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).
Construction and drawbacks of existing mica sparking plugs and proposing an improved H-T plug design.
| Identifier | ExFiles\Box 58\2\ Scan021 | |
| Date | 7th December 1931 | |
| - 2 - A.J. Rowledge, Esq., (contd) 7th. December, 1931. sparking plug and mica plugs of other manufacture, I will ask you to be good enough to consider the following. In all existing mica sparking plugs the manu-facture starts with the center electrode and winds a number of layers of mica, known as the cigarette, around this electrode, and then they insert mica washers over the electrode, and then apply extreme pressure on the two ends to compress the washers closely together. The actual method of securing a gas tight seal is to compress a small cone of brass, where it comes through the coupling nut. This construction has perhaps three bad features. One, that it places a sudden strain on the mica cigarette over a relatively small area, and, in the constant heating and cooling of the plug, is very likely to cause insulation failure at that point. Secondly, there is no real close contact between the center electrode and the inner edges of the mica washer, which produces poor heat flow from the firing nose of the plug. Thirdly, in order to dissipate the heat away from the nose, it is necessary to weld on a tip to the center electrode of special alloy, to a shank or shaft, of carbon steel so as to conduct the heat away rapidly from the nose of the plug. In the construction of the H-T plug, we feel we have produced a decided improvement over other types of con-struction, for the mica cigarette is first built around a small copper tube, over which the mica washers are placed in the usual way. The whole assembly is then placed in a special fixture and, by a special expander the copper cooling jacket is expanded outwardly so that it forms a perfect contact with the washers throughout the entire length of the plug, and does not cause any undue strain at the coupling nut. Furthermore, copper has the quality of conducting heat ten times as rapidly as steel so that the copper materially helps in drawing the heat away from the nose of the plug, and with this construction, as engine compressions get higher and higher, it will be perfectly possible to use thicker and thicker copper without making the nose of the plug unduly short, which is so apt to cause fouling. If you will take an H-T sparking plug and remove the shell, I would like to point out the fact that, after the copper has been expanded, the mica washers are in tight contact with the copper. This should be compared with the ordinary mica plug in which it will be found that, if the terminal is removed, or the nose taken off, the washers can easily be dropped from the center electrode. This is a most important point in achieving perfect heat flow. Furthermore, it is not necessary to apply such terrific end pressure to secure gas tightness, and this immediately remedies the usual difficulty of washers cracking down near the nose of the plug. Furthermore, the H-T plug, having the perfect | ||