TECHNICAL FIELD

The present invention relates to stepping motors, and in particular, relates to PM-type (Permanent Magnet-type) stepping motors having a rotor which is a permanent magnet and a stator having multiple pole teeth extending in an axial direction at an outer circumferential side of the rotor.

BACKGROUND ART

Conventionally, in the abovementioned PM-type stepping motor, a bearing rotatably supporting the rotor is installed, and there is a gap along the axial direction between the rotor and the bearing. For example, in a two-phase PM-type stepping motor, there is a phenomenon due to combination of excitation of two stators, in which the magnetic centers of the rotor and the stator are misaligned during driving of the stepping motor, and the rotor moves along an axial direction, thereby causing abnormal noise by collision with the bearing.

To solve this phenomenon, a stepping motor is known in which a magnetic attractive force generating member attracting a rotor in the axial direction is arranged, and the rotor is attracted to one bearing, thereby reducing striking noise by collision with the other bearing (see Patent Document 1)

The stepping motor disclosed in Patent Document 1 is a two-phase PM-type stepping motor including a rotor magnet which is a permanent magnet and a stator having multiple pole teeth extending in an axial direction at a circumferential side of the rotor, and having a structure in which a magnetic washer as the magnetic attractive force generating member made of magnetic material affixed to a case plate metal fixing the bearing. In this stepping motor, outer diameter DM1of the magnetic washer and outer diameter DRof the rotor magnet are set to satisfy a relationship DM1=<DR(see claim3). In addition, the rotor magnet is attracted to the magnetic washer side so as to reduce striking noise during rotation driving of the stepping motor.

Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2014-3832

SUMMARY OF THE INVENTION

However, in a case in which the stepping motor is used in an electronic device, such as a mobile phone, such as a smart phone, an outer diameter of the stepping motor is not more than 6 mm, and an outer diameter of the rotor of such a stepping motor is very small, being not more than 3 mm. In such a case of a very small diameter motor, it is difficult to obtain a space to arrange the magnetic washer of the Patent Document 1. Therefore, it can be considered that the bearing itself is formed by a magnetic material instead of the magnetic washer.

On the other hand, the Patent Document 1 also discloses that instead of the magnetic washer and the bearing, a bearing made of a magnetic material and enlarged in a radial direction can be used as the magnetic attractive force generating member. (see [0041]). In that case, since it is assumed that an outer diameter of the bearing made of a magnetic material and enlarged in a radial direction is similar to an outer diameter DM1of the magnetic washer, DM1(outer diameter of the bearing) and DR(outer diameter of the rotor magnet) are set so as to satisfy the relationship DM1=<DR.

However, in Patent Document 1, the rotor magnet is magnetized at the bottom surface thereof facing the magnetic washer in addition to the side surface thereof facing the excitation stator. Therefore, it is unclear whether or not the abovementioned effect can be obtained even in a case in which the bottom surface of the rotor magnet is not magnetized.

The present invention has been completed in view of the above circumstances, and an object of the present invention is to provide a stepping motor in which a relationship between the outer diameter of a rotor and the outer diameter of a bearing made of magnetic material are appropriately adjusted in a case in which the bearing made of magnetic material is used as a magnetic attractive force generating member instead of a magnetic washer and a bearing.

The present invention is a stepping motor including: a rotor made of a permanent magnet, a stator including multiple pole teeth extending in an axial direction of the rotor arranged at an outer circumferential side of the rotor, a primary bearing rotatably supporting one end portion in the axial direction of the rotor, and a secondary bearing rotatably supporting the other end portion in the axial direction of the rotor, in which magnetic permeability of the secondary bearing is greater than that of the primary bearing, the rotor includes multiple magnetic poles on an outer circumferential surface along a circumferential direction of the rotor, and an outer diameter DRof the rotor, an outer diameter DBof a portion of the secondary bearing which is facing the rotor, and an inner diameter DSIof the stator are set to satisfy the following relationship of Formula 1.
DR<DB<DSI(formula 1)

In the present invention, since magnetic permeability of the secondary bearing is set higher than that of the primary bearing, magnetic force (magnetic flux) of the rotor acts more strongly at the secondary bearing than at the primary bearing, and magnetic attractive force to the rotor is generated at the secondary bearing. Here, since the outer diameter DBof the portion of the secondary bearing facing the rotor is set greater than the outer diameter DRof the rotor, a part of magnetic force (magnetic flux) at magnetic poles (N pole and S pole) formed on an outer circumferential surface of the rotor in addition to magnetic force (magnetic flux) leaked from the end surface of the rotor affects the secondary bearing over the end surface of the rotor, and the secondary bearing acts as a magnetic attractive force generating member. Therefore, the rotor is attracted to the secondary bearing, thereby reducing the striking noise occurring during rotation driving of the stepping motor.

In the present invention, action and effect based on Formula 1 can be obtained only in a case in which the magnetic poles are arranged only on the outer circumferential surface of the rotor. However, in a case in which the magnetic poles are arranged on an end surface of the rotor, since greater magnetic force acts at the secondary bearing, the present invention does not exclude such an embodiment.

In the present invention, it is desirable that the secondary bearing include a small diameter portion and a large diameter portion, the small diameter portion be affixed to a covering member, and the large diameter portion face an end surface of the rotor. In this way, the size of the secondary bearing can be reduced, thereby reducing material cost, and the secondary bearing can be attached to the covering member via a step portion between the small diameter portion and the large diameter portion.

In the present invention, by arranging a resin washer between the rotor and the secondary bearing, metals of the rotor and the secondary bearing are prevented from making contact with each other, thereby avoiding the occurrence of abnormal noise.

According to the present invention, the stepping motor in which the relationship between the outer diameter of a rotor and the outer diameter of a bearing made of magnetic material is adjusted appropriately can be provided in a case in which the bearing made of magnetic material is used as a magnetic attractive force generating member instead of a magnetic washer and a bearing.

