Operational Differences

A basic operational difference between the lubricated and non-lubricated cylinder should be explained. The piston works against pressure and should form a sliding seal so that it can compress the gas without leakage. So perhaps, in the lubricated cylinder, the simplest piston would be a plug piston with a very close fit to the cylinder bore. But, because of temperatures and other engineering and economic reasons, this is not practical. Piston rings are therefore used for sealing.

These piston rings have many variations, but all follow the basic principle of a thin metallic split ring fitted into a groove around the piston. The ring is made with "spring," or tension, which tends to push out against the cylinder wall and make a tight sliding fit.

It is important to note that piston rings float in the ring grooves of the piston and that they only seal. THEY DO NOT SUPPORT THE PISTON, nor is there any other device to support the piston. The piston is supported off the cylinder wall by the liquid lubrication film only.

In the non-lube or oil-free piston and piston ring assembly there is no oil film to support the piston, so the metallic piston must be kept off the cylinder bore by other means or serious damage will result. Note that this is the difference between lube and non-lube principle.

In the conventional non-lubricated compressor, the piston is kept off the cylinder wall by a guide ring which is referred to as a bull, wear, or rider ring. This rider is of a low friction material, such as carbon or Teflon, and of low unit loading relative to the piston weight.

The outside diameter of the piston is smaller than that of the piston in the lube compressor; this creates clearance between the piston outside diameter and the cylinder bore. This clearance allows for rider band wear before metal contact is achieved with the cylinder bore.




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Figure 2-35. Conventional non-lubricated compressor, vertically, oriented (Worthington Type VBB) (Source: Plant Engineering, October 18,1979).

The crankcase components are conventional; both compressors contain a forged crankshaft, anti-friction bearings, a forged connecting rod, and a cast crosshead.

Both the lube and non-lube compressors have the same lubrication system for these components—an oil pump, which flows oil through the crankshaft, passes through the connecting rod to lubricate the crosshead pin bushing and the crosshead. This oil is retained in the frame with a frame oil head and metallic scraper rings.

On the totally non-lubricated compressors a housing is used between the frame and the cylinder, along with a longer piston rod. This ensures that no part of the piston which is in contact with the frame-lubricating oil will pass into the cylinder.

Generally an oil deflector assembly is used on the piston rod to prevent any oil from "creeping" up the piston rod by capillary action and entering the cylinder.

The cylinder bore is honed to an 8-16 RMS finish to reduce wear of the piston rings and rider band. In addition to the normal honing to achieve this sort of finish, the non-lube cylinder is given an additional treatment of "teflon honing." In this treatment, blocks of virgin teflon are used in place of the abrasive stones on the hone sets, which impregnates the pores of the iron bore of the cylinder.

The piston rod surface finishes through the packing travel are very important. On a lubricated compressor, the piston rod is finished to 16 RMS, while on the non-lube cylinder the piston rod is micro finished to 8-10 RMS.

In addition, the piston rod will have a surface hardness in the packing travel area of 50-55 Rc.

Piston rod alignment and eccentricity must very carefully be checked in order to minimize packing wear. Excessive lateral movement or runout exceeding .003" will prevent packing from sealing.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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