Air compressor

Abstract

Provided is a technique capable of recovering oil stored in a gear casing without using a pump.　A compressor 100 is provided with: a rotor 9 for compressing air; an electric motor 42 for driving the rotor 9; a gear for transmitting the drive of the electric motor 42 to the rotor 9; a gear casing 22 for storing the gear; an oil tank 11 for storing oil separated from the compressed air; an oil supply flow path 25 communicating from the oil tank 11 to the gear casing 22; an oil recovery flow path 26 communicating from the gear casing 22 to the compressor intake part 20; and an oil pressurizing flow path 27 communicating from a discharge port 14b to the gear casing 22. Oil is supplied from the oil tank 11 to the gear casing 22 via the oil supply flow path 25 by the discharge pressure of the compressor 100, and the gear casing 22 is pressurized via the oil pressurizing flow path 27 by the discharge pressure to recover the oil in the gear casing 22 to the compressor intake part 20 via the oil recovery flow path 26.

Description

air compressor

[0001] The present invention relates to an air compressor.

[0002] Air compressors that compress air by rotating a pair of male and female rotors arranged inside a casing are known. Among these air compressors is a rotary screw compressor, in which the male rotor is rotated by a drive source such as a motor, and the male rotor rotates the female rotor via a timing gear or a helical groove.

[0003] The transmission path between the motor and male rotor can be a direct drive structure or a structure in which the drive is transmitted via gears to adjust the rotation speed. The gears consist of a bull gear connected to the drive source and a pinion gear connected to the rotor. The grooves of the male and female rotors and the casing form a compression chamber, which reduces in volume as the rotor rotates, creating a negative pressure in the casing intake section, as the pressure is increased from atmospheric pressure to the user's specified pressure.

[0004] The gear casing, which houses the gears, is adjacent to the casing intake section, and oil is stored inside the gear casing to lubricate and cool the gears. Since the stored oil is used to cool and lubricate the rotor compression chamber, a known structure is to provide a communication hole between the gear casing and the casing, and use the negative pressure in the casing intake section to recover the oil inside the gear casing to the casing intake section.

[0005] On the other hand, as mentioned above, there are cases where the negative pressure at the casing intake is not enough to recover the oil inside the gear casing. For example, in a two-stage compression rotary screw compressor with two pairs of male and female rotors arranged one above the other, the number of pinion gears connected to the male rotor increases, which increases the volume of the gear casing that houses the bull gear and pinion gear, and increases the amount of oil that accumulates inside the gear casing. As a result, the negative pressure at the casing intake alone is not enough to recover the oil in time, and there are concerns that the gears will be immersed in oil, increasing stirring resistance and causing oil leakage to the drive source.

[0006] Patent Document 1 describes a system in which lubricating oil stored in a gear case is pressure-fed to a lubricating oil heat exchanger via a lubricating oil pipe using an oil pump. The lubricating oil is cooled to a predetermined temperature or lower in the lubricating oil heat exchanger, and then sent to the compressor bearings on the discharge side and intake side via the lubricating oil pipe and an oil filter, and then collected in the gear case.

[0007] International Publication No. 2015 / 198647

[0008] In the technology of Patent Document 1, a pump is used to recover oil from inside the gear casing, but the rotor and dedicated motor that drive the pump often rotate at a constant speed, and the compressor often maintains the same rotation speed during no-load operation as during loaded operation. Meanwhile, the amount of oil supplied to the gears is determined by the discharge pressure acting on the oil, so the amount of oil that accumulates in the gear casing during no-load operation is less than during loaded operation.

[0009] Therefore, during no-load operation, the pump's suction volume is greater than the amount of oil collected by the pump, causing the oil to contain air, which increases the pump's vibration.The pump's vibrations are transmitted to the piping connected to the pump, which can damage the piping.

[0010] Therefore, an object of the present invention is to provide a technique that can recover oil accumulated in a gear casing without using a pump.

[0011] In order to solve the above problems, one representative air compressor of the present invention is an air compressor that compresses air taken in by an intake section and discharges it from a discharge port, and is equipped with a compressor that compresses the air, a drive source for driving the compressor, a gear for transmitting drive from the drive source to the compressor, a gear casing for storing the gear, an oil tank that stores oil separated from the compressed air, an oil supply passage connecting the oil tank to the gear casing, an oil recovery passage connecting from the gear casing to the intake section, and a pressurized passage connecting from the discharge port to the gear casing, and oil is supplied from the oil tank to the gear casing through the oil supply passage by the discharge pressure of the air compressor, and the gear casing is pressurized through the pressurized passage by the discharge pressure, thereby recovering the oil in the gear casing through the oil recovery passage to the intake section.

[0012] According to the present invention, it is possible to recover oil accumulated in the gear casing without using a pump. Objects, configurations, and effects other than those described above will become apparent from the following description of the embodiments.

[0013] Fig. 1 is a diagram showing an example of the configuration of an air compressor according to a first embodiment; Fig. 2 is a diagram showing an example of the internal configuration of a compressor main body according to a first embodiment; Fig. 3 is a diagram showing an example of the internal configuration of a compressor main body according to a second embodiment; Fig. 4 is a diagram showing an example of the internal configuration of a compressor main body according to a third embodiment;

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0015] FIG. 1 is a diagram illustrating an example of the configuration of an air compressor 100 (hereinafter referred to as "compressor 100") according to a first embodiment.

[0016] FIG. 2 is a diagram illustrating an example of the internal configuration of the compressor body 2 according to the first embodiment.

[0017] In this example, air is used as the compressed gas, but this is not limited to this and other gases may be used. The compressor 100 is a liquid-feed type positive displacement compressor that supplies a liquid (e.g., oil) to the compression working chamber, and in this example, an oil-feed type screw compressor is applied. However, the present invention is not limited to this and may also be a turbo type.

[0018] The compressor 100 includes a package 1, a compressor body 2 disposed inside the package 1, and an electric motor 42 that drives the compressor body 2. The compressor body 2 is connected to the electric motor 42 via a bull gear 3, a first-stage gear 4, and a second-stage gear 5. The rotation of the electric motor 42 rotates the bull gear 3, which is transmitted to the first-stage gear 4 and the second-stage gear 5, causing the first-stage rotor 6 and the second-stage rotor 7 to rotate, thereby taking in outside air from the compressor intake section 20 of the compressor body 2 and compressing it.

[0019] Outside air may be taken in from outside the air compressor 100 using the compressor intake passage 8b, which is connected to the outside of the air compressor 100, or air may be taken in from inside the air compressor 100 without providing a compressor intake passage 8b.

[0020] The rotor 9 shown in FIGS. 1 and 2 is a set of male and female rotors arranged one above the other, with the male rotor on the front side and the female rotor on the rear side from the front to the back of the figure.

[0021] Although the diagram in this embodiment shows an oil-cooled two-stage compressor, the present invention is not limited to this and may also be a single-stage liquid-cooled air compressor. In a two-stage compressor, the upper male and female rotors are called the first-stage compressor, and the lower male and female rotors are called the second-stage compressor. It is desirable for both the first-stage and second-stage compressors to compress air at the same compression ratio. For example, when compressing air from atmospheric pressure to 0.75 MPa, the first-stage compressor compresses the air from atmospheric pressure to 0.18 MPa, and the second-stage compressor compresses the air to 0.75 MPa. To suppress temperature rise during compression, oil is supplied during the compression process.

[0022] The supplied oil is transported together with compressed air to the oil separator element 10 and separated (this is called secondary separation). Before reaching the oil separator element 10, the oil undergoes swirl separation in a cylindrical region formed by the skirt 12 disposed inside the oil tank 11 and the inner periphery of the oil tank 11 (this is called primary separation).

[0023] The primary separated oil is stored in the oil tank 11. Meanwhile, the secondary separated compressed air passes through the pressure regulating check valve 43 and the aftercooler 13, and is discharged from the compressed air discharge port 17 to the outside of the air compressor 100 and supplied to the user.

[0024] A pressure acting within the oil tank 11 is lower than that at the discharge port 14b of the compressor body 2 and higher than the pressure of the compressed air supplied to the user (for example, 0.7 MPa). As a result, oil stored in the oil tank 11 is transported from the oil tank 11 to the oil cooler 15 via an oil flow path 14a connected to the oil cooler 15. The oil transported to the oil cooler 15 exchanges heat with cooling water 16 inside the oil cooler 15. The oil that has exchanged heat passes through an oil filter 18 and is then supplied to the compressor body 2 via an oil supply flow path 25 connected to the oil filter 18 to cool the compressor body 2 and gears (bull gear 3, first gear 4, second gear 5).

[0025] The compressor 100 of this embodiment is provided with a temperature control valve 19 at the outlet of the oil cooler 15. When the temperature of the oil reaching the oil cooler 15 is lower than a predetermined temperature, the temperature control valve 19 is switched to supply oil to the compressor body 2 without performing heat exchange in the oil cooler 15. In this way, the generation of drain is prevented by controlling the lower limit temperature of the compressed air.

[0026] The secondary separated oil is stored inside the oil separator element 10. The oil separator element 10 is provided with an oil recovery passage 2 (8a) that communicates with the compressor intake section 20 of the compressor main body 2. Since the pressure inside the oil separator element 10 is always higher than the pressure at the compressor intake section 20 while the compressor 100 is in operation, this differential pressure is used to recover the secondary separated oil in the compressor intake section 20.

[0027] A suction throttle valve 21 is provided in the compressor intake section 20 of the compressor body 2. The suction throttle valve 21 opens and closes depending on the pressure increase and decrease in a pipe (not shown) that communicates from the oil separator element 10 to the pressure chamber of the suction throttle valve 21, thereby switching between load operation and no-load operation.

[0028] 2 is a diagram showing the state in which the electric motor 42, gear casing 22, and compressor body 2 are connected, and only the flange portion of the electric motor 42 that connects to the gear casing 22 is shown. A bull gear 3 is provided at the end of the shaft 23 connected to the electric motor 42. The bull gear 3 is meshed with a first-stage gear 4 connected to the male rotor of the first-stage rotor 6 and a second-stage gear 5 connected to the female rotor of the second-stage rotor 7. Although not shown, male and female rotors are meshed with the first-stage rotor 6 and the second-stage rotor 7, respectively. Rotation of the electric motor 42 rotates the male rotor from the bull gear 3 via the first-stage gear 4 and second-stage gear 5. Rotation of the male rotor rotates the female rotor meshed with the male rotor. Rotation of both the male and female rotors forms a compression volume between them and the casing 24 that houses the rotors, and air is compressed as the compression volume changes due to the rotation of the male and female rotors.

[0029] On the other hand, the temperature of the bull gear 3, first gear 4, and second gear 5 rises due to friction between the gears caused by rotation, so it is necessary to apply oil to the gears to cool and lubricate them, and therefore it is desirable to constantly oil the gears. In this embodiment, the gears are arranged inside the gear casing 22 to prevent contamination from adhering to the gears and to store the oil that has been supplied to the gears.

[0030] As a means for supplying oil to the gears, oil that has passed through an oil filter 18 is supplied to the inside of the gear casing 22 via an oil supply passage 25 connected to the gear casing 22. In addition, the oil stored in the gear casing 22 is recovered into the compressor intake section 20 via an oil recovery passage 26 that is connected from the gear casing 22 to the compressor intake section 20.

[0031] However, since the connection point of the oil recovery path 26 with the compressor intake section 20 is located higher than the connection point with the gear casing 22, it is difficult to recover oil from the gear casing 22 simply by arranging the oil recovery flow path 26.

[0032] Therefore, an oil pressurizing passage 27 is provided which communicates from the discharge port 14b of the two-stage rotor 7 to the inside of the gear casing 22. The inside of the gear casing 22 is pressurized through the oil pressurizing passage 27 by the discharge pressure of the compressor 100. As a result, the oil accumulated inside the gear casing 22 is recovered to the compressor intake section 20 through the oil recovery passage 26.

[0033] At this time, in order to increase the pressure inside the gear casing 22, sealing mechanism 1 (30), sealing mechanism 2 (31), and sealing mechanism 3 (32), such as mechanical seals or oil seals, are provided in the parts of the gear casing 22 through which the shafts (shafts 23, 28, and 29) pass, thereby creating a structure in which the inside of the gear casing 22 is sealed.

[0034] Oil that passes from the oil tank 11 through the oil cooler 15 and oil filter 18 is supplied to the gear casing 22 to cool the gears inside the gear casing 22, as well as seal mechanism 1 (30), seal mechanism 2 (31), and seal mechanism 3 (32).

[0035] Here, the pressure at the compressor intake section 20 is P1, the discharge pressure of the compressor 100 is P2, the pressure inside the gear casing 22 is P3, and the pressure of the oil supplied from the oil tank 11 to the inside of the gear casing 22 is P4.

[0036] If the pressure P3 inside the gear casing 22 becomes greater than the pressure P4 of the oil supplied from the oil tank 11 to the inside of the gear casing 22, the oil flowing from the oil tank 11 to the gear casing 22 will flow backward, preventing oil from flowing into the inside of the gear casing 22. Therefore, P3 must be lower than P4.

[0037] Therefore, a pressure reducing valve 33 is provided in the oil pressure passage 27, and the set pressure of the pressure reducing valve 33 is adjusted so that P3<P4.

[0038] If the amount of compressed air used by the user decreases during full-load operation, the pressure on the compressed air discharge port 17 side increases. When the detection value of the pressure sensor 2 (39) located at the outlet of the aftercooler 13 exceeds a certain threshold, pressure is supplied from the oil tank 11 to the pressurizing chamber (not shown) of the suction throttle valve 21, and no-load operation is initiated to close the suction throttle valve 21.

[0039] During no-load operation, the pressure P1 at the compressor intake section 20 is close to vacuum pressure, so even when the inside of the gear casing 22 is not pressurized, i.e., P3 is at atmospheric pressure, the relationship P1 > P3 can be maintained, and oil can be recovered from inside the gear casing 22 through the oil recovery passage 26.

[0040] During no-load operation, no outside air is taken in, so discharge pressure P2 is expected to be atmospheric pressure or lower, but because the suction throttle valve 21 is not completely closed, discharge pressure P2 is higher than atmospheric pressure (for example, 0.15 MPa). At this time, P4 is, for example, 0.03 MPa, which is lower than the pressure during full-load operation (0.75 MPa), but remains above atmospheric pressure (0 MPa gauge pressure). Therefore, oil can be supplied from the oil tank 11 to the inside of the gear casing 22.

[0041] The flow paths connected to the inside of the gear casing 22 include an oil supply flow path 25 that supplies oil from the oil tank 11 to the inside of the gear casing 22, an oil recovery flow path 26 that recovers oil from the inside of the gear casing 22 to the compressor intake section 20, and an oil pressure flow path 27 that pressurizes the inside of the gear casing 22 with the discharge pressure of the compressor 100.

[0042] During no-load operation, the compressor intake section 20 is brought to a state close to vacuum pressure, which recovers oil from inside the gear casing 22, and the pressure at P3 falls below atmospheric pressure due to the negative pressure of the suction throttle valve 21. This maintains P4 > P3, preventing backflow of oil through the oil supply passage 25.

[0043] On the other hand, the oil pressurizing passage 27 communicates with the inside of the gear casing 22 from the discharge port 14b. When the pressure of the discharge port 14b pressurizes the inside of the gear casing 22 through the oil pressurizing passage 27, the pressure at P3 increases and P4 becomes smaller than P3, which may cause the oil flowing from the oil tank 11 to the gear casing 22 to flow back. To prevent this, a solenoid valve 35 with an opening / closing function is provided in the oil pressurizing passage 27. During no-load operation, the solenoid valve 35 is closed to prevent compressed air from the discharge port 14b from flowing into the inside of the gear casing 22.

[0044] As a method for closing the solenoid valve 35 during no-load operation, for example, in order to control the opening and closing of the suction throttle valve 21, the solenoid valve 35 is operated at the same timing as a solenoid valve (not shown) provided in an operating chamber pressurization flow path (not shown) that connects the oil tank 11 to the pressurization chamber of the suction throttle valve 21. This solenoid valve (not shown) is connected to a control board (not shown) by wiring, and the solenoid valve (not shown) is operated by a no-load determination flag determined by the control board (not shown).

[0045] According to the first embodiment, an oil pressurizing flow path 27 is provided that connects the discharge port 14b of the two-stage rotor 7 to the inside of the gear casing 22, and the inside of the gear casing 22 is pressurized through the oil pressurizing flow path 27 by the discharge pressure of the compressor 100, so that oil stored in the gear casing can be recovered without using a pump.

[0046] This eliminates vibration caused by the pump sucking in air during no-load operation, preventing damage to the piping due to pump vibration.Furthermore, power to drive the pump is no longer required, reducing power consumption.

[0047] The second embodiment takes into consideration the case where the pressure values ​​of P2 and P4 fluctuate during load operation depending on the usage state of the compressor 100.

[0048] The discharge pressure of the compressor 100 varies depending on the user's usage of compressed air. The discharge pressure of the compressor 100 is determined by the amount of air flowing through a discharge flow path (not shown) at the outlet of the aftercooler 13 and the flow path diameter of the discharge flow path (not shown) at the outlet of the aftercooler 13. When the user uses air, the diameter of the discharge flow path (not shown) at the outlet of the aftercooler 13 increases, and the discharge pressure decreases. On the other hand, when the user stops using air and the diameter of the discharge flow path (not shown) at the outlet of the aftercooler 13 decreases or the discharge flow path (not shown) at the outlet of the aftercooler 13 is blocked, the discharge pressure increases.

[0049] The electric motor 42 used in the compressor 100 may be an induction motor that operates at a constant rotational speed or a variable-speed motor that uses inverter control. In a variable-speed electric motor, the rotational speed of the rotor 9 is changed according to the amount of compressed air used by the user (load factor). In a screw compressor, the higher the discharge pressure and the higher the rotational speed of the rotor 9, the higher the output of the electric motor 42.

[0050] Therefore, in a variable speed screw compressor, when the pressure exceeds a certain rated point (rotor rotation speed, discharge pressure) as a reference point, the rotation speed of the rotor 9 is reduced to suppress an increase in the output of the electric motor 42. Also, in a variable speed screw compressor, when the pressure falls below the rated point, the rotation speed of the rotor 9 is increased within the range of the allowable output of the electric motor 42.

[0051] As described above, when operating a constant-speed or variable-speed compressor, if the pressure at P2 drops below the rated point, it is conceivable that the pressure at P4 will also drop at the same time. For example, if the rated point of P2 is 0.75 MPa, the pressure at P4 is 0.3 MPa, and the set pressure of the pressure reducing valve 33 is 0.2 MPa, i.e., P3 is 0.2 MPa, when P2 drops to 0.45 MPa, the pressure at P4 will fall below the set pressure of the pressure reducing valve 33, 0.2 MPa, and the relationship P4 < P3 will be established. As a result, there is a possibility that oil will flow back from the oil filter 18 to the gear casing 22, preventing oil from being supplied.

[0052] To eliminate such concerns, the set pressure of the pressure reducing valve 33 is made variable according to the pressure of P4.

[0053] FIG. 3 is a diagram showing an example of the internal configuration of the compressor body 2 according to the second embodiment.

[0054] The compressor body 2 of FIG. 3 is configured by adding a control board 37, a pressure sensor 3 (38), and a pressure sensor 2 (39) to the configuration of FIG.

[0055] The pressure sensor 2 (39) is the same as the pressure sensor 2 (39) in FIG. 1, and is disposed at the outlet of the aftercooler 13 to detect the discharge pressure of the compressor 100.

[0056] The pressure sensor 3 (38) is disposed at the discharge port 14b and detects the discharge pressure P2 of the compressor 100.

[0057] The set pressure of the pressure reducing valve 33 is changed according to the value of the pressure sensor 2 (39). Alternatively, the set pressure of the pressure reducing valve 33 may be changed according to the value of the pressure sensor 3 (38) provided at the discharge port 14b.

[0058] Pressure sensor 2 (39) or pressure sensor 3 (38) is connected to a control board 37. The control board 37 determines the set pressure of the pressure reducing valve 33 based on the value of pressure sensor 2 (39) or pressure sensor 3 (38) using a table (not shown) prepared in advance.

[0059] When the set pressure of the pressure reducing valve 33 is determined by the control board 37, the value is converted into, for example, a voltage, and an instruction to change the set pressure is sent to the pressure reducing valve 33. The pressure reducing valve 33 may be, for example, a mechanism that adjusts the flow path diameter of the oil pressure flow path 27 using a solenoid (not shown), or a mechanism (not shown) that automatically converts the adjustment handle of a diaphragm-type pressure reducing valve.

[0060] The set pressure of the pressure reducing valve 33 is changed depending on the pressure of P4. For example, if P2 is reduced from 0.75 MPa to 0.45 MPa and P4 is reduced to 0.15 MPa, the set pressure of the pressure reducing valve 33 is set to a value lower than P4. This maintains the relationship P4 > P3, making it possible to prevent oil from flowing back from the oil filter 18 to the gear casing 22.

[0061] The value of P4 may be read, for example, by pressure sensor 1 (40) arranged in the oil supply flow path 25. Alternatively, without using pressure sensor 1 (40), the relationship between the pressures of P2 and P4 may be known in advance, and the pressure of P4 may be predicted from the actual pressure value of P2 read by pressure sensor 3 (38).

[0062] According to the second embodiment, the control board 37 determines the set pressure of the pressure reducing valve 33 based on the value of the pressure sensor 2 (39) or the pressure sensor 3 (38), thereby maintaining the relationship P4 > P3 and preventing the flow of oil from the oil filter 18 to the gear casing 22 from backflowing.

[0063] FIG. 4 is a diagram showing an example of the internal configuration of the compressor body 2 according to the third embodiment.

[0064] The compressor body 2 of FIG. 4 is configured by adding a pressure reducing valve 2 (41) to the oil supply passage 25 from the oil filter 18 to the gear casing 22 to the configuration of FIG.

[0065] As with the second embodiment, this is an alternative embodiment to the second embodiment, and is structured to maintain the relationship P4 > P3 even if P2 fluctuates, assuming that the discharge pressure P2 of the compressor 100 fluctuates depending on the user's usage of compressed air.

[0066] In this embodiment, the set pressures of the pressure reducing valve 33 and the pressure reducing valve 2 (41) are set so that the set pressure of the pressure reducing valve 33 is lower than the set pressure of the pressure reducing valve 2 (41). This makes it possible to maintain the relationship P4 > P3 even if the pressure of P2 fluctuates, without adding a function to automatically change the set pressure of the pressure reducing valve 33 in accordance with the value of the pressure sensor 2 (39) or the pressure sensor 3 (38) as in the second embodiment.

[0067] According to the third embodiment, the set pressures of the pressure reducing valve 33 and the pressure reducing valve 2 (41) are set so that the set pressure of the pressure reducing valve 33 is lower than the set pressure of the pressure reducing valve 2 (41). Therefore, even if the pressure of P2 fluctuates, it is possible to maintain the relationship P4 > P3.

[0068] 1...package, 2...compressor body, 3...bull gear, 4...first-stage gear, 5...second-stage gear, 6...first-stage rotor, 7...second-stage rotor, 8a...oil recovery passage 2, 8b...compressor intake passage, 9...rotor, 10...oil separator element, 11...oil tank, 12...skirt, 13...aftercooler, 14a...oil passage, 15...oil cooler, 16...cooling water, 17...compressed air discharge port, 18...oil filter, 19...temperature control valve, 20...compressor intake Gas section, 21... suction throttle valve, 22... gear casing, 23... shaft, 24... casing, 25... oil supply passage, 26... oil recovery passage, 27... oil pressurization passage, 28... shaft, 29... shaft, 30... sealing mechanism 1, 31... sealing mechanism 2, 32... sealing mechanism 3, 33... pressure reducing valve, 35... solenoid valve, 37... control board, 38... pressure sensor 3, 39... pressure sensor 2, 40... pressure sensor 1, 41... pressure reducing valve 2, 42... electric motor, 43... pressure regulating check valve

Claims

1. An air compressor which compresses air taken in by an intake section and discharges it from a discharge port, comprising: a compressor which compresses the air; a drive source for driving the compressor; a gear for transmitting drive from the drive source to the compressor; a gear casing for housing the gear; an oil tank for storing oil separated from the compressed air; an oil supply passage communicating from the oil tank to the gear casing; an oil recovery passage communicating from the gear casing to the intake section; and a pressurized passage communicating from the discharge port to the gear casing, wherein the oil is supplied from the oil tank to the gear casing through the oil supply passage by the discharge pressure of the air compressor, and the gear casing is pressurized through the pressurized passage by the discharge pressure, thereby recovering the oil in the gear casing through the oil recovery passage to the intake section.

2. An air compressor as claimed in claim 1, wherein the gear comprises a drive shaft portion connected to the drive source and a compressor shaft portion connected to the compressor, and the gear casing comprises a seal mechanism for sealing the portions through which the drive shaft portion and the compressor shaft portion pass.

3. An air compressor according to claim 2, wherein the pressurized passage is provided with a pressure reducing valve for adjusting the pressure of the gear casing so that the pressure of the gear casing is lower than the pressure of the oil supply passage.

4. An air compressor as claimed in claim 3, wherein the pressure reducing valve adjusts the pressure of the gear casing so that P1 < P3 < P4 < P2, where P1 is the pressure of the intake section, P2 is the discharge pressure, P3 is the pressure of the gear casing, and P4 is the pressure of the oil supply passage.

5. An air compressor according to claim 3, wherein the intake section is provided with an intake valve which closes the intake section when the air compressor is operating at no load, and the pressurized flow passage is provided with a solenoid valve which closes the pressurized flow passage when the air compressor is operating at no load.

6. An air compressor according to claim 3, further comprising: a pressure sensor for detecting the discharge pressure; and a control unit for controlling the pressure reducing valve in accordance with the discharge pressure.

7. An air compressor according to claim 3, wherein the oil supply passage is provided with a second pressure reducing valve which adjusts the pressure in the oil supply passage, and the pressure reducing valve and the second pressure reducing valve adjust the pressure in the gear casing and the pressure in the oil supply passage so that the pressure in the gear casing is lower than the pressure in the oil supply passage.