Magnetic Purification of Gearbox Oil
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A Contributor Argues that a Device to Remove Ferrous Particles from Gearbox Oil is More than a
Refinement
MO engineer will deny the vital i necessity of filtering the oil that circulates through the lubricating system of any petrol or oil engine. The impurities, comprising metallic particles, caused by wear of the moving parts, fragments of carbon, resulting from combustion, and road dust, inhaled with the air supply to the cylinders, all render the lubricant less effective. Thus, an oil filter is an essential item of equipment.
It has long been recognized that the lubricant in the gearbox is also the better for filtration, although by a system of different character. Practically all the nearing surfaces here are steel. and thus magnetic attraction is effective.
At the outset it seemed that a permanent magnet, or magnets, as opposed to an electro-magnet, would suffice. This proved to be the case, for the first tests. made on the gearbox of a small car, showed that there was a considerable amount of fine ferrous matter together with comparativelylarge brokenoff fragments of steel in gearbox oil, and that this material could be picked up and held out of harm's way by a magnet or group of magnets.
Scope for Experiment.
Actually, this was only a beginning. Quite a number of considerations presented themselves in the course of later tests. Questions arose, for example, as to the length and number of magnets, their relative polar arrangement and the poeition of the filter in relation to the gears and to the oil-stream, whilst cheap production was a further pertinent matter.
There is a characteristic applying to the magnetic system of oil cleaning xvhich does not apply to the engine filter in its usual form. The latter may sometimes become clogged, so that its filtering element requires periodic removal and replacement or cleaning. Passage through the filter may, in deed, altogether cease. This cannot occur in the case of a magnetic filter, for impurities build up on to, or bebetween, the magnets, almost indefinitely arid with no appreciable effect upon the circulation of the oil. Some engineers consider filtration ot gearbox oil rather as a refinement than as art essential. But, if those who doubt its importance were to examine the material adhering to a magnetic filter after use t preferably under a magnifying glass) or were to stir with a magnet the emptied contents of a gearbox, they would assuredly be convinced of the useful work done by the magnetic oil filter. tin close examination, the adhering particles are found to consist of finely ground material which may, perhaps, be termed " ferrous sludge," pieces of steel chipped from tooth edges, as large, perhaps, as a match head and not infrequently steel frag ments as much as *-in. long, • In the simplest form of magnetic filter, a single magnet is held in a gunmetal plug, which usually replaces the drain plug of the gearbox. This position may not be the best one for the filter, /Hit it is convenient for the manufacturer. A more elaborate type is that which has four magnets pitched about one and a half or two diameters apart. Not only has this the advantage of greater range of action and more satisfactory retaining property, but the space between adjacent magnets constitutes a convenient system of pockets in which the foreign particles collected are more or less sheltered from the oilstreatn and, therefore, are not readily washed off.
The accompanying plan view of a filter of this type, after use, indicates clearly how the collected particles arrange themselves. As before, the four-magnet type is incorporated in the drain plug, which may be as large as B.S.P., so that a filter of adequate size can be accommodated.
When special provision is made for the filter, an altogether better type of device is applicable. In this case, a single bar-magnet is used, perhaps 6 ins, or more in length and with a diameter of in. The magnet is surrounded by a helix formed from material of square section which is conveniently screwed on to a single pin in the magnet. A particularly interesting feature is that the helix is made of a ferrous material which does not retain any appreciable magnetism after removal from the main magnetic source. Thus, foreign matter can readily be washed off before replacing it.
Remarkable Durability.
No part of these oil purifiers suffers from wear, and the magnetic property may last 30 or 40 years. If necessary, however, re.magnetization is cheaply and easily performed. Two classes of steel are used—tungsten and chrome-cobalt, the latter being, generally, preferable. Gunmetal is usually employed for the screwed end plug.
It is difficult to call to mind any device used by the automobile engineer that is of more simple construction and freer from partial or complete breakdown. Indeed, the filter was once described by a well-satisfied user as one of the cleverest little inventions of which he had knowledge. E.C.J.