Abstract:
In rollers used in a radial needle bearing, the circularity of crowning sections provided at opposite ends of each roller is 0.6 μm or smaller. This allows for reduction of sound (noise) produced when the radial needle bearing rotates by itself and can also allow for reduction of sound (noise) of a rotational machine when the radial needle bearing is installed and rotates therein.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of Japanese Application No. 2010-105200 filed in Japan on Apr. 30, 2010, the content of which is hereby incorporated by its reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radial needle bearings suitably used in, for example, rotational machines, such as electric compressors. 
     2. Description of Related Art 
     A known example of a rotational machine equipped with a radial needle bearing is an electric compressor disclosed in Japanese Unexamined Patent Application, Publication No. 2009-293523. 
     In recent years, there has been a further demand for reducing sound (noise) produced when a rotational machine, such as an electric compressor, is rotated. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in view of the circumstances described above, and an object thereof is to provide a radial needle bearing that allows for reduction of sound (noise) produced when it rotates by itself and that can also allow for reduction of sound (noise) of a rotational machine when the radial needle bearing is installed and rotates therein. 
     In order to achieve the aforementioned object, the present invention provides the following solutions. 
     In rollers used in a radial needle bearing according to a first aspect of the present invention, the circularity of crowning sections provided at opposite ends of each roller is 0.6 μm or smaller. 
     With the rollers for a radial needle bearing according to the first aspect of the present invention, when the sound, which was produced by the radial needle bearing fitted with the rollers therefor while rotating the radial needle bearing by itself, was measured, it was demonstrated that the rotation sound (noise) can be reduced as compared with a radial needle bearing fitted with rollers therefor, in which the circularity of crowning sections provided at opposite ends thereof is between 1.1 μm and 1.5 μm. 
     A radial needle bearing according to a second aspect of the present invention includes the aforementioned rollers for the radial needle bearing. 
     With the radial needle bearing according to the second aspect of the present invention, when the sound, which was produced by the radial needle bearing fitted with the rollers therefor while rotating the radial needle bearing by itself, was measured, it was demonstrated that the rotation sound (noise) can be reduced as compared with a radial needle bearing equipped with rollers in which the circularity of crowning sections provided at opposite ends thereof is between 1.1 μm and 1.5 μm. 
     A rotational machine according to a third aspect of the present invention includes the aforementioned radial needle bearing. 
     With the rotational machine (e.g., electric compressor) according to the third aspect of the present invention, when the sound, which was produced by the rotational machine fitted with the aforementioned radial needle bearing while rotating the rotational machine, was measured, the results shown in  FIG. 4  were obtained. Specifically, with the rotational machine according to the present invention, the rotation sound (noise) can be reduced by about 3.5 dB as compared with a rotational machine fitted with a radial needle bearing that includes rollers therefor, in which the circularity of crowning sections provided at opposite ends thereof is between 1.1 μm and 1.5 μm. 
     The radial needle bearing according to the present invention advantageously allows for reduction of sound (noise) produced when it rotates by itself and can also allow for reduction of sound (noise) of a rotational machine when the radial needle bearing is installed and rotates therein. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a partially-cutaway perspective view of a radial needle bearing according to an embodiment of the present invention; 
         FIG. 2A  is a perspective view showing a part of an outer ring of the radial needle bearing according to the embodiment of the present invention; 
         FIG. 2B  is a perspective view showing an entire cage of the radial needle bearing according to the embodiment of the present invention; 
         FIG. 2C  is a plan view of an entire roller of the radial needle bearing according to the embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of an electric compressor equipped with the radial needle bearing according to the embodiment of the present invention; and 
         FIG. 4  is a graph for explaining the advantages of the electric compressor equipped with the radial needle bearing according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A radial needle bearing according to an embodiment of the present invention will be described below with reference to  FIGS. 1 to 4 . 
       FIG. 1  is a partially-cutaway perspective view of the radial needle bearing according to this embodiment.  FIGS. 2A to 2C  are exploded views of the radial needle bearing according to this embodiment. Specifically,  FIG. 2A  is a perspective view showing a part of an outer ring,  FIG. 2B  is a perspective view showing an entire cage, and  FIG. 2C  is a plan view of an entire roller.  FIG. 3  is a cross-sectional view of an electric compressor equipped with the radial needle bearing according to this embodiment.  FIG. 4  is a graph for explaining the advantages of the electric compressor equipped with the radial needle bearing according to this embodiment. 
     A radial needle bearing  30  according to this embodiment is a bearing suitable for use in, for example, an electric compressor (rotational machine)  1 , as shown in  FIG. 3 . 
     As shown in  FIG. 3 , the electric compressor  1  includes a cylindrical housing  2  that constitutes an outer shell. The housing  2  is constituted of a compressor housing  3  and a motor housing  4  that are both formed into a bowl shape by aluminum die-casting, and is formed by joining together flanges  3 A and  4 A thereof, respectively, with an O-ring  6  therebetween by using bolts  5 . 
     An upper outer-peripheral surface of the motor housing  4  is integrally provided with an inverter accommodating section  7 , and an inverter device (not shown) that converts direct-current power supplied from a high-voltage power source to three-phase alternating-current power and feeds the power to an electric motor  10  provided within the motor housing  4  via a glass-sealed terminal  8  is accommodated in the inverter accommodating section  7 . 
     The inverter device may be of a known type, and a detailed description thereof will be omitted here. 
     The electric motor  10  provided within the motor housing  4  is constituted of a stator  11  and a rotor  12 , and the stator  11  is fixed to an inner peripheral surface of the motor housing  4  by press-fitting or the like. Refrigerant channels (not shown) extending in the axial direction through multiple locations in the circumferential direction are provided between the stator  11  and the motor housing  4 . Through these refrigerant channels, a refrigerant taken into a space  14 , located between the base of the motor housing  4  and an end surface of the electric motor  10 , from a refrigerant intake port (not shown) provided at a rear end (i.e., a right end in  FIG. 3 ) of the motor housing  4  can flow in the axial direction toward the front side (i.e., the left side in  FIG. 3 ). 
     A rotating shaft (crankshaft)  15  is integrally joined to the rotor  12 , and a rear end of this rotating shaft  15  is rotatably supported by a bearing  16  provided at the base of the motor housing  4 , whereas a front end thereof is rotatably supported by a main bearing (bearing)  18  provided in a bearing support member  17 . The front end of the rotating shaft  15  is provided with a crankpin  15 A at a position decentered from the axis of the rotating shaft  15  by a predetermined dimension ρ. 
     The bearing support member  17  is securely supported by the motor housing  4  by means of bolts  41 . 
     On the other hand, a scroll compression mechanism  20  is provided within the compressor housing  3 . This scroll compression mechanism  20  is a known compression mechanism formed by engaging a fixed scroll member  21  and orbiting scroll member  22  pair with each other, and is configured to compress a refrigerant gas by causing a compression chamber  23  formed between the scroll members  21  and  22  to be displaced while reducing its volume from the outer periphery toward the center due to an orbiting motion of the orbiting scroll member  22 . 
     The fixed scroll member  21  is fixed to the base of the compressor housing  3  by means of a bolt  24 , and a discharge chamber  25  is formed between a back surface of an end plate of the fixed scroll member  21  and the base of the compressor housing  3 . The refrigerant gas compressed to a high-temperature high-pressure state is discharged into this discharge chamber  25  from the compression chamber  23  via a discharge hole  26  and a discharge valve  27 , and this refrigerant gas is discharged outward from the discharge chamber  25  via a discharge port  28 . 
     A back surface of an end plate of the orbiting scroll member  22  is supported by a thrust surface of the bearing support member  17 , and the crankpin  15 A of the rotating shaft  15  is linked with a boss section  29 , provided at the back surface of the end plate, via the needle bearing  30  and a drive bush  31 . Rotation of the rotating shaft  15  causes the orbiting scroll member  22  to revolve in an orbit based on an orbital radius ρ relative to the fixed scroll member  21 . The orbiting scroll member  22  is prevented from self-rotating by an Oldham ring  32  interposed between the back surface of the end plate and the bearing support member  17 . 
     The drive bush  31  is integrally provided with a balance weight  33  for cancelling out an unbalanced load occurring when the orbiting scroll member  22  is driven in an orbit. 
     As shown in  FIG. 1  and  FIGS. 2A to 2C , the radial needle bearing  30  according to this embodiment is constituted of a cylindrical outer ring  30 A, a plurality of (needle-shaped) rollers  30 B disposed inside the outer ring  30 A, and a cage  30 C that holds these rollers  30 B at a predetermined pitch. 
     In this embodiment, a radial clearance (outer-ring orbital diameter (outer-ring inner diameter)−inner-ring orbital diameter (inner-ring inner diameter)−2×roller outer-diameter) is set between 10 μm and 30 μm. 
     The outer ring  30 A and the cage  30 C may be of known types, and detailed descriptions thereof will be omitted here. 
     Regarding each roller  30 B, the circularity of crowning sections  30 D provided at opposite ends thereof is 0.6 μm or smaller (0.2 μm or greater, or 0.0 μm or greater). A radial needle bearing fitted with rollers in which the circularity, measured at positions corresponding to 10% to 20% of the overall length of each roller  30 B from extremities  30 E thereof, is varied within a range between 0.2 μm and 1.5 μm was rotated by itself so as to measure the sound produced by the radial needle bearing. As a result, with the radial needle bearing  30  according to this embodiment, the rotation sound (noise) can be reduced as compared with a radial needle bearing fitted with rollers in which the circularity of the crowning sections  30 D provided at the opposite ends thereof is between 1.1 μm and 1.5 μm. 
     On the other hand, an electric compressor fitted with a radial needle bearing that includes rollers in which the circularity, measured at positions corresponding to 10% to 20% of the overall length of each roller  30 B from extremities  30 E thereof, is varied within a range between 0.2 μm and 1.5 μm was rotated so as to measure the sound produced by the electric compressor. The results obtained are shown in  FIG. 4 . Specifically, with the electric compressor  1  fitted with the radial needle bearing  30  that includes the rollers  30 B according to this embodiment, the rotation sound (noise) can be reduced by about 3.5 dB as compared with an electric compressor fitted with a radial needle bearing that includes rollers in which the circularity of the crowning sections  30 D provided at the opposite ends thereof is between 1.1 μm and 1.5 μm. 
     With the radial needle bearing  30  according to this embodiment, after measuring the sound produced by the radial needle bearing  30  fitted with the rollers  30 B while rotating the radial needle bearing  30  by itself, it was demonstrated that the rotation sound (noise) can be reduced as compared with a radial needle bearing equipped with rollers in which the circularity of crowning sections provided at opposite ends thereof is between 1.1 μm and 1.5 μm. 
     Furthermore, with the electric compressor  1  equipped with the radial needle bearing  30  according to this embodiment, after measuring the sound produced by the electric compressor  1  fitted with the radial needle bearing  30  while rotating the electric compressor  1 , the results shown in  FIG. 4  were obtained. Specifically, with the electric compressor  1  equipped with the radial needle bearing  30  according to this embodiment, the rotation sound (noise) can be reduced by about 3.5 dB as compared with an electric compressor fitted with a radial needle bearing that includes rollers in which the circularity of crowning sections provided at opposite ends thereof is between 1.1 μm and 1.5 μm. 
     The present invention is not limited to the above-described embodiment, and modifications and alterations are permissible where appropriate. 
     Furthermore, the present invention is applicable not only to electric compressors but also to other rotational machines that require a radial needle bearing.