Patent Publication Number: US-2015078938-A1

Title: Air compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2013-193538 filed on Sep. 18, 2013, the entire subject-matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an air compressor suitable for generation of compressed air required to drive a pneumatic tool such as a nailing machine. 
     BACKGROUND 
     In a building site and the like, a portable pneumatic tool to drive a nail and a screw into a wooden material by a pressure of compressed air has been widely used. In general, an air compressor for driving the pneumatic tool and the like is configured to convert a rotational motion of a rotational shaft of a driving part such as a motor into a reciprocal motion of a piston in a cylinder through a crankshaft of a compression part and to compress air sucked from a suction valve of the cylinder by the reciprocal motion of the piston. The compressed air compressed in the cylinder is discharged from an exhaust valve of the cylinder to an air tank through a pipe and is stored in the air tank. When compressing the gas to a high pressure, a multistage reciprocating compressor of increasing a pressure in a stepwise manner has been generally used. The high-pressure compressed air stored in the air tank is adjusted to an appropriate pressure by a decompression valve attached to the air tank and is then supplied to the pneumatic tool and the like through an air hose. The air compressor is disclosed in JP-A-2013-40586. 
     SUMMARY 
     The air compressor has a plurality of heat generation parts in which the temperature thereof is high in accordance with using the motor, the compression part (particularly, the cylinder), the control circuit and the like. Thus, in many cases, the heat generation parts should be thus arranged at spaced positions. Therefore, it is necessary to securely guide the cooling air generated by the cooling fan attached to the rotational shaft of the motor to the respective heat generation parts. However, the heat generation part may be positioned at a place that is difficult to be cooled by the general cooling fan, due to a layout, which is a problem to be solved with respect to the cooling efficiency. 
     It is therefore an object of the present invention to provide an air compressor capable of improving cooling efficiency by a cooling fan. 
     According to one illustrative aspect of the present invention, there is provided an air compressor comprising: an air tank configured to store therein compressed air; a compression part configured to compress air sucked from an outside and to supply the air to the air tank; a motor configured to drive the compression part; a cooling fan provided at one end-side of a rotational shaft of the motor, and a cover configured to cover at least the compression part, the motor and the cooling fan, wherein the cooling fan comprises a ring part provided at an outer peripheral part thereof and having a substantially cylindrical shape, and wherein the cooling fan is configured to change a direction of an air stream by the ring part. 
     Incidentally, any combination of the above-described elements, and a method, a system and the like converted from the expressions of the present invention are also effective as the aspects of the present invention. 
     According to the illustrative aspects of the present invention, it is possible to provide an air compressor capable of improving cooling efficiency by a cooling fan. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an air compressor  1  according to a first illustrative embodiment of the present invention; 
         FIG. 2  is a plan view of the air compressor  1  in which a cover  26  is a cross-sectional surface; 
         FIG. 3  is a plan sectional view of the air compressor  1 ; 
         FIG. 4  is a IV-IV sectional view of  FIG. 2 ; 
         FIG. 5  is a front sectional view of the air compressor  1 ; 
         FIG. 6  is a first perspective view from below of the cover  26  of the air compressor  1 ; 
         FIG. 7  is a second perspective view from below of the cover  26  of the air compressor  1 ; 
         FIG. 8  is an VIII-VIII sectional view of  FIG. 4  relating to the cover  26  of the air compressor  1 ; 
         FIG. 9  is a IX-IX sectional view of  FIG. 4  relating to the cover  26  of the air compressor  1 ; 
         FIG. 10  is a perspective view of a cooling fan  8   b  of the air compressor  1 ; 
         FIG. 11  is a perspective view of a cooling fan  8   a  of the air compressor  1 ; 
         FIG. 12  is a perspective view of a seventh baffle plate  306  of the air compressor  1 ; 
         FIG. 13  is a plan view of an air compressor according to a second illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface; 
         FIG. 14  is a plan view of an air compressor according to a third illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface; and 
         FIG. 15  is a plan view of an air compressor according to a fourth illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred illustrative embodiments of the present invention will be described with reference to the drawings. Incidentally, the same or equivalent elements, members and the like shown in the respective drawings are denoted with the same reference numerals and the overlapping descriptions are appropriately omitted. Also, the illustrative embodiments are just exemplary, not to limit the present invention, and all features and combinations thereof described in the illustrative embodiments cannot be considered as the essentials of the present invention. 
     First Illustrative Embodiment 
       FIG. 1  is a perspective view of an air compressor  1  according to a first illustrative embodiment of the present invention.  FIG. 2  is a plan view of the air compressor  1  in which a cover  26  is a cross-sectional surface. Incidentally, in  FIG. 2 , gripping parts  31   a ,  31   b  are not shown.  FIG. 3  is a plan sectional view of the air compressor  1 .  FIG. 4  is a IV-IV sectional view of  FIG. 2 .  FIG. 5  is a front sectional view of the air compressor  1 .  FIG. 6  is a first perspective view from below of the cover  26  of the air compressor  1 .  FIG. 7  is a second perspective view from below of the cover  26  of the air compressor  1 .  FIG. 8  is an VIII-VIII sectional view of  FIG. 4  relating to the cover  26  of the air compressor  1 .  FIG. 9  is a IX-IX sectional view of  FIG. 4 .  FIG. 10  is a perspective view of a cooling fan  8   b  of the air compressor  1 .  FIG. 11  is a perspective view of a cooling fan  8   a  of the air compressor  1 .  FIG. 12  is a perspective view of a seventh baffle plate  306  of the air compressor  1 . 
     The air compressor  1  includes: a pair of air tanks  2   a ,  2   b  arranged to be parallel at a predetermined interval therebetween and storing therein compressed air; a compression part  3  for compressing air sucked from an outside and supplying the compressed air to the air tanks  2   a ,  2   b ; and a motor  4 , which is connected to the compression part  3 , for driving the compression part  3 . The motor  4  and the compression part  3  are arranged above the pair of air tanks  2   a ,  2   b  such that an axis direction of the motor  4  is substantially perpendicular to a longitudinal direction of the air tanks  2   a ,  2   b.    
     As shown in  FIG. 3 , a rotational shaft  5  of the motor  4  penetrates the compression part  3 , and a cooling fan  8   a  (a first cooling fan) is provided at a motor-side end portion of the rotational shaft  5  of the motor  4  and a cooling fan  8   b  (a second cooling fan) is provided at a non-motor-side end portion of the rotational shaft  5  of the motor  4 . The cooling fans  8   a ,  8   b  are rotated integrally with the motor  4 . 
     As shown in  FIG. 2 , decompression valves  9   a ,  9   b , pressure meters  10   a ,  10   b  for displaying a pressure of the decompressed compressed air and couplers  11   a ,  11   b  functioning as outlets of the compressed air are provided in the vicinity of the compression part  3  in the longitudinal direction of the pair of air tanks  2   a ,  2   b . An operator connects the couplers  11   a ,  11   b  and a pneumatic tool (not shown) such as a nailing machine by an air hose for high pressure (not shown) and operates the pneumatic tool by using the compressed air to appropriately perform an operation. 
     As shown in  FIG. 2 , the air tanks  2   a ,  2   b  are provided with a safety valve  12  and a drain discharge device  13 . The safety valve  12  is configured to discharge a part of the compressed air to the outside when a pressure is abnormally increased. The drain discharge device  13  has an operation part  14 . When the operation part  14  is operated, the compressed air and moisture in the air tanks  2   a ,  2   b  can be appropriately discharged to the outside. 
     As shown in  FIG. 3 , the compression part  3  is provided therein with a crank mechanism. A rotational motion of the rotational shaft  5  of the motor  4  is converted into reciprocal motions of a first piston  34   a  and a second piston  34   b  through a first connecting rod  33   a  and a second connecting rod  33   b , respectively. The first piston  34   a  is accommodated in a first cylinder  15   a  and a first cylinder head  16   a . The second piston  34   b  is accommodated in a second cylinder  15   b  and a second cylinder head  16   b . The first cylinder  15   a  and the second cylinder  15   b  are horizontally opposed to each other with the rotational shaft  5  of the motor  4  being interposed therebetween. The first cylinder  15   a  and the second cylinder  15   b  are arranged to be substantially parallel with the air tanks  2   a ,  2   b , respectively. When the exterior air is sucked in the compression part  3 , the air is first compressed by the second cylinder  15   b  (a low pressure-side cylinder) and the air compressed by the second cylinder  15   b  is supplied to the first cylinder  15   a  (a high pressure-side cylinder) through a piping  19   a  (refer to  FIG. 2 ). The air further compressed by the first cylinder  15   a  is supplied to the air tank  2   a  through a piping  19   b . The air tanks  2   a ,  2   b  are connected to each other by a connection pipe  20 , so that the pressures of the air tanks  2   a ,  2   b  are uniform. 
     As shown in  FIG. 3 , a control circuit  21  (a control circuit board) for driving the motor  4  is accommodated in a case  22 . The control circuit  21  is arranged to face the cooling fan  8   b  at the non-motor-side of the compression part  3 . The control circuit  21  is fixed to the air tank  2   a . The motor  4 , which is a DC brushless motor, has a stator coil  23 , a rotor  24  arranged in the stator coil  23  and a Hall element board  25  (refer to  FIG. 2 ) for detecting a rotating position of the rotor  24 . The motor  4  is inverter-controlled by the control circuit  21 . The control circuit  21  includes a heat generation component  204  (refer to  FIG. 3 ) such as a semiconductor switching element for inverter control. A surface of the case  22  on which the heat generation component  204  is attached is configured as a heat generation surface  203 , from which a heat is to be radiated or which is to be cooled. 
     As shown in  FIG. 1 , a cover  26  for covering the components of the air compressor such as the compression part  3 , the motor  4 , the control circuit  21  and the like is arranged above the air tanks  2   a ,  2   b  and is fixed to the air tanks  2   a ,  2   b . Both end portions of the air tanks  2   a ,  2   b  in the longitudinal direction are provided with gripping parts  31   a ,  31   b  for carrying the air compressor  1 . The cover  26  is provided with an operation panel  26  having a power supply switch (not shown) for operating the air compressor  1  and the like. The cover  26  is provided with ventilating windows  29   a ,  29   b  (refer to  FIGS. 6 and 7 ) on wall surfaces facing the cooling fans  8   a ,  8   b . A cover  30  (refer to  FIG. 4 ) for preventing foreign materials from being introduced is additionally attached between the air tanks  2   a ,  2   b . The air tanks  2   a ,  2   b  are provided with leg parts  32  for preventing the air tanks  2   a ,  2   b  from directly contacting a ground to protect the air tanks  2   a ,  2   b.    
     During an operation of the air compressor  1 , the motor  4  is alternately applied with a compression load upon compression of the air by the reciprocal motions of the first piston  34   a  and the second piston  34   b . For this reason, load currents are generated in the stator coil  23  and the control circuit  21 , so that temperatures of the stator coil  23  and the control circuit  21  are increased by Joule heats accompanied by the load currents. Also, temperatures of the first cylinder  15   a , the first cylinder head  16   a , the second cylinder  15   b  and the second cylinder head  16   b  are increased by compression heat of the compressed air. Temperatures of the pipings  19   a ,  19   b  and the air tanks  2   a ,  2   b  are also increased because the compressed air whose temperature is increased by the compression heat flows therein. For this reason, it is necessary to suppress the increase in the temperature of the respective parts by the cooling, with a focus on the heat generation parts such as the first cylinder  15   a , the first cylinder head  16   a , the second cylinder  15   b , the second cylinder head  16   b , the stator coil  23 , the control circuit  21 , the case  22  and the like. Hereinafter, configurations relating to the cooling will be described. 
     (Cooling by Cooling Fan  8   a ) 
     In a typical axial flow fan, a generated air stream has a tendency to flow in an outer peripheral direction due to a high centrifugal force thereof. Therefore, it is difficult to promote the air stream in the adjacent motor, and thus the heat is likely to be accumulated in the motor. Here, a configuration for improving cooling performance of the motor  4  will be described. 
     As shown in  FIG. 11 , the cooling fan  8   a  has outer blades  104  and inner blades  105 . The outer blades  104  and the inner blades  105  are connected to each other by a cylindrical partition part (cylindrical part)  102 . The partition part  102  extends in a direction along the rotational shaft of the motor  4 . An attaching part  103  is formed at a further inner side of the inner blades  105 . The cooling fan  8   a  can be attached to the rotational shaft  5  of the motor  4  by the attaching part  103 . The inner blades  105  are formed integrally with the partition part  102  and the attaching part  103 . The inner blades  105  are formed into a curved plate shape extending substantially parallel with the rotational shaft  5  of the motor  4 , a so-called centrifugal type. The cooling fan  8   a  has a through-hole, which is formed by the adjacent inner blades  105 , partition part  102  and attaching part  103 . The through-hole is formed such that an opening area of a non-motor-side opening  107  is smaller than an opening area of a motor-side opening  106  at an opposite side (refer to  FIG. 3 ). The outer blades  104  are formed into a curved shape extending obliquely relative to the direction along the rotational shaft  5  of the motor  4 , a so-called axial flow type. As shown in  FIG. 3 , an outer diameter D2 of the outer blades  104  is set to be larger than an outer diameter D1 of the motor  4 . The cover  26  is formed with the ventilating window  29   a  facing the cooling fan  8   a . A central portion of the ventilating window  29   a  is formed with a shield plate  101  (refer to  FIGS. 7 and 8 ) facing the inner blades  105  and serving as a shield part. 
     When the motor  4  is operated, the cooling fan  8   a  is rotated to generate an air stream. That is, the exterior air is sucked from an outside through the ventilating window  29   a  by the outer blades  104 , and an air stream CA 1  flowing towards the first cylinder  15   a , the first cylinder head  16   a , the second cylinder  15   b  and the second cylinder head  16   b , as shown in  FIG. 3 , is generated. Also, a part of the air stream generated by the outer blades  104  flows towards the stator coil  23 , like an air stream CA 2 . At this time, by a negative pressure P1 additionally generated by the inner blades  105 , the air in the vicinity of the stator coil  23  is sucked to the inner blades  105 , like an air stream CA 3 . The air sucked to the inner blades  105  is enabled to flow out from the motor-side opening  106  towards the non-motor-side opening  107  by a negative pressure P2 generated by the outer blades  104 , and is then sucked to the outer blades  104 , like an air stream CA 4 . Thereafter, the air sucked to the outer blades  104  like the air stream CA 4  is enabled to flow out together with the air stream CA 1 . As shown in  FIG. 4 , the cooling wind having completed the cooling is discharged to the outside of the cover  30  through between the air tanks  2   a ,  2   b.    
     According thereto, it is possible to generate the air streams CA 3 , CA 4  by the inner blades  105  (the air stream generated by the inner blades  105  is restrained by a cylindrical inner surface of the partition part  102  and is securely enabled to flow in a direction along a rotational shaft of the cooling fan  8   a ). Therefore, a synergetic effect with the air streams CA 1 , CA 2  of the outer blades  104  can be exhibited, and it is possible to generate a high flow rate in the vicinity of the stator coil  23 , in which the air stream is stagnant in the related art. According thereto, it is possible to effectively suppress the temperature increase of the stator coil  23 . That is, it is possible to realize the auxiliary effect or synergetic effect for the air stream of the outer blades  104 , which is likely to flow in the outer peripheral direction due to the high centrifugal force, by the inner blades  105 , so that it is possible to improve the cooling performance of the motor  4  by the cooling fan  8   a.    
     Further, the rotational shaft  5  of the motor  4  is alternately applied with the compression load upon the compression of the air in the first cylinder  15   a  and the second cylinder  15   b , so that a rotational variation is generated. Thus, a distortion vibration due to the rotational variation is generated for the cooling fan  8   a . However, the attaching part  103  and the partition part  102  are strongly connected using the plurality of inner blades  105 . According thereto, it is possible to disperse and reduce the stress resulting from the distortion vibration, thereby increasing the strength and reliability of the cooling fan  8   a . Further, since the inner blades  105  also serve as a connection part (a frame) connecting the partition part  102  and the attaching part  103  each other, it is not necessary to separately form a connection part, which does not contribute to the air stream, so that the structure is efficient. Incidentally, by increasing the number of the inner blades  105 , it is possible to improve the performance of the inner blades  105  as a fan and to improve the connection strength of the partition part  102  and the attaching part  103 . 
     Further, the cooling fan  8   a  has a difference in the opening area between the non-motor-side opening  107  and the motor-side opening  106 . According thereto, it is easy to control the flowing direction of the air stream generated by the inner blades  105 . That is, it is possible to appropriately control the flowing direction of the air stream generated by the inner blades  105  by appropriately adjusting an opening area ratio between the non-motor-side opening  107  and the motor-side opening  106 . 
     Further, the outer blades  104  suck the air from the non-motor-side (the outside of the cover  26 ) and enable the air stream to flow out towards the motor  4  and the compression part  3 . Therefore, it is possible to suck a large amount of the exterior air having a temperature lower than the temperature in the cover  26  into the cover  26  and to extensively cool the motor  4 , the compression part  3  and the like. 
     Further, since the shield plate  101  is provided, the negative pressure P2 between the cooling fan  8   a  and the shield plate  101  by the action of the outer blades  104  is enhanced. According thereto, it is possible to further promote the air streams CA 3 , CA 4 , thereby considerably improving the cooling efficiency. That is, it is possible to enable the air in the vicinity of the motor  4 , which is sucked to the central part of the cooling fan  8   a  by the negative pressure (suction) by the inner blades  105 , to smoothly flow towards the non-motor-side of the inner blades  105  and to further improve the cooling effect in the vicinity of the motor by the synergetic effect of the negative pressures of the outer blades  104  and the inner blades  105 . 
     Further, the outer diameter D2 of the outer blades  104  is set to be larger than the outer diameter D1 of the motor  4 . According thereto, a part of the air stream generated by the outer blades  104  is directly supplied to the compression part  3  without via the motor  4 . As a result, it is possible to improve the cooling efficiency of the compression part  3 . 
     Further, the air volume by the outer blades  104  is set to be larger than the air volume enabled to flow through the inner side of the partition part  102  by the inner blades  105 . According thereto, it is easy to enable a part of the air stream, which is generated by the inner blades  105  mainly generating the air stream in the vicinity of the motor  4 , to flow together with the air stream generated by the outer blades  104 . Therefore, it is possible to securely supply the air stream after the cooling of the motor to the compression part  3  and to finally discharge the air stream to the outside of the cover. Thus, it is possible to exclude the bad influence on the cooling performance, which is caused as the heat is accumulated in the vicinity of the motor  4 , thereby improving the cooling performance. 
     Incidentally, it may be possible to appropriately change the area ratio between the motor-side opening  106  and the non-motor-side opening  107  of the cooling fan  8   a  and to appropriately change the shapes of the inner blade  105  and the outer blade  104 . 
     (Cooling by Cooling Fan  8   b ) 
     In general, the air compressor has a plurality of heat generation parts in which the temperature thereof is high in accordance with using the motor, the compression part (particularly, the cylinder), the control circuit and the like. Thus, in many cases, the heat generation parts should be arranged at spaced positions. Therefore, it is necessary to securely guide the cooling air generated by the cooling fan attached to the rotational shaft of the motor to the respective heat generation parts. However, the heat generation part may be positioned at a place that is difficult to be cooled by the general cooling fan, due to a layout, which is a problem to be solved with respect to the cooling efficiency. Regarding this problem, a configuration for improving the cooling efficiency will be described. 
     As shown in  FIGS. 3 and 4 , the case  22  and the control circuit  21  are arranged to face the cooling fan  8   b  that is attached to the end portion of the non-motor-side of the rotational shaft  5  of the motor  4 . In the case  22 , the heat generation surface  203  of the accommodated control circuit  21 , on which the heat generation component  204  is mounted, faces the cooling fan  8   b . The heat generation component  204  faces an outer peripheral part (a ring part  201  that will be described later) of the cooling fan  8   b  with the heat generation surface  203  being interposed therebetween. The heat generation component  204  is an IGBT (insulated gate bipolar transistor), a diode bridge, an IPM (intelligent power module) and the like. 
     As shown in  FIG. 10 , the cooling fan  8   b  has a back plate  202  serving as a plate shape part formed on a flat plate, a normal line of which is set to a direction along the rotational shaft  5  of the motor  4 . Blades  200  extending substantially parallel with the rotational shaft  5  of the motor  4  from an inner side of the back plate  202  towards an outer periphery thereof are centrifugally formed on the back plate  202 . An attaching part  205  is formed at an inner side of the blades  200 . The cooling fan  8   b  can be attached to the rotational shaft  5  of the motor  4  by the attaching part  205 . The outer peripheral end of the cooling fan  8   b  is formed with the ring part  201 . The ring part  201  has a cylindrical shape substantially parallel with the rotational shaft  5  of the motor  4 . The ring part  201  extends from the back plate  202  towards the heat generation surface  203 . The ring part  201  and the attaching part  205  are connected and integrated over an entire circumference by the back plate  202 . The back plate  202  entirely closes the motor-side between the ring part  201  and the attaching part  205 . 
     Further, as shown in  FIG. 2 , a fifth baffle plate  304  (a baffle wall part), a sixth baffle plate  305  (a baffle wall part) and a seventh baffle plate  306  (a baffle wall part) are provided in the vicinity of the cooling fan  8   b . The fifth baffle plate  304  and the sixth baffle plate  305  are formed as ribs hanging from the cover  26 . The seventh baffle plate  306  shown as a unitary member in  FIG. 12  is attached to the case  22  by a screw material. The seventh baffle plate  306  is formed from the case  22  towards the non-motor-side of the second cylinder  15   b , as shown in  FIG. 2 . The fifth baffle plate  304  hangs from the cover  26  towards the case  22 . The fifth baffle plate  304  is formed with an opening  207  (a notched portion) having a diameter that is smaller than a diameter D3 (refer to  FIG. 4 ) of the cooling fan  8   b  (refer to  FIGS. 4 and 6  and the like). The sixth baffle plate  305  is formed integrally with the fifth baffle plate  304  and is smoothly formed from the fifth baffle plate  304  towards the non-motor-side of the first cylinder  15   a . A wall surface of the cover  26  facing the cooling fan  8   b  is formed with the ventilating window  29   b.    
     As shown in  FIG. 3 , when the motor  4  is operated, the exterior air is sucked from the outside through the ventilating window  29   b  by the cooling fan  8   b , like an air stream CA 10 , and is guided to the central part of the cooling fan  8   b  by the opening  207 . The air stream CA 10  flows towards the outer periphery of the cooling fan  8   b  along the back plate  202 , like an air stream CA 11 . Then, as shown in  FIG. 3 , the air stream CA 11  is deflected (a flowing direction thereof is changed) by the ring part  201  such that the flowing direction of the air stream CA 11  closes to parallel with the suction direction of the air stream CA 10  by the cooling fan  8   b , flows towards and collides with the heat generation surface  203  (a first heat generation part) and further flows radially from the cooling fan  8   b  along the heat generation surface  203 . A part of the air stream CA 11  is guided towards the non-motor-sides (a second heat generation part) of the first cylinder  15   a  and the second cylinder  15   b  by the fifth baffle plate  304 , the sixth baffle plate  305  and the seventh baffle plate  306 , like air streams CA 12 , CA 13 . The cooling air having completed the cooling is discharged to the outside of the cover  30  through between the air tanks  2   a ,  2   b , as shown in  FIG. 4 . 
     In this way, the cooling air is deflected by the ring part  201  and is thus enabled to securely collide with the heat generation surface  203 . According thereto, it is possible to remarkably improve the cooling efficiency of the heat generation surface  203 . Further, by providing the back plate  202 , it is possible to enable the more cooling air to securely flow towards the non-motor-side (the heat generation surface  203 -side), so that it is possible to further improve the cooling efficiency of the heat generation surface  203 . Further, since the air flow can be enabled to flow out so as to be ejected towards the heat generation surface  203  that is arranged in front of the cooling fan  8   b , it is possible to further improve the cooling efficiency. That is, since the cooling fan  8   b  sucks centrally the air and enables the air to flow out in the outer peripheral direction by using the blades  200  and the back plate  202  and applies an axial component (a component facing the non-motor-side) in the outflow direction of the air stream by the ring part  201 , it is also possible to securely supply the cooling air to the heat generation surface  203 , which exists on a plane different from the cooling fan  8   b . The cooling air supplied to the heat generation surface  203  is mainly the exterior air introduced through the ventilating window  29   b  and not used yet for another cooling. Therefore, the cooling efficiency of the heat generation surface  203  is favorable. 
     Further, it is possible to utilize the cooling air having cooled the heat generation surface  203  and the cooling air having not reached the heat generation surface  203  for cooling the first cylinder  15   a  and the second cylinder  15   b  by the fifth baffle plate  304 , the sixth baffle plate  305  and the seventh baffle plate  306 . The effect of this configuration is very advantageous, because the non-motor-sides of the first cylinder  15   a  and the second cylinder  15   b  are difficult to be cooled by the air stream CA 1  generated by the cooling fan  8   a  and the temperatures thereof are likely to increase. 
     Further, the rotational shaft  5  of the motor  4  is alternately applied with the compression load upon the compression of the air in the first cylinder  15   a  and the second cylinder  15   b , so that a rotational variation is generated. Thus, a distortion vibration due to the rotational variation is generated for the cooling fan  8   b . However, since the ring part  201  is connected by the back plate  202 , a section modulus of a cross-section, a normal line of which extends in the direction along the rotational shaft  5  of the motor  4 , is remarkably increased, so that it is possible to obtain the sufficient strength against the centrifugal load and the distortion vibration load. Further, since the back plate  202  is provided with the blades  200 , it is also possible to increase the strength of the blades  200 . Incidentally, it is preferable to connect the ring part  201  and the attaching part  205  such that the back plate  202  entirely closes the motor-side between the ring part  201  and the attaching part  205 , from a standpoint of increasing the cooling efficiency of the first heat generation part. Alternatively, a structure where the ring part  201  and the attaching part  205  are connected such that the back plate  202  partially closes the motor-side may also be possible. 
     Further, the ring part  201  has not only the function of changing the direction of the cooling air but a flywheel ring function of increasing the inertia force of the cooling fan  8   b  to thus relieve the rotational variation, thereby reducing the load to the motor  4  due to the rotational variation, which is structurally efficient. From another standpoint, the ring part  201  functioning as the flywheel ring is provided with the function of changing the direction of the cooling air, so that the ring part  201  can be used to improve the cooling efficiency, which is also structurally efficient. 
     Incidentally, the blades  200  are not limited to the centrifugal type and may be appropriately changed. The ring part  201  is not necessarily a complete cylindrical shape and may be formed to deflect the cooling air towards a heat generation part in addition to the control circuit  21 , the case  22  and the heat generation surface  203 . The ring part  201  is not necessarily parallel with the rotational shaft  5  of the motor  4  and may be inclined relative to the back plate  202  to guide the cooling air towards the first heat generation part. The cooling fan  8   b  may be attached such that the back plate  202  faces towards the ventilating window  29   b  of the cover  26 . 
     (Cooling of Cylinder Head) 
     During the compression process by the piston in the compression part  3 , the compressed air, which has been heated by the compression, is supplied into the cylinder head at a high flow rate and is then stored in the air tank through the piping. Therefore, since the high-temperature air flows at the high flow rate in the cylinder head, the heat transfer from the high-temperature air to the cylinder head is made to a remarkable extent, and the cylinder head of the compression part  3  becomes high temperatures. Thus, for the efficient cooling, it is required to intensively cool the cylinder head becoming the highest temperature. Hereinafter, a configuration for efficiently cooling the cylinder head will be described. 
     As shown in  FIG. 2 , a first baffle plate  300  (a baffle wall part) is arranged in a substantially linear shape in the cover  26  such that a virtual extension line inclined relative to the rotational shaft  5  of the motor  4  passes above the first cylinder head  16   a , when seen from above, towards the first cylinder  15   a  that is positioned at a downstream side in the rotating direction from the cooling fan  8   a . Here, the virtual extension line of the first baffle plate  300  coincides with a linear approximation straight line of the first baffle plate  300 . An air path or virtual extension line of the air path following a wall surface of the first baffle plate  300  facing towards the cooling fan  8   a  intersects with the first cylinder head  16   a , when seen from above. 
     A second baffle plate  301  (a baffle wall part) is arranged above the first cylinder head  16   a  such that the second baffle plate  301  hangs from the cover  26  towards the first cylinder head  16   a  (refer to  FIG. 5 ). The second baffle plate  301  is provided to intersect with the air path or virtual extension line of the air path following the first baffle plate  300  or the linear approximation straight line of the first baffle plate  300 , and to enable the air stream guided to the first baffle plate  300  to flow towards the first cylinder head  16   a  or a vicinity thereof. 
     A third baffle plate  302  (a baffle wall part) is arranged in a substantially linear shape at the second cylinder head  16   b -side such that a virtual extension line inclined relative to the rotational shaft  5  of the motor  4  passes above the second cylinder head  16   b , when seen from above. Here, the virtual extension line of the third baffle plate  302  coincides with a linear approximation straight line of the third baffle plate  302 . An air path or virtual extension line of the air path following a wall surface of the third baffle plate  302 , which faces towards the cooling fan  8   a , intersects with the second cylinder head  16   b , when seen from above. 
     A fourth baffle plate  303  (a baffle wall part) is arranged to face the second cylinder head  16   b  (refer to  FIG. 5 ). The fourth baffle plate  303  is provided to intersect with the air path or virtual extension line of the air path following the third baffle plate  302  or the linear approximation straight line of the third baffle plate  302 , such that the fourth baffle plate  303  enables the air stream guided to the third baffle plate  302  to flow towards the second cylinder head  16   b  or a vicinity thereof. 
     The first baffle plate  300  and the third baffle plate  302  are connected above the rotational shaft  5  of the motor  4  to thus form a substantial V shape. Further, as shown in  FIGS. 6 and 7 , the first baffle plate  300 , the second baffle plate  301  and the third baffle plate  302  are formed as ribs extending (protruding downwardly) integrally from the cover  26 , and the fourth baffle plate  303  is formed on a part of the wall surface of the cover  26 . As shown in  FIG. 2 , the first baffle plate  300  and the third baffle plate  302  are provided to form different angles relative to the rotational shaft  5  of the motor  4  so as to favorably cool the first cylinder head  16   a  and the second cylinder head  16   b.    
     An eighth baffle plate  307  and a ninth baffle plate  308  (refer to  FIG. 7 ) are provided so as to prevent the air from going round from the motor-side of the cooling fan  8   a  towards the non-motor-side and to prevent the air from flowing out more outwards than the first cylinder head  16   a  and the second cylinder head  16   b  in the cover  26 . The eighth baffle plate  307  and the ninth baffle plate  308  are formed as ribs extending (hanging) integrally from the cover  26 . 
     During the operation of the air compressor  1 , the rotational shaft  5  of the motor  4  is rotated to generate the compressed air, and the cooling fan  8   a  is rotated to suck the air from the ventilating window  29   a  into the cover  26 , as shown in  FIG. 2 . The cooling air turns along the rotating direction of the cooling fan  8   a , like an air stream CA 20  (refer to  FIG. 2 ) and flows along the rotational shaft  5  of the motor  4  towards the compression part  3 . Then, the cooling air is guided to the first cylinder head  16   a  by the first baffle plate  300  (and the upper surface of the cover  26 ), like an air stream CA 21 , and is enabled to further flow to be ejected to the first cylinder head  16   a  by the second baffle plate  301 , like an air stream CA 22 , as shown in  FIG. 5 . Therefore, since it is possible to form the flowing so that the cooling air securely reaches the first cylinder head  16   a  by the first baffle plate  300  and the second baffle plate  301 , it is possible to cool the first cylinder head  16   a  very effectively. Here, since the air stream flows along the wall surface at the high flow rate, by making the linear approximation straight line or virtual extension line of the first baffle plate  300  to face towards the first cylinder head  16   a  when seen from above, it is possible to securely guide the air stream of high flow rate to the first cylinder head  16   a  whose temperature is likely to increase. As a result, it is possible to realize the high cooling effect. 
     Further, a part of the air stream CA 20  is guided to the second cylinder head  16   b  by the third baffle plate  302  (and the upper surface of the cover  26 ), like an air stream CA 23 , and is then enabled to flow to be ejected to a vicinity of the second cylinder head  16   b  by the fourth baffle plate  303 , like an air stream CA 24 , as shown in  FIG. 5 . Therefore, since it is possible to form the flowing such that the cooling air securely reaches the second cylinder head  16   b  by the third baffle plate  302  and the fourth baffle plate  303 , it is possible to cool the second cylinder head  16   b  very effectively. Here, since the air stream flows along the wall surface at the high flow rate, by making the linear approximation straight line or virtual extension line of the third baffle plate  302  to face towards the second cylinder head  16   b , when seen from above, it is possible to securely guide the air stream of high flow rate to the second cylinder head  16   b  whose temperature is likely to increase. As a result, it is possible to realize the high cooling effect. 
     Further, the first baffle plate  300  and the third baffle plate  302  are connected above the rotational shaft  5  of the motor  4  to thus distribute an amount of the cooling air to the first cylinder head  16   a  and the second cylinder head  16   b , so that an air volume of the first baffle plate  300  along the rotating direction of the cooling fan  8   a  is set to be large. Therefore, it is possible to prevent an air path resistance due to the third baffle plate  302 , which is arranged against the rotating direction of the cooling fan  8   a , from being excessively high, so that it is possible to favorably cool the first cylinder head  16   a  and the second cylinder head  16   b . That is, since the air volume guided by the first baffle plate  300  following the turning direction of the air stream is set to be larger than the air volume guided by the third baffle plate  302  arranged against the turning direction of the air stream, it is possible to cool both the first cylinder head  16   a  and the second cylinder head  16   b  while suppressing the increase in the air path resistance due to the third baffle plate  302 . The cooling air having completed the cooling is discharged to the outside of the cover  30  mainly through between the air tanks  2   a ,  2   b.    
     Incidentally, the first baffle plate  300  and the third baffle plate  302  are ideally formed to have a linear shape so as to minimize the air path resistance (the wall surfaces facing towards the cooling fan  8   a -side are formed to be planar). However, even though the first baffle plate  300  and/or the third baffle plate  302  are partially curved/bent so as to avoid other components, for example, it is possible to guide the cooling air to the first cylinder head  16   a  or second cylinder head  16   b  inasmuch as the linear approximation straight line is formed to pass above the first cylinder head  16   a  or second cylinder head  16   b.    
     Second Illustrative Embodiment 
       FIG. 13  is a plan view of an air compressor according to a second illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface. The air compressor of this illustrative embodiment is the same as the first illustrative embodiment, except that the first baffle plate  300  and the third baffle plate  302  are curved to be convex towards the cooling fan  8   a -side. The linear approximation straight lines and virtual extension lines of the first baffle plate  300  and the third baffle plate  302  intersect with the first cylinder head  16   a  and the second cylinder head  16   b , when seen from above. Also in this illustrative embodiment, it is possible to obtain the same effects as the first illustrative embodiment. 
     Third Illustrative Embodiment 
       FIG. 14  is a plan view of an air compressor according to a third illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface. The air compressor of this illustrative embodiment is the same as the first illustrative embodiment, except that the connection part between the first baffle plate  300  and the third baffle plate  302  is shifted from the upper of the rotational shaft  5  of the motor  4  towards the second cylinder head  16   b . In this illustrative embodiment, the air volume to the first cylinder head  16   a  is increased and the air volume to the second cylinder head  16   b  is decreased, as compared to the first illustrative embodiment. However, it is possible to reduce the air path resistance due to the third baffle plate  302  arranged against the turning direction of the air stream. 
     Fourth Illustrative Embodiment 
       FIG. 15  is a plan view of an air compressor according to a fourth illustrative embodiment of the present invention, in which the cover  26  is a cross-sectional surface. The air compressor of this illustrative embodiment is the same as the first illustrative embodiment, except that the third baffle plate  302  is omitted and the first baffle plate  300  extends up to the second cylinder head  16   b -side. In this illustrative embodiment, the air volume to the first cylinder head  16   a  is increased and the air volume to the second cylinder head  16   b  is decreased, as compared to the first illustrative embodiment. However, since the third baffle plate  302  arranged against the turning direction of the air stream is omitted, the air path resistance is reduced. 
     Although the present invention has been described with reference to the illustrative embodiments, it can be understood by one skilled in the art that the respective elements and respective processes of the illustrative embodiments can be variously modified within the scope defined in the claims. 
     The following matters may also be disclosed in this specification. 
     (1) An air compressor comprising: an air tank configured to store therein compressed air; a compression part configured to compress air sucked from an outside and to supply the air to the air tank; a motor configured to drive the compression part; a cooling fan provided at one end-side of a rotational shaft of the motor, and a cover configured to cover at least the compression part, the motor and the cooling fan, wherein the cooling fan comprises a ring part provided at an outer peripheral part thereof and having a substantially cylindrical shape, and wherein the cooling fan is configured to change a direction of an air stream by the ring part. 
     (2) The air compressor according to (1), wherein the ring part is configured to guide the air stream to a first heat generation part. 
     (3) The air compressor according to (1) or (2), wherein the cooling fan comprises a plate-shaped part partially bridging between an attaching part to the rotational shaft and the ring part. 
     (4) The air compressor according to (3), wherein the attaching part and the ring part are connected and integrated over an entire circumference by the plate-shaped part. 
     (5) The air compressor according to (3) or (4), wherein the cooling fan comprises a blade for generating an air stream, and wherein the blade is integrated with the plate-shaped part. 
     (6) The air compressor according to any one of (1) to (5), wherein the cooling fan is configured to generate an air stream flowing from a center-side towards an outer peripheral direction by a rotation of the blade and to change a direction of the air stream such that the air stream has a component in a direction along the rotational shaft. 
     (7) The air compressor according to (6), wherein a first heat generation part is arranged at a position which faces the cooling fan and with which the air stream which direction has been changed by the ring part collides. 
     (8) The air compressor according to (7), wherein a baffle wall part is provided at a position facing a part except for a central part of the cooling fan at the same side as the first heat generation part, wherein the cooling fan is configured to suck the air stream through between the first heat generation part and the baffle wall part, and wherein the ring part enables an outflow direction of the air stream of the cooling fan to be close to parallel with the suction direction of the air stream. 
     (9) The air compressor according to (7) or (8), wherein the first heat generation part is a control circuit board or a holding member of the control circuit board. 
     (10) The air compressor according to any one of (1) to (9), wherein the cover comprises a ventilating window at a position facing a suction-side of the cooling fan. 
     (11) The air compressor according to any one of (1) to (10), further comprising a baffle wall part configured to change a direction of the air stream outflowing from the cooling fan and to guide the air stream towards a second heat generation part. 
     (12) The air compressor according to (11), wherein the second heat generation part is a cylinder of the compression part. 
     (13) The air compressor according to (12), further comprising another cooling fan provided at the other end-side of the rotational shaft of the motor, wherein the baffle wall part is configured to guide the air stream outflowing from the cooling fan to a side of the cylinder with which an air stream from said another cooling fan does not collide.