Powder Metallurgy and the Motor Industry
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By H. W. Greenwood, M.Inst.Met.
(Powder Metallurgy. Ltd.) ALTHOUGH a good deal of pub. licity has been given to the use, by the motor industry, of certain components produced by powder metallurgy, it is sa10 to say that few people in the industry, and still fewer
outside it, have any conception of the potentialities of the technique applied to the particular requirements of both heavy and light motor vehicles.
Possibly the most highly publicized product has been the pump gear wheel, which has now become standard equipment on a number of vehicles. It is an interesting example and a good illustration of how the process can , serve the industry and of the specific advantages it can offer over other methods.
The simplest type of oil pump consists of two involute gears in a closed housing, the function of the assembly being to force oil under pressure to where it is wanted. Such gears were originally produced from cast blanks:.
this involved a machining operation in which some 64 per cent, of the metal had to be removed as chips, etc.
These gears might he made by diecasting, but even then would require some skilled machinist work, owing to,
the accuracy demanded for the perfect involute form. By powder metallurgy, not only is high precision obtained with high output, but, in addition, no serious waste of metal is entailed The saving in cost is not sensational, but quite appreciable. 'Another component widely used in the motor industry is the door snubber, a part which, if not properly lubricated, gives annoying squeaking, whilst if it be liberally lubricated, manages to distribute oil over the hands or the clothing of persons using the vehicle. An obvious suggestion was to combine graphite with the metal and so produce a self-lubricating snubber which would be free alike from squeaks and risks of dily hands or
clothes. Iron powder and graphite were mixed, pressed and sintered, and there resulted a component formed in a single pressing operation which not only provided an ideal solution to the snubber problem, but, incidentally, eliminated 17 operations.
A similar solution was found in connection with window winders, which were also prone to squeak.. Production of such components by powder metallurgy is interesting, but is hardly to be classed as revolutionary. This term is much snore applicable when we come to suCh examples as the coppernickel-lead bearings developed by the
General Motors Corporation, of U.S.A. A full account of the original work which led up to the production of steel-backed copper-nickel-lead running surfaces would comprise a description, of years of research work on the bearings of automobile engines. To say that modern motor vehicles call for high-duty bearing materials is to utter a platitude, but the story of their evolution; is _much nearer a romance than many people imagine. Practically every problem that arises in the design, production, and per'forinance of bearings fOr either crankshaft or connecting rod had to be re-viewed, studied from new angles, and solved in' the light of modern requirements.
There finally emerged a bearing material of a type never dreamed of before; it consisted of a copper-nickel sponge first firmly attached, by sintering, to a steel strip and then filled with a lead-tin-antimony alloy. in which the tin and antimony were present. in much smaller proportions than in a normal babbitt metal.
In practice; now, the sponge is saturated with the bearing metal and a layer of from one to two thousandths of an inch of the alloy left over the top of the porous matrix; this provides a bearing medium suitable for duty at average speeds.
If very heavy duty be called for. then the bearings are finished off by 'diamond tools, so that the coppernickel sponge is exposed and thus becomes available -to support heavy loads.
Quite a variety of bearing materials can be made available by machining the finished bearing surface to different depths from the :steel backing, and it is possible from one and the same material to provide bearings for lowspeed, heavily loaded shafts, right away to those for operation at very high speeds with either high or moderate loading.
Self-oiling Bearings The part played by powder metallurgy in supplying the porous or selflubricating bearing is, probably, well known and hardly requires mention here, although it is, perhaps, its bestknown application and provides the largest tonnage of product a year.
A more recent but no longer novel development for both the motor and the aero industries is the production of brake blooks, in particular, those subjected to severe duty, as called for in a heavy lorry. Here, -powder metallurgy, by permitting the incorporation of highly abrasive material in a metal base, gives good grip combined with good heat conductivity allied to resistance to high temperatures. In this category there are already many varying types, and it is safe to say that there will he many additions as post-war development progresses.
As will be realized from the qualities such materials can possess, .a tremendous range of properties can. be attained by altering the nature of the constituents or their ratio one to. another: It is in these possibilities of varying properties over a tremendously wide range that powder metallurgy offers such ample opportunity in so many directions. . • Another important field upon .which powdet-metalturgy has encroached is in the production of piston rings, pistons, and even cylinder heads. So far, progress has hardly developed beyond the experimental stage, but promising possibilities are already envisaged and there is little doubt that, so far as piston rings are concerned, powder metallurgy is -likely not only to produce a 'standard article, but to allow a wider variation in physical properties under perfect control than is possible with present methods.
Sparking plugs have also received. contributions from powder metallurgy, and another important advance, so far, of limited applicability, concerns the
production of special apparatus for regulating fluid-evaporation rate; here, it is not unlikely that a demand may come for use on motor vehicles oPerating in northern latitudes.
In connection with cheaper massproduced vehicles, it is probable that the technique will be called upon to supply a large number of small parts where good wearing properties are esiential, and, in some cases, where self-lubrication is an added advantage. Materials of this type are being, increasingly used in the 'United States for bearings, small machine parts, cams, 'gears and in all cases where Machining is to be eliminated and a strong, stable wear-resisting material is wanted. Somewhat more specialized articles are valve guides.
Another category of special articles are compound metals used as heavyduty contacts for starters on motors, relays and the like. An example is the silver-nickel tungsten material which is arc-resisting, hard, and has a low contact resistance. It can be taken as typical of many other products of powder metallurgy of a not dissimilar character where combinations of metals, only possible by the technique of powder Metallurgy, can be utilized. Wide differences in melting point and other physical properties render the normal methods 'of manipulation, such as melting and casting or die-casting. quite out of the question, yet sintering allows the combining of widely.. different metals and even non-metals. This is a sphere, the possibilities of which have by no means been realized. and which will in the. future,' undoubtedly, be of tremendous service to the aero and motor industries.
Special Porous Pg
. .ates
Yet another field where this system can supply a unique product is in the cleaning of oils, etc. Here filtering media can be made from powders, such as bronze, iron, nickel, monel or other metals, the degree of porosity of which can be most accurately controlled. The filter plates can be produced in any desired thickness and are ideal for the clarification of oils or other liquids under high pressure. They are also capable of other applications, such as the measuring or metering of fluids.
So 'far, only what might be termed pure powder metallUrgical products have been mentioned.' There are, however, numerous ancillary branches where the method plays a vital part in the motor industry; examples may be found in the diamond or other impregnated wheel, and in certain types of diamond wheel dressers and -cutting tools. Then there are itie bard-metal tools, without which our present ideas of production would have no sound basis; they are wholly produced by powder-metallurgical methods.
Finally may be mentioned as an example of the application of powder metallurgy the production of rotor blades. for an internal-combustion turbine—a unit calltng for a unique combination of corrosion resistance and hot strength.