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 report discussing the design and operation of a contact breaker with an auxiliary rocker for safeguarding against back-firing.
| Identifier | ExFiles\Box 35\5\ scan 029 | |
| Date | 17th April 1918 | |
| Contd. -6r Oy6/FMF17.4.18. Two rockers are employed on each contact breaker, merely as a guarantee of good firing and as a safeguard against, accident. I put it to them that it was doubling the risk of fail-ure through wear of the rubbing contact. They do not appear to have thought of this, but state that the gray fibre used did not wear appreciably. At the Packard Co. they told me that id did wear a few thousandths in the first hour or so of the engines life, vut that subsequently there was no wear. I believethat this is due to the roughness left on the cam by the grinding wheel. The cam is about 1-1/4" diameter, which, running at half engine speed gives a very low rubbing velocity compared with that which we had on the Bosch magneto, running at 6000 revs. One of the most ingenious points of the contact breaker is the auxiliary rocker, which is a safeguard against back-firing. The auxiliary contact opens immediately before the two main contacts, and therefore closes immediately befroe the two main contact. If the engine turns backwards the auxiliary contact will open immediately after the two main contact. When the main contacts open, therefore, in the reverse direction, the fall of voltage in the primary is gradual, due to the shortcircuit through the auxiliary contact. When the auxiliary contact breaks, the current has decreased sufficiently, due to the resistance in seri with the latter contact, so that no spark is produced. Mr. Kett-ering said considerable experimenting had been necessary before t correct resistance, etc. had been found, I give herewith a dia-gram of the arrangement as I understand it:- Resistances A and B are exposed and are of nickel wire because:- Contd. | ||