1. Field of the Invention
The present invention relates to a mechanism for transmitting driving force between two or more rotary shafts, specifically to a drive transmission mechanism with which rotors of a dry-sealed mechanical vacuum pump such as Roots type, screw type, and claw type vacuum pump having two or more rotors can be rotated in synchronism with one another without a need for lubrication oil for lubricating the mechanism thereby eliminating occurrence of oil contamination and to an oil-free fluid machine equipped with the mechanism.
2. Description of the Related Art
Fluid machines having two or more counter-rotating meshed lobed rotors accommodated in a rotor casing to expel air trapped in a space between the wall of the casing and the rotor surface by rotating the rotors in synchronization with one another are widely used as vacuum pumps such as roots vacuum pumps, claw vacuum pumps, and screw vacuum pumps.
In these dry mechanical vacuum pumps having two or more lobed rotors, synchronization gears made of metal is usually adapted to allow meshing lobed rotors to rotate in directions opposite to each other. The synchronization gears made of metal are needed to be lubricated with oil, grease, or a solid lubricant, etc. Further, noise occurs due to contact meshing of the synchronization gears.
Lubrication of the synchronization gears is performed with oil, grease, or a solid lubricant, etc. Oil lubrication deteriorates quality of vacuum. In a case of low rotation speed of the rotors, grease may be used, but refilling of grease is not easy. Solid lubricants are not adequate when the gears experience large loads. Grease is poor in friction heat removing performance, and solid lubricants can not remove friction heat.
In a case lubricating oil is reserved in a gear case and supplied to where needed when operating the vacuum pump, there are problems that oil leaks through oil seals of drive shafts of the rotors. Particularly, oil molecules leaked to the pump chamber defuse into the vessel to be evacuated and deteriorate quality of vacuum.
To deal with the problems, it is thinkable to use plastic gears or toothed belt (synchronous belt) in order to transmit driving force without lubricating the synchronization gears. However, there is a disadvantage that large torque can not be transmitted, since the plastic gears and toothed belt are lower in strength as compared with metal gears, resulting in decreased operation life.
In Japanese Laid-Open Patent Application No. 6-185483 (Patent literature 1) is disclosed a dry mechanical vacuum pump of roots type, in which an annular magnet is attached to an end of the drive shaft of a drive motor and to an end of one of the rotary shaft respectively, and a partition member made of electrical insulating material is provided to run in the gap between the outer periphery and inner periphery of the annular magnets so that the pump body side where the annular magnet attached to the rotary shaft exists is separated from the outside of the pump body where the annular magnet attached to the drive shaft of the motor exists. Synchronization gears consisting of a metal gear and a plastic gear for allowing the two rotors to rotate in direction opposite to each other in synchronization with each other are provided at the other ends of the rotary shafts respectively. With this construction, lubricating oil for lubricating the synchronization gears is not needed, oil seals for preventing oil leak from the gear chamber to the pump chamber and for preventing oil leak from the gear chamber to outside are eliminated, and power loss due friction is decreased.
However, with the dry mechanical vacuum pump of roots type disclosed in the patent literature 1, driving torque of the drive motor is transmitted via the annular magnets to one of the rotor and this driving torque is transmitted to the other rotor by way of the synchronization gears consisting of the metal gear and plastic gear. Therefore, when increased driving torque is transmitted from the drive motor to one of the rotors, all of the driving torque is transmitted to the other rotor by way of the synchronization gears and the plastic gear may be fractured or decreased in operation life due to the increased torque.