Method for manufacturing magnetic encoder

Abstract

Provided is a method for a magnetic encoder having a magnetic body comprised of a magnetic rubber molded article, comprising a mixing step of mixing and then kneading a nitrile rubber (A), a ferrite magnetic powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition; and a molding step of molding and vulcanizing the magnetic rubber composition in a mold to which a magnetic field is applied to provide the magnetic rubber molded article, wherein a content of the ferrite magnetic powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A); and a compressed density of the ferrite magnetic powder (B) is 3.5 g/cm.sup.3 or more. According to this method, a magnetic encoder having a magnetic body with high coercivity and residual magnetic flux density can be produced by vulcanizing a magnetic rubber composition having favorable moldability.

Claims

1. A method for manufacturing a magnetic encoder having a magnetic body comprised of a magnetic rubber molded article, comprising a mixing step of mixing and then kneading a nitrile rubber (A), a ferrite magnetic powder (B) and a vulcanizing agent (C) to provide a magnetic rubber composition; and a molding step of molding and vulcanizing the magnetic rubber composition in a mold to which a magnetic field is applied to provide the magnetic rubber molded article, wherein a content of the ferrite magnetic powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A); a compressed density of the ferrite magnetic powder (B) is 3.5 g/cm.sup.3 or more; and a minimum torque ML of the magnetic rubber composition is 3 to 8 kgf.Math.cm as determined in a vulcanization curve at 180 C.

2. The method for manufacturing a magnetic encoder as claimed in claim 1, wherein the ferrite magnetic powder (B) has a particle size distribution with a plurality of peaks.

3. The method for manufacturing a magnetic encoder as claimed in claim 2, wherein the ferrite magnetic powder (B) is an anisotropic magnetic powder.

4. The method for manufacturing a magnetic encoder as claimed in claim 1, wherein the ferrite magnetic powder (B) is an anisotropic magnetic powder.

5. The method for manufacturing a magnetic encoder as claimed in claim 1, wherein in the mixing step, the nitrile rubber (A), the ferrite magnetic powder (B) and the vulcanizing agent (C) are mixed and then kneaded at 60 to 130 C. for 10 to 60 min to provide the magnetic rubber composition.

6. The method for manufacturing a magnetic encoder as claimed in claim 1, wherein vulcanization is conducted at 140 to 250 C. for 1 to 30 min in the mold to which a magnetic field is applied.

7. The method for manufacturing a magnetic encoder as claimed in claim 1, wherein the magnetic encoder comprises a supporting member attachable to a rotator and an annular magnetic rubber molded article mounted on the supporting member, in which the magnetic rubber molded article is circumferentially alternately magnetized in N-pole and S-pole.

8. A magnetic encoder having a magnetic body comprised of a magnetic rubber molded article, wherein the magnetic rubber molded article is obtained by vulcanizing a magnetic rubber composition comprising a nitrile rubber (A) and a ferrite magnetic powder (B), a content of the ferrite magnetic powder (B) is 700 to 1500 parts by mass based on 100 parts by mass of the nitrile rubber (A), a compressed density of the ferrite magnetic powder (B) is 3.5 g/cm.sup.3 or more, an average particle diameter of the ferrite magnetic powder (B) is 0.5 to 2 m, the ferrite magnetic powder (B) has a particle size distribution with a plurality of peaks, and a minimum torque ML of the magnetic rubber composition is 3 to 8 kgf.Math.cm as determined in a vulcanization curve at 180 C.

9. The magnetic encoder as claimed in claim 8, wherein the ferrite magnetic powder (B) is an anisotropic magnetic powder.

10. The magnetic encoder as claimed in claim 8, comprising a supporting member attachable to a rotator and an annular magnetic rubber molded article mounted on the supporting member, in which the magnetic rubber molded article is circumferentially alternately magnetized in N-pole and S-pole.

11. A method for manufacturing the magnetic encoder as claimed in claim 8, comprising a mixing step of mixing and then kneading the nitrile rubber (A), the ferrite magnetic powder (B) and a vulcanizing agent (C) to provide the magnetic rubber composition; and a molding step of molding and vulcanizing the magnetic rubber composition to provide the magnetic rubber molded article.

12. The method for manufacturing the magnetic encoder as claimed in claim 11, wherein in the mixing step, the nitrile rubber (A), the ferrite magnetic powder (B) and the vulcanizing agent (C) are mixed and then kneaded at 60 to 130 C. for 10 to 60 min to provide the magnetic rubber composition.

13. The method for manufacturing the magnetic encoder as claimed in claim 11, wherein in the molding step, the vulcanization is conducted at 140 to 250 C. for 1 to 30 min.

14. The method for manufacturing the magnetic encoder as claimed in claim 11, wherein in the molding step, the magnetic rubber composition is molded and vulcanized in a mold to which a magnetic field is applied.

Description

EXAMPLES

Example 1

(1) [Preparation of an Unvulcanized Rubber Sheet]

(2) The starting materials below were kneaded using an open roll with a diameter of 8 inch while the composition was kept at 60 to 100 C. for 35 min, to prepare unvulcanized rubber sheets with a thickness of 1 mm, 1.5 mm and 2 mm. Nitrile rubber (unhydrogenated: NBR): 100 parts by mass Acrylonitrile content 34%, Moony viscosity (ML.sub.1+10, 100 C.) 45 Strontium ferrite magnetic powder A (for magnetic field orientation): 1100 parts by mass

(3) Average particle diameter: 1.2 m (a particle size distribution has a plurality of peaks.)

(4) Compressed density: 3.6 g/cm.sup.3

(5) Residual magnetic flux density of a compressed body: 196 mT

(6) Coercivity of a compressed body: 236 kA/m Plasticizer TOTM [tris(2-ethylhexyl) trimellitate]: 3 parts by mass Zinc oxide: 4 parts by mass Stearic acid: 3 parts by mass Anti-aging agent: [4,4-bis(,-dimethylbenzyl)diphenylamine]: 5 parts by mass Solid paraffin: 2 parts by mass Sulfur: 0.4 parts by mass Vulcanization accelerator MBTS (2,2-dibenzothiazolyl disulfide):2 parts by mass Vulcanization accelerator TETD (tetraethylthiuram disulfide): 1.5 parts by mass
[Vulcanization Properties]

(7) The unvulcanized rubber sheet obtained as a sample was measured for vulcanization properties using Curelastometer 7 from A&D Company, Limited in accordance with JIS K6300-2. A vulcanization curve was formed at a measurement temperature of 180 C. for 5 min, and from a graph in which a vertical axis is torque and a horizontal axis is time, a minimum torque ML (kgf.Math.cm), a maximum MH (kgf.Math.cm), a time to 10% torque of MH t10 (min) and a time to 90% torque of MH t90 (min) were determined.

(8) [Mechanical Properties]

(9) A tensile test was conducted in accordance with JIS K6251. The unvulcanized rubber sheet obtained was press-vulcanized at 170 C. for 10 min to give a vulcanized rubber sheet with a thickness of 1 mm. A tensile strength (MPa) and an elongation (%) were determined at 23 C. and a relative humidity of 50% and at a tension rate of 500 mm/min, using a Dumbbell No.3 type test piece prepared by cutting the vulcanized rubber sheet obtained. As a result, a tensile strength was 4.0 MPa and an elongation was 30%.

(10) [Hardness]

(11) Hardness was determined in accordance with JIS K6253. A test piece prepared by laminating three vulcanized rubber sheets with a thickness of 2 mm as prepared for the tensile test was measured for hardness at a temperature of 23 C. and a relative humidity of 50% using a type A durometer to read a peak value. As a result, an A-hardness was 90.

(12) [Magnetic Properties]

(13) From the unvulcanized rubber sheet obtained, a disk test piece with a diameter of 18 mm and a thickness of 6 mm was prepared and then press-vulcanized at 170 C. for 10 min under a magnetic field in a direction of test-piece thickness, to prepare a vulcanized rubber test piece. The molded article obtained was measured for a residual magnetic flux density and a coercivity using a direct-current magnetization property testing device BH curve tracer from METRON Inc. As a result, a residual magnetic flux density was 300 mT and a coercivity was 270 kA/m.

(14) [Adhesiveness to a Supporting Member]

(15) An SUS430 annular supporting member (slinger) with a plate thickness of 0.6 mm and an L-shaped cross-section was used. The supporting member had a dimension; an inner diameter of an inner cylinder: 55 mm, an outer diameter of an outer circular-ring: 67 mm, and an axial length of the inner cylinder: 4.0 mm. Separately, an unvulcanized rubber sheet obtained with a thickness of 1.5 mm was cut into a toroidal sheet with an inner diameter of 56 mm and an outer diameter of 67 mm, which was then placed on the supporting member precoated with an adhesive made of a phenol resin. Subsequently, it was press-vulcanized at 180 C. for 3 min, to form a magnetic body with an inner diameter of 56 mm, an outer diameter of 67 mm and a thickness of 1.0 mm. The magnetic body was firmly bonded to the supporting member and adhesiveness was good. The above results are summarized in Table 1.

Example 2

(16) An unvulcanized rubber sheet was produced as described in Example 1, substituting a strontium ferrite magnetic powder B for a strontium ferrite magnetic powder A. The properties of the strontium ferrite magnetic powder B are as follows. Using the unvulcanized rubber sheet obtained, vulcanization properties, magnetic properties and adhesiveness to a supporting member were measured as described in Example 1. The results are summarized in Table 1.

(17) Average particle diameter: 1.14 m (a particle size distribution has one peak.)

(18) Compressed density: 3.5 g/cm.sup.3

(19) Residual magnetic flux density of a compressed body: 185 mT

(20) Coercivity of a compressed body: 273 kA/m

Example 3

(21) An unvulcanized rubber sheet was produced as described in Example 1, except that a hydrogenated nitrile rubber (HNBR) was substituted for a nitrile rubber (NBR), and the added amount of stearic acid was 2 parts by mass and the added amount of sulfur was 0.5 parts by mass. The properties of the hydrogenated nitrile rubber used herein are as described below. Using the unvulcanized rubber sheet obtained, vulcanization properties, magnetic properties and adhesiveness to a supporting member were measured as described in Example 1. The results are summarized in Table 1.

(22) Acrylonitrile content: 36%

(23) Moony viscosity (ML.sub.1+10, 100 C.): 57

(24) iodine value: 28 g/100 g

Comparative Example 1

(25) An unvulcanized rubber sheet was produced as described in Example 1, substituting a strontium ferrite magnetic powder C (for magnetic field orientation) for a strontium ferrite magnetic powder A. The properties of the strontium ferrite magnetic powder C are as described below. Using the unvulcanized rubber sheet obtained, vulcanization properties, magnetic properties and adhesiveness to a supporting member were measured as described in Example 1. The results are summarized in Table 1.

(26) Average particle diameter: 1.4 m (a particle size distribution has one peak.)

(27) Compressed density: 3.4 g/cm.sup.3

(28) Residual magnetic flux density of a compressed body: 185 mT

(29) Coercivity of a compressed body: 207 kA/m

Comparative Example 2

(30) An unvulcanized rubber sheet was produced as described in Example 1, except that a strontium ferrite magnetic powder D (for mechanical orientation) was substituted for a strontium ferrite magnetic powder A, and vulcanization was conducted without applying a magnetic field. The properties of the strontium ferrite magnetic powder D are as described below. Using the unvulcanized rubber sheet obtained, vulcanization properties, magnetic properties and adhesiveness to a supporting member were measured as described in Example 1. The results are summarized in Table 1.

(31) Average particle diameter: 1.1 m (a particle size distribution has one peak.)

(32) Compressed density: 3.2 g/cm.sup.3

(33) Residual magnetic flux density of a compressed body: 193 mT

(34) Coercivity of a compressed body: 235 kA/m

(35) TABLE-US-00001 TABLE 1 Comparative Comparative Example Example Example 1 Example 2 Example 3 1 2 Composition NBR 100 100 100 100 HNBR 100 Ferrite A (3.6 g/cm.sup.3) 1100 1100 Ferrite B (3.5 g/cm.sup.3) 1100 Ferrite C (3.4 g/cm.sup.3) 1100 Ferrite D (3.2 g/cm.sup.3) 1100 Plasticizer TOTM 3 3 3 3 3 Zinc oxide 4 4 4 4 4 Stearic acid 3 3 2 3 3 Anti-agent agent 5 5 5 5 5 Solid paraffin 2 2 2 2 2 Sulfur 0.4 0.4 0.5 0.4 0.4 Vulcanization accelerator 2 2 2 2 2 MBTS Vulcanization accelerator 1.5 1.5 2 1.5 1.5 TETD Vulcanization T10 [min] 1.13 1.25 1.40 0.88 0.90 curve T90 [min] 2.52 3.01 2.91 2.40 3.29 ML [kgf .Math. cm] 5.28 8.02 6.51 9.13 10.40 MH [kgf .Math. cm] 37.89 60.10 22.32 47.53 81.20 Residual magnetic flux density [mT] 300 290 298 292 260 Coercivity [kA/m] 270 273 260 229 280 Adhesiveness to a supporting member Good Good Good Good Good

(36) As seen in Table 1, in Examples 1 to 3 where a magnetic powder with a compressed density of 3.5 g/cm.sup.3 or more was vulcanized under a magnetic field, a magnetic body with a high residual magnetic flux density, and a high coercivity was obtained. Furthermore, in Examples 1 to 3, an ML value in a vulcanization curve was small and fluidity during molding was favorable. In particular, it can be seen that in Example 1 where a particle size distribution of the magnetic powder had a plurality of peaks, an ML value is particularly low and fluidity is significantly improved. In contrast, in the magnetic body of Comparative Example 1 where a magnetic powder with a compressed density of less than 3.5 g/cm.sup.3 was vulcanized under a magnetic field, a coercivity was reduced by shear force during kneading. In the magnetic body of Comparative Example 2 with mechanical orientation without applying a magnetic field during press vulcanization, the magnetic powder was insufficiently orientated, and thus a residual magnetic flux density was reduced.