Laminate carbon brush for fuel pump motor

Abstract

A laminated carbon brush for a liquid pump motor slides in a liquid fuel on a disk-like commutator. The laminated carbon brush includes two layers of a lower resistivity layer and a higher resistivity layer. In both the lower resistivity layer and the higher resistivity layer, circular directional resistivities of the brush along a circular direction in the rotation of the commutator are higher than non-circular directional resistivities of the brush along a radial direction of the commutator and a perpendicular direction to the sliding surface of the commutator. A non-circular directional resistivity of the higher resistivity layer are higher than or equal to 90,000 .Math.cm. No spark discharges occur if the fuel pump motor is operated to output a high power.

Claims

1. A laminated carbon brush for a liquid pump motor sliding on a sliding surface of a disk-like commutator in a liquid fuel comprising: at least two layers of a lower resistivity layer and a higher resistivity layer, wherein regarding both the lower resistivity layer and the higher resistivity layer, circular directional resistivities of the brush along a circular direction in the rotation of the commutator are higher than non-circular directional resistivities of the brush along a radial direction of the commutator and along a perpendicular direction to the sliding surface of the commutator, and wherein a non-circular directional resistivity of the higher resistivity layer is higher than or equal to 90,000 .Math.cm.

2. The laminated carbon brush for a liquid pump motor according to claim 1, wherein the higher resistivity layer contains an insulative inorganic layered compound.

3. The laminated carbon brush for a liquid pump motor according to claim 2, wherein the higher resistivity layer contains an insulative self-lubricating inorganic layered compound as the insulative inorganic layered compound.

4. The laminated carbon brush for a liquid pump motor according to claim 2, wherein the higher resistivity layer contains the insulative inorganic layered compound in an amount of no less than 10 mass % and no greater than 70 mass %.

5. The laminated carbon brush for a liquid pump motor according to claim 2, wherein the higher resistivity layer contains talc.

6. The laminated carbon brush for a liquid pump motor according to claim 1, wherein both the lower resistivity layer and the higher resistivity layer contain at least a thermoplastic resin.

7. The laminated carbon brush for a liquid pump motor according to claim 2, wherein both the lower resistivity layer and the higher resistivity layer contain at least a thermoplastic resin.

8. The laminated carbon brush for a liquid pump motor according to claim 1, wherein the non-circular directional resistivity of the higher resistivity layer is lower than or equal to 800,000 .Math.cm.

9. The laminated carbon brush for a liquid pump motor according to claim 8, wherein the circular directional resistivity of the higher resistivity layer is lower than or equal to 4,000,000 .Math.cm.

10. The laminated carbon brush for a liquid pump motor according to claim 1, wherein the non-circular directional resistivity of the lower resistivity layer is lower than or equal to 10,000 .Math.cm.

11. The laminated carbon brush for a liquid pump motor according to claim 9, wherein the circular directional resistivity of the lower resistivity layer is lower than or equal to 50,000 .Math.cm.

12. The laminated carbon brush for a liquid pump motor according to claim 1, wherein the commutator is circular and disk-like, and the sliding surface of the commutator is circular.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a side view showing a laminated carbon brush for a fuel pump motor according to an embodiment together with a commutator.

(2) FIG. 2 is a plan view showing the laminated carbon brush for a fuel pump motor according to the embodiment together with the commutator.

(3) FIG. 3 is a diagram showing the manufacturing process of the laminated carbon brush for a fuel pump motor according to the embodiment, where 1) shows the filling of the higher-resistivity material, 2) shows the filling of the lower-resistivity material, 3) shows pressing, 4) shows thermal processing at a temperature higher than or equal to the melting point of the thermoplastic resin, 5) shows cutting, and 6) shows the attachment of a lead wire.

(4) FIG. 4 is a characteristics diagram showing the energy of spark discharges generated between the laminated carbon brush and the commutator when the fuel pump motor is operated with a DC voltage of 18 V.

(5) FIG. 5 is a photograph showing the sliding surface of a brush a after the test shown in FIG. 4.

(6) FIG. 6 is a photograph showing the sliding surface of a brush c after the test shown in FIG. 4.

(7) FIG. 7 is a photograph showing the sliding surface of a brush d after the test shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The best embodiment for carrying out the present invention is described in the following. The present invention should not be limited to the embodiment and should be construed based on the claims to include modifications of the embodiments with known matters in the art.

Embodiment

(9) Structure of Laminated Carbon Brush for Fuel Pump Motor

(10) FIG. 1 and FIG. 2 show a laminated carbon brush for a fuel pump motor (a carbon brush) 2 and a commutator 20 on which the carbon brush 2 slides. The carbon brush 2 includes a lower resistivity layer 4 that extends from the front end 5 to a central portion along the sliding direction and a higher resistivity layer 6 from the central portion to the rear end 9 along the sliding direction. A lead wire 8 is attached to the lower resistivity layer 4. The thickness ratio of the lower resistivity layer 4 and the higher resistivity layer 6 is preferably 10:1 to 2:1, for example, and the front end 5 along the sliding direction (the front end) and the rear end 9 along the sliding direction (the rear end) of the carbon brush are cut in conformity with the radial direction of the commutator 20.

(11) The commutator 20 is circular and disk-like. Its sliding surface in the front surface of the commutator 20 is made of carbon, for example, and the commutator is cut into a plurality of segments 22 along the circumferential direction. Gaps 24 are provided between the segments 22, and 26 denotes a center hole. The rotation direction of the commutator 20 is indicated by the white arrow in FIGS. 1 and 2. Relative to the carbon brush 2, the direction parallel to the rotation direction (the circular direction) is denoted as an x-direction, the direction extending along the sliding surface 23 of the commutator and perpendicular to the x-direction is denoted as a y direction, and the direction orthogonal to the sliding surface 23 of the commutator is denoted as a z direction. The y-direction and the z-direction are both non-circular directions. The lower resistivity layer 4 and the higher resistivity layer 6 of the carbon brush 2 respectively have resistivities along the x-direction approximately four times higher than the resistivities along the y-direction and the z-direction, and the resistivities along the y-direction and the z-direction are the same basically. In the higher resistivity layer 6, the resistivity along the x direction is four times or higher than the resistivities along the y-direction and the z-direction and, for example, five times or higher. 29 denotes the rear end along the sliding direction of the segments 22.

(12) In FIGS. 1 and 2, the higher resistivity layer 6 is just to remove from a segment 22a, and the lower resistivity layer 4 has come into contact with a segment 22b. Since the resistivity along the x direction is high, the resistance of the electrical current path from the segment 22a to the lower resistivity layer 4 via the higher resistivity layer 6 is high. Therefore, the electrical current flowing between the higher resistivity layer 6 and the segment 22a is reduced, and the occurrence of spark discharges is suppressed.

(13) By making the resistivity of the higher resistivity layer 6 higher than the resistivity of the lower resistivity layer 4, the electrical current becomes to gradually decrease when the carbon brush 2 moves away from the segments 22, and thus, spark discharges are prevented. Generally, it is preferable that the resistivity of the lower resistivity layer 4 for a fuel pump motor is lower than or equal to 10,000 .Math.cm, and is 2,000 .Math.cm in the embodiment. Note that the resistivity means that along the non-circular direction unless otherwise noted. However, in the prior art, the prevention of spark discharges by laminated carbon brushes has not been considered, it has not been clear what is the adequate range for the resistivity of the higher resistivity layer 6 in order to prevent the spark discharges. Therefore, the inventors of the present invention manufactured laminated carbon brushes having various resistivities in the higher resistivity layers 6 in the following manner, and observed how the spark discharges occur.

(14) Manufacturing of Carbon Brushes

(15) 92 mass % natural graphite having an average particle size of 30 m, and 8 mass % thermoplastic resin (PPS) powder having an average particle size of 10 m were mixed, the particle size distribution was adjusted, and thus a mixed powder for the lower resistivity layer having an average particle size of 100 m was prepared. For the higher resistivity layer, a mixed powder comprising 56 mass % natural graphite having an average particle size of 30 m, 8 mass % amorphous carbon having an average particle size of 70 m, and phenol resin solution was prepared. The amount of phenol resin added was 8 mass % of the total amount of the higher resistivity layer material. 72 mass % of the above-described mixed powder, 4 mass % thermoplastic resin (PPS) powder having an average particle size of 10 m, and 24 mass % talc powder having an average particle size of 10 m were mixed, the particle size distribution was adjusted, and thus a mixed powder for the higher resistivity layer having an average particle size of 150 m was obtained. Talc is an inorganic insulator, is stable in fuels such as gasoline and alcohol, has a low Mohs hardness, and is self-lubricating.

(16) The amount of talc content in the higher resistivity layer was changed in order to change the resistivity, and thus mixed powders for the higher resistivity layers having various resistivities were obtained. Also, the amount of amorphous carbon content, the particle sizes of graphite and amorphous carbon, and so on were changed in order to change the resistivity of the higher resistivity layer. Further, mixed powders for the higher resistivity layer containing, instead of talc, molybdenum disulfide, tungsten disulfide, boron nitride, molybdenum trioxide, and mica were obtained. Although the resistivity may be adjusted by another method not changing the talc content, talc having a low hardness reduced the amount of wears of the brush and the commutator.

(17) The carbon brush 2 was manufactured as shown in FIG. 3. A mold provided with a fixed mold 30 and a lower movable mold 31 was filled with a mixed powder 32 for the higher resistivity layer, then, a mixed powder 34 for the lower resistivity layer was filled above, and an upper movable mold 35 was lowered for compression molding. The direction of pressure was perpendicular to the interface between the mixed powder 32 for the higher resistivity layer and the mixed powder 34 for the lower resistivity layer. After compression molding, the molded pieces were heated to 300 C., which is higher than the melting point (280 C.) of PPS (the thermoplastic resin), and thus the material powders were combined by the PPS binder. Then, the molded pieces were shaped according to the shape of the commutator, the lead wire 8 was attached, and thus the carbon brush 2 was formed.

(18) Test

(19) The manufactured carbon brushes 2 were built into fuel pump motors and operated in a regular gasoline. To operate the fuel pump motors with high output powers and to make spark discharges easily occur, the fuel pump motors were operated for one hour by a DC power supply of 18 V, which was higher than the ordinary 12 V power supply. The resistivity (the non-circular directional resistivity) of the higher resistivity layer of the carbon brush 2 was changed within the range of 2000 .Math.cm (the resistivity of a single layer brush having only the lower resistivity layer) to 235,000 .Math.cm with changing the amount of talc content.

(20) Spark noises were measured by a current probe and an oscilloscope, and the spark energies were calculated based on the waveform of the spark noises. Further, the sliding surfaces of the carbon brush (the contact surfaces with the commutator) after the test were observed. The amounts of wears of the carbon brushes and the amounts of wears of the commutators were measured. Within the tested range, the motor outputs were approximately constant, regardless of the resistivity of the higher resistivity layer.

(21) FIG. 4 and TABLE 1 show the results. When the resistivity of the higher resistivity layer 6 was lower than or equal to approximately 70,000 .Math.cm, spark discharges occurred. However, when the resistivity of the higher resistivity layer 6 was higher than or equal to 100,000 .Math.cm, no spark discharges occurred. Namely, the occurrence of spark discharges changed critically, according to the resistivities of the higher resistivity layer 6, between 70,000 .Math.cm and 100,000 .Math.cm.

(22) The sliding surfaces of samples a, c, and d after the test are shown in FIGS. 5, 6, and 7. The sample a (FIG. 5) and the sample c (FIG. 6) had remarkably rough sliding surfaces at their rear ends. However, the sample d (FIG. 7) had a smooth sliding surface at the rear end, and wears by spark discharges were not observed. Furthermore, while spark discharges occurred on a sample g which had the high resistivity along the radial direction of the commutator by changing the press, no spark discharges occurred on the sample d having the same manufacturing conditions other than the press direction. This shows that spark discharges may be suppressed by increasing the resistivity along the circular direction.

(23) TABLE-US-00001 TABLE 1 Proportion of talc Resistivity of higher Resistivity of higher content in higher resistivity layer (in resistivity layer (in Spark energy Amount of wear Amount of wear of resistivity layer non-press direction) press direction) (A .Math. ms) of brush (mm) commutator (mm) Sample Single layer brush 2000 10000 45589 0.22 0.05 a 5 25000 128000 33756 0.12 0.03 b 9 70000 369000 21888 0.05 0.01 c 18 100000 534000 0 0 0 d 24 128000 700000 0 0 0 e 40 235000 1290000 0 0 0 f 18 100000 534000 10896 0.04 0.01 g *1 The unit of talc content is mass %. *2 The unit of resistivity of the higher resistivity layer is .Math. cm, and the resistivity along the non-press direction and the press direction is indicated. *3 The unit of spark energy is A .Math. ms. *4 The unit of the amount of wears of the brush and the commutator is mm. *5 The sample g is a comparative example, where the press direction was made the y direction (non-circular direction) increase the resistivity. In other points, it is the same as the sample d. *6 In the samples other than the sample g, the higher resistivity layer have resistivity along the circular direction approximately five times the resistivity along the non-circular direction. *7 In each sample, the lower resistivity layer have the resistivity along the non-circular direction of 2000 .Math. cm and the resistivity along the circular direction of 10000 .Math. cm.

(24) Note that when the press direction was the circular direction, no spark discharges occurred, if the resistivity of the higher resistivity layer was higher than or equal to 100,000 .Math.cm, when the resistivity was adjusted by the talc content, or by other self-lubricants such as molybdenum disulfide, or additionally by the particle sizes or the like of graphite or the like.

LIST OF REFERENCE NUMERALS

(25) 2 laminated carbon brush for a fuel pump motor (carbon brush) 4 lower resistivity layer 5 front end along the sliding direction 6 higher resistivity layer 8 lead wire 9, 29 rear end along the sliding direction 20 commutator 22 segment 23 sliding surface 24 gap 26 center hole 30 fixed mold 31, 35 movable mold 32 mixed powder for the higher resistivity layer 34 mixed powder for the lower resistivity layer