1. Field of the Invention
The present invention relates to a shaft structure in a screw rotor used in a screw fluid assembly, and more particularly to a joint structure of a shaft of the screw rotor and a mounting structure of the shaft of the screw rotor to the rotor for the screw rotor used in a compressor, an expander, a vacuum pump and the like.
Conventionally, in a screw rotor of this kind, the rotor and the rotor shaft are integrally formed. For example, the rotor and the shaft comprising the center shaft and the rotor shaft are worked out by milling from the same metal material. Alternatively, it is proposed to mold a rotor made of synthetic resin material, for example, injection molding around the shaft made of metal to improve a processing efficiency and to reduce a gross weight.
2. Description of the Prior Art
Even if a size of the rotor is common to different kinds of machines, since a discharging pressure of the screw fluid assembly or a prime driving means are changed, or length, diameter of the shaft or surface treatment of the shaft is changed in accordance with usage of the screw fluid assembly, the manufacturing process of the rotor is complicated, and a large number of kinds of processes is required.
For such reasons, there are various problems. If the case of diameter of various portions of the screw rotor is taken as an example, in general, a diameter of tooth portion of the rotor is three or four times greater than a diameter of the shaft. Therefore, there are following problems concerning manufacture:
(1) When the rotor and the shaft are worked out by lathing from the same metal material, it is necessary to use a material having the size corresponding to the diameter of the rotor tooth portion, and to lathe the material to a desired shaft diameter by lathing operation. Therefore, this wastes materials and necessitates a number of steps for the process. PA1 (2) When a rotor made of thermosetting synthetic resin is integrally molded around the shaft made of metal, this case only requires a metal material having a diameter of the shaft. However, since the axial length of such metal material is too long with respective to its diameter, it is necessary to prepare a remedy for a prevention of oscillation of center of a work at the time of working, which lowers the productivity. PA1 (3) Further, the axial length of the rotor is also long, a mold of the rotor is increased in size and price, and the workability at the time of molding is also inferior. Further, the operating performance for a surface treatment is likewise inferior. PA1 (4) For example, when the rotor and the shaft are used for a water-injection type compressor or the like in which water is injected into a compressed space for cooling in the space and tightly sealing up the space, and in which the shaft contacts with atmosphere or fluid including moisture, rust is generated when they are used for a long time, which may cause a leakage of the shaft sealing portion or abnormal abrasion of the sliding portion. PA1 (5) Even if the tooth size of the rotor is common, since the diameter or length of the shaft is different, parts can not be used in common and the manufacture and management are complicated.
As countermeasures or remedies for the above problems, Japanese Utility Model Application Laid-open (Kokai) No.S57-105418 proposed to separately work out a hollow small-diameter portion and a hollow large-diameter portion of the screw rotor so as to reduce the weight, and to joint corresponding opposed ends of both the portions by friction welding, thereby providing a screw compressor. This proposal is to hollow the shafts, and joint them by friction welding, and is not to solve the above described various problems. Japanese Utility Model Application Laid-open No.S56-49311 proposed to simply spline-connect crankshafts, and to charge elastic material to the spline-connected portions, thereby reducing both the friction and noise. Further, Japanese Utility Model Application Laid-open No.H2-71108 proposed to form a groove in one of a propeller shaft or a yoke, and to form, on the other, a projection to be meshed with the groove. Furthermore, Japanese Patent Publication (Kokoku) No.S52-25562 proposed to integrally provide an impeller of a compressor with a sleeve having a threaded hole, and a threaded portion of a shaft is received in the threaded hole and assembled. However, these proposals are only related to a general mounting operation for enhancing the reliability, and are not for solving the above described various problems.
A conventionally known screw rotor of this type is shown in FIG. 12 as an example relating to a screw rotor formed by molding a rotor made of synthetic resin by injection molding around the above described shaft made of metal.
Each of FIGS. 12 (A) and 12 (B) show a female rotor, wherein FIG. 12 (A) is a partial sectional view taken along the line A--A in FIG. 12 (B), and FIG. 12 (B) is a front view thereof. A shaft 41 comprises a center shaft 38 and a rotor shaft 36 of these screw rotors. The shaft 41 is formed at its peripheral surface with a single or a plurality of grooves 39 or projections each having a square cross section (a quadrilateral) with a lead angle in the torsion direction and its opposite direction of teeth 46 of a rotor 1f, thereby reinforcing a connecting force between the shaft 41 and the rotor 1f to prevent them from leaving off (see, e.g., Japanese Patent Applications Laid-open No.H6-123292 and No.1-301976).
In these screw rotors, rotor surfaces are formed of synthetic resin, and therefore rust is not generated.
Therefore, the screw rotor of this kind is most suitable for a screw compressor (water-injection type screw compressor) in which water is injected in a space where compression action is performed (hereinafter also called as a compression space).
In a screw rotor in which the groove 39 having a square cross section is formed on the peripheral surface of the shaft 41 and synthetic resin material is molded thereon to connect the shaft 41 and the rotor 1f, stress concentration is generated on an angle portion of the groove of the rotor (synthetic resin portion) corresponding to a crest of the groove 39 provided on the shaft surface by a difference in a coefficient of linear thermal expansion based on difference in material of the shaft and the rotor at the time of cooling the rotor made of synthetic rotor after injection molding, and by a difference in thermal shrinkage between the shaft 41 made of metal and the rotor made of synthetic resin (thermal shrinking amount of the shaft is small, and thermal shrinking amount of the rotor is large) due to thermal change, at the further time of driving the compressor and at the time of stopping the compressor, and by a large variation in revolutional torque due to variation in load during the driving.
By the above described reasons, in the screw rotor of this type, a crack 51 is prone to be generated from the angle portion of the crest of the groove 39 provided on the shaft surface toward the bottom of the tooth of the rotor 1f, and while driving and stopping operations of the screw rotor are repeated for a long time of period, there is a problem that the crack is spread and fixing force between the shaft 41 and the rotor 1f is lowered and finally the shaft and the rotor are separated.
On the other hand, when the case of the water-injection type screw compressor is taken with reference to FIG. 6, if the screw rotor of the above-described type is employed, since the rotor is made of synthetic resin, there is a characteristic that rust can be prevented. However, there is a problem that rust may be generated on the shaft surface because the shaft 1 is made of metal, and the shaft surface between the compression chamber and the shaft sealing portion (Water-shaft sealing device) contacts with water supplied in the compression chamber.
As described above, in the conventional screw rotor of the water-injection type screw compressor, if rust is generated on the shaft portion, especially on the shaft surface into which a shaft sealing device for water is fitted, the shaft surface is corroded by the rust, and a diameter of such portion is reduced to cause a clearance between the shaft sealing device, and the shaft surface or the shaft sealing device itself is damaged to lower the shaft-sealing performance.
As a result, water supplied to the compression chamber leaks from the clearance between the shaft sealing device and the shaft surface toward the end of the shaft of the screw rotor, and flows into a bearing portion adjacent to the shaft sealing portion, which induces a deterioration of lubricant oil or emulsification to shorten life span of the bearing, and remedy therefor has been required.
Further, there are various problems that since the performance of the shaft sealing portion is lowered due to the generated rust, the lubricant oil supplied to the bearing portion flows into the compression chamber from the clearance between the shaft sealing portion and the shaft surface and is mixed into the cooling and sealing water to contaminate the water, and oil is also mixed in the discharged air, which exerts a harmful influence on apparatuses which consume the discharged air.
Therefore, rust prevention remedies have been applied by using stainless steel as material for the screw rotor shaft, or plating or coating a material having high rust prevention effect on the shaft surface to which the shaft sealing device is fitted. However, such remedies increase the prices and working costs of the screw rotors.
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