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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).
Diagrams and explanation of an oil filter's function and the properties of filtered oil.

Identifier  ExFiles\Box 135\3\  scan0219
Date  27th August 1937 guessed
  
Fig. 1. Cut away view of washer showing raised pips to permit oil flow to central spindle.

Fig. 2. General arrangement of oil filter.
A.{Mr Adams} Inlet.
B. Outlet.
C. Slot in outlet tube.
D.{John DeLooze - Company Secretary} Filter pad.
E.{Mr Elliott - Chief Engineer} Compression spring.
F.{Mr Friese} Oil diffuser.

For a long time oil technologists have been satisfied that oil retains its lubricating efficiency until the oil is completely used up. Discriminating engineers also agree that the useful life of an oil is not ended when it has been extensively used, although it may have collected a large quantity of impurities. While, however, the lubricating properties of an oil are indestructible, yet oil in service accumulates impurities. These consist of carbon from decomposition of the oil and extraneous matter such as dust, grit and metal particles.

Figs. 9, 10 and 11 are reproductions of untouched photomicrographs of oil samples. Due to the efficiency of the Smith Filter, there was nothing to be seen on a photomicrograph. To remove any doubt and to prove this point the filtered oil photomicrograph, Fig. 11, was taken with the cover glass arranged to blank off a portion of the photomicrograph. The large surface is the filtered oil and the line across is the edge of the cover glass.

There is no reason why an oil should not be better after use than unused oil— in fact, the results of the latest research work shows that certain lubricating oils do not acquire their full lubricating efficiency until they have been in service for some time under conditions of heating and contact with air. Impurities of all sorts suspended in the oil, however, are always objectionable, and efficiency of purification is obviously of vital importance.

It is also suggested that a filtered oil does not take up such a large percentage of volatile matter, and that during its use the unstable hydrocarbons in the oil which are the cause of breakdown, and are also particularly responsible for oxidisation, are removed while the more stable molecules remain. Further, it is generally agreed by the majority of oil technologists that oil after use and efficiently filtered a number of times is really a super-refined oil.

It has been stated that a bypass connected filter takes a considerable time to clean up the oil in the engine sump. This may be the case with other types of filters, but this does not apply to the Smith Filter, because it is extremely efficient.

When this filter is new it filters the whole of the oil in the sump many times faster than the impurity collects. This rate of cleaning gradually decreases until the impurity in the oil collects faster than the filter cleans it. It is at this stage that the oil in the sump commences to lose its clearness, which is an indication that the filter requires changing.
  
  


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