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).
Premature failure of Watford EW6 magneto contact breaker points due to an incorrect spring design.
| Identifier | ExFiles\Box 65\3\ scan0344 | |
| Date | 9th February 1929 | |
| X7791 To ROY.{Sir Henry Royce} from EFC. c.c. BY.{R.W. Bailey - Chief Engineer} EFC4/T9.2.29. WATFORD EW6 MAGNETOS. We hand you herewith a contact breaker from an EW6 magneto, the platinum points of which have burnt away prematurely after only 4000 miles. We also hand you an additional lever and platinum screw and shall be pleased if you will have these sent to the makers for reconditioning and handed back to us on return. With regard to the failure of EW6 contact points, we are of the opinion that this is largely due to the incorrect arrangement of the semi-circular lever spring. This spring as arranged on the magnetos is not in pure circular bending as it should be, a fact which can readily be demonstrated by releasing one end of the spring and attaching to that end a long thin wire whose direction is normal to the plane of the contact breaker, and applying to the other end of that wire a pure turning moment. The spring does not then take up the same shape as it is constrained to take up on the contact breaker. It is important that this contact breaker spring should be arranged in the right way because the given amount of couple on the lever is thereby obtained with the minimum maximum stress of the material. I do not think Messrs. North have properly appreciated this point - it is another instance of the non-application of the fundamental principle to a product which is otherwise good from a manufacturing point of view. The shape of the spring both in its set up position and absolutely free position should be absolutely circular; this will ensure that the action along the spring is a pure twisting moment, as distinct from a twisting moment plus a force. EFC. | ||