From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The optimal pressure and type of electrical brush for minimizing loss in starter motors.
| Identifier | ExFiles\Box 35\1\ scan 021 | |
| Date | 25th September 1919 | |
| Contd. -2- EFC3/T25.9.19. (at a given speed) For each grade of brush there is a pressure at which the total loss (electrical resistance loss plus mechanical friction loss) is least; above that pressure the loss is greater owing to increased mechanical friction; below that pressure the loss is greater due to the increase of electrical contact resistance. The more graphitic brush requires a rather larger pressure to ensure the contact resistance being kept down; on the other hand, owing to its lower coefficient of mechanical friction, the frictional loss may still be approximately the same as with the C.M. type of brush, the lower coefficient being due to the increased graphite contained. It appears as though the correct combination will be C.M.3 brush used with a pressure of 6 lbs/sq.". We have already put in hand experimentally, one dozen springs which will give approximately this pressure. At the present moment we have a considerable number of brushes of quality C.M. on order, and there is not much doubt about these being perfectly satisfactory with the higher brush pressure, as they are so nearly perfectly satisfactory at present with only 3 lbs/sq.". During our visit to Messrs. Smiths on Monday last, we spoke about the question of starter motors and starter motor brushes, and we were told that they had experienced some trouble in this direction which, however, they entirely got rid of by using a machine for bedding in the brushes. This machine consisted of an arrangement of a revolving copper cylinder of the same diameter as the commutator, upon which the brushes would be placed in special holders arranged to give a pressure of 10 times | ||