GB2279819A - Noise and vibration damped brush gear for an electric motor - Google Patents

Abstract

Brush gear assemblies 80 are mounted within a resilient insulating ring 44 which is secured to the motor end plate 40 be means of screws and recessed eyelets 46. The gap "b" isolates the assemblies electrically and mechanically from the end plate. Cover member 82 closes the open side of the brush channel and an extension 86a closes the contact end of the channel to retain the brush 82 and spring 84. Resilient rotor mounting is also disclosed. <IMAGE>

Description

NOISE AND VIBRATION DAMPENED ELECTRIC MOTOR SUCH AS FOR USE WITH A SOUND MOVIE CAMERA The field of the present invention relates to electric motors, particularly to small electric motors used in equipment such as synchronous sound motion picture cameras, in which it is desireable to minimize the noise and vibration produced by the motor. In the motion picture industry, the electric motor which drives the film advance mechanism is desirably small but requires a relatively high power output. In addition and particularly in a synchronous sound camera, it is highly desirable to minimize noise and vibration produced by the camera to avoid recording the noise on the sound track or transfer vibration to the filming mechanisms.

There have been many attempts to dampen noise and vibration in electric motors. In U.S. Patent No. 3,546,504, layers of elastic material having high inherent dampen properties are positioned (a) between the motor shaft and the bearings, (b) between the bearings and the bearing flange and (c) within the rotor shaft itself. An additional layer of elastic damping material is also placed between the circumference of the stator lamination packet and the supporting arm.

Similarly, Great Britain Patent No. 1,207,145 discloses an electric motor having a layer of resilient material interposed between the bearing seat and the outer ring of the ball bearing.

In the devices of both U.S. 3,546,504 and GB 1,207,145, a layer or sheet of resilient material is positioned between certain parts.

However these layers of resilient material must be specially manufactured or cut to size and the motors themselves must be substantially modified in order to accommodate these layers. In addition, the amount of damping is highly dependent upon the damping properties of the resilient material itself as well as the thickness chosen for the material.

In addition, the installation of the layers of resilient material would not be readily implemented upon an existing motor but would likely require design considerations at the manufacturing stage.

According to the present invention, there is provided an electric motor assembly, comprising: a motor case; a rotor shaft concentrically positioned along a centre axis of the motor case; a bearing means connected at either end of the rotor shaft for rotationally supporting the rotor shaft; a stator; a rotor; a front endcap connected to the motor case; and a brush mount assembly mounted to the front endcap, the brush mount assembly being comprised of a flat, ring-shaped disc of non-metallic, resilient material within which a brush assembly is mounted, the brush assembly having no metal-to-metal contact with the front endcap.

The invention will be further described by way of non-limitative example, with reference to the accompanying drawings, in which: Figure 1 is a sectional side elevation view of an electric motor according to the present invention; Figure 2 is a detailed view of a portion of Figure 1 showing details of the rear bearing support; Figure 3 is a cross-sectional view of Figure 1 taken along line 3-3; Figure 4 is a cross-sectional view of Figure 3 taken along line 4-4; and Figure 5 is a cross-sectional view of a bearing support configuration of an alternate embodiment.

The preferred embodiment will now be described with respect to the drawings. To simplify the description, any numeral identifying an element in one Figure will represent the same element in any other Figure.

Figure 1 illustrates a sectional side elevation view of an electric motor modified according to the present invention. Except for the modifications, the motor 10 is a motor, this particular motor being a DC servo model which produces 80 oz-in (0.565 Nm) at 1440 rpm available from Magnetic Technology Inc, Canoga Park, California, USA. The motor 10 is essentially comprised of an outer case 15, a rear endcap 20, a front endcap 40, a rotor shaft 50, magnetic field assembly 60, an armature assembly 70, and the brush assembly 80.

The magnetic field assembly 60 is attached to the endcap 20 by a plurality of screws 22, the endcap 20 being secured to the motor case 15. r The armature assembly 70 is comprised of an epoxy cup 72 in which the armature windings 74 are embedded. The armature assembly 70 is molded to the commutator 58, both of which are molded onto the shaft 50 and rotate therewith. The armature assembly 70 has a cylindrical shape positioned coaxially with the rotor shaft 50 and the magnetic field assembly 60. The armature assembly 70 is concentrically positioned between the motor case 15 and the magnetic field assembly 60.

The armature assembly 70 is provided with flat annular or ring shaped brass balancing plates 76 and 78. The front balancing plate 76 is positioned along an inner face of the epoxy cup 72 and the rear balancing plate 78 is molded to the rear lip of the epoxy cup 72. By selectively removing some of the metal at certain positions of the balancing plates 76 and 78, the armature assembly 70 may be precisely balanced. Such precise balance is desired when the 10 motor is adapted with the vibration isolation means described below.

Electrical contact is provided by the brush assembly 80 which includes brushes 82 biased by springs 84 against the commutator 58 attached to the shaft 50. The brush assembly 80 is mounted to the front endcap 40 by a brush mount assembly 44, the brush mount assembly 44 being preferably constructed from a nonmetallic resilient material such as neoprene to isolate the motor case 15 from the vibration of the brush assembly 80.

Details of the construction for the brush mount assembly 44 and brush assembly 80 are illustrated in Figs. 3 and 4. The brush mount assembly 44 is comprised of an annular or flat donut-shaped ring portion 45 constructed from a resilient material, such as neoprene, having high vibration dampening properties. A plurality of threaded eyelets 46 are embedded in the annular ring 45 and spaced thereabout. Mounting screws 41, passing through corresponding holes in the front cap 40, engage the threaded eyelets 46 and secure the annular ring 45 to the front cap 40. As best viewed in Fig. 4, the eyelet 46 is recessed within the annular ring 45 to maintain a spacing "b" of neoprene material for vibration isolation of the eyelet 46 and the front cap 40.

The brush mount 80 is comprised of a housing 81 which is embedded in the annular ring 45 in a recessed position to maintain a spacing "b" of neoprene material for vibration isolation of the housing 81 and the front cap 40. The housing 81 includes a radial channel or cavity for the brush (contact) 82. A spring 84 urges the brush 82 into contact with the copper contacts 59 of the commutator 58. A cap plate 86 covers the brush channel in the housing 81 to contain the brush 82 and spring 84 therein. The cap plate 86 also includes an end flap 86a which encloses the outer end of the brush channel. The cap plate 86 is secured to the brush housing 81 by screws 89.

A flexible wire 87 is attached at one end to the brush 82 and at the other end to the end flap 86a. The wire 87 retains the brush 82 at least partially within the brush channel so that the brushes 82 do not become separated from the brush assembly 80 when the components are disassembled, thereby facilitating assembly. The wire 87 also provides superior electrical connection between the brush 82 and the cap plate 86, The shaft 50 extends through the front endcap 40 and is connected to a coupling 56 for connection to, for example, the camera film advance mechanism. The coupling 56 is mounted to the shaft 50 by mounting screw 56a and rotationally secured by key 57.

A bearing cover 42 is provided between the front endcap 40 and the coupling 56 to inhibit the transmission of noise out through the front endcap 40. The bearing cover 42 is also preferably constructed from an elastic material such as neoprene. The bearing cover 42 provides further sound insulation.

An encoder cover 30 is attached to the endcap 20 to close off noise and vibration. A layer of insulation 32 is provided along inside surface of the encoder cover 30 to provide further sound insulation.

The rotor shaft 50 is supported by front and rear ball bearings 52a and 52b with spacers 54a and 54b positioned therebetween. A wave washer 55 is inserted between the spacers 54a and 54b. The ball bearing 52a is secured by a retaining ring 51 on the rear end of the rotor shaft 50. The ball bearings 52a and 52b and the spacers 54a and 54b are mounted to the magnetic field assembly 60 and support the rotor shaft 50.

The electric motor 10 is also modified by providing vibration isolation between (a) the magnetic field assembly 60 and (b) the bearings 52a, 52b and spacers 54a, 54b, the vibration isolation being comprised of a plurality of O-rings 26a, 26b, 26c, 26d, 26e, 26f, 26g, 26h therebetween. Each of the respective O-rings 26a to 26g is positioned in a respective channel 26a' to 26g' within the inner circumference of the magnetic field assembly.

Figure 2 is an enlarged view of a portion of Figure 1 illustrating construction details.

In Figure 2, the O-ring 26g is positioned within its corresponding channel 26g' along the inner circumference of the magnetic field assembly 60. The channels 26a' to 26fZ corresponding to O-rings 26a to 26f are of similar configuration. The O-ring 26h is wedged in a channel formed by shoulder 26h' in the magnetic field assembly 60 and shoulder 26h" in the housing of the ball bearing 52a which form opposed side walls of the channel. The O-rings 26a to 26h maintain a gap "G" both between the magnetic field assembly 60 and the bearings 52a & 52b and between the magnetic field assembly 60 and the spacers 54a & 54b.

Additional vibration isolation is accomplished by installing an O-ring 24 between the rear ball bearing 52a and the endcap 20. A circular channel 24' of generally rectangular crosssection is formed in the front face of the endcap 20. The channel of 24' extends past the bearing 52a maintaining a gap "a" between the endcap 20 and the bearing 52a. The end rings 26h and 24 cooperate with the rear end of bearing 52a to provide axial stability for the bearing assembly.

Unlike the electric motor configurations of the prior art such as in U.S. Patent No. 3,546,504, this arrangement does not use a layer or sheet of elastic material to provide for the noise and vibration insulation, but uses a plurality of o- rings to minimize the area of contact between rotating parts. In contrast to supplying a contact surface over the entire surface area of the insulation layer, an 0-ring establishes more of a point contact, minimizing the area of contact through which vibration and/or noise may be produced or propagated.

Alternately the channels 26a'-h' and 24' may be positioned within the bearings or spacer housings.

In contrast to using insulation sheet material which requires special molding or manufacture, the Rings of the preferred embodiment may be standard off the shelf items available from a variety of vendors.

The preferred 0-ring for the motor used in a motion picture camera is made from a nitril compound (such as Buna N), such a material having chemical resistance to fuel and oil which may be encountered in the motor environment. The material also resists breakdown from exposure to ozone, a gas produced by some motors. The material selection will depend upon the particular application as well as the anticipated motor service and environment. The O-ring size and O-ring channel size need to be selected for the particular size motor being modified. For the Magnetic Technology motor illustrated, the O-ring size selected is 2.19 cm (0.864") i.d. with a width of 0.18 cm (0.07") and a compression strength of 70 durometers. When assembled, the O-rings 26a to 26h are compressed approximately 0.018 cm (0.007"). The channels 26a to 26h' are approximately 0.19 cm (0.075") wide by 0.131 cm (0.0515") deep which would result in a gap "G" of about 0.029 cm (0.0115").

There is a trade-off with respect to the hardness of the O-ring material selected - the softer the material, the more damping, but if the material is too soft, misalignment may occur. Similarly, the number of O- rings needs to be enough to provide adequate support for the components, but the more O-rings, the more surface contact area for vibration/noise transfer.

A standard motor may be readily modified to the above described design by (1) forming a plurality of axially spaced channels (26a' to 26h') along the interior circumference of the magnetic field assembly 60, (2) installing a respective O-ring 26a to 26h within each channel 26a' to 26h' in concentric position between the bearing means (the shaft bearings 52a and 52b and spacers 54a and 54b) and the magnetic field assembly 60 to form a gap "G" therebetween, (3) forming a circular channel 24' of rectangular cross-section in the front face of the endcap 20, and (4) inserting an O-ring 24 in the circular channel 24' to form a gap "a" between the rear ball bearing 52a and the endcap 20.

An electric motor modified according to the preferred embodiment described above achieves a substantial reduction in operating noise level. Table A lists a group of motors modified according to the preferred embodiment of the invention comparing noise levels of each motor before and after modification according to the above method.

STABLE a Motor Ser. No. Oria. dBA Mod. dBA 1027A 37.5 28.5 1208 36.0 29.5 1229 36.0 25.5 1527 34.0 24.5 1603 36.5 23.5 The tests in Table A were conducted in an anechoic chamber with the motor mounted firmly to the mounting pad. The motor was run at 1440 RPM which corresponds to 24 frames per second operation for the motion picture camera. The noise level was measured by a microphone placed one meter from the motor. Test results indicate that the motors modified according to the present invention operated at reduced noise levels. The modified motors tested achieved noise level reductions of at least 6.5 dBA to as much as 13 dBA when compared to unmodified motor operation.

Fig. 5 illustrates an alternative embodiment for an electric motor 110 having a more conventional rotor and stator configuration. The motor 150 has a stator (not shown) which is of cylindrical shape and concentrically surrounds the rotor 150. The motor shaft 120 is mounted on a roller bearing 152 which in turn is mounted in the motor housing 140. Vibration isolation means are provided between the motor housing 140 and the bearing 152 comprised of rings 126a and 126b positioned in annular channels formed within the inner circumferential surface of the housing 140 adjacent to the bearing 152. The rear bearing (not shown) will also have an O-ring support structure of similar configuration.

Other features from the first embodiment may also be incorporated into the alternate motor 110. A bearing cover 142 may be provided between the housing 140 and the shaft 120 to inhibit the transmission of noise out the shaft 120. Though not illustrated, the alternate motor 110 may be modified with the brush assembly isolation means previously described.

Thus, a quiet camera and camera modification method have been shown and described. Though certain examples and advantages have been disclosed, further advantages and modifications may become obvious to one skilled in the art from the disclosures herein. The invention therefore is not to be limited except in the claims that follow.

This application describes matter described and claims in co-pending Application No 92 00695.6 from which this was divided.

Claims (7)

1. An electric motor assembly, comprising: a motor case; a rotor shaft concentrically positioned along a centre axis of the motor case; a bearing means connected at either end of the rotor shaft for rotationally supporting the rotor shaft; a stator; a rotor; a front endcap connected to the motor case; and a brush mount assembly mounted to the front endcap, the brush mount assembly being comprised of a flat, ring-shaped disc of non-metallic, resilient material within which a brush assembly is mounted, the brush assembly having no metal-to-metal contact with the front endcap.

2. An electric motor assembly as in claim 1, wherein the brush mount assembly includes a plurality of threaded eyelets embedded in the ring-shaped disc and arranged circumferentially at a given radial distance, the brush mount assembly being secured to the front endcap by screws engaging the threaded eyelets.

3. An electric motor assembly according to claim 2, wherein the eyelets are recessed into the resilient material to be spaced from the front endcap.

4. An electric motor assembly as in claim 1, 2 or 3, wherein the brush assembly comprises a plurality of brush units spaced about an inner perimeter of the ring shaped disc, each brush unit being comprised of (a) a brush contact disposed in a channel, (b) a spring disposed in the channel and biasing the brush contact radially inward, and (c) a wire disposed in the channel and attached at one end to the brush contact and at the other end to the brush unit.

5. An electric motor assembly according to claim 4, wherein said channel is closed by a cover extending over the length and one end of the channel.

6. An electric motor assembly according to any one of the preceding claims, wherein the brush assembly is recessed into the side of the resilient material opposite the front endcap to be spaced therefrom by said resilient material.

7. A synchronous sound movie camera including an electric motor assembly according to any one of the preceding claims.