Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2000-06-14
2001-08-14
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C419S040000
Reexamination Certificate
active
06274083
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of hydrostatics, more particularly, hydraulic pistons used in the cylinder block of a hydrostatic power unit. The present invention relates to hollow or reduced oil volume pistons and methods of producing the same.
It is conventional to use solid steel pistons in the rotating cylinder block of a hydrostatic power unit. Solid steel pistons are durable, reliable, and inexpensive to make, but their weight tends to impose limitations on the speed of operation for the cylinder blocks in which they are used. They also develop more operational frictional forces.
Consequently, some manufacturers have constructed hollow pistons that weigh less than the solid steel pistons. One common and conventional method of making hollow pistons involves rough turning a hollow piston body and a separate piston cap from steel barstock. Then the body and cap are joined together by conventional welding techniques, such as inertia welding, to form a piston with a hollow interior. However, many fabrication steps with tight tolerances or process controls are required to make such welded hollow pistons, which makes them relatively costly to manufacture. Hollow pistons weigh less than solid pistons but don't necessarily reduce the compressed oil volume of the hydrostatic unit unless they are welded shut to enclose an internal void. Reduced compressible oil volume provides better control of swashplate moments and efficiency in swashplate controlled hydrostatic power units.
Therefore, a principal objective of this invention is the provision of an improved method for producing hollow pistons used in the cylinder block of a hydrostatic power unit.
Another objective of this invention is the provision of an easy and inexpensive method for producing hollow pistons.
Another objective of this invention is the provision of a method for producing hollow pistons by utilizing a unique combination of new and conventional metal injection molding techniques.
Another objective of this invention is the provision of a method for producing hollow pistons that involves the steps of securing a core insert of a non-metallic material in a mold, filling the mold with a mixture of non-metallic binding material and metal particles, then eradicating the non-metallic binding material to leave a void in the cast piston.
Another objective of this invention is the provision of a method for producing hollow pistons that reduces the number and difficulty of the subsequent machining operations required to finish the piston.
These and other objectives will be apparent to one skilled in the art from the drawings, as well as from the following description and claims.
SUMMARY OF THE INVENTION
The present invention relates to methods for producing a hollowed piston for use in the cylinder block of the hydrostatic power unit.
The method includes the steps of fixing a core insert formed of a non-metallic material in a desired location within the cavity of a mold, injecting a mixture of a non-metallic binder material and metal particles into the cavity so as to surround at least a portion of the core insert and form a piston, and eradicating the core insert from the piston. The core insert can be eradicated by various methods, including raising its temperature to melt it or chemically disintegrating it. The cast piston can be further heated to debind the non-metallic binder material from the metal particles and metallurgically bond the metal particles to each other. The core insert can be formed by injection molding a thermoplastic material and the filling step can be carried out by conventional metal injection molding techniques.
Of course, once the core insert has been eradicated, it could also be replaced in whole or part with a lighter-than-oil filler material suitable to withstand the typical operating conditions of the hydrostatic unit. This would reduce the volume of compressible oil in the piston without significantly increasing the weight of the piston. In low pressure hydrostatic units or applications, there is little risk of the filler material being forced out by the oil pressure experienced by the piston.
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Mai Ngoclan
Sauer-Danfoss Inc.
Zarley McKee Thomte Voorhees & Sease
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