MPLE MECHANICS
Page 73
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by Preceptor Lubrication
ICTION is a force resisting or eventing the movement of 'o bodies, the surfaces of 'doh are in contact. However lI finished a surface may be, ider a microscope it will be en to consist of a series of hills id valleys. It is the roughness two moving surfaces in con^t which causes friction. _ The force of friction is often eful: brakes rely on friction tween the brake lining and a drum to stop or retard a hide; a vehicle would not be le to move if it were not for the ction which exists between ai tyres and the road. surface. -Itches make use of friction to ce up the drive between the gine and the gearbox.
In other cases, however, ction is wasteful and the heat inerated in overcoming the ctional force may cause mage. The friction between ;ton and a cylinder wall, for ample, is considerable and a ston would soon seize in a linder if the friction was not Pt to a minimum.
To reduce this unwanted ction a lubricant, oil or grease, introduced between the oving surfaces. If these eying surfaces can be kept mpletely apart by a film of oil, ction, heat and wear are kept a minimum and this is what is med at, but not always :hieved, in motor vehicle brication.
When discussing oil, the rm -viscosity' is often used id sometimes misunderstood, ie viscosity of an oil is the easure of its resistance to flow. high viscosity oil is a "thick" I, or one which does not flow iry freely, while a low viscosity I denotes a "thin" oil.
To make practical irnparisons, the viscosity of oil measured by noting the time a von volume of oil, at a stated mperature, takes to flow 'rough a calibrated jet. The 3paratus used to measure scosity is called a viscometer id, in this country, the edwood Viscometer is 3nerally used.
The viscosity of an oil ianges with its temperature: as 3temperature increases, so the
oil "thinsand the viscosity is lowered; as the temperature falls, the oil thickens and so the viscosity is raised.
The Society of Automotive Engineers (SAE), an American institution, has classified oils into grades dependent on their viscosity. Each number defines, within specified limits, the viscosity of the oil at a given temperature. The lower numbers, eg SAE 5 or SAE 10, indicate a low Viscosity and the higher numbers a high viscosity.
In some cases, the letter W is added to the number; this denotes that the oil in addition to falling within that particular grade at a low temperature, is also within the acceptable minimum limits at high temperature.
In order to prevent oil drag, assist easy starting and allow the oil to circulate quickly when the engine is cold, it is desirable that an engine oil has a low viscosity at low temperatures and, at the same time, provides adequate lubrication when the engine is hot.
To meet both these conditions, multigrade oils have been introduced. These oils contain an additive that reduces the effect of temperature on viscosity. Such oils may be classified, for example, as 10W/30, which means the oil comes within the viscosity rating of SAE 10 at 19•'• C and 30 SAE at 99 C. It must be emphasised that the SAE rating only denotes an oil's viscosity and has no relation to the quality of the oil.
All lubrication systems on modern motor vehicle engines are of the force feed type where the oil is fed, under pressure, from an engine-driven pump to all the crankshaft main and big end bearings and to the other components needing lubrication. Most engines have "wet sump" type systems, where the oil is carried in a sump at the bottom of the crankcase, Occasionally "dry sump" lubrication is employed and, in this system, two pumps are used. One forces oil to the bearings and the other, slightly larger and known as the scavenge pump, draws the surplus oil from the engine sump and delivers it to an external tank.
This system, used, incidentally for many years on motor cycles, allows larger quantities of oil to be used and for the oil to pass through coolers on its way from the sump to an external tank. As there is very little oil in the sump, the engine is not affected by extreme inclination In both systems, oil is forced by the pump to an oil gallery running the whole length of the crankcase. The main bearings are fed with oil from the oil gallery through holes drilled in the crankcase. Holes are drilled in the crankshaft webs from the main bearing journals to the big end journals so that the oil fed into the main bearings, can also reach the big end bearings.
Further drillings • and pipework takes the oil from the oil gallery to the camshaft bearings and rocker arms Sometimes, the connecting roc is drilled from the big end to thE small end so that the gudgeor pin can be pressure fed with oil.
• In other cases, the gudgeor pin relies on oil which escape! from the big end being splashec on to it; the cylinder walls anc pistons are lubricated in 'IN same way. In some designs, z• small hole is drilled in the bic. end bearing specifically for thi! purpose,
Oil is sometimes encouragec to reach the underside of thc crown of the piston to assist ir cooling. Surplus oil from thi overhead valve rockers allowed to return to the sump Icr gravity, travelling via the holei which accommodate the pusi rods lubricating the cams am cam followers en route.
The drawing below is ; diagram showing the oil circi, lation on an overhead valv push rod type engine.