JP5807436B2 - Bearing device design method and bearing device - Google Patents

Description

  The present invention relates to a bearing device design method and a bearing device that rotatably supports a rotating shaft on a housing via a cylindrical bearing member. More specifically, the bearing member has a bearing portion having bearing gaps at both ends in the axial direction. And a design method and a bearing device of a semi-float type bearing device that is restricted in rotation relative to a housing.

  Conventionally, as a bearing device that rotatably supports a rotor shaft in a supercharger, a cylindrical bearing member used as a floating bush disposed between the rotor shaft and the bearing housing is restricted from rotating with respect to the bearing housing. A semi-float type bearing device may be used (see, for example, Patent Documents 1 and 2 below).

  The bearing member used in this semi-float type bearing device is provided with a bearing gap between the rotor shaft at both ends in the axial direction, and an oil film is formed by the lubricating oil supplied to the bearing gap, thereby providing a bearing function. Is demonstrated.

JP-A-9-242553 JP 2010-133530 A

  By the way, the lubricating oil supplied to the bearing gap is discharged to the outside through an oil passage formed in the bearing housing after being used for lubrication, so that the lubricating oil is smoothly discharged. In order to achieve this, it is necessary to allow lubricating oil at an optimal flow rate to flow in the bearing gap.

  That is, if the amount of lubricating oil flowing through the bearing gap is too large, the lubricating oil after lubrication may flow out into the turbine housing or the compressor housing through the seal portions at both ends in the axial direction. Although the outflow to the inside can be suppressed, the function as a bearing is impaired due to incomplete lubrication.

  Therefore, for example, the amount of lubricating oil flowing through the bearing gap can be adjusted by changing the shape of the oil passage in the bearing housing so that the lubricating oil flows smoothly from the bearing gap to the outlet of the bearing housing. However, since the space in the bearing housing is limited, it is difficult to change the shape of the oil passage.

  In view of the above, an object of the present invention is to allow lubricating oil having an optimum flow rate to flow in the bearing gap without changing the shape in the bearing housing.

  The present invention relates to a design method of a bearing device that rotatably supports a rotating shaft on a housing via a cylindrical bearing member, and the bearing member has a bearing gap between the rotating shaft at both ends in the axial direction. A semi-float type bearing member that includes a bearing portion that is restricted in rotation relative to the housing, and is a cross-section in a direction perpendicular to the axial length L of the bearing portion and the axial direction of the bearing gap The relationship with the area S is such that the following equation is satisfied.

y = ax + b
However, a = 0.3 ± 0.1, b = 0.7 ± 0.1, and x is the ratio of the axial length L to the reference value L0 of the axial length L of the bearing portion (x = L / L0), y is the ratio of the area S of the bearing gap to the reference value S0 of the area S of the bearing gap (y = S / S0).

  According to the present invention, the relationship between the axial length L of the bearing portion and the area S of the bearing gap is expressed by the ratio of the axial length L to the reference value L0 of the axial length L of the bearing portion (L / L0). Where x is the ratio of the bearing clearance area S to the reference value S0 of the bearing clearance area S (S / S0), and y = ax + b, where a = 0.3 ± 0.1, b = 0. 7 ± 0.1], it is possible to allow the lubricating oil to flow at an optimum flow rate in the bearing gap without changing the shape in the bearing housing.

It is sectional drawing to which the principal part of the bearing apparatus of FIG. 2 was expanded. It is sectional drawing of a supercharger provided with the bearing apparatus concerning one Embodiment of this invention. (A) is sectional drawing of the semi-floating metal used for the bearing apparatus of FIG. 2, (b) is AA sectional drawing of FIG. It is a graph which shows the relationship between the bearing width of the bearing part of the semi-floating metal used for the bearing apparatus of FIG. 2, and the cross-sectional area of a bearing clearance gap.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 2, a turbocharger 1 according to an embodiment of the present invention has a turbine impeller 2 that is rotated by exhaust from, for example, an automobile engine (not shown), and the turbine impeller 2 serves as a rotating shaft. The compressor impeller 4 rotates integrally with the rotor shaft 3 to supercharge the air supplied to the engine.

  The turbine impeller 2 is rotatably accommodated in a turbine housing 5, and a plurality of turbine blades 7 are provided along the circumferential direction on the outer peripheral side of the turbine wheel 6 at the center. The rotor shaft 3 is integrally connected to the center of the end of the turbine wheel 6 on the compressor impeller 4 side.

  The turbine housing 5 described above has an exhaust intake (not shown) for taking in exhaust from the engine at an appropriate position. The exhaust intake port can be connected to an engine exhaust manifold (not shown). In addition, a turbine scroll passage 8 communicating with the above-described exhaust intake port is formed inside the turbine housing 5 so as to surround the turbine impeller 2. Further, an exhaust outlet 9 for discharging the exhaust gas flowing into the turbine scroll flow path 8 to the outside is formed outside the turbine impeller 2 in the turbine housing 5 in the axial direction (left side in FIG. 2). That is, the turbine scroll flow path 8 and the exhaust discharge port 9 communicate with each other within the turbine housing 5.

  On the other hand, the compressor impeller 4 is rotatably accommodated in the compressor housing 10, and a plurality of compressor blades 12 are provided along the circumferential direction on the outer peripheral side of the compressor wheel 11 at the center.

  An air suction port 13 for sucking air is formed outside the compressor impeller 4 in the compressor housing 10 in the axial direction (right side in FIG. 2). The air suction port 13 is connected to an air cleaner (not shown). Is possible.

  A bearing housing 15 as a housing is provided between the turbine housing 5 and the compressor housing 10. An annular diffuser passage 16 for increasing the pressure of the compressed air is formed on the outer peripheral side (outlet side) of the compressor impeller 4 between the bearing housing 15 and the compressor housing 10. It communicates with the suction port 13.

  Further, a compressor scroll passage 17 is formed inside the outer periphery of the compressor housing 10 so as to surround the compressor impeller 4, and the compressor scroll passage 17 communicates with the diffuser passage 16. That is, the compressor scroll passage 17 and the air inlet 13 are communicated with each other by the diffuser passage 16.

  An air discharge port (not shown) for discharging compressed air is formed at an appropriate position of the compressor housing 10, and this air discharge port communicates with the compressor scroll flow path 17, It can be connected to an intake manifold (not shown).

  A through-hole 11a that penetrates in the extending direction (axial direction) of the rotor shaft 3 is formed at the center of the compressor wheel 11 described above, and the rotor shaft 3 is inserted into the through-hole 11a so as to penetrate the rotor shaft 3. A nut 18 is fastened and fixed to the male screw portion 3a on the end portion protruding outward from the hole 11a.

  Here, as shown in FIG. 1 in an enlarged manner, the rotor shaft 3 has a journal portion 3b at a position corresponding to the bearing housing 15 via a semi-floating metal 20 as a bearing member constituting the radial bearing 19. The housing 15 is rotatably supported. Further, the rotor shaft 3 is rotatably supported with respect to the bearing housing 15 by a thrust bearing 21 in a vicinity of the compressor impeller 4.

  As shown in FIG. 3, the semi-floating metal 20 has a cylindrical shape as a whole, and has bearing portions 22 and 23 that are thicker than other portions (center portions) at both axial end portions. Between the bearing portions 22 and 23 and the journal portion 3b are bearing gaps 25 and 27 in which an oil film is formed.

  As shown in FIG. 3A, the bearing portions 22 and 23 of the semi-floating metal 20 are formed as a thin portion 29 thinner than the bearing portions 22 and 23, and the rotor shaft 3 at a position corresponding to the thin portion 29 is The central small diameter portion 3c is smaller in diameter than the journal portion 3b at a position corresponding to the bearing portions 22 and 23.

  The outer diameter of the bearing portions 22 and 23 of the semi-floating metal 20 is larger than the outer diameter of the thin portion 29, and the inner diameter of the bearing portions 22 and 23 is smaller than the inner diameter of the thin portion 29. As a result, an inner circumferential gap 31 having a gap size larger than the bearing gaps 25 and 27 is formed between the thin portion 29 and the central small diameter portion 3 c of the rotor shaft 3, and between the thin portion 29 and the bearing housing 15. An outer peripheral gap 32 is formed in the outer periphery.

  In addition, a through hole 29 a that communicates between the outer peripheral gaps 31 and 32 is formed in the thin portion 29 in the vicinity of the upper bearing portion 23 in FIGS.

  Further, a pin insertion hole 29b is formed in the thin portion 29 of the semi-floating metal 20 almost immediately below the through hole 29a, and the thrust pin 33 is inserted into the pin insertion hole 29b. The thrust pin 33 is screwed and fixed to a female thread portion 15 a formed in the bearing housing 15 from a gap 34 formed in the bearing housing 15, whereby the semi-floating metal 20 is fixed to the bearing housing 15. Rotation is restricted. Note that the above-described gap 34 communicates with an oil drainage passage 35 formed in the lower portion of the bearing housing 15.

  The thrust pin 33 has a tip pin portion 33 a inserted so as to be movable with respect to the pin insertion hole 29 b, so that the semi-floating metal 20 is movable in the radial direction of the rotor shaft 3.

  Further, as shown in FIG. 3, grooves 22 a and 23 a having substantially semicircular cross sections extending along the axial direction are formed on the inner peripheral surfaces of the bearing portions 22 and 23 of the semi-floating metal 20 along the circumferential direction. A plurality (three in this case) are formed at equal intervals.

  As shown in FIG. 2, the bearing housing 15 is provided with a lubricating oil supply port 36 for supplying lubricating oil to the radial bearing 19 and the thrust bearing 21 at an upper portion opposite to the above-described oil discharging port 35. As shown in FIG. 1, the lower end of the lubricating oil supply passage 37 extending downward from the lubricating oil supply port 36 passes through the vertical communication passage 38 into the outer peripheral gap 32 between the bearing housing 15 and the semi-floating metal 20. Communicate.

  The vertical communication passage 38 has a smaller passage cross-sectional area than the lubricating oil supply passage 37, is located on the turbine housing 5 side of the lubricating oil supply passage 37, and has a through hole 29 a formed in the thin portion 29 of the semi-floating metal 20. Part and end overlap each other. Further, in the middle of the lubricating oil supply passage 37, one end of the lateral communication passage 39 communicates with the other end of the lateral communication passage 39 and communicates with the side portion of the thrust bearing 21.

  In the thrust bearing 21, a turbine side thrust ring 40, an intermediate thrust ring 41, and a compressor side thrust ring 42 are arranged in this order along the axial direction from the turbine impeller 2 side to the compressor impeller 4 side.

  The turbine-side thrust ring 40, the intermediate thrust ring 41, and the compressor-side thrust ring 42 are all bearing members formed in a substantially plate-like ring shape having a through hole into which the rotor shaft 3 is inserted at the center. .

  Here, the rotor shaft 3 includes a tip small-diameter portion 3d including the male screw portion 3a, which has a small diameter with respect to the journal portion 3b located in the bearing housing 15. That is, the journal portion 3b has a large diameter portion with respect to the small tip end diameter portion 3d, and the end portion of the journal portion 3b on the tip small diameter portion 3d side is a surface perpendicular to the axial direction of the rotor shaft 3. A surface 3e is provided.

  The intermediate thrust ring 41 is fitted and fixed to the tip small-diameter portion 3d so that the side surface of the inner circumferential side end portion of the intermediate thrust ring 41 in the thrust bearing 21 is substantially in contact with the abutting surface 3e. That is, the intermediate thrust ring 41 rotates integrally with the rotor shaft 3.

  A generally cylindrical oil draining member 43 is interposed between the intermediate thrust ring 41 and an end surface 11b of the compressor impeller 4 facing the inner peripheral side of the compressor wheel 11 in the axial direction. Therefore, by fastening the nut 18, the compressor impeller 4 has the intermediate thrust ring 41 and the oil draining member 43 sandwiched between the abutting surface 3 e of the journal portion 3 b of the rotor shaft 3 and the rotor shaft 3. 3 to be fastened and fixed.

  The oil draining member 43 includes an annular protrusion 43a that protrudes radially outward on the outer peripheral portion, and suppresses the outflow of the lubricating oil on the thrust bearing 21 side to the compressor impeller 4 side. An end portion on the inner peripheral side of the partition wall 45 that separates the compressor impeller 4 and the thrust bearing 21 is substantially in contact with the protrusion 43a of the oil draining member 43 so as to be rotatable. The side end is fixed to the inner wall of the bearing housing 15.

  On the other hand, the bearing housing 15 in the vicinity of the position corresponding to the abutting surface 3e of the journal portion 3b in the rotor shaft 3 is formed with an annular recess 15b that opens to the compressor impeller 4 side, and the thrust bearing 21 is provided in the recess 15b. The turbine side thrust ring 40 is fixed.

  At this time, the turbine-side thrust ring 40 has its inner peripheral surface opposed to the outer peripheral surface in the vicinity of the abutting surface 3e of the journal portion 3b so as to be relatively rotatable, and the side surface of the intermediate thrust ring 41 is Can be contacted so as to be relatively rotatable. Further, most of the intermediate thrust ring 41 is located in the recess 15b.

  On the other hand, the compressor side thrust ring 42 has its outer peripheral side fixed to the inner wall of the bearing housing 15. The compressor side thrust ring 42 is formed with an oil passage 47 through which the above-described lateral communication passage 39 communicates. The oil passage 47 is entirely formed such that an annular oil passage 47a formed as an annular groove on the surface opposite to the compressor impeller 4 and an end communicating with the annular oil passage 47a and the other end reaching the vicinity of the hot water draining member 43. An inclined oil passage 47b that is inclined, and an axial oil passage 47c having one end communicating with the other end of the inclined oil passage 47b and the other end opening toward the intermediate thrust ring 41 are provided.

  A plurality of the inclined oil passages 47b and the axial oil passages 47c are provided along the circumferential direction, and the lubricating oil that has flowed into the annular oil passage 47a from the lateral communication passage 39 is a plurality of the inclined oil passages 47b and the axial oil passages. Via 47c, it is supplied to a minute gap between the intermediate thrust ring 41 and the compressor side thrust ring 42 and lubricates between them.

  The thrust bearing 21 including the turbine-side thrust ring 40, the intermediate thrust ring 41, and the compressor-side thrust ring 42 has the compressor-side thrust ring 42 side exposed in the air gap 34 in the bearing housing 15 so that the above-described lubrication is performed. The supplied lubricating oil flows out into the gap 34 directly or through a discharge passage 44 formed in the lower part of the bearing housing 15. The air gap 34 communicates with an annular passage 48 opened on the outer peripheral surface of the rotor shaft 3 between the turbine impeller 2 and the radial bearing 19, and lubrication supplied from the lubricating oil supply passage 37 to the radial bearing 19. Oil flows out into the gap 34 through the annular passage 48.

  In the turbocharger configured as described above, the exhaust from the engine flows into the turbine scroll passage 8 from the exhaust intake port of the turbine housing 5, whereby the turbine impeller 2 rotates. The compressor impeller 4 rotates. As the compressor impeller 4 rotates, air is sucked from the air suction port 13 of the compressor housing 10 and compressed. The compressed air is discharged from the air discharge port via the diffuser flow path 16 and the compressor scroll flow path 17 to supercharge the air supplied to the engine.

  At this time, the rotor shaft 3 rotates while the radial bearing 19 receives a load in the diameter direction of the rotor shaft 3 while the thrust bearing 21 receives the load in the axial direction of the rotor shaft 3. Here, regarding the thrust bearing 21, when the rotor shaft 3 receives a load toward the turbine side (leftward in FIGS. 1 and 2), the intermediate thrust ring 41 slides against the turbine side thrust ring 40. A sliding bearing is formed by moving and exerts a bearing effect. On the other hand, when the rotor shaft 3 receives a load toward the compressor side (rightward in FIGS. 1 and 2), the intermediate thrust ring 41 slides with respect to the compressor side thrust ring 42. Configure to demonstrate the bearing effect.

  At this time, the lubricating oil is supplied to the lubricating oil supply passage 37, and a part of this lubricating oil is supplied to the radial bearing 19 through the longitudinal communication passage 38 to lubricate the radial bearing 19. That is, the lubricating oil flowing from the longitudinal communication path 38 toward the radial bearing 19 flows toward the outer side in the axial direction on the inner peripheral side and the outer peripheral side of the semi-floating metal 20 as indicated by arrows in FIG. Thereafter, the lubricating oil directed to one turbine impeller 2 flows toward the gap 34 through the annular passage 48, and the other lubricating oil enters a minute gap between the turbine side thrust ring 40 and the intermediate thrust ring 41, and mutually It flows toward the gap 34 after being lubricated. The lubricating oil that has flowed into the gaps 34 is discharged to the outside through the oil discharge port 35.

  Here, in the radial bearing 19, lubricating oil enters the bearing gaps 25 and 27 on the inner periphery of the bearing portions 22 and 23 of the semi-floating metal 20 to form an oil film, and this oil film causes the rotor shaft 3 to move to the semi-floating metal 20 ( Smooth rotation with respect to the bearing housing 15).

  Further, another part of the lubricating oil supplied to the lubricating oil supply passage 37 flows from the lateral communication passage 39 into the annular oil passage 47a of the compressor side thrust ring 42, and the inclined oil passage 47b and the axial oil passage 47c. Then, it flows into a minute gap between the intermediate thrust ring 41 and the compressor side thrust ring 42 and lubricates between them. The lubricating oil after this lubrication also flows out into the gap 34 of the bearing housing 15 and is then discharged to the outside through the oil outlet 35 at the lower end.

  The lubricating oil that lubricates the radial bearing 19 and the thrust bearing 21 in this way smoothly flows in the bearing housing 15 to the oil discharge port 35 while suppressing the outflow into the turbine housing 5 and the compressor housing 10 after lubrication. It is necessary to flow towards.

  Therefore, in this embodiment, the amount of lubricating oil flowing through the bearing gaps 25 and 27 on the inner periphery of the bearing portions 22 and 23 of the semi-floating metal 20 is optimized. In this embodiment, in order to optimize the amount of lubricating oil flowing through the bearing gaps 25 and 27, the bearing width L that is the axial length (the axial length on the inner peripheral side) of the bearing portions 22 and 23, and the groove The clearance cross-sectional area (cross-sectional area in the direction orthogonal to the axial direction) S of the bearing gaps 25 and 27 including 22a and 23a was considered.

  Since the bearing portions 22 and 23 have the same shape, only the bearing portion 22 will be described below.

  In other words, in the present embodiment, the bearing structure including the semi-floating metal 20 is set so that the relationship between the bearing width L of the bearing portion 22 and the clearance cross-sectional area S of the bearing clearance 25 satisfies the following expression shown by a straight line in FIG. Designing.

y = ax + b
However, a = 0.3 ± 0.1, b = 0.7 ± 0.1, x is the ratio of the bearing width L to the reference value L0 of the bearing width L (x = L / L0), and y is The ratio of the gap cross-sectional area S to the reference value S0 of the gap cross-sectional area S (y = S / S0).

Here, the reference value L0 of the bearing width L is 3.6 mm, and the reference value S0 of the gap cross-sectional area S is 0.6 mm ² .

The straight line [y = ax + b] in FIG. 4 is a point P corresponding to x = 1.00, y = 1.00, that is, bearing width L = L0 = 3.6 mm, clearance cross-sectional area S = S0 = 0.6 mm ^2. It is confirmed by experiments that the amount of lubricating oil flowing through the bearing gap 25 is optimal when L = 3.6 mm and S = 0.6 mm ² . That is, when L = 3.6 mm and S = 0.6 mm ² , the semi-floating metal 20 effectively functions as the radial bearing 19 due to the oil film formed in the bearing gap 25, and at the same time the seal portion at the axial end. Can be efficiently suppressed from flowing out to the turbine housing 5.

If the relationship between the bearing width L corresponding to the straight line [y = ax + b] and the clearance cross-sectional area S is based on the above L = 3.6 mm and S = 0.6 mm ² , the oil flowing through the bearing clearance 25 The amount can be secured constant as in the case of L = 3.6 mm and S = 0.6 mm ² . That is, if the relationship between the bearing width L corresponding to the straight line [y = ax + b] and the clearance sectional area S is satisfied, the semi-floating metal 20 effectively functions as the radial bearing 19 by the oil film formed in the bearing clearance 25, The outflow of the lubricating oil into the turbine housing 5 through the seal portion at the axial end can be efficiently suppressed.

  Such an effect can be similarly obtained for the bearing portion 23. That is, the semi-floating metal 20 effectively functions as the radial bearing 19 in the bearing portion 23, and at the same time, the outflow of the lubricating oil into the compressor housing 10 through the seal portion at the axial end portion can be efficiently suppressed. In particular, by suppressing the outflow of the lubricating oil into the turbine housing 5, the lubricating oil is heated by high-temperature exhaust gas flowing from the turbine scroll flow path 8 toward the exhaust outlet 9 and flows out as smoke. Can be suppressed.

The linear equation [y = ax + b] represents the oil flowing through the bearing gap 25 in the bearing structure using the actual values of the bearing width L and the gap cross-sectional area S at a plurality of other points in addition to the point P described above. The amount is created by confirming that a constant amount can be secured as in the case of L = 3.6 mm and S = 0.6 mm ² .

  Here, in designing the supercharger 1, when the design of the bearing structure of the radial bearing 19 is changed, it is considered that the bearing width L needs to be shortened with respect to the reference value L0 = 3.6 mm. . In this case, from the point P on the straight line [y = ax + b] in FIG. 4, a gap cross-sectional area S at an appropriate position in the left direction indicated by an arrow B in FIG. 4 is obtained along the straight line.

For example, when the bearing width L is 3.0 mm, y = ax + b = 0.3 (3.0 / 3.6) + 0.7 = 0.95, and y = S / 0.6. From S / 0.6 = 0.95, S = 0.57 mm ² . That is, when the bearing width L is 3.0 mm with respect to the bearing width reference value L0 = 3.6 mm and the clearance sectional area reference value S0 = 0.6 mm ² , the clearance sectional area S is 0.57 mm ² . Become.

  The reason why the bearing width L is shortened is to improve the mechanical loss. In this case, if the gap cross-sectional area S is kept constant, the amount of lubricating oil increases and there is a concern about the influence on the oil seal performance. However, by setting the gap cross-sectional area S to be small as described above, an increase in the amount of lubricating oil can be suppressed and ensured appropriately.

  On the contrary, when it is necessary to increase the bearing width L, an appropriate position in the right direction indicated by an arrow C in FIG. 4 from the point P on the straight line [y = ax + b] in FIG. The gap cross-sectional area S at is obtained in the same manner as described above.

  The reason why the bearing width L is increased is to increase the bearing load capacity. In this case, if the gap cross-sectional area S is kept constant, the amount of lubricating oil is reduced, which may affect the durability. By setting the gap cross-sectional area S as described above (the gap cross-sectional area S becomes larger), it is possible to suppress the decrease in the amount of lubricating oil and ensure the amount of lubricating oil appropriately.

  The clearance cross-sectional area S is changed by changing the cross-sectional area of the grooves 22a and 23a with the bearing gaps 25 and 27 other than the grooves 22a and 23a remaining unchanged. The change of the cross-sectional area by the grooves 22a and 23a can be easily performed by changing the shape and number thereof. At this time, the bearing gaps 25 and 27 other than the grooves 22a and 23a are not changed.

  Thus, in the present embodiment, the amount of lubricating oil flowing through the bearing gaps 25 and 27 of the radial bearing 19 using the semi-floating metal 20 without changing the shape of the bearing housing 15 when designing the turbocharger 1. The semi-floating metal 20 is effectively functioned as the radial bearing 19 at the same time, and at the same time, the outflow of lubricating oil into the turbine housing 5 and the compressor housing 10 through the seal portion at the axial end is efficiently suppressed. can do.

1 Turbocharger 2 Turbine impeller 3 Rotor shaft (rotary shaft)
4 Compressor impeller 15 Bearing housing (housing)
20 Semi-floating metal (bearing member)
22, 23 Bearing part 25, 27 Bearing gap L Bearing width (Axial length of bearing part)
S Gap cross-sectional area (area of cross section in the direction perpendicular to the axial direction of the bearing gap)

Claims (5)

A design method of a bearing device that rotatably supports a rotating shaft on a housing via a cylindrical bearing member, wherein the bearing member has a bearing portion having a bearing gap between the rotating shaft at both ends in the axial direction. A semi-float type bearing member that is rotationally restricted with respect to the housing, and an axial length L of the bearing portion and an area S of a cross section in a direction perpendicular to the axial direction of the bearing gap The bearing device design method is characterized in that the relationship of
y = ax + b
However, a = 0.3 ± 0.1, b = 0.7 ± 0.1, and x is the ratio of the axial length L to the reference value L0 of the axial length L of the bearing portion (x = L / L0), y is the ratio of the area S of the bearing gap to the reference value S0 of the area S of the bearing gap (y = S / S0). ^{2. The} bearing device according to claim 1, wherein the bearing device is designed such that a reference value L0 of the axial length L of the bearing portion is 3.6 mm and a reference value S0 of the area S of the bearing gap is 0.6 mm ² . Design method of bearing device. A bearing device that rotatably supports a rotating shaft on a housing via a cylindrical bearing member, the bearing member including a bearing portion having a bearing gap between the rotating shaft at both axial ends, It is a semi-float type bearing member whose rotation is restricted with respect to the housing, and a relationship between an axial length L of the bearing portion and an area S of a cross section in a direction orthogonal to the axial direction of the bearing gap. A bearing device that satisfies the following formula.
y = ax + b
However, a = 0.3 ± 0.1, b = 0.7 ± 0.1, and x is the ratio of the axial length L to the reference value L0 of the axial length L of the bearing portion (x = L / L0), y is the ratio of the area S of the bearing gap to the reference value S0 of the area S of the bearing gap (y = S / S0). The reference value L0 of the axial length of the bearing portion is 3.6 mm, the bearing device according to claim 3, wherein the reference value S0 of the area S of the bearing gap is 0.6 mm ^2.   The bearing member has a turbine impeller that is rotated by exhaust from the engine at one end, and a rotating shaft that has a compressor impeller that rotates integrally with the turbine impeller at the other end. 5. The bearing device according to claim 3, wherein the bearing device is provided in a supercharger that is inserted in the turbocharger and supercharges air supplied to the engine by rotation of the compressor impeller. 6.

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