Seal structure and hub unit bearing

Abstract

Provided is a seal structure capable of reducing seal torque. A seal structure 3 comprises: at least one seal lip 23; at least one lip sliding-contact surface 24; and grease 25. The at least one seal lip 23 and the at least one lip sliding-contact surface 24 slide by means of an oil film of the grease 25. The at least one lip sliding-contact surface 24 has a plurality of recesses 35. With respect to the plurality of recesses 35, the average value of the difference in height between the bottom sections and the opening edge sections is 1-10 μm inclusive, and the percentage of the total opening area of the plurality of recesses 35 to a certain area of the at least one lip sliding-contact surface 24 is 30% or more but less than 100%.

Description

Seal structure and hub unit bearing

[0001] The present disclosure relates to a seal structure and a hub unit bearing.

[0002] An automobile wheel is rotatably supported relative to a suspension by a hub unit bearing, which includes an outer ring having a double-row outer ring raceway on its inner peripheral surface, a hub having a double-row inner ring raceway on its outer peripheral surface, and a plurality of rolling elements disposed freely rollably between the double-row outer ring raceway and the double-row inner ring raceway.

[0003] To prevent the intrusion of muddy water and the like from the outside, the hub unit bearing is provided with a seal structure at the opening of the internal space that exists between the inner peripheral surface of the outer ring and the outer peripheral surface of the hub.

[0004] Japanese Patent Application Laid-Open Publication No. 2022-155415 discloses a seal structure in which an opening of an internal space of a hub unit bearing is closed by sliding a plurality of seal lips against lip sliding surfaces.

[0005] In addition to hub unit bearings, single-row rolling bearings, double-row rolling bearings, and other bearings may also be provided with a seal structure at the opening of the internal space. Also, in various rotating machines, a seal structure may be provided at the boundary between the external space and the internal space.

[0006] Japanese Patent Application Laid-Open No. 2022-155415

[0007] In recent years, there has been an increasing demand for lower torque in hub unit bearings, and there is a demand for reduced seal torque in the seal structure of hub unit bearings, as well as in other bearings and various rotating machines.

[0008] An object of the present disclosure is to provide a seal structure that can reduce seal torque, and a hub unit bearing that includes the seal structure.

[0009] A seal structure according to one aspect of the present disclosure includes at least one seal lip, at least one lip sliding surface, and grease, and the at least one seal lip and the at least one lip sliding surface slide against each other via an oil film of the grease.

[0010] The at least one lip sliding surface has a plurality of recesses.

[0011] The average height difference between the bottom and the opening edge of the plurality of recesses is 1 μm or more and 10 μm or less.

[0012] The ratio of the total opening area of ​​the plurality of recesses to the fixed area of ​​the at least one lip sliding contact surface is equal to or greater than 30% and less than 100%.

[0013] In the seal structure according to one aspect of the present disclosure, the ratio can be preferably 30% or more and 90% or less, and more preferably 30% or more and 80% or less.

[0014] In the seal structure according to one aspect of the present disclosure, at least a portion of the opening edge of the recess can be raised above a reference surface of the lip sliding surface where the recess is not formed.

[0015] Alternatively, in a seal structure according to one aspect of the present disclosure, the entire opening edge of the recess can be positioned at the same height as a reference surface of the lip sliding surface on which the recess is not formed.

[0016] In the seal structure according to one aspect of the present disclosure, the kinematic viscosity of the base oil constituting the grease is set to 5.0 mm at 40°C. ２ / s or more 460.0mm ２ / s or less, and 1.9 mm at 100 °C ２ / s or more 56.0mm ２ The kinematic viscosity of the base oil is preferably 9.0 mm / s or less at 40°C. ２ / s or more 75.0mm ２ / s or less, and 2.0 mm at 100°C ２ / s or more 11.0mm ２ / s or less, and more preferably 9.0 mm ２ / s or more 30.0mm ２ / s or less, and 2.0 mm at 100°C ２ / s or more 6.0mm ２ / s or less.

[0017] In the seal structure according to one aspect of the present disclosure, the grease may be a urea-based grease having a consistency of 200 or more and 300 or less.

[0018] A hub unit bearing according to an aspect of the present disclosure includes a seal structure according to an aspect of the present disclosure.

[0019] According to the seal structure according to one aspect of the present disclosure, the seal torque can be reduced.

[0020] FIG. 1 is a half-sectional view showing a seal structure and a hub unit bearing according to a first example of an embodiment of the present disclosure. FIG. 2 is a cross-sectional view (corresponding to an enlarged view of portion X in FIG. 1 ) showing an example of the seal structure of the first example. FIG. 3 is a schematic diagram of the lip sliding surface of the seal structure of the first example, viewed from the front. FIG. 4 is a cross-sectional view taken along line Z-Z in FIG. 3. FIG. 5 is a cross-sectional view (corresponding to an enlarged view of portion Y in FIG. 1 ) showing another example of the seal structure of the first example. FIG. 6 is a schematic diagram used to explain the mechanism by which cavitation occurs in the seal structure of the first example. FIGS. 7(A) to 7(I) are plan views showing nine examples of recess opening shapes. FIGS. 8(A) and 8(B) are diagrams showing two examples in which multiple recesses are regularly arranged on the circumferential surface. FIGS. 9(A) to 9(C) are diagrams showing three examples in which multiple recesses are regularly arranged on the axial side surface. Figures 10(A) and 10(B) are diagrams showing two examples in which a plurality of recesses are arranged so that the ratio of the total opening area of ​​the plurality of recesses to a fixed area of ​​the lip sliding surface is large. Figure 11 is a diagram corresponding to Figure 4 showing a seal structure according to a second example of an embodiment of the present disclosure. Figure 12 is a schematic diagram showing a state in which cavitation occurs in the seal structure of the second example. Figure 13 is a diagram corresponding to Figure 12 showing a seal structure according to a third example of an embodiment of the present disclosure.

[0021] [First Example] A seal structure 3 and a hub unit bearing 1 according to a first example of an embodiment of the present disclosure will be described with reference to Figs. 1 to 10(B).

[0022] [Sealing Structure] The sealing structure 3 of this example is a machine element having the function of preventing muddy water and the like from entering the internal space 2 from the outside, and can be applied to bearings such as hub unit bearings 1, single-row rolling bearings, double-row rolling bearings, or various rotating machines. Note that, although not limited thereto, the following description will refer to an example in which the sealing structure 3 is applied to a sealing structure that closes an axially inner opening of the internal space 2 of an inner-ring-rotating hub unit bearing 1 for a driving wheel, as shown in FIG.

[0023] The seal structure 3 includes at least one seal lip 23, at least one lip sliding surface 24, and grease 25. The at least one seal lip 23 and the at least one lip sliding surface 24 slide against each other via an oil film of the grease 25. The basic configurations of the seal lip 23, lip sliding surface 24, and grease 25 that constitute the seal structure 3 are the same as those of the conventional seal structure. The seal lip 23 is installed on one of the rotating element or non-rotating element that constitutes a bearing or various rotating machines, and the lip sliding surface 24 is installed on the other of the rotating element or non-rotating element.

[0024] If the device to which the seal structure 3 is applied has a structure in which rotating elements and non-rotating elements are arranged radially, such as the hub unit bearing 1, the rotating elements are formed by one of the inner member or the outer member, and the non-rotating elements are formed by the other of the inner member or the outer member. If the device to which the seal structure 3 is applied has a structure in which rotating elements and non-rotating elements are arranged axially, the rotating elements are formed by members on one axial side, and the non-rotating elements are formed by members on the other axial side.

[0025] At least one seal lip 23 is supported and fixed to the one element, or is configured as a part of a seal member 29 that constitutes a seal ring 26 supported and fixed to the one element. The seal lip 23 or the seal member 29 including the seal lip 23 is configured from an elastic material (sealing material) such as rubber or thermoplastic elastomer. The structure and arrangement of each seal lip 23 are arbitrarily set depending on the location where the seal lip 23 is installed on the one element and the positional relationship with the lip sliding surface 24 installed on the other element.

[0026] At least one lip sliding surface 24 is formed on the circumferential surface and / or side surface of the other element that faces the seal lip 23, or is provided on a separate member that is supported and fixed to the circumferential surface and / or side surface of the other element. That is, the lip sliding surface 24 is constituted by a circumferential surface facing in the radial direction and / or a side surface facing in the axial direction.

[0027] In either case, the lip sliding surface 24 preferably has good surface properties to ensure sliding properties with the seal lip 23. When the lip sliding surface 24 is provided on a separate member such as the slinger 27, the separate member is preferably made of a metal plate having rust-resistant properties, such as a ferritic stainless steel plate (SUS430) or an austenitic stainless steel plate (SUS304). When the lip sliding surface 24 is formed directly on the peripheral surface and / or side surface of the other element, the peripheral surface and / or side surface are preferably subjected to a surface treatment.

[0028] In the seal structure 3, the seal lip 23 and the lip sliding surface 24 slide against each other via an oil film of grease 25. Therefore, as is well known in the technical field of oil seals, the seal structure 3 has a dimensionless characteristic coefficient G (= μ u b / P ｒ ) is 2 x 10 －８ and the fluid lubrication state is reached. ｒ : sealing lip tension force (N {kgf}) μ: sealing fluid viscosity (N s / cm ２ {kgf·s / cm ２}) u: peripheral speed (cm / s) b: contact width of seal lip (cm)

[0029] Under hydrodynamic lubrication conditions, if Φ is a constant determined by the state of the oil film, the relationship between the friction coefficient f and the dimensionless characteristic coefficient G is f = ΦG １／３ This is expressed by the following formula. Therefore, the friction coefficient f increases as the 1 / 3 power of the viscosity (μ) of the sealed fluid. Therefore, if the viscosity of the sealed fluid can be reduced, it will be possible to reduce the friction coefficient.

[0030] The seal structure 3 of this example is particularly characterized in that at least one lip sliding surface 24 has a plurality of recesses 35. That is, when the seal structure 3 includes a plurality of seal lips 23, at least one of the plurality of seal lips 23 slides against the lip sliding surface 24 having a plurality of recesses 35 via an oil film of grease 25. All of the plurality of lip sliding surfaces 24 corresponding to the plurality of seal lips 23 may have a plurality of recesses 35, or only some of the plurality of lip sliding surfaces 24 may have a plurality of recesses 35. That is, the number of seal lips 23 and the number of lip sliding surfaces 24 are determined appropriately depending on the application and required performance of the device to which the seal structure 3 is applied, the arrangement of the seal structure 3 in the device, and the like. Furthermore, when a plurality of lip sliding surfaces 24 are present corresponding to a plurality of seal lips 23, which lip sliding surface 24 should have a plurality of recesses 35 is also determined appropriately depending on the application and required performance of the device.

[0031] When the seal lip 23 and the lip sliding surface 24 slide against each other via an oil film of grease 25, the plurality of recesses 35 have the function of reducing the pressure of the oil film of grease 25 passing through the recesses 35 and increasing the pressure of the oil film of grease 25 at the position of the opening edge (ridge) on the front side of the recess 35 in the direction of movement of the seal lip 23. When the pressure of the oil film in the recess 35 falls below the cavitation pressure, cavitation occurs within the recess 35, and bubbles 45 are generated in the oil film.

[0032] When bubbles 45 are generated in the oil film, the viscosity of the grease 25 decreases, and the shear resistance of the grease 25 decreases. This provides the same effect as when the viscosity of the base oil constituting the grease 25 is reduced. In other words, the apparent viscosity of the base oil constituting the grease 25 can be reduced.

[0033] In the seal structure 3 of this example, by providing a plurality of recesses 35 on the lip sliding surface 24, when the seal lip 23 and the lip sliding surface 24 slide against each other via an oil film of the grease 25, the sliding resistance of the grease 25 is reduced and the seal torque is reduced.

[0034] The configuration (shape, size, number) of the plurality of recesses 35 is appropriately set within a range in which the function of generating cavitation is achieved, depending on the application and required performance of the bearing or rotary machine to which the seal structure 3 of one embodiment of the present disclosure is applied. However, in the seal structure 3 of this example, from the viewpoint of appropriately exerting this function, the configuration of the plurality of recesses 35 is controlled by the average value of the height difference between the bottom and the opening edge of the plurality of recesses 35 and the proportion of the total opening area of ​​the plurality of recesses to a certain area of ​​the at least one lip sliding surface.

[0035] In the seal structure of this example, the average height difference between the bottom and the opening edge of the plurality of recesses 35 is 1 μm or more and 10 μm or less. Also, the ratio of the total opening area of ​​the plurality of recesses 35 to the fixed area of ​​at least one lip sliding surface 24 is 30% or more and less than 100%.

[0036] Below, a more specific structure of the seal structure 3 of this example will be explained with reference to an example in which the seal structure 3 is applied to an inner ring rotating type seal structure that blocks the axially inner opening of the internal space 2 of a hub unit bearing 1 for a drive wheel, and the mechanism by which the apparent viscosity of the grease 25 decreases will be explained.

[0037] The seal structure 3 of this example includes at least one seal lip 23 and at least one lip sliding surface 24. The number of seal lips 23 is set appropriately depending on the application of the device to which the seal structure 3 is applied, and is not limited thereto, but is preferably between one and three. Depending on the device to which the seal structure 3 is applied, if the number of seal lips 23 is more than three, the seal torque may increase. However, in the present disclosure, the number of seal lips 23 may be four or more.

[0038] When the at least one seal lip 23 is constituted by a single seal lip 23 , one lip sliding contact surface 24 with which the single seal lip 23 slides has a plurality of recesses 35 .

[0039] When at least one seal lip 23 is composed of multiple seal lips 23, all of the multiple lip sliding surfaces 24 against which the multiple seal lips 23 slide can be configured to have multiple recesses 35, or only some of the multiple lip sliding surfaces 24 can be configured to have multiple recesses 35.

[0040] Furthermore, at least one seal lip 23 may be either a radial lip that is in sliding contact with a radially facing circumferential surface, i.e., an outer or inner circumferential surface, or a thrust lip that is in sliding contact with an axially facing side surface. When at least one seal lip 23 is configured as a radial lip, at least one lip sliding contact surface 24 is configured as a radially facing circumferential surface, and when at least one seal lip 23 is configured as a thrust lip, at least one lip sliding contact surface 24 is configured as a axially facing side surface.

[0041] In the seal structure 3 of this example, at least one seal lip 23 is made up of three seal lips 23a to 23c, and at least one lip sliding surface 24 is made up of three lip sliding surfaces 24a to 24c. All of the three lip sliding surfaces 24a to 24c have a plurality of recesses 35. Note that Figures 1 and 2 show the shapes of the seal lips 23a to 23c in their free states.

[0042] The seal lips 23a to 23c are provided on a seal member 29 of the seal ring 26. The seal ring 26 includes a core metal 28 in addition to the seal member 29.

[0043] The core metal 28 is made of a metal plate such as a mild steel plate, and has a substantially L-shaped cross section.

[0044] The core wire 28 comprises a fixed tube portion 30 having a cylindrical shape that is fitted and fixed to the axially inner end portion of the outer member 5, and an inward flange-shaped fixed circular ring portion 31 that extends radially inward from the axially outer end portion of the fixed tube portion 30.

[0045] The fixed ring portion 31 in this example has a substantially crank-shaped cross section.

[0046] The entire seal member 29 is made of an elastic material (sealing material) such as rubber or thermoplastic elastomer, and has a seal base 32 in addition to three seal lips 23a to 23c.

[0047] The seal base 32 covers the surface of the core metal 28. Specifically, the seal base 32 covers the inner peripheral surface, the axially inner end portion and axially inner end face of the outer peripheral surface of the fixed cylindrical portion 30, and also covers the axially inner side surface, the radially inner end portion and inner peripheral surface of the axially outer peripheral surface of the fixed circular ring portion 31.

[0048] The three seal lips 23 a to 23 c are provided on the radially inner side of the seal ring 26 , and their respective base ends are connected to the seal base 32 .

[0049] Of the three seal lips 23a to 23c, the seal lip 23a located closest to the internal space 2 has its base end connected to a portion of the seal base 32 that covers the radially inner end of the fixed ring portion 31, and extends axially outward as it moves radially inward. The seal lip 23a is configured as a radial lip. Specifically, the radially inner end of the seal lip 23a, which is its tip, is in sliding contact with a lip sliding surface 24a, which is the outer peripheral surface. This type of seal lip 23a is called a grease lip, and its main function is to prevent bearing grease sealed in the internal space 2 from leaking into the external space.

[0050] Of the three seal lips 23a to 23c, the seal lip 23b, which is located adjacent to the seal lip 23a on the axially inner side, has its base end connected to a portion of the seal base 32 that covers the radially inner end of the fixed ring portion 31, and extends axially inward as it moves radially inward. The seal lip 23b is configured as a radial lip. The radially inner end of the seal lip 23b, which is its tip, is in sliding contact with the lip sliding surface 24b. This seal lip 23b is called a main lip, and prevents the bearing grease sealed in the internal space 2 from leaking into the external space and prevents foreign matter from entering the internal space 2 from the external space.

[0051] Of the three seal lips 23a to 23c, the seal lip 23c located farthest from the internal space 2 has its base end connected to a portion of the seal base 32 that covers the axially inner side surface of the fixed ring portion 31, and extends radially outward as it moves axially inward. The seal lip 23c is configured as a thrust lip. The axially inner end portion, which is the tip of the seal lip 23c, is in sliding contact with the lip sliding surface 24c. This type of seal lip 23c is called a side lip, and primarily prevents foreign matter from entering the internal space 2 from the external space.

[0052] The lip sliding surfaces 24 a to 24 c can be provided on the outer peripheral surface of the inner member 6 or on a separate member supported and fixed to the inner member 6 .

[0053] The lip sliding surfaces 24a to 24c are composed of a reference surface 36 where no recesses 35 are formed, and a plurality of recesses 35 formed on the reference surface 36. When the lip sliding surfaces 24a to 24c are composed of peripheral surfaces facing in the radial direction, the reference surface 36 is composed of a cylindrical surface centered on the central axis of the inner member 6 or the separate member, and when the lip sliding surfaces 24a to 24c are composed of side surfaces facing in the axial direction, the reference surface 36 is composed of a plane perpendicular to the central axis of the inner member 6 or the separate member.

[0054] In this example, all three lip sliding surfaces 24a to 24c correspond to the separate members and are provided on a slinger 27 that is fitted and fixed to the outside of the inner member 6. That is, the seal structure 3 of this example is composed of the seal lips 23a to 23c that constitute the seal ring 26 and the lip sliding surfaces 24a to 24c that constitute the slinger 27. The seal ring 26 and the slinger 27 constitute a combined seal ring.

[0055] The slinger 27 is an element that facilitates ensuring good surface quality at the portions where the seal lips 23a to 23c slide. The slinger 27 is formed by bending and shaping a rust-resistant metal plate, such as a ferritic stainless steel plate (SUS430) or an austenitic stainless steel plate (SUS304), into a circular ring shape and has a cross-sectional shape such as a substantially L-shape or a substantially U-shape. In this example, the slinger 27 has a substantially L-shape in cross section.

[0056] The slinger 27 has a rotating cylindrical portion 33 and a rotating circular ring portion 34 .

[0057] The rotary cylinder portion 33 has a cylindrical shape and is fitted and fixed to the outside of the inner ring 17 .

[0058] The rotating cylinder portion 33 has, on its outer peripheral surface, a lip sliding contact surface 24a against which the seal lip 23a slides, and a lip sliding contact surface 24b against which the seal lip 23b slides. That is, although the outer peripheral surface of the rotating cylinder portion 33 is configured as a cylindrical surface macroscopically (macroscopically), due to the presence of a plurality of recesses 35 provided on the lip sliding contact surface 24a and the lip sliding contact surface 24b, it is configured as a non-cylindrical surface microscopically (microscopically).

[0059] The rotary ring portion 34 has a disk shape and extends radially outward from the axially inner end of the rotary cylindrical portion 33 .

[0060] The rotating ring portion 34 has, on its axially outer surface, a lip sliding contact surface 24c with which the seal lip 23c slides. That is, although the axially outer surface of the rotating ring portion 34 is configured to be flat when viewed macroscopically, due to the presence of multiple recesses 35 provided in the lip sliding contact surface 24c, it is configured to be non-flat when viewed microscopically.

[0061] The range of the surface of the slinger 27 with which the seal lips 23a to 23c slide changes due to the relative displacement between the seal ring 26 and the slinger 27, as well as the elastic deformation and wear of the seal lips 23a to 23c. Therefore, the lip sliding contact surfaces 24a to 24c refer to the range with which the seal lips 23a to 23c may slide, including portions that may not be in sliding contact at the start of use but will come into sliding contact after a predetermined time has passed. In other words, the lip sliding contact surfaces 24a to 24c refer to portions with which the seal lips 23a to 23c may slide at any time during the use of the hub unit bearing 1.

[0062] The lip sliding contact surface 24a and the lip sliding contact surface 24b have a cylindrical reference surface 36 with a plurality of recesses 35. The lip sliding contact surface 24c has a planar reference surface 36 with a plurality of recesses 35.

[0063] The recesses 35 may be formed in any part of the lip sliding surfaces 24a to 24c with which the seal lips 23a to 23c slide. For example, the recesses 35 may be formed only on the lip sliding surfaces 24a to 24c of the surface of the slinger 27, or may be formed in a predetermined axial or radial range of the surface of the slinger 27 that includes the lip sliding surfaces 24a to 24c.

[0064] In this example, the entire outer peripheral surface of the rotating cylindrical portion 33, including the lip sliding contact surface 24a and the lip sliding contact surface 24b, is provided with a recess 35. In addition, the entire axially outer surface of the rotating annular portion 34, including the lip sliding contact surface 24c, is provided with a recess 35.

[0065] Each of the recesses 35 is composed of a depression 37 recessed below a reference surface 36 of the lip sliding surfaces 24a to 24c, and an opening edge (ridge) around the depression 37. In this example, the opening edge of the recess 35 is located at the same height as the reference surface 36. However, the recess 35 may be provided with a raised portion as an optional component, which is provided around the recess 35 and raised above the reference surface 36. In this case, the opening edge of the recess 35 is located at the top of the raised portion, which is located at a higher height than the reference surface 36.

[0066] In this example, each of the recesses 35 is composed only of a depression 37. That is, the opening edge of the recess 35 is located at the same height as the reference surface 36.

[0067] In the seal structure 3 of this example, the surface properties of the lip sliding surface 24 having a plurality of recesses 35 (the configuration of the plurality of recesses 35) are controlled by the average value of the height difference (H) between the bottom and the opening edge of the plurality of recesses 35, and the proportion of the total opening area of ​​the plurality of recesses 35 to the fixed area of ​​the lip sliding surface 24 having a plurality of recesses 35.

[0068] Specifically, for the plurality of recesses 35, the average value of the height difference (H) between the bottom and the opening edge is 1 μm or more and 10 μm or less, and preferably 2 μm or more and 5 μm or less. If the average value of the height difference (H) is less than 1 μm, the pressure of the oil film of the grease 25 cannot be sufficiently reduced at the position of the depression 37 of the recess 35, cavitation does not occur in the depression 37, and the viscosity of the grease 25 cannot be reduced. If the average value of the height difference (H) is greater than 10 μm, the processing time may become longer.

[0069] The method for determining the height difference (H) of the recesses 35 and its average value is not particularly limited, and any method can be used. For example, it can be determined as follows. First, the lip sliding surface 24 (in this example, each of the three lip sliding surfaces 24a to 24c) is scanned with a surface roughness meter to measure the roughness curve of the lip sliding surface 24. After measuring the roughness curve of the lip sliding surface 24, the height difference (H) (see FIG. 4) between the bottom and the opening edge of each recess 35 is determined. That is, for each recess 35, the height difference between the bottom, which is the deepest part of the recess 37, and the opening edge is determined. In this example, the height difference between the bottom of the recess 37 and the reference surface 36 is determined.

[0070] When there are a plurality of lip sliding surfaces 24, the average value of the height differences (H) for all the recesses 35 present is calculated for each lip sliding surface (in this example, for each of the three lip sliding surfaces 24a to 24c). When each of the plurality of lip sliding surfaces 24 has a plurality of recesses 35, the average value of the height differences (H) can be the same for all of the lip sliding surfaces 24, or can be different for each lip sliding surface 24. In this case, the surface properties of the plurality of lip sliding surfaces 24, each of which has a plurality of recesses 35, can also be managed collectively by the average value of the height differences (H) of the recesses 35 for all of the plurality of lip sliding surfaces 24.

[0071] The ratio of the opening area of ​​the plurality of recesses 35 to the constant area S of the lip sliding surface 24 is 30% or more and less than 100%. The ratio is preferably 30% or more and 90% or less, and more preferably 30% or more and 80% or less. If the ratio is less than 30%, the amount of bubbles 45 generated by cavitation is small, which may cause a problem in that the viscosity of the grease 25 cannot be sufficiently reduced.

[0072] The method for determining the proportion of the opening area occupied by the recesses 35 is not particularly limited, and any method can be used. For example, in the case where the opening shapes of the recesses 35 are all circular and have the same diameter, as in this example, the diameter D of the opening of the recesses 35 is determined based on the roughness curve of the lip sliding contact surfaces 24a to 24c measured with a surface roughness meter, and the opening area (A) of the recesses 35 is calculated from the diameter. Then, the proportion can be determined by multiplying this by the number of recesses 35 present within the fixed area S of the lip sliding contact surfaces 24a to 24c.

[0073] The opening shape, height difference (H), and size of each recess 35 are not particularly limited, as long as the average value of the height difference (H) can be restricted within a predetermined range and the ratio can be restricted within a predetermined range. The opening shape of each recess 35 can be any shape. For example, the opening shape of the recess 35 can be a circle ( FIG. 7(A) ), an ellipse ( FIG. 7(B) ), a triangle ( FIG. 7(C) ), a square ( FIG. 7(D) ), a rectangle ( FIG. 7(E) ), a partial ring ( FIG. 7(F) ), an arrowhead ( FIG. 7(G) ), or similar shapes. Alternatively, the opening shape of the recess 35 can be a sine wave ( FIG. 7(H) ), a triangular wave ( FIG. 7(I) ), or similar shapes.

[0074] The opening shapes of the recesses 35 may all be the same, or a mixture of different shapes may be used. The size of the recesses 35 may all be the same, or a mixture of different sizes may be used.

[0075] The width of the narrowest portion of the opening of the recess 35 can be any width as long as it is possible to generate cavitation within the recess 35, but it is preferably larger than the height difference (H) of the recess 35. For example, the width of the narrowest portion of the opening of the recess 35 can be preferably 30 μm or more and 100 μm or less. However, the width of the narrowest portion of the opening of the recess 35 can also be smaller than the height difference (H) of the recess 35.

[0076] The arrangement of the plurality of recesses 35 is not particularly limited and can be any arrangement as long as it can regulate the average value of the height difference (H) within a predetermined range and regulate the ratio within a predetermined range. Specifically, the plurality of recesses 35 can be arranged regularly or irregularly. In either case, it is preferable that the plurality of recesses 35 be formed so that the effect of reducing the viscosity of the grease 25 due to the occurrence of cavitation is approximately the same regardless of the circumferential position.

[0077] When the recesses 35 are arranged regularly, for example, the recesses 35 can be arranged at equal intervals.

[0078] More specifically, when the recesses 35 are arranged on the peripheral surface facing the radial direction, the recesses 35 can be arranged so as to be located at the intersections of a square lattice when viewed from the radial direction, as shown in Fig. 8(A) , or can be arranged so as to be located at the intersections of an equilateral triangular lattice when viewed from the radial direction, as shown in Fig. 8(B) .

[0079] When multiple recesses 35 are arranged on the side surface facing the axial direction, the recesses 35 can be arranged so as to be located at the intersections of a square lattice when viewed from the axial direction, as shown in Fig. 9(A) , or can be arranged so as to be located at the intersections of an equilateral triangular lattice when viewed from the axial direction, as shown in Fig. 9(B) .

[0080] When the recesses 35 are arranged at equal intervals, for example, when the openings of the recesses 35 are circular and have a diameter (D) of 50 μm, the pitch (P), which is the distance between the centers of adjacent recesses 35, can be 50 μm or more and 80 μm or less. In this case, when the sliding contact diameter, which is the diameter of the portion where the seal lip 23 slides, is 60 mm, the number of recesses 35 will be 2,300 or more and 3,768 or less.

[0081] Alternatively, when multiple recesses 35 are arranged regularly on the side surface facing the axial direction, they can also be arranged at multiple locations at equal radial intervals and at a fixed angle in the circumferential direction, as shown in Figure 9 (C).

[0082] 10(A) and 10(B), it is also possible to combine and arrange a plurality of types of recesses 35, each having a circular opening shape and different diameters, in order to increase the proportion of the total opening area of ​​the plurality of recesses 35 relative to the fixed area of ​​the lip sliding surface 24, specifically to about 90% or more (substantially less than 100%).

[0083] In this example, the recesses 35 each have a circular opening shape with the same diameter. The recesses 35 provided on the outer peripheral surface of the rotating cylindrical portion 33, including the lip sliding contact surface 24a and the lip sliding contact surface 24b, are arranged so as to be located at the intersections of a square lattice when viewed from the radial direction. The recesses 35 provided on the axial outer surface of the rotating annular portion 34, including the lip sliding contact surface 24c, are arranged so as to be located at the intersections of a square lattice when viewed from the axial direction.

[0084] The method for forming the recesses 35 is not particularly limited, and they can be formed by, for example, shot peening, laser processing, embossing transfer, etching, or the like.

[0085] Grease 25 is applied to the seal lip 23 to reduce seal torque. Although not limited thereto, the grease 25 is typically a seal grease that does not cause swelling or hardening of elastic materials (sealing materials) such as rubber or thermoplastic elastomer. In this example, the grease 25 is applied to the inner circumferential surfaces of each of the seal lips 23a to 23c. Therefore, the grease 25 lubricates the sliding contact portions between the seal lips 23a to 23c and the lip sliding surfaces 24a to 24c. In this example, the base oil of the grease 25 accumulates in an annular interlip space 38a formed in the area surrounded by the seal lip 23a, the seal lip 23b, and the outer circumferential surface of the fixed cylindrical portion 30. An annular interlip space 38b is also formed in the area surrounded by the seal lip 23b, the seal lip 23c, and the axially outer surface of the rotating annular portion 34.

[0086] The kinematic viscosity of the base oil of the grease is preferably lower than the kinematic viscosity of the base oil of the bearing grease sealed in the internal space 2 (see FIG. 1) formed in the bearing or various rotating machines. For this reason, it is preferable to use a grease (seal grease) having a different composition from the bearing grease as the grease 25. In this case, it is more preferable that the grease (seal grease) 25 is incompatible with the bearing grease. However, the composition of the grease 25 can also be the same as the composition of the bearing grease. In this example, a seal grease having a different composition from the bearing grease and incompatible with the bearing grease is used as the grease 25.

[0087] The kinematic viscosity of the base oil of the grease 25 is, but is not limited to, 5.0 mm at 40°C. ２ / s or more 460.0mm ２ / s or less, and 1.9 mm at 100 °C ２ / s or more 56.0mm ２ The kinematic viscosity of the base oil is preferably 9.0 mm / s or less at 40°C. ２ / s or more 75.0mm ２ / s or less, and 2.0 mm at 100°C ２ / s or more 11.0mm ２ / s or less, and more preferably 9 mm ２ / s or more 30mm ２ / s or less, and at 100°C, ２ / s or more 6mm ２ / s or less.

[0088] By setting the kinematic viscosity of the base oil of the grease 25 within the above range, it is possible to achieve an appropriate balance between ensuring lubrication at the sliding contact portions between the seal lips 23a to 23c and the lip sliding surfaces 24a to 24c and the generation of bubbles 45 due to cavitation.

[0089] The base oil of the grease 25 may be, but is not limited to, a mineral oil, a synthetic hydrocarbon oil, or a mixture of a mineral oil and a synthetic hydrocarbon oil.

[0090] The grease 25 may be, but is not limited to, a urea-based grease containing a urea compound as a thickener. The use of a urea-based grease makes it easier to repel water, thereby preventing water from entering from the outside. In this case, the consistency of the grease 25 may be 200 or more and 300 or less. If the consistency of the grease 25 is less than 200, the grease 25 may become too hard, which may result in a deterioration in the lubrication of the sliding contact portion between the seal lips 23a to 23c and the lip sliding surfaces 24a to 24c. If the consistency of the grease 25 is 300 or more, the grease 25 may become too soft, which may result in leakage of the grease 25 from the sliding contact portion between the seal lips 23a to 23c and the lip sliding surfaces 24a to 24c to the outside.

[0091] In the seal structure 3 of this example, the lip sliding surface 24 (24a to 24c) is provided with a plurality of recesses 35, so the pressure of the oil film of the grease 25 is low at the position of the recessed portion 37 of the recessed portion 35, as shown in Figure 6, and is high at the position of the opening edge (ridge portion) on the front side (left side in Figure 6) of the recessed portion 35 in the direction of movement of the seal lip 23 (23a to 23c). When the pressure of the oil film at the position of the recessed portion 37 falls below the cavitation pressure, cavitation occurs in the recessed portion 37, and air bubbles 45 are generated inside the recessed portion 37.

[0092] When bubbles 45 are generated in the oil film, the viscosity of the grease 25 decreases, and the shear resistance of the grease 25 decreases. This provides the same effect as when the viscosity of the base oil of the grease 25 is reduced. In other words, the apparent viscosity of the base oil of the grease 25 can be reduced. As a result, the sliding resistance at the position of the recessed portion 37 decreases, and the seal torque decreases.

[0093] In particular, in this example, the height difference of the recesses 35 and the proportion of the recesses 35 in the lip sliding contact surface 24 (24a to 24c) are restricted to values ​​that make it easy for cavitation to occur.

[0094] Specifically, since the average height difference between the bottom and the opening edge of the plurality of recesses 35 is 1 μm or more and 10 μm or less, the variation in the height difference of the recesses 35 is suppressed and the height difference of the recesses 35 is kept within a range that is likely to cause cavitation. Therefore, bubbles 45 can be effectively generated in the oil film, and the seal torque can be sufficiently reduced.

[0095] In this example, the ratio of the total opening area of ​​the recesses 35 to the fixed area of ​​the lip sliding surface 24 (24a to 24c) is 30% or more and 100% or less, so that the amount of bubbles 45 generated can be ensured, and therefore the effect of reducing the seal torque can be sufficiently obtained.

[0096] [Structure of Hub Unit Bearing] As described above, the seal structure 3 of this example can be suitably applied to bearings such as single-row rolling bearings and double-row rolling bearings, as well as various rotating machines, in addition to the hub unit bearing 1. When the seal structure 3 of this example is applied to the seal structure of the hub unit bearing 1, it may be applied to either the seal structure of a hub unit bearing for a driving wheel or the seal structure of a hub unit bearing for a driven wheel. In this case, the hub unit bearing can adopt either an inner-ring rotating type structure in which the outer member constitutes the stationary ring and the inner member constitutes the rotating ring, or an outer-ring rotating type structure in which the outer member constitutes the rotating ring and the inner member constitutes the stationary ring.

[0097] Below, a hub unit bearing 1 to which the seal structure 3 of this example is applied will be described using an inner ring rotating type hub unit bearing for a drive wheel as an example. The hub unit bearing 1 is provided with two seal structures that close openings on both axial sides of the internal space 2 to prevent leakage of bearing grease (not shown) sealed in the internal space 2 and to prevent foreign matter such as muddy water from entering the internal space 2. The seal structure 3 of this example can be applied to both of the two seal structures, or to only one of the two seal structures. Which seal structure the seal structure 3 of this example is applied to is determined depending on the performance required of the hub unit bearing 1, etc.

[0098] In the hub unit bearing 1 of this example, the seal structure 3 of this example is applied only to the seal structure that closes the opening on the axially inner side of the internal space 2, and is not applied to the seal structure 4 that closes the opening on the axially outer side of the internal space 2. However, additionally or alternatively, the seal structure 3 of this example can also be applied to the seal structure that closes the opening on the axially outer side of the internal space 2.

[0099] In addition to the seal structures 3 and 4, the hub unit bearing 1 of this example further comprises an outer member 5, an inner member 6, and a plurality of rolling elements 7a and 7b.

[0100] In the following description of the hub unit bearing 1, the left side of Figures 1, 2 and 5, which is located on the outer side in the width direction of the vehicle when the hub unit bearing 1 is assembled to the vehicle, is referred to as the outer axial side, and the right side of Figures 1, 2 and 5, which is located on the central side in the width direction of the vehicle when the hub unit bearing 1 is assembled to the vehicle, is referred to as the inner axial side.

[0101] The outer member 5 has a substantially cylindrical shape and has double-row outer ring raceways 8a and 8b on its inner peripheral surface.

[0102] Each of the outer ring raceways 8 a, 8 b has a generatrix shape that corresponds to the shape of the plurality of rolling elements 7 a, 7 b. When the plurality of rolling elements 7 a, 7 b are balls, each of the outer ring raceways 8 a, 8 b has an arc-shaped generatrix shape, and when the plurality of rolling elements 7 a, 7 b are tapered rollers, each of the outer ring raceways 8 a, 8 b has a linear generatrix shape that is inclined with respect to the central axis of the outer member 5.

[0103] In the hub unit bearing 1 of this example, the plurality of rolling elements 7a, 7b are made up of balls, and therefore, each of the outer ring raceways 8a, 8b has an arc-shaped generatrix shape.

[0104] In this example, the outer member 5 has, as an optional component, a stationary flange 9 protruding radially outward from an axially intermediate portion. The stationary flange 9 has support holes 10 penetrating in the axial direction at a plurality of circumferential positions in the radially intermediate portion. The stationary flange 9 and the support holes 10 are elements for supporting and fixing the outer member 5 to the suspension device.

[0105] In this example, the support holes 10 are configured as screw holes. The outer member 5 is supported and fixed to the suspension system by threading a support bolt, which is inserted through a through hole provided in the knuckle of the suspension system, into the support hole 10 of the stationary flange 9 from the inside in the axial direction, so that the outer member 5 does not rotate even when the wheel rotates.

[0106] The outer member 5 is made of a hard metal such as medium carbon steel.

[0107] The inner member 6 is disposed radially inside the outer member 5 and coaxially with the outer member 5. The inner member 6 has double-row inner ring raceways 11a, 11b on its outer circumferential surface.

[0108] Each of the inner ring raceways 11 a, 11 b has a generatrix shape that corresponds to the shapes of the multiple rolling elements 7 a, 7 b. When the multiple rolling elements 7 a, 7 b are balls, each of the inner ring raceways 11 a, 11 b has an arc-shaped generatrix shape, and when the multiple rolling elements 7 a, 7 b are tapered rollers, each of the inner ring raceways 11 a, 11 b has a linear generatrix shape that is inclined with respect to the central axis of the inner member 6.

[0109] In the hub unit bearing 1 of this example, the plurality of rolling elements 7a, 7b are made up of balls, and therefore, each of the inner ring raceways 11a, 11b has a generatrix shape that is an arc.

[0110] The inner member 6 has, as optional components, a rotating flange 12 that protrudes radially outward and is provided at a portion that protrudes axially outward beyond the outer member 5, and a cylindrical pilot portion 13 that is provided at the axially outer end. The rotating flange 12 is an element to which a braking rotor such as a disk or drum and a wheel that constitutes a wheel are coupled and fixed, and the pilot portion 13 is an element for radially positioning the braking rotor and the wheel with respect to the inner member 6.

[0111] The rotary flange 12 has mounting holes 15 that penetrate the rotary flange 12 in the axial direction at a plurality of circumferential positions in a radially intermediate portion of the rotary flange 12. Each of the mounting holes 15 is configured as a cylindrical hole or a threaded hole.

[0112] When each of the mounting holes 15 is formed as a cylindrical hole, a stud 16 is press-fitted and serrated into each of the mounting holes 15. The braking rotor and the wheel are coupled and fixed to the rotating flange 12 by inserting a pilot portion 13 into a central hole provided in the center of each of them, inserting studs 16 into through-holes provided at multiple locations in the circumferential direction in the radially intermediate portion of each of them, and then screwing a hub nut onto the tip of the stud 16.

[0113] When each of the mounting holes 15 is formed as a threaded hole, the braking rotor and the wheel are connected and fixed to the rotating flange 12 by inserting the pilot portion 13 into the central hole provided in the center of each of them, and by threading a hub bolt that has passed through a through hole provided in the braking rotor and a through hole provided in the wheel into the mounting hole 15 from the outside in the axial direction.

[0114] In this example, each of the mounting holes 15 is configured as a cylindrical hole, and therefore, a stud 16 is press-fitted into each of the mounting holes 15 with serrations.

[0115] The inner member 6 has, as an optional component, a spline hole 14 at the radial center thereof that passes through the inner member 6 in the axial direction.

[0116] The spline hole 14 is an element with which the tip of a drive shaft, which is driven to rotate by an engine or an electric motor as a drive source, is spline-engaged. When the vehicle is running, the drive shaft drives the inner member 6 to rotate, thereby driving the wheels and braking rotors that are coupled and fixed to the rotating flange 12 of the inner member 6 to rotate.

[0117] In this example, the inner member 6 is configured as a hub formed by combining an inner ring 17 and a hub wheel 18 .

[0118] The inner ring 17 is made of a hard metal such as bearing steel and has an annular shape. The inner ring 17 has an inner ring raceway 11b on the axially inner side on its outer circumferential surface.

[0119] The hub ring 18 is made of a hard metal such as medium carbon steel, and includes an inner ring raceway 11 a on the axially outer side, a rotation flange 12, a pilot portion 13, and a spline hole 14.

[0120] The hub ring 18 has a small diameter step 19 at a portion axially more inward than the axially outer inner ring raceway 11a, the small diameter step 19 having an outer diameter smaller than that of an adjacent portion on the axially outer side. The hub ring 18 has a step surface 20 at the axially outer end of the small diameter step 19, facing axially inward.

[0121] The inner ring 17 is fitted tightly onto the small diameter step 19 of the hub ring 18, and its axially outer end face abuts against a step surface 20 of the hub ring 18. In this way, the hub ring 18 and the inner ring 17 are joined and fixed together.

[0122] In addition, the hub wheel 18 and the inner ring 17 can also be joined and fixed by clamping the inner ring 17 from both axial sides between the stepped surface of the hub wheel 18 and a crimped portion provided on the axially inner end of the hub wheel 18, or a nut threaded onto the axially inner end of the hub wheel 18.

[0123] The inner member 6 can also be configured by externally fitting and fixing two inner rings, each having inner ring raceways 11 a and 11 b on its outer circumferential surface, onto a shaft member having a rotating flange 12, a pilot portion 13, and a spline hole 14. Furthermore, if the inner member 6 does not include the rotating flange 12 and the pilot portion 13, the inner member 6 can be configured by two inner rings, each having inner ring raceways 11 a and 11 b on its outer circumferential surface.

[0124] A plurality of rolling elements 7a, 7b are arranged between the double-row outer ring raceways 8a, 8b and the double-row inner ring raceways 11a, 11b. Specifically, the rolling elements 7a, 7b are held in cages 22a, 22b and arranged at equal intervals in the circumferential direction.

[0125] In this example, each of the rolling elements 7a and 7b is made up of a ball.

[0126] The hub unit bearing 1 of this example has an equal diameter PCD type structure in which the pitch circle diameter of the rolling elements 7 a in the axially outer row is equal to the pitch circle diameter of the rolling elements 7 b in the axially inner row, and the diameter (ball diameter) of the rolling elements 7 a in the axially outer row is equal to the diameter (ball diameter) of the rolling elements 7 b in the axially inner row.

[0127] In the hub unit bearing 1 of this example, the pitch circle diameters of the rolling elements in the axially outer row and the pitch circle diameters of the rolling elements in the axially inner row may be different from each other. Additionally or alternatively, the diameters (ball diameters) of the rolling elements 7a in the axially outer row and the diameters (ball diameters) of the rolling elements 7b in the axially inner row may be different from each other. Furthermore, tapered rollers may be used instead of balls as the rolling elements 7a, 7b.

[0128] Each of the rolling elements 7a and 7b is made of an iron alloy such as bearing steel, or ceramics.

[0129] The bearing grease sealed in the internal space 2 lubricates the rolling contact portions between the rolling surfaces of the rolling elements 7a, 7b and the outer ring raceways 8a, 8b and the inner ring raceways 11a, 11b.

[0130] The bearing grease may be, but is not limited to, a grease containing a hydrocarbon oil, such as a mineral oil or a synthetic hydrocarbon oil, as a base oil. The bearing grease may contain, as a thickener, a metal soap such as lithium soap, urea, a fluorine compound, bentonite, or the like. The bearing grease may contain additives such as an antioxidant, a rust inhibitor, and an extreme pressure agent, as necessary.

[0131] In the hub unit bearing 1 of this example, the seal structure 3 of this example is applied to the seal structure that closes the axially inner opening of the internal space 2. For this reason, in the hub unit bearing 1 of this example, when the vehicle is running, the seal lips 23a to 23c and the lip sliding surfaces 24a to 24c that constitute the axially inner seal structure 3 slide against each other via an oil film of grease (seal grease) 25, causing cavitation to occur in the multiple recesses 35, reducing the viscosity of the grease 25 and decreasing the shear resistance of the grease 25, making it possible to reduce the seal torque.

[0132] The seal structure 4 that blocks the axially outer opening of the internal space 2 is also in a fluid lubricated state when the vehicle is running, and the seal lips 39a to 39c and the lip sliding surfaces 40a to 40c slide against each other via an oil film of the seal grease 41.

[0133] The seal structure 4 includes a seal ring 42 having seal lips 39a to 39c.

[0134] The seal ring 42 is fixed to the outer member 5. The seal ring 42 is composed of a core metal 43 and a seal member 44 having seal lips 39a to 39c.

[0135] The lip sliding surfaces 40a to 40c are provided on the inner member 6. Specifically, the lip sliding surfaces 40a and 40b are provided on a portion of the outer peripheral surface of the hub wheel 18 that is adjacent to the axially outer side of the inner ring raceway 11a on the axially outer side. In contrast, the lip sliding surface 40c is provided on the axially inner surface of the rotating flange 12.

[0136] In this example, the lip sliding contact surfaces 40a to 40c are not provided with multiple recesses 35. However, in the hub unit bearing 1 of this example, multiple recesses 35 similar to those in the seal structure 3 can also be provided in any of the lip sliding contact surfaces 40a to 40c.

[0137] The seal grease 41 is applied to the seal lips 39a to 39c to reduce the seal torque. The seal grease 41 may have the same composition as the grease (seal grease) 25, or may have a different composition.

[0138] Second Example A second example of the embodiment of the present disclosure will be described with reference to FIGS. 11 and 12. FIG.

[0139] In this example, the structure of the recess 35a provided in the lip sliding contact surface 24 (24a to 24c) is changed from the structure of the recess 35 in the first example.

[0140] The recess 35 a of this example includes a recessed portion 37 and a raised portion 46 that is provided around the recess 35 a and raised above the reference surface 36 .

[0141] The raised portions 46 may be formed on the opening edges of all of the recesses 35a, or may be formed only on the opening edges of some of the recesses 35a. Furthermore, the raised portions 46 may be formed around the entire periphery of the opening edges of the recesses 35a, or may be formed only on some of the peripheries. In this example, the raised portions 46 are formed on the opening edges of all of the recesses 35a. Furthermore, each raised portion 46 is formed around the entire periphery of the recess 35a.

[0142] In this example, the average height difference (H) between the bottom and the top of the protrusion 46, which is the opening edge, of the plurality of recesses 35a is 1 μm to 10 μm, preferably 1 μm to 5 μm.

[0143] In this example, for recess 35a, the height difference between the bottom, which is the deepest part of recess 37, and the apex of protrusion 46 located on one side of recess 37 in the sliding contact direction of seal lips 23a to 23c is defined as a1, and the height difference between the bottom of recess 37 and the apex of protrusion 46 located on the other side of recess 37 in the sliding contact direction of seal lips 23a to 23c is defined as a2, and the height difference of recess 35 is calculated as {(a1 + a2) / 2}. Then, the average value of the height differences (H) for all recesses 35a present on lip sliding surfaces 24a to 24c is calculated.

[0144] In this example, for each of the three lip sliding surfaces 24a to 24c, the ratio of the total opening area of ​​the multiple recesses 35a to the fixed area of ​​the lip sliding surfaces 24a to 24c is set to 30% or more and 100% or less.

[0145] In this example, the diameter (D) is determined, which is the circumferential distance between the raised portions 46 located on both sides of the recessed portion 37 in the sliding direction of the seal lips 23a to 23c, and the opening area of ​​the recessed portion 35a is calculated from the diameter.Then, the total opening area of ​​all the recessed portions 35 present in a certain area of ​​the lip sliding surfaces 24a to 24c is determined.

[0146] In this example, the method for processing the recesses 35a is not particularly limited as long as it is possible to form the protrusions 46. The recesses 35a can be formed by, for example, shot peening or laser processing.

[0147] When recess 35a is formed by shot peening, material ejected when recess 37 is formed forms protrusion 46 on a part or all of the circumferential surface of recess 37. When recess 35a is formed by laser processing, material (debris) melted when recess 37 is formed adheres to a part or all of the circumferential surface of recess 37 as protrusion 46.

[0148] In this example, as in the first example, the pressure of the oil film of grease 25 is low at the position of recessed portion 37 of recessed portion 35a and is high at the position of the opening edge on the front side of recessed portion 35a (left side in FIG. 12) in the direction of movement of seal lips 23a to 23c. When the pressure of the oil film at the position of recessed portion 37 falls below the cavitation pressure, cavitation occurs in recessed portion 37, and air bubbles 45 are generated inside recessed portion 37.

[0149] In particular, in this example, even at the position of the reference surface 36 that exists on the rear side of the raised portion 46 (the right side in FIG. 12) in the direction of travel of the seal lips 23a to 23c, the pressure of the oil film is likely to drop below the cavitation pressure, making it easier for bubbles 45 to occur. Therefore, in this example, the region in which bubbles 45 occur is wider than in the first example, and the seal torque reduction effect is enhanced.

[0150] In this example, the lip sliding surfaces 24a to 24c on which the plurality of recesses 35a are formed are provided on the surface of a metal slinger 27.

[0151] The other configurations and effects of the second example are the same as those of the first example.

[0152] Third Example A third example of the embodiment of the present disclosure will be described with reference to FIG.

[0153] In this example, the ratio of the total opening area of ​​the plurality of recesses 35 to the fixed area of ​​the lip sliding contact surfaces 24a to 24c is changed from the structure of the first example.

[0154] That is, in this example, the ratio of the total opening area of ​​the plurality of recesses 35 to the fixed area of ​​the lip sliding surface 24 (24a to 24c) is made larger than in the structure of Example 1, and the recesses 35 are provided more densely. This makes it possible to increase the amount of bubbles 45 generated due to cavitation, thereby enhancing the effect of reducing the seal torque.

[0155] The other configurations and effects of the third example are the same as those of the first example.

[0156] Although the first to third examples of the embodiment of the present disclosure have been described above, the structures of the first to third examples can be implemented in any suitable combination as long as no contradiction occurs.

[0157] REFERENCE SIGNS LIST 1 hub unit bearing 2 internal space 3 seal structure 4 seal structure 5 outer member 6 inner member 7a, 7b rolling elements 8a, 8b outer ring raceway 9 stationary flange 10 support hole 11a, 11b inner ring raceway 12 rotating flange 13 pilot portion 14 spline hole 15 mounting hole 16 stud 17 inner ring 18 hub ring 19 small diameter step portion 20 step surface 22a, 22b cage 23, 23a, 23b, 23c seal lip 24, 24a, 24b, 24c lip sliding surface 25 grease 26 seal ring 27 slinger 28 core metal 29 seal member 30 fixed cylinder portion 31 fixed circular ring portion 32 seal base portion 33 rotating cylinder portion 34 Rotating ring portion 35, 35a Recessed portion 36 Reference surface 37 Depressed portion 38a, 38b Space between lips 39a, 39b, 39c Seal lip 40a, 40b, 40c Lip sliding surface 41 Seal grease 42 Seal ring 43 Core metal 44 Seal member 45 Air bubble 46 Protruding portion

Claims

1. A seal structure comprising at least one seal lip, at least one lip sliding surface, and grease, wherein the at least one seal lip and the at least one lip sliding surface slide against each other via an oil film of the grease, the at least one lip sliding surface has a plurality of recesses, With respect to the plurality of recesses, an average value of a height difference between a bottom portion and an opening edge portion is 1 μm or more and 10 μm or less, a ratio of the total opening area of ​​the plurality of recesses to a certain area of ​​the at least one lip sliding surface is 30% or more but less than 100%; Seal structure.

2. 2. The seal structure according to claim 1, wherein a ratio of a total opening area of ​​said plurality of recesses to a fixed area of ​​said at least one lip sliding surface is 30% or more and 90% or less.

3. 2. The seal structure according to claim 1, wherein at least a portion of an opening edge of the recess is raised above a reference surface of the lip sliding surface where the recess is not formed.

4. The kinematic viscosity of the base oil constituting the grease is 5.0 mm at 40°C. ２ / s or more 460.0mm ２ / s or less and 1.9 mm at 100 ° C. ２ / s or more 56.0mm ２ The seal structure according to claim 1, wherein the resistance is equal to or less than 1 / s.

5. 2. The seal structure according to claim 1, wherein the grease is a urea-based grease having a consistency of 200 or more and 300 or less.

6. A hub unit bearing comprising the seal structure according to any one of claims 1 to 5.