Title of Invention: Rolling Bearing for Air Turbine
Technical field
[0001]
The present invention relates to rolling bearings for air turbines.
Background technology
[0002]
Small and lightweight air turbine handpieces are often used in dental treatment. FIG. 4 shows an example dental air turbine handpiece 120 . This dental air turbine handpiece 120 includes a grip portion 121 and a head portion 122 provided at the tip portion of the grip portion 121 . The operator holds the grip part 121 and cuts teeth, for example.
[0003]
As shown in FIG. 5, this type of air turbine handpiece 120 has a rotary shaft 101 having turbine blades 103 for receiving compressed air from the air supply port inside a housing 105 having an air supply port and an air exhaust port. Housed in a rotatable manner. Rotating shaft 101 is supported by housing 105 via a pair of rolling bearings 130 so as to be rotatable at high speed. The operator operates the air turbine handpiece 120 while rotating the treatment tool attached to the rotating shaft 101 at high speed, thereby performing tooth cutting and the like.
[0004]
Each rolling bearing 130 is supported by the housing 105 via rubber rings 113 fitted in the annular recesses 109 and 111 of the housing 105 . Also, one rolling bearing 130 is biased toward the other rolling bearing 130 by a spring washer 115 .
[0005]
In the rolling bearing 130 described in Patent Document 1, a seal member 132 is fixed by a snap ring 133 to a groove formed in the inner peripheral surface of an outer ring 131, and the inner peripheral portion is inclined axially outward toward the inner diameter side. It has a portion so that it can be elastically deformed in the axial direction and radial direction of the bearing.
Depending on the presence or absence of compressed air, the inner peripheral portion of the seal member 132 is brought into contact with or out of contact with the outer peripheral surface of the inner ring 134, so that both ultrahigh-speed rotation and quick stoppage can be achieved.
prior art documents
patent literature
[0006]
Patent Document 1: Japanese Patent Application Laid-Open No. 2017-211076
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0007]
By the way, in a rolling bearing for an air turbine handpiece, when a seal member and a snap ring are attached to the groove of the outer ring, a circumferential gap is formed between both ends of the snap ring in the circumferential direction. It is required to prevent the seal member from being lifted up by the gap when the seal member is pressed, and to prevent the seal member from coming off the groove of the outer ring.
[0008]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and its object is to float a seal member by means of a circumferential gap formed between both circumferential ends of a retaining ring when compressed air is supplied. An object of the present invention is to provide a rolling bearing for an air turbine capable of suppressing friction and preventing a seal member from coming off a groove of an outer ring.
Means to solve problems
[0009]
The above objects of the present invention are achieved by the following configuration.
(1) Outer ring and
Inner ring and
a plurality of rolling elements arranged to be free to roll between the outer ring and the inner ring;
a substantially annular seal member provided at one end in the axial direction of the space inside the bearing between the outer ring and the inner ring and made of an elastic body without a core metal,
The outer circumference of the seal member is fixed by a snap ring to a groove formed in the inner circumference of the one axial end of the outer ring, and the inner circumference is elastically deformable,
The one end in the axial direction is on the side opposite to the inlet of the compressed air supplied to the inner space of the bearing,
The seal member has an inclined portion inclined toward one end in the axial direction on the inner diameter side,
When the seal member and the snap ring are attached to the groove portion of the outer ring, the circumferential gap of the snap ring is Ts, and the axial thickness of the portion of the seal member that contacts the snap ring is Sct,
1 ≤ Ts/Sc ≤ 10
A rolling bearing for air turbines.
(2) Letting Lss be the radial length of the two axial side surfaces of the sealing member that are in contact with and sandwiched between the retaining ring and the groove of the outer ring, and Φdg be the inner diameter of the outer ring,
0.018≦Lss/Φdg≦0.093
The rolling bearing for an air turbine according to (1), wherein:
Effect of the invention
[0010]
According to the air turbine rolling bearing of the present invention, when compressed air is supplied, the circumferential gap formed between the circumferential ends of the retaining ring prevents the seal member from floating, and the seal member detachment from the groove can be suppressed.
Brief description of the drawing
[0011]
1 is a partial cross-sectional view showing a stopped state of a rolling bearing according to an embodiment of the present invention; FIG.
2 is a cross-sectional view of the sealing member shown in FIG. 1; FIG.
3 is a cross-sectional view along line III-III in FIG. 1, showing the entire circumference of the retaining ring inserted into the outer ring;
4 is a schematic side view of a dental air turbine handpiece; FIG.
5 is a fragmentary cross-sectional view of the dental air turbine handpiece of FIG. 4; FIG.
MODE FOR CARRYING OUT THE INVENTION
[0012]
　Hereinafter, an embodiment of a rolling bearing for an air turbine according to the present invention will be described in detail based on the drawings.
[0013]
FIG. 1 is a partial cross-sectional view showing a stopped state of the rolling bearing of the first embodiment.
The rolling bearing 1 includes an outer ring 10 having an outer ring raceway surface 10a, an inner ring 20 having an inner ring raceway surface 20a, and a plurality of balls (rolling elements) 3 disposed between the outer ring 10 and the inner ring 20 so as to be free to roll. ,
and a retainer 5 that holds a plurality of balls 3 in a rollable manner. The ball bearing is not limited to the illustrated example, and may be an angular type ball bearing.
[0014]
The retainer 5 is a so-called crown-shaped retainer, and the substantially annular rim portion 7 is located on the upstream side of the ball 3 in the supply direction of the compressed air, that is, on the right side in FIG. An arrow P in the drawing indicates the direction in which the compressed air flows.
[0015]
An annular sealing member 30 is provided between the outer ring 10 and the inner ring 20 . The seal member 30 is made of an elastic body that does not include a cored bar and is made of only an elastic material. The outer peripheral portion of the seal member 30 is fixed by a retaining ring 40 to a groove portion 13 formed in the inner peripheral surface of the outer ring 10, so that the inner peripheral portion is elastically deformable in the axial and radial directions of the bearing.
[0016]
As the elastic member constituting the seal member 30, for example, water-resistant acrylic rubber with a Shore A hardness (JIS K 6253) of 60 to 90, general water-resistant fluororubber with a Shore A hardness of 60 to 90, or the like can be used. By using the above material for the seal member 30, appropriate elastic characteristics can be obtained, and durability and wear resistance are also improved.
[0017]
The seal member 30 is provided on the downstream side of the ball 3 in the compressed air supply direction, that is, on the left side in FIG. That is, the seal member 30 is provided at one axial end portion of the bearing internal space S opposite to the compressed air supply side (compressed air inlet). A groove portion 13 for fixing the seal member 30 is formed in the inner peripheral surface 11 of the outer ring 10 . A seal member 30 is fixed to the groove portion 13 by a retaining ring 40 . The shape of the seal member 30 is not limited to an annular shape, and may be any other shape as long as it is substantially annular as will be described later.
[0018]
The outer peripheral surface 21 of the inner ring 20 has an inclined surface 23 on the downstream side in the compressed air supply direction, that is, on the left end in FIG. The inclined surface 23 is preferably formed in an annular shape (conical surface shape) inclined so that the diameter becomes smaller toward the shaft end on the side of the axial seal member, but it may be formed in a cylindrical shape. .
[0019]
FIG. 2 is a cross-sectional view of the sealing member shown in FIG.
The seal member 30 has an annular base portion 31 extending in the radial direction, and an inclined portion 33 integrally formed radially inside the base portion 31 and inclined toward one end side in the axial direction with respect to the base portion 31 . . The inclination angle θ of the inclined portion 33 with respect to the base portion 31 of the sealing member 30, that is, the angle formed by the radial direction of the base portion 31 and the extending direction of the inclined portion 33 is 10° to 80°. If the inclination angle .theta. The inclination angle θ is preferably 20° to 60°, more preferably 25° to 50°.
[0020]
The base portion 31 is inserted into the groove portion 13 together with the retaining ring 40 and fixed to the groove portion 13, as shown in FIG. The groove portion 13 has a tapered surface 15 with which the retaining ring 40 is in contact and which is axially inward of the tapered surface 15 and is in contact with the axial side surface of the base portion 31 . and a directional inner surface 17 . The tapered surface 15 is in line contact with the outer diameter side end of the retaining ring 40 along the circumferential direction. Since the retaining ring 40 is made of an elastic member that is biased radially outward, it generates a force that presses the seal member 30 axially inward. As a result, the base portion 31 of the seal member 30 is strongly sandwiched between the snap ring 40 and the axial inner side surface 17 , and is firmly fixed to the outer ring 10 . Note that the retaining ring 40 has a rectangular cross section,
It may have a circular cross section. Alternatively, the retaining ring 40 may be provided with an inclined surface, the groove 13 may have a rectangular cross section, and the corners of the groove 13 having a rectangular cross section may be pressed against the inclined surface of the retaining ring 40 .
[0021]
The inclined portion 33 is inclined downstream in the direction of supply of compressed air (outward in the axial direction) as it goes radially inward, and is capable of coming into contact with the inclined surface 23 of the inner ring 20 . The shape of the inner peripheral surface 35 of the inclined portion 33 is an annular shape (conical surface shape). The inclined surface 23 of the inner ring 20 with which the inclined portion 33 can abut also has an annular shape (conical surface shape). Therefore, the inner peripheral surface 35 of the inclined portion 33 can contact the inclined surface of the inner ring 20 over the entire circumference. In other words, the seal member 30 can seal the bearing internal space S between the inner peripheral surface 11 of the outer ring 10 and the outer peripheral surface 21 of the inner ring 20 over the entire circumference.
[0022]
Also, as shown in FIG. 1 , the supplied compressed air flows into the bearing internal space S, and the pressure of the compressed air acts on the seal member 30 . Then, the inclined portion 33 is elastically deformed toward the downstream side of the compressed air flow. As a result, the contact area between the inner peripheral surface 35 of the inclined portion 33 and the inclined surface 23 of the inner ring 20 becomes smaller than when the pressure of the compressed air does not act. In other words, the inclined portion 33 is in an open state allowing the compressed air to communicate.
[0023]
Since the sealing member 30 does not have a metal core and is made of only an elastic material, it has a structure that is elastically deformable as a whole. In particular, since the inclined portion 33 does not interfere with the snap ring 40 at all, the seal member 30 is supported by the outer ring 10 so as to be easily elastically deformed. Therefore, when compressed air acts on the seal member 30 beyond a certain pressure, the inner peripheral portion of the seal member 30 elastically deforms outward in the axial direction, and the inner peripheral surface 35 of the inclined portion 33 and the inner ring 20 are inclined. A contact area with the surface 23 is reduced.
[0024]
Thus, in this configuration, even when the supply pressure of compressed air is relatively low, the inclined portion 33 of the seal member 30 is reliably elastically deformed, and the contact area can be reduced.
[0025]
As a result, the air turbine can be started smoothly, the frictional resistance between the seal member 30 and the inner ring 20 can be reduced, and the rotating shaft 101 can be rotated at an ultra-high speed of about 400,000 min-1. Furthermore, since the inclined surface 23 is provided at the end portion of the outer peripheral surface 21 of the inner ring 20 on the downstream side in the supply direction of the compressed air, the flow of the compressed air passing between the inclined portion 33 and the inclined surface 23 becomes smooth. , it is possible to achieve ultra-high-speed rotation, which is even faster than before.
[0026]
Further, as shown in FIGS. 1 to 3, when the seal member 30 and the retaining ring 40 are attached to the groove portion 13 of the outer ring 10, the circumferential gap between the retaining ring 40 and the retaining ring 40 is defined as Ts. Assuming that the thickness of the contact portion in the axial direction is Sct, the ratio between Ts and Sct is formed so as to satisfy the following formula (1).
1≦Ts/Sct≦10 (1)
[0027]
By setting Ts/Sct to be 10 or less, the seal member 30 is prevented from rising from the circumferential clearance Ts of the retaining ring 40 when compressed air is supplied, and the seal member 30 is disengaged from the groove portion 13 of the outer ring 10. can be made difficult. Therefore, Ts/Sct is preferably 6 or less, more preferably 5 or less.
[0028]
In addition, when Ts/Sct is less than 1, when the diameter of the retaining ring 40 is reduced, the circumferential Both ends in the direction contact each other, and the diameter of the retaining ring 40 cannot be sufficiently reduced. Therefore, Ts/Sct should be 1 or more, preferably 2 or more, and more preferably 2.5 or more.
[0029]
Further, in a state in which the seal member 30 is fixed to the groove portion 13 of the outer ring 10 by the retaining ring 40 , both axial side surfaces of the base portion 31 of the seal member 30 are aligned with the axial inner side surfaces 17 of the groove portion 13 of the retaining ring 40 and the outer ring 10 . Lss is the length in the radial direction and Φdg is the inner diameter of the outer ring 10 .
0.018≦Lss/Φdg≦0.093 (2)
[0030]
This is because by setting Lss/Φdg to 0.018 or more, the base portion 31 of the seal member 30 can secure the radial length that is sandwiched between the retaining ring 40 and the groove portion 13 of the outer ring 10, and compressed air is supplied. In some cases, it is possible to reliably prevent the seal member 30 from coming off the groove portion 13 of the outer ring 10 . In order to more reliably prevent the seal member 30 from coming off the groove 13 of the outer ring 10 when compressed air is supplied, Lss/Φdg is preferably 0.027 or more, and 0.035. It is more preferable to be above.
[0031]
Also, by setting Lss/Φdg to 0.093 or less, it is possible to appropriately open and close the seal member 30 with compressed air. From the above viewpoint, Lss/Φdg is preferably 0.074 or less, more preferably 0.047 or less.
In this embodiment, when the seal member 30 is fixed to the groove portion 13 of the outer ring 10 by the retaining ring 40, the inner diameter of the retaining ring 40 is smaller than the inner diameter of the outer ring 10, and the inner peripheral surface of the retaining ring 40 is It is located on the inner diameter side of the inner peripheral surface of the outer ring 10 .
[0032]
In addition, as shown in FIG. 1, the seal member 30 has a radial length of Skn from the axially inner inclination start portion P of the inclined portion 33 to the outermost diameter position of the contact portion with the outer peripheral surface 21 of the inner ring 20 . year,
Assuming that the thickness of the inclined portion 33 in the inclined direction is Skt, it is formed so as to satisfy the following formula (3).
0.25≦Skn/Skt≦2.5 (3)
[0033]
By setting Skn/Skt to 0.25 or more, the length of the inclined portion 33 that receives the compressed air can be secured, and the seal member 30 can be brought into contact with the outer peripheral surface 21 of the inner ring 20 at an appropriate angle. Since the inclined portion 33 is also easily deformed, the seal member 30 is easily out of contact with the outer peripheral surface 21 of the inner ring 20 even with a small amount of compressed air. Skn/Skt is preferably 0.65 or more, and more preferably 0.90 or more, so that the seal member 30 can easily be out of contact with the outer peripheral surface 21 of the inner ring 20 .
[0034]
On the other hand, when Skn/Skt is greater than 2.5, the length of the inclined portion 33 becomes too long,
The pressing force against the outer peripheral surface 21 of the inner ring 20 increases, and it becomes difficult to contact the outer peripheral surface 21 of the inner ring 20 at an appropriate angle. Hard to come in contact with. Therefore, Skn/Skt should be 2.5 or less, preferably 2.1 or less, and more preferably 1.75 or less.
In this embodiment, the outermost diameter position of the contact portion of the seal member 30 with the outer peripheral surface 21 of the inner ring 20 is the boundary portion between the inclined surface 23 and the cylindrical surface on the outer peripheral surface 21 of the inner ring 20. The outermost diameter position may be the intermediate portion of the inclined surface 23 .
Further, in the present embodiment, the portion of the seal member 30 that contacts the retaining ring 40, that is, the thickness Sct in the axial direction of the base portion 31 and the thickness Skt in the direction of inclination of the inclined portion 33 may be the same thickness. and may be different.
[0035]
Also, in the seal member 30, the radial direction between the inclination start portion P on the inner side of the inclined portion 33 in the axial direction and the inner diameter of the snap ring 40 when the seal member 30 and the snap ring 40 are attached to the groove portion 13 of the outer ring 10 When the length is Scn, the ratio between the above-mentioned radial length Skn and Scn is formed so as to satisfy the following formula (4).
0.21≦Skn/Scn≦4.7 (4)
[0036]
By setting Skn/Scn to 0.21 or more, the length of the inclined portion 33 that receives the compressed air can be secured between the inner diameter of the retaining ring 40 and the outer peripheral surface 21 of the inner ring 20, and the seal member 30 can be brought into contact with the outer peripheral surface 21 of the inner ring 20 at an appropriate angle, the seal member 30 is easily out of contact with the outer peripheral surface 21 of the inner ring 20 even with a small amount of compressed air. It should be noted that Skn/Scn is preferably 0.42 or more so that the seal member 30 is easily kept out of contact with the outer peripheral surface 21 of the inner ring 20 .
[0037]
On the other hand, when Skn/Scn is greater than 4.7, the length of the inclined portion 33 becomes too long,
If it is not possible to contact the outer peripheral surface 21 of the inner ring 20 at an appropriate angle and the amount of compressed air is small, it is difficult for the seal member 30 to come out of contact with the outer peripheral surface 21 of the inner ring 20 . Therefore, Skn/Scn should be 4.7 or less, preferably 1.6 or less.
[0038]
Further, when the radial length between the outermost diameter position of the contact portion of the seal member 30 with the outer peripheral surface 21 of the inner ring 20 and the axial center X of the rolling bearing 1 is Ngn, the above-mentioned radial length Skn and
Ngn is formed so as to satisfy the following formula (5).
0.025≦Skn/Ngn≦0.25 (5)
[0039]
By satisfying this formula (5), the length of the inclined portion 33 that receives the compressed air can be secured even with a bearing of a predetermined size, and the sealing member 30 is positioned at an appropriate angle with the outer peripheral surface 21 of the inner ring 20. Therefore, even with a small amount of compressed air, the seal member 30 is likely to be out of contact with the outer peripheral surface 21 of the inner ring 20 . In order to easily keep the seal member 30 out of contact with the outer peripheral surface 21 of the inner ring 20, it is preferable that 0.065≦Skn/Ngn≦0.21, and 0.090≦Skn/Ngn≦0.18. is more preferable.
[0040]
Further, when the supply of compressed air to the turbine blades 103 is stopped by stopping the dental air turbine handpiece 200, the pressure of the compressed air acting on the inclined portion 33 decreases. Then, the inclined portion 33 returns to the state shown in FIG. 1, and the inner peripheral surface 35 of the inclined portion 33 contacts the inclined surface 23 of the inner ring 20 over the entire circumference. That is, the inclined portion 33 is closed, and the inclined portion 33 functions as a brake for the inner ring 20 . In this case, since the inner peripheral surface 35 of the inclined portion 33 is in contact with the inclined surface 23 of the inner ring 20 over the entire circumference, the greatest braking effect is obtained due to the frictional resistance between the seal member 30 and the inner ring 20 . This makes it possible to stop the rotary shaft 101 fixed to the inner ring 20 most quickly.
[0041]
In addition, since the inclined portion 33 of the seal member 30, which is particularly susceptible to elastic deformation, is configured to contact the inclined surface 23 of the inner ring 20, the contact pressure between the seal member 30 and the inclined surface 23 is less than in the case of contact in the radial direction. can be reduced by As a result, the opening and closing operation of the seal member 30 by compressed air can be performed smoothly and with high responsiveness. In addition, the contact pressure can be reduced with less compressed air pressure than in the conventional structure, so that the rotational speed of the rotating shaft 101 can be further increased and the stop time can be shortened at the same time.
[0042]
A contact surface that makes surface contact with the inclined surface 23 of the inner ring 20 is provided on the inclined portion 33 at the tip of the seal member 30, thereby reducing the surface pressure acting on the seal member 30 and reducing wear. In addition, the increased contact area improves the sealing performance.
Furthermore, the contact surface of the inclined portion 33 may be a surface that makes line contact with the inclined surface 23 of the inner ring 20 . In that case, the frictional resistance is reduced as compared with the case of surface contact, which is advantageous for high-speed rotation.
[0043]
Particularly in dental air turbine handpieces, extremely high-speed rotation is required when grinding teeth, and abrupt rotation stop performance is required within 2 seconds, preferably within 1 second when stopping. According to this configuration, the effect of increasing the rotation speed and shortening the stop time can be stably obtained, so that the usability of the dental air turbine handpiece can be greatly improved.
[0044]
And when the dental air turbine handpiece is driven, compressed air is less likely to leak from the inside of the bearing than when there is no sealing member, so noise during driving is reduced and high quietness can be obtained.
[0045]
Further, as shown in FIG. 5, a pair of rolling bearings are arranged on the rotary shaft 101, and the seal member 30 is arranged at one axial end of the outer ring 10 opposite to the inlet of the compressed air. be. As a result, lubricating oil can be supplied into each rolling bearing from the bearing end side where the seal member 30 is not arranged by spraying oil from between the pair of rolling bearings. Further, since the seal member 30 is arranged on the opposite side of the spray lubricating side, liquid leakage from each rolling bearing to the outside of the head portion 122 does not occur.
[0046]
Usually, dental air turbine handpieces are autoclaved for high-temperature cleaning and sterilization after use. Although this process reduces the amount of lubricating oil in the rolling bearing, since the seal member 30 is arranged only at one axial end of the rolling bearing, the lubricating oil can be easily supplied from the other axial end. Therefore, the rolling bearing can always be kept in a good lubricating state, and the rotating shaft 101 can be stably driven to rotate.
[0047]
The present invention is not limited to the above embodiments, and can be modified, improved, etc. as appropriate.
For example, in the retainer 5 used in the rolling bearing 1 of the above embodiment, the rim portion 7 on the one end side is arranged upstream of the balls 3 in the supply direction of the compressed air. may be arranged on the axially opposite side of the sealing member.
[0048]
In addition, since the seal member is disposed only on one axial end side of the rolling bearing 1, the groove portion 13 of the outer ring 10 and the inclined surface 23 of the inner ring 20 are formed only on one axial end side, but the present invention is not limited to this. , and may be formed symmetrically on the other end side in the axial direction. In that case, one of the pair of inclined surfaces is not used, but in the process of assembling the rolling bearing, there is no need to be aware of the assembling direction, and the work process can be simplified.
[0049]
In addition, although the sealing member may have a constant thickness at the inclined portion, it may gradually decrease toward the inner side in the radial direction. In this case, the thickness Skt of the inclined portion 33 in the inclined direction is the thickness of the thickest portion.
[0050]
This application is based on a Japanese patent application (Japanese Patent Application No. 2019-238708) filed on December 27, 2019 and a Japanese patent application (Japanese Patent Application No. 2019-238709) filed on December 27, 2019. The contents are incorporated into this application by reference.
Code explanation
[0051]
1 Rolling bearing
　　3 balls (rolling elements)
5 Cage
　　7　Rim part
10 Outer ring
11 inner peripheral surface
13 Groove
　15 tapered surface
17 Axial inner surface
20 Inner ring
21 outer peripheral surface
23 Inclined surface
30 Seal member
31 base
　33 Inclined part
35 Inner peripheral surface
40 Retaining ring
100 Air turbine bearing unit
120 dental air turbine handpiece
122 head part
S Bearing internal space
Ts Clearance in the circumferential direction of the retaining ring when the sealing member and retaining ring are attached to the groove of the outer ring
Sct Thickness in the axial direction of the part in contact with the retaining ring of the seal member
Lss The length in the radial direction where both sides in the axial direction of the seal member are in contact with and sandwiched between the retaining ring and the groove of the outer ring
Φdg Internal diameter of outer ring
The scope of the claims
[Claim 1]
the outer ring and
Inner ring and
a plurality of rolling elements arranged to be free to roll between the outer ring and the inner ring;
a substantially annular seal member provided at one end in the axial direction of the space inside the bearing between the outer ring and the inner ring and made of an elastic body without a core metal,
The outer circumference of the seal member is fixed by a snap ring to a groove formed in the inner circumference of the one axial end of the outer ring, and the inner circumference is elastically deformable,
　The one end in the axial direction is opposite to the inlet of the compressed air supplied to the inner space of the bearing side and
The seal member has an inclined portion inclined toward one end in the axial direction on the inner diameter side,
When the seal member and the snap ring are attached to the groove of the outer ring, the circumferential gap between the snap ring is Ts, and the axial thickness of the portion of the seal member that contacts the snap ring is Sct,
1 ≤ Ts/Sc ≤ 10
A rolling bearing for air turbines.
[Claim 2]
Let Lss be the radial length of the two axial side surfaces of the sealing member that are in contact with and sandwiched between the retaining ring and the groove of the outer ring, and Φdg be the inner diameter of the outer ring,
0.018≦Lss/Φdg≦0.093
The rolling bearing for an air turbine according to claim 1, wherein: