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).
Technical letter discussing the design and materials for brush holders, springs, and commutators, including two diagrams.
| Identifier | ExFiles\Box 180\M6\ img040 | |
| Date | 12th June 1928 guessed | |
| (2) (1) Naturally you use pigtail, but it is not clear where they are fastened to the brush holders. The spring pressure in the specimen appears to be quite inadequate, and from the dimensions of the spring one fears that it could not be adjusted to give sufficient spring pressure, the proportions as they stand being very different to that of the R-R specimen which I send. I suggest that the brush holder spring should be wider and thicker. I am not sure of the metal at present used, but suggest that plated steel or stainless steel be adopted. Salter's might make you some good springs from Brunton tempered strip. I send you our little brush holder which shews the pressure we find gives reliability in service. (2) The contacts of the connections and brush holders to the terminals appear to depend on the bakelite not shrinking. Our experience is that it will shrink in heat. Probably I refer to wood powder filled bakelite, and perhaps not to the type you are using, which one believes is made of pressed paper or cotton. I think that bolts would be more suitable than set screws. It should be realised that bakelite is not very rigid; in a dry hot atmosphere it becomes brittle. I quite agree that it has the defect of tracking. I should much prefer that your terminals and brush mountings should depend on micanite sheeting washers, square tubes, etc, a clamped square micanite tube being quite permanent and suitable for fixing or making contact. It provides longer insulating surfaces than tubes and washers of the older construction, which I agree are quite unsuitable for high voltage at present used. If a new carbon brush is the same length as the guide box, plus the distance to the commutator, (it should be slightly longer) then the spring does not rest upon it effectively - i.e. the centre of the spring is about .2" too near the commutator, and rather too short, (ours is slightly too short.) The flexibles seem better if they can lie parallel with the motor shaft; we have done this in our later work. When the commutator gets worn how do you move the brush holder nearer the commutator; is this not a requirement? The longer box and carbon is good; we have no more radial depth. Yours faithfully, F.H. Royce | ||