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).
Paper on engine design from a maintenance viewpoint, discussing cylinder wear from dust, the importance of air cleaners, and the need for designers to be informed by real-world conditions.
| Identifier | ExFiles\Box 126\4\ scan0138 | |
| Date | 1st April 1934 | |
| DESIGN FROM MAINTENANCE VIEWPOINT 135 oil company but must include in his design features that will keep the temperature within reason. The selection of a proper air cleaner is a difficult problem on account of the wide range of conditions under which his product must operate. An air cleaner that may be fairly satisfactory in the eastern or northern states is often entirely inadequate in the South and West, and it is doubtful if he can meet these conditions with one type and size of air cleaner. The tractor industry is thoroughly conscious of the scrupulous attention that must be paid to the subject of air cleaners, but the passenger-car fraternity is too often inclined to form its idea of air-cleaner requirements from local atmosphere. The local atmosphere, by the way, is not as clean as may appear. During winter, the average monthly fall of soot in Detroit will vary from 600 to 900 tons per square mile. This soot, especially that from industrial stacks operating under forced draft, is hard, abrasive material, and air cleaners on compressors in power-house service often triple the cylinder life. Let the designer drive his product mile after mile through Idaho roads with the dust 8 to 10 in. deep, or through a California Santa Ana sand storm, which removes the finish from a car as would a first-class job of sand blasting, and his perspective on air cleaners undergoes a remarkable change. Damage Caused by Dust An experience that came to our notice in Oregon is enlightening in this connection. A bus engine had just been equipped with new pistons and cylinders and started on a trip up the Columbia River highway. It encountered an exceptionally severe dust storm, a storm so severe that is was noticed 500 miles out at sea. The bus made a round trip of 380 miles. When it returned to Seattle, a 0.035-in. feeler would fall through the clearance between the piston and cylinder. Another case was that a fleet of ten trucks operating on a road-building project near Spokane. The engines were continually laid up due to wear. The owner, becoming exasperated and suspicious of his truck drivers, finally instructed his car driver to take the car and follow the trucks all day long to see if he could learn anything. He did. In ten days the car's operation deteriorated so that it was in worse condition than were the truck engines. The contractor drove it into the dealer's place for a check-up. Later, he was called to the dealer's shop and cross-questioned as to what he had been doing with his car. The damage done by trailing his trucks in clouds of dust was so appalling that he promptly stationed one man at the loading pit with 200 heavy woolen bags which just fitted over the air cleaner supplied as original equipment. This man's sole duty was to lift the hood on every truck as it drove up, remove the dirty bag and install a clean one. He finished his contract without further delay due to engine wear. All of this wear could have been eliminated in both cases by adequate air cleaners of the proper type for this territory, though not necessarily penalizing eastern cars with expensive air cleaners. Wear of cylinders and pistons caused by dirt and dust can only be combated successfully by eliminating the cause. Very hard irons, hardened steel, and even nitralloy, offer little resistance to abrasives. In some recent tests of various materials in tractor cylinders, the wear of the hardest material was 70 per cent as much as that of medium-grade iron. Special alloys are of great value in resisting the corrosive action described previously in this paper, and the designer must keep these two principal causes of wear clearly segregated in his analysis if he is to combat them successfully. Still another cause of cylinder wear, while recognized by engine builders for many years, has only recently been receiving active consideration from designers. This is wear caused by explosion pressure getting back of the top ring and forcing the ring out against the barrel. Wear due to this cause is particularly severe in engines which are subject to frequent and rapid applications of load, as in city bus-service where heavily loaded buses must be accelerated many times per hour. Explosion pressures applied to pistons moving at high speeds do not seem nearly so disastrous as the same pressures applied suddenly to pistons moving at low idling speeds. If you wish to show up wear rapidly in cylinders in a test, do not set the engine up and run it constantly at a high speed. Put it on a cycle that changes abruptly from a low idling speed to full load at moderate speed 20 or 30 times per hr., and the wear will be all that could be desired. Some special rings have appeared which, in a large measure, neutralize this wear. The wafer rings, apparently because of the downward pressure on the upper face being far more than the relatively lower total pressure back of the ring, are effecting considerable reductions in cylinder wear. Due to their narrow width, they are less subject to rounding over on the face, which causes wide rings to lose their efficiency. The load effect of blow-by due to cylinder distortion has been graphically described in a recent paper by Ralph Teetor. I believe that anyone who has done much investigating with dummy heads in measuring barrel distortion after the cylinder-head nuts have been pulled down tightly will agree with his findings. A careful check-up of this nature should be included as standard procedure in any new model, provided that the engineering department can keep two jumps and a half ahead of the sales department. Designers Should be Fully Informed If an engine designer is steadily to improve his technique in his work, he must be fully conscious of the foregoing points and many others of a similar nature. I do not mean that he must constantly be reminded of his mistakes and shortcomings until he loses confidence in himself and secretly doubts his ability to design a corn sheller. I believe it to be the duty of management to see that the designer is kept thoroughly posted on exactly what his product is supposed to do by first-hand observations, and to aid him in discovering and eliminating the reasons for wear or customer dissatisfaction. Too often the designer is in about the fourth or fifth-line trenches, when he should be in the first, to receive at first hand the reaction from the field. A great aid to such an end is a proper grouping of the departments with which he should be closely affiliated. An ideal engineering department should have the pattern shop, casting inspection and returned-goods department, located so close to each other that they would be sitting in one another's laps. Conscientious designers are keenly alert to measure the degree of success their product is achieving. We should strive, therefore, to see that they are actually watching their designs take shape in the pattern shop, noting the foundry and production problems involved, and seeing most clearly and intimately the causes and effects of wear and breakage. This is a final responsibility that rests on management, in acquainting and carefully schooling its designers with the thousand and one little details on which a successful design must rest. April, 1934 | ||