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Improved Worm-gear Lubrication
Page 73
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Lower Friction, Reduced Wear and Better Oxidation Stability Claimed for New Oil
TN a paper entitled " Lubrication of Automotive Worm Gears," read to the Automobile Division of the Institution of Mechanical Engineers on Tuesday, Mr. J. Whittle, B.Sc., A.M.1.Mech.E.. of Shell Research, Ltd., gave details of investigations carried but on worm-gear hffirication, which have led to the development of a premium-grade lubricant, which appeared to have qualities superior to -those of a straight mineral oil.
These include lower friction and a resultant lower oil temperature and power loss; reduced wear and pitting; and oxidation stability better than that of an inhibited mineral oil. Mr. Whittle dealt with these three aspects separately in his paper.
High Temperatures
Referring to friction, be. said the results reaffirmed that power loss was directly proportional to temperature rise. It was found in general that low-viscosity mineral oils gave extremely high temperatures, , accompanied by excessive wear of the bronze wheel, which agreed with the general field experience of these oils. High-viscosity oils gave more stable temperatures, although at a fairly high level, but the addition of . up to 15 per cent. of acidless tallow to the thinner oils gave lower temperatures than the highest viscosity oils. Tests carried out with poly-alkylene oxide derivatives showed that the friction and resulting temperature rise with this material as a worm-gear lubricant was similar to mineral oils of comparable viscosity under mild conditions, but much lower under moderate and severe conditions.
Wear and Pitting
The tests concerned with wear and pitting were not fully conclusive, said Mr. Whittle, due to the difficulty and time involved in getting reliable results. Whilst in certain tests the synthetic oil showed advantages over S.A.E. 140 mineral oil, in others there was no significant difference between them in this respect. The disc machine used in the tests proved to be the most reliable guide available, but was capable of detecting only large differences in the performance of different oils. It was shown, however, that under test conditions, oils with an active extreme-pressure additive were more prone to produce pitting than straight mineral oils, and also that high oil acidity increased the pitting tendency of the bronze.
Extensive field tests with regard to oxidation were carried out on both straight mineral and inhibited mineral oils. These were concerned mainly with the relationship between viscosity changes, acidity, operating temperature and mileage. An important finding of the tests was that reducing operating-oil temperature effectively reduced the rate of oxidation and consequently the degree of oil acidity.
Better Stability
Laboratory and field tests using the synthetic oil gave results which confirmed that it had better stability„ and that this, with its lower operating temperature. would give an appreciably longer life than an S.A.E. 140 mineral oil. In the field tests the synthetic oil covered mileages of over 70,000 without changing, and was still in a reasonable condition.
In the conchisions to his paper, Mr. Whittle said that the •decline in the popularity of the worm-gear was partly, because present lubricants' were not satisfactory for the increased powers being transmitted. He said that the synthetic lubricant which had been developed appeared to meet most of the requirements for a superior type of worm-gear lubricant.