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Design and Production Faults Revealed
Page 114
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Manufacturers Welcome Services Type Test: Severe Trial of Engines
ENGINE failures and troubles caused by design faults and manufacturing mistakes resulting from deviations from makers' original intentions, were disclosed last Friday in a paper, "Type Testing," by Lt.-Col. R. A. Laurence and Mr. W. H. A. Collins.
It was presented to the Institution of Mechanical Engineers in London and formed part of a symposium on engine testing. The authors thought that makers' apparent interest in, and desire for, the joint services type test—in spite of its severity and limitations in its field— seemed to indicate a demand for an independent type test and certificate which would gain recognition and standing.
Conditions of Test
Implying, as it did, a test to prove the whole performance of a machine as finally designed and developed for manuture, type testing had to be devised with that end in view. It had clearly to demonstrate the ability of equipment to perform its function in accordance with. specified requirements under specified conditions for a given length of time and with specified maintenance.
Its aim was separate from that of relatively brief routine production tests. An important requirement of any type test was that • it should command the respect and confidence of all concerned. To secure that, the three most important characteristics seemed to be relevance, rigour and impartiality.
It was believed that the joint services type test of internal-combustion engines was the only independent trial of its kind now regularly available to manufacturers. and seemed to he keenly sought.
Standard Types Used
Except in a few particular cases, the Services and the Ministry of Supply depended upon basically standard commercial engines. The margin of capacity commonly allowed by engineers in civil practice was often not admissible to the same extent for much military equipment, with its emphasis on high performance and power-to-weight ratio.
Moreover, the standard of operating skill and knowledge was often likely to fall short of that in civilian use. This stressed the need for thorough proof of the selected engines by type testing.
Because the service type test was strict and with conditions which were not always in accord with civilian usage and the maker's previous proving tests. manufacturers had to be asked to give a pre F16 liminary demonstiation run of some 250 hours' duration. This was carried out at the manufacturer's works under the conditions of the type test to show that the engine had a reasonable chance of passing the official test. This had proved well worth while in reducing troublesome and delaying failures in the official test.
The Services wanted several thousand hours' life between workshop overhauls. Consequently, the main part of the type test was an endurance run, which had been fixed at the practicable figure of 1,000 hours minimum for oil engines. and 500 hours minimum for petrol engines. In practical use, more was to be expected, cr to be hoped for, unless conditions were particularly bad.
Coolant and, if separately controllable, lubricating-oil temperatures were deliberately kept up to the makers' recommended maximum. Inlet depression and exhaust back-pressure were adjusted to figures appropriate to suitable air cleaners and exhaust silencers for the particular engine, in reasonably dirty condition in service.
Load cycles of the endurance test were designed to flog" the engine, but not inajustifiably. They consisted of alternate diaity cycles comprising 12 hours at full rated load—adjusted to site conditions— with an hour at 10-per-cent, overload, and 12 hours on a cycling load including overload, full load and light load.
Oil-engine Rating Change
The basis of the British Standard rating of oil engines in B.S.649 was being altered in the new edition to a continuous rating only, with an assessable reduction in cases where periods of service betweep overhauls in excess of makers' recommendations were required, in place of the old 12-hour and 24-hour ratings, By no means every engine would, as it stood, satisfactorily complete the services type test. Many of the troubles encountered arose because the grades of fuel and lubricating oil that had to be used might differ from those normally recommended by the makers of the engines. Nevertheless, there remained a residue of cases in which failures had manifestly been caused by inherent faults in detail of design.
An engine completed 1,000 hours of running on type test and, on dismantling, it was found that the water-pump drive coupling was badly worn. The drive consisted of a tongue at the end of one shaft engaging with a slot in the end of the other shaft. The coupling was redesigned in the form of a robust dog and several of the new forms of dog coupling satis
factorily completed a similar period a! running with no sign of wear.
After running for 1,000 hours the bigends of an engine had failed severely; in two cases the bearing housings had frac
tured completely. These housings had originally been -of phosphor bronze. white-metal lined. Various remedies had been tried, but the trouble was finally overcome by fitting steel housings carrying steel shells, the upper shells being lined with lead-bronze and the lowei ones with white metal.
A four-cylindered engine had been put up for the joint services type test, but was found to suffer from excessive vibra tion. The makers had withdrawn the engine and fitted it with balancer shafts. The modified unit had then satisfactorily completed the type test.
The defects quoted were not indicated by any particular aspect of the services type test. They would have become apparent in any prolonged trial under realistically envisaged and applied conditions.
Errors in Manufacture
By far the greater proportion of troubles encountered in the course of type testing had been caused by various faults which might be broadly termed mistakes in manufacture and were due to departures from the makers' details of design and/or intended methods of production.
An oil-pump cover plate and the corresponding gear-wheel face had both been badly scored as a result of swarf left in an engine after erection. In another instance the timing wheels of an engine had been damaged through too-close meshing. The pressure had been so great that pieces of metal had been torn out of the faces of the teeth.
A set of bearings was badly scored and fused after 1,000 hours of running—thc result of excessive clearance in the oil pump which brought about loss of prim. ins and lack of lubrication for a short time whenever the engine was started.
Engine Seizure
A piston and cylinder bore had been badly damaged by a severe seizure resulting from swelling of the liner jacket sealing rings, which were of the wrong grade of material.
In yet another instance the rocker pads of an engine had been found to be worn after 1.000 hours' running. This was the result of the ends of the valve stems not having been hardened as specified in the design.
These mistakes in manufacture, which were a few examples, would have become evident in any prolonged -test.
It appeared that manufacturers were often grateful that defects, such as those quoted, had been tirought to their notice. They had generally been quick to act to rectify the faults and follow up weaknesses indicated by the service type test.
[Two other papers were presented. They were. "Test Bed 'Installations and Engine Test Equipment." by Mr. H. G. Freeston, and "Statistical Methods Applied to Fngine Testing." by Mr. W. R. Bucklandi