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).
Page from an engineering journal discussing the development and timing method for a pivotless breaker in high-speed engines.
| Identifier | ExFiles\Box 37\1\ scan 167 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 36 THE JOURNAL OF THE SOCIETY OF AUTOMOTIVE ENGINEERS Fig. 11—Method of “Timing” Contact Separation in Pivotless Breaker A Straight-Edge Is Laid across the End of the Cam against the Raised Shoulder c and the Rotor Is Turned until the 0-Deg. Mark on the Breaker-Housing Lines Up with the Straight-Edge, Indicating Full-Advance Position. Then the Two Clamping-Screws Are Loosened and the Contact Assembly Moved until the Contacts Separate. fully faced off, the points touch only on two of the highest points, which wear away rapidly, and this necessitates a second adjustment in a much shorter time than the first period. Experience shows that such breakers require frequent attention when operated on high-speed engines. In an attempt to eliminate the various troubles encountered in service with the lever-type breaker, the experimental engineering section of the Materiel Division undertook the development of a breaker to meet the following requirements: (1) Ability to operate continuously at speeds equivalent to 3000 r.p.m. on a 12-cylinder engine, at 18,000 sparks per min. (2) Minimum impact-loads on the rubbing-block and contacts (3) Elimination of the pivot-bearing and lubrication of the breaker-mechanism (4) Simple adjustment of the breaker without special tools and without disturbing the alignment or refacing the contacts (5) Elimination of flexible lead-wires from the primary winding to the breaker After the design and construction of a number of experimental breakers and several thousand hours of testing, the two types of pivotless breaker shown in Figs. 9 and 10 were developed. To meet the requirement of high-speed operation, it was necessary to reduce the moment of inertia of the moving parts to the minimum. Since a spring is required to return the contacts to the closed position, the obvious method of mounting the moving contact is to attach it to the spring itself and eliminate the necessity for a pivot-bearing and a breaker-lever at the same time. Thus, practically the entire mass of the moving contact-support is usefully employed. Some sort of rubbing-block is required to operate against the cam, and, to eliminate the impact and spring loads on the contacts, if this rubbing-block were carried with the moving contact-spring, the lifter-spring is introduced. This is given sufficient tension to cause the rubbing-block to follow the cam and relieve the contacts of any unnecessary spring-pressure. It has been found that the spring-tension of both the contact-spring and the lifter-spring can be varied over a wide range without materially affecting the operation of the breaker, as the natural periods of both springs are well above any required operating-speeds. The contact pressure in itself does not materially affect the contact wear, if the impact loads are small. One of the most important features of the design is the method of adjusting the contact separation without disturbing the relation of the contact surfaces. By mounting both contacts integrally on one support in a permanent assembly, the contact separation can be adjusted, or the breakers synchronized, by moving the whole assembly in relation to the cam without in any way disturbing the contacts. This makes it unnecessary for the contacts to be faced off or aligned after each adjustment, as they always fit perfectly even when badly pitted. Roughness itself is not detrimental so long as the contacts always meet in exactly the same way. The only tools required to adjust the contacts are a screw-driver and a feeler gage. But the primary object of adjusting the contact separation is not to obtain a given lift on the breaker, but to interrupt the primary circuit when the rotor is in a certain relation to the pole-pieces. If nothing in the breaker has worn, except the contacts, and all the parts have been properly made and assembled, a specified contact-separation will cause the circuit to be interrupted, with the rotor in the desired position. But, after a time in service, the wear on the rubbing-block and the cam may allow this position to shift, even though the contact separation is maintained at the specified amount. The effect of breaker wear is more pronounced with the slow-lift cams required for high-speed magnetos. To assure the correct timing of the break under all conditions, the breaker shown in Fig. 9 was designed to be “timed” rather than adjusted for a given lift. As shown by the sketch in Fig. 11, a scale or straight-edge is laid across the end of the cam against the raised shoulder c and the rotor is turned until the scale lines up with the 0-deg. mark on the breaker-housing, when in the full-advance position. This gives the proper location of the rotor with respect to the pole-pieces. It is then necessary only to loosen the two clamping-screws and to move the contact assembly until the contacts separate. The only tools required are a scale and a screw-driver. In the breaker shown in Fig. 9, the primary connection to the coil is made through a laminated brush that bears on a segment in the terminal block. This eliminates all flexible leads in the breaker and allows the entire breaker-assembly to be easily removed by taking off two nuts in the breaker housing. The pivotless type of breaker has shown its superiority over the lever type from the very first experimental model. Three conventional lever types of magneto were bench-tested at 3800 r.p.m. and ran continuously without adjustment for 15, 100 and 116 hr. respectively. In each case, the contacts were worn sufficiently to require either refacing or renewal. The first pivotless type ran for more than 500 hr. on the same set of contacts at 4200 r.p.m. and was still in good condition, the total wear on the points being about 0.008 in. A pair of pivotless breakers on a vertical double-magneto recently ran for 1015 hr. at 4200 r.p.m. without | ||