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).
The process and considerations for drying and impregnating articles with varnish and compounds.
| Identifier | ExFiles\Box 24\2\ Scan289 | |
| Date | 21th February 1928 guessed | |
| -2- Contd. Some woods are much easier to penetrate than others, but as the wood is usually of much greater thickness than the covering of the wires a greater pressure is required to ensure penetration of the varnish. If the wood is to be dyed only, then care must be taken to select a dye that will penetrate without being filtered out of its carrier, or solvent. An important point that was not touched upon was the heating of the articles to be dried and impregnated. Neither perfect drying nor impregnation will ensue unless the articles are thoroughly heated first. The articles may be heated in the impregnator, but that usually takes a long time to ensure good results, and it is not good practice to waste the time of the impregnator when the preliminary heating can be carried out better in an oven that costs far less money, leaving the impregnator free for the work for which it is specially designed. Another important point that should be emphasized is the quality, as it may be called, of the vacuum to which the articles are subjected when being dried for impregnation. The highest vacuum possible, under ordinary commercial conditions, does not remove all the air from the impregnator. There is always a slight amount left which may be imprisoned by the varnish, in the article being impregnated, unless special precautions are taken. To eliminate as much risk as possible from that cause a vacuum of 29 in. to 29½ in. (with the barometer at 30 in.) should be produced in the impregnator when the articles are being dried, to reduce the liability of imprisoning any air in the windings. The important matter of ascertaining when the articles in the vacuum chamber are absolutely free from moisture is now solved. Previous to our placing a humidity gauge on the market, it was a matter of guesswork - sometimes called the attendant's judgement led him to subject the articles to vacuum for a long enough period, the results were good, otherwise not, and in many cases there was a great deal of trouble afterwards due to moisture being imprisoned by the varnish and damaging the insulation. We have now a humidity gauge which can be relied upon to show, under vacuum, when all of the moisture has been drawn off from the articles in the impregnator. It is then safe to flood them with the impregnating varnish, or compound, and apply pressure to cause saturation of the covering and penetration of the windings. With regard to drying after impregnation, if solid compound is used no drying is required, and the articles merely need to cool to become a solid mass. For varnish impregnation, the writer patented, many years ago, the introduction of oxygen in connection with certain varnishes | ||