Accurate powder metal component, assembly and method

The invention provides an accurate powder metal component having a body with opposed surfaces at least one of which has at least one projection of smaller cross-sectional area than the main portion of the body of the component. The distance from the free end of the projection to the opposite end of the component defines one dimension of the component that must be relatively accurate, in one embodiment to slide against and form a seal that inhibits liquid flow against another component of an assembly. In the manufacturing method, the powdered metal component is made by compaction to form the body including a projection at the end, is sintered and thereafter is reduced in the dimension between the end of the projection and the opposite end of the component by coining the free end of the projection so as to reduce the dimension to within a tolerance of the nominal specified dimension.

Not applicable.

FIELD OF THE INVENTION

This invention relates to sintered powered metal components that must be made to precise dimensions.

BACKGROUND OF THE INVENTION

Sintered powder metal components are well known in the art. An advantage of making a component of sintered powder metal is that it can be formed to near net shape. Usually, due to variability in the manufacturing process, if a powder metal part must have precise dimensions, it must have material removed by a machining process to obtain the required precision. Such processes result in time and tooling expense, and do not necessarily improve the material characteristics of the component, other than to make it of precise dimensions. This invention addresses these issues in some types of sintered powder metal components.

SUMMARY OF THE INVENTION

The invention provides an accurate powder metal component, assembly and method in which the powder metal component has a body with opposed surfaces at least one of which has at least one projection of smaller cross-sectional area than the main portion of the body of the component. The distance from the free end of the projection to the opposite end of the component defines one dimension of the component that must be accurate. The powdered metal component is made by compaction to form the body including a projection at the end, is sintered and thereafter is reduced in the dimension between the end of the projection and the opposite end of the component by coining the free end of the projection so as to reduce the dimension to within a tolerance of the nominal specified dimension.

The invention may advantageously be applied to a pump gear, for example a gerotor gear.

The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to powder metal components that require a narrow range of dimensional tolerances. Several types of example components are described in the following paragraphs. One application that requires components of accurate dimensional tolerances is gerotor pumps. In this application, gerotor gears have a narrow thickness tolerance to ensure adequate sealing of the pump, since the ends of the gear seal against surfaces of the pump housing as illustrated inFIG. 10, at the sliding interface of the end of the gear against the pump housing. This example application is addressed inFIGS. 1 through 6. Referring toFIG. 1, a sintered powder metal gerotor pump gear10includes opposed surfaces12, tooth surfaces14, a bore16, a chamfer18between the opposed surfaces12and the bore16, and coining projections20. The shape of coining projections20is preferably tapered, for example semi-circular, in cross-section to minimize surface area toward the free end. The free end of the projection is flattened and slides against the adjacent surface of the housing to create a sliding seal. Alternatively, the shape of coining projections20may be triangular, trapezoidal, or any other cross-sectional shape that tapers from a root or base of the projection that is integral with the body of the gear to a free end21.

Coining projections20are located, in general, along the periphery of opposed surfaces12and extend axially. Preferably, coining projections20are located at locations on the opposed surfaces12to ensure adequate sealing against adjacent surfaces of a mating component, such as a pump housing, that the free ends of the projections slide against. For example, coining projections20may be located along the edges of the gear teeth, in a circular form adjacent to the root diameter of the gear teeth and in another circular form spaced radially inward of the root of the gear teeth, adjacent to the chamfer18that leads in to the axial bore16through the gear10.

The gerotor gear size, i.e. bore diameter, number of teeth, diametral pitch, tooth pressure angle, gear thickness, and tooth profile shape, etc., may be determined as appropriate for the application in which the sintered powder metal gerotor gear10is used.

Referring toFIG. 2, a coined sintered powder metal gerotor gear110includes the above features with coined projections120formed from the coining projections20. Coined projections120include coined free ends121as shown inFIG. 6. Inclusion of coined free ends121results in highly accurate overall thickness relative to a nominal specified dimension. Coined sintered powder metal gerotor gear110can meet a tolerance of less than 50 microns for the nominal specified dimension. It should be understood that overall thickness refers to the distance between coined free ends121on opposite opposed surfaces12of the coined sintered powder metal gerotor gear110. Alternatively, overall thickness refers to the distance between a coined free end121and the opposite opposed surface12if coined projections120are only included on a single opposed surface12.

Coined projections120have a trapezoid-like shape as shown inFIG. 6. The sides of coined projections120adjacent to the opposed surfaces121are arcuate due to the original semi-circular shape of coining projections20. Coined projections120have a relative density of approximately 95% at their free ends compared to the original powder metal material. Advantageously, coined projections120have increased strength properties compared to a non-coined PM component, and are not as porous to provide improved sealing characteristics. Another benefit is that there is a lower surface contact area between the pump gears and the housing. Advantageously, the pockets or recesses between or surrounded by coined projections120create reservoirs for containing fluid, which in the case of hydraulic fluid is a lubricant, resulting in increased performance, efficiency, and durability.

For sub-assemblies such as hydraulic pumps, gears need to have a very tight overall thickness control to ensure adequate sealing surfaces for proper operation of the pump (minimal leakage or pressure loss). The thickness tolerances that are required for these pumps are less than 50 microns, usually less than 30 microns. Current manufacturing methods can't meet these tolerance levels without the addition of a secondary machining operation. The ribs of the current invention may be provided in powder metal punches (top and/or bottom) along the outer profile or perimeter or other surfaces on the ends of the parts. This feature provides a raised region that projects beyond the surface of the part faces, formed by the punches, that can be subsequently plastically deformed to a very accurate overall thickness tolerance during a coining operation. This deformation region provides the sealing surface for a pump assembly in the case of gerotor gears. During the application of a stress field on the porous material at the ends of the ribs during the coining operation causes a flattening of pores causing a decrease in thickness or elevation of the raised region, as shown in the micrograph ofFIG. 9. With increasing thickness reduction, after attaining about 95% of full density, the compressive strength of material increases rapidly. The plastic deformation becomes the predominant densification mechanism with limited further closing of the pores.

The process for creating a coined sintered powder metal gerotor gear110is as follows: a powder metal mixture is created which may include iron, steel, aluminum, bronze, brass, or any mixture that is well known in the art for making conventional PM gerotor gears, or other parts if the invention is applied to a different part. The powder metal mixture is placed in a forming die and compacted in the die with punches from above and/or below. The forming die includes the negative shape of the tooth surfaces14on the powder metal part. The punches include the negative shape of the opposed surfaces12of the part, including the coining projections20. Such apparatuses are well known in the art, and may be driven by hydraulic or mechanical devices. Next, the compacted powder metal gerotor gear is sintered. Sintering time, temperature, and environment can be chosen based on the selected powder metal mixture. Such combinations are well known in the art. Sintering results in a near net shape sintered powder metal gerotor gear10. Typically after being cooled, sintered powder metal gerotor gear10is coined in a coining apparatus22as shown inFIG. 3. Coining apparatus22includes tooling components24(which may be carbide), a hydraulic actuator or press element26, a fixture27, a positive stop28, and an internal diameter support30. Tooling components24may include movable upper tooling component32and lower tooling component34. Tooling components24also include coining surfaces36which contact and deform coining projections20. Upper tooling component32includes flange38which contacts the positive stop28. Positive stop28causes motion of upper tooling component32to cease, and therefore determines the overall thickness of the coined sintered powder metal gerotor gear110.

Other embodiments for the coining apparatus are possible. For example, a mechanical press can replace the hydraulic press. Also, the bore16can include coining projections20which form an internal spline. These projections can be deformed by the internal diameter support30in the coining operation. In addition, modifications to the sintering process are possible. For example, the coined sintered powder metal gerotor gear110may be subjected to an infiltration process to further improve the strength characteristics of the component. Such modifications to the sintering process are well known in the art. In any case, the process results in coined sintered powder metal gerotor gear110with coined projections120and coined free ends121.

The coining process is much quicker and less expensive than a machining operation. In addition, it can be used with a variety of powder metal materials that can be plastically deformed during a coining operation, such as iron, steel, aluminum, bronze, and brass.

This process can also be used to manufacture other types of components which have a narrow range of dimensional tolerances. For example,FIG. 7ashows a sintered powder metal bushing component210with coining projections220. Coining the component results in coined sintered powder metal bushing component310with coined projections320, as shown inFIG. 7c. Additionally,FIG. 8ashows a sintered powder metal spacer component410with coining projection420. Coining the component results in coined sintered powder metal spacer component510with coined projection520. In addition, there are other types of pumps and other machines, that have gears, gear-like components or other components with ends that slide or rotate against a surface and that must be sealed against that surface, that this invention could advantageously be applied to, for example a P-pump or a ringed pump.

A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiment described but should be defined by the claims that follow.