1. Field of the Invention
The present invention relates to a method of treating surfaces of rotors of a screw-type rotary fluid machine suitable for use as a compressor, blower or a vacuum pump and to a method for treating surfaces of rotors of a corrosion-resistant, low-noise and low-friction screw-type rotary fluid machine which is suitable for use as a dry-type fluid machine
2. Description of the Related Art
In general, a screw-type rotary fluid machine has a male rotor and a female rotor which are accommodated in a casing and which rotate in meshing engagement with each other, so that a space defined by both rotors and the casing is progressively shifted in the direction of axis of the casing while reducing its volume, thereby compressing or pressurizing a fluid confined in the space.
There are two types of screw rotary fluid machines, namely, an oil-cooled type machine in which a lubricating oil is supplied into the machine together with the fluid to be compressed so as to cool the rotors and the casing while providing lubrication between the rotors and the casing and between the meshing rotors, and a dry-type machine in which no oil is supplied into the machine
In the oil-cooled type machine, the male rotor and the female rotor contact each other through the intermediary of an oil film. As the male rotor is driven by a motor through a drive gear, the female rotor is driven by the male rotor so as to rotate in synchronization with the male rotor. Thus, the construction of the machine is comparatively simple because no independent driving mechanism is necessary for driving the female rotor.
In addition, the oil supplied to the machine effectively cools the rotors so as to prevent seizure of both rotors which may otherwise be caused by the friction heat generated during rotation of these rotors.
The oil-cooled type machine, however, has a in that it cannot be used in handling fluids which are strictly required to be clean, e.g., fluids used in food industries and high-technology industries.
In contrast, a dry-type machine can provide clean fluid because no oil is suspended in the fluid discharged from this type of machine. However, in this type of machine, both rotors rotate without contacting each other. It is therefore necessary to employ synchronizing gears on one end of each of the rotors so as to synchronize the rotational phases of both rotors. Consequently, the construction of this type of machine. is complicated as compared with oil-cooled type machine Another problem is that, since both rotors do not contact each other, fluid tends to leak through the gap between both rotors so as to reduce compression efficiency.
Thus, both the oil-cooled type machine and the dry-type machine have advantages and disadvantages.
In for example, Japanese Patent Laid-Open No. 56-75992, a screw-type fluid machine is disclosed which can be constructed both as an oil-cooled type machine and dry-type machine. This type of machine will be referred to hereinafter as the as "first machine". In this first machine, the surfaces of both rotors are coated with a soft metal or a plastic so that the rotors can smoothly contact each other while preventing leak of fluid, thus achieving a high compression efficiency.
In Japanese Patent Laid-Open No. 58-148292 a method is disclosed in which a plastic, rubber or molybdenum disulfide is applied to the surfaces of the rotor so as to reduce the gap between the rotors, thus improving the compression efficiency. This method will be hereinafter referred to as "first method.".
In Japanese Patent Laid-Open No. 48-2308, a dry-type machine is disclosed in which both rotors are formed of sintered bodies and is impregnated with an oil such that the oil is held in the micro-pores between particles of the sintered bodies, thereby reducing friction between both rotors and, hence, wear of these rotors. This machine will hereinafter be referred to as the "second machine".
Japanese Patent Publication No. 61-47992 discloses a machine wherein synchronizing gears, provided on the end of each of the rotors, mesh with each other so as to prevent any contact between both rollers which are designed to rotate without contacting each other, wherein the shapes and sizes of both rotors are determined beforehand so as to avoid an occurrence of mutual contact of the rotors due to rise of temperature of both rotors during operation of the machine. This machine will be referred to hereinafter as "fourth known art," the "third machine".
In the "first machine" explained above, the surfaces of the rotors are coated with a soft metal or a plastic. Therefore, when a corrosive fluid is handled, the soft metal or the plastic and, finally, the surfaces of the rotors are corroded. Thus, the "first machine" could not be used practically for corrosive gases.
In the "first method" described above, the surfaces of both rotors are coated with plastic, rubber or molybdenum disulfide. As a result of repeated contact between both rotors during the operation of the machine, the films on both rotors are damaged and finally cause excoriation from the rotor surfaces. In order that the required resistances to wear and corrosion may be provided by the plastic or rubber coating layer alone, the coating layer is required to have a thickness of 0.1 mm to several centimeters. Such a thick coating layer inherently has a risk of separation from the rotor material due to difference in the thermal expansion coefficient. Thus, the second known art is still unsatisfactory from the view point of performance, reliability and noise.
In the first machine and "first method" as described above, the surfaces of the rotors are directly treated by coating or application, and the required resistances to wear and corrosion are provided solely by the layer formed by the surface treatment. Consequently, the surface layer is liable to be damaged or separated in a short time after the start of operation of the screw-type fluid machine, causing a reduction in the compression performance and increase in the noise level. In the worst case, the operation of the screw-type fluid machine has to be stopped.
The "second machine" described above, a disadvantage resides in the fact the machine cannot provide clean fluid which is an essential requirement for dry-type machines, due to the fact the rotors made of sintered bodies are impregnated with an oil.
With regard to the "third machine" described above, mutual contact between the rotors during the operation is materially unavoidable due to, for example, a machining error, even though the rotation of both rotors are synchronized to effect non-contact rotation of both rotors. In addition, there is a risk that any foreign matter in the fluid is jammed between the rotors. If the rotor surfaces are not suitably treated, therefore, the rotor surfaces are soon damaged to cause problems such as a reduction in the compression efficiency and generation of noise.