Patent Publication Number: US-2023151803-A1

Title: Air compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-186204, filed on Nov. 16, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an air compressor for supplying compressed air to an air tool, such as a compressed air-driven nailer or an air duster. 
     2. Description of the Background 
     A technique for an air compressor is described in Japanese Patent No. 5186799 (hereafter, Patent Literature 1). The air compressor includes a reciprocating compression assembly for producing compressed air. A known compression assembly converts rotational output from an electric motor with a crank assembly into reciprocation of a piston in a cylinder to produce compressed air. The compressed air produced in the compression assembly is stored into a tank. The compressed air stored in the tank is supplied to an air tool. 
     Outside air to produce such compressed air is drawn into a crankcase accommodating the crank assembly. Thus, a filter for reducing noise and dust is installed on an intake path for drawing outside air into the crankcase. With the technique in Patent Literature 1, the filter is attached to a crankcase cover that hermetically closes an end of the crankcase. The filter covers multiple inlets in the crankcase cover. The dust filter is held by a filter cover coupled to the crankcase cover. Outside air is then drawn into the inlets through a space between the peripheries of the filter cover and the crankcase cover. The intake path is thus bent, reducing noise resulting from external leakage of the operation noise of the compression assembly. 
     BRIEF SUMMARY 
     In addition to reducing noise, the air compressor is to protect the inside of the crankcase from dust. In particular, the air compressor with its intake path being protected from dust has higher maintainability and durability. 
     One or more aspects of the present disclosure are directed to a technique for improving the dustproof performance of an intake path to a crankcase. 
     A first aspect of the present disclosure provides an air compressor, including: 
     a compression assembly configured to reciprocate a piston in a cylinder to produce compressed air; 
     a crankcase accommodating the compression assembly and having an inner inlet to allow outside air to be drawn; 
     a filter located in the crankcase and covering the inner inlet from outside; 
     a filter cover having an outer inlet and covering the filter from outside; and 
     a dust cover covering the filter cover from outside, covering the outer inlet, and including an outer circumferential portion, the outer circumference portion and the filter cover having a space to draw outside air between the outer circumference portion and the filter cover. 
     The air compressor according to the above aspect of the present disclosure improves the dustproof performance of an intake path to the crankcase. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an external perspective view of an air compressor. 
         FIG.  2    is a view of the air compressor with a body cover removed and a compression assembly exposed as viewed diagonally from the left rear in the direction indicated by arrow II in  FIG.  1   . 
         FIG.  3    is a sectional view of the compression assembly. 
         FIG.  4    is a perspective view of an intake unit in a first embodiment. 
         FIG.  5    is an exploded perspective view of the intake unit in the first embodiment. 
         FIG.  6    is a longitudinal sectional view of the intake unit in the first embodiment, taken along line VI-VI in  FIG.  5    as viewed in the direction indicated by arrows. 
         FIG.  7    is a longitudinal sectional view of the intake unit in the first embodiment, taken along line VII-VII in  FIG.  5    as viewed in the direction indicated by arrows. 
         FIG.  8    is a longitudinal sectional view of the intake unit in the first embodiment, taken along line VIII-VIII in  FIG.  5    as viewed in the direction indicated by arrows. 
         FIG.  9    is an enlarged view of portion IX in  FIG.  7   . 
         FIG.  10    is a perspective view of a crankcase cover as viewed from its outer surface. 
         FIG.  11    is a perspective view of a filter cover as viewed from its inner surface. 
         FIG.  12    is a perspective view of the filter cover as viewed from its outer surface. 
         FIG.  13    is a perspective view of a dust cover as viewed from its inner surface. 
         FIG.  14    is a perspective view of an intake unit in a second embodiment. 
         FIG.  15    is a longitudinal sectional view of the intake unit in the second embodiment. 
         FIG.  16    is a perspective view of an intake unit in a third embodiment. 
         FIG.  17    is a longitudinal sectional view of the intake unit in the third embodiment. 
         FIG.  18    is a perspective view of an intake unit in a fourth embodiment. 
         FIG.  19    is a longitudinal sectional view of the intake unit in the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments 
     As shown in  FIGS.  1  and  2   , an air compressor  1  includes two cylindrical tanks  2  extending in the front-rear direction. The two tanks  2  store produced compressed air. The two tanks  2  include four legs  3  in total on their front and rear portions. Each leg  3  is formed from high vibration-proof rubber. Each leg  3  is adjacent to a side protector  3   a . A drain cock  2   a  is located between the front portions of the two tanks  2 . The two tanks  2  include upper portions coupled to each other with a base  4 . The base  4  receives a compression assembly  10  mounted on its upper surface. Carrying handles  5  are located in front of and behind the base  4  and each extend across the upper portions of the two tanks  2 .  FIG.  1    shows the compression assembly  10  covered with a body cover  6 . 
     The body cover  6  has two outlet ports  7  for high-pressure air on its front left surface. The body cover  6  has two outlet ports  8  for low-pressure air on its front right surface. Through the outlet ports  7  for high-pressure air, for example, compressed air of 2.5 MPa is supplied. Through the outlet ports  8  for low-pressure air, for example, compressed air of 1 MPa is supplied. Adjustment dials  7   a  and  8   a  are located above the outlet ports  7  and  8  to set their respective outlet pressures. The body cover  6  includes, on its upper front surface, an operation unit  9  operable for activation. The operation unit  9  includes various displays. 
     As shown in  FIG.  2   , the body cover  6  is removed to expose the compression assembly  10 . As shown in  FIGS.  2  and  3   , the compression assembly  10  includes a cylindrical crankcase  21  including a front first compressor  11  and a rear second compressor  12 . The crankcase  21  supports, on its right, an electric motor  22  between the first compressor  11  and the second compressor  12 . The crankcase  21  is fixed on the base  4 . 
     The electric motor  22  is a brushless motor that produces relatively large activation torque. The electric motor  22  includes an annular rotor  22   a  and an annular stator  22   b . The stator  22   b  is located inward from the circumference of the rotor  22   a . The stator  22   b  is fixed to a right portion of the crankcase  21 . A motor shaft  25  is coupled to the rotor  22   a  at the center. The motor shaft  25  has a right end receiving a heat-dissipating fan  23 . The heat-dissipating fan  23  rotates to dissipate heat generated in the electric motor  22  and cool the electric motor  22 . The motor shaft  25  extends leftward through the center of the stator  22   b . The motor shaft  25  is rotatably supported across right and left portions of the crankcase  21  with a right bearing  25   a  and a left bearing  25   b . The motor shaft  25  has a left end protruding leftward through an intake unit  30 . The motor shaft  25  has the left end receiving an intake fan  24 . The intake fan  24  rotates to blow outside air against the intake unit  30 . 
     The cylindrical crankcase  21  includes a front portion coupled to a first cylinder  11   a  in the first compressor  11 . The crankcase  21  includes a rear portion coupled to a second cylinder  12   a  in the second compressor  12 . The internal space of the crankcase  21  allows passage of outside air. 
     The first cylinder  11   a  accommodates a first piston  11   b  to allow reciprocation in the front-rear direction. The first cylinder  11   a  extends frontward from the front portion of the crankcase  21 . The first piston  11   b  is coupled to a first crank  26  in the motor shaft  25  with a first rod  11   c.    
     The second cylinder  12   a  accommodates a second piston  12   b  to allow reciprocation in the front-rear direction. The second cylinder  12   a  extends rearward from the rear portion of the crankcase  21 . The second piston  12   b  is coupled to a second crank  27  in the motor shaft  25  with a second rod  12   c.    
     The first crank  26  and the second crank  27  are decentered in the same direction at the same position about the axis of the motor shaft  25 . As the motor shaft  25  rotates once, one of the first compressor  11  and the second compressor  12  performs a compression process, and the other compressor performs an intake process at the same time. In the compression process in which the first piston  11   b  in the first compressor  11  moves forward, the second piston  12   b  in the second compressor  12  moves forward to perform the intake process. In the intake process in which the first piston  11   b  in the first compressor  11  moves backward, the second piston  12   b  in the second compressor  12  moves backward to perform the compression process. 
     The first cylinder  11   a  includes a first compression chamber  11   d  that connects to a second compression chamber  12   d  in the second cylinder  12   a  with a supply pipe  13 . The supply pipe  13  has an upstream end connected to the first compression chamber  11   d  with an auxiliary check valve  11   e . The auxiliary check valve  11   e  blocks compressed air without allowing flowing back from the supply pipe  13  into the first compression chamber  11   d . The supply pipe  13  has a downstream end connected to the second compression chamber  12   d . The compressed air flowing from the first compression chamber  11   d  through the auxiliary check valve  11   e  into the supply pipe  13  is directly supplied to the second compression chamber  12   d.    
     In response to activation of the electric motor  22 , compressed air is produced in two steps, which are through the first compressor  11  and the second compressor  12 . The compressed air supplied to the second compression chamber  12   d  is compressed to a higher pressure in response to retraction of the second piston  12   b . The high-pressure compressed air of, for example, about 4.5 MPa produced in the second compression chamber  12   d  flows into an air channel  15  extending to the tanks  2  through a first check valve  14 . The first check valve  14  blocks the compressed air flowing into the air channel  15  without flowing back into the second compression chamber  12   d.    
     The crankcase  21  is adjacent to the intake unit  30  on its left. Outside air is drawn into the crankcase  21  through the intake unit  30 . 
     First Embodiment 
     An example intake unit according to each of embodiments will be described.  FIGS.  2  to  8    show the intake unit  30  in a first embodiment.  FIGS.  4  to  8    show the intake unit  30  in detail.  FIGS.  4  and  5    show the motor shaft  25  with the intake fan  24  removed from the motor shaft  25 . In  FIGS.  6  to  8   , outside air flows from left to right with respect to the intake unit  30  and is drawn into the crankcase  21 . The flow of intake air being upstream is hereafter also referred to as being toward an outer surface (outside), and being downstream is also referred to as being toward an inner surface (inside). 
     The intake unit  30  in the first embodiment includes a crankcase cover  31 , a filter  32 , a filter cover  33 , and a dust cover  34 . The crankcase cover  31  hermetically covers an intake opening  21   a  of the crankcase  21 . The filter  32  covers inner inlets  31   e  in the crankcase cover  31 . The filter cover  33  covers the filter  32 . The dust cover  34  covers the filter cover  33 . 
     The opening  21   a  of the crankcase  21  is substantially circular. As shown in  FIGS.  5  and  9   , the crankcase cover  31  is substantially disk-shaped. The crankcase cover  31  is coupled to the opening  21   a  of the crankcase  21  with six mount screws  35  in total arranged on its circumference. The crankcase cover  31  hermetically covers the opening  21   a . The crankcase cover  31  has a cylindrical bearing recess  31   a  on a central portion of its inner surface. The bearing recess  31   a  holds the bearing  25   b.    
     As shown in  FIG.  8   , the bearing recess  31   a  has an opening to receive a single restriction plate  25   c  fastened with four fixing screws  36 . Thus, the restriction plate  25   c  covers the opening of the bearing recess  31   a . The bearing  25   b  is held between the bottom of the bearing recess  31   a  and the restriction plate  25   c  and thus restricted from being displaced in a direction of a motor axis J. 
     As shown in  FIGS.  5  to  8   , the crankcase cover  31  has a filter-receiving recess  31   b  accommodating the filter  32  on its outer surface. The filter-receiving recess  31   b  includes a cylindrical protrusion  31   c  on its central portion. The protrusion  31   c  defines the bearing recess  31   a  on the inner surface. The protrusion  31   c  has a through-hole  31   d  at its center to receive the motor shaft  25 . The protrusion  31   c  has four threaded holes  31   g  equally spaced on its periphery. The fixing screws  36  fastening the restriction plate  25   c  are screwed into the threaded holes  31   g . The filter-receiving recess  31   b  surrounds the protrusion  31   c.    
     As shown in  FIGS.  5  to  7   , the crankcase cover  31  has the multiple inner inlets  31   e  and multiple screw bosses  31   f  on its bottom surface. Four screw bosses  31   f  are arranged in the present embodiment. Each screw boss  31   f  has an internal thread  31   h  on its inner periphery. As described later, a fixing screw  37  is screwed onto the internal thread  31   h  on each of the four screw bosses  31   f  to fasten the filter cover  33  and the dust cover  34  together to the crankcase cover  31 . 
     Each screw boss  31   f  protrudes leftward from the outer surface of the crankcase cover  31 . Each screw boss  31   f  has a stepped outer diameter and includes a larger-diameter portion on its basal end and a smaller-diameter portion on its distal end. Each screw boss  31   f  has the inner inlets  31   e  on both sides. Eight inner inlets  31   e  are arranged in total in the present embodiment. The single filter  32  is accommodated in the filter-receiving recess  31   b  to cover the eight inner inlets  31   e  from outside. 
     The filter-receiving recess  31   b  is deep enough to accommodate the filter  32  substantially entirely in the thickness direction. Thus, as shown in  FIGS.  6  to  8   , the outer circumferential end face of the crankcase cover  31  is substantially flush with the outer surface of the filter  32 . 
     The filter  32  is a felt filter for reducing noise and dust. The filter  32  is substantially disk-shaped. The filter  32  has a through-hole  32   a  at its center. The protrusion  31   c  on the crankcase cover  31  extends through the through-hole  32   a . The through-hole  32   a  has four through-holes  32   b  equally spaced on its periphery. Each threaded hole  31   g  in the crankcase cover  31  extends through the corresponding through-hole  32   b . The through-hole  32   a  is surrounded by four through-holes  32   c . The larger-diameter portion of each screw boss  31   f  in the crankcase cover  31  extends through the corresponding through-hole  32   c.    
     The filter cover  33  is coupled to the outer surface of the filter  32 . The filter cover  33  is a disk having substantially the same diameter as the filter  32 . The filter cover  33  covers the entire outer surface of the filter  32 . The filter cover  33  includes a cylindrical insertion portion  33   a  at its center. The insertion portion  33   a  receives the motor shaft  25  on its inner circumference. 
     As shown in  FIG.  11   , the filter cover  33  includes an inner circumferential holder  33   b  surrounding the insertion portion  33   a  on its inner surface. The inner circumferential holder  33   b  is a ridge protruding inward (toward the filter  32 ) along the peripheries of the through-holes  32   a  and  32   b  in the filter  32 . The filter cover  33  also includes an outer circumferential holder  33   c  on the circumference of the inner surface. The outer circumferential holder  33   c  is a ridge protruding inward along the circumference of the filter  32 . The inner circumferential holder  33   b  and the outer circumferential holder  33   c  respectively abut against the inner and outer circumferences of the outer surface of the filter  32 . The filter  32  is thus less likely to be displaced in the filter-receiving recess  31   b  on the crankcase cover  31 . 
     The filter cover  33  has multiple outer inlets  33   d . As shown in  FIGS.  5 ,  7 ,  9 , and  12   , the filter cover  33  includes multiple annular walls  33   e  on its outer surface. The annular walls  33   e  are cylindrical. Each annular wall  33   e  has an inner circumference defining the outer inlet  33   d  extending through the annular wall  33   e  in the thickness direction. Each outer inlet  33   d  with the corresponding annular wall  33   e  has a depth greater than its hole diameter. This reduces intake noise. 
     Each annular wall  33   e  enters a recess  34   d  on the dust cover  34  described later. As indicated by arrow W (intake path W) in  FIG.  9   , each annular wall  33   e  bends the intake path W extending in a plane direction of the filter cover  33  to extend in a thickness direction (a direction along the motor axis J). Each annular wall  33   e  is included in a path bender that bends the intake path W of outside air in the thickness direction. The outside air flowing toward the outer surface of the filter cover  33  flows through the path bent in the thickness direction by each path bender into the outer inlet  33   d . The outside air flowing into the outer inlet  33   d  is blown toward the filter  32 . 
     The filter cover  33  has four through-holes  33   f  equally spaced on its circumference. The smaller-diameter portion of each screw boss  31   f  in the crankcase cover  31  extends through the corresponding through-hole  33   f  As shown in  FIG.  11   , the filter cover  33  includes middle holders  33   g  surrounding the through-holes  33   f  on the inner surface. Similarly to the inner circumferential holder  33   b  and the outer circumferential holder  33   c , each middle holder  33   g  is a ridge protruding toward the filter  32 . The middle holders  33   g  hold middle areas in the radial direction of the filter  32 . 
     As shown in  FIGS.  5  and  12   , the filter cover  33  has, on the outer surface, positioning recesses  33   h  each surrounding the opening of the corresponding through-hole  33   f . Each positioning recess  33   h  is coaxial with the corresponding through-hole  33   f  and is a circular recess with a predetermined depth. Each positioning recess  33   h  receives a protrusion  34   b  on the dust cover  34 . Thus, the dust cover  34  is positioned relative to the filter cover  33  about the motor axis J. 
     The filter cover  33  includes an outer circumferential portion  33   i  having a uniform width and bent toward the filter  32  at an angle of substantially 45° along its entire circumference. 
     As shown in  FIG.  13   , the dust cover  34  is a disk having substantially the same diameter as the filter cover  33 . The dust cover  34  obstructs the flow of outside air produced by the intake fan  24  to prevent the outside air from being blown directly against the filter cover  33 . This prevents dust or other matter contained in the outside air from being blown directly against the filter cover  33 . The dust cover  34  includes a cylindrical insertion portion  34   a  in the central portion of its inner surface. The insertion portion  34   a  protrudes toward the filter cover  33 . The insertion portion  33   a  of the filter cover  33  is received in the insertion portion  34   a . The motor shaft  25  has a distal end protruding outward through the inner circumference of the insertion portion  33   a  received in the insertion portion  34   a . The protruding distal end of the motor shaft  25  supports the intake fan  24 . 
     As shown in  FIGS.  6  to  9   , the insertion portion  33   a  in the filter cover  33  is received (fitted through recess-protrusion engagement) in the insertion portion  34   a  in the dust cover  34 . The filter cover  33  and the dust cover  34  are thus positioned relative to each other coaxially with the motor shaft  25 . 
     The dust cover  34  includes the four cylindrical protrusions  34   b  equally spaced in the circumferential direction on the inner surface. The protrusions  34   b  protrude toward the filter cover  33 . The four protrusions  34   b  are received in the corresponding positioning recesses  33   h  on the filter cover  33 . The dust cover  34  is thus positioned relative to the filter cover  33  in the direction of the motor axis J. In the positioning state, the dust cover  34  and the filter cover  33  have a space between them in the direction of the motor axis J. The space serves as the intake path W for drawing outside air into the crankcase  21 . 
     Each protrusion  34   b  has a through-hole  34   c  at its center. Each through-hole  34   c  receives the corresponding fixing screw  37 . As shown in  FIG.  5   , the dust cover  34  has, on its outer surface, circular indentations  34   e  surrounding the openings of the through-holes  34   c . The dust cover  34  has four indentations  34   e  recessed toward the inner surface and thus includes the corresponding protrusions  34   b  protruding on the backs of the indentations  34   e.    
     The dust cover  34  has multiple recesses  34   d  on the inner surface. The multiple recesses  34   d  are aligned with the annular walls  33   e  on the filter cover  33 . Each recess  34   d  is circular and has a diameter large enough to receive the corresponding annular wall  33   e . Each recess  34   d  is deep enough to receive the corresponding annular wall  33   e  with a narrow space left. The space between the bottom surface of each recess  34   d  and the corresponding annular wall  33   e  defines a part of the intake path W for drawing outside air. 
     Each annular wall  33   e  on the filter cover  33  enters (overlaps in the direction of the motor axis J) the corresponding recess  34   d  on the dust cover  34 , thus bending the intake path W at substantially right angles. This defines the intake path W with a labyrinth structure. 
     The dust cover  34  includes an outer circumferential portion  34   f  having a uniform width and bent toward the filter cover  33  at an angle of substantially 45° along its entire circumference. As shown in  FIGS.  6  to  9   , the outer circumferential portion  34   f  of the dust cover  34  and the outer circumferential portion  33   i  of the filter cover  33  thus have a uniform space between them. The space defines an inlet port  38  of the intake path W. 
     As shown in  FIGS.  5  and  6   , the dust cover  34  and the filter cover  33  are fastened together to the crankcase cover  31  with the four fixing screws  37 . The larger-diameter portion of each screw boss  31   f  in the crankcase cover  31  is in the corresponding through-hole  32   c  in the filter  32 . The smaller-diameter portion of each screw boss  31   f  is placed through the corresponding through-hole  33   f  in the filter cover  33  and the corresponding through-hole  34   c  in the dust cover  34 . The internal thread  31   h  on each screw boss  31   f  receives the corresponding fixing screw  37  to fasten the dust cover  34  and the filter cover  33  together to the crankcase cover  31 . The filter  32  is thus held inside the filter-receiving recess  31   b  on the crankcase cover  31 . Each fixing screw  37  has a head in the corresponding indentation  34   e . This prevents the heads of the fixing screws  37  from protruding from the outer surface of the dust cover  34 . 
     In response to activation of the electric motor  22 , the intake fan  24  rotates to blow outside air against the intake unit  30 . As indicated by arrow Win  FIG.  9   , the blown outside air flows into the space (inlet port  38 ) between the dust cover  34  and the filter cover  33  through the inlet port between the outer circumferential portion  34   f  of the dust cover  34  and the outer circumferential portion  33   i  of the filter cover  33 . The outer circumferential portion  34   f  of the dust cover  34  and the outer circumferential portion  33   i  of the filter cover  33  are bent in the same direction. Thus, the intake path W of the intake unit  30  is bent at the inlet port  38 . 
     The outside air flowing into the space between the dust cover  34  and the filter cover  33  through the inlet port  38  is blown against the annular walls  33   e  on the filter cover  33  and flows into the recesses  34   d  on the dust cover  34 . In this state, the flow of the outside air (intake path W) is bent at substantially right angles. After being bent at two positions on the intake path W, the outside air flowing into the recesses  34   d  flows into the outer inlets  33   d  (the inner circumferences of the annular walls  33   e ) and is blown against the filter  32 . 
     The outside air passes through the filter  32  to filter dust. Clean outside air resulting from the dust filtering with the filter  32  flows into the crankcase  21  through the inner inlets  31   e  in the crankcase cover  31 . Outside air flowing in through the intake path W is supplied to the first compressor  11 . The outside air flowing in is supplied into the first cylinder  11   a  and compressed by the first piston  11   b.    
     In the air compressor  1  according to the first embodiment, outside air flows into the outer inlets  33   d  through the inlet port  38  between the outer circumferential portion  34   f  of the dust cover  34  and the filter cover  33 . Thus, the intake path W of the outside air extending in the plane direction of the filter cover  33  is bent in the thickness direction (the direction of the motor axis J) to reduce the likelihood that dust contained in the outside air is blown directly against the filter  32 . The filter  32  is thus less likely to be clogged. 
     The path benders (annular walls  33   e ) are located between the filter cover  33  and the dust cover  34 . The intake path W of the outside air passing between the filter cover  33  and the dust cover  34  is bent in the thickness direction of the filter cover  33  (the direction of the motor axis J). This reduces the likelihood that dust contained in the outside air is blown directly against the filter  32  and thus reduces clogging of the filter  32  more reliably. 
     Each path bender includes the annular wall  33   e  protruding from the circumference of the corresponding outer inlet  33   d  in the filter cover  33  toward the dust cover  34 , and the recess  34   d  located on the dust cover  34  and receiving the annular wall  33   e . Thus, the intake path W is bent in the thickness direction of the filter cover  33  reliably. 
     The dust cover  34  and the filter cover  33  are fastened together to the crankcase cover  31  with the fixing screws  37 . This simplifies the connection of the dust cover  34  and the filter cover  33  to the crankcase cover  31  (crankcase  21 ). 
     The dust cover  34  has, on its outer surface, the indentations  34   e  accommodating the heads of the fixing screws  37 . The dust cover  34  includes the protrusions  34   b  on the backs of the corresponding indentations  34   e  (on the inner surface). The protrusions  34   b  are received in the corresponding positioning recesses  33   h  on the filter cover  33 . Thus, the dust cover  34  is positioned relative to the filter cover  33  about the motor axis J. The engagement of the protrusions  34   b  with the positioning recesses  33   h  positions the dust cover  34  relative to the filter cover  33 . The dust cover  34  is thus attached easily. 
     The insertion portion  34   a  of the dust cover  34  receives the insertion portion  33   a  of the filter cover  33 , and the filter cover  33  and the dust cover  34  are thus positioned relative to each other coaxially with the motor shaft  25 . The insertion portion of the dust cover may be received on the inner circumference of the insertion portion of the filter cover. 
     Each outer inlet  33   d  is surrounded by the corresponding annular wall  33   e  and thus has a depth greater than the hole diameter. This prevents noise leakage in the crankcase  21 , thus reducing noise in the compression assembly  10 . 
     The first embodiment described above may be modified variously. For example, although each path bender includes the annular wall  33   e  on the filter cover  33  and the recess  34   d  on the dust cover  34  in the first embodiment, the recesses  34   d  on the dust cover  34  may be eliminated. 
     Although each annular wall  33   e  surrounds the corresponding outer inlet  33   d  in the filter cover  33 , the dust cover  34  may include, for example, annular or curved walls protruding toward the outer inlets  33   d  as the path benders on the inner surface, instead of the annular walls  33   e  on the filter cover  33 . 
     The inclination angles of the outer circumferential portion  33   i  of the filter cover  33  and the outer circumferential portion  34   f  of the dust cover  34  may be changed. One or both of the inclined outer circumferential portions  33   i  and  34   f  may be eliminated. 
     Second Embodiment 
       FIGS.  14  and  15    show an intake unit  40  in a second embodiment. The air compressor  1  has the same basic structure as in the above embodiment without any modification except the intake unit  40 , and the components are given the same reference numerals and will not be described. The components and the structure of the intake unit  40  that are the same as those in the first embodiment are also given the same reference numerals and will not be described. 
     In the second embodiment, a dust cover  42  differs from the dust cover  34  in the first embodiment. The dust cover  42  in the present embodiment has a diameter larger than the dust cover  34  in the first embodiment. The dust cover  42  includes an outer circumferential portion  42   a  bent toward the crankcase  21  at substantially 90°. The outer circumferential portion  42   a  of the dust cover  42  covers outer circumferential portions of a filter cover  41  and the crankcase cover  31  laterally. 
     As shown in  FIG.  14   , the outer circumferential portion  42   a  bent toward the crankcase  21  extends along an area about the motor axis J excluding an area of substantially 90° in a lower portion of the dust cover  42 . Thus, as shown in  FIG.  15   , the circumferences of the filter cover  41  and the crankcase cover  31  are open downward in an area of substantially 90° in a lower portion of the intake unit  40 . 
     In the second embodiment, the dust cover  42  includes multiple thick portions  42   b  on its outer surface. The thick portions  42   b  correspond to the recesses  34   d  (refer to  FIG.  13   ) on the inner surface. The thick portions  42   b  thicken the bottoms of the recesses  34   d . Thus, the recesses  34   d  are deeper, and the filter cover  41  includes annular walls  41   a  protruding by a length greater than the annular walls  33   e  in the first embodiment. Each annular wall  33   e  has an inner hole defining the outer inlet  33   d  extending through its inner surface. The longer annular walls  41   a  allow the path benders to separate dust contained in drawn outside air more reliably. The outer inlets  33   d  being deeper allow less noise in the intake unit  40 . 
     In the second embodiment, the air compressor  1  includes multiple path benders with a labyrinth structure having a locally narrowed space between the filter cover  41  and the dust cover  42  in addition to the path benders including the annular walls  41   a  on the filter cover  41  and the recesses  34   d  on the dust cover  42 . 
     As shown in  FIG.  14   , the outer circumferential portion  42   a  of the dust cover  42  includes six screw covers  42   c . Each screw cover  42   c  covers the head of the corresponding mount screw  35 . The outer circumferential portion  42   a  extends along an area of substantially 270° covering the heads of the mount screws  35  about the motor axis J. 
     In the second embodiment, the outer circumferential portion  42   a  of the dust cover  42  covers the outer circumferential portions of the filter cover  41  and the crankcase cover  31  laterally. Thus, when flowing into an inlet port  43  between the outer circumferential portion  42   a  of the dust cover  42  and the outer circumferential portion of the filter cover  41 , outside air flows from the filter cover  41  toward the dust cover  42  (opposite to the flow direction in the outer inlets  33   d ). The intake path W of the outside air toward the dust cover  42  is thus bent in a plane direction of the filter cover  41 . In the second embodiment, the outer circumferential portion  42   a  of the dust cover  42  bends the intake path W more reliably than the inlet port  38  in the first embodiment. 
     Third Embodiment 
       FIGS.  16  and  17    show an intake unit  50  in a third embodiment. The components and the structures that are the same as those in the above embodiments without any modification are given the same reference numerals and will not be described. The structure in the third embodiment includes a second filter  53  in addition to the structure in the second embodiment. The second filter  53  is annular and extends along the circumference of a filter cover  51 . The second filter  53  is formed from felt, similarly to the filter  32 . 
     As shown in  FIG.  17   , the filter cover  51  includes a flat basal portion  51   a  along the entire circumference of its outer surface. The second filter  53  is held between the basal portion  51   a  and an inner surface of a dust cover  52 . The dust cover  52  includes an outer circumferential portion  52   a  covering the circumferences of the filter cover  51  and the crankcase cover  31  laterally as in the second embodiment. The outer circumferential portion  52   a  in the third embodiment extends along the entire circumference of the dust cover  52 . The outer circumferential portion  52   a  includes screw covers  52   b  covering the heads of the mount screws  35  as in the second embodiment. 
     Outside air is drawn through an inlet port  54  between the outer circumferential portion  52   a  of the dust cover  52  and a circumferential portion of the filter cover  51 . The second filter  53  extends along the entire circumference of the inlet port  54 . 
     In the third embodiment, the second filter  53  is installed between the filter cover  51  and the dust cover  52 . This reduces clogging of the filter  32  still more reliably and also allows still less noise in the intake unit  50  (compression assembly  10 ). 
     Fourth Embodiment 
       FIGS.  18  and  19    show an intake unit  60  in a fourth embodiment. The intake unit  60  in the present embodiment excludes the dust cover  34  from the intake unit  30  in the first embodiment. Thus, a filter cover  61  has an exposed outer surface. As in the first embodiment, the filter cover  61  includes multiple cylindrical annular walls  61   a  on the outer surface. Each annular wall  61   a  has an outer inlet  61   b  extending through its inner surface. 
     The intake unit  60  has recesses  61   c  accommodating the heads of the fixing screws  37 . The recesses  61   c  are shallower than the positioning recesses  33   h  in the first embodiment. 
     In the intake unit  60  in the present embodiment, the intake fan  24  rotates to blow outside air against the entire outer surface of the filter cover  61 . The outside air is blown in a direction substantially along the motor axis J. Thus, although a portion of the outside air flows directly into the outer inlets  61   b , the other portion of the outside air is mostly blown against the outer surface of the filter cover  61 . 
     The outside air blown against the outer surface of the filter cover  61  flows along the outer surface and then is blown against the annular walls  61   a  to bend the intake path. The path benders remove dust from the outside air. The clean outside air with the dust removed flows into the outer inlets  61   b.    
     Although the filter cover  61  is exposed, the annular walls  61   a  surrounding the corresponding outer inlets  61   b  serve as the path benders. This removes dust in the outside air effectively and reduces clogging of the filter  32 . 
     The air compressor  1  according to any one of the first to fourth embodiments is an example of an air compressor in an aspect of the present disclosure. The compression assembly  10  in any one of the first to fourth embodiments is an example of a compression assembly in an aspect of the present disclosure. The crankcase  21  in any one of the first to fourth embodiments is an example of a crankcase in an aspect of the present disclosure. The inner inlets  31   e  in any one of the first to fourth embodiments each are an example of an inner inlet in an aspect of the present disclosure. 
     The filter  32  in any one of the first to fourth embodiments is an example of a filter in an aspect of the present disclosure. The filter cover  33  in the first embodiment, the filter cover  41  in the second embodiment, and the filter cover  51  in the third embodiment each are an example of a filter cover in an aspect of the present disclosure. The outer inlets  33   d  in the first embodiment and the outer inlets  33   d  in the second embodiment each are an example of an outer inlet in an aspect of the present disclosure. 
     The dust cover  34  in the first embodiment, the dust cover  42  in the second embodiment, and the dust cover  52  in the third embodiment each are an example of a dust cover in an aspect of the present disclosure. The outer circumferential portion  34   f  in the first embodiment, the outer circumferential portion  42   a  in the second embodiment, and the outer circumferential portion  52   a  in the third embodiment each are an example of an outer circumferential portion in an aspect of the present disclosure. The inlet port  38  in the first embodiment, the inlet port  43  in the second embodiment, and the inlet port  54  in the third embodiment each are an example of a space in an aspect of the present disclosure. 
     REFERENCE SIGNS LIST 
     
         
           1  air compressor 
           2  tank 
           3  leg 
           3   a  side protector 
           4  base 
           5  handle 
           6  body cover 
           7  outlet port (for high-pressure air) 
           7   a  adjustment dial 
           8  outlet port (for low-pressure air) 
           8   a  adjustment dial 
           9  operation unit 
           10  compression assembly 
           11  first compressor 
           11   a  first cylinder 
           11   b  first piston 
           11   c  first rod 
           11   d  first compression chamber 
           11   e  auxiliary check valve 
           12  second compressor 
           12   a  second cylinder 
           12   b  second piston 
           12   c  second rod 
           12   d  second compression chamber 
           13  supply pipe 
           14  first check valve 
           15  air channel 
           21  crankcase 
           21   a  opening 
           22  electric motor 
           22   a  rotor 
           22   b  stator 
           23  heat-dissipating fan 
           24  intake fan 
           25  motor shaft 
           25   a ,  25   b  bearing 
           25   c  restriction plate 
           26  first crank 
           27  second crank 
           30  intake unit (first embodiment) 
         W intake path 
           31  crankcase cover 
           31   a  bearing recess 
           31   b  filter-receiving recess 
           31   c  protrusion 
           31   d  through-hole 
           31   e  inner inlet 
           31   f  screw boss 
           31   g  threaded hole 
           31   h  internal thread 
           32  filter 
           32   a ,  32   b ,  32   c  through-hole 
           33  filter cover 
           33   a  insertion portion 
           33   b  inner circumferential holder 
           33   c  outer circumferential holder 
           33   d  outer inlet 
           33   e  annular wall 
           33   f  through-hole 
           33   g  middle holder 
           33   h  positioning recess 
           33   i  outer circumferential portion 
           34  dust cover 
           34   a  insertion portion 
           34   b  protrusion 
           34   c  through-hole 
           34   d  recess 
           34   e  indentation 
           34   f  outer circumferential portion 
           35  mount screw 
           36  fixing screw 
           37  fixing screw 
           38  inlet port 
           40  intake unit (second embodiment) 
           41  filter cover 
           41   a  annular wall 
           42  dust cover 
           42   a  outer circumferential portion 
           42   b  thick portion 
           42   c  screw cover 
           43  inlet port 
           50  intake unit (third embodiment) 
           51  filter cover 
           51   a  basal portion 
           52  dust cover 
           52   a  outer circumferential portion 
           52   b  screw cover 
           53  second filter 
           54  inlet port 
           60  intake unit (fourth embodiment) 
           61  filter cover 
           61   a  annular wall 
           61   b  outer inlet 
           61   c  recess