Electrical brushes with wear sensors

#This invention relates to electrical brushes of the type having a wear sensor in the form of an electrical conductor or lead within the brush body, normally insulated, but adapted to emit a detectable signal when the insulation is worn away upon wear of the brush to a predetermined point.

#As is well known, electrical brushes are merely devices for conducting current to or from a rotating part. Typically, the brush is stationary, and is held and guided by a fixed brush holder in which it slides freely. There may be several brushes side by side to form a single brush set. The rotating member may be the commutator of a direct current machine or the slip rings or the like of an alternating current machine, but in general brushes are usable in any instance where current is to be passed from one member to a relatively movable member. A typical brush material is carbon, to which circuit connections are made by means of short flexible cables. Brushes wear and must be replaced periodically, and it is known in the art to provide brush wear sensors, typically in the form of an electrical wire or the like incorporated in but insulated from the brush body, and so located as to activate a sensor or signal when the insulation is worn away when brush wear reaches a predetermined point.

#A typical previous approach has been to use a length of standard Teflon (trademark) coated wire, cut square at one end and sealed with an insulating cement to insulate the exposed wire tip. This wire is then cemented into a blind hole, typically slightly deeper than, and parallel to, the main conductors extending into the brush body. When the brush wears to the wire tip, and the wire tip sealant wears away, the circuit to a signal is completed. Difficulties have been encountered in anchoring such single wires because of the nature of the required high temperature insulating material, typically being polymers of fluoroethylene, having a smooth, generally inert surface that is difficult to bond in the brush body. Additionally, there has been the problem of insulating the cut end of the wire to insulate this cut end from the brush. Silicone sealants have been tried and have been successful to some extent, but silicone can be detrimental to brush performance. Additionally, there is the possibility of faulty application of the insulating sealant, which will result in a false signal. Additionally, creep of the original insulation adjacent its cut end can result in exposure of the conducting wire to the brush material.

#FIG. 1 is an elevational view of a typical carbon brush incorporating a brush wear sensor in accordance with the invention.

#FIG. 2 is a frontal view corresponding to FIG. 1, but partially in section.

#FIG. 3 is an enlarged fragmentary view corresponding to FIG. 2, but showing more detail.

#As shown, a carbon brush body 1 has a commutator contacting, wear surface 3 and an upper or outer surface 4. Silastomer sleeved flexible conductors 2 of conventional type lead from and are connected to the brush body, being located therein by, for example, tamped copper powder connections. The brush itself and its main conductors are entirely conventional. A blind bore 6 extends from the upper surface of the brush body to the point at which brush wear is to be sensed, typically slightly below the lower ends of the main conductors to ensure sensing of brush wear prior to exposure of the main conductor ends. Typically this clearance might be of the order of 0.062 inches. A brush wear sensor 5 comprises a length of wire insulated with Teflon (trademark) doubled back on itself intermediate its ends 8 and 9 to form a looped portion such that the end 12 of the doubled wire extends to the bottom of the blind bore 6. The entire length of doubled wire within the blind bore is secured therein by filling the bore with a cement 7. To enhance retention, the double wire preferably is twisted as illustrated diagrammatically in FIG. 4 such that the wires are twisted about each other at spaced points 10 which separate adjacent looping portions 11 from each other. The stripped free ends can be connected to a common terminal so as to provide a double signal path, or they can be separately connected to individual sensors or other devices (not shown in the drawing).

#From the foregoing, it will be readily apparent that when the brush wears to the bottom of the blind bore 6, the insulation at tip 12 of the wire sensor will begin to wear away, resulting in contact of the conducting wire with the commutator, and the generation of a signal or signals at terminal ends 8 and 9. Even if the brush is not replaced promptly thereafter, the signal will continue, and, since the sensor wires are firmly embedded in the brush body by the cement, the wires will not become loose so as to contact other moving electrically live surfaces.

#An exemplary wire material is No. 30 AWG Teflon-coated wire, the Teflon insulation preferably being etched, although standard unetched insulation is entirely workable. Any suitable cement, such as epoxy resin, can be used to anchor the wire sensor in the blind bore. As between etched and unetched wire insulation, the etched is preferred in that it provides a higher pullout resistance, and wire with etched Teflon insulation is much easier to handle. In pull tests using unetched wire, the results were 6 pounds average, 2.5 pounds minimum and 10 pounds maximum, relative to a probably acceptable level of 2 pounds pull resistance. Using etched wire, the corresponding results were 16 pounds average, 12 pounds minimum and 22 pounds maximum. In the twisted embodiment, an average of 6 twists per inch has been found to be entirely satisfactory.