DC MOTOR FOR STARTER

Abstract

DC motor (40) for a starter (S) that starts a machine, wherein the DC motor (40) comprises: a commutator (20) formed of copper or a copper alloy, the percentage of copper being greater than or equal to 99 wt%; and a brush (30) in sliding contact with a surface of the commutator (20), wherein the brush (30) is formed of a sintered body containing graphite, copper and at least one metal sulfide solid lubricant, the percentage of copper in the sintered body being 30-70 wt%, the sintered body comprising at least one metal sulfide solid lubricant tungsten disulfide, and the percentage of tungsten disulfide in the sintered body being greater than or equal to 6.5 wt%.

Description

BACKGROUND OF THE INVENTION1. Technical FieldThe present disclosure relates to DC motors for starters that start engines.2. Description of the Prior ArtIn recent years, as vehicles equipped with idle stop systems (or engine automatic stop / restart systems) have been spread, the number of engine starts of vehicles by starters has been increased.Accordingly, it has been desired to extend the life of brushes of DC motors (direct current) used in the starters; various techniques have been developed to suppress the wear of the brushes.Moreover, it has also been desired to increase the surface hardness of commutators on which the brushes slide, thereby suppressing the wear of the commutators. For example, Japanese Patent Application Publication No. 59185138A discloses a technique of forming a hard coating layer whose main component is a nitride or carbide of Ti or Ta and whose thickness is 1 to 2 μm on the surface of a commutator by sputtering or ion plating.However, it is impossible to keep the hard coat layer of 1 to 2 μm for a long period of time; therefore, it is impossible to ensure the durability of the commutator.That is, it is necessary to suppress the wear of commutators as well as the wear of brushes in DC motors.Moreover, formation of the hard coat layer by sputtering or ion plating lowers productivity and increases material cost, resulting in increase in manufacturing cost.SummaryAccording to the present disclosure, a DC motor is provided for a starter that starts an engine. The DC motor includes a commutator and a brush. The commutator is formed of copper or a copper alloy whose copper percent content is higher than or equal to 99% by mass. The brush is disposed in sliding contact with a surface of the commutator. Moreover, the brush is formed of a sintered body including 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 includes tungsten disulfide. The percentage of tungsten disulfide in the sintered body is higher than or equal to 6.5 mass %.Conventionally, DC motors configured to form a graphite film on the surface of a commutator by transferring graphite included in brushes to the surface of the commutator via sliding movement between the brushes and the commutator have been known. With the graphite film, it is possible to suppress generation of sparks between the brushes and the commutator. However, the hardness of the graphite film is not enough to reliably suppress wear of the commutator.In addition, metal sulfide solid lubricants such as tungsten disulfide have been used as lubricants for improving sliding between brushes and commutators in DC motors. However, the percentage of metal sulfide solid lubricants in brushes was set to be lower than or equal to 5 mass %. This is because it is common knowledge in the art to set the metal sulfide solid lubricant content percentage in brushes to less than or equal to 5 mass % to ensure lubricity.In contrast, the present inventors have found by experimental investigation that a graphite film having a higher hardness than conventional graphite films can be obtained by setting the percentage of tungsten disulfide in a sintered body forming a brush to be higher than 5 mass %.Specifically, based on the discovery of the inventors, in the DC motor according to the present disclosure, the percentage of tungsten disulfide in the sintered body is set to be higher than or equal to 6.5 mass %.In setting the percentage of tungsten disulfide in the sintered body as above, during operation of the DC motor, the tungsten disulfide contained in the sintered body reacts with graphite due to heat generated by supplying electric power to the DC motor and sliding movement between the brush and the commutator. Consequently, the tungsten disulfide is converted into tungsten carbide which is a hard compound. As a result, tungsten carbide is contained in the graphite film formed on the surface of the commutator, thereby improving the hardness of the graphite film to become higher than the hardness of conventional graphite films.That is, in the DC motor according to the present disclosure, the graphite film is not originally formed on the surface of the commutator. Instead, the graphite film is formed by operating the DC motor without performing any additional process. Consequently, it becomes possible to form the graphite film at a low cost. Moreover, the graphite film is continuously formed with the sliding movement between the brush and the commutator. Therefore, the graphite film can be maintained until the end of the life of the brush.Further, a part of the tungsten disulfide transferred from the brush to the graphite film remains in its original form (i.e., not converted into tungsten carbide). That is, the graphite film formed on the surface of the commutator contains tungsten disulfide, which is a metal sulfide solid lubricant. Consequently, the graphite film is excellent in lubricity as well as in hardness.Accordingly, with the graphite film formed on the surface of the commutator, it is possible to suppress wear of the commutator while prolonging the life of the commutator. Further, by suppressing wear of the commutator, it is possible to suppress the dimensional accuracy of the commutator from being lowered due to uneven wear of the commutator. Consequently, it is possible to suppress generation of cracks due to lowering of dimensional accuracy of the commutator, while also prolonging the life of the brush.Brief Description of the DrawingsIn the accompanying drawings:FIG. 1 is a partial cross-sectional view of a starter including a DC motor according to an exemplary embodiment;FIG. 2 is an enlarged cross-sectional view of a part of the DC motor;FIG. 3 is a circuit diagram illustrating the configuration of a starter circuit electrically connected to the DC motor via an electromagnetic switch;FIG. 4 is a schematic view illustrating sliding movement between a brush and a commutator of the DC motor;FIG. 5 is a schematic view illustrating both the composition of the brush and the composition of a graphite film formed on the surface of the commutator;FIG. 6 is a cross-sectional view of the graphite film and the surface area of the commutator taken by an SEM;FIG. 7 is an enlarged cross-sectional view of the graphite film taken by a TEM;FIG. 8 is a graph illustrating the relationship between the depth from the surface of the graphite film and the hardness of the graphite film;FIG. 9 is a graph illustrating the relationship between the percentage of tungsten disulfide in a first sintered body forming a first layer of the brush and the hardness of the graphite film;FIG. 10 is a graph illustrating the relationship between the percentage content of molybdenum disulfide in the first sintered body and the hardness of the graphite film;FIG. 11 is a graph illustrating the relationship between the percentage of copper in the first sintered body and the durability of the brush;FIG. 12 is a classical view illustrating the relationship between the ratio of the percentage of brass in the first sintered body to the total percentage of metal sulfide solid lubricant in the first sintered body and brush resistance; andFIG. 13 is a graph illustrating the relationship between the number of times the starter starts the motor and the surface hardness of the commutator.DESCRIPTION OF EMBODIMENTSFIG. 1 shows the overall construction of a starter S including a DC motor 40 according to an embodiment.In the present embodiment, the starter S is designed for use in a vehicle equipped with an idle stop system for starting an engine of the vehicle.As shown in FIG. 1, the starter S further includes a speed reducer 50 and an electromagnetic switch 60 in addition to the DC motor 40.The speed reducer 50 is configured to transmit rotation of the DC motor 40 to a pinion gear 73 through speed reduction. The speed reducer 50 is implemented by, for example, a planetary gear mechanism.The electromagnetic switch 60 is configured to selectively establish and shut off an electrical connection for the power supply of the DC motor 40.Specifically, the electromagnetic switch 60 includes: an exciting coil (or a solenoid); a plunger slidably provided on an inner periphery of the exciting coil; a fixed core; a movable contact held by the plunger; and a pair of fixed contacts provided inside the electromagnetic switch 60 and respectively connected to a battery-side terminal 61 and a motor-side terminal. The battery-side terminal 61 is electrically connected to a battery 80 (see FIG. 3 ), while the motor-side terminal is electrically connected to the DC motor.In operation, the exciting coil is excited when electric power is supplied from the battery 80 provided outside the starter S to a switch terminal (not shown) when an ignition switch is turned on by, for example, a key operation by a user. Upon energization of the exciting coil, the plunger is attracted by the fixed core, whereby a shift lever 71 shifts the pinion gear 73 to a side opposite to the DC motor 40, thereby engaging the pinion gear 73 with a ring gear of the motor. Moreover, when the plunger is attracted to the fixed core, the movable contact is brought into contact with a pair of fixed contacts, causing electric power to be supplied from the battery 80 to the DC motor 40 via the battery-side terminal 61 and the motor-side terminal. Consequently, the DC motor 40 rotates with the electric power supplied from the battery 80; the rotation of the DC motor 40 is transmitted to the ring gear of the motor via the speed reducer 50, one-way clutch 72, and the pinion gear 73, thereby starting the motor.The DC motor 40 includes a rotor 10 serving as an armature of the DC motor 40. The rotor 10 includes a rotating shaft 12, a rotor coil 15 provided around the rotating shaft 12, and a commutator 20 provided at an end portion of the rotating shaft 12 and electrically connected to the rotor coil 15.In the present embodiment, the DC motor 40 is configured as a magnetic field motor. Specifically, the DC motor 40 includes magnets 16 serving as a field of the DC motor 40. The magnets 16 are fixed to an inner circumferential surface of a yoke of the DC motor 40. Moreover, the rotor coil 15 is disposed radially inside the magnets 16.The commutator 20 is composed of a plurality of commutator segments 21 spaced apart from each other at equal intervals in the circumferential direction and each electrically connected to the rotor coil 15.In the present embodiment, the commutator 20 is formed of copper or a copper alloy whose copper percentage is higher than or equal to 99% by mass. Specifically, the commutator 20 is formed of a copper alloy containing silver or phosphorus-deoxidized copper.Note that the commutator 20 has at least one surface 20A and an electrically conductive portion formed of copper or a copper alloy whose copper percentage is equal to or greater than 99% by mass. Moreover, the surface 20A of the commutator 20 is made up of the surfaces of the commutator segments 21.The DC motor 40 also includes a plurality of brushes 30 disposed in sliding contact with the commutator 20 (i.e., the commutator segments). As shown in FIG. 2, an uneven portion is formed at a radially outer periphery of the commutator. The uneven portion 22 includes protrusions and recesses that are alternately formed in an axial direction of the rotating shaft 12 and extend along the sliding direction of the brushes 30 (or the circumferential direction of the rotating shaft 12), respectively. With the uneven portion 22, it is possible to stabilize the sliding contact between the brushes 30 and the commutator 20, thereby suppressing generation of sparks due to the sliding contact.In operation, electric power is supplied from the battery 80 to the rotor coil 15 via the sliding contact between the brushes 30 and the commutator 20, whereby the rotor 10 rotates. Moreover, during idling of the DC motor 40, the rotor 10 and the commutator 20 rotate at a circumferential speed higher than 30 m / s.FIG. 3 shows the configuration of a starter circuit 81 electrically connected to the DC motor 40 via the electromagnetic switch 60.In the conventional starter circuits, a single lead-acid battery is used to supply an electric power to a starter. In this case, the open circuit voltage of the lead acid battery is in the range of 11-14 V; the circuit resistance composed of the line resistance and the internal resistance of the lead acid battery is in the range of 6-8 mΩ.On the other hand, in recent years, lithium ion batteries have been used in starter circuits to reduce weight and improve charging efficiency.In the present embodiment, the battery 80 included in the starting circuit 81 may be implemented by a single lithium ion battery (LiB) as shown in FIG. 3( a), a pair of lead-acid batteries (PbB) connected in parallel to each other as shown in FIG. 3( b), or a lithium ion battery (LiB) and a lead-acid battery (PbB) connected in parallel to each other as shown in FIG. 3( c). In any of these cases, the open circuit voltage of the battery 80 is in the range of 11-14 V; the circuit resistance R, which is the sum of the line resistance Rw and the internal resistance Rb of the battery 80, is lower than 5 mΩ. Consequently, it is possible to increase the voltage applied to the DC motor 40, thereby increasing the rotational speed of the DC motor 40.Specifically, in the present embodiment, the voltage applied to the DC motor 40 when the motor is started at an ambient temperature of, for example, 25° C. is about 12 V. The electric current flowing in the DC motor 40 during the operation of the DC motor may exceed 100A.As described above, in the present embodiment, the DC motor 40 is configured as a magnetic field motor. Therefore, more sparks are generated between the commutator 20 and the brushes 30 in the DC motor 40, and the commutator 20 and the brushes 30 wear more than in a field winding motor. Moreover, with the above-described configuration of the starter circuit 81, the rotational speed of the DC motor 40 is increased, thereby generating more sparks between the commutator 20 and the brushes 30, and the wear of the commutator 20 and the brushes 30 increases.In addition, as spark generation between the commutator 20 and the brushes 30 increases, wear of the uneven portion 22 of the commutator 20 may easily occur. Consequently, it may become possible to lose the effect of the uneven portion 22 on stabilizing the sliding contact between the brushes 30 and the commutator 20 at an early stage.In order to solve the above problem, in the present embodiment, the brushes 30 are configured to form a graphite film (or a graphite thin layer) 25 on the surface 20A of the commutator 20 with a sliding movement between the brushes 30 and the commutator 20; the graphite film 25 includes hard compounds.FIG. 4 illustrates the sliding movement between each of the brushes 30 and the commutator 20. FIG. 5 both shows the composition of each of the brushes 30 and the composition of the graphite film 25 formed on the surface 20A of the commutator 20.In the present embodiment, each of the brushes 30 is formed of a sintered body containing graphite and copper.As shown in FIG. 4, each of the brushes 30 has a lead wire 35 embedded in a side surface thereof; the lead wire 35 is electrically connected to the motor-side terminal of the electromagnetic switch 60.In the present embodiment, each of the brushes 30 is multilayer-structured to include a first layer 31 and a second layer 32 that are aligned with each other in the rotational direction of the commutator 20 (or the rectification of the commutator 20 with respect to the brushes 30). During the rotation of the commutator 20 with respect to each of the commutator segments 21, the first layer 31 first comes into sliding contact with the commutator segment 21, and then the second layer 32 comes into sliding contact with the commutator segment 21.The first layer 31 has a higher copper content and a lower resistance 25 than the second layer 32. Moreover, the second layer 32 has a thickness smaller than the thickness of the first layer 31 in the rotational direction of the commutator 20.Specifically, in the present embodiment, for each of the brushes 30, the sintered body constituting the brush 30 is composed of a first sintered body constituting the first layer 31 and a second sintered body constituting the second layer 32. The percentage of copper (or copper powder) in the first sintered body (i.e., in the first layer 31) is 30-70% by mass, whereas the percentage of copper in the second sintered body (i.e., in the second layer 32) is 10-30% by mass.It is preferable that the first layer 31 of each of the brushes 30 contains, as metal sulfide solid lubricants, molybdenum disulfide (MoS 2) and tungsten disulfide (WS 2). It is also preferred that the second layer 32 of each of the brushes 30 contains molybdenum disulfide but no tungsten disulfide as a metal sulfide solid lubricant.With the sliding movement of the first layer 31 of each of the brushes 30 containing graphite, tungsten disulfide, and molybdenum disulfide on the commutator 20, the graphite film 25 is formed on the surface 20A of the commutator 20. As the name suggests, the main constituent of the graphite film 25 is graphite. The graphite film 25 has such a hardness as to be suitable for suppressing generation of sparks between the brushes 30 and the commutator 20 and thus wear of the brushes 30 and the commutator 20.Specifically, the graphite film 25 contains graphite, tungsten disulfide, and molybdenum disulfide transferred from the brushes 30. Moreover, the graphite film 25 also contains hard compounds formed by chemical reactions of tungsten disulfide and molybdenum disulfide, which are solid lubricants, with graphite; these hard compounds include carbides such as molybdenum carbide (Mo 2 C) and tungsten carbide (W 2 C).FIG. 6 is a cross-sectional view of the graphite film 25 and the surface 20A of the commutator 20 taken with the SEM (scanning electron microscope).As can be seen from FIG. 6, after the operation of the DC motor 40, the graphite film 25 is formed on the surface 20A of the commutator.FIG. 7 is an enlarged cross-sectional view of the graphite film 25 taken with a TEM (transmission electron microscope).In FIG. 7, "I" enclosed by broken lines denotes molybdenum carbide and tungsten carbide which are hard compounds. "II" enclosed by solid lines denotes tungsten disulfide and molybdenum disulfide which are metal sulfide solid lubricants. "III" enclosed by chain line indicating copper derived from the base metal of the commutator or brushes.As can be seen from FIG. 7, the graphite film 25 contains the hard compounds, the metal sulfide solid lubricants and copper in addition to graphite. Moreover, the main component of the graphite film 25 is graphite. Further, a portion of the metal sulfide solid lubricants transferred from the brushes 30 remains in their original form (i.e., not converted to the compounds by chemical reaction with graphite). Consequently, the graphite film 25 is excellent in both of the sliding property and the hardness.Fig. 8 shows the relationship between the depth from the surface 25A of the graphite film 25 and the hardness of the graphite film 25.In FIG. 8, the solid line represents the hardness of the graphite film 25 containing the hard compounds according to the present embodiment. The broken line represents the hardness of the commutator 20 without the graphite film 25 formed thereon. The two-dot chain line represents the hardness of the hard compound alone.The hardness of the graphite film 25 shown in FIG. 8 was measured by a continuous nanoindentation stiffness measurement using a nanoindenter Xp, which is a product of MTS Systems Corporation.Specifically, in the measurement, a triangular pyramid-shaped indenter (or Berkovich indenter) made of diamond was pressed into a surface 25A of the graphite film 25 at a constant strain rate of 0.05 / sec and a Z-direction attenuation vibration frequency of 45 Hz. Then, the profile of the hardness in the depth direction was obtained by analyzing the load displacement curve determined during pressing of the indenter into the surface 25A of the graphite film 25.In addition, before measuring the hardness of the graphite film 25, measurement was performed on a standard sample made of quartz glass; the analysis results were such that the hardness was 9.5 GPa±5% when the depth was 100 nm or more.In the measurement of the hardness of the graphite film 25, as the pressing depth of the indenter increases, the hardness of copper, which is the base metal of the commutator 20, was detected, and thus the measured hardness was decreased. Therefore, the hardness in a flat portion of the hardness observed when the pressing depth was small was detected when the hardness of the graphite film 25.Specifically, the measurement was performed at a plurality of points (e.g., 20 to 50 points) in a front half of the graphite film 25 formed on a commutator segment 21 (i.e., the half of the graphite film 25 on the front side in the sliding direction of the commutator 20 with respect to the brushes 30); these 25 points are spaced apart from each other in both the longitudinal and transverse directions at intervals of, e.g., 500 μm or more. Then, from all the measured values of the hardness of the graphite film 25, a predetermined number of the measured values were sequentially removed to minimize variation of the measured values at a predetermined depth (e.g., 150 nm). Thereafter, an averaging process was performed for the remaining measurement values.As can be seen from FIG. 8, the hardness of the graphite film 25 containing the hard compounds according to the present embodiment is higher than the hardness of the base metal of a commutator 20 and lower than the hardness of the hard compounds. Moreover, as the depth from the surface 25A of the graphite film 25 increases, the hardness of the graphite film 25 approaches the hardness of copper that is the base metal of the commutator 20. Moreover, although not shown in FIG. 8, the hardness of a graphite film 25 according to the present embodiment is also higher than the hardness of the conventional graphite film containing only graphite.Next, the composition of the first sintered body constituting the first layer 31 of each of the brushes 30 will be described with reference to FIGS. 9 to 12.FIG. 9 shows the relationship between the percentage of tungsten disulfide in the first sintered body and the hardness of the graphite film 25 formed on the surface 20A of the commutator 20.Moreover, the relationship shown in FIG. 9 was determined by experimental examination performed by the inventor of the present application. Specifically, in the experimental evaluation, the percentage of tungsten disulfide in the first sintered body was varied; the hardness of the graphite film 25 was measured at each of a plurality of values of the percentage of tungsten disulfide in the first sintered body.Conventionally, metal sulfide solid lubricants such as tungsten disulfide are used as lubricants for improving sliding between brushes and commutators in DC motors. However, the percentage of metal sulfide solid lubricants in brushes was set to equal to 5 mass % or less. This is because it is well known technical knowledge of the art that it is enough to set the percentage of metal sulfide solid lubricants on brushes to less than or equal to 5 mass % to ensure lubricity.However, as can be seen from FIG. 9, when the percentage of tungsten disulfide in the first sintered body is less than or equal to 5 mass %, it is impossible to improve the hardness of the graphite film 25.In contrast, in the present embodiment, the percentage content of tungsten disulfide in the first sintered body is preferably set to be higher than or equal to 6 mass %, and more preferably set to be higher than or equal to 7 mass %.When the percentage of tungsten disulfide in the first sintered body is set as above, the tungsten disulfide contained in the first sintered body reacts with graphite during operation of the DC motor 40 due to heat generated by supplying electric power to the DC motor 40 and the sliding movement between the brush 30 and the commutator 20. As a result, tungsten carbide is contained in the graphite film 25 formed on the surface 20A of the commutator 20, whereby the hardness of the graphite film 25 is improved to become higher than the hardness of conventional graphite films.FIG. 10 shows the relationship between the percentage of molybdenum disulfide in the first sintered body and the hardness of the graphite film 25.Moreover, the relationship shown in FIG. 10 was also determined by experimental examination performed by the inventor. Specifically, in the experimental evaluation, the percentage of molybdenum disulfide in the first sintered body was varied with the percentage of tungsten disulfide in the first sintered body set to 7.2 mass %; the hardness of the graphite film 25 was measured at each of a plurality of values of the percentage of molybdenum disulfide in the first sintered body.In the present embodiment, the percentage of molybdenum disulfide in the first sintered body is set to be higher than or equal to 2 mass %.By adjusting the percentage of molybdenum disulfide in the first sintered body as above, the molybdenum disulfide contained in the first sintered body reacts with graphite during operation of the DC motor 40 due to heat generated by supplying electric power to the DC motor 40 and the sliding movement between the brush 30 and the commutator 20. As a result, molybdenum carbide together with tungsten carbide is contained in the graphite film 25 formed on the surface 20A of the commutator 20, thereby further improving the hardness of the graphite film 25.In addition, in the example shown in FIG. 10, the percentage of tungsten disulfide in the first sintered body is set to 7.2 mass %. Therefore, even if the percentage of molybdenum disulfide in the first sintered body is less than 2% by mass, it is still possible to secure the hardness required for the graphite film 25.It is preferable that the mass of tungsten disulfide contained in the first sintered body is larger than the mass of molybdenum disulfide contained in the first sintered body. It is more preferred that the mass of tungsten disulfide contained in the first sintered body is about three times the mass of molybdenum disulfide contained in the first sintered body. Moreover, it is also preferable that the total mass of the metal sulfide solid lubricants contained in the first sintered body is smaller than the graphite mass contained in the first sintered body.In the present embodiment, the resistance of the brush 30 may increase as the percentage of the metal sulfide solid lubricants in the first sintered body increases.On the other hand, in recent years, in DC motors of starters used in vehicles having idle-stop systems, the percentage of copper in the brushes has been reduced to limit the amount of electric current supplied in DC motors via the brushes. However, as the percentage of copper in the brushes is decreased, the influence of the oxidation of the copper contained in the brushes becomes large, thereby increasing the electric resistance of the brushes. Therefore, in order to prevent oxidation of copper in a high temperature and high humidity environment, brass containing zinc is generally added to the brushes by about 5-6 mass %.Moreover, as the percentage of metal sulfide solid lubricants in the brushes increases, the copper contained in the brushes can be sulfurized, thereby increasing the electrical resistances of the brushes.FIG. 11 shows the relationship between the percentage of copper in the first sintered body constituting the first layer 31 of each of the brushes 30 and the brush resistance (i.e., the resistance of each of the brushes 30).Moreover, the relationship shown in FIG. 11 is also determined by the experimental investigation performed by the inventor. Specifically, in the experimental evaluation, the percentage of copper in the first sintered body was varied, the total percentage of the metal sulfide solid lubricants in the first sintered body is set to 7.2 mass %, and the percentage of brass in the first sintered body is set to 5.4 mass %; for each of a plurality of values of the percentage of copper in the first sintered body, brush resistance was measured after the brush 30 was allowed to stand in a high temperature and high humidity environment for a long period of time, particularly at a temperature of 80° C. and a humidity of 90% for 250 hours.As can be seen from FIG. 11, when the percentage of copper in the first sintered body is lower than 45 mass %, the influence of sulfurization of the copper contained in the first sintered body becomes large, which increases brush resistance. Moreover, it is apparent from FIG. 11 that, with the percentage content of brass in the first sintered body being in the conventional range (i.e., about 5-6 mass %), the copper contained in the first sintered body cannot be prevented from being sulfurized.FIG. 12 shows the relationship between the ratio of the mass percent content in the first sintered body to the total content in mass % of the metal sulfide solid lubricants in the first sintered body and the brush speed.Moreover, the relationship shown in FIG. 12 was also determined by the experimental investigation performed by the inventor. Specifically, in the experimental evaluation, the percentage of brass in the first sintered body was varied, the percentage of copper in a first sintered body being set to 45 mass %, and the total percentage of metal sulfide solid lubricants in the first sintered body being set to 7.2 mass %; for each of a plurality of values of the percentage of brass in the first sintered body, brush resistance was measured after the brush 30 was allowed to stand in a high temperature and high humidity environment for a long period of time, particularly at a temperature of 80° C. and a humidity of 90% for 250 hours.As can be seen from FIG. 12, when the ratio of the content in mass % of brass in the first sintered body to the total content in mass % of the metal sulfide solid lubricants in the first sintered body is higher than or equal to 0.8, it is possible to suppress an increase in brush resistance even when the percentage content of copper in the first sintered body is not higher than 45 mass %. For example, when the percentage of copper in the first sintered body is 45 mass %, the percentage of tungsten disulfide in the first sintered body is 7.2 mass %, and the percentage of molybdenum disulfide in the first sintered body is 2.1 mass % (i.e., the total percentage of metal sulfide solid lubricants in the first sintered body is 9.3 mass %), addition of brass to the first sintered body by 7.5 mass %, it is possible to suppress an increase in brush resistance. Moreover, in this case, the mass of copper contained in brass is not taken into account in a calculation of the percentage content of copper in the first sintered body.Of the components of brass, zinc can be sulfurized more easily than copper, thus contributing to both suppression of oxidation of copper and suppression of sulfurization of copper. Therefore, it is also possible to formulate the measurement results shown in FIG. 12 with respect to the mass percent content of zinc in the first sintered body. Specifically, the amount of zinc contained in the brass used in the experimental study was about 2 / 5 of the mass of brass. Accordingly, when the ratio of the content in mass % of zinc in the first sintered body to the total content in mass % of metal sulfide solid lubricants in the first sintered body is higher than or equal to 0.3, it is possible to suppress an increase in brush resistance even when the percentage of copper in the first sintered body is not higher than 45 mass %.Moreover, as described above with reference to FIG. 4, in the present embodiment, each of the brushes 30 is multilayer-structured to include the first layer 31 and the second layer 32 having a low copper content and thus a higher resistance than the first layer 31. Moreover, the second layer 32 is disposed behind the first layer 31 with respect to the rotational direction of the commutator 20. Consequently, during the rotation of the commutator 20, with respect to each of the commutator segments 21, the first layer 31 first comes into sliding contact with the commutator segment 21 and then the second layer 32 comes into sliding contact with the commutator segment 21; thus, the second layer 32 whose resistance is higher than the resistance of the first layer 31 is brought out of sliding contact with the commutator segment 21 later than the first layer 31. As a result, it is possible to suppress generation of sparks due to the sliding contact.Further, in the present embodiment, the composition of the second sintered body constituting the second layer 32 is set to be different from the composition of the first sintered body constituting the first layer 31. Specifically, the percentage of copper in the second sintered body is 10-30 mass %. The percentage content of molybdenum disulfide in the second sintered body is about 5 mass %. However, tungsten disulfide is not contained in the second sintered body.Tungsten disulfide is more hydrophilic than molybdenum disulfide.Therefore, if tungsten disulfide were contained in the second sintered body whose percentage of copper is less than the percentage of copper of the first sintered body, the amount of moisture absorbed by the second layer 32 in a high humidity environment would be large. Consequently, the moisture absorbed by the second layer 32 can be rapidly evaporated by heat generated by electric current flowing through the second layer 32 whose resistance is higher than the resistance of the first layer 31, thereby causing cracking of the brush.Although tungsten disulfide is not contained in the second sintered body in the present embodiment, it is nevertheless possible to form a sufficient amount of the hard compounds by chemical reactions of tungsten disulfide contained in the first sintered body and molybdenum disulfide contained in both the first and second sintered bodies with graphite.FIG. 13 shows the relationship between the number of times the starter S starts the motor of the vehicle and the surface hardness of the commutator 20 when the first layer 31 of each of the brushes 30 is formed of the first sintered body having the composition described above.By a durability test of the starter S performed by the inventor of the present application, it was confirmed that after thousands of operation times of the starter S, the graphite film 25 containing the hard compounds was formed.As can be seen from FIG. 13, in the durability test, as the operation frequency of the starter S increases, the graphite film 25 was formed on the surface 20A of the commutator 20, and thus the surface hardness of the commutator 20 was gradually increased from the hardness of the base material of the commutator 20 (i.e., copper).That is, in the present embodiment, the graphite film 25 is formed with the operation of the DC motor 40 (or the operation of the starter S) without performing an additional process. Consequently, it becomes possible to form the graphite film 25 at a low cost. Moreover, the materials of the graphite film 25 are originally contained in the brushes 30, and are transferred to the surface 20A of the commutator 20 to form the graphite film 25 in operation of the DC motor 40. Therefore, the graphite film 25 can be maintained until the end of the life of the brushes 30.With the graphite film 25 formed on the surface 20A of the commutator 20 and having both excellent hardness and lubricity, it is possible to suppress the wear of the commutator 20, thereby prolonging the life of the commutator 20. Further, by suppressing the wear of the commutator 20, it becomes possible to suppress the dimensional accuracy of the commutator 20 (more specifically, the roundness in the case of the cylinder-type commutator 20 or the flatness in the case of the surface-type commutator 20) from being lowered due to uneven wear of the commutator 20. Consequently, it is possible to suppress generation of spark due to reduction in dimensional accuracy of the commutator 20, thereby also extending the life of the brushes 30.Moreover, with the graphite film 25, it is possible to effectively suppress the wear of the uneven portion 22 of the commutator 20 formed on the radially outer periphery of the commutator 20. Consequently, it is possible to stabilize the sliding contact between the brushes 30 and the commutator 20 over a long period of time, further prolonging the life of the brushes 30.In addition, it is preferable that in the first sintered body constituting the first layer 31 of each of the brushes 30, the percentage of copper is 40 mass %, the percentage of tungsten disulfide is 7.8 mass %, the percentage of molybdenum disulfide is 2.6 mass %, the percentage of brass is 10.4 mass %, and the balance is graphite and additives such as an abrasive. Moreover, the voltage applied to the DC motor 40 when the motor is started at an ambient temperature of, for example, 25° is about 12 V; the electric current flowing in the DC motor 40 during the operation of the DC motor 40 may exceed 100A. By using the brushes 30 described above in such a DC motor, it is possible to reliably form the graphite film 25 on the surface 20A of the commutator 20.According to the present embodiment, it is possible to achieve the following advantageous effects.In the present embodiment, the DC motor 40 is used in the starter S configured for use in a vehicle equipped with an idle stop system to start an engine of the vehicle. The DC motor 40 includes the commutator 20 and the brushes 30. the commutator 20 is formed of copper or a copper alloy whose percentage of copper is equal to or greater than 99% by mass. The brushes 30 are disposed in sliding contact with the commutator 22. Each of the brushes 30 is formed on the sintered body made up of the first sintered body constituting the first layer 31 and the second sintered body constituting the second layer 32. The first sintered body contains graphite, copper, and the metal sulfide solid lubricants (i.e., tungsten disulfide and molybdenum disulfide). The percentage of copper in the first sintered body is 30-70% by mass. The percentage of tungsten disulfide in the first sintered body is higher than or equal to 6.5 mass %.With the above configuration, during operation of the DC motor 40, the tungsten disulfide contained in the first sintered body reacts with graphite due to heat generated by supplying electric power to the DC motor 40 and the sliding movement between the brush 30 and the commutator 20. Consequently, the tungsten disulfide is converted into tungsten carbide which is a hard compound. As a result, tungsten carbide is contained in the graphite film 25 formed on the surface 20A of the commutator 20, whereby the hardness of the graphite film is improved to become higher than the hardness of conventional graphite films.That is, in the DC motor 40, the graphite film 25 is not originally formed on the surface 20A of the commutator 20. Instead, the graphite film 25 is formed with the operation of the DC motor 40 without performing an additional process. As a result, it becomes possible to form the graphite film 25 at a low cost. Moreover, the graphite film 25 is continuously formed with the sliding movement between each of the brushes 30 and the commutator 20. Therefore, the graphite film 25 can be maintained until the end of the life 30.Moreover, a part of the tungsten disulfide transferred from the brushes 30 onto the graphite film 25 remains in its original form (i.e., not converted into tungsten carbide). That is, the graphite film 25 contains tungsten disulfide, which is a metal sulfide solid lubricant. Consequently, the graphite film 25 is excellent in both of the sliding property and the hardness.Accordingly, with the graphite film 25 formed on the surface 20A of the commutator 20, it is possible to suppress the abrasion of the commutator 20, thereby prolonging the life of the commutator 20. Further, by suppressing the abrasion of the commutator 20, it is possible to prevent the dimensional accuracy of the commutator 20 from decreasing due to uneven abrasion of the commutator 20. Consequently, it is possible to suppress generation of spark due to reduction in dimensional accuracy of the commutator, thereby also extending the life of the brushes 30.It is preferable that the percentage of tungsten disulfide in the first sintered body constituting the first layer 31 of each of the brushes 30 is higher than or equal to 7 mass %.By setting the percentage of tungsten disulfide in the first sintered body as above, it is possible to facilitate the generation of tungsten carbide, thereby increasing the hardness of the graphite film 25. Consequently, it is possible to more reliably suppress the abrasion of the commutator 20.In the present embodiment, the first sintered body constituting the first layer 31 of each of the brushes 30 includes molybdenum disulfide in addition to tungsten disulfide. The percentage content of molybdenum disulfide in the first sintered body is higher than or equal to 2 mass %.With the above configuration, during operation of the DC motor 40, the molybdenum disulfide contained in the first sintered body reacts due to heat generated by supplying electric power to the DC motor 40 and the sliding movement between the brush 30 and the commutator 20. Consequently, the molybdenum disulfide is converted into molybdenum dicarbide which is a hard compound. As a result, molybdenum carbide together with tungsten carbide is contained in the graphite film 25 formed on the surface 20A of the commutator 20. Therefore, compared with the case of the graphite film 25 containing tungsten carbide as a hard compound, it is possible to further improve the hardness of the graphite film 25, thereby more reliably suppressing the abrasion of the commutator 20.With the increase in the percentage of the metal sulfide solid lubricants (i.e., tungsten disulfide and molybdenum disulfide) in the brushes 30, desulfurization of the copper contained in the brushes 30 in a high temperature and high humidity environment by the sulfur contained in metal sulfide solid lubricants can become remarkable. In particular, when the percentage of copper in the brushes 30 is low, the brush resistance (i.e., the resistance of each of the brushes 30) can be significantly increased due to sulfurization of the copper contained in the brushes 30.In view of the above, in the present embodiment, when the percentage of copper in the first sintered body is not higher than 45 mass %, zinc is added to the first sintered body to such an extent that the ratio of the percentage of zinc in the first sintered body to the total content in mass % of the metal sulfide solid lubricants in the first sintered body is higher than or equal to 0.3. Specifically, in the present embodiment, brass containing zinc is supplied to the first sintered body to such an extent that the ratio of the content in mass % of brass of the first sintered body to the total content in mass % of the metal sulfide solid lubricants in the first sintered body is higher than or equal to 0.8.With the above configuration, since zinc can be sulfurized more easily than copper, it is possible to suppress an increase in brush resistance due to sulfurization of copper contained in the brushes 30, even if the percentage of copper in the first sintered body is not higher than 45 mass %. Consequently, it is possible to suppress the abrasion of the commutator 20 when an increase in brush resistance is suppressed.In the present embodiment, each of the brushes 30 is multilayer-structured to include the first layer 31 and the second layer 32 aligned with each other in the rotational direction of the commutator 20. The second layer 32 has a thickness smaller than the thickness of the first layer 31 in the rotational direction of the commutator 20, and the first layer 31 is formed of the first sintered body, and the second layer 32 is formed on the second sintered body. The content in mass % of copper in the second sintered body is less than the content in mass % of copper in the first sintered body. In the second sintered body, no tungsten disulfide is contained.That is, in the present embodiment, in order to suppress generation of sparks between the commutator 20 and the brushes 30, each of the brushes 30 includes the second layer 32 formed of the second sintered body whose content in mass % of copper is less than the content in mass % of copper of the first sintered body forming the first layer 31. On the other hand, tungsten disulfide is more hydrophilic than molybdenum disulfide. Therefore, if tungsten disulfide were contained in the second sintered body, the amount of moisture absorbed by the second layer 32 in a high humidity environment would be large. Consequently, the moisture absorbed by the second layer 32 can be rapidly evaporated by heat generated by electric current flowing through the second layer 32 whose resistance is higher than the resistance of the first layer 31, thereby causing cracking of the brush. In view of the above, in the present embodiment, tungsten disulfide is not contained in the second sintered body.Moreover, in the present embodiment, although tungsten disulfide is not contained in the second sintered body, it is still possible to form a sufficient amount of the hard compounds by chemical reactions of tungsten disulfide contained in the first sintered body and molybdenum disulfide contained in the first and second sintered bodies with graphite. Consequently, it is still possible to reliably suppress the abrasion of the commutator 20.While the above specific embodiment has been shown and described, it will be understood by those skilled in the art that various modifications, changes and improvements may be made without departing from the spirit of the present disclosure.For example, in the above-described embodiment, each of the brushes 30 is multi-layeredly structured to include the first and second layers 31 and 32 formed of the first and second sintered bodies, respectively. Alternatively, each of the brushes 30 may be single-layered structured to include only the first layer 31 formed of the first sintered body.In the above-described embodiment, the first sintered body constituting the first layer 31 of each of the brushes 30 contains both tungsten disulfide and molybdenum disulfide. Alternatively, the first sintered body may contain only tungsten disulfide as a metal sulfide solid lubricant; the percentage of tungsten disulfide in the first sintered body is higher than or equal to 6.5 mass %.A DC motor is provided for use in a starter that starts an engine. The DC motor includes a commutator and a brush. The commutator is formed of copper or a copper alloy whose percentage of copper is 99% by mass or more. The brush is disposed in sliding contact with a surface of the commutator. Moreover, the brush is formed of a sintered body containing 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 includes tungsten disulfide. The percentage content of tungsten disulfide in the sintered body is higher than or equal to 6.5 mass %.

Claims

A DC motor (40) for a starter (S) that starts an engine, the DC motor (40) comprising: a commutator (20) formed of copper or a copper alloy, the percentage of copper of which is higher than or equal to 99% by mass; and a brush (30) that is in sliding contact with a surface of the commutator (20), the brush (30) being formed of a sintered body containing 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, and a percentage of tungsten disulfide in the sintered body is higher than or equal to 6.5% by mass.The DC motor (40) according to claim 1, wherein the percentage of tungsten disulfide in the sintered body is higher than or equal to 7 mass%.The DC motor (40) according to claim 2, wherein the at least one metal sulfide solid lubricant further comprises molybdenum disulfide, and a percentage of molybdenum disulfide in the sintered body is higher than or equal to 2 mass%.The DC motor (40) according to claim 3, wherein the sintered body further contains zinc, the percentage content of copper in the sintered body is not higher than 45 mass%, and a ratio of a content in mass% of zinc in the sintered body to a total content in mass% of the at least one metal sulfide solid lubricant in the sintered body is higher than or equal to 0.3.The DC motor (40) according to claim 4, wherein the sintered body is a first sintered body, the brush (30) includes a first layer (31) and a second layer (32) that are aligned with each other in a rotational direction of the commutator (20), the second layer (32) has a thickness in the rotational direction of the commutator (20) smaller than the first (31), the first layer (31) is formed of the first sintered body, and the second layer (32) is formed of a second sintered body, a mass percentage of copper in the second sintered body is lower than the mass percentage of copper in the first sintered body, and no tungsten disulfide is contained in the second sintered body.The DC motor (40) according to claim 3, wherein the sintered body further contains brass, the percentage of copper in the sintered body is not higher than 45 mass%, and a ratio of a content in mass% of brass in the sintered body to a total content in mass% of the at least one metal sulfide solid lubricant in the sintered body is higher than or equal to 0.8.The DC motor (40) according to claim 6, wherein the sintered body is a first sintered body, the brush (30) includes a first (31) and a second layer (32) that are aligned with each other in a rotational direction of the commutator (20), the second layer (32) has a thickness smaller than the first layer (31) in the rotational direction of the commutator (20), the first layer (31) is formed of the first sintered body, and the second layer (32) is formed of a second sintered body, a mass% content of copper in the second sintered body is lower than the mass% content of copper in the first sintered body, and no tungsten disulfide is contained in the second sintered body.The DC motor (40) according to claim 1, wherein the at least one metal sulfide solid lubricant further comprises molybdenum disulfide, and the percentage of molybdenum disulfide in the sintered body is higher than or equal to 2 mass%.The DC motor (40) according to claim 1, wherein the sintered body further contains zinc, the percentage content of copper in the sintered body is not more than 45 mass%, and a ratio of the content in mass% of zinc in the sintered body to a total content in mass% of the at least one metal sulfide solid lubricant in the sintered body is higher than or equal to 0.3.The DC motor (40) according to claim 1, wherein the sintered body further contains brass, the percentage of copper in the sintered body is not more than 45 mass%, and a ratio of the percentage by mass of brass in the sintered body to a total percentage by mass of the at least one metal sulfide solid lubricant in the sintered body is higher than or equal to 0.8.The DC motor (40) according to claim 1, wherein the sintered body is a first sintered body, the brush (30) includes a first layer (31) and a second layer (32) that are aligned with each other in a rotation direction of the commutator (20), the second layer (32) has a thickness smaller than the first layer (31) in the rotation direction of the commutator (20), the first layer (31) is formed of the first sintered body, and the second layer (32) is formed of a second sintered body, a percentage content of copper in the second sintered body is lower than the content in mass% of copper in the first sintered body, and no tungsten disulfide is contained in the second sintered body.

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