JP2013204808A - Sliding bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sliding bearing configured to improve conformability following an opposite shaft without reducing wear resistance of a resin coating layer containing solid lubricant.SOLUTION: In a sliding bearing, a resin coating layer 103 containing binder resin and solid lubricant is provided on a sliding bearing substrate including back metal steel 101 and a bearing alloy layer 102 provided on the back metal steel. When an axial width of the resin coating layer 103 is 1, a ratio between the widths of both axial end portions A, A is 0.3 to 0.7. The thickness of the resin coating layer 103 is decreased gradually from an axial center portion toward axial terminal ends, and thickness C of both axial ends is smaller than the thickness D of the axial center portion. In the resin coating layer, hardness of both axial end portions A, A is higher than that of the axial center portion B.

Description

  The present invention relates to a sliding bearing used for an engine for automobiles and other industrial machines, and more particularly to solid lubrication on a bearing base material having a back metal and a bearing alloy layer disposed on the back metal. The present invention relates to a plain bearing provided with a resin coating layer containing an agent.

  As a material for a sliding bearing (sometimes simply referred to as a “bearing”) for an automobile engine, a bearing in which a bearing alloy layer (lining) is provided on a back metal is generally used. In recent years, the performance of automobile engines with high output and high revolution has been remarkably improved, and excellent sliding performance such as initial conformability, seizure resistance, durability, and heat resistance is desired for these bearing materials.

  A bearing provided with a bearing alloy layer (lining) of aluminum or copper on the back metal can ensure fatigue resistance and seizure resistance, but has insufficient initial conformability. In addition, in order to ensure conformability, a copper lead alloy bearing in which a soft metal film such as Sn or Pb is further provided on the sliding surface is not sufficient in wear resistance and requires a complicated manufacturing process, resulting in high cost. There was also a problem.

  Therefore, in order to ensure excellent wear resistance, a resin coating layer including a solid lubricant and a binder resin has been proposed in place of the soft metal coating (Patent Documents 1 and 2). The resin coating layer containing the solid lubricant has better wear resistance than the soft metal coating, and has obtained good results even under recent engine operating conditions in which the engine is repeatedly started and stopped.

JP-A-4-83914 Japanese Patent No. 3133209

However, both ends of the bearing sliding surface in the axial direction tend to make strong contact between the shaft and the bearing, and the load during sliding is larger than that at the center. In addition, since the resin coating layer containing a solid lubricant is harder than the soft metal coating, it tends to have a low property of changing the shape of the coating layer along the other axis due to deformation and wear during sliding, so-called conformability. . Thus, there was room for improvement, such as the occurrence of initial baking due to local contact with both ends and insufficient conformability of the resin coating layer.
Therefore, the present invention provides a plain bearing that improves the conformability to the counterpart shaft without impairing the wear resistance of the resin coating layer containing the solid lubricant, and achieves both wear resistance and conformability. It is intended.

  As a result of earnest study to achieve the above object, the present inventor has made the resin coating layer thinner at the axial end portions than at the axial central portion, and has the hardness at the axial end portions. It has been found that, by increasing the height from the center in the axial direction, it is possible to impart conformability and abrasion resistance that can withstand strong surface pressure during sliding, and the present invention has been completed.

That is, the present invention relates to the following <1> to <7>.
<1> A sliding bearing in which a resin coating layer containing a binder resin and a solid lubricant is provided on a sliding bearing base material having a backing metal and a bearing alloy layer provided on the backing steel,
When the axial width dimension of the resin coating layer is 1, the ratio of the width dimension at both ends in the axial direction is 0.3 or more and 0.7 or less,
The thickness of the resin coating layer is gradually decreased as the thickness of both axial ends is smaller than the thickness of the axial central portion, and approaches the axial end from the axial central portion,
A sliding bearing in which the hardness of the resin coating layer is higher at both axial end portions than at the axial central portion.
<2> The plain bearing according to <1>, wherein a difference in thickness between the both axial ends of the resin coating layer and the central portion in the axial direction is 2 μm or more and 15 μm or less.
<3> The plain bearing according to <1> or <2>, wherein the nanoindenter hardness at both axial end portions of the resin coating layer is 0.08 GPa or more higher than the nanoindenter hardness at the axially central portion.
<4> Any of <1> to <3>, wherein a ratio of a width dimension of the other end portion to a width dimension of the one end portion in the axial direction of the resin coating layer is 0.8 or more and 1.25 or less. Or a plain bearing according to claim 1.
<5> The ratio <1> in which the ratio of the nanoindenter hardness at the other end to the nanoindenter hardness at one end in the axial direction of the resin coating layer is 0.8 or more and 1.25 or less. The sliding bearing according to any one of ~ <4>.
<6> The above <1> to <5>, wherein the solid lubricant contains at least one of molybdenum disulfide and graphite, and the content of the solid lubricant in the resin coating layer is 20 to 60% by volume in total. The plain bearing of any one.
<7> The plain bearing according to any one of <1> to <6>, wherein the content of the solid lubricant in the resin coating layer is larger in the axial end than in the axial center. .

  The plain bearing according to the present invention is formed by reducing the thickness of both end portions in the axial direction of the resin coating layer as compared with the central portion in the axial direction and increasing the hardness of both end portions in the axial direction from the central portion in the axial direction. The compatibility with the shaft is good, initial seizure can be prevented, and the wear resistance can be improved.

FIG. 1 is a schematic view showing an embodiment of the slide bearing of the present invention. FIG. 2 is a sectional view showing an embodiment of the slide bearing of the present invention. FIG. 3 is a sectional view showing an embodiment of the slide bearing of the present invention. FIG. 4 is a sectional view showing an embodiment of the slide bearing of the present invention. FIG. 5 is a sectional view showing an embodiment of the plain bearing of the present invention. FIG. 6 is a sectional view showing an embodiment of the slide bearing of the present invention.

Hereinafter, the present invention will be described in detail.
In the present invention, the axial end, the axial center, and the axial end of the slide bearing all mean the axial end, the axial central, and the axial end of the inner peripheral surface. In the present specification, these may be referred to only as an end, a center, and a terminal.

1 and 2 show an embodiment of a slide bearing according to the present invention.
The sliding bearing 1 according to the present invention is provided with a resin coating layer 103 containing a solid lubricant on a sliding bearing base material having a backing metal 101 and a bearing alloy layer 102 disposed on the backing metal 101. Yes. This resin coating layer is characterized in that the thickness at both ends in the axial direction is smaller than the thickness at the central portion in the axial direction, and the hardness at both axial end portions is higher than the hardness at the central portion in the axial direction. A symbol A in FIGS. 1 and 2 corresponds to an axial end portion, and is a portion occupying a certain range from the axial end to the central portion. Both axial ends indicate the axial ends A and A. The axial central portion B is a portion excluding the axial end portions A and A.

  The lower end of the ratio of the width dimension (A + A) at both ends in the axial direction is 0.3 or more, assuming that the width dimension of the entire axial width (A + B + A) in the resin coating layer is 1. 0.4 or more is preferable. Moreover, an upper limit is 0.7 or less, and it is preferable that it is 0.6 or less.

  If the width dimension of both ends is equal to or higher than the lower limit of the range, it is preferable from the viewpoint of the effect obtained on the conformability and improvement of seizure resistance, and if it is equal to or lower than the upper limit of the range, the surface applied to the sliding surface. It is preferable from the viewpoint of the effect on pressure and wear resistance.

  In addition, the width dimension of axial direction edge part A may be the same, or may differ. If they are different, the width dimension of each end can be determined appropriately within the above range, but if the width dimension of one end is extremely small, the familiarity performance cannot be ensured and the wear at both ends is uniform. Therefore, the ratio of the width dimension of one end to the width dimension of the other end is preferably in the range of 0.8 to 1.25.

  As shown in FIG. 2, the resin coating layer 103 has a smaller thickness C at both axial ends than the thickness D at the axial center. Further, the thickness of the resin coating layer gradually decreases from the axial center to the axial end. Since both ends in the axial direction of the bearing are in strong contact with the shaft, the surface pressure applied to both ends becomes large if the sliding surface of the bearing is flat. However, as in the present invention, the central portion of the sliding surface has a high shape, that is, a so-called crowning shape, and the thickness gradually decreases from the central portion to the end. And the local contact is relaxed, and the seizure resistance is improved.

  The difference between the thickness D at the central portion in the axial direction of the resin coating layer and the thickness C at both axial ends is preferably 2 to 15 μm, and more preferably 3 to 8 μm. If the difference in thickness is within the range, seizure due to high contact with the shaft at the axial end portion is unlikely to occur, and the load only on the central portion becomes large, resulting in high surface pressure on the sliding surface. This is preferable because there is no increase in wear.

  Moreover, it is preferable that the thickness D of the axial direction center part in a resin coating layer is 3-15 micrometers, and it is more preferable that it is 3-9 micrometers. If it is within such a range, it is too thin to lose the effect of wear resistance, and is not easily worn at an early stage, and is too thick to easily cause peeling in the layer. In addition, the thickness of the layer is considered to affect the adhesive strength with the base material, the strength within the layer, the thermal conductivity, and the like.

  Further, the thickness C at both ends in the axial direction in the resin coating layer is preferably 2 μm or more from the viewpoint of wear resistance.

  The shape of the resin coating layer is not particularly limited, and the cross-sectional shape at both ends in the axial direction is a tapered shape as shown in FIG. 2, a convex curved shape on the inner peripheral side as shown in FIG. 3, and as shown in FIG. A concave curved surface shape on the inner circumference side, a two-stage taper shape having a convex inflection point on the inner circumference side as shown in FIG. 5, and a concave inflection point on the inner circumference side as shown in FIG. The two-stage taper shape is exemplified.

  The resin coating layer in the present invention is characterized in that the hardness at both axial end portions is higher than that in the axial central portion. By making the hardness of both ends higher than that of the central portion, the wear resistance against local contact with the counterpart shaft is improved.

  The hardness of both axial end portions is nanoindenter hardness, and is preferably 0.08 GPa or more higher than the axial central portion, more preferably 0.1 GPa or more, and 0.2 to 0.3 GPa higher. Further preferred. If the difference in hardness is within such a range, it is preferable from the viewpoint of wear resistance and conformability.

  Moreover, it is preferable from a viewpoint of abrasion resistance that the hardness of axial both ends is 0.4 GPa or more in terms of nanoindenter hardness, and the hardness of the axial center is 0.1 GPa or more in terms of nanoindenter hardness. Less than 0.7 GPa is preferable from the viewpoint of conformability and adhesion.

  In addition, the hardness of the axial end portions A may be the same or different, but in order to ensure the conformability with the shaft and to wear both ends uniformly, the nanoindenter hardness of one end portion The ratio of the nanoindenter hardness at the other end to the thickness is preferably in the range of 0.8 to 1.25. The above-mentioned nanoindenter hardness is an average value at both end portions, and it is preferable that both end portions are within the numerical range.

  The resin coating layer in the slide bearing of the present invention contains a binder resin and a solid lubricant.

The solid lubricant is not particularly limited, and a commonly used solid lubricant can be used regardless of the presence or absence of orientation. Specific examples include molybdenum disulfide (MoS ₂ ), graphite, hexagonal boron nitride (h-BN), WS ₂ , polytetrafluoroethylene (PTFE), etc. Among them, strength and solid lubricant with resin From the viewpoint of adhesion, molybdenum disulfide (MoS ₂ ) and graphite are preferably used. Moreover, a solid lubricant may be used by 1 type, or may be used in combination of 2 or more type.

The total content of the solid lubricant is preferably 20 to 60% by volume with respect to the entire resin coating layer.
Solid lubricant comprises either at least MoS ₂ and graphite, in the case where the other MoS ₂ or graphite further comprises a solid lubricant, MoS ₂ or the total amount of graphite of at least solid lubricant overall 50% Is preferably exceeded.
The solid lubricant content, blend type, and blend ratio may be the same or different at the axial center and axial end, but from the standpoint of compatibility, the solid lubricant content is axial. It is preferable that the end portion is larger than the central portion in the axial direction.

  As the binder resin, a resin having mechanical strength and high heat resistance is preferably used. Specific examples include thermosetting resins such as polyimide resin (PI), polyamideimide (PAI) resin, epoxy resin, and phenol resin, and thermoplastic resins such as polybenzimidazole (PBI) and PES.

  The binder resin may be used alone or in combination of two or more. In the case of combining, a PAI resin or PI resin may be combined with a PA resin or an epoxy resin to add a high shear force to form a polymer alloy. A coupling agent or the like may be added to the binder resin.

  The resin coating layer is provided on a bearing base material having a backing metal and a bearing alloy layer disposed on the backing metal. Here, as the back metal and the bearing alloy layer, various materials conventionally used in the field can be used under various conditions.

The bearing alloy layer includes Al, Cu and the like as main components, and among them, an aluminum bearing alloy, a copper bearing alloy and the like are preferable.
As the bearing alloy layer, an aluminum bearing alloy or a copper bearing alloy can be used.

The composition of the aluminum-based bearing alloy is not particularly limited, but is a group consisting of 10% by mass or less of Cr, Si, Mn, Sb, Sr, Fe, Ni, Mo, Ti, W, Zr, V, Cu, Mg, Zn, and the like. An alloy containing one or more elements selected from the group consisting of 20% by mass or less and one or more elements selected from the group consisting of Sn, Pb, In, Tl, Bi, and the like can be preferably used.
The former group of elements mainly imparts strength and wear resistance, and the latter group of elements mainly imparts conformability, and exhibits bearing characteristics depending on the type and amount of elements added.
Further, the resin coating layer of the present invention can be used with the skirt portion of a piston made of a high Si—Al alloy such as AC8A or AC9B, which is an aluminum alloy casting, as a base, and the wear resistance can be improved.

The composition of the copper alloy is not particularly limited, but an alloy containing 25% by mass or less of Bi, 10% by mass or less of Sn, and 2% by mass or less of P, Ag, In, Ni, Al, or the like is preferably used. can do.
Among these elements, Bi, which is a soft metal, exhibits conformability, Sn, which is a basic component of bronze, exhibits high strength and wear resistance, and other components supplementarily improve characteristics. In particular, P is effective for deoxygenation, promotion of sintering, strengthening, etc. Ag forms a compound effective for improving sliding characteristics by reaction with lubricating oil or impurity component S in copper, and In is corrosion resistance and lubrication. Oil and wettability are improved, and Ni and Al have effects such as strengthening copper.

  The bearing alloy layer generally has a thickness of 0.2 to 0.5 mm. The thickness of the backing metal that supports this can generally be selected according to the shaft diameter.

  The shape of the plain bearing of the present invention may be cylindrical or halved. When the bearing is halved, two halved plain bearings are combined into a cylindrical shape.

  When the sliding bearing of the present invention is used, the lubrication conditions are not particularly limited, and it can be applied under oil lubrication using a lubricating oil or under non-lubrication.

Next, a method for manufacturing the plain bearing according to the present invention will be described.
The manufacturing method of a slide bearing includes the following processes, for example.
(A) A step of obtaining a plain bearing base material in which a bearing alloy layer is provided on the back metal steel by press-contacting the alloy to the back metal steel, or sintering and press-contacting the alloy metal powder after being dispersed on the back metal steel;
(B) A step of applying a treatment for ensuring adhesion between the base material and the resin coating layer on the surface of the bearing alloy layer of the plain bearing base material;
(C) applying a resin composition containing a binder resin and a solid lubricant to the bearing alloy layer surface;
(D) a step of curing the binder resin by heating,
(E) A step of crushing both axial ends of the resin coating layer.

  Examples of the step (b) include physical treatment represented by roughening treatment such as sand blasting, and chemical treatment such as etching and chemical conversion treatment.

In applying the resin composition in the step (c), a coating liquid in which a solid lubricant, a binder resin and other optional components are mixed is prepared. Further, a solvent such as methyl pyrrolidone can be used as necessary for enhancing the dispersibility of the solid lubricant and the binder resin or for adjusting the viscosity of the coating solution.
The method for applying the resin composition is not particularly limited, and examples include an air spray method, an airless spray method, a roll method, a pad method, a screen printing method, electrostatic coating, and tumbling.

  Thereafter, the solvent is evaporated by drying. In drying, the method is not particularly limited as long as the solvent evaporates, but it is preferable to perform the drying at 40 to 120 ° C. for 5 to 30 minutes from the viewpoint of adhesion between the lining and the resin. Note that the temperature during drying may be constant within the above temperature range, or may be changed by raising the temperature.

  The heating step (d) is not particularly limited as long as the binder resin is cured, but may be performed at 150 to 400 ° C. for 30 to 60 minutes. Note that the temperature during firing may be constant within the above range, or may be changed by raising the temperature.

  The resin coating layer in the present invention can be a layer having a smaller thickness and a higher hardness than the central portion in the axial direction by the step (e) in which both axial end portions are crushed.

  The crushing means crushing by applying pressure, and the crushing method is not limited. For example, a method of rolling down the end portion with a roll is used.

A preferred embodiment in the case of rolling down with a roll will be described below.
For example, it is a cylindrical roll, and an inner surface roll that rolls the sliding surface (inner peripheral surface) side of the bearing and a rear roll that rolls the rear surface (outer peripheral surface) side of the sliding surface are used in combination. The inner surface roll on the sliding surface side is loaded only at both ends in the axial direction of the slide bearing and the central portion is not loaded, and the desired shape defined in the present invention, that is, the thickness of the central portion in the axial direction. A recess is made in the center portion of the inner surface roll so that the thickness at both ends in the axial direction is smaller than the thickness. By rotating both rolls and passing the slide bearing through the gap between both rolls, a bearing that is crushed only at both ends of the inner surface (sliding surface side) can be manufactured. For example, the recess shape of the inner surface roll can be appropriately changed so that the thickness and hardness of the resin coating layer are within the scope of the present invention.

  By crushing, both ends of the resin coating layer in the axial direction of the sliding bearing can be gently inclined, and the shape follows the misalignment of the shaft and the inclination of the shaft. It becomes good and seizure resistance is improved. Further, since the resin at the end portion is compressed, the density of the resin at the portion increases and the hardness increases, resulting in excellent wear resistance.

  EXAMPLES The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

Example 1
-Manufacture of plain bearings (half bearings) Bearing base materials were manufactured by press-contacting back metal and aluminum half bearing alloys. In order to ensure the adhesion between the bearing substrate and the resin coating layer, the surface was roughened with sand blasting, and then washed to remove the blasting powder.
A coating solution of a resin composition in which a binder resin, a solid lubricant, and a solvent were mixed with the composition shown in Table 1 was prepared. N-methyl-2pyrrolidone (NMP) was used as the solvent.
The bearing substrate obtained above is preheated in a 70 ° C. atmosphere, and both ends in the axial direction are masked, and the above coating solution is applied by a spray method at a film thickness of 6 μm at the center in the axial direction on the bearing alloy layer. Applied. Thereafter, drying was performed at 100 ° C. for 30 minutes. Furthermore, the resin coating layer was obtained by baking at 250 degreeC for 1 hour.
Subsequently, both ends of the resin coating layer were crushed with a roll. Specifically, a cylindrical inner surface roll that rolls on the sliding surface (inner peripheral surface) side of the bearing and a cylindrical rear roller that rolls on the rear surface (outer peripheral surface) side of the sliding surface are combined and rotated. By passing a slide bearing through the gap between the rolls, only the both ends on the inner surface (sliding surface side) were crushed. In the center part of the inner surface roll, the width dimension ratio of both ends is 0.5 (the width dimension ratio of the end is 0.25 respectively), the difference in thickness between the center and both ends is 4 μm, and the cross section is illustrated. A recess designed to have a tapered shape as shown in FIG.
Thus, the slide bearing of Example 1 was produced.

(Examples 2-3, Comparative Examples 1-2)
A plain bearing was produced in the same manner as in Example 1 except that the composition of the resin coating layer and the recess shape of the inner surface roll were changed to the results shown in Table 1.

(Comparative Example 3)
A slide bearing was produced in the same manner as in Example 1 except that the composition of the resin coating layer was changed to the result shown in Table 1 and both ends were not crushed.

As Examples 1 to 4 and Comparative Examples 1 and 2, the nano-indata hardness of the resin coating layer in the slide bearing produced according to each condition was measured, and the seizure surface pressure and the wear amount were tested.
The nano indenter hardness is a value measured at the axial center of the bearing in the axial center, and both axial ends are 10% of the axial length of the bearing from the axial end of the bearing. The average value of the measured values at the respective positions on the axial center side of the bearing. About the comparative example 3, it measured in the axial direction center.
The seizure surface pressure and the amount of wear were measured by a seizure test and a wear test, respectively.

(Nanoindenter hardness test)
The nanoindenter test of the resin coating layer was performed under the following conditions using an ultra-micro hardness meter (manufactured by Elionix Co., Ltd.), and the nanoindenter hardness at the axial center and both axial ends was measured.
Measurement temperature: 23 ° C
Applied load: 1000mg
Step interval: 20 msec
Number of divisions: 500

(Baking test)
The seizure test was performed under the following conditions using a bearing seizure tester.
Rotation speed: 8000rpm
Lubricating oil: 0W-20
Lubrication temperature: 120 ° C
Load: Gradually increase by 3 kN every 3 minutes

(Abrasion test)
The wear test was performed using the wear tester per piece under the following conditions. In addition, a measurement location is an axial direction center part.
Rotational speed: 0 rpm (1 minute hold) → 1200 rpm (1 minute hold) → 0 rpm (1 minute hold) cycle test Lubricating oil: 0W-20
Lubrication temperature: 100 ° C
Load: 4.41kN
Test time: 100 hours

  The results of the above tests are summarized in Table 1.

From Table 1, seizure resistance is achieved by providing both end portions with thicknesses at both ends smaller than the central portion in the axial direction compared to a conventional plain bearing (Comparative Example 3) in which both end portions in the axial direction are not crushed. It was found that the property was remarkably improved. Also, wear resistance was improved.
In the present invention, by providing both ends with thicknesses at both ends smaller than the central portion in the axial direction, and by providing gentle slopes at both ends, the surface pressure on the concentrated local area is reduced, and seizure resistance It is thought that the compatibility improved. Further, it is considered that the wear resistance is improved by improving the hardness of both ends by crushing.
On the other hand, when the ratio of the width dimension at both ends in the axial direction exceeded 0.8, seizure resistance and wear resistance were lowered.

The present invention reduces the initial seizure and improves the conformability by reducing the thickness of both ends in the axial direction of the resin coating layer in the slide bearing from the axial central portion, and has high hardness at both ends. Therefore, the present invention provides a plain bearing having excellent wear resistance.
By giving such characteristics to both ends in the axial direction, high conformability and wear resistance can be obtained. Therefore, the slide bearing of the present invention is used in high-performance automobiles such as start / stop engines and engines for other industrial machines. It is particularly useful.

DESCRIPTION OF SYMBOLS 1 Slide bearing 101 Back metal steel 102 Bearing alloy layer 103 Resin coating layer A Axial end part B Axial center part C Resin coating layer Axial thickness of both ends D Resin coating layer axial center part thickness E Axial direction F Sliding direction (circumferential direction)

Claims (7)

A sliding bearing provided with a resin coating layer containing a binder resin and a solid lubricant on a sliding bearing base material having a backing metal and a bearing alloy layer provided on the backing steel,
When the axial width dimension of the resin coating layer is 1, the ratio of the width dimension at both ends in the axial direction is 0.3 or more and 0.7 or less,
The thickness of the resin coating layer is gradually decreased as the thickness of both axial ends is smaller than the thickness of the axial central portion, and approaches the axial end from the axial central portion,
A sliding bearing in which the hardness of the resin coating layer is higher at both axial end portions than at the axial central portion.   The slide bearing according to claim 1, wherein a difference in thickness between the axially opposite ends and the axially central portion of the resin coating layer is 2 µm or more and 15 µm or less.   3. The plain bearing according to claim 1, wherein the nanoindenter hardness at both axial end portions of the resin coating layer is 0.08 GPa or more higher than the nanoindenter hardness at the axial central portion.   The ratio of the width dimension of the other edge part with respect to the width dimension of the one edge part of the axial direction of the said resin coating layer is 0.8 or more and 1.25 or less, The any one of Claims 1-3. Slide bearing.   The ratio of the nano indenter hardness of the other edge part with respect to the nano indenter hardness of the one edge part of the axial direction of the said resin coating layer is 0.8 or more and 1.25 or less, Any of Claims 1-4 2. A plain bearing according to item 1.   6. The solid lubricant according to claim 1, wherein the solid lubricant contains at least one of molybdenum disulfide and graphite, and the total content of the solid lubricant in the resin coating layer is 20 to 60% by volume. Plain bearings.   The plain bearing according to any one of claims 1 to 6, wherein a content of the solid lubricant in the resin coating layer is larger in an axial end portion than in an axial central portion.

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