JP2648050B2 - Brushless motor for media drive - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brushless motor for driving a radial gap type medium used in a device for rotating a disk type medium such as a magnetic disk or an optical disk, for example, a hard disk drive.

[0002]

2. Description of the Related Art FIG. 8 shows a DC brushless motor for driving a medium conventionally used as a hard disk drive.

[0003] In the figure, reference numeral 1 denotes a fixed base made of a non-magnetic metal material, and a stator support 2 is integrally provided on a peripheral portion thereof.
The fixed base 1 is press-fitted into a fixing hole 3a of a lower frame 3 of the hard disk drive. The stator support portion 2 includes a ring-shaped first portion 2a which engages with an inner peripheral surface of a stator core described later, and a step-like second portion 2 which is connected to a lower end of the portion 2a and engages with an end surface of the stator core.
b.

[0004] A stator 4 is mounted on the stator support 2 as described later. The stator 4 is provided with exciting windings on each of salient poles radially projecting outward in the radial direction of a stator core 5 formed by laminating a plurality of core plates 5a having the same structure as shown in FIG. It is formed by winding the wire 6. Therefore, the inner peripheral surface of the stator 4 has a configuration in which the diameter does not change. Reference numeral 7 denotes slot insulation for insulating between the stator core 5 and the winding 6.

Further, as shown in FIG. 7, half of the salient pole portions 5c protruding from the ring portion 5b are subjected to half blanking processing so that each core plate 5a is formed on one surface of the salient pole portions 5c. The convex portion is provided with a concave portion on the other surface (in FIG. 7, reference numeral 5d indicates a half blanking portion), and the concave portion and the convex portion are press-fitted between the core plates 5a adjacent in the laminating direction. Thus, they are connected to form the stator core 5. The stator 4 has a lower end surface of the stator core 5 abutting on the second portion 2b and a lower inner peripheral surface of the core 5 pressed into the outer peripheral surface of the first portion 2a, so that the stator support portion 2 Fixed.

At the center of the fixed base 1, a fixed shaft 8 that projects vertically through the center of the stator 4 is provided. The upper end of the fixed shaft 8 is connected to the upper frame 10 of the hard disk drive by means of a screw 9 screwed thereto, whereby the fixed shaft 8 is supported at both ends. The upper frame 10 and the lower frame 3 are connected to each other to form a shield case.

A rotor 12 for supporting a hard disk 11 is rotatably supported on the fixed shaft 8 via a first ball bearing 13 and a second ball bearing 14. The rotor 12 has a cup shape that has a lower surface opened and accommodates the stator 4 therein. A rotor magnet 16 is attached to an inner peripheral surface of a peripheral cylindrical wall 15 of the rotor 12, and the rotor magnet 16 faces downward at the center. A projecting cylindrical bearing support 17 is integrally provided. The outer diameter Z of the peripheral cylindrical wall 15 of the rotor 12 is 1
It is about 0 mm to 20 mm.

The rotor magnet 16 is a plastic magnet formed in a ring shape, and is magnetized along its radial direction. With this magnetization, an N pole and an S pole are formed on the inner peripheral portion of the magnet 16 in the circumferential direction. Are provided alternately.
The inner peripheral surface of the rotor magnet 16 is closely opposed to the distal end surface of each salient pole of the stator 4 via a slight air gap.

The bearing support 17 is inserted between the inner peripheral surface of the stator 4 and the fixed shaft 8. This support 1
A first ball bearing 13 is provided between the base 17a of the fixed shaft 7 and the tip of the fixed shaft 8, and a second ball is provided between the tip 17b of the bearing support 17 and the base of the fixed shaft 8. Bearing 14
Is provided. A step portion 17c is formed on the inner peripheral surface of the intermediate portion of the bearing support portion 17, and the outer races of the two ball bearings 13 and 14 are axially supported by upper and lower ends of the step portion 17c. 14 can be mounted under preload. The distal end of the bearing support 17 is slightly smaller in diameter than the other portions, and is inserted and arranged inside the first portion 2 a of the stator support 2. In addition,
In FIG. 8, reference numeral 18 denotes a closing ring fixed to the upper end of the outer race of the first ball bearing 13.

An annular flange portion 15a is integrally formed on the outer surface of the peripheral cylindrical wall 15, and, for example, two hard disks 11 are mounted above the flange portion 15a. . These disks 11
Is mounted by providing a spacer ring 19 between the two hard disks 11 fitted to the peripheral cylindrical wall 15 and screwing a ring-shaped nut 20 to a thread formed at the upper end of the peripheral cylindrical wall 15. Combine this with the flange 15
This is achieved by sandwiching the two hard disks 11 and the spacer ring 19 between them.

In the brushless motor having the above-described structure, the thickness A of the base 17a of the bearing support 17 in which the first ball bearing 13 is disposed inside and the thickness A of the bearing support 17 in which the second ball bearing 14 is disposed inside. The thickness B of the distal end portion 17b secures the required strength of the bearing support portion 17, and the bearing support portion 1
7 is set to be within the allowable value. Further, the width C (see FIG. 7) of the ring portion 5b of the stator core 5 ensures a stable engagement area with the two portions 2b and secures a necessary magnetic path cross-section formed between the rotor magnet 16 and the portion. In order to achieve this, it is formed in a predetermined size.

[0012]

In the conventional brushless motor described above, the thickness A of the base 17a is adjusted in order to obtain the strength required for the base 17a of the bearing support 17 inserted and arranged inside the stator 4. It is secured inside the stator 4. For this reason, the first ball bearing 13 disposed inside the base 17a has to be used with a small outer diameter. Similarly, in order to obtain the strength required for the tip 17b of the bearing support 17, the tip 1
The thickness B of 7b is ensured inside the first portion 2a of the stator support portion 2 located so as to cover the lower inner peripheral surface of the stator 4. For this reason, it has been necessary to use a second ball bearing 14 having a small outer diameter for the second ball bearing 14 disposed inside the distal end portion 17b.

The runout of the rotor 12 and the hard disk 11 supported by the rotor 12 is caused by ball bearings 13 and 14.
It is known that the rigidity greatly depends on the rigidity of the steel ball, and the rigidity increases in proportion to the diameter of the steel ball provided in the ball bearings 13 and 14. Therefore, in the above-described conventional configuration in which the ball bearings 13 and 14 having a small diameter must be used, the rigidity is small,
Of the disk 11 is large, which becomes an obstacle when increasing the recording density of the disk 11.

As one of the measures to make it possible to use large-diameter ball bearings 13 and 14 under conditions where the diameter of rotor 12 is limited, a method of increasing the entire inner diameter of stator 4 is known. Conceivable. However, in this case, the width C of the ring portion 5b of the core plate 5a becomes thinner, so that the ring portion 5b is easily magnetically saturated, and the torque is greatly reduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide a brushless motor for driving a medium in which a large-diameter ball bearing for rotatably supporting a rotor can be used without causing a large decrease in torque, thereby reducing surface run-out. Is to get

[0016]

SUMMARY OF THE INVENTION According to the present invention, there is provided a fixed base having a stator support, a stator mounted on the stator support, and projecting from the fixed base and penetrating the center of the stator. A cylindrical bearing having a fixed shaft and a cup shape for accommodating the stator, having a rotor magnet mounted on an inner peripheral surface of a peripheral cylindrical wall thereof, and a central portion disposed between the fixed shaft and the stator; A first ball bearing provided between a base of the bearing support and a distal end of the fixed shaft, the rotor having a support and having a disk-shaped medium mounted on the outer periphery of the peripheral cylindrical wall; When,
A radial gap type provided between a tip portion of the bearing support portion and a base portion of the fixed shaft, the second ball bearing rotatably supporting the rotor on the fixed shaft together with the first ball bearing. This is implemented in the medium drive brushless motor.

In order to achieve the above object, the inner diameter of the end of the stator on the side of the first ball bearing is made larger than the inner diameter of the intermediate part in the thickness direction of the stator, so that the first ball bearing of the stator is formed. A relief portion is formed on the inner surface of the side end portion, and an outer diameter of a base portion of the bearing support portion in which the first ball bearing is disposed inside is larger than an inner diameter of an intermediate portion in a thickness direction of the stator. Is inserted into the escape portion. In the present invention, the inner diameter of the second ball bearing side end located opposite to the relief portion with respect to the middle portion in the thickness direction of the stator is equal to or larger than the inner diameter of the middle portion in the thickness direction of the stator. Is the diameter.

In order to achieve the same object, the inner diameter of the end of the stator on the side of the second ball bearing is made larger than the inner diameter of the intermediate part in the thickness direction of the stator. Form a relief on the inner surface of the ball bearing side end,
The stator support portion is engaged with the relief portion, and the inner peripheral surface of the support portion and the inner peripheral surface of the intermediate portion in the thickness direction of the stator are substantially flush with each other. The tip of the bearing support may be provided. In the present invention, the inner diameter of the first ball bearing side end located opposite to the relief portion with respect to the middle portion in the thickness direction of the stator is equal to or larger than the inner diameter of the middle portion in the thickness direction of the stator. Is the diameter.

In order to achieve the same object, the inner diameter of the end of the stator on the side of the first ball bearing is made larger than the inner diameter of the intermediate part in the thickness direction of the stator, so that the first ball of the stator is formed. A first relief portion is formed on an inner surface of the bearing side end portion, and an outer diameter of a base portion of the bearing support portion in which the first ball bearing is disposed is larger than an inner diameter of a thickness direction intermediate portion of the stator. , The outer peripheral portion of the base portion to the first
The inner diameter of the end of the stator in the second ball bearing is made larger than the inner diameter of the intermediate part in the thickness direction of the stator, and the inner diameter of the end of the second ball bearing on the side of the second ball bearing is increased. A second relief portion is formed on the inner surface, and the stator support portion is engaged with the second relief portion, and the inner peripheral surface of the support portion and the inner peripheral surface of the intermediate portion in the thickness direction of the stator are substantially flush with each other. And the distal end of the bearing support may be disposed inside the stator support. Further, in order to secure a necessary and sufficient magnetic path, it is preferable that the stator supporting portion is made of a magnetic material.

[0020]

According to the first aspect of the present invention, the outer peripheral portion of the base of the bearing support portion is inserted into the relief portion formed at the end of the first ball bearing on the side of the stator. Can be provided so as not to be arranged in the radial direction but to overlap, and the first ball bearing arranged inside the base can be enlarged in accordance with the penetration. In addition, since the inner diameter of the portion for securing the magnetic path in the inner peripheral portion of the stator is smaller than the inner diameter of the escape portion, the width is large and a necessary magnetic path cross section can be secured.

According to the second aspect of the present invention, the stator support portion is inserted into the relief portion formed at the end of the stator on the side of the second ball bearing, so that the support portion and the inner peripheral portion of the stator are radially aligned. Instead, the tip of the bearing support having the required thickness can be disposed inside the stator support, so that the second ball bearing is disposed inside the tip. Can be increased in accordance with the penetration. In addition, since the inner diameter of the portion for securing the magnetic path in the inner peripheral portion of the stator is smaller than the inner diameter of the escape portion, the width is large and a necessary magnetic path cross section can be secured.

According to a third aspect of the present invention, the first and second ball bearings having a large diameter can be used for both the first and second ball bearings by obtaining the effects of the first and second aspects. Due to the axial middle part of the stator, the inner diameter of which is smaller than the part,
A necessary magnetic path cross section can be secured.

According to a fourth aspect of the present invention, since the stator supporting portion engaged with the relief portion formed at the end of the second ball bearing of the stator is made of a magnetic material, this supporting portion is used as a part of the magnetic path. Can be used.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIGS. In the description, the same components as those of the conventional DC brushless motor described with reference to FIGS. 7 and 8 are denoted by the same reference numerals as those in the conventional example, description thereof will be omitted, and different configurations will be described. Only the portions will be described below.

The difference from the conventional example is that the fixed base 21, the stator core 25 of the stator 24, the cylindrical bearing support 27 of the rotor 12, and the first and second ball bearings 43, 4 are provided.
4.

A fixed base 21 having the same diameter as the conventional fixed base.
Is formed of a magnetic metal material, and the stator support portion 22 provided on the periphery thereof includes a ring-shaped first portion 22a and a second portion 22b as in the conventional example. The first part 22
“a” is formed to have a thickness substantially equal to that of a second escape portion described later. The thickness of the first portion 22a can be thicker than that of the conventional example, and the second portion 22b is correspondingly narrower.

The stator core 25 of the stator 24 has core plates 25X and 25Y made of two kinds of silicon steel plates having different inner diameters.
Are laminated. The first core plate 25X is shown in FIG.
As shown in (2), a plurality of salient pole portions 5c are radially projected outward from the outer circumference of the ring portion 25e. The second core plate 25Y has a ring portion 25f as shown in FIG.
A plurality of salient pole portions 5c are radially projected outward on the outer periphery of. The ring portions 25e and 25f have the same outer diameter, but the inner diameter is larger in the ring portion 25f. The first core plate 25X is the same as the conventional core plate. As shown in FIG. 2, a plurality of second core plates 25 </ b> Y are stacked to form an intermediate core portion 31 located at an axially intermediate portion of the stator core 25. A plurality of first core plates 25X are stacked to form a first end core portion 32 stacked on one surface of the intermediate core portion 31, and a second end core portion 33 stacked on the other surface of the intermediate core portion 31. Has made.

Therefore, the first and second end core portions 32,
The inner diameter D of 33 is formed larger than the inner diameter E of the intermediate core portion 31, whereby a first relief portion 34 is provided at one end of the stator 24 located on the first ball bearing 23 side, Stator 2 located on second ball bearing 24 side
The second escape portion 35 is provided at the other end of the fourth.

The stator 24 having such a configuration is attached to the fixed base 1 by engaging the first portion 22a of the stator support portion 22 with the second relief portion 35. With this attachment, the inner peripheral surface of the stator support portion 22 is
The first inwardly protruding portion 31a is substantially flush with the inner peripheral surface. The stator 24 is attached by press fitting.
The tip of the first portion 22a is the inward projecting portion 3 of the intermediate core portion 31
1a while being in contact with the lower surface of the first portion 22a.
The inner peripheral surface of the second end core portion 33 is pressed against the outer peripheral surface of, and the lower surface of the second end core portion 33 is pressed against the second portion 22b.

The bearing support 27 has a base 27a larger than the inner diameter E of the inwardly protruding portion 31a.
The distal end portion having a smaller diameter than this is formed as a straight structure in which the diameter does not change. The outer peripheral portion of the base portion 27a enters the first escape portion 34 and
The thickness A between the first mounting groove 36 provided inside 7a and the outer peripheral surface of the base 27a has a thickness necessary for design. The first ball bearing 43 fitted in the first mounting groove 36 and mounted between the base 27a and the distal end of the fixed shaft 8 has a larger outer diameter in accordance with the first clearance 34. It is used.

The tip 27b of the bearing support 27 extends inside the stator support 22. This tip 2
7b and the tip 27b provided inside the inner groove 7b.
Has a thickness necessary for design. A second ball bearing 44 fitted in the second mounting groove 37 and mounted between the distal end portion 27b and the base of the fixed shaft 8 has a larger outer diameter in accordance with the second relief portion 35. It is used.

According to the DC brushless motor having the configuration described above, the inwardly projecting portion 3
1a, first and second escape portions 34 and 35 are provided, respectively, and the first escape portion 34 is provided at the base 27a of the bearing support 27.
The outer peripheral part of the is inserted. Therefore, the base 27a and the stator 2 having the thickness A for obtaining the required strength are provided.
4 can be provided so as not to be arranged in the radial direction but to overlap. Therefore, the diameter of the first mounting groove 36 provided inside the base 27 can be increased in accordance with the entry of the base 27a into the first escape portion 34, and accordingly, a large-diameter first ball provided with a steel ball having a large diameter. A bearing 43 can be used.

At the same time, by allowing the stator support portion 22 to enter the second escape portion 35 of the stator 24,
The support portion 22 and the stator 24 are provided so as to overlap with each other instead of being arranged in the radial direction, and a distal end portion 27a of a bearing support portion 27 having a necessary thickness B is provided inside the stator support portion 22. it can. Therefore, this tip 27
Since b can be made thicker, the second mounting groove 37 provided on the inside thereof can be made larger in diameter in accordance with the entry, and accordingly, a large-diameter second ball bearing 44 provided with a steel ball having a larger diameter can be used.

As described above, since the first and second bearings 43 and 44 can be of large diameter, their rigidity can be greatly improved, and the runout of the rotor 12 and the hard disk 11 can be reduced. Note that this surface runout preventing action can be realized even when one of the first and second ball bearings 43 and 44 has a large diameter. High degree.

The first and second escape portions 34, 3
5, the first and second end core portions 31,
Although the width f (see FIG. 5) and the cross-sectional area of the ring portion 25f at 32 become smaller, the stator 24 has the inwardly protruding portion 31a formed by the ring portion 25e having the wide width C of the intermediate core portion 31. Therefore, the magnetic flux can mainly pass through this portion. Therefore, since a sufficient and sufficient magnetic path cross-sectional area can be secured, and magnetic saturation does not occur,
The torque does not decrease significantly.

Moreover, since the stator support portion 22 made of a magnetic material is engaged with the second escape portion 35, a magnetic flux can pass through the ring portion 25f and the first portion 22a as shown by the arrow in FIG. Therefore, the narrowing of the ring portion 25f by the first portion 22a can be compensated for, which is particularly advantageous in eliminating a substantial decrease in torque.

Further, since the second relief portion 35 can be provided with a thickness within the width of the ring portion 25f, the thickness B of the first portion 22a of the stator support portion 22 engaged with the second relief portion 35 can be increased. In the case of the related art shown in FIG. 8, the first portion 2a overlapping the inner peripheral surface of the lower end portion of the stator 4 is
Due to the required thickness, it had to be quite thin. ). Therefore, the strength of the stator support 22 is increased, and the stator 24 can be supported straight and stably.

As shown in FIG. 1, in the configuration in which the distal end portion 27a of the bearing support portion 27 is inserted and arranged inside the stator support portion 22, as long as the support strength of the fixed shaft 8 with respect to the fixed base 21 is not impaired. Insertion depth can be increased. In other words, it is possible to increase the distance between the pair of upper and lower ball bearings 43 and 44, thereby further reducing the runout of the rotor 12 and the hard disk 11.

[0039]

Since the present invention is configured as described above, the following effects can be obtained.

In the brushless motor for driving a medium according to the first aspect, since the first ball bearing provided between the tip of the fixed shaft and the base of the bearing support portion of the rotor can have a large diameter, the rotor and the like can be used. The surface runout can be reduced, and the required magnetic path cross section at the inner peripheral portion of the stator can be ensured, so that the torque is not greatly reduced.

In the brushless motor for driving a medium according to the second aspect, since the second ball bearing provided between the base of the fixed shaft and the tip of the bearing support of the rotor can have a large diameter, the rotor and the like can be used. The surface runout can be reduced, and the required magnetic path cross section at the inner peripheral portion of the stator can be ensured, so that the torque is not greatly reduced.

According to a third aspect of the present invention, in the brushless motor for driving a medium, the first ball bearing is provided between the tip of the fixed shaft and the base of the bearing support of the rotor, and the base of the fixed shaft and the bearing support of the rotor. Each of the second ball bearings provided between the top and the bottom can use a large-diameter bearing, so that the runout of the rotor and the like can be reduced, and the necessary magnetic path cross section at the inner peripheral portion of the stator is secured. The ability does not significantly reduce torque.

In the brushless motor for driving a medium according to the fourth aspect, the stator supporting portion can be used as a magnetic path in the inner peripheral portion of the stator. .

[Brief description of the drawings]

FIG. 1 is a vertical sectional side view of an entire brushless motor according to one embodiment of the present invention.

FIG. 2 is a vertical sectional side view of a stator core included in the brushless motor of the embodiment.

FIG. 3 is a plan view of the stator core of the embodiment.

FIG. 4 is a plan view of a first core plate included in the stator core of the embodiment.

FIG. 5 is a plan view of a second core plate included in the stator core of the embodiment.

FIG. 6 is a sectional view for explaining a magnetic path on the stator core side of the brushless motor of the embodiment.

FIG. 7 is a plan view of a core plate provided in a conventional stator.

FIG. 8 is a vertical sectional side view of the entire brushless motor according to the conventional example.

[Explanation of symbols]

8: fixed axis, 11: hard disk (media), 12
... rotor, 15 ... peripheral cylindrical wall, 16 ... rotor magnet, 21 ... fixed base, 22 ... stator support, 24 ...
Stator, 27: bearing support, 27a: base, 27b ...
Tip portion, 34: first escape portion, 35: second escape portion, 43 ...
1st ball bearing, 44 ... 2nd ball bearing.

Claims (4)

(57) [Claims] A fixed base having a stator support;
A stator mounted on the stator support, a fixed shaft protruding from the fixed base and penetrating through the center of the stator, and a cup-shaped housing for accommodating the stator; And a cylindrical bearing support portion disposed between the fixed shaft and the stator at a central portion, and a disk-shaped medium is mounted on the outer periphery of the peripheral cylindrical wall. A first ball bearing provided between the rotor and the base of the bearing support and the tip of the fixed shaft; and a first ball bearing provided between the tip of the bearing support and the base of the fixed shaft; A radial gap type medium driving brushless motor including a first ball bearing and a second ball bearing rotatably supporting the rotor on the fixed shaft, wherein the first ball bearing side end of the stator is provided. The inner diameter of the stator is made larger than the inner diameter of the middle part in the thickness direction of the stator to form a clearance portion on the inner surface of the first ball bearing side end of the stator, and the first ball bearing is disposed inside. A brushless motor for driving a medium, wherein an outer diameter of a base portion of a bearing support portion is made larger than an inner diameter of an intermediate portion in a thickness direction of the stator, and an outer peripheral portion of the base portion is inserted into the relief portion. 2. A fixed base having a stator support,
A stator mounted on the stator support, a fixed shaft protruding from the fixed base and penetrating through the center of the stator, and a cup-shaped housing for accommodating the stator; And a cylindrical bearing support portion disposed between the fixed shaft and the stator at a central portion, and a disk-shaped medium is mounted on the outer periphery of the peripheral cylindrical wall. A first ball bearing provided between the rotor and the base of the bearing support and the tip of the fixed shaft; and a first ball bearing provided between the tip of the bearing support and the base of the fixed shaft; A radial gap type medium driving brushless motor, comprising: a second ball bearing rotatably supporting the rotor on the fixed shaft together with the first ball bearing; and a second ball bearing side end of the stator. The inner diameter of the stator is made larger than the inner diameter of the intermediate part in the thickness direction of the stator, and a clearance is formed on the inner surface of the end of the stator on the side of the second ball bearing, and the stator support is engaged with the clearance. The inner peripheral surface of the support portion and the inner peripheral surface of the intermediate portion in the thickness direction of the stator are substantially flush with each other, and the distal end portion of the bearing support portion is disposed inside the stator support portion. A brushless motor for driving a medium, characterized by the following. 3. A fixed base having a stator support,
A stator mounted on the stator support, a fixed shaft protruding from the fixed base and penetrating through the center of the stator, and a cup-shaped housing for accommodating the stator; And a cylindrical bearing support portion disposed between the fixed shaft and the stator at a central portion, and a disk-shaped medium is mounted on the outer periphery of the peripheral cylindrical wall. A first ball bearing provided between the rotor and the base of the bearing support and the tip of the fixed shaft; and a first ball bearing provided between the tip of the bearing support and the base of the fixed shaft; A radial gap type medium driving brushless motor including a first ball bearing and a second ball bearing rotatably supporting the rotor on the fixed shaft, wherein the first ball bearing side end of the stator is provided. The inner diameter of the stator is made larger than the inner diameter of the middle part in the thickness direction of the stator to form a first relief portion on the inner surface of the first ball bearing side end of the stator, and the first ball bearing is disposed inside. The outer diameter of the base of the bearing support portion is made larger than the inner diameter of the middle portion in the thickness direction of the stator, and the outer periphery of the base is inserted into the first relief portion.
The inner diameter of the ball bearing side end portion is made larger than the inner diameter of the thickness direction intermediate portion of the stator, and a second clearance portion is formed on the inner surface of the second ball bearing side end portion of the stator. The stator support portion is engaged with the portion, and the inner peripheral surface of the support portion and the inner peripheral surface of the intermediate portion in the thickness direction of the stator are substantially flush with each other, and the bearing support is provided inside the stator support portion. A brushless motor for driving a medium, characterized in that a leading end of the section is disposed. 4. The brushless motor for driving a medium according to claim 2, wherein the stator supporting portion is made of a magnetic material.