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LUBRICATION
How the Oil Companies Keep Well Abreast of Developments in• Commercial Vehicle Chassis Design
ACOMPREHENSIVE lecture on "Modern Developments in Automobile Lubrication" was read• last week by Mr. 0. T. Jones,
M.I.Mech.E., before the London members of the Institute of Road Transport Engineers.
On engine oils, he first reviewed their development from 1918 to the
present time. So-called "conventionally refined" motor oils only were available. These were lubricants made from distilled products blended with refined residual oils to provide the required viscosities. After refining, they were treated with sulphuric acid, washed with caustic soda and water to remove acidic material, and filtered through special clays, such as Fuller's earth. Often they were treated to remove wax.
Difficult Starting From Cold
Compared with present-day standards, such oils had relatively steep viscosity curves and the pour tests were higher. Consequently, cold starting was difficult and oil circulation sluggish until engines had warmed up. Impurities often caused excessive deposits in crankcases and on piston crowns. With improvements in engine performance, internal temperatures rose and deposits in the oil systems became more pronounced as a result of decomposition under the influence of heat, air and combustion products.
It became necessary to increase rates of oil circulation to improve the cooling of bearings and pistons, resulting in further oxidation and pointing to the need for higher refining. These needs were partly met by using more acid to remove impurities, together with other modifications, but an excess of. acid reduces lubricating efficiency and thus imposes a practical limit.
Reducing Oxidation
The next important step was the introduction of "solvent refining," one method being the Duosol, employing propane and selecto to remove impurities and obviate the use of acid. Such oils gave flatter viscosity curves, easier starting and less oxidation at high temperatures.
With the further development of engines, crankcase temperatures in hot climates rose as high as 300 degrees F. It is interesting to note in this connection that the rate of oxidation doubles for each rise in temperature of 18 degrees F.
This problem was partly met by improving engine design, particularly cooling systems, and it brought to light the problem of bearing corrosion. This occurred occasionally in bearings of hard alloy, where the crankcase temperatures were in excess of 250 degrees F., and chemical additives, known as anti-oxidants, were developed to stabilize the oils and reduce , the corrosive effect of petroleum acids formed under high temperature.
• These additives were frequently phosphorus and sulphur compounds, which also had some slight film-strength properties. They virtually put an end to bearing corrosion.
The practice abroad of running heavily laden vehicles, sometimes with trailers, over long hauls at high speeds caused excessive sludge deposits in crankcases and overhead-valve covers, and frequent failures occurred. After much research, oils containing antioxidants and detergents were developed, the former maintaining oil stability and the latter dispersing the heavy carbonaceous and varnish-like deposits —suspending them as fine particles floating in the oil, to be removed when crankcases were drained. Such oils were later adopted for certain types of oil engine where excessive deposits occurred.
Additive-type Oils
Mr. A. T. Wilford, of London Transport Executive, recently read a paper entitled "Lubrication of Engines in Public Service Vehicles," referring mainly to oilers. He described an extensive series of road tests of additivetype oils. From the results it seems that cylinder wear was substantially reduced, engine life showed an average increase of 20,000 miles and, he said. it is a fair prediction that for high-speed oil engines operating under strenuous conditions, detergent-type oils will become the lubricant of the future, and their use may eventually be extended to cover the majority of engines.
The use of straight mineral oils is, however, unlikely to be discontinued. Where conditions are not severe, some users may prefer them, for they respond more easily to filtering and reclamation.
Some scepticism is expressed by certain operators who may believe that exaggerated claims are made for the properties of additive oils. In this connection, the lecturer showed a series of interesting lantern slides indicating the absence of deposits and the general cleanliness after mileages varying from 22,000 up to 40.000 on such oils. There were also pictures of units taken out of passenger vehicles in regular service, in some cases, after 40,000 and 80,000 miles, respectively, without cleaning. They showed results substantially better than were _obtainable in the same fleet with similar engines, running on goodquality mineral oil.
There are indications that in 011engine lubrication, the higher sulphur content of fuels may accentuate lubrication problems, and it is to be hoped that there will be no need to. use fuels containing more sulphur than those employed at present.
It is probable that new oil additives can be found to meet the adverse conditions set up by higher-sulphur fuels, but a considerable Increase in cost would have to be faced and more frequent crankcase drainage might be necessary. Only practical experience of operation under service conditions can give the answers to these questions.
The Lubrication of Gears
Early type crash gearboxes used a heavy oil of SAE 140 viscosity. • This was a straight mineral oil of good stability necessary to reduce deposits caused by temperature rise, churning and aeration due to gear rotation, resulting in oxidation. In some boxes, similar oils having extreme-pressure characteristics were specified.
In certain vehicles Wilson epicyclic gearboxes are used, often in conjunction with hydraulic flywheels. There are no problems of high-tooth pressure, and to Maintain maximum efficiency it is desirable to use an oil of low viscosity similar to medium or winter-grade engine oil as used on cars. In some cases, special additive-type oils are used to reduce thickening in service and formation of deposits. For gearboxes of this kind, ptimp circulation is normally employed, and it is important to avoid such effects. For synchromesh boxes a light gear oil of SAE 90 type or engine oil of the SAE 50 class is normally required to permit squeezing from the cone surfaces. Heavy oils, in addition to reducing the efficiency, delay synchromesh action.
Sometimes a hypoid-type oil is used also for the gearbox when a hypoid gear is provided in the back axle.
Hydraulic transmissions require a low-viscosity oil with one or more carefully selected additives.
The spiral-bevel axles on light and heavy vehicles normally employ a straight-mineral SAE 140 gear oil, but there is a tendency to use lowerviscosity types owing to improvements in oil seals, etc Worm and Hypoid Gears
With worm drive, minute bronze particles tend to act as catalysts and to increase oil oxidation. He also believed there was some merit in the use of higher-viscosity oils than those normally employed for worm gears, especially when heavy trailers were drawn.
With the hypoid type there is need for specially developed oils of lower viscosity which can carry away the sludge more rapidly. The additives used are usually of the sulphur and chlorine type, but others may be defrothing agents, pour-point depressants and anti-corrosion materials.
Water pumps require special greases to seal against water leakage, and these must be non-soluble. •