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Over 80 years ago, piston compressors were mainly used in tire-fillingtechnology. Users wanted to have compact, robust and reliable compressors – and that has not changed until today.
What has changed,though, are demands on the economical efficiency and the quality of compressed air itself. In many modern areas of application, compressed air has to be absolutely oil-free, from the food industry to breathingair for hospitals, and from chip production to the pharmaceutical industry.
For such sensitive areas, direct, oil-free compression isthe well-tried way to get unchangingly excellent quality air. Becausewhat is not in the air from the beginning does not have to be removedagain with effort. So, the condensate does not have to be conditionedbefore being disposed of in the sewer.
Fig. 1 Oil-free compressing stroke piston with push-rod principle.
The transformationof a reciprocating motion (oscillation) into the circular motion of awheel (rotation) creates a great variety of possible applications. Thenew oil-free compressor presented here has a lot of constructive features, which enable the owner to use it safely and economically. For the engine, a special gearing concept has been chosen, which has a straightforward construction and is tailor-made for the properties of an oil-free compressor.
Generally, simple crank gears for oil-free compressors consist of the three main components crankshaft, drive rodand piston.
In the design of the compressors presented here, oil-lubricated gearing with crossheads and separate oil-free cylinders driven by plungers were deliberately done without. The feature oil-free operation of the compressor is this way guaranteed from the beginning.
Fig. 2 Hermetically sealed crankshaft and drive rod bearings
The current construction places particular care on a simple and easily serviced design. Here, too, the gear consists of the main components crankshaft, drive rod and piston described above. All bearing points are provided with sealed bearings. For the structural shape of thecompressor, a design with two cylinders has been chosen. Through this,single-stage as well as two-stage compression is made possible.
Both cylinders are arranged horizontally and have a common centerline. Only in one of the cylinders, a normal piston oscillates, driven via the drive rod by the crankshaft. For the second piston, the gearingis effected by a push rod construction connected to the first piston.As a result, both pistons are synchronically moved back and forth asa unit in the cylinders. Both pistons are geared by only one drive rod, which has to alternately transfer tractive and pressure forces.
The piston driven by the drive rod is designed as a plunger. Not only gas and mass forces impact on this piston lengthwise to the cylinder axis, but also normal forces developing by the deflection of the drive rod act perpendicularly on the sliding surface. These forces are transferred with the oil-free piston by a guide band support in the direction of the cylinder bore.
However, the strain of such supports by pressure, sliding speed and temperature has certain definitive limits. Because in crank engines the highest normal pressure impacts onthe guide band support just at the same time as the sliding speed reaches a maximum as well, the guide band support has to be respectivelyamply dimensioned in order to avoid untimely wear. If the piston – e.g. because of a too small diameter or length – does not present the possibility for normal force transfer, it has to be effected with the help of a crosshead. Such crossheads clearly need more construction expense, expand mounting conditions and augment the masses to be moved. Piston compressors with a crosshead design are commonly only built in anoil-lubricated construction method. Then, however, the procedural security of absolutely oil-free compression is no longer guaranteed. The design chosen by BOGE renders a crosshead design superfluous and safelyproduces completely oil-free compressed air without having any oil-lubrication in the system.
The push rod between the two pistons described above effects a piston practically free of normal force on the side not connected to the drive rod. This piston is used as additional guide and only needs a small bearing surface. Because of the absence of normal forces, a piston with a small diameter and bearing surfacecan operate here.
The gear is therefore especially well suitedfor two-stage compressors, whereby the piston of the second stage isinstalled on the side that is free of normal forces. In a conventionalengine, a crosshead would be necessary here because of the piston’s smaller diameter. The oil-free piston on the normal force side takes onthe function of the usually oil-lubricated crosshead for the piston of the second stage. For single-stage designs with two equal cylinder diameters, a short and light disc piston with a small bearing surface is installed on the side free of normal forces.
Because of thehigh thermic expansion of the guide band support, the piston of an oil -free compressor has to have an optimally defined cold clearance. By the long overall bearing surface of the double-piston arrangement, a potential lateral buckling of the pistons is prevented. Excitations of structure-borne noise, which are usually generated by such a tilting movement, cannot develop. Uneven and untimely wear of the guide band support is also avoided by the prevention of lateral buckling.
Bythe double-sided piston arrangement, the drive rod is loaded with pressure and traction. The normal forces developing on the driven piston affect therefore also the two sides of the piston. With a conventionalplunger, acting only in one cylinder, normal forces occur to a great extent only on one side of the piston. That is why the guide band supports are worn only on one side. However, they usually surround the piston skirt on the whole circumference. In double-sided strain, the guideband support is evenly used on both sides and therefore utilized fully.
Special attention was given to the bearing of the drive rod.The stroke pivot bearing on the crankshaft is subject to very strongradial acceleration. This makes a well thought-out construction of thebearing necessary. Particularly selected lubricating greases, especially pure bearing steel and the exact presetting of the bearing play all contribute to the long service life of the bearing.
A furtherconstructive particularity is the piston pin bearing. Here only pivoting movements occur, but these place high demands on bearings and bearing seals. Surface properties are aimed for especially high demands, too. The piston pin bearing is provided with first-class lubricating grease, which effects a very long service life of the bearing.
Asconstruction method for the crankshaft an overhung construction has been chosen, in which the bearings of the gear serve as main crankshaftbearings. Another advantageous feature of this arrangement is the possibility of easily exchanging the whole double-piston construction inaxial direction during maintenance.
The sealing of the piston to the compression chamber is effected by a piston ring. The ring jointis practically gas-tight and reduces annoying leakages.
Special importance has been attached to the wear-resistance of the piston rings, because they consist of a particular special compound. However, not only the wear-resistance of the piston ring material is important but also the material of the counter rotation surface. The surface geography of the cylinder bores is achieved by a complex honing process. By this method, especially defined surface roughness values are achieved.
The material of the counter rotation surface is made of a special alloy, which gives the cylinder bore surface the necessary hardnessand wear-resistance.
With the K-series, BOGE has broken new ground in the area of compressor valves, too. The suction valves are particularly flow-favorably built and provide the entering air an ample conduit. The valve construction developed this way has optimally proveditself in thousands of service hours and under the most extreme conditions.
The construction presented here makes it possible to operate a completely oil-free piston compressor without complex crossheadconstructions or similar additional expenses and effort. Lubricating-oil is completely superfluous, and so are the duties of constant oil level controls and oil changes. By the absence of oil in the whole system the possibility of oil contamination of the compressed air is eliminated. This provides the highest possible safety, which is especially decisive for demanding and sensitive applications.
The downstream treatment of the compressed air can be designed in a respectively less complex and expensive way. With a considerably smaller loss of pressure in conditioning, the amount of energy needed for compression is reduced, so that the use of an oil-free piston compressor provides notonly more safety but also considerable long-term energy savings.
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