Torque sensor

Abstract

The present invention improves the performance of a torque sensor for detecting torque in a rotational direction, in which the influence of forces other than torque about a central axis is reduced and stress at the edges of a bonding part of a strain detection element is relieved. As a means of said improvement, a torque sensor is used that has an outer ring, an inner ring, two or more spokes connecting the outer ring and the inner ring to each other, and a strain detection element provided on the spokes. The spokes each comprise a mounting part for the strain detection element, a first peripheral part connecting the mounting part to the outer ring, and a second peripheral part connecting the mounting part to the inner ring. In the axial direction of the outer ring, the thickness of each of the first peripheral part and the second peripheral part is less than the thickness of the mounting part, a first surface of the strain detection element is adhered to a second surface of the mounting part via a bonding agent, and the second surface is the surface along the radial and circumferential directions of the outer ring and has a smaller area than the first surface.

Description

Torque Sensor

[0001] The present invention relates to a torque sensor, and more particularly to a technique that is effective when applied to a torque sensor that detects strain in the rotation direction of a central axis between an outer ring and an inner ring.

[0002] For example, as shown in Patent Document 1 (JP 2003-83824 A), a torque sensor is composed of an outer ring, an inner ring, and spokes equipped with strain sensors, and in many cases, the strain sensors are mounted with adhesive. In addition, two pairs of such spokes are provided symmetrically so that strains generated by moments other than torque around the central axis can be canceled out by output calculation. This allows only torque around the central axis to be measured.

[0003] Furthermore, Patent Document 2 (Japanese Patent Laid-Open Publication No. 2007-40774) describes a torque meter that reduces the influence of a load received by a load member on a torque member.

[0004] JP 2003-83824 A JP 2007-40774 A

[0005] For example, due to the influence of contact with the object to which the torque sensor is attached, variations in the bolt axial force and misalignment when the torque sensor is fastened, or the asymmetric shape of the object to which the torque sensor is attached, moments other than the torque around the central axis (crosstalk) are superimposed as asymmetric strain components that are difficult to offset by output calculations, resulting in a problem of reduced measurement accuracy of the torque sensor.

[0006] Therefore, in order to avoid the influence of forces other than the torque about the central axis, it is necessary to devise a way to prevent the measurement value of the torque sensor from being influenced by these loads.The technology described in Patent Document 2 is thought to have a certain effect of reducing moments other than the torque about the central axis.

[0007] However, when a torque sensor with a sensing element mounted on a spoke with adhesive is subjected to repeated loads, the repeated stress generated inside the adhesive used to bond the strain sensor causes fatigue cracks to form at the edge of the adhesive. These cracks propagate and expand mainly along or near the interface between the adhesive and the mounting part, reducing the area that transmits strain to the strain sensor and ultimately reducing its sensitivity.

[0008] In order to improve the long-term reliability of the torque sensor, it is necessary to devise a way to alleviate the stress at the adhesive end of the strain sensor.

[0009] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

[0010] A brief summary of a representative embodiment of the present invention will be given below.

[0011] A torque sensor according to one embodiment includes an outer ring, an inner ring, two or more spokes connecting the outer ring and the inner ring, and strain detection elements provided on the spokes, wherein the spokes include a mounting portion for the strain detection element, a first peripheral portion connecting the mounting portion to the outer ring, and a second peripheral portion connecting the mounting portion to the inner ring. In the axial direction of the outer ring, the thicknesses of the first peripheral portion and the second peripheral portion are each smaller than the thickness of the mounting portion, and a first surface of the strain detection element is affixed to a second surface of the mounting portion via an adhesive, and the second surface is a surface that extends radially and circumferentially around the outer ring and has a smaller area than the first surface.

[0012] According to one embodiment, the performance of the torque sensor can be improved.

[0013] FIG. 1 is a plan view showing a torque sensor according to an embodiment. FIG. 2 is a perspective view showing a portion of a torque sensor according to an embodiment. FIG. 3 is a cross-sectional view taken along line A-A in FIG. 1. FIG. 4 is a cross-sectional view showing a torque sensor according to an embodiment. FIG. 5 is a graph showing the relationship between the width of the mounting surface of the mounting portion and the adhesive equivalent stress. FIG. 6 is a graph showing the relationship between the width of the mounting surface of the mounting portion and the adhesive equivalent stress. FIG. 7 is a cross-sectional view showing a torque sensor as a comparative example. FIG. 8 is a perspective view showing a torque sensor attached to a robot arm.

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, components having the same functions are designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, in the embodiments, explanations of the same or similar parts will not be repeated unless particularly necessary.

[0015] The thickness here refers to the length of each component in the direction along the central axis of the outer ring and inner ring that make up the torque sensor (hereinafter referred to as the axial direction). The plan view refers to the case where each component is viewed in the axial direction.

[0016] <Structure of Torque Sensor> FIG. 1 shows an example of a disk-shaped torque sensor 1 according to this embodiment. The torque sensor (torque meter) 1 includes an outer ring (first structure) 2, an inner ring (second structure) 3, multiple spokes (third structure) 4, and multiple mounting portions (measuring portions, sensor portions) 4a. The inner ring 3 and the outer ring 2 are formed into annular shapes, with the diameter of the outer ring 2 being larger than the diameter of the inner ring 3. The outer ring 2 is arranged concentrically with the inner ring 3. The inner ring 3 and the outer ring 2 are connected to each other by multiple spokes 4 serving as beam portions arranged radially around their axes (hereinafter referred to as central axes). The spokes 4 transmit torque between the inner ring 3 and the outer ring 2. Each spoke 4 has a mounting portion 4a equipped with a strain detection element (sensor element, strain sensor) 5. In other words, the mounting portion 4a is where the strain detection element 5 is attached.

[0017] The mounting portion 4a is also a portion that generates strain due to an external force. This external force is a force (torque around the central axis) that rotates the inner ring 3 or the outer ring 2 about its axis. When this external force is applied to one or both of the inner ring 3 and the outer ring 2, strain is generated in the mounting portion 4a, and this strain is detected by the strain detection element 5, and the torque is measured by calculation.

[0018] For example, if the inner ring 3 or outer ring 2 of the torque sensor is attached to a robot arm 10 (see FIG. 8 ), when the robot arm 10 is driven, a force about the central axis is applied to the torque sensor 1. The outer ring 2 of the torque sensor 1 is displaced in the direction about the central axis (circumferential direction) relative to the inner ring 3. When the outer ring 2 is displaced relative to the inner ring 3, electrical signals are output from the two strain detection elements 5 on either side of the central axis, allowing the torque sensor 1 to detect torque. Here, the inner ring 3 has an annular structure, but the inner ring 3 may also be a disk without a central hole.

[0019] The torque sensor 1 of this embodiment is used by being assembled to, for example, a rotatable joint of an articulated robot, i.e., a robot arm 10. For example, one of the outer ring 2 or the inner ring 3 is fixed to a first arm 11 constituting the robot arm 10, and the other is fixed to a second arm 12 constituting the robot arm 10 and rotatable relative to the first arm 11. A hand or various tools is attached to the second arm 12. The outer ring 2 or the inner ring 3 is fixed to the first arm 11 or the second arm 12, to which the torque sensor 1 is attached, by a plurality of bolts. The outer ring 2 and the inner ring 3 have a plurality of circular holes 6 that penetrate them in the axial direction (central axis direction), and the bolts fasten each arm to the outer ring 2 or the inner ring 3 through these holes 6. The plurality of circular holes 6 are arranged in a circumferential direction on each of the outer ring 2 and the inner ring 3.

[0020] The outer ring 2, inner ring 3, and spokes 4 are made of metal (e.g., stainless steel), but materials other than metal can be used as long as they have sufficient mechanical strength to withstand the applied torque. The outer ring 2 and inner ring 3 have, for example, the same thickness in the axial direction. In the radial direction of the outer ring 2 and inner ring 3, one end of each spoke 4 is joined to the inner ring 3, and the other end is joined to the outer ring 2. The outer ring 2, inner ring 3, and spokes 4 are integral with each other here, and are formed, for example, by cutting out from a single member. In order to increase the strength of each connection, the width of the spoke 4 in the circumferential direction is increased near the connection between the outer ring 2 and the inner ring 3, and the outer shape of these connection parts is rounded.

[0021] As shown in FIG. 2 , each spoke 4 has a mounting portion 4a and an annular peripheral portion 4b that surrounds the entire periphery of the mounting portion 4a in a plan view. The peripheral portion 4b includes a peripheral portion (connection portion) 42 that connects the mounting portion 4a to the outer ring 2 and a peripheral portion (connection portion) 43 that connects the mounting portion 4a to the inner ring 3. In other words, the peripheral portions 42 and 43 constitute the peripheral portion 4b. The thickness, fill level, and material of the peripheral portion 4b are the same as those of the peripheral portions 42 and 43, respectively. As shown in FIG. 3 , the mounting portion 4a has a substantially constant overall thickness and a rectangular cross-sectional shape. The peripheral portion 4b also has a substantially constant overall thickness. However, the corners of the connections between the mounting portion 4a and the peripheral portion 4b may be rounded (connection portions with curved surfaces) to avoid stress concentration. FIG. 3 also illustrates a configuration in which the boundaries between the peripheral portion 4b and the outer ring 2 and the inner ring 3 are rounded. The thickness of the peripheral portion 4b is smaller than the thickness of the mounting portion 4a. The mounting portion 4a and the peripheral portion 4b are integral with each other here, and are formed, for example, by cutting out from a single member.

[0022] Of the parts constituting the spoke 4, the parts other than the mounting part 4a (peripheral part 4b) have a constant thickness except for the reinforcing part 4c described below. Here, the thickness of both ends of the spoke 4 in the circumferential direction is smaller than the thickness of the mounting part 4a except for the reinforcing part 4c.

[0023] 3, the peripheral portion 4b and the peripheral portions 42 and 43 shown in FIG. 2 each have a first surface b1 in the axial direction and a second surface b2 opposite the first surface b1. The mounting portion 4a has a first protruding portion that contacts the protruding strain detection element 5 with respect to the first surface b1 of the peripheral portion 4b in the axial direction, and a second protruding portion that is spaced apart from the protruding strain detection element 5 with respect to the second surface b2 of the peripheral portion 4b. However, the second protruding portion may not be present, and the surface opposite the first protruding portion in the axial direction, including the second surface b2, may be flat.

[0024] Furthermore, each spoke 4 has a pair of reinforcing portions 4c that sandwich the mounting portion 4a and the peripheral portion 4b in the circumferential direction of the outer ring 2 and the inner ring 3. Each of the pair of reinforcing portions 4c is formed between the outer ring 2 and the inner ring 3. The peripheral portion 4b connects the mounting portion 4a and the reinforcing portion 4c. The thickness of the reinforcing portion 4c is approximately equal to the thickness of each of the outer ring 2 and the inner ring 3. As shown in FIG. 2 , the thickness of the mounting portion 4a is smaller than the thickness of the reinforcing portion 4c. The mounting portion 4a, the peripheral portion 4b, and the reinforcing portion 4c are integral with one another. Here, the reinforcing portion 4c constitutes both ends of the spoke 4 in the circumferential direction. In a plan view, the mounting portion 4a is spaced apart from the reinforcing portion 4c.

[0025] If there is no reinforcing portion 4c, there may be no peripheral portion 4b adjacent to the mounting portion 4a in the radial direction. In this case, the peripheral portion 4b is composed only of a peripheral portion 42 between the outer ring 2 and the mounting portion 4a, and a peripheral portion 43 between the inner ring 3 and the mounting portion 4a. In this case, the peripheral portions 42, 43 are spaced apart from each other, and a portion of the end of the spoke 4 in the circumferential direction is composed of the mounting portion 4a.

[0026] As shown in FIG. 1 , the spokes 4 each having a mounting portion 4 a are formed as a pair on either side of the central axis in a plan view, and have a symmetrical structure, so that distortions caused by moments other than torque about the central axis can be canceled out by output calculation. For the same purpose, another pair of spokes 4 is provided on either side of the central axis, for a total of four spokes 4. The number of spokes 4 each having a mounting portion 4 a does not have to be two pairs, and may be only one pair arranged on either side of the central axis. Two or more pairs of spokes 4 each having a mounting portion 4 a may also be provided. In addition to the spokes 4 each having a mounting portion 4 a, multiple spokes that connect the outer ring 2 and the inner ring 3 and do not have a mounting portion 4 a may also be provided.

[0027] In a plan view, the periphery of the mounting portion 4a is completely surrounded by the peripheral portion 4b, so the mounting portion 4a is separated from both the outer ring 2 and the inner ring 3. The peripheral portion 4b, which is made of the same material as the mounting portion 4a, has a smaller thickness than the mounting portion 4a, so the rigidity of the peripheral portion 4b is lower than the rigidity of the mounting portion 4a. In this way, by providing a difference in thickness between the mounting portion 4a and the peripheral portion 4b, the second moment of area of ​​the peripheral portion 4b can be made lower than that of the mounting portion 4a. This allows the peripheral portion 4b to elastically deform, reducing the effect of asymmetric deformation on the mounting portion 4a and preventing a decrease in measurement accuracy.

[0028] The reinforcing portion 4c may be omitted. In this case, the end of the spoke 4 in the circumferential direction is formed by the end of the peripheral portion 4b, which is thinner than the mounting portion 4a. Whether or not the reinforcing portion 4c is present, by making the rigidity of the peripheral portion 4b lower than the rigidity of the mounting portion 4a, the influence of asymmetric deformation due to moments other than torque around the central axis can be reduced, and a decrease in measurement accuracy can be suppressed.

[0029] A strain detection element 5 is attached (bonded) via adhesive 5a (see FIG. 3) to a mounting surface 4d, which is one surface (the surface of the first protrusion) of the mounting portion 4a in the axial direction. In FIG. 1, the outline of the mounting surface 4d covered by the strain detection element 5 is shown by a dashed line. The mounting surface 4d is a surface that extends along the radial and circumferential directions. The strain detection elements 5 are arranged symmetrically with respect to the center (the center of torque action, the central axis) of the outer ring 2 and the inner ring 3. The two strain detection elements 5 are arranged diametrically, sandwiching the center of the annular inner ring 3 and the outer ring 2. The strain detection elements 5 may be metal or semiconductor strain gauges, or optical fiber elements, etc.

[0030] 1, the planar shape of the strain detection element 5 is shown as a rectangle, and the planar shape of the mounting portion 4a, i.e., the planar shape of the mounting surface 4d shown in Fig. 3, is also shown as a rectangle. The planar shapes of the strain detection element 5 and the mounting portion 4a are not limited to these, and may be, for example, a rectangle extending in the radial direction, or a circle or an ellipse.

[0031] One of the main features of this embodiment is that, among the opposing surfaces of the strain detection element 5 and the mounting portion 4a, the area of ​​the mounting surface 4d of the mounting portion 4a is smaller than the area of ​​the attachment surface 5b of the strain detection element 5. Here, with regard to the radial length of the opposing surfaces of the strain detection element 5 and the mounting portion 4a, the length of the surface of the mounting portion 4a (mounting surface 4d) is smaller than the length of the surface of the strain detection element 5 (attachment surface 5b). This means that, in the radial direction, both ends of the strain detection element 5 protrude outward beyond both ends of the mounting portion 4a. Furthermore, in the radial direction, both ends of the adhesive 5a protrude outward beyond both ends of the mounting portion 4a. This is true not only in the radial direction but also in the circumferential direction.

[0032] Since the strain detection element 5 has a larger area than the mounting portion 4a, in a plan view, the entire mounting surface 4d to which the strain detection element 5 is attached is covered by the strain detection element 5. Here, the entire end (entire periphery) of the strain detection element 5 in a plan view is separated from the end of the mounting portion 4a.

[0033] In the radial and circumferential directions, the ends of the strain detection element 5 and the adhesive 5a are located outside the corners 8 of the mounting portion 4a (first protrusion). The corners 8 of the mounting portion 4a here refer to the corners at the boundaries between the mounting surface 4d of the mounting portion 4a, to which the strain detection element 5 is attached, and the side surfaces of the mounting portion 4a in the radial and circumferential directions. The side surfaces of the mounting portion 4a are surfaces that extend in a direction perpendicular to the mounting surface 4d of the mounting portion 4a (axial direction). As long as the side surfaces have surfaces that extend in a direction perpendicular to the mounting surface 4d, a radius may be provided at the boundary between the side surfaces and the first surface b1 of the peripheral portion 4b.

[0034] The torque sensor of the present embodiment includes an outer ring, an inner ring, two or more spokes connecting the outer ring and the inner ring, and strain detection elements provided on the spokes, and the spokes include a mounting portion for the strain detection elements and a peripheral portion that surrounds the mounting portion in a plan view. Therefore, the rigidity of the peripheral portions that make up the spokes is lower than the rigidity of the mounting portion.

[0035] If the outer ring or inner ring undergoes asymmetric deformation due to a moment other than torque around the central axis, the second moment of area of ​​the peripheral part can be made lower than that of the mounting part, causing the peripheral part to elastically deform, thereby reducing the effect of asymmetric deformation on the mounting part and preventing a decrease in measurement accuracy.

[0036] That is, in this embodiment, there is a difference in thickness between the peripheral portion and the mounting portion in the axial direction (torque measurement axis direction) (mounting portion > peripheral portion). Here, strain due to crosstalk is divided by the square of the spoke thickness and becomes smaller. On the other hand, strain due to torque around the central axis decreases in inverse proportion to the spoke thickness. In this embodiment, the peripheral portion of the spoke is made thinner than the mounting portion. Therefore, deformation caused by measurement torque (torque around the central axis) is transmitted to the strain detection element, but deformation caused by moments other than measurement torque is less likely to be transmitted to the strain detection element. In other words, by making the rigidity of the peripheral portion lower than the rigidity of the mounting portion, the influence of asymmetric deformation caused by moments other than torque around the central axis can be reduced, and a decrease in measurement accuracy can be suppressed.

[0037] Here, Fig. 7 shows a cross section of a torque sensor of the comparative example. The cross section in Fig. 7 is taken at a position corresponding to the cross section shown in Fig. 3. As shown in Fig. 7, the torque sensor of the comparative example includes a mounting portion 4x corresponding to the mounting portion 4a in Fig. 1 and a strain detection element 5x corresponding to the strain detection element 5 in Fig. 1. However, the torque sensor of the comparative example differs from the present embodiment in that the area of ​​the strain detection element 5x facing the mounting surface 4d is smaller than the mounting surface 4d of the mounting portion 4x. Furthermore, the width of the adhesive 5y connecting the strain detection element 5x and the mounting portion 4x in the radial and circumferential directions is smaller than the width of the mounting portion 4x.

[0038] 7, the lines of force of the load applied to the torque sensor are indicated by arrows. The load is transmitted to the mounting portion 4x via the peripheral portion 4b. At this time, the stress distribution near the surface of the mounting portion 4x is a distribution that gradually slopes toward the peripheral portion 4b, with peaks near the centers of the mounting portion 4x, the adhesive 5y, and the detection element 5x in the radial direction.

[0039] As in the comparative example, when the strain detection element 5x is mounted on a mounting portion 4x that is larger in area than the attachment surface 5b of the strain detection element 5x, the end 7 of the adhesive 5y is located inside the corner 8 of the mounting portion 4x. In this case, more force flows through the end 7 of the adhesive 5y than through the corner 8 of the mounting portion 4x, making stress concentration more likely to occur. As a result, fatigue cracks may occur from the end 7 due to stress generated inside the adhesive 5y, which may result in a decrease in the sensitivity of the torque sensor.

[0040] Therefore, in this embodiment, the strain detection element 5 is mounted on a mounting portion 4a having a mounting surface 4d that is smaller in area than the attachment surface 5b of the strain detection element 5. In FIG. 4 , the force lines of the load applied to the torque sensor are indicated by arrows. The load is transmitted to the mounting portion 4a via the peripheral portion 4b. The stress distribution near the surface of the mounting portion 4a is a distribution that gradually slopes toward the peripheral portion 4b, with peaks near the centers of the mounting portion 4a, adhesive 5a, and detection element 5 in the radial direction. The number of force lines flowing through the corners 8 of the mounting portion 4a is smaller than the force lines flowing through the mounting portion 4a located inside the corners 8 in the radial and circumferential directions. Furthermore, the number of force lines flowing through the ends 7 of the adhesive 5a located outside the corners 8 of the mounting portion 4a is even smaller than the force lines flowing through the corners 8 of the mounting portion 4a.

[0041] In this embodiment, the area of ​​the mounting surface 4d of the mounting portion 4a is smaller than the area of ​​the attachment surface 5b of the strain detection element 5, and the entire outer periphery of the mounting surface 4d in plan view is located inside the outer periphery of the attachment surface 5b. As a result, the end 7 of the adhesive 5a is inevitably located near the corner 8 of the mounting portion 4a or outside the corner 8 in the radial and circumferential directions, so force is less likely to be transmitted to the end 7 of the adhesive 5a than in the comparative example, resulting in a structure in which stress concentration is less likely to occur at the end 7.

[0042] 5 and 6 are graphs showing the relationship between the radial width (horizontal axis) of the mounting surface 4d of the mounting portion 4a and the adhesive equivalent stress (vertical axis) in the adhesive 5a below the strain detection element 5. Assuming that the size of the attachment surface 5b of the strain detection element 5 is 7 × 7 mm, as shown in Fig. 5, if the circumferential width dimension of the mounting surface 4d is reduced from approximately 8.3 mm, the adhesive equivalent stress also decreases, but if the width dimension is smaller than 7 mm, there is a limit to the reduction in the adhesive equivalent stress.

[0043] In addition, assuming that the size of the attachment surface 5b of the strain detection element 5 is 12 x 12 mm as shown in Figure 6, if the circumferential width dimension of the mounting surface 4d is reduced from approximately 13.2 mm, the adhesive equivalent stress will also decrease, but if the width dimension is smaller than 12 mm, there will be a limit to the reduction in the adhesive equivalent stress.

[0044] From this, it can be seen that when the area of ​​the mounting surface 4d is smaller than the area of ​​the attachment surface 5b, the stress at the end 7 of the adhesive 5a can be alleviated, improving the reliability of the torque sensor.

[0045] 1, the positional relationship between the mounting surface 4d of the mounting portion 4a and the attachment surface 5b of the strain detection element 5 is desirably equal in both the radial and circumferential directions. In other words, the shortest distance between the end of the mounting surface 4d of the mounting portion 4a and the end of the attachment surface 5b of the strain detection element 5 in a plan view is desirably constant, including in the radial and circumferential directions. The above-mentioned effect can be particularly achieved by making the width dimension of the mounting surface 4d 5% or more smaller than the width dimension of the attachment surface 5b of the strain detection element 5 in each direction, including the radial and circumferential directions, in a plan view.

[0046] The invention made by the inventor has been specifically described above based on an embodiment, but it goes without saying that the present invention is not limited to the above embodiment and can be modified in various ways without departing from the gist of the invention.

[0047] REFERENCE SIGNS LIST 1 Torque sensor 2 Outer ring 3 Inner ring 4 Spokes 4a, 4x Mounting portion 4b Peripheral portion 4c Reinforcement portion 4d Mounting surface 5, 5x Detection element 5a, 5y Adhesive 5b Adhesion surface 6 Hole portion 7 End portion 8 Corner portion 10 Robot arm 11 First arm 12 Second arm 42, 43 Peripheral portion (connection portion) b1 First surface b2 Second surface

Claims

1. A torque sensor comprising an outer ring, an inner ring, two or more spokes connecting the outer ring and the inner ring, and a strain detection element provided on the spokes, wherein the spokes have a mounting portion for the strain detection element, a first peripheral portion connecting the mounting portion and the outer ring, and a second peripheral portion connecting the mounting portion and the inner ring, wherein in the axial direction of the outer ring, the thicknesses of the first peripheral portion and the second peripheral portion are smaller than the thickness of the mounting portion, a first surface of the strain detection element is affixed to a second surface of the mounting portion via an adhesive, and the second surface is a surface that follows the radial and circumferential directions of the outer ring and has an area smaller than that of the first surface.

2. A torque sensor according to claim 1, wherein an end of the adhesive is located outside an end of the second surface in the radial and circumferential directions of the outer ring.

3. A torque sensor according to claim 1, wherein the rigidity of each of the first peripheral portion and the second peripheral portion is lower than the rigidity of the mounting portion.

4. A torque sensor according to claim 1, further comprising a third peripheral portion surrounding the mounting portion in a plan view, the first peripheral portion and the second peripheral portion constituting the third peripheral portion.

5. A torque sensor according to claim 1, wherein, in a plan view, the second surface is entirely covered by the first surface.

6. A torque sensor according to claim 1, wherein the width of said second surface is smaller than the width of said first surface in the radial and circumferential directions of said outer ring by 5% or more.

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