SLIDING CONTACT MATERIAL FOR MOTOR BRUSH, MOTOR BRUSH, AND DIRECT CURRENT MOTOR

Abstract

The present invention is drawn to a sliding contact material for a motor brush, the sliding contact material containing: pure Ag as a matrix; and ZnO particles and Ta.sub.2O.sub.5 particles dispersed in the matrix, wherein the sliding contact material has a ZnO particle content of 0.1% by mass or more and 12% by mass or less and a Ta.sub.2O.sub.5 particle content of 0.1% by mass or more and 6.0% by mass or less. The present inventive sliding contact material is preferable as a constituent material of motor brushes of small DC motors, satisfactory in mechanical wear resistance and spark resistance, and superior in low-noise characteristics. In addition, the present invention is drawn to a sliding contact material that is capable of substituting for AgPd-based alloys, which have become expensive because of the recent increase in palladium price.

Claims

1. A sliding contact material for a motor brush, the sliding contact material comprising: pure Ag as a matrix; and ZnO particles and Ta.sub.2O.sub.5 particles dispersed in the matrix, wherein the sliding contact material has a ZnO particle content of 0.1% by mass or more and 12% by mass or less and a Ta.sub.2O.sub.5 particle content of 0.1% by mass or more and 6.0% by mass or less.

2. The sliding contact material for a motor brush according to claim 1, wherein the pure Ag as a matrix is pure Ag having an Fe/Cr/Ni total content of 0.3% by mass or less.

3. A composite material for a motor brush, the composite material comprising: a base material comprising a Cu-based material; and a sliding contact material jointed to at least a part of the base material, wherein the sliding contact material jointed is the sliding contact material defined in claim 1.

4. A motor brush for a DC motor, the motor brush comprising the composite material defined in claim 3.

5. A DC motor comprising: a motor brush; and a commutator herein the motor brush is the motor brush defined in claim 4.

6. A composite material for a motor brush, the composite material comprising: a base material comprising a Cu-based material; and a sliding contact material jointed to at least a part of the base material, wherein the sliding contact material jointed is the sliding contact material defined in claim 2.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0040] The FIGURE is a photograph showing the material structure of a sliding contact material (Ag-1.2% by mass Ta.sub.2O.sub.5-3.0% by mass ZnO) produced in First Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] First Embodiment: Preferred embodiments of the present invention will be described with examples shown below. In the present embodiment, a sliding contact material in which 1.2% by mass of Ta.sub.2O.sub.5 and 3.0% by mass of ZnO were dispersed in a pure Ag matrix, and a composite material including the sliding contact material were produced. Then, the composite material was processed into a motor brush, which was installed in a small DC motor, and the wear resistance was evaluated.

[Production of Sliding Contact Material]

[0042] In the present embodiment, a sliding contact material was produced by powder metallurgy. Pure Ag powder (average particle size: 7 m), 1.2% by mass of Ta.sub.2O.sub.5 powder (average particle size: 1 m), and 3.0% by mass of ZnO powder (average particle size: 1 m) were mixed with a ball mill for 5 hours. A cylindrical container was filled with that powder mixture, which was compressed by the application of a pressure of 510.sup.2 MPa from the longitudinal direction to form a cylindrical billet of 50 mm in diameter. The cylindrical billet was then sintered through heating in the atmosphere at 850 C. for 4 hours. Those compression and sintering were repeated three times, giving a sliding contact material. This sliding contact material was hot-extruded into a coarse wire of 6 mm in diameter, which was repeatedly drawn and annealed, giving a wire rod of 1 mm in diameter. The wire rod was further rolled with a roller, giving a sliding contact material in the form of a tape. The Figure shows a photograph of the metal structure of the sliding contact material produced in the present embodiment.

[Production of Composite Material for Motor Brush]

[0043] Subsequently, the sliding contact material in the form of a tape and a base material were jointed together by cladding (inlaying) to give a composite material. The base material used was nickel silver for springs (C7701). The cladding was performed with a roller, and the rolled material was subjected to a heat treatment at 750 C. to provide the composite material. The thickness of the sliding contact material in the composite material, which was to be processed into a motor brush, was 30 m.

Reference Example

[0044] In order to evaluate the durability of the sliding contact material (Ag-1.2% Ta.sub.2O.sub.5-3% ZnO) produced in the present embodiment, a composite material including a sliding contact material of a Ag-50% Pd alloy was produced as Reference Example. The reason why a AgPd alloy was applied as Reference Example to serve as an evaluation reference is that the AgPd alloy has very satisfactory wear resistance as described above, and that the AgPd alloy is a precious metal-based contact material satisfactory also in terms of spark resistance, which is a key in use under high loads, and hence can serve as a reference for evaluation of spark resistance. With considering improvement in material cost, which is one of the objects of the present application, and the increased Pd price, the thickness of the AgPd alloy of Reference Example was set to 10 m so as to match the material costs. Then, the same base material as in the present embodiment was cladded with the Ag-50% Pd alloy, giving a composite material.

[Production of Small DC Motor and Durability Test]

[0045] The composite materials produced in the present embodiment and Reference Example were processed into motor brushes, with which small DC motors were actually assembled, and durability test was carried out for the sliding contact materials under two sets of test conditions shown below. Commutators for the small DC motors were produced with composite materials in each of which a base material including a copper-based material was cladded with a contact material, wherein different contact materials were used for different test conditions.

[0046] Among the two sets of test conditions in the present embodiment, Test Conditions 1 were ones simulating a low-load small DC motor to be used in an air-conditioning system (HVAC) for vehicles, and intended for evaluation of durability to mechanical wear. Test Conditions 2 were ones simulating a high-load small DC motor to be used in an electric folding mirror in an automobile, and intended for evaluation of durability to sparking wear. In the durability test, an operation mode as a combination of counterclockwise rotation (CCW) and suspension (OFF) and clockwise rotation (CW) and suspension (OFF) was defined as one cycle, and the number of cycles until a motor broke down was evaluated.

TABLE-US-00001 TABLE 1 Test Conditions 1 Test Conditions 2 Voltage DC 13.5 V Current 83 mA 200 mA Load 1.96 mN-m 2.0 mN-m Rotational frequency 3400 rpm 7000 rpm Commutator AgCuZnNi AgCuNiTa.sub.2O.sub.5 + ZnO Mode CCW 6 seconds - CCW 3 seconds - OFF 2 seconds OFF 5 seconds CW 6 seconds - CW 3 seconds - OFF 2 seconds OFF 5 seconds Test environment 25 C. 65% 20% RH

[0047] The results of the durability test under the two sets of test conditions showed that Ag-1.2% Ta.sub.2O.sub.5-3% ZnO, which was the sliding contact material of the present embodiment, exhibited a durability of 683000 cycles under Test Conditions 1. For the AgPd alloy as Reference Example, by contrast, the motor was found to break down after 473000 cycles. The evaluation test under Test Conditions 1 is one on the mechanical wear characteristics of a low-load motor for HVAC, etc., and the sliding contact material of the present embodiment has higher durability than the AgPd alloy as Reference Example.

[0048] The evaluation results under Test Conditions 2 showed that motors did not break down even after 100000 cycles for both the Ag-1.2% Ta.sub.2O.sub.5-3% ZnO and the AgPd alloy. Motors for electric folding mirrors are required to have a durability of 50000 cycles under Test Conditions 2 according to the standard, and those sliding contact materials were confirmed to have lifetimes two times or more longer than specified in the standard. The test under Test Conditions 2 is one for evaluating the spark resistance of a high-load small DC motor, and the Ag-1.2% Ta.sub.2O.sub.5-3% ZnO was confirmed to be superior in spark resistance. The AgPd alloy as Reference Example is also deemed to be superior in spark resistance similarly to the present embodiment.

[0049] Thus, the evaluation test under the two sets of test conditions in the present embodiment confirmed that the Ag-1.2% Ta.sub.2O.sub.5-3% ZnO, which was the sliding contact material of the present embodiment, had preferable wear resistance and spark resistance even over the AgPd alloy as Reference Example.

[0050] Second Embodiment: In the present embodiment, a plurality of sliding contact materials were produced with AgTa.sub.2O.sub.5ZnO having different Ta.sub.2O.sub.5 and ZnO contents, and evaluated for their durability. The sliding contact materials were produced by powder metallurgy as in First Embodiment. Here, the same pure Ag powder, Ta.sub.2O.sub.5 powder, and ZnO powder were used under the same production conditions as in First Embodiment to produce sliding contact materials (thickness: 30 m) each including a AgTa.sub.2O.sub.5ZnO alloy, and motor brushes were produced with the sliding contact materials.

[0051] Each motor brush produced was then installed in the same small DC motor as in First Embodiment, and subjected to durability test. Durability to mechanical wear and that to sparking wear were checked also in the present embodiment, but the evaluation was performed by accelerated test under severe test conditions. Table 2 shows the test conditions in the present embodiment.

TABLE-US-00002 TABLE 2 Test Conditions 3 Test Conditions 4 Voltage DC 12.5 V DC 13.5 V Current 90 mA 160 mA Load 0.78 mN-m 1.96 mN-m Rotational frequency 7000 rpm 7000 rpm Commutator AgCuNiTa.sub.2O.sub.5 + ZnO AgCuZnNi Mode CCW 6 seconds - OFF 2 seconds CW 6 seconds - OFF 2 seconds Test environment 25 C. 65% 20% RH

[0052] The two sets of test conditions in the present embodiment were respectively ones simulating a low-load small DC motor for HVAC application. Test Conditions 3 were ones primarily for evaluating mechanical wear, and the test was accelerated test with the rotational frequency increased from that of Test Conditions 1 in First Embodiment. Test Conditions 4 were ones primarily for evaluating spark resistance under increased electric loads, and the test was accelerated test with the rotational frequency and current increased from those of Test Conditions 1 in First Embodiment. Table 3 shows the results of the durability test. Also in the present embodiment, a motor brush with a contact material (thickness: 10 m) including a Ag-50% Pd alloy was prepared as Reference Example and subjected to the durability test. The results of the durability test are shown in Table 3.

TABLE-US-00003 TABLE 3 Test Test Conditions 3 Conditions 4 (wear (spark Composition resistance) resistance) Example 1 Ag1.2Ta.sub.2O.sub.53ZnO 110000 cycles 20000 cycles Example 2 Ag1.8Ta.sub.2O.sub.53ZnO 159000 cycles 51000 cycles Example 3 Ag1.2Ta.sub.2O.sub.56ZnO 110000 cycles 83000 cycles Example 4 Ag1.8Ta.sub.2O.sub.56ZnO 151000 cycles 100000 cycles Reference Ag50Pd 87000 cycles 9600 cycles Example *Example 1 is the contact material of First Embodiment.

[0053] The results of the durability test in the present embodiment confirmed that the AgTa.sub.2O.sub.5ZnO alloys of the examples were all superior in both wear resistance and spark resistance to the AgPd alloy as Reference Example in the accelerated test under any set of the test conditions. Accordingly, the AgTa.sub.2O.sub.5ZnO alloys of the present embodiment enable production of high-durability sliding contact materials in combination with cost reduction due to being Pd-free.

[0054] Comparison among the contact materials of Examples 1 to 4 found enhancement of spark resistance by increase in either one of the Ta.sub.2O.sub.5 and ZnO contents. This suggested that Ta.sub.2O.sub.5 and ZnO each have an effect of improving spark resistance. Comparison on mechanical wear between Example 1 and Example 2 suggested that Ta.sub.2O.sub.5 has an effect of improving wear resistance. On the other hand, comparison between Example 2 and Example 4 found that increase in the ZnO content did not result in change in the number of cycles before breaking down, suggesting that ZnO has a weaker effect of improving mechanical wear. Nevertheless, having a proper ZnO content is essential for a contact material for motor brushes, ensuring wear resistance, under a harsh environment in terms not only of mechanical wear but also of sparking wear.

[0055] If the Ag-50% Pd alloy tested as Reference Example in First and Second Embodiments has the same thickness of a contact material as the AgTa.sub.2O.sub.5ZnO alloys of the present embodiment, the Ag-50% Pd alloy is expected to exhibit longer endurance time than in the present embodiment. The AgPd alloy exhibits a very wide range of reinforcement by solid-solution strengthening. On the other hand, the present inventive AgTa.sub.2O.sub.5ZnO alloy has wear resistance enhanced by reinforced dispersion with metal oxide particles, but contains Ag, which is relatively soft, as a matrix, and hence the range of reinforcement is inferred to be narrower than that by solid-solution strengthening. However, the increased Pd price has made the AgPd alloy unbalanced between cost and performance as described above. Being Pd-free, motor brush materials including the present inventive AgTa.sub.2O.sub.5ZnO alloy require significantly lower material cost than the AgPd alloy. As shown in First and Second Embodiments, the AgTa.sub.2O.sub.5ZnO alloys of the present embodiment were confirmed to be very useful as an alternative material to the AgPd alloy in view of material cost.

[0056] Third Embodiment: In the present embodiment, evaluation simulating a motor brush of a low-load universal small DC motor, which is used for shavers, etc., was carried out. In the present embodiment, the same AgTa.sub.2O.sub.5ZnO alloys as Examples 1 to 3 in Second Embodiment were produced by powder metallurgy. Then, a sliding contact material in the form of a tape and a base material were jointed together by cladding in the same manner as in Second Embodiment, giving a composite material (thickness: 30 m). The base material used was beryllium copper (C1741). The composite material produced was processed into a motor brush, with which a small DC motor was assembled and subjected to durability test.

[0057] For comparison, durability test was carried out for two commercially available universal small DC motors. The commercially available small DC motors as comparative examples were one for high voltages and one for low voltages. The motor for high voltages included a motor brush material of a Ag-30% Pd alloy having a thickness of 5 m, and the motor for low voltages included a motor brush material of a Ag-50% Pd alloy having a thickness of 5 m. Also in the present embodiment, the thickness of the contact material including the AgTa.sub.2O.sub.5ZnO alloy (30 m) was set so as to match the material cost with those of the contact materials of the comparative examples.

TABLE-US-00004 TABLE 4 For high voltages For low voltages Voltage DC 3.4 V DC 1.1 V Current 0.85 A 2 A Load 1.27 mN-m Rotational frequency 12500 rpm 10000 rpm Commutator AgPdCuZnNi AgPdCuZnNi Mode CCW 2 minutes - OFF 2 seconds Test environment 25 C. 65% 20% RH

TABLE-US-00005 TABLE 5 Brush material For high voltages For low voltages Example 1 Ag1.2Ta.sub.2O.sub.53ZnO 436 hours 342 hours Example 2 Ag1.8Ta.sub.2O.sub.53ZnO 525 hours Example 3 Ag1.8Ta.sub.2O.sub.56ZnO 566 hours Universal Ag30Pd 352 hours motor Ag50Pd 169 hours

[0058] It was confirmed from Table 5 that the small DC motors each including a motor brush material of any of the AgTa.sub.2O.sub.5ZnO alloys of Examples 1 to 3 exhibited operation times comparable to or longer than those of the universal motor for high voltages and that for low voltages as comparative examples, thus being satisfactory in durability. Also from the results in the present embodiment, the AgPd alloy is expected to exhibit longer endurance time than the AgTa.sub.2O.sub.5ZnO alloys of the examples for the same contact material thickness. However, the evaluation carried out in the present embodiment was based on the concern about material cost, and confirmed from this viewpoint that the AgTa.sub.2O.sub.5ZnO alloys of the present embodiment are useful as an alternative material to the AgPd alloy.

[0059] Fourth Embodiment: Here, motor brush materials were produced with other types of oxides to be dispersed in the Ag matrix, and their durability to sparking wear was evaluated. In the present embodiment, contact materials based on the AgTa.sub.2O.sub.5ZnO alloy of First Embodiment (Example 1) (Ta.sub.2O.sub.5: 1.2% by mass, ZnO: 3% by mass) with different amounts of Ta.sub.2O.sub.5, and those in which MgO or SnO was dispersed in place of ZnO were produced, and their durability when being installed in a small DC motor was evaluated. The contact materials were produced by powder metallurgy as in First Embodiment. For ZnO and SnO in producing mixed powder, ZnO powder and SnO powder each of 1 m in particle size were used.

[0060] Composite materials were produced with the contact materials, and processed into motor brushes, with which small DC motors were assembled. Then, wear test was carried out under conditions shown below. The test conditions were ones simulating an automatic door lock in an automobile, and intended to primarily evaluate spark resistance.

TABLE-US-00006 TABLE 6 Voltage DC 13.5 V Current 3 A Mode CCW 0.1 seconds - OFF 2.4 seconds CW 0.1 seconds - OFF 2.4 seconds Commutator material AgCuNiMgZn Load 50 g-cm Test environment 298K, 50% RH

[0061] In the wear test in the present embodiment, each motor operated for 100000 cycles in that mode was disassembled and the brush was taken out, and the wear depth was measured with a contact roughness meter (number of testing: 3). Table 7 shows the mean values of wear depth measured for the motor brush materials.

TABLE-US-00007 TABLE 7 Composition (% by mass) Ag Ta.sub.2O.sub.3 ZnO MgO SnO Wear depth Example 1 balance 1.2 3.0 14 m Example 2 balance 1.8 3.0 16 m Example 5 balance 0.6 3.0 16 m Example 6 balance 0.6 5.0 15 m Comparative balance 1.2 18 m example 1 Comparative balance 1.2 2.3 22 m example 2 Comparative balance 1.8 2.3 25 m example 3 Comparative balance 0.6 2.3 22 m example 4 Comparative balance 1.2 3.0 22 m example 5 Comparative balance 1.2 5.0 26 m example 6 Comparative balance 1.2 3.0 2.3 22 m example 7 Comparative balance 1.2 3.0 3.0 20 m example 8

[0062] As confirmed in Second Embodiment, among the oxides dispersing in the Ag matrix, Ta.sub.2O.sub.5 is deemed to primarily ensure the durability of the contact material to mechanical wear, and ZnO is deemed to have an effect of improving spark resistance. The results in the present embodiment (Table 7) showed that the contact materials in which a non-ZnO oxide (MgO, SnO) was dispersed were inferior in spark resistance to Example 1 and so on. The contact materials in which ZnO was contained but MgO or SnO was simultaneously dispersing did not have satisfactory spark resistance. This suggests that not all types of metal oxide should be dispersed. The present embodiment revealed that ZnO is preferable for enhanced durability to mechanical wear and sparking wear.

INDUSTRIAL APPLICABILITY

[0063] The present inventive sliding contact material has wear resistance and spark resistance preferable for brushes of small DC motors, and exerts durability in combination with noise reduction effect. The present invention can be preferably applied to small DC motors in various fields including automotive applications, precision instruments, and home appliances. In automotive applications, the present invention can be applied to small DC motors in electric components such as audio devices, air conditioner dampers, electric folding mirrors, and automatic door locks. For home appliances, the present invention can be applied to small motors in shavers, electric toothbrushes, small vacuum cleaners, and so on.