Field of the Invention
The present invention relates to the field of air compressors adapted for use on rail vehicles for the purpose of supplying compressed air to pneumatic units associated with the rail vehicle and, in particular, to an oil-free air compressor on a rail vehicle for supplying compressed air to various pneumatic units associated with the rail vehicle.
Description of Related Art
Normally, a pneumatic system is provided for a rail vehicle by which the brakes of the rail vehicle are operated. An air compressor is used to supply compressed air to one or more pneumatic units associated with the rail vehicle involved in the operation of the brakes. The air compressor usually consists of a driving unit, such as an electric motor, and of a compressor unit, which typically consists of several piston-cylinder arrangements that are driven by a crankshaft. The crankshaft is driven by the driving unit and includes connecting rods to convert the rotating movement of the driving unit into linear movement for each piston to supply compressed air to the downstream units. Screw-type air compressors are also generally known in the field for this purpose and are also included within the scope of the present invention. Furthermore, air compressor units for use on rail vehicles may have a single-stage or a multi-stage construction with at least one low-pressure stage and one high-pressure stage.
The air compressors used in the rail vehicle field may be subjected to continuous operation or to frequent on-and-off operation. In either mode of operation, friction during operation of the compressor leads to high heat development. As a result, in the past, air compressors that were predominantly used in the rail vehicle field used oil lubrication to ensure sufficient cooling during operation. However, oil lubrication carries a risk that the lubricating oil, usually situated in the housing of the compressor unit in the case of a piston air compressor, can penetrate past the piston-cylinder interface and into the pneumatic system, which may result in oil fouling the pneumatically operated brake units on the rail vehicle. Furthermore, condensate, which occurs during the required air drying of a pneumatic system, will typically contain some oil that has to be collected for environmental protection reasons. This condensate is typically stored in heatable containers and has to be drained and disposed of at regular intervals. This collection process leads to increased maintenance and disposal expenditures as well as to high oil consumption. In addition to the foregoing difficulties, emulsion formations in the oil circuit of these oil-lubricated compressor units can occur if the oil-lubricated compressor units are used infrequently or for limited periods of time as during cold weather operation.
Recently, dry-running air compressors have found increased usage in the rail vehicle field. A dry-running air compressor operates without lubricating oil situated in the housing and is said to be “oil-free”. In the case of oil-free air compressors, the lubrication on the piston travel path is replaced by a particularly low-friction dynamic sealing arrangement. All rotating components are normally disposed in roller bearings. The encapsulated roller bearings are provided with a temperature-stable long-lived grease filling. In the valve area, slidably guided components are largely avoided. Because of these measures, oil lubrication is not required in the air compressor unit. The risk of fouling by oil of the compressed air can therefore also be excluded. As a result of the elimination of an oil circuit, the oil-free air compressor can have a relatively light construction. In the rail vehicle field, there is a current trend toward lighter construction, and light carrier structures are also increasingly used for frame constructions. However, such light carrier structures frequently have a number of unfavorable natural frequencies that are close to the rotational speed of the air compressor of the pneumatic system which is arranged thereon. Therefore, it is difficult to sufficiently observe the required specifications concerning permissible structure-born noise levels.
U.S. Pat. No. 6,776,587 to Hartl et al. and U.S. Pat. No. 7,059,841 to Meyer et al. are patents directed to oil-free air compressor technology. The Meyer et al. patent discloses an arrangement of an oil-free compressor apparatus on a rail vehicle for supplying compressed air to pneumatic units assigned to the rail vehicle. The arrangement includes an oil-free air compressor and a cooler unit connected with the air compressor. The arrangement also includes a rail vehicle having a floor with at least one opening. The air compressor is fastened on at least one side to the vehicle floor such that a main axis of rotation of the air compressor is arranged essentially vertical with respect to the vehicle floor. The Hartl et al. patent discloses a piston arrangement for a dual-stage piston air compressor that includes a crankshaft and several piston-cylinders. The arrangement allows two or more low-pressure stages and at least one high-pressure stage to be formed. The arrangement allows the two or more low-pressure cylinders to be arranged in relation to the high-pressure stage in such a way that said two or more low-pressure cylinders are in phase or are offset by less than a predetermined amount and compress in a position which is offset by another predetermined amount in relation to one or more of the high-pressure cylinders.
United States Patent Application Publication No. 2007/0292289 to Hartl et al. discloses a compressor piston including a piston and a cylinder, a connecting rod connecting the piston to a crankshaft in a crankcase by a roller bearing, an air inlet line, and an air outlet line in a cylinder head. A tube connection between the air inlet line and the crankcase transports cooling air from the inlet line to the crankcase. The tube connection is exterior of the cylinder. An inlet valve is connected to the tube connection which opens when the pressure in the crankcase is less than the pressure in the air inlet line, and an outlet valve is connected to the crankcase which opens when the pressure in the crankcase exceeds a predetermined value.
Further, United States Patent Application Publication No. 2009/0016908 to Hartl et al. discloses a multi-cylinder dry-running piston compressor for generating compressed air. The piston compressor includes a crankcase having an interior and a crankshaft rotatably mounted in the crankcase. Also included are two connecting rods mounted on the crankshaft and configured to run counter to one another. Further included are two cylinders mounted in the crankcase and a piston arranged at an end of each of the connecting rods and configured to run in a respective one of the two cylinders.