Piston for water pump and related method

Disclosed is a piston for a water pump, e.g., a pressure washer pump. The piston includes a piston body and a piston head and in the improvement, the piston body is solid, homogeneous ceramic. That is, the piston body has a generally cylindrical surface and the body is circumscribed by the surface and is homogeneous ceramic. A method for making a piston for a water pump includes the steps of providing a piston body made of a ceramic material, and making a piston head of steel and having a pocket formed therein. The head is heated, the pocket and the body are engaged in overlapping relationship and the head is allowed to cool and "shrink-fit" to the piston body. The piston head is formed to have a relief groove circumscribing the pocket for reducing compression forces resulting from such cooling.

FIELD OF THE INVENTION 
The invention relates generally to power plants and, more particularly, to 
pressure washers having a water pump driven by a prime mover such as an 
electric motor or an internal combustion engine. 
BACKGROUND OF THE INVENTION 
Products known as pressure washers are used for a wide variety of washing 
applications. A few such applications include washing building walls, 
removing paint and stains, washing such items as bicycles and golf carts, 
and cleaning sidewalks and driveways. Some pressure washers incorporate a 
venturi device whereby chemical cleaning liquids may be aspirated into and 
mixed with the water stream. Pressure washers are used by contractors, 
homeowners and equipment rental businesses, as examples. Generac 
Corporation, Waukesha, Wis., U.S.A. is a leading manufacturer of pressure 
washers. 
The primary components of a pressure washer include a water pump connected 
to a source of water such as a garden hose and providing a high-pressure 
stream or spray of water. The pump is coupled to and driven by a prime 
mover such as an electric motor or an internal combustion engine. 
Because of its high pressure capability, a pump of the reciprocating piston 
type is usually chosen for pressure washer service. An exemplary piston 
pump has several (e.g., three) pistons reciprocally moving in respective 
bores of the pump barrel. The pistons are reciprocated by a rotating 
"wobble plate"-type cam powered by the prime mover. 
A well known water pump arrangement has a housing, the wobble plate cavity 
of which contains a quantity of oil for lubricating bearings and for 
maintaining the proximal portions of the reciprocating pistons at an 
acceptably-low operating temperature, e.g., 260.degree.-270.degree. F. 
The cavity and the distal portions of the pistons (which come in contact 
with and deliver high-pressure water out of the pump) are isolated from 
one another by appropriate seal arrangements. Such arrangements form a 
barrier between the oil in the cavity and around the proximal ends of the 
pistons and the water being pumped by the distal ends of the pistons. 
A high pressure water pump represents a severe operating environment for 
the pistons in it. Special care must be taken to design pistons which not 
only have acceptable life in the application but which also evidence a 
cost consistent with the cost and selling price of the pressure washer 
equipment. 
One type of known piston is made of stainless steel with a thin 
plasma-applied ceramic coating or is made of hardened steel with a 
phosphatized or "Parkerized" surface treatment. It is believed that 
ceramic coating of pistons was implemented because one or more of the 
seals being used therewith is inordinately abrasive and quickly attacks 
the piston surface finish during pump operation. 
Ceramic pistons are disclosed in a number of patent documents including 
U.S. Pat. No. 4,759,110 (Rieger et al.). The Rieger et al. patent 
discloses a process for attaching a metal "holder" (shoe) to a ceramic 
piston. At ambient temperature, the bore of the shoe has a diameter 
smaller than that of the piston. Attachment is by heating the holder until 
its bore diameter is greater than that of the piston, placing the pieces 
together and allowing the shoe to cool. 
The patent explains that "optimum" locking of the piston and the holder to 
one another occurs when (a) the bore and piston diameters are each within 
a particular range, and (b) the piston end and the holder bore are each 
prepared to have a particular roughness Ra. 
As to structural shape, the end portion of the holder which is 
perpendicular to the piston long axis is flat. The patent notes that such 
holder has a relief notch" in order that "uniform stresses are produced in 
the region of clamping." 
U.S. Pat. No. 5,038,673 (Schulze), shows a pump piston, the body of which 
is ceramic. In the version shown in FIGS. 1, 3a and 4, the piston body has 
a groove at the head end and a cap-shaped metal contact element over such 
end. In the arrangements of FIGS. 6, 7, 8, 10, 11 and 14, the contact 
element is a metal button on the piston end. 
U.S. Pat. No. 5,094,150 (Russner et al.) discloses a ceramic piston to 
which is attached what the patent calls a "metallic drive part," i.e., a 
shoe. In one arrangement, the ceramic is metallized and the shoe is 
solder-attached using relatively-common solder. In another arrangement, 
there is no metallization and the shoe is solder-attached using solder 
containing, e.g., titanium or zirconium. 
U.S. Pat. No. 5,392,693 (Engel et al.) describes a piston assembly having a 
ceramic body with a spherical head over which is fitted a slipper or shoe, 
also made of ceramic. As shown in FIGS. 4-6, the piston may be made in two 
parts, a body and a head, to permit a ceramic ring to be slipped over the 
shaft of the inverted-key-hole-shaped head. 
While known pistons for pressure washer pumps have been generally 
satisfactory for the purpose, at least those having plural-layer coated 
construction are not without disadvantages. Chief among them is cost. Such 
pistons require special, higher-cost materials (stainless or hardened 
steel) and a special, separately-provided surface coating or treatment. 
Such coatings and treatments have adverse implications for the 
manufacturing process and for the manufactured cost of the 
ready-to-assemble piston. 
And pistons made of steel have mass (and, therefore, inertia) which, in 
view of the invention, is relatively high. Depending in part upon the 
rotational speed at which the pump is being driven, high-mass pistons may 
have adverse implications for the size and type of piston return spring 
and/or for the bearings used in conjunction with the wobble-plate type 
cam. 
An improved piston and method for making such piston and a water pump 
having an improved piston, all of which address some of the disadvantages 
and shortcomings of the prior art would represent distinct technological 
advances in the art. 
OBJECTS OF THE INVENTION 
An object of the invention is to provide an improved piston for a water 
pump which overcomes some of the problems and shortcomings of prior art 
pistons. 
Another object of the invention is to provide an improved piston which 
incorporates very-low-cost material. 
Yet another object of the invention is to provide an improved piston which 
requires no surface coating. 
Another object of the invention is to provide an improved water pump 
piston, the ceramic body of which has uniform surface finish along its 
length. 
Another object of the invention is to provide an improved water pump piston 
which is easy to manufacture. 
Still another object of the invention is to provide a water pump 
incorporating an improved piston having reduced mass and inertia. 
Another object of the invention is to provide an improved method for making 
a piston for a water pump. How these and other objects are accomplished 
will become apparent from the following descriptions and from the 
drawings. 
SUMMARY OF THE INVENTION 
The invention involves a water pump of the type having at least one piston 
(and typically several pistons) reciprocating in a pump barrel. The piston 
includes an elongate, cylindrical piston body extending along a long axis 
and a piston head attached to the body. In the improvement, the piston 
body is made entirely of ceramic. Stated another way, the piston body has 
a generally cylindrical surface and that portion of the body bounded by 
the surface is homogeneous. Such portion is free of discontinuities, i.e., 
is solid, both along its length and through its cross-sectional area. 
In another aspect of the invention, the body and the head are made of 
dissimilar materials such as ceramic and steel, respectively. The body has 
a proximal end and a distal end and the body is substantially cylindrical 
at the ends and between the ends. The pump has at least one seal and, 
typically, a plurality of seals around and contacting the piston body. 
Seal materials including graphite-filled polytetrafluoroethylene (PTFE) 
and nitrile with molybdenum di-sulfide work well because of the 
"slipperiness" of such seals. The piston head includes a pocket having a 
generally cylindrical pocket wall and the proximal end of the piston body 
is received in such pocket. 
In a more specific aspect of the invention, the piston head has a spring 
retainer flange extending parallel to the long axis. The piston head 
includes an annular relief groove between the wall and the flange. Such 
relief groove circumscribes the wall and is instrumental in reducing the 
forces tending to crush the distal end of the piston body when the head 
cools after heat treating as described below. 
And "shrink-fit" attachment of the head and body to one another is not the 
only means of attachment. In another embodiment, the piston body and the 
piston head include, respectively, a body adhesive bonding region and a 
head adhesive bonding region. Adhesive adheres to both bonding regions and 
secures the body and the head to one another. 
In a variant of the second embodiment, the piston head includes a pocket 
and the head adhesive bonding region is in the pocket. In another variant, 
the piston body includes a passage and the body adhesive bonding region is 
in the passage. 
A new method for making a piston for a water pump includes the steps of 
providing a piston body made of a first material, making a piston head of 
a second material and having the above-noted pocket formed in it. The head 
is heated and then the pocket and the body are engaged in overlapping 
relationship. The head is then allowed to cool. 
In a highly preferred aspect of the method, the providing step includes 
providing a body having a first diameter and the making step includes 
forming the pocket to have, at ambient temperature, a second diameter 
slightly less than the first diameter. The heating step includes heating 
the head until the second diameter is greater than the first diameter. 
In a more specific aspect of the method, the head includes a crown portion 
and a nose portion extending from the crown portion and the heating step 
includes heat-treating the nose portion to a Rockwell C hardness of at 
least 40 and preferably to a hardness in the range of 50 to 55 Rockwell C. 
Heat is permitted to propagate to the crown portion (including the pocket 
wall) to enlarge the pocket to a size to accept the proximal end of the 
piston body. 
In a highly preferred method, the making step includes forming the piston 
head to include (a) a relief groove circumscribing the pocket, (b) a wall 
defining the pocket and having a root portion and a distal lip spaced from 
the root portion, and (c) a relief groove circumscribing the wall and 
extending generally to the root portion. The making step also includes 
forming the piston head to have a radially-outwardly-extending retainer 
ring and a retainer flange extending generally parallel to the axis and 
coacting with the ring for positionally retaining a spring. 
But the piston need not be made only by using shrink-fit attachment. A 
variant method for making a piston for a water pump includes the steps of 
providing a piston body made of a first material and having a body 
adhesive bonding region thereon, making a piston head of a second material 
and having a head adhesive bonding region thereon and applying adhesive to 
at least one of the bonding regions. The pocket and the body are engaged 
in overlapping relationship and the adhesive is allowed to harden.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Before describing the new piston 10 and related method, it will be helpful 
to have an understanding of some aspects of a pressure washer 11. 
Referring to FIGS. 1 and 2, the pressure washer 11 includes a prime mover 
13 (an internal combustion engine in the drawing) to which is coupled a 
high-pressure water pump 15. The pump 15 has an inlet 17 to which a source 
of water, e.g., a garden hose, is attached and water at high pressure is 
discharged from the outlet port 19 to a hose 21 connected to a 
hand-manipulated spray wand 23. 
The new piston 10 is used in the water pump 15 and a few aspects of such 
pump 15 will now be described. The water pump 15 is attached to the prime 
mover 13, the drive shaft of which is coupled to an angled cam 25. The 
pistons 10 are slideably received in respective bores 27 and the cam 25 
sequentially reciprocates the pistons 10 in their bores. The pump 15 
includes a cam bearing 29 with a flat, ring-like, annular thrust plate 31. 
The shoes 33 of the pistons 10 contact and "ride on" such plate 31. 
In sequence, the pistons 10 are moved leftwardly by the rotating cam 25 and 
returned rightwardly by respective piston return springs 35 acting upon 
the piston head 37. It is to be appreciated that spring force retains the 
piston shoe 33 against the plate 31. The general arrangement of the pump 
described above is known per se and it is to be appreciated that only one 
of the pump pistons 10 is shown in FIG. 2. An exemplary pump 15 has three 
pistons 10 spaced equidistant from the pump axis of rotation 39. 
For a given piston 10, piston spring 35 and piston spacing from the pump 
rotational axis 39, there is an upper limit to the speed at which the pump 
15 can be driven and still reliably spring-retain the shoes 33 in contact 
with the plate 31. As will become apparent from the description below, the 
new piston 10 has favorable implications for increasing such upper limit. 
Because the plate 31 is not coupled to the prime mover 13 and because there 
is very little viscous drag imposed on such plate 31, the plate 31 rotates 
at very low speed. As a consequence, friction-related forces which 
otherwise may impose significant bending loads upon the ends of the 
pistons 10 are very low. Stated another way, the force imposed upon each 
piston 10 by the plate 31 is substantially entirely axial. 
The pump cavity 41 has a quantity of oil therein for lubrication and 
cooling purposes. The pump 10 has at least one seal and, typically, a 
plurality of seals 43, 45, 47 around and contacting the piston body 49. 
The seal 47 is preferably Parker Packing Div. FS-1078 which has 
graphite-filled PTFE. The seal 45 is preferably Parker Seal 
4274-8506006625 which includes 85 Durometer nitril loaded with molybdenum 
disulfide. The oil seal 43 is preferably Transcom TC4 which includes 
carboxylated nitril, 80 Durometer. The seals 43, 45, 47 function as a 
barrier between the cavity 41 and the proximal end 51 of the piston 10 on 
one hand and, on the other hand, the water being pumped at the piston 
distal end 53. 
Referring next to FIGS. 3 through 7, details of the new piston 10 and 
method for making such piston 10 will now be set forth. The piston 10 
includes an elongate, cylindrical piston body 49 extending along a long 
axis 55. But for small chamfers 57 at the proximal and distal ends 51 and 
53, respectively, the body 49 is of uniform diameter D1 along its length. 
The surface 59 is preferably finished to 0.14 to 0.22 micrometer Ra along 
its entire length. 
The piston body 49 is made entirely of ceramic, i.e., aluminum oxide powder 
and a binder. In other words, the piston body 49 has a generally 
cylindrical surface 59 and that portion of the body 49 bounded by the 
surface 59 is homogeneous and is free of discontinuities, i.e., the body 
49 is solid ceramic. A suitable ceramic is Coors Ceramics FG-995. 
The piston head 61 includes a crown portion 63, a nose 65 extending from 
such portion 63 in a direction away from the body 49. The head 61 also has 
a pocket 67 with an annular pocket wall 69 extending from the wall root 
portion 71 toward the distal end 53 and terminating in a distal wall lip 
73. A suitable finish for the interior wall surface 75 is about 0.8 
micrometers. An annular spring retainer flange 77 is generally parallel to 
the body long axis 55 and, like the wall 69, also extends toward the 
distal end 53 in a direction generally parallel to the axis 55. 
Notably, the piston head 61 includes an annular relief groove 79 between 
the wall 69 and the flange 77. Such relief groove 79 circumscribes the 
wall 69, is interposed between the wall 69 and the flange 77, is 
concentric with the wall 69, the flange 77 and when mounted on the body 
49, with the axis 55. Such groove 79 is instrumental in reducing the 
forces otherwise tending to crush the proximal end 51 of the piston body 
49 when the head 61 cools after heat treating as described below. 
Another embodiment of the new piston 10 will now be described. Referring to 
FIG. 8, the piston body 49 includes a body adhesive bonding region 81 
around the circumferential surface 59 at the proximal end 51. The head 83 
includes a pocket 67 and the head adhesive bonding region 85 is in the 
pocket 67 and, specifically, is on the wall 69 of the pocket 67. Adhesive 
adheres to both bonding regions 81, 85 and secures the body 49 and the 
head 83 to one another. The preferred thickness of adhesive between the 
head 83 and the body 49 is about 0.002 inches and a preferred adhesive is 
Permabond ESP-308 epoxy available from Permabond International of 
Englewood, N.J. 
A third embodiment of the new piston 10 is shown in FIG. 9. The piston head 
87 includes a spring retainer plate 89 and a plate retention stem 91 
attached concentrically to the plate 89. The stem 91 has a nose 93 and a 
stem securing portion 95 and the plate 89 is interposed between the nose 
93 and the portion 95. The portion 95 has a plurality of cylindrical lands 
97 separated by annular grooves 99, such lands 97 and grooves 99 comprise 
the stem bonding region 101 and adhesive adheres to both the lands 97 and 
the grooves 99. The piston body 49 includes a hole 103 sized to receive 
the retention stem 91 with slight clearance. The body adhesive bonding 
region 105 is in the hole 103 and the head 87 and body 49 may be secured 
to one another using the Permabond epoxy noted above. 
The embodiment shown in FIGS. 2 through 7 works well in operating 
environments of 260.degree.-270.degree. F. and the embodiments of FIGS. 8 
and 9 and work well in such environments up to about 
200.degree.-210.degree. F. But as new adhesives are developed, it is 
expected that the temperature-related operating capabilities of the latter 
embodiments will increase. 
A new method for making a piston 10 for a water pump 15 includes the steps 
of providing a piston body 49 made of a first material, making a piston 
head 37 of a second material and having the above-noted pocket 67 formed 
in it. The head 61 is heated and then the pocket wall 69 and the body 49 
are engaged in overlapping relationship. The head 37 is then allowed to 
cool. 
In a highly preferred aspect of the method, the providing step includes 
providing a body 49 having a first diameter D1 and the making step 
includes forming the pocket 67 to have, at ambient temperature, a second 
diameter D2 slightly less than the first diameter D1. The heating step 
includes heating the head 37 until the second diameter D2' is greater than 
the first diameter D1. 
In a more specific aspect of the method, the head 37 includes a crown 
portion 63 and a nose 65 extending from the crown portion 63. The heating 
step includes heat-treating the nose 65 to a Rockwell C hardness of at 
least 40 and preferably to a hardness in the range of 50 to 55 Rockwell C. 
Heat is permitted to propagate to the crown portion 63 and to the pocket 
wall 69 to enlarge the diameter D2 of the pocket 67 to a diameter D2' to 
accept the proximal end 51 of the piston body 49 with sliding clearance. 
In a highly preferred method, the making step includes forming the piston 
head 61 to include (a) a relief groove 79 circumscribing the pocket 67, 
(b) a wall 69 defining the pocket 67 and having a root portion 71 and a 
radially distal lip 73 spaced from the root portion 71. The relief groove 
79 circumscribes the wall and extends generally to the root portion 71. 
The making step also includes forming the piston head 67 to have a 
radially-outwardly-extending retainer ring 107 and a retainer flange 77 
extending generally parallel to the axis 55 and coacting with the ring 107 
for positionally retaining a piston return spring 35. 
As noted above, the pocket diameter D2 prevailing at, say, ambient 
temperature, increases to D2' when the head 61 is heated. It is to be 
appreciated that as the head 61 is allowed to cool after the proximal end 
51 of the body 49 has been seated in the pocket 67, very substantial 
radially-inwardly-directed forces are developed if "shrinkage" or 
contraction of the wall 69 back to a pocket diameter D2 is resisted. The 
presence of the body proximal end 51 in the pocket 67 provides such 
resistance. It has been discovered that the inclusion of the relief groove 
79 as described above has a marked effect in reducing the magnitude of 
such forces and preventing the proximal end 51 from being "hairline 
fractured" or crushed. 
Another aspect of the invention involves a method using adhesive for making 
a piston 10. Such method includes the steps of providing a piston body 49 
made of a first material and having a body adhesive bonding region 81. A 
piston head 83 is made of a second material and has a head adhesive 
bonding region 85. Adhesive is applied to at least one of the bonding 
regions 81, 85 and the bonding regions 81, 85 are urged into engagement 
with one another. The adhesive is then allowed to harden. 
In a more specific aspect, the piston head 83 includes a pocket 67 and the 
head adhesive bonding region 85 is in the pocket 67. And in an alternative 
aspect, the piston body 49 includes a hole 103 and the body adhesive 
bonding region 105 is in the hole 103. 
While the principles of the invention have been shown and described in 
connection with specific embodiments, it is to be understood clearly that 
such embodiments are by way of example and are not limiting.