GB2151856A - Electric motor - Google Patents

Abstract

An electric motor, preferably a miniature electric motor for a magnetic tape player or recorder has a non-magnetic rotor shaft 1. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors and more particularly, though not exclusively, to their employment in magnetic tape recorders or players.

So-called miniature electric motors are widely employed in electrically driven devices of all kinds. For example, miniature electric motors are widely employed in magnetic tape recorders and players where they are used for causing the tape to pass the magnetic recording or playing head. In a typical such arrangement, the rotating shaft of the motor causes tape travel by direct contact with the tape, for example with the tape passing through the nip between the shaft and a co-operating capstan roller. Since, so far as the Applicants are aware, the shafts of miniature electric motors have in the past always been made of a ferromagnetic or paramagnetic material such as a hardened steel or stainless steel rod, which materials generally possess remanence, there is a tendency for the magnetism of the magnetic tape to magnetise the shaft.The present Applicants have appreciated that the effect of the remanence of the rotatable shaft is to tend to degrade the tape proper or the magnetic record thereon.

The Applicants have appreciated that this problem is essentially overcome if the conventional ferromagnetic or paramagnetic rotor shaft is substituted by a shaft made of a non-magnetic material. So far as Applicants are aware, this relatively simple substitution has never previously been suggested or used.

Accordingly, the present invention provides an electric motor having a rotor which is mounted on a rotor shaft made of a non-magnetic material for rotataion relative to a permanent magnet stator, and brush gear comprising brushes in sliding contact with a commutator fixedly fitted to the rotor shaft.

The invention also provides a magnetic tape player or recorder comprising a magnetic recording or playing head, and drive means for causing magnetic tape to pass the said head which drive means comprises an electric motor according to any preceding claim.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1A is a part-sectional side elevational view of an embodiment of electric motor constructed in accordance with the present invention; Figure 1B is an end elevational view of the motor of Figure 1A; and Figure 2 is a largely schematic view of an embodiment of magnetic tape recorder or player incorporating the motor of Figures 1A and 1 B.

The miniature electric motor of Figure 1A has a rotatable shaft 1 formed of a non-magnetic material preferably with the composition described in detail below. The stator comprises permanent magnets 2, and the rotor 3 is mounted on the shaft 1 which is supported in bearings 4 mounted respectively in the end wall of a generally cylindrical motor case 5 and in a motor case cover or end cap 6. Terminals 7 protrude through the end cap 6 as best shown in Figure 1 B. A commutator 8 is mounted on the rotor shaft. The motor is provided with brush gear comprising brushes 9 respectively integrally formed with a terminal 7 and in sliding contact with the commutator 8.

The shaft 1 is made of a non-magnetic material. We have found that suitable shafts can be made from high-manganese steel alloys with the composition shown in Table 1 below.

TABLE 1 Ingredient C Si Mn Cr Ni Wt.% 0.02-0.2 1.0-2.0 25.0-35.0 5.0-10.0 2.0-5.0 (Balance, apart from impurities: Fe) Using three motor shafts manufactured from a conventional stainless steel SUS420J2 (JIS Standard), which material is generally considered paramagnetic, and three different motor shafts manufactured from high manganese steel with compositions as set out in Table 1 above, the remanence was compared under different conditions and the results are set out in Table 2 below.

TABLE 2 High-Mn Rotating shaft material SUS420J2 Steel Remanence in an individual motor 1, 25, 3 0,1,0 Remanence when the individual motor was magnetized 200,190,210 0,1,0 Remanence after the motor was assembled into equipment 110, 40,165 0, 1,0 The results, expressed in Gauss show that within the limits of measurement error, the remanence of the high manganese steel is essentially zero.

Figure 2 shows a typical configuration of a magnetic tape recorder or tape player utilizing a motor as shown in Figures 1A and 1 B and having the high manganese steel shaft. The generally schematically illustrated magnetic recording system 10 employs a capstan roller 11 between which and the rotor shaft 1 the magnetic tape 12 is adapted to pass in operation of the recorder or player. The tape is thus driven to pass magnetic head 13 for recording or playback. We have found that with this arrangement, even though the motor shaft 1 is in direct contact with the surface of the magnetic tape, since the shaft 1 is non-magnetic it is immune to magnetic effects caused by the tape. As the shaft does not become magnetized, it cannot magnetically degrade the record on the tape or physically degrade the tape proper by magnetically lifting the surface layers of the tape. As a result, the use of an electric motor in accordance with the present invention in a magnetic tape recorder or tape player will serve as a significant contribution to continuing quality of playback from a pre-recorded magnetic tape and to the extended life of the tape itself.

Claims (7)

1. An electric motor having a rotor which is mounted on a rotor shaft made of a non-magnetic material for rotation relative to a permanent magnet stator, and brush gear comprising brushes in sliding contact with a commutator fixedly fitted to the rotor shaft.

2. A motor according to Claim 1, wherein the said non-magnetic material consists of a high manganese steel.

3. A motor according to Claim 2, wherein the said high manganese steel comprises an alloy with the following composition C : 0.02 - 0.2% Si : 1.0 - 2.0% Mn : 25.0 - 35.0% Cr ~ 5.0 - 10.0% Ni : 2.0 - 5.0% Fe : Balance (apart from impurities).

4. An electric motor substantially as hereinbefore described with reference to and as shown in Figures 1A and 1 B of the accompanying drawing.

5. A magnetic tape player or recorder comprising a magnetic recording or playing head, and drive means for causing magnetic tape to pass the said head which drive means comprises an electric motor according to any preceding claim.

6. A magnetic tape recorder or player according to Claim 5, wherein the said drive means comprises a capstan roller adapted to co-operate with the rotor shaft whereby magnetic tape operatively passes through the nip between the said capstan roller and the rotor shaft in driving contact therewith.

7. A magnetic tape recorder or player substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.