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).
Article explaining the workings of a magneto for beginners.
| Identifier | ExFiles\Box 4\5\ 05-page040 | |
| Date | 9th November 1912 | |
| THE AUTOCAR. November 9th, 1912 DC 1190 925 How the Magneto Works. A Colloquial Explanation for Beginners. By Eric W. Walford. (Concluded from page 795.) " Now we come to the distributer, which determines which cylinder is to be supplied with the current each time the contact breaker operates. As stated previously, two cylinders fire per revolution, and during that period the contact breaker operates twice, once for each cylinder. Therefore, two revolutions of the armature must be completed before all four cylinders have been fired. As I said before, the current from the armature passes to the brass slip ring upon which presses a carbon brush (fig. 13). [Image of a carbon brush] Fig. 13.—Carbon brush, acting on the slip ring, and its holder detached from the magneto. This is a bigger affair than the carbon brush at the back of the contact breaker. You will see (fig. 14) that its holder projects out of the cover plate which encloses the armature, and this projecting part is black, being made of vulcanite, which is a non-conductor of electricity. It is also roughened so that one can grip it with the fingers and unscrew it. If you unscrew this black brush-holder and lift it out of its place, you will see the carbon brush, which is like a large pencil lead, projecting from the holder. On pressing the end of this you will find there is a light spring at the back, the object of which is to keep the carbon brush always rubbing up against the slip ring. The slip ring receives the current from the armature as stated earlier, and the current then passes up the carbon brush to the brass terminal at the top of the carbon brush holder; from here it passes by means of a conductor to the distributer. This conductor (fig. 15) is slightly different in detail in the three magnetos (Bosch, Simms, and Eisemann), but it is the same in principle in each case. It is made in two telescopic parts with a spring in between, and one end hooks on the brass terminal on the carbon brush holder while the other end projects into the distributer. By pushing the hooked end inwards it can be removed. " To get at the distributer the cover has to be removed. In the case of the Eisemann, the black diamond-shaped box is the cover (see fig. 16), and this is pulled away from the brown vulcanite base. It is held in place by two invisible spring catches, and consequently requires a somewhat strong pull. In the Simms, three screws have to be undone when the cover will come away. In the case of the Bosch, there are two spring clips, one on each side. These have to be pressed towards one another and the cover pulled off with the other hand. It will be seen that the construction of the three distributers is somewhat different, and I shall have to describe them separately. [Image of high tension conductors] Fig. 15.—High tension conductors of Bosch, Eisemann, and Simms magnetos. In the case of the Simms (fig. 17), it will be seen that there is a large gear wheel, and fixed to it is a diamond-shaped block or holder. Centrally in this is a carbon brush of, roughly, cruciform shape. This brush is made with a white centre. The gear wheel meshes with the gear wheel on the armature shaft, but it is twice the size of it. Consequently as the armature revolves one complete revolution the distributer goes through only half a revolution. For a complete revolution of the distributer the gear wheel of the armature has, therefore, to complete two rotations. As two rotations are necessary for the firing of all four cylinders, and inasmuch as the armature revolves at the same speed as the engine crankshaft, it will be clear that only one rotation of the distributer gear wheel is necessary. Thus during each revolution of the distributer all four cylinders are fed and then the first is ready to fire again." "Are all distributers like this?" "The distributers of four-cylinder magnetos are all the same in the sense that they run at half the speed of the armature. If you examine the interior of the distributer cover which we have removed from the Simms magneto, you will see that it has a brass centre surrounded by a ring with four brass plates or segments. The conductor (fig. 15) from the carbon brush presses against the brass centre, and by it the current from the armature is led to this brass disc. Now as the diameter brush rotates, it passes over this brass disc and the ring, and as it revolves [Image of a coupling end of a magneto] Fig. 14. The coupling end of the Eisemann magneto. [Image labels: CARBON BRUSH HOLDER, TERMINAL, SAFETY SPARK GAP, LUBRICATOR, COVER PLATE] | ||