OCTOBER 1937 295

THE APPLICATION OF MAGNETIC OIL FILTERS TOLUBRICATING SYSTEMS

by L. H. DE LANGEN.

Summary. An apparatus is described which can remove small magnetic particles froman oil-stream. The decrease in wear on shafts and the like obtained with this apparatusis discussed.

After the lubrication of bearings and other partsof machinery had been the exclusive concern ofthe engineer for many years, this domain oftechnology was brought within the reach of scienceabout the end of the last century. The problemwas at first attacked chiefly from the mathematicalpoint of view. In setting up the hydrodynamiclubrication theory 1) Reynolds began with theproportionality between the sliding stress appearingin the lubricant and the fall in velocity.So m m erf'e l d, Michell, Gümbel"and otherslater developed the theory further. Of the various.assumptions which form the basis of the hydro-dynamic lubrication theory the following. two areof importance for our subject:_1) that the surfaces which slide over each other

are quite smooth and-2) that the lubricant is a pure liquid.

In practice neither of these two conditions isentirely fulfilled. Recent investigations have shownthat most surfaces, as obtained in the workshopby the usual means, even when they are observedunder moderate magnification, resemble a mountainlandscape. The heights of the ·mountains in thislandscape are of about the same order of magnitudeas the thickness of the lubricating oil layer. It is .therefore not surprising that when these surfaces. are allowed to slide over each other the theoryof lubrication is scarcely applicable; the oil film isbroken, and considerable wear occurs.The writer has ascertained on various occasions

that even with highly-finished bushes in which ashaft had run for several hours, the .diameterhad become several hundredths of a millimetregreater; the tops of the mountains had been wornaway. Because of this wear the second assumption,namely that the lubrication is carried out with apure liquid, is no longer justified. The particles ofmetal worn off circulate with the liquid a?d causefurther wear. This .experience leads to thestatement of the following paradox: a bearingmust b e lubricated with as small an amount

1), o. Rey n ol d s, Phil. Trans. Roy. Soc. London'177, 157(1886).

of oil as possible. When the oil is contaminateddue to wear it is actually better to use little oil,so that only a few. solid particles take part in thefurther wear and tear of the surfaces. Of two. .

approximately equally loaded bearings of a certainconstruction, one of which 'received very much' oiland the other, because of accidental circumstances,almost nothing from the same oil reservoir, thefirst showed by far the greater wear.With the above in mind it is not difficult to

indicate methods for improving lubrication anddecreasing wear and tear. In the first place moreideal surfaces must he aimed at, and in the' secondplace methods must be found of removing themetal particles carried by the oil before they cause'further' wear.

It is now possible, even in ordinary production.by making use of modern methods of treatmentsuch as lapping and honing 2) to give the surfacesof shafts and bearings such a finish that the above-mentioned first condition is fairly well satisfied.This article will describe a method developed

in this laboratory of continually removing .theparticles of iron from the oil, so that the secondcondition may also be ~ulfilled.· This method isas follows: the circulating lubricating oil flowscontinuously through a strong magnetic field, sothat the iron particles are attracted to one ofthe poles and separated from the oil.The latest development of magnet steel has made

it possible, with permanent magnets of moderatedimensions, to attain fields of such strength that'even very small iron particles can he taken out inthis way. .

Fundamentally the magnetic oil filters consistof a cylindrical' permanent magnet placed in acylindrical space enclosed by iron walls. A simple

2) Lapping is a method of polishing in which a metal surface'is polished with another softer metal surface, with theaddition of an abrasive suspended in a liquid. By givingthe correct form to the polishing surface it is possiblein this way, not only to make the parts smooth, butalso true in shape and dimensions. Honing is a similarprocess, in which however stone is used for the polishingsurface 'instead of a fine abrasive embedded in' a soft :métal. The finished surface is somewhat less smooth.

296 PHILIPS TECHNICAL REVIEW Vol. 2, No. 10

construction IS drawn in jig.1. The permanentmagnet A is placed in a steel tube B. Three brasswires C soldered to the magnet hold it in the middle

A

8

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Fig. 1. Simple magnetic oil filter.

of the tube. The lines of force of the magnet aredrawn in the left-hand half. If oil contaminated withparticles of iron flows through this tube, the ironis deposited mainly on the upper side of the magnetat the points of highest concentration of magneticflux, as may be seen in the photograph (jig. 2) ofa contaminated magnet.

Fig. 2. Magnet with iron filings.

An oil filter of this construction is very satis-factory, but is somewhat limited in capacity;because the impurities are deposited as a nar-row ring, the filter is quickly full.

An oil filter of more perfect construction is shownin jig. 3. The permanent magnet A is fastened toan iron cap B, which is screwed into the iron housingC. A brass cylinder D with holes E is placed aroundthe magnet in order to direct the oil which entersat F and leaves at G. This construction has 'three

important advantages. Firstly a good magneticcircuit is formed here with only one air gap sothat with a given length of the permanent magnet

c

Fig. 3. Cross section of an oil filter. The magnetic lines offorce are indicated by continuous lines and the stream linesare dotted.

the strongest possible field is generated. Secondlythe convex pole of the magnet is so placed withrespect to the housing that the particles of ironare deposited over the whole convex surface, andthe filter can therefore collect a large amount ofimpurities. Thirdly the air gap is of such a formthat the oil entering flows in about the samedirection as that in which the iron par'ticles areattracted. The lines of force and the stream linescross each other at small angles; therefore the ironparticles are subject to the attractive effect of themagnet over a long distance.Various magnetic filters have been employed in

the Philips organisation for some time. The firstgeneral application is to be found in the filmprojectors of the Cinema Department, which arenow equipped with magnetic oil filters. Successfulattempts have been made along two lines to limitthe wear and tear on film projectors. The first stepwas that of making the surfaces of the parts truerand smoother. A great deal has been achieved inthis respect. All shafts and bearing bushes arelapped or honed, the Maltese cross also is finishedby a special lapping treatment to give great

OCTOBER 1937 MAGNETIC OILFILTERS 297

smoothness and high precisIOn (tolerances 1 to2 microns). The toothed wheels, however, havean ordinary finish, which is obtained by hobbingwith a fine milling cutter. Grinding or lapping ofthese little toothed wheels is not yet possible.During working the toothed wheels produced irondust which, acting as an abrasive, spoiled the smoothsurfaces of the lapped shafts, the Maltese cross etc.In this case the use of simple magnetic filterseffected a great improvement. Two of t.hese filtersare built into the housing of the Maltese cross.Fig.4 is a cross section through one of these filters.

E

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Fig. 4 The oil filters. C are here introduced into holes B inthe housing A of the Maltese cross of a film projector. Inthe front view (left) they are dotted, and to the right is across section through a magnet.

A is a part of the housing, B is a hole in which thepermanent. magnet C is placed. D is a wire. ofnon-magnetic material which is cast in. E is anoil reservoir which is continually refilled by theoil pump. The oil flows through the hole F intothe housing. Of the oil passing through the pumpsonly that portion is filtered which is intended forthe Maltese cross. Since, however, the circulation israpid, the whole of the oil eventually finds its waythrough the filter t.o the Maltese cross. This methodis very satisfactory; if a magnetic filter is introducedinto a projector whose oil has become dark-colouredfrom the ground-offiron particles, the oil may beseen to become clear again after a little time. Thelength of the captured particles of iron seems tolie between 0.4 and 4 microns. The grinding offdecreased rapidly after the initial working in, asmay be seen from the graph of jig. 5.

Especially in the case of the Maltese cross isthe limitation of the wear and tear extremelyimportant; a Maltese cross in which there is play

due to wear begins to knock harder and thenwears off even more rapidly.

mg150

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V100

50

oo 100 wo 700h200 300 400 500

Fig. 5. Wear and tear on a projector: weight of the iron particlesdeposited by the magnetic oil filter as a function of the numberof hours during which the projector had run.

We have also introduced a magnetic filter intoan overhauled automobile motor, whose cylindershad been honed and whose pistons had beenprovided with new piston rings. A filter liket.he one in fig. 3 was placed in the pressure oilsystem in such a way that all the oil passedthrough it. After t.he car had run 500 km the filtercontained 673 mg of iron particles whose size variedbetween 0.4 and 5 microns. Fig. 6 is a photograph

Fig. 6. Oil filter magnet covered with iron filings.

of the magnet after it was taken out of the filterhousing. Later on the wear became less, as maybe seen from the graph of jig. 7. The graph repre-senting the wear and tear has the same form asthat found in the case of the cinema projectors.Magnetic oil filters also have been used with

success in various industrial machines. In the

298 PHILlPS TECHNICAL REVIEW Vol. 2, No. 10

isolated case of an oil filter introduced into thelubricating system of a steam turbine, which hadbeen running for some time, only a small amountof worn-off metal was found, as would be expected.

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Fig .. 7. Wear and tear on an overhauled automobile motor:weight of the iron particles deposited on the oil filter as afunction of the distance the car had run. The magnet offig. 6. was taken out of the filter housing at the momentrepresented in this figure by the point A.

In the employment of magnetic oil filters thereare various methods to be followed. In the caseof machines which are not too large it is best tointroduce an oil filter into the main feedpipe ofthe pressure system. With larger machines havinga large amount of circulating oil it may be suf-ficient to pass only a portion of the oil flowthrough the filter by installing it in a branch pipeof the supply. Another possibility is the intro-duetion of a simple oil filter directly before eachlubrication. In many cases it is not difficult toarrange the holes for the lubricant in such away that a magnet can be introduced, for examplein the way done in fig. 4.With a magnetic oil filter of course only

magnetic particles can be captured; particles ofbronze or white metal' thus remain in the oil.Fortunately the parts which are most subject toinitial wear and tear are as a rule made of ironor steel, such as gear wheels, piston rings,cylinders etc.In many cases where bronze is now often em-

ployed, cast iron may be used to advantage. Thusfor example the use of cast iron bearings in thePhilips proj ector was a success,and as an additionaladvantage any particles from the bearings werecaptured in the magnetic filter.The wear due to metal particles in the

lubricating oil has also been investigated by meansof the following simple test. A silver steel shaftlapped as in normal production, 10 mm in diameter,ran for P/2 hours in smooth bronze bearings.In order to be sure that the lubrication would notbe unfavourably affected be too tight a fitthe play was arranged to be large namelyÓ.06 mm (difference of the diameters). The shaft,

which made about 1000 revolutions per minute,was driven by a belt of string; the tension of thestring was the only load on the shaft. The bearingswere lubricated with oil which had been artificiallycontaminated with iron particles taken from themagnetic oil filter of a projector. After the con-clusion of this test the shaft showed ringshapedgrooves visible to the naked eye. With a specialBusch microscope for the examination of surfaces,a photograph was made of the worn part of theshaft and of a portion which lay outside the bearings.This photograph is shown in .fig. 8. Two objects

Fig. 8. Photomicrograph of the surface of a shaft withillumination from one side. A is the new and B the wornportion. Magnification 102 diameters.

which are illuminated obliquily may be placedunder the microscope for comparison of the surfaces.If the surfaces are completely smooth the light isreflected to the other side; the perpendicularlyplaced microscope receives no light. If there aregrooves or inequalities in the surface, light is alsoreflected in a vertical direction. Looking throughthe microscope, light streaks Ol' points may beobserved. Part A of the photograph shows theundamaged portion of the shaft. The fine criss-cross scratches of the polishing treatment may beclearly seen. It is not difficult to produce a muchmore perfect surface which appears practicallyblack in the microscope, but for ordinary purposesthis extreme quality is not necessary.

Part B is a picture of the worn shaft, Nothingcan be seen of the original polishing scratches;instead there are now the much coarser groovesdue to wear. By means of this simple test whichcan easily be repeated it is shown convincinglythat attention must be devoted to meaures forthe purification of circulating lubricating oiLTests will be carried out on a still larger scale

on the application of magnetic oil filters. We hopeto announce the results uf such tests in the future.