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The Articulated Piston Pump
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The Articulated Piston Pump

By: Benjamin

The Articulated Piston
By Benjamin Kendall

A two-piece piston composed of independent head and skirt portions, wherein the skirt portion is mounted by means of a wrist pin on bosses located on the lower end of members pending from the head portion, the head portion being provided with an annular recess between the combustion bowl and the ring zone, and the skirt provided with a tray on its upper end, the annular recess and the tray defining a semi-open cooling chamber. With the piston assembled, the lower end of the head portion below the ring zone and the upper end of the skirt portion, which constitutes the outer wall of the tray, define an oil baffle designed to prevent the cooling oil from flowing from the tray toward the space between the piston and the cylinder.

The methodology allows the characterization of conventional and articulated piston secondary motions in the thrust plane of the cylinder. Motions of the piston, pin, rod and (for articulated pistons) skirt are separately calculated, by integrating equations of motion for individual components and dynamic degrees of freedom. Various configurations with respect to rigid attachment of the wristpin to other components can also be represented. In the equations of motions solved, all gas pressure, inertia, friction and oil or contact pressure forces are accounted for. Detailed submodels of skirt and wristpin lubrication are utilized to calculate the effect of these oil films. All pertinent operating parameters (engine speed and cyclic pressure variation) as well as design parameters, such as component masses, moments of inertia, mass centers, pin offsets etc.

The model was applied, in conjunction with the skirt and wristpin lubrication submodels, to conventional and articulated pistons, in a number of parametric studies. Effects of speed, load and piston configuration were investigated.

In the present days there is a growing tendency toward the use of articulated, or two-piece, pistons on diesel engines. In the articulated piston the head and the skirt are independent members: the skirt is assembled by means of a wrist pin on bosses located at the lower end of members depending from the head. With the piston fitted in the cylinder there is no contact between the head and the skirt portions. The basic feature of the articulated piston is that its components perform independent functions: the head, provided with the ring zone, bears the pressure of the combustion gases, while the skirt functions as a guide of the piston in the cylinder and supports the side loads transmitted by the inclination of the connecting rod.

The mounting of this type of piston in the cylinder can be made with small clearances due to the use of ferrous metals, which have a lower coefficient of thermal expansion, approximately that of the cylinder liner, which favors a better piston ring stability owing to a smaller lateral movement of the piston, thereby affording a lower lubricating oil consumption and a reduction of blow-by. Moreover, the absence of contact between the skirt and the head results in a reduced temperature on the skirt, thereby affording smaller mounting clearances and accordingly a decrease of the piston noise level.

Generally, the top portion, or head, of articulated pistons is made of a ferrous metal which is more resistant to the high thermal and mechanical loads, the skirt being generally of a cast or forged aluminum alloy. The use of a ferrous metal for the head affords placing the upper compression ring groove very close to the piston top, which would be virtually impracticable on conventional aluminum alloy pistons.

On high output diesel engines, the high temperatures on the piston top, especially on the ring zone and combustion chamber, make these areas extremely susceptible to problems. On the ring zone, very high temperatures may cause an excessive and early wear of the upper compression ring groove, degradation of the lubricating oil, build-up of carbon deposits and ring sticking. On the combustion chamber, these problems are represented by thermal cracks which tend to propagate and ultimately cause a breakage of the piston top. Therefore, it is necessary to provide a decrease of temperature of these regions in order to solve this problem.

Thomas DC-Powered Articulated Piston Pumps Engineered for Long Life Powered by rugged permanent magnet less design, permanently lubricated bearings, cast iron cylinders, and PTFE piston rings and skirts. Models TA-2101DC, TA-3101DC, TA-4101DC max operating and restart pressure is 100 PSI. TA Series articulated piston pumps from Thomas combine a range of heavy-duty features design, permanently lubricated bearings, cast iron cylinders, and polytetrafluoroethylene (PTFE) piston rings and skirts.

During the development of the higher grade steel piston crown, the material was found to be extremely difficult to machine. Since the material is extremely tough, it has the characteristic of causing gumming of the tooling, and it was necessaryto develop a unique machining method which is the subject of this invention.

In one aspect of the invention, the method of producing a crown of an articulated piston is provided. The crown has a circular top face, a combustion crater extending from and below the top face, a sidewall depending from the top face which hasa cylindrical surface. A plurality of ring grooves on the sidewall are provided, which are disposed at spaced locations below the top face. The sidewall has an inner surface forming a first side of a cooling gallery that extends adjacent to top faceand the combustion crater. A depending center section has a lateral extending pin-receiving bore, therethrough, and a surface adjacent to the crater forming a second wall of the cooling gallery. The sidewall has a bottom face extending between thecenter of the sidewall and the cooling gallery and is disposed above the pin receiving bore.

With a view to minimizing these problems, applicant devised and developed a solution to keep the cooling oil from escaping in the direction of the cylinder. It is, therefore, the object of the present invention to provide a two-piece, or articulated, piston wherein the lower end of the head and the upper end of the skirt are shaped in such a manner as to define a baffle designed to render difficult the exit of cooling oil through the gap between the head and the skirt.

A second aspect of the invention is a method of the rough machining wherein one portion of the outer surface is grasped in a chuck and the rough machining of that surface proceeds along the outer surface, ending in an area overlying the positionof one of the ring grooves. The crown is then removed from the chuck, reinserted on the machined surface, and the remaining material is removed from the outer surface, following which the ring groove is then machined, removing the juncture at theinterrupted cut.

For more information on the articulated piston, or other products by Gardner Denver, visit http://www.gd.thomas.com

Article Source: http://articlenexus.com

For more information on the articulated piston, or other products by Gardner Denver, visit www.gd.thomas.com

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