US2948795A

US2948795A - Commutator brush assembly

Info

Publication number: US2948795A
Application number: US749208A
Authority: US
Inventors: John F Brinster; Everett B Garretson
Original assignee: GENERAL DEVICES Inc
Current assignee: GENERAL DEVICES Inc
Priority date: 1958-07-17
Filing date: 1958-07-17
Publication date: 1960-08-09
Anticipated expiration: 1977-08-09

Description

Aug. 9, 1960 Filed July 17, 1958 J. F. BRINSTER EIAL COMMUTATOR ssusu ASSEMBLY 2 Sheets-Sheet 1 IN VENTORS.

JOHN F. BRINSTER EVERETT B. GARRETSON AGENT Aug. 9, 1960 J. F. BRINSTER ETAL 2,948,795

COMMUTATOR BRUSH ASSEMBLY Filed July 17, 1958 l 2 Sheets-Sheet 2 INVENTORS.

JOHN F. BRINSTER EVERETT B. GARRETSON PM KM AGENT United States PatentOfificc COMMUTATOR BRUSH ASSEMBLY John F. Brinster and Everett B. Garretson, Princeton, NJ., assignors to General Devices, 'Inc., Princeton,

NJ., a corporation of New Jersey Filed July 17, 1958, Ser. No. 749,208

7 Claims. (Cl. 200-166) This invention relates to high speed commutators or switching devices for electrical signals such as may be used, for example, in time-division multiplex systems of signal transmission, and it refers more particularly to an improved assembly of brush and brush holder for such devices.

The art of high speed time-division multiplexing, or signal sampling, whereby a plurality of signal channels are sequentially assigned the use of a common trans mission path, such as a wire or radio circuit, makes severe demands on the commutator which connects the sources of the channel signals, in turn, to the common path. A prime consideration is that there shall be no significant distortion of the transmitted signals as a result of the commutation process. This distinguishes commutators for such service, where the informationbearing signals are at a low level, from commutators for. electric generator and motor service where high current carrying capacity is the principal design consideration and minor distortions of the commutated voltage wave can be tolerated.

In a high speed mechanical commutator which comprises a plurality of spaced contacts over which a resiliently biased brush sweeps, a common source of signal or wave distortion is a variation in contact pressure as the brush traverses a contact, with a resultant variation in brush contact resistance. This, in turn, produces a modulation of the transmitted signals which may affect the intelligence carried by them or completely destroy their utility. In some cases the brush actually leaves the surface of a contact, momentarily, while traversing the contact, thus breaking the channel circuit. Among other causes, variable pressure and contact bounce apparently are related to incipient vibrations of a brush which may be set up normal to the contact surface in the oscillatory system comprising the brush and the resilient biasing means. The present invention, according to one feature thereof, provides a brush assembly for a mechanical commutator of the above type which includes improved means for damping or preventing such vibrations, thereby significantly improving signal transmission.

Accordingly, it is one object of the invention to provide improved commutator means for the switching of low-level information-bearing signals.

Another object is to provide a brush assembly for a speed mechanical commutator which reduces variations in brush contact resistance.

Another object is to provide a brush assembly of the above type which has improved means of simple construction for damping or preventing brush vibrations.

These and other ojects and advantages of the invention will be more clearly apparent by reference to the following specification and to the accompanying drawings in which:

Fig. 1 is a fractional view, to a greatly enlarged scale, of a commutator assembly showing a contact thereof engaged by a cooperative brush assembly constructed in accordance. with the principles of the invention;

Fig. 2 is a fractional plan view of the brush-carrying rotor of the commutator assembly of Fig. 1;

Fig. 3 is an end view of the brush forming a part of the brush assembly of Fig. 1;

Fig. 4 is a side view, partly in section, of the brush of Fig. 3;

Fig. 5 is a plan view of the stator of the commutator assembly of Fig. 1; and

Fig. 6 is a plan view of the rotor of said commutator assembly.

The form of the brush assembly of the invention illustrated and described herein is one that finds utility, more particularly, in a commutator of the flat disc type, but is not limited to such use. In a common construction for a high speed disc type commutator a plurality of contacts are arranged in spaced circular manner on the face of a disc of insulating material, usually the stator disc, and are'sw'ept over by at least one brush mounted on an adjacent rotor disc, or its equivalent.- In addition, a second brush carried by the rotor engages a collector ring on the stator, the two brushes being in electrical connection. Springs individual to the brushes maintain a predetermined contact pressure and allow limited displacement of the brushes to accommodate slight variations in spacing of the brush holders and engaged contacts during relative displacement thereof. The stator contacts and collector ring usually are made of a precious metal, for example of silver alloy, or have a facing of such a metal and the contact material of the brush is chosen to have a low electrical resistance and a low coeflicient of friction against the surface of such a contact. For multiplexing or signal sampling service such commutators operate at high switching rates, a speed of 2000 contacts swept over by a brush in a second not being unusual. Precise positioning of the brushes and their dynamic stability, therefore, assume an importance not present in low speed switching operations if distortion of the signal samples, whose duration is measured in microseconds, is to be avoided.

Referring, now, to the figures and particularly to Fig. 1, there are shown as components of a commutator assembly a stator disc 11 and a rotor disc 13, both of insulating material. Stator disc 11 mounts a ring of spaced contacts 15 having shanks pressed or molded into the disc and having the heads thereof elevated above the surface of the disc, better seen in their assembled relationship in Fig. 5. Disc 11 also carries collector ring 17. Rotor disc 13 mounts brush assemblies 19 and 19', the latter seen in Fig. 6, comprising brushes engaging contacts 15 and collector ring 17, respectively. Brush assembly 19 comprises an outer shell constituting a brush retainer or brush holder 21, preferably of a silver alloy or a silver plated metal, which has a tubular body portion fitting tightly in a hole in disc 13 and an arcuate end flange held to disc 13 by screws 23. This construction permits the holes in disc 13 in which the brush assemblies are inserted to be jig-drilled in positions which accurately define the positions of the brushes themselves. The arcuate shape of the end flange facilitates close spacing of multiple brushes, where used.

Brush 25 comprises a sleeve portion 27 of ground finished aluminum tubing and a brush head 29 of high conductivity amorphous carbonaceous material such as a metallized graphite preferably a silver-bearing graphite. The brush head has a cylindrical end portion 30 which is pressed into sleeve 27 and which is of reduced diameter in comparison with an adjacent cylindrical portion 31. The latter portion forms a narrow band of substantially less axial extent than sleeve 27, possibly one tenth to one third the length of the sleeve, with a diameter that preferably is of the order of but is not Patented Aug. 9, 1960 one or more thousandths of an inch greater. The next brush head portion 33 may be square as shown in Figs. 3 and 4 or rectangular or. even round. The endportion 35 of brush head. 29 is of square (or rectangular) cross section, this sectional form being best seen irrFig. 3. The form and extent of the brush contact are a are in part determined by the timing and timing tolerances required. Since the spaced contacts are raised above the surface of disc 11, as shown, the brush must always produce shorting action between adjacent contacts. Non-shorting electrical operation is achieved by providing an electrically isolated contact between each pair of live contacts. By way of example of a brush for non-shorting service the active brush contact surface may be a square 0.040 inch on a side for engagement with contacts inch in diameter. This dimension will vary with the duty cycle of the switching operation. The brush heads are precisely machined with particular reference of the relation of the rectilinear edge of the brush contact surface to the longitudinal axis of the brush, to the end that precise phasing of the edges of the commutated waves may be attained. The phasing of the leading edges of the waves, as determined by the position of the leading brush contact edge, usually is of major importance.

Brush 25, comprising head 29 and sleeve 27, has an easy sliding fit, without appreciable play, in the bore of brush holder 21, where it engages the holder, and is urged away from the holder or retainer toward engagement with a contact 15 by compression spring 37. A limit to the displacement of brush in one direction is defined by an inwardly projecting end portion of a retainer ring 39 seated in a groove in holder 21 which is adapted to engage, as a stop, the radially extending surface or step between

brush head portions

31 and 33. Spring 37 has as a stationary abutment a dowel pin 41' fast in holder 19 along a diameter of the bore thereof to accommodate which a slot 43, is provided in sleeve 27 (Fig. 4). Pin 41 fits closely in slot 43 and thereby prevents twisting of the brush which otherwise would skew the leading edge of the contact surface thereof, with a resultant change in phasing of the commutated wave. Brush assembly 19', cooperative with collector ring 17, may be similar in construction to assembly 19.

'Brush 25 is held to a low mass by employing a minimum amount of material, for the function performed, in head 29 as well as in sleeve 27. This sleeve, in a size compatible with the dimensions of head 29 given above, may have an outside diameter of A; inch and a wall thickness of 0.010 inch. An appreciable axial extent is necessary for the smooth displacement of the brush, without chatter, in the bore of brush holder 21. Spring 37 has considerable stiffness to produce a satisfactory brush pressure, which may be of the order of 80 to 100 grams. The natural frequency of axial vibrations of the brush is thus raised to a relatively high value, generally above 2000 cycles per second, which facilitates damping. The slight degree of damping then required to inhibit vibration is introduced principally by the rubbing of the narrow band cylindrical portion Ell of the brush head against the inner surface of the bore of brush holder 21. An axial length of only 0.040 for the rubbing surface of brush head portion 31 has been found to be effective with a brush of the given dimensions, under certain conditions. The same type of material, a metallized graphite, which provides low electrical brush resistance has been found to provide a suitable coefficient of friction against the metallic brush retainer for damping purposes, allowing one member of simple construction to have these two functions.

As a result of investigation it is believed that axial brush vibrations result from a repeated sticking and release of the, brush relative to the contact over which it ispassing. This stick-slip manner of inducing vibrations 4 is known in other manifestations, for example in the bowing of stringed musical instruments. In high speed commutator practice it usually appears to a noticeable extent only after an initial period of operation which results in the deposition of some of the brush material on the contacts. The sticking possibly therefore may be attributed to seizure of two like surfaces. Again, in the presently disclosed brush assembly construction a portion of the same brush head member which deposits material on the contacts serves as an effective means for damping or preventing vibrations resulting from such deposition.

The description herein of a preferred form of the invention is not to be construed as limiting the invention to the described form, equivalent means being Within the scope of the invention.

What is claimed is: V

1. In a commutator brush assembly for high speed switching of low level information signals the combine tion of a metallic cylindrical body brush retainer having an axial bore, a brush partially mounted within said bore for limiteddisplacement axially thereof comprising a brush head of high conductivity metallized graphite material having an end portion defining a brush" contact surface for engagement with opposed commutator contacts and a portion spaced therefrom of cylindrical section in close sliding frictional engagement with the inner surface of said bore together with a metallic tubu=- lar extension of said head coaxial with said bore of substantially greater axial extent than said cylindrical head portion and having a longitudinal slot therein, the outside diameter of said extension being of the order of but not greater than the diameter of said cylindrical head portion, a pin monted on said brush retainer and diametrically disposed within said bore for engagement with the slotted portion of said tubular extension'to prevent rotation of the brush within said bore; and compression spring means having an abutmenton said' pin and engaging at the free end thereof said brush head to urge the brush toward the exterior of the bruslrre tainer.

2. The combination defined in claim 1 wherein the end brush contact surface of said brush head is bounded by at least one rectilinear edge, to provide the operational leading edge of said surface. 1 i

3. In a commutator brush assembly for high speed switching of low level information signals the combina? tion of a brush retainer having an end-flanged cylindrical; body portion and a bore coaxial therewith, a pin fast in said retainer and extending. along a diameter of said bore, a brush at least partially positioned within said bore comprising a tubular member longitudinally slotted adjacent one end thereof to receive said pin and a. brush. head of metallized graphite material fixedly mounted by said tubular member at the other end thereof and extending axially beyond the member, said brush head having a cylindrical portion adjacent to and coaxial with" said tubular member of at least as great adiameter; as the outside dimeter of said member, said tubular member and said cylindrical brush head portion both having surfaces with a close sliding fit within said here, the" proportions of said surfaces being selected to optimize. damping, said brush head further having. an end portion remote from said tubular member of rectangular section and of reduced area relative to said cylindrical portion thereof providing the brush contact surface; and compression spring means acting between saidpin'and' said brush to urge the brush axially away from said brush retainer. Y L

4. The combination defined in claim 3 wherein; said" compression spring means is positioned within said tubular member.

5. In a commutator brush assembly the combination- Of a metallic brush retainer having a cylindrical bore, a brush at least partially positioned within said: bore hav 5 ing a cylindrical body portion and an axial extension thereof of rectangular section and of reduced area relative to said body portion, said extension having an end brush contact surface of silver graphite material and said cylindrical body portion having a band thereof of relatively small axial extent comprising like material, said band being in close rubbing contact with the inner surface of said bore, means preventing rotation of said brush within said bore, resilient means tending to displace said brush axially of said bore, and stop means limiting such displacement.

6. The combination defined in claim 5 wherein said end brush contact surface is square in form.

7. In a commutator brush assembly for high speed operation the combination of a metallic brush retainer having a cylindrical bore, a brush having an elongated cylindrical portion thereof mounted Within said bore for sliding contact with the surface of the bore and an externally extending end portion thereof of reduced sectional area having at least one axially extending plane surface defining a rectilinear edge of the external end surface of said end portion, said edge being designed to serve as the operational leading edge of the brush, axially acting spring means urging said brush away from said retainer, and means preventing rotation of the brush within said retainer, said cylindrical brush portion comprising a relatively narrow band of metal-bearing graphite material to provide a predetermined coefiicient of friction for rubbing against the surface of said bore, and said end portion being of like material to provide a brush contact surface of high electrical conductivity.

References Cited in the file of this patent UNITED STATES PATENTS 728,273 Mix May 19, 1903 1,821,405 Riese Sept. 1, 1931 2,254,277 Ehlers Sept. 2, 1941 2,813,179 Rugg Nov. 12, 1957 FOREIGN PATENTS 805,904 France Sept. 7, 1936