Gleichstrommotor

Abstract

DC motor comprising: a commutator (20) formed of copper or a copper alloy of 99% or more copper; and a brush (30) which is pressed against the commutator and is in contact with it, wherein the brush is composed of a sintered body containing graphite and copper powder, Hard compound particles (33) having a higher hardness than the copper or the copper alloy and the graphite or the copper powder are contained in at least one of the commutator and the brush and are scattered on or near a contact surface (20A) of the commutator with the brush at least during use, wherein the hard compound particles are contained in the brush, and in the brush, an area ratio of the hard compound particles in a plane perpendicular to a direction in which the brush is pressed against the commutator is 0.4% to 5%.

Description

Background of the inventionTechnical field of the invention

[0001] The present disclosure relates to a DC motor. Description of the state of the art

[0002] Generally, in a DC motor, a brush is held pressed against a commutator to establish electrical contact between the commutator and the brush. In this case, the brush is worn by the rotation of the commutator. In order to prolong the service life of the DC motor, various methods have recently been proposed to prevent brush wear. On the other hand, in order to prolong the service life of the DC motor, it has been desired to also prevent commutator wear. To prevent commutator wear, a technique is known in which a 1 µm to 2 µm thick hard coating layer mainly containing a nitride or a carbide of Ti, Ta, or the like is formed on a surface of the commutator by sputtering or ion plating (e.g., JP 1984-185138 A).

[0003] In JP 1984-185138 A, only 1 µm to 2 µm of a hard coating layer is formed on the commutator surface. Therefore, the commutator has poor durability, and the hard coating layer cannot last for an extended period of time. In other words, the commutator may wear beyond a wear limit. Furthermore, sputtering and ion plating are low in productivity and involve high material costs, resulting in high production costs.

[0004] DE 10 2019 107 876 A1 (Section 3 (2) of the Patent Act) relates to a starter including a motor configured with a DC motor, wherein the DC motor has a brush (30) and a commutator (20), the brush (30) is a sintered material formed from graphite and a copper powder, the commutator (20) is formed from either copper or a copper alloy having a copper content of 99% or more, and a surface of a sliding portion of the commutator (20) that is in sliding motion with the brush (30) is provided with a graphite coating layer containing graphite as a main component, and the graphite coating layer contains a hard compound having a Vickers hardness of more than 10 GPa and a metal sulfide solid lubricant.

[0005] DE 10 2019 116 600 A1 (Section 3 (2) PatG) relates to a DC motor for a starter, which includes a commutator and a brush, wherein the commutator is made of copper or a copper alloy whose copper content is greater than or equal to 99% by mass, the brush is arranged in sliding contact with a surface of the commutator, the brush is made of a sintered body which contains graphite, copper and at least one metal sulfide solid lubricant, the percentage of copper in the sintered body is 30-70% by mass, the at least one metal sulfide solid lubricant comprises tungsten disulfide, the percentage of tungsten disulfide in the sintered body is greater than or equal to 6.5% by mass.

[0006] In view of these problems, a main object of the present disclosure is to provide a DC motor that can suppress the wear of the commutator. Summary

[0007] A first aspect of the present disclosure includes a commutator formed of copper or a copper alloy containing 99% or more copper, and a brush pressed against and in contact with the commutator. The brush includes a sintered body containing graphite and copper powder. Hard compound particles having a higher hardness than the copper or copper alloy and the graphite or copper powder are contained in at least one of the commutator and the brush, and are scattered on or near the contact surface of the commutator with the brush, at least during use.

[0008] At least one of the commutator and the brush contains the hard compound particles. The hard compound particles are scattered on or near the contact surface of the commutator and the brush at least during use, so that hard portions containing the hard compound particles and non-hard portions different from the hard portions exist on the contact surface of the commutator and the brush. The hard portions can suppress wear of the non-hard portions, that is, prevent wear of the copper or copper alloy portions of the commutator. In this way, wear of the commutator can be suppressed, enabling an improvement in the service life of the commutator. It should be noted that "during use" refers to at least a state in which the commutator is used by a user and which follows the start of use.

[0009] The hard compound particles are at least contained in the brush.

[0010] In a case where the hard bond particles are contained in the brush, the sliding between the brush and the commutator causes the hard bond particles to be transferred to the commutator. As a result, the hard bond particles are scattered at or near the contact surface of the commutator and the brush. In addition, the hard bond particles contained in the brush continue to be transferred as the brush wears. Therefore, even in a case where the hard bond particles are lost from the contact surface of the commutator and the brush, new hard bond particles are supplied so that the hard bond particles are continuously scattered at or near the contact surface of the commutator and the brush.

[0011] In the brush, an area ratio of the hard connection in a plane perpendicular to a direction in which the brush is pressed against the commutator is 0.4% to 5%.

[0012] Not all of the hard compound particles in the brush are transferred, and some of the hard compound particles are lost instead of being transferred. Therefore, the hard compound particles must be included with a certain area ratio. On the other hand, the hard compound particles with too high an area ratio undesirably reduce the proportion of graphite, thus hindering the effect of a lubricating function of the graphite. Therefore, in a case where, in the brush, an area ratio of the hard compound in a plane perpendicular to a direction in which the brush is pressed against the commutator is 0.4 to 5%, the lubricity can be maintained with the hard compound particles appropriately transferred to the contact surface of the commutator and the brush.

[0013] In a second aspect of the present disclosure, the hard compound particles are contained in the brush within a predetermined assumed wear range from the contact surface with the commutator.

[0014] The hard compound particles are contained in the brush within the predetermined assumed wear range from the contact surface to the commutator. In other words, the hard compound particles are contained in the area where the brush wears. Thus, the hard compound particles can be supplied to the contact surface of the commutator and the brush until the assumed wear range of the brush is worn and the brush reaches the end of its service life. The hard compound particles can be kept scattered on or near the contact surface of the commutator and the brush, thereby suppressing commutator wear.

[0015] In a third aspect of the present disclosure, an amount of the hard compound particles contained in the sintered body is 1 mass% or less.

[0016] An increased amount of hard compound particles contained in the sintered body (brush) will scrape the commutator or reduce the amount of graphite contained in the sintered body, hindering the lubrication function of graphite. Limiting the amount of hard compound particles contained in the sintered body to 1% or less by mass can prevent scraping of the commutator by the hard compound and suppress the deterioration of the lubrication between the commutator and the brush.

[0017] In a fourth aspect of the present disclosure, a metal sulfide solid lubricant is included in the brush.

[0018] Due to the sliding action between the commutator and the brush, the graphite in the brush is transferred to the commutator, forming a graphite film on the contact surface between the commutator and the brush. At this time, in a case where the metal sulfide solid lubricant is contained in the brush, the metal sulfide solid lubricant is also contained in the graphite film formed on the contact surface of the commutator and the brush. The graphite film containing the metal sulfide solid lubricant provides excellent lubricity. Thus, in addition to suppressing wear caused by the hard joint particles, the lubricity of the metal sulfide solid lubricant also protects the contact surface of the commutator and the brush.

[0019] In a fifth aspect of the present disclosure, the hard bond particles are scattered on or near the contact surface of the commutator and the brush.

[0020] In the commutator, hard compound particles with a relatively high hardness are scattered on or near the contact surface of the commutator and the brush. In the commutator, the hard compound particles are scattered on or near the contact surface of the commutator and the brush before use, which can reduce commutator wear. In addition, pre-scattering the hard compound particles in the commutator, rather than transferring them from the brush, makes it easier to control the ratio in which the hard compound particles are scattered in the commutator.

[0021] In a sixth aspect of the present disclosure, an abundance rate of the hard compound particles present on or near the contact surface of the commutator is a ratio at which, on average, 10 to 100 hard compound particles are present within an area of 100 μm × 100 μm from the contact surface of the commutator and the brush.

[0022] As a result of repeated experiments with the changed frequency of the hard bond particles, the present inventor found that 10 or more hard bond particles present in an area of 100 µm × 100 µm improved the wear-reducing effect on the commutator. The present inventor also found that more than 100 hard bond particles excessively increased the effect of brush scraping. Therefore, in the present configuration, 10 to 100 hard bond particles were present in an area of 100 µm × 100 µm from the contact surface of the commutator and the brush. This made it possible to suppress brush scraping while reducing commutator wear.

[0023] In a seventh aspect of the present disclosure, a ratio of an exposed area of the hard bond particles exposed from the contact surface of the commutator to a surface area of the commutator is 0.4% or more.

[0024] As a result of repeated experiments in which the ratio of the exposed area of the hard bond particles to the commutator surface was varied, the present inventor found that the wear-reducing effect on the commutator is enhanced when the exposed hard bond particles account for 0.4% or more of the commutator surface. Therefore, in the present configuration, the exposed hard bond particles account for 0.4% or more of the commutator surface. This enables a reduction in commutator wear.

[0025] In an eighth aspect of the present disclosure, the hard compound particles each have a size of 1 to 6 µm.

[0026] Due to the sliding action between the commutator and the brush, the graphite in the brush is transferred to the commutator, forming a graphite film on the contact surface of the commutator and the brush. The graphite film generally has an average thickness of about 1 µm. This causes excessively small hard compound particles to become embedded in the graphite film.

[0027] On the other hand, in a case where the hard compound particles are excessively large, the hard compound particles offer sliding resistance to the brush during sliding between the hard compound particles and the brush and are likely to detach from the contact surface of the commutator and the brush. In addition, the sliding resistance causes excessive scraping of the brush.

[0028] Therefore, the hard compound particles each have a size of 1 to 6 µm. Hard compound particles larger than 1 µm are likely to be exposed by the graphite layer. Additionally, hard compound particles smaller than 6 µm can prevent the sliding resistance from increasing. This can provide appropriate support to the brush and commutator, thereby reducing commutator wear.

[0029] In a ninth aspect of the present disclosure, the hard compound particles have a resistivity of 16 µΩm or less.

[0030] The electrical contact resistance between the brush and the commutator depends largely on the contact resistance of the graphite film formed on the contact surface of the commutator and the brush. The contact resistance of the graphite film depends on the resistivity of carbon, which is a major component, and carbon has a resistivity of about 16 μΩm. Therefore, if the resistivity of the hard bond particles is equal to or less than the resistivity of carbon, that is, equal to or less than 16 μΩm, the contact resistance between the brush and the commutator will be prevented from increasing despite the scattered hard bond particles. By using hard bond particles with a resistivity of 16 μΩm or less, commutator wear can be suppressed while preventing an increase in the resistance of the motor.

[0031] In a tenth aspect of the present disclosure, an outer peripheral surface of the commutator has an uneven shape parallel to a sliding direction across the brush.

[0032] In the prior art, an uneven shape parallel to the sliding direction is provided across the brush on the outer peripheral surface of the commutator to stabilize the sliding contact state of the brush. However, in a conventional commutator and brush, the uneven shape gradually closes with repeated sliding and also reduces the stabilizing effect on the contact state.

[0033] Therefore, the hard compound particles are scattered on or near the contact surface of the commutator and the brush to suppress commutator wear and maintain the uneven shape of the commutator's outer peripheral surface. This stabilizes the sliding contact with the brush over a longer period of time. Short description of the drawings

[0034] In the accompanying drawings: Fig. 1 is a partially cut-away cross-section of a starter motor; Fig. 2 is a partially enlarged cross-sectional view of a commutator; Fig. 3 is a schematic diagram showing the sliding between a brush and the commutator; Fig. 4(a), Fig. 4(b) and Fig. 4(c) are diagrams showing examples of a configuration of a starting circuit connected to a DC motor; Fig. Figure 5 is a schematic representation of the commutator and brush during use; Fig. 6 is a graph illustrating a relationship between the occurrence rate of hard joint particles and the service life of the commutator; Fig. 7 is a graph illustrating a relationship between the exposed area of the hard joint particles and the commutator lifetime; Fig. 8 is a schematic diagram of a commutator according to another embodiment; and Fig. 9 is a schematic diagram of the commutator during use according to the other embodiment. Detailed description of the preferred embodiments

[0035] An embodiment of a starter motor configuration for starting a vehicle engine will be described with reference to the drawings. It is assumed that the starter motor according to the present embodiment is used in a vehicle with an idle stop system.

[0036] Fig. 1 is a schematic configuration diagram of a starter S. The starter S includes a DC motor 40, a speed reduction unit 50 that reduces the rotation of the DC motor 40 and transmits the reduced rotation to a pinion gear 73, and a magnetic switch 60 that serves as a switch for supplying power to the DC motor 40. The speed reduction unit 50 includes, for example, a planetary gear.

[0037] The solenoid switch 60 corresponds to a switch that causes the starter S to rotate a ring gear of the motor. For example, when a user operates a key to turn on an IG switch, the solenoid switch 60 causes a shift lever 71 to push out the pinion gear 73 relative to the DC motor 40, whereby the ring gear of the motor meshes with the pinion gear 73. The DC motor 40 is electrically connected via the solenoid switch 60 to transmit the rotation of the DC motor 40 to the pinion gear 73 via the speed reduction unit 50 and a one-way clutch 72.

[0038] The DC motor 40 includes a rotor 10 corresponding to an armature, and the rotor 10 includes a rotating shaft 12 provided at the center of the rotor 10 and a coil 15 provided around the rotating shaft 12. The rotating shaft 12 is provided with a commutator 20 connected to coils 15. A field system for the DC motor 40 is of the magnetic field type, and a magnet 16 for magnetic fields is attached to an inner peripheral surface of a yoke. The coils 15 of the rotor 10 are arranged inside the magnet 16.

[0039] The commutator 20 is made using Fig. 1 and Fig. 2 described. Fig. 2 is a partially enlarged cross-sectional view of the commutator 20. The commutator 20 includes a plurality of segments 21 arranged at regular intervals in the circumferential direction, and each of the segments 21 is connected to a corresponding one of the coils 15. The commutator 20 is formed of copper or a copper alloy containing 99% or more copper. More specifically, the commutator 20 is formed of silver-containing copper or phosphorus-deoxidized copper. Note that it is sufficient that an exposed outer peripheral surface of the commutator 20 and portions of the commutator 20 related to electrical connection, that is, the segments 21, are formed of copper or a copper alloy.

[0040] A brush 30 is pressed against and into sliding contact with the commutator 20. The outer peripheral surface of the commutator 20 is provided with an uneven portion 22 having an uneven shape. The uneven portion 22 is formed parallel to the sliding direction across the brush 30, that is, the rotational direction of the commutator 20, and has a shape alternately provided with circular arc-shaped depressions and sharp projections. Each of the grooves in the uneven portion 22 has a depth of, for example, approximately 0.1 mm to 0.3 mm, and the distance between the projections is approximately 1 mm. The uneven portion 22 forms grooves in the brush 30 into which the uneven portion 22 is fitted. By the fitting between the brush 30 and the commutator 20, the contact state of the brush 30 can be stabilized, thereby suppressing sparking.

[0041] A configuration of the brush 30 is determined using Fig. 3 described. Fig. 3 is a schematic diagram showing the sliding between the brush 30 and the commutator 20. An arrow in Fig. 3 shows the sliding direction of the commutator 20 over the brush 30.

[0042] The brush 30 includes a sintered body containing graphite, copper, and the like. The brush 30 includes a pigtail 35 implanted into a side surface of the brush 30 and connected to an external device. Note that, although not shown, the brush 30 is pressed against the commutator 20 by a spring provided opposite the commutator 20.

[0043] The brush 30 is a multi-layer brush including a first layer 31 and a second layer 32 arranged in the rotation direction of the commutator 20. During the rotation of the commutator 20, the first layer 31 first comes into contact with each of the segments 21, and the second layer 32 subsequently comes into contact with the segment 21. In this case, the second layer 32 is the last to come into contact with each segment 21. The first layer 31 has a high copper content and a small resistance value, while the second layer 32 has a lower copper content and a larger resistance value than the first layer 31. The second layer 32 is thinner than the first layer 31. Specifically, the amount of copper powder (hereinafter referred to as "copper amount") in the first layer 31 is 30 to 60 mass% of the sintered body, while the copper amount of the second layer 32 is less than 30 mass% of the sintered body.

[0044] Now, a circuit for supplying power to the DC motor 40 using Fig. 1 and Fig. 4 described. Fig. 4 is a diagram showing a configuration of a starting circuit 81 connected to the DC motor 40 via the magnetic switch 60. The commutator 20 is powered by a battery 80 via the brush 30 and supplies the coil 15 with the current supplied by the brush 30. The coil 15 is powered to rotate the rotor 10. In the DC motor 40, for example, the commutator 20 rotates at a peripheral speed of more than 30 m / s under no-load conditions.

[0045] In the prior art, a lead-acid battery is often used to power the starter S. A starting circuit using a lead-acid battery typically has an open circuit voltage in the range of 11 V to 14 V and a circuit resistance of approximately 6 to 8 milliohms, with the circuit resistance being calculated by adding the wiring resistance and the internal resistance of the battery.

[0046] On the other hand, in recent years, the battery 80 contains a lithium-ion battery or the like to reduce weight and increase charging efficiency. For example, the battery 80 has a configuration with a lithium-ion battery (LiB) as shown in Fig. 4(a), a configuration with two lead batteries (PbB) connected in parallel as in Fig. 4(b) or a configuration with a lithium-ion battery and a lead-acid battery connected in parallel as in Fig. 4(c). These batteries 80 have an open-circuit voltage in the range of 11 V to 14 V and a circuit resistance R of less than 5 milliohms, the circuit resistance being calculated by adding the wiring resistance Rw and the internal resistance Rb of the battery 80. Therefore, using the batteries 80 increases a voltage applied to the DC motor 40 and also increases the rotational speed of the DC motor 40. The DC motor 40 according to the present embodiment has an applied voltage of approximately 12 V at cold start and a current of 100 A or more flowing when the DC motor 40 is activated.

[0047] In the DC motor 40 according to the present embodiment, which is arranged in a circuit configuration as shown in Fig. 4, the wear is likely to be caused by sparking. Generally, sparking and wear between the commutator 20 and the brush 30 occur more frequently in a magnetic field system such as the DC motor 40 according to the present embodiment than in the winding field system. In the starting circuit 81 using a lithium-ion battery or the like, the DC motor 40 has an increased rotational speed, resulting in more frequent sparking and wear between the commutator 20 and the brush 30.

[0048] More frequent sparking may cause the uneven portion 22 to be easily scraped. In general, the sliding between the commutator 20 and the brush 30 causes gradual wear of the uneven portion 22 and accordingly gradually deteriorates the stabilizing effect for the sliding contact of the brush 30. Particularly, in a configuration with thin grooves in the uneven portion 22 and a high probability of sparking, as in the present embodiment, the uneven shape is worn prematurely, thereby canceling out the stabilizing effect for the sliding contact of the brush 30.

[0049] Therefore, in the present embodiment, at least during use of the starter S after the start of use, hard compound particles 33 are scattered on or near a contact surface 20A of the commutator 20 with the brush 30, as shown in Fig. 5 shown. Fig. Figure 5 is a schematic diagram of the commutator 20 and the brush 30 during use. The hard compound particles 33 are contained in the brush 30, are transferred by sliding between the brush 30 and the commutator 20, and are scattered on or near the contact surface 20A of the commutator 20. Note that "near" indicates a predetermined depth from the contact surface 20A of the commutator 20, e.g., a depth of 20 µm from the contact surface 20A.

[0050] A graphite film 25 is formed on the contact surface 20A of the commutator 20 with the brush 30 due to the sliding between the commutator 20 and the brush 30. The graphite film 25 is formed by transferring the graphite contained in the brush 30 to the contact surface 20A of the commutator 20. The formed graphite film 25 imparts lubricity to the contact surface 20A of the commutator 20. The graphite film 25 generally has an average thickness of about 1 µm.

[0051] In addition to graphite and copper powder, a metal sulfide solid lubricant is also contained in the brush 30. The metal sulfide solid lubricant includes, for example, tungsten disulfide or molybdenum disulfide. In a case where the metal sulfide solid lubricant is contained in the brush 30, the metal sulfide solid lubricant is transferred simultaneously with the transfer of the graphite, and the same metal sulfide solid lubricant as contained in the brush 30 is contained in the graphite layer 25. The graphite film 25 in which the metal sulfide solid lubricant is contained provides excellent lubricity. Note that it is sufficient that one metal sulfide solid lubricant is contained in at least one of the first layer 31 and the second layer 32 of the brush 30. Alternatively, different metal sulfide solid lubricants may be contained in the first layer 31 and the second layer 32 of the brush 30.For example, tungsten disulfide can be contained in the first layer 31 and molybdenum disulfide in the second layer 32.

[0052] Furthermore, the brush 30 contains the hard compound particles 33. The hard compound particles 33 have a higher hardness than copper or a copper alloy, which is a base material of the commutator 20, and graphite, copper powder, and the like contained in the brush 30. The hard compound particles 33 have a Vickers hardness of at least 5 GPa and desirably more than 10 GPa. The hard compound particles 33 are inorganic compounds of carbon (carbides), and are, for example, molybdenum carbide, tungsten carbide, boron carbide, or silicon carbide.

[0053] It is desirable that the hard bond particles 33 each have a size of 1 to 6 µm. Even more desirable is that the hard bond particles 33 each have a size of 4.5 µm to 5 µm. The graphite film 25 has an average thickness of about 1 µm, and therefore, the hard bond particles 33 each having a size of less than 1 µm are embedded in the graphite film 25. When the hard bond particles 33 slide over the brush 30 on the contact surface 20A of the commutator 20, the hard bond particles 33 each larger than 6 µm, and particularly larger than 10 µm, offer sliding resistance and are likely to fall off the contact surface 20A. In addition, the sliding resistance may cause the brush 30 to be excessively scraped. It should be noted that a small amount of hard compound particles 33 each less than 1 µm or more than 6 µm in size may be included.

[0054] It is desirable that the hard bond particles 33 have a resistivity equal to or less than the resistivity of carbon, that is, equal to or less than 16 μΩm. The electrical contact resistance between the brush 30 and the commutator 20 depends largely on the resistivity of the graphite film 25 formed on the contact surface 20A of the commutator 20. The resistivity of the graphite film 25 depends on the resistivity of carbon, which is a main component. Carbon has a resistivity of about 16 μΩm. Therefore, as long as the resistivity of the hard bond particles 33 is equal to or less than the resistivity of carbon, that is, equal to or less than 16 μΩm, even with the hard bond particles 33 scattered on the contact surface 20A, an increase in the contact resistance between the commutator 20 and the brush 30 is prevented.

[0055] When the brush 30 is pressed against the commutator 20, the brush 30 and the commutator 20 rub against each other, so that the hard compound particles 33 contained in the brush 30 are transferred to the contact surface 20A of the commutator 20. In other words, the hard compound particles 33 are harder than the other materials used for the commutator 20 and the brush 30, and the force with which the brush 30 is pressed against the commutator 20 and the heat caused by friction act on the hard compound particles 33. Therefore, the hard compound particles 33 are scattered on or near the contact surface 20A of the commutator 20 and implanted into the contact surface 20A.

[0056] The hard compound particles 33 are scattered on or near the contact surface 20A of the commutator 20, and thus, on the contact surface 20A of the commutator 20, there are hard portions including the hard compound particles 33 and non-hard portions other than the hard portions. The brush 30 pressed against the commutator 20 is supported by the hard compound particles 33, thereby reducing the force with which the brush 30 is pressed against the non-hard portions (the portions other than the hard compound particles 33). This prevents the brush 30 from rubbing strongly against the non-hard portions, thereby suppressing scraping of the non-hard portions, that is, the base material of the commutator 20.

[0057] Now, based on Fig. 6 and Fig. 7 a relationship between the presence rate of the hard compound particles 33 and the degree of wear of the commutator 20 is discussed. Fig. Figure 6 is a graph illustrating the relationship between the frequency rate of the hard bond particles 33 and the service life of the commutator 20. Note that the frequency rate refers to the average number of the hard bond particles 33 scattered in a predetermined area. Fig. Figure 7 is a graph illustrating the relationship between the exposed area of the hard bond particles 33 and the service life of the commutator 20. Note that the "exposed area" refers to the surface area of the portions of the hard bond particles 33 that protrude from the contact surface 20A of the commutator 20.

[0058] The present inventor repeated experiments on the frequency rate of the hard compound particles 33 in the contact surface 20A of the commutator 20 to determine the appropriate frequency rate. As a result of the repeated experiments on the frequency rate, the present inventor discovered that 10 or more hard compound particles 33 in an area of 100 μm × 100 μm from the contact surface 20A of the commutator 20 have a wear-reducing effect on the commutator 20 to extend the service life of the commutator 20, that is, the period until the commutator 20 reaches a wear limit, as shown in Fig. 6. The present inventor also found that more than 100 hard compound particles 33 present in the 100 µm × 100 µm area excessively enhances the scraping action of the brush 30. Therefore, in the present embodiment, 10 to 100 hard compound particles 33 are present in the 100 µm × 100 µm area of the contact surface 20A of the commutator 20. This enables the suppression of scraping of the brush 30 while reducing wear of the commutator 20.

[0059] It should be noted that when the hard bond particles 33 are transferred from the brush 30, the hard bond particles 33 are likely to be transferred in a case where the hard bond particles 33 have a low frequency, and in a case where the hard bond particles 33 have a high frequency, they are lost instead of being transferred because the contact surface 20A of the commutator 20 contains many hard portions. Therefore, the appropriate frequency is likely to be achieved during transfer from the brush 30.

[0060] In addition, as a result of repeated experiments with changing the ratio of the exposed area of the hard compound particles 33 to the surface area of the contact surface 20A of the commutator 20, the present inventor has found that the exposed hard compound particles 33, which account for 0.4% or more of the surface area of the contact surface 20A of the commutator 20, improve the wear reduction effect on the commutator 20, as shown in Fig. 7. Therefore, in the present embodiment, the exposed hard bond particles 33 account for 0.4% or more of the surface area of the commutator 20. This enables a reduction in wear of the commutator 20. Note that in a case where the hard bond particles 33 have a suitable abundance rate and suitable size, the exposed area of the hard bond particles 33 is 0.4% or more of the surface area.

[0061] The ratio and amount of the hard compound particles 33 contained in the brush 30 to achieve the above-described abundance rate of the hard compound particles 33 and the exposed area of the hard compound particles 33 will now be discussed. Not all of the hard compound particles 33 in the brush 30 are transferred, but some of the hard compound particles 33 are lost instead of being transferred. Therefore, a surface of the brush 30 perpendicular to the direction in which the brush 30 is pressed against the commutator 20 must contain the hard compound particles 33 with a certain area ratio. On the other hand, the hard compound particles 33 with too high an area ratio undesirably reduce the graphite rate to hinder the effect of a lubricating function of the graphite.Therefore, in the present embodiment, in the brush, an area ratio of the hard compound in a plane perpendicular to a direction in which the brush is pressed against the commutator is 0.4% to 5%. In a case where the area ratio of the area occupied by the hard compound particles 33 is 0.4% to 5%, lubricity can be maintained, and the hard compound particles 33 are properly transferred to the contact surface 20A of the commutator 20.

[0062] In addition, an increased amount of the hard compound particles 33 contained in the brush 30, which includes a sintered body, scrapes the commutator 20 or reduces the amount of graphite contained in the sintered body, hindering the effect of the lubricating function of the graphite. Therefore, in the present embodiment, the amount of the hard compound particles 33 contained in the brush 30 is limited to 1 mass % or less to prevent scraping of the commutator 20 by the hard compound particles 33 and suppress deterioration of the lubricity between the commutator 20 and the brush 30.

[0063] It should be noted that the metal sulfide solid lubricant and the hard compound particles 33 are mixed with graphite and copper powder, and the mixture is molded. Therefore, the metal sulfide solid lubricant and the hard compound particles 33 are contained in the brush 30 throughout the entire area of the brush 30 in the direction in which the brush 30 is pressed against the commutator 20. In other words, the hard compound particles 33 are located within a predetermined assumed wear range from the contact surface of the brush 30 to the commutator 20. This allows the hard compound particles 33 to be supplied to the contact surface 20A of the commutator 20 until the assumed wear range of the brush 30 is worn and the brush 30 reaches the end of its service life.It should be noted that the assumed wear area in the pressing direction extends from the end face of the brush 30 on the side of the commutator 20 to a position of the brush 30 from which the connecting wire 35 protrudes.

[0064] As described above, an operation test was conducted in which the brush 30 containing the hard compound particles 33 at a predetermined ratio was used in the DC motor 40 to which a voltage of about 12 V is applied during cold start and through which a current of 100 A or more can flow during use. In the operation test, it was confirmed that after several thousand operations, the hard compound particles 33 are present on the contact surface 20A of the commutator 20 at a frequency rate of 10 to 100 hard compound particles 33 within the area of 100 μm × 100 μm, and that the exposed area of the hard compound particles 33 is 0.4% or more.

[0065] The hard compound particles 33 thus scattered onto the contact surface 20A of the commutator 20 during use make it possible to suppress wear of the commutator 20, thereby improving the service life of the commutator 20. In addition, suppressing wear of the commutator 20 makes it possible to inhibit the deterioration of shape accuracy caused by uneven wear of the commutator 20, that is, the deterioration of roundness in a cylindrical commutator and the deterioration of flatness in a face commutator. Therefore, sparking caused by the reduced shape accuracy can be suppressed, thereby extending the service life of the brush 30.

[0066] Additionally, the hard bonding particles 33 are scattered on the contact surface 20A of the commutator 20. As a result, the uneven shape of the uneven section 22 of the commutator 20 is significantly less worn, so that the stabilizing effect for the sliding contact of the brush 30 is maintained over a longer period of time. A service life-enhancing effect on the brush 30 can also be maintained over a longer period of time.

[0067] According to the present embodiment described in detail above, the following excellent effects are obtained.

[0068] The hard compound particles 33 are contained in the brush 30. The hard compound particles 33 are scattered on or near the contact surface 20A of the commutator 20 with the brush 30, at least during use. Thus, on the contact surface 20A of the commutator 20, there are hard portions containing the hard compound particles 33 and non-hard portions different from the hard portions. The hard portions can suppress the non-hard portions, i.e., the copper or copper alloy portions of the commutator 20, from wearing. In this way, the wear of the commutator 20 can be suppressed, enabling an improvement in the service life of the commutator 20.

[0069] In a case where the hard compound particles 33 are contained in the brush 30, the sliding between the brush 30 and the commutator 20 causes the hard compound particles 33 to be transferred to the commutator 20.

[0070] As a result, the hard bond particles 33 are scattered on or near the contact surface 20A of the commutator 20. In addition, the hard bond particles 33 contained in the brush 30 continue to be transferred as the brush 30 wears. Therefore, even if the hard bond particles 33 are lost from the contact surface 20A of the commutator 20, new hard bond particles 33 are supplied, so that the hard bond particles 33 are continuously scattered on or near the contact surface 20A of the commutator 20.

[0071] The hard compound particles 33 are contained in the brush 30 within the predetermined assumed wear range from the contact surface with the commutator 20. In other words, the hard compound particles 33 are contained in the area where the brush 30 wears. Thus, the hard compound particles 33 can be supplied to the contact surface 20A of the commutator 20 until the assumed wear range of the brush 30 is worn and the brush reaches the end of its service life. The hard compound particles 33 can be kept scattered on or near the contact surface 20A of the commutator 20, so that wear of the commutator 20 can be suppressed.

[0072] Not all of the hard compound particles 33 in the brush 30 are transferred, and some of the hard compound particles 33 are lost instead of being transferred. Therefore, the hard compound particles 33 must be included with a certain area ratio. On the other hand, the hard compound particles 33 with too high an area ratio undesirably reduce the proportion of graphite and hinder the effect of the lubricating function of the graphite. Therefore, in the present embodiment, in a case where, for the surface of the brush 30 perpendicular to the direction in which the brush 30 is pressed against the commutator 20, the area ratio of the portion of the surface occupied by the hard compound particles 33 to the surface area is 0.4 to 5%, the lubricity can be maintained with the hard compound particles 33 appropriately transferred to the contact surface 20A of the commutator 20.

[0073] An increased amount of hard compound particles 33 contained in the sintered body (brush 30) scrapes the commutator 20 or reduces the amount of graphite contained in the sintered body, hindering the lubricating function of the graphite. Therefore, the amount of hard compound particles 33 contained in the brush 30 is limited to 1 mass% or less to prevent the hard compound particles 33 from scraping the commutator 20 and suppressing the deterioration of the lubricity between the commutator 20 and the brush 30.

[0074] Due to the sliding between the commutator 20 and the brush 30, the graphite in the brush 30 is transferred to the commutator 20, forming the graphite film 25 on the contact surface 20A of the commutator 20. At this time, in a case where the metal sulfide solid lubricant is contained in the brush 30, the metal sulfide solid lubricant is also contained in the graphite film 25 formed on the contact surface 20A of the commutator 20. The graphite film 25 containing the metal sulfide solid lubricant provides excellent lubricity. Thus, in addition to the wear suppression by the hard bond particles 33, the contact surface 20A of the commutator 20 is protected by the lubricity of the metal sulfide solid lubricant.

[0075] The graphite film 25 generally has an average thickness of about 1 µm. Therefore, excessively small hard bond particles 33 are embedded in the graphite film 25. On the other hand, in a case where the hard bond particles 33 are excessively large, the hard bond particles 33 offer sliding resistance to the brush 30 during sliding between the hard bond particles 33 and the brush 30 and are likely to detach from the contact surface 20A of the commutator 20. In addition, the sliding resistance causes excessive scraping of the brush 30.

[0076] Therefore, in the present embodiment, the hard compound particles 33 each have a size of 1 to 6 µm. Hard compound particles 33 each larger than 1 µm are likely to be exposed by the graphite layer 25. Additionally, hard compound particles 33 each smaller than 6 µm can be prevented from providing sliding resistance. This allows the brush 30 and the commutator 20 to be properly supported, thereby reducing wear on the commutator 20.

[0077] The electrical contact resistance between the brush 30 and the commutator 20 depends largely on the contact resistance of the graphite film 25 formed on the contact surface 20A of the commutator 20. The contact resistance of the graphite film 25 depends on the resistivity of carbon, which is a main component, and carbon has a resistivity of about 16 μΩm. Therefore, when the resistivity of the hard bond particles 33 is equal to or less than the resistivity of carbon, that is, equal to or less than 16 μΩm, the contact resistance between the brush 30 and the commutator 20 is prevented from increasing despite the scattered hard bond particles 33. Using the hard bond particles 33 with a resistivity of 16 μΩm or less makes it possible to suppress wear of the commutator 20 while preventing the resistance of the DC motor 40 from increasing.

[0078] In the prior art, the uneven shape is provided parallel to the sliding direction across the brush 30 on the outer peripheral surface of the commutator 20 to stabilize the sliding contact state of the brush 30. In this case, the hard compound particles 33 are scattered on or near the contact surface 20A of the commutator 20 to suppress wear of the commutator 20 and maintain the uneven shape of the outer peripheral surface of the commutator 20. In this way, the sliding contact with the brush 30 can be stabilized for a longer period of time. <Andere Ausführungsformen>

[0079] The present disclosure is not limited to the above-described embodiments and may be implemented, for example, as follows. The following alternative configurations may be applied separately to the configurations of the above-described embodiments or may optionally be combined for application.

[0080] - The hard compound particles 33 may be scattered on or near the contact surface 20A of the commutator 20 with the brush 30, as in Fig. 8. In this case, the hard compound particles 33 are preliminarily embedded within a predetermined area including the surface of the commutator 20 (contact surface 20A with the brush 30) or near the surface. Then, the commutator 20 is shaped into a perfect circle by polishing, and the uneven portion 22 is subsequently formed. Note that the area where the hard compound particles 33 are scattered can be from the surface of the commutator 20 to the wear limit of the commutator 20, for example, less than 0.7 mm from the surface.

[0081] During use, the non-hard portions of the commutator 20 wear slightly, and the graphite film 25 forms on the contact surface 20A, as shown in Fig.9. Then, the hard compound particles 33 exposed from the contact surface 20A of the commutator 20 provide support between the commutator 20 and the brush 30. Note that even in a case where the commutator 20 is worn, the hard compound particles 33 embedded to the wear limit continue to provide support between the commutator 20 and the brush 30, so that the wear of the commutator 20 can be suppressed.

[0082] In the commutator 20, the hard bond particles 33 are scattered on or near the contact surface 20A of the commutator 20 before use, so that wear of the commutator 20 can be reduced from the start of use. In addition, the hard bond particles 33 are scattered in the commutator 20 in advance instead of being transferred from the brush 30, which facilitates control of a scattering ratio of the hard bond particles 33 and the like. Note that it is desirable that the same hard bond particles 33 embedded in the commutator 20 are also included in the brush 30. In this case, even in a case where the pre-embedded hard bond particles 33 are lost, new hard bond particles 33 can be supplied by being transferred from the brush 30.

[0083] In addition, the frequency of the pre-embedded hard bond particles 33 may be such that, on average, 10 to 100 hard bond particles 33 are present within the area of 100 μm × 100 μm. As described above in the embodiments, in the configuration where 10 to 100 hard bond particles 33 are present within the area of 100 μm × 100 μm of the contact surface 20A of the commutator 20, the scraping of the brush 30 can be prevented and the wear of the commutator 20 can be reduced.

[0084] The ratio of the exposed area of the pre-embedded hard bond particles 33 to the surface area of the commutator 20 may be 0.4% or more. As described above in the embodiments, in the configuration where the exposed hard bond particles 33 account for 0.4% or more of the surface area of the commutator 20, wear of the commutator 20 can be reduced.

[0085] - In the direction in which the brush 30 is pressed against the commutator 20, the end surface of the brush 30 on the commutator 20 side may have a higher content of the hard compound particles 33 than the opposite end of the brush 30. In this configuration, the portion of the brush 30 having a high content of the hard compound particles 33 comes into sliding contact with the contact surface 20A of the commutator 20 with a small amount of the hard compound particles 33, and thus the hard compound particles 33 are more likely to be transferred. In addition, in a case where the hard compound particles 33 are scattered on the contact surface 20A of the commutator 20, a small amount of the hard compound particles 33 and a large amount of graphite are contained in the brush 30. Therefore, after the required hard compound particles 33 are transferred, the lubricity can be improved.

Claims

[1] DC motor comprising: a commutator (20) formed of copper or a copper alloy of 99% or more copper; and a brush (30) which is pressed against the commutator and is in contact with it, wherein the brush is composed of a sintered body containing graphite and copper powder, Hard compound particles (33) having a higher hardness than the copper or the copper alloy and the graphite or the copper powder are contained in at least one of the commutator and the brush and are scattered on or near a contact surface (20A) of the commutator with the brush at least during use, wherein the hard compound particles are contained in the brush, and in the brush, an area ratio of the hard compound particles in a plane perpendicular to a direction in which the brush is pressed against the commutator is 0.4% to 5%. [2] A DC motor (40) according to claim 1, wherein the hard compound particles are contained in the brush within a predetermined assumed wear range from the contact surface with the commutator. [3] The DC motor (40) according to claim 1 or 2, wherein in the brush, an amount of the hard compound particles contained in the sintered body is 1 mass% or less. [4] A DC motor (40) according to any one of claims 1 to 3, wherein a metal sulfide solid lubricant is contained in the brush. [5] A DC motor (40) according to any one of claims 1 to 4, wherein the hard bond particles are scattered on or near the contact surface of the commutator and the brush. [6] The DC motor (40) according to claim 5, wherein an abundance rate of the hard compound particles present on or near the contact surface of the commutator is a ratio at which an average of 10 to 100 hard compound particles are present within an area of 100 µm × 100 µm of the contact surface of the commutator and the brush. [7] The DC motor (40) according to claim 5 or 6, wherein a ratio of an exposed area of the hard bond particles exposed from the contact surface of the commutator to a surface area of the commutator is 0.4% or more. [8] DC motor (40) according to one of claims 1 to 7, wherein the hard compound particles each have a size of 1 to 6 µm. [9] A DC motor (40) according to any one of claims 1 to 8, wherein the hard compound particles have a specific resistance of 16 µΩm or less. [10] A DC motor (40) according to any one of claims 1 to 9, wherein an outer peripheral surface of the commutator has an uneven shape parallel to a sliding direction over the brush.

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