packings in reciprocating fluid pumps. 
    
    
     DETAILED DESCRIPTION OF PREFERRED AND ALTERNATE EMBODIMENTS 
     As shown in FIGS. 1A and 1B, one type of machine in which the principles of the invention can be employed is a reciprocating fluid delivery pump, indicated generally at  10 . The pump  10  includes an air motor section  15  and a hydraulic section  20 . The air motor section  12  includes a cylinder  16  in which a piston  17  is mounted to reciprocate in response to air pressure introduced through an air valve, indicated generally at  18 . The piston  17  is connected to a connecting rod  19  which passes through the cylinder  16 , and is connected to a plunger  21 . The plunger  21  includes an upper section  22  and a lower section  23 . As used herein, the terms “upper” and “lower” are merely illustrative of one particular orientation of the described pump and are not limiting to other possible orientations of the pump or pump components, or otherwise limiting to the scope of the invention. The plunger  21  is mounted for linear reciprocation within a hydraulic housing. The upper section  22  of the plunger  21  passes through an opening  25  in the hydraulic housing  24  which leads into a solvent chamber  30 , to reciprocate both within the housing  24  and within the solvent chamber  30 , which is located between the air cylinder  16  and the housing  24 . The lower section  23  of the plunger  21  remains within the housing  24  throughout the reciprocation cycle of the pump. 
     The lower section  23  of the plunger  21  includes an internal passageway or bore  26  which extends from a lower end of the section  23  to an upper end where it joins with the upper section  22 . A pressure ball check  27  is mounted within the bore  26  near the lower end of the lower section  23 . The housing  24  further includes a fluid intake port  28  in which a siphon ball check  29  is mounted, and a fluid exit port  14 , which is the pressurized delivery point of the pump. 
     Upper packings  31 , and lower packings  32 , (also referred to herein as “seals” and “packing seals”) are mounted within the walls of the housing  24 , to form a seal against the outer diameters of the upper and lower sections  22 ,  23  of the plunger  21 . The upper packings  31  prevent fluid from exiting the housing  24  through opening  25  during both the siphon and pressure strokes of the pump. The lower packings  32  form the siphon force which draws liquid into the housing during the siphon stroke of the pump shown in FIG. 1A, and force liquid through the plunger bore  26  during the pressure stroke of the pump shown in FIG. 1B, by preventing passage of fluid between the outer diameter of the lower section of the plunger and the interior surface of the walls of housing  24 . 
     The packings  31  and  32  are, in one embodiment, formed of a plurality of rings  33  having a generally V-shaped cross-section with a chamfered edge on the inner diameter which bears against the outer diameter of the plunger sections. The rings are preferably made of any of the below listed non-metallic materials which , in accordance with the principles of the invention, are well-suited for continuous and repeated sliding contact with a chromium nitride coated machine component such as the described plunger. The non-metallic ring material may be selected from the representative group of: polyurethane; polyurethane with molybdenum disulfide; carboxylated nitrile; ethylene propylene (EPR or EPDM); polypropylene; nylon; neoprene; fluorocarbon; Buna-N (Nitrile); Kalrez; Polysulfide; styrene butadiene; ultra high molecular weight (UHMW) polyethylene; teflon, leather; and combinations of these materials, such as buna-N on cotton duck, teflon impregnated buna-N or nylon fabric. 
     A ring adapter  34  applies pressure to the rings  33  to bias the internal diameter against the plunger. A solvent such as a glycol such as polyether or polypropylene glycol, mixed aliphatic dimethyl esters, liquid anionic floculant, vitalizer oil or epoxidized soybean oil, is added to the solvent chamber  35  primarily to lubricate the upper packings  31 . The lower packings  32  are lubricated by the fluid drawn into the hydraulic housing  24 . 
     In order to dramatically increase the performance life of the seal formed by the packings  31  and  32 , the outer diameter surfaces of the upper section  22  and lower section  23  of the plunger  21  are coated with chromium nitride (CrN) by a physical vapor deposition (PVD) process. The coating is applied to thickness in a preferred approximate range of 7 to 10 microns, and the surfaces to be coated are pre-ground to this extent. Because this dimension is substantially less than that required for conventional chrome plating, the invention provides significant manufacturing cost savings. No post-coating grinding is required, which also reduces manufacturing costs. Prior to coating, the plunger is heat treated at approximately 1150° F. for a period of approximately four hours and allowed to air cool. 
     The repeated sliding contact of the chromium nitride coated plunger against the packings  31  and  32  does not cause any cracks in the coating, or coating particles to leave the plunger surface and lodge in the packings. In testing, pumps with chromium nitride coated plungers have performed up to four million cycles (one cycle being defined as the complete travel of the plunger through the pressure stroke and siphon stroke—(add approx. length of plunger and extent of travel, e.g. 6 inches) without any abrasion of the of the plunger surfaces detectable at 5000× magnification. 
     FIG. 2 illustrates the hydraulic section, indicated generally at  40 , of another embodiment of a reciprocating pump constructed in accordance with the invention. In this type of pump, the plunger  41  has a single section with a constant outer diameter, and reciprocates within the hydraulic housing  42  past a single packing or seal  43  near an opening  44  of the housing  42 . Fluid is drawn into the housing through an inlet  45  which includes a siphon ball check  46 , which prevents the flow of liquid out of the inlet during the pressure stroke of the plunger. The plunger  41  includes an internal bore  47  which extends from a distal end  48  of the plunger, to a point approximately aligned with an outlet port  49  of the housing  42 . A ball check  50  is incorporated into the internal bore  47  of the plunger near the distal end  48  to prevent flow of fluid through the plunger during the pressure stroke of the plunger. A sliding gasket  51  (also referred to herein as a “sliding seal”), such as in the form of a U-cup made of polyurethane or other suitable material, reinforced by opposing back-up washers, is mounted upon the plunger  41  near distal end  48 , and in contact with the internal walls of the housing  42 , to prevent fluid from entering the open area  52  between the plunger  41  and the housing  42 . Alternatively, one or more rings having a V-shaped cross-section, similar to the packings described with reference to FIGS. 1A and 1B, may be used as the sliding gasket  51  attached to the plunger  41 . 
     In this embodiment, the internal walls of the housing are coated with chromium nitride, to similar thickness&#39; as described above, to dramatically increase the performance life of the seal formed by the sliding gasket  51 . Repeated sliding contact of the gasket  51  against the interior walls of the housing  42 , for up to four million cycles in testing by the inventors, does not cause any cracking or failure of the chromium nitride coating, or production of coating or metallic particles which would become lodged within the gasket  51  and score the housing walls. 
     In this type of pump also, it has been found through testing that the described chromium nitride coating of the outer surface of the plunger  41  has greatly improved the performance life of the seal formed by packing  43 , due to the fact that the chromium nitride coating does not crack after application or during the repeated sliding contact with the packing  43 , and therefore does not produce metal particles which become lodged in the packings and score the plunger. As with the previously described pump, the pump of FIG. 2 has been tested up to four million cycles without an seal failure, and without producing any detectable scoring of the plunger. Other types of coatings which can be applied by physical vapor deposition and which provide similar performance enhancements to the described pumps include titanium nitride (TiN), titanium aluminum nitride (TiAIN), aluminum titanium nitride (AlTiN), titanium carbon nitride (TiCN), and zirconium nitride (ZrN). 
     The invention thus provides dramatically improved machine performance in cases where a dynamic seal is formed between a metallic surface and a non-metallic surface, with manufacturing cost savings over traditional plating processes.