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).
Description of the B.T.H. Type 'MM' Magneto.
| Identifier | ExFiles\Box 4\5\ 05-page133 | |
| Date | 24th January 1921 | |
| G.M. 855A (FOR T.D.R.) 1M. 10-12-20 (D.L. 2047) X.430. (Issued 24-1-21) The B.T.H. Co., Ltd. Coventry, England. X.3515 DESCRIPTION. X 430 of the B.T.H. TYPE "MM" MAGNETO. This type of magneto is made suitable for four and six cylinder engines and can be supplied for base or spigot mounting. In external appearance it is very similar to the type "G" magneto, as will be observed from Fig. 1 which is a photograph of the assembled machine. The essential difference from the type "G" magneto is clearly shewn in Fig. 2 and 3, from which it will be noted that in addition to the usual laminated pole-pieces, a non-magnetic sleeve provided with two laminated pole segments 180 degrees apart, is accurately fitted between the laminated poles secured to the magnet. The outer and inner surfaces of this sleeve are accurately machined so as to fit closely between the pole-pieces and at the same time give a minimum clearance between the sleeve and the armature. The timing lever and cam ring are fitted to an extension at one end of the sleeve, so that any adjustment of the timing lever position will result in a corresponding movement of the polar segments in the sleeve, which project more or less beyond the tips of the pole-pieces secured to the permanent magnet. Thus for all positions of the timing lever the small air gap between the trailing edge of the armature core and the leading edge of the laminated polar segments, remains constant and in consequence the spark intensity is uniform over the whole timing range. Magnetos have been constructed on somewhat similar lines in the past, but the B.T.H. company are the first to adapt this construction to a laminated pole machine. The sleeve member linked to the timing lever is in our design an aluminium or brass tube into which the laminated polar segments are cast. This gives a thoroughly sound mechanical construction and is from this point of view superior to designs hitherto developed, in which solid iron segments are attached to non-magnetic end caps. The electrical results secured with type MM magneto have shown it to be vastly superior to any post-war Bosch magneto, and we would substantiate this assertion by giving results recently obtained upon Contd. | ||