PROTECTING THE BIG ENDS

Shannon News, 7 December 1928, Page 2

PROTECTING THE BIG ENDS

SCHEMES TO AVOID OVERHEATING. A consistent increase of engine speeds has been one of the most strongly marked trends of development during the past few years; gear ratios have become lower and., wheel sizes have decreased so that :on top gear 1200 r.p.m. at 3Q ni.p.h. i- now quite common, which means that 3500 r.p.m. mayA>e reached or even excelled, at maximum speed in man,’ touring cars. On the indirect gear conditions are still more onerous, partly owing to the smooth running qualities of the modern engine—particularly the “six” — which enables the driver to reach, ->av 10 m.p.h. on second gear without real ising that,he is overdoing it. Narrow Safety Margin.

It, would, in fact, be no exaggerationto sav that the safety margin has become very narrow in many cases so that the engine bearings are saved from disaster only by the time element —i.e., road conditions are such that only for a brief period is the engine kept running at or near its maximum rate of revolutions. Were it possible or desirable to keep it “flat out” for more than a few minutes, it is highly probable that one of the big ends would fail.

Conditions are, of course, particularly acute in the case of sports, models in which the mechanical balance of the engine is particularly good and the valve gear is especially arranged to permit of ultra high rates of r.p.m. Furthermore, the driver of the sports car is, more .likely to make full use of the gear box than is the driver of the touring model, so that the big-ends are liable to be loaded almost to the limits of their endurance upon quite numerous occasions. In such circumstances the slightest disability, such as a partial stoppage in the oil line or a bypass valve cutting in at too low a pressure, will result in heating the white metal to melting point so causing bearing failure. Before going further, it seems desirable to explain that in a high-speed engine the bearing loads caused by inertia of the reciprocating parts are much more serious than those produced by the compression and the explosion of the mixture. Each piston is being flung up and down the cylinder with great rapidity and must be brought to a standstill and restarted at the end of each stroke; at the top dead-centre the piston tries to maintain its speed and continue to move upwards and the bottom dead centre’,the same, tendency occurs only in a downward direction. These tendencies can only be curbed by the control exerted by the connecting rod, so that a very heavy load is produced on the big-end bearing at the ends ■ of each stroke, which may easily exceed one ton in, say a threelitre six-cylinder engine. This enormous force is trying to pull the connecting rod upwards and push it downwards alternately many thousand times per minute, and is exerted on a relatively small bearing encircling the steel crank-pin and lined with white metal. • .

/ Continuous Oil Supply. Under such trying conditions the bearings depend for safety upon a continuous supply of oil which, by forming a film between the white-metal and the crank-pin, cuts down friction to a minimum, and which also serves the purpose of a heat-carrying agent to prevent the temperature of the white metal from rising to danger point. The film of oil is only about one-thou-sandth of an inch in .thickness, but so long as the supply of lubricant is copious this film proves remarkably tenacious, ' and, of bourse, tne more rapidly the oil is pumped through tne bearing the more neat will it prove capable of carrying away. Much the same conditions apply to the main bearings which support the crankshaft in the crankcase, but, here two factoi*s help the bearings considerably; first of all, the crankcase, with its large area of heat-conducting arid heat-radiating aluminium, enables the temperature to be kept down to a low figure, while, secondly, the main bearings receive oil straight from the pump,, which is, therefore, relatively cool. 1 In the fully forced systems which are so'general to-day, ho ver, the oil which has done duty in a main bearing and has become heated by frSc- ' tion and by .at picked up from V steel journal (which may be running at a temperature as highas 200 deg Fahr.) passes through a narrow passage drilled in the hot crankshaft web, and therefore readies the big-end bearing at quite a high temperature. The unfortunate big-end, therefore, receives preheated oil instead of cool oil, so that the margin of safety is much riarnovrer than in the case of the main bearing. The strength of white metal decreases considerably at temperatures in excess of 200 deg. Fahr., and the metal actually fuses (i.c., becomes fluid) at about 300 dog. Fahr', so that it does not require much technical knowledge to understand that the big-ends of an engine Avhich is driven at high speed for a lengthy period of time may come near to disaster.

Reducing Oil Temperature. One way of reducing the oil temperature is to employ a sump of generous size, ribbed on tile outer surface and exposed to the air passing under the car, in Avhich the oil will cool down rapidly. Another schc'me is to discard the principle of delivering oil to the big ends via the main bearings, and to supply the big ends direct, either by dipper sand troughs, or by qil jets. This may seem a reversion to an old method, but it is used by several prominent manufacturers. Other manufacturers have turned their attention to oil radiators in which the lubricant circulated and cooled^

so reducing the average temperature of the oil supplied to the pumjp to a reasonably loav figure. The use of oil radiators has been a common practico for racing cars for some time past; they are now employed in many sports models, and certain Continental makers are trying them for fast touring cars. In some experiments carried out the fitting of an oil radiator Avas found to reduce the temperature of the lubricant from 195dcg. to IQOdeg. Fahr., other conditions being identical. Furthermore, it has been proved that the oil radiator causes a very marked decrease in the consumption of lubricant, amounting to 15 per cent, or more. ,