GB1561559A - Cut-off carbon brush - Google Patents

Description

(54) CUT-OFF CARBON BRUSH (71) We, ROBERT BOSCH GMBH, a German company of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement:- There is provided by the present invention a cut-off carbon brush which serves to protect commutators of electric motors from damage which can occur as a result of excessively worn carbon brushes.

The cutoff carbon brush is of the kind which includes a lifting device comprising a lifting pin which is made from insulating material and which is arranged in a recess in the brush body. A spring clamped in the recess moves out the lifting pin through an opening which is formed in the recess when the carbon brush is worn, wherein the lifting pin removes the worn carbon brush from the surface of the commutator located opposite the brush.

A cut-off carbon brush of this type is already known in which the lifting device includes a lifting pin made from an electrically insulating plastics material.

However, in the case of the types of plastics material normally used nowadays for this purpose, the lifting pin frequently cannot withstand the mechanical stresses, thermal stresses or the stresses caused by the arc produced by the magnetic energy stored in the eletric motor at the commencement of the cut-out operation. The lifting pin can thus break, shatter, carbonise or burn out in the arc, whereby the flow of current is not interrupted, so that the electric motor continues to run. Thus, the lifting device does not fulfil its purpose.

There is provided by the present invention a cut-out carbon brush for an electric motor having a lifting pin which is made from insulating material, which is arranged in a recess in the brush body and which, by means of a spring held in the recess, is moved out of the recess through an opening therein which is coated when the carbon brush is worn, whereby the lifting pin raises the worn carbon brush from the commutator surface located opposite thereto, wherein the pin is formed of an insulating material having ceramic properties.

In contrast to this, the cutoff carbon brush in accordance with the invention has the advantage that it reliably withstands the above-described stresses during the lifting operation, so that it is able to lift the carbon brush from the collector during any operating state of the motor when the carbon brush attains the admissible limit of wear. In this manner, any substantial mechanical damage to the collector caused by the worn carbon brush is reliably prevented.

An embodiment of the invention is illustrated in the accompanying drawings in which: Fig. 1 is a partially sectioned view of a cutoff carbon brush; Fig. 2 is a side elevation of the cut-off carbon brush of Fig. 1 and Fig. 3 shows the cut-off carbon brush after the admissible limit of wear has been attained.

Figures 1 and 2 show a cut-off carbon brush 1 of substantially parallelpiped configuration. The bottom end face 2, as viewed in the drawing, of the brush is of arcuate in cross section so as to seat against the surface of an oppositely located commutator 3 of an electric motor otherwise not shown in the drawing. A flexible lead 5 is soldered in a conventional manner to the top end face 4 remote from the cylindrical shape end face 2 of the cut-off carbon brush.

A recess 6 is formed in the interior of the brush body from the top end face 4 thereof. It is of substantially cylindrical construction and its axis extends in the direction of the longitudinal axis of the brush body. The bottom end of the recess 4 is provided with a truncated cone shaped member 7; the bottom 8 of the recess 6 being contiguous with the bottom edge of the member 7. As may readily be seen in Figures 1 and 2, the configuration of the bottom 8 is that of a full, upwardly directed cone.

A lifting pin 9 is loosely fitted in the recess 6 and its rounded bottom end rests on the tip of the conical bottom 8 of the recess.

The lifting pin 9 has a flared collar 10 the angle of the wall of which corresponds with the angle of the wall of the member 7. The bottom end of a compression spring 11 abuts against a shoulder formed by the rear of the collar 10 remote from the bottom 8.

The compression spring 11 is in the form of a helical spring and surrounds the slightly tapered stem of the lifting pin 9. The top end of the compression spring 11 abuts against a cover 13 pressed into an annular, widened portion 12 of the recess 6. The cover is in turn externally protected by an insert 14 of plastics material.

As may be seen in Figures 1 and 2, which show the cut-out carbon brush in its new, non-worn state, the compression spring 11 is in the first instance interposed in a compressed state between the cover 13 and the bottom 8 of the recess 6.

The cut-out carbon brush 1 is guided in a brush guide 15 which is only indicated by dash-dot lines in the drawing and which is of conventional construction.

Figure 3 shows the cut-out carbon brush 1 in a worn state. Owing to the wear on the carbon brush, the bottom end face 2' has been worn to the extent where it is in contact with points on the bottom end of the member 7 of the recess 6. The remaining portion of the frusto-conical bottom 8 has been broken away, whereby an opening 16 (Figure 3) for the lifting pin 9 has been created. The force of the compressed spring 11 now moves the lifting pin 9 out of the opening 16 to the extent where the conical collar 10 comes into abutment against the conical portion 7 of the recess 6. The bottom, conical end of the lifting pin 9 thereby comes into contact with the outer surface of the commutator 3 and lifts the worn carbon brush from the surface of the commutator 3. Since the lifting pin 9 is made from an insulating material, the electrical contact between the cut-out carbon brushes 1 and the surface of the collectors 3 is interrupted, whereby the electric motor stops.

The insulating material, from which the lifting pin 9 is manufactured, is a plastics material having so-called ceramic properties. A particularly important property of the insulating material is also its resistence to arcing. An unsaturated polyester resin has proved to be particularly suitable.

As a result of the ceramic properties of the electrically insulating material used, the lifting pin 9 withstands the high thermal stresses to which it is subjected by the high temperature of the cut-out carbon brush on the one hand and, on the other hand, by the friction occurring between the lifting pin and the surface of the commutator 3 at the instant at which they come into contact with one another. Owing to the fact that the material is an arc-resistant plastics material having ceramic properties, the lifting pin 9, during the lifting operation, also withstands the arc which is fed by the magnetic energy stored in the electric motor. Thus, its electrical and mechanical properties are neither impaired nor destroyed, so that, when the motor is in any operating state, the lifting pin is able to raise the cut-out carbon brush 1 from the commutator 3, and reliably interrupt the flow of current, when the cut-out carbon brush reaches the admissible limit of wear.

WHAT WE CLAIM IS: 1. A cut-out carbon brush for an electric motor, having a lifting pin which is made from insulating material, which is arranged in a recess in the brush body and which, by means of a spring held in the recess, is moved out of the recess through an opening therein which is created when the carbon brush is worn, whereby the lifting pin raises the worn carbon brush from the commutator surface located opposite thereto, wherein the pin is formed of an insulating material having ceramic properties.

2. A cut-out carbon brush as claimed in claim 1, wherein the insulating material is a plastics material having ceramic properties.

3. A cut-out carbon brush as claimed in claim 2, wherein the plastics material is one resistant to arcing.

4. A cut-out carbon brush as claimed in claim 2 or 3, wherein the plastics material is an unsaturated polyester resin.

5. A cut-out carbon brush substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. may readily be seen in Figures 1 and 2, the configuration of the bottom 8 is that of a full, upwardly directed cone. A lifting pin 9 is loosely fitted in the recess 6 and its rounded bottom end rests on the tip of the conical bottom 8 of the recess. The lifting pin 9 has a flared collar 10 the angle of the wall of which corresponds with the angle of the wall of the member 7. The bottom end of a compression spring 11 abuts against a shoulder formed by the rear of the collar 10 remote from the bottom 8. The compression spring 11 is in the form of a helical spring and surrounds the slightly tapered stem of the lifting pin 9. The top end of the compression spring 11 abuts against a cover 13 pressed into an annular, widened portion 12 of the recess 6. The cover is in turn externally protected by an insert 14 of plastics material. As may be seen in Figures 1 and 2, which show the cut-out carbon brush in its new, non-worn state, the compression spring 11 is in the first instance interposed in a compressed state between the cover 13 and the bottom 8 of the recess 6. The cut-out carbon brush 1 is guided in a brush guide 15 which is only indicated by dash-dot lines in the drawing and which is of conventional construction. Figure 3 shows the cut-out carbon brush 1 in a worn state. Owing to the wear on the carbon brush, the bottom end face 2' has been worn to the extent where it is in contact with points on the bottom end of the member 7 of the recess 6. The remaining portion of the frusto-conical bottom 8 has been broken away, whereby an opening 16 (Figure 3) for the lifting pin 9 has been created. The force of the compressed spring 11 now moves the lifting pin 9 out of the opening 16 to the extent where the conical collar 10 comes into abutment against the conical portion 7 of the recess 6. The bottom, conical end of the lifting pin 9 thereby comes into contact with the outer surface of the commutator 3 and lifts the worn carbon brush from the surface of the commutator 3. Since the lifting pin 9 is made from an insulating material, the electrical contact between the cut-out carbon brushes 1 and the surface of the collectors 3 is interrupted, whereby the electric motor stops. The insulating material, from which the lifting pin 9 is manufactured, is a plastics material having so-called ceramic properties. A particularly important property of the insulating material is also its resistence to arcing. An unsaturated polyester resin has proved to be particularly suitable. As a result of the ceramic properties of the electrically insulating material used, the lifting pin 9 withstands the high thermal stresses to which it is subjected by the high temperature of the cut-out carbon brush on the one hand and, on the other hand, by the friction occurring between the lifting pin and the surface of the commutator 3 at the instant at which they come into contact with one another. Owing to the fact that the material is an arc-resistant plastics material having ceramic properties, the lifting pin 9, during the lifting operation, also withstands the arc which is fed by the magnetic energy stored in the electric motor. Thus, its electrical and mechanical properties are neither impaired nor destroyed, so that, when the motor is in any operating state, the lifting pin is able to raise the cut-out carbon brush 1 from the commutator 3, and reliably interrupt the flow of current, when the cut-out carbon brush reaches the admissible limit of wear. WHAT WE CLAIM IS:

1. A cut-out carbon brush for an electric motor, having a lifting pin which is made from insulating material, which is arranged in a recess in the brush body and which, by means of a spring held in the recess, is moved out of the recess through an opening therein which is created when the carbon brush is worn, whereby the lifting pin raises the worn carbon brush from the commutator surface located opposite thereto, wherein the pin is formed of an insulating material having ceramic properties.

2. A cut-out carbon brush as claimed in claim 1, wherein the insulating material is a plastics material having ceramic properties.

3. A cut-out carbon brush as claimed in claim 2, wherein the plastics material is one resistant to arcing.

4. A cut-out carbon brush as claimed in claim 2 or 3, wherein the plastics material is an unsaturated polyester resin.

5. A cut-out carbon brush substantially as hereinbefore described with reference to the accompanying drawings.