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).
Recent developments in aircraft ignition systems and high-speed breaker mechanisms.
| Identifier | ExFiles\Box 37\1\ scan 166 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 RECENT AIRCRAFT-IGNITION DEVELOPMENTS 35 (8) Both bearings supported in insulated cages (9) Breaker assembly, or individual breakers, easily removable The first Splitdorf Model-VA magneto, as well as its predecessor, the Model-PA{Mr Paterson}, and the first experimental Scintilla Model-SB{Mr Bull/Mr Bannister} double-magneto, all gave trouble because of the short life of the breaker-mechanism. All three of these breakers were of the lever type with quick-lift cams, resulting in rapid contact-wear. It was apparent from the continued trouble with lever-type breakers on all makes of magnetos, when run at high speed, that any marked improvement in the life of the magneto would require a much better breaker-mechanism than any so far produced. The fact that practically all lever-type breakers use a bumper, or buffer spring, is sufficient evidence to show that these breakers are all operating above their safe speed. As a result of this situation, the Materiel Division undertook the development of a breaker that would overcome the difficulties encountered with the lever type. HIGH-SPEED BREAKER-MECHANISM The conventional lever-type breaker-mechanism of good design is satisfactory at sparking rates up to about 10,000 sparks per min. Above this speed, the inertia of the breaker lever is usually sufficient to prevent it from following the movement of the cam, and chattering occurs. Rather than overcome this chattering by increasing the spring tension on the lever, with the resulting increased contact and rubbing-block pressures, a solid bumper, or a buffer-spring, is placed behind the lever to limit its travel and to allow the spring to return the lever to the closed position in time for another spark. This chattering greatly increases the wear of the contacts by introducing high impact-loads as the contacts close, and by causing additional arcing at the contacts when the lever bounces several times before coming to rest. So long as the lever follows the cam, the closing velocity is governed by the contour of the cam and can be made much less than is the case when chattering occurs. A cylindrical pivot-bearing is usually provided for the contact-lever, either near the center or at one end. This bearing must have an insulating-bushing in the lever to prevent the pitting of the pivot-pin by electrical discharges from the lever to the pin. These are usually high-frequency static discharges that occur in spite of the fact that the lever is grounded through the spring. The lubrication of this bearing is particularly difficult, as the motion is oscillatory, and foreign particles do not work out of the bearing; consequently, any rust, moisture or wear tends to cause the lever to stick. Some means for lubricating the pivot-bearing and cam must always be provided. In practically all breaker-mechanisms, the contact separation is adjusted by mounting one of the contacts on a screw. After a short period of operation, the contacts form irregular surfaces that match perfectly so long as the moving contact returns to the same closed position. But, if the separation must be corrected, due to wear of the cam, rubbing-block, pivot-bearing, or the contacts themselves, the contact mounted on the screw must be rotated, and the matched surfaces no longer register accurately. Unless the surfaces are again care- FIG. 10—PIVOTLESS CONTACT-BREAKER DEVELOPED BY MATERIEL DIVISION OF THE AIR CORPS The Moving Contact Is Attached to the Spring That Returns the Contact to Closed Position, Thus Eliminating the Need for a Pivot-Bearing and a Breaker-Lever and Reducing to the Minimum the Moment of Inertia of the Moving Parts To Meet the Requirements of High-Speed Operation. Virtually the Entire Mass of the Moving Contact-Support Is Employed Usefully. A Lifter-Spring Holds the Rubbing-Block against the Cam and Relieves the Contact Element of Impact and Spring Loads. As Both Contacts Are Mounted Integrally on One Support in a Permanent Assembly, the Contact Separation Can Be Adjusted or the Breaker Synchronized by Moving the Whole Assembly in Relation to the Cam, without Disturbing the Contacts FIG. 9—DISASSEMBLED PARTS OF SCINTILLA MODEL-SC DOUBLE MAGNETO | ||