Marine Diesel Engine Lubrication Technologies
We have talked quite a bit about the lubrication aspects of the main propulsion plant on ships including properties of lub oil, cylinder lubrication and crankcase lubrication. Follow this link if you want to know about the importance of lubrication in marine engines. Of course these concepts apply more or less equally to all engines since the basic working principles are the same. In this article I will talk in detail about the two main types of lubrication technologies (rather techniques) used in marine diesel engines.
Lubrication of machine parts takes place in two ways namely
- fluid film lubrication
- boundary lubrication
In the first type, copious supply of suitable grade of lubricant is fed to the bearing in which the journal rotates at moderate speed at a designed load. However, when the load on the journal is large and the journal rotates at slow speed, it presses on the bottom part of the bearing and tends to squeeze the oil out of the clearance space between the journal and the bottom of the bearing until a stage is reached when metal surfaces of the journal and the bearing are about to touch. This is termed as ‘boundary lubrication’. During boundary lubrication the heat generated is higher and there is increase in the rate of wear.
Theoretically, in an ideal situation, when a bearing is perfectly lubricated and operates normally, the bearing friction is independent of load on the journal. This does not happen in practice, as, when a bearing gets overheated, one of the obvious steps to be taken is to reduce the load. The reason for this is that an overheated bearing is not perfectly lubricated. Therefore, in practice, there is a maximum limit for the load that can be put on a bearing.
When this limit is exceeded, the bearing will get overheated. The reason is not the friction that increases with the load but it is the tendency of the clearance space getting reduced due to overload squeezing out the oil from between the rubbing surfaces and causing overheating due to reduced supply of lubricant. Further increase in load may completely destroy the oil film between the rubbing surfaces causing quick rise in temperature resulting in melting and wiping out of the bearing metal.
FLUID FILM LUBRICATION
In a properly lubricated bearing, a film of oil is maintained between the rubbing surfaces. This is formed as a result of relative movement between the surfaces and depends upon the viscosity of the oil and the relative speed between the moving parts. The oil is drawn between the most heavily loaded parts of the bearing. More the relative speed and higher the viscosity of the oil, greater will be tendency to draw the oil between the most heavily loaded parts of the bearings. This is the reason when the load on an engine is increased, it is advisable to increase the pressure of lubricating oil so that any tendency to reduce the oil film thickness due to overload will be nullified by increased pressure of the oil, which tends to increase the clearance and maintain the oil film.
From the above it is clear that whenever two metallic surfaces slide against each other in a machine, an effort should made so that these sliding surfaces are prevented from coming in physical contact with each other so that. The medium that brings about this separation of the sliding surfaces should offer minimum friction and is termed as ‘Lubricant’. Therefore, while designing a machine with sliding parts, arrangement should be made to produce and maintain a thin film of lubricant between the sliding surfaces. The thickness of the film varies with the operating conditions and will depend on factors like load on the bearing, the viscosity of oil and the operating pressure of the oil.
In order to understand the mechanism of how such thin film of oil is formed between the sliding metal surfaces and what makes it to remain as such even when subjected to heavy loads, let us consider a simple shaft revolving within a bearing. Since an oil film has a certain thickness and it covers the rubbing surfaces completely, the inside diameter of the bearing should be greater than the diameter of the journal by at least twice the thickness of the oil film. When the shaft is at rest, it makes a contact with the bearing at its lowest point and the clearance will be greatest at the point diametrically opposite.
As the shaft begins to revolve in the direction shown in the sketch below, it tends to climb up the bearing and moves slightly to the left as shown. The shaft will tend to climb and move to right if it rotates in reverse direction. The oil under the clearance space on left will be squeezed and, if the medium is water, the shaft will make contact with the bearing. Since oil is used as lubricating medium, a thin film of oil will be formed under the most heavily loaded part of the bearing indicated by X. The oil has good adhesion properties to the metal surfaces and is drawn between the metal surfaces at the most heavily loaded part X forming a film the thickness of which depends upon various factors like the load on the shaft, the speed of the shaft and the viscosity of oil. Thus the motion of the shaft in the bearing acts as pump and produces its own pressure; the pressure in the oil that reaches the bearing only ensures that the oil reaches the bearing in sufficient quantity.
All the fluids, including gases, act as lubricants under suitable conditions of speed, load and bearing clearances. Air will act a suitable lubricant for spindles running at very high speeds. Thus higher the velocity of the rubbing surfaces, lower is the viscosity of the lubricating medium. This is the reason lightest of the lubricating oils are also called spindle oils.
Boundary Lubrication Explained
From the above discussion it is clear that the friction between the shaft and the bearings depends on the oil film thickness, which in turn depends upon the viscosity of the oil, speed of the shaft and load on the bearing. Therefore, the clearance space between the shaft and the bearing or the oil film thickness will reduce with any increase in load or with reduction in speed of the shaft. It may reduce so much that it is unable to keep the rubbing surfaces apart. Such a condition is called “Boundary Lubrication" and is useful in engine lubrication systems.
Under the boundary lubrication, the friction between the rubbing surfaces increases several fold compared to the friction under “Fluid Film Lubrication". Under such a condition, the friction is independent of the viscosity of the oil or speed of the shaft but will depend upon the load on the bearing and the property called the ‘oiliness’ of the lubricant, which is the tendency of the oil molecules to stick to metal surface.
Apart from the above two types of lubrication, intermediate states of lubrication are possible depending on the load on the bearing, the speed and the form of motion. Best example of this is reciprocating motion of piston rings inside the cylinder in which the formation of oil film between the rubbing surfaces is difficult. Pattern of lubrication of gear drives and chain drive for camshaft is more complicated to understand.
The engine bearings operate under “Fluid Film Lubrication" under normal conditions. However, as the engine of the speed is reduced, the pumping action of journals and crankpins too reduces making the oil film between the rubbing surfaces of the bearing thinner and, when the engine is about to stop, the lubrication reaches the state of “Boundary Lubrication". At this stage, the lubrication is dependent on the “oiliness" of the oil and not on its viscosity. It is the quality of “oiliness" which can save the bearing from excessive wear under these conditions of engine operation.
The above statement explains the importance of using good quality lubricating oil in an engine. Under the conditions of boundary lubrication, the friction produced by the fatty oils and fatty acids is about one third the friction produced by straight mineral oil. Under fluid film lubrication conditions, the distance between the rubbing surfaces (bearing clearances) depends upon the viscosity of the lubricant which may be same for both, the straight mineral oils as well as the fatty oils and fatty acids. Therefore, use of oils compounded with both fatty and mineral oils will prove advantageous under all operating conditions of a diesel engine.
From the above, it may be expected that the lubrication of crosshead guides, camshaft driving chains and gear wheels etc. where boundary lubrication conditions may exist, use of compounded oils would be preferred. However, the use of straight mineral oils has been found to be equally effective without any appreciable wear in the parts lubricated.
For fluid film lubrication, the viscosity of the oil plays an important role. The oil film thickness or the distance rubbing surfaces are kept apart is dependant on the viscosity of the lubricating oil. In a properly adjusted bearing, the shaft floats within the bearing while rotating, preventing any metallic contact and, only frictional resistance to be overcome by the shaft is the resistance of the oil film to shear and no wear should occur under such ideal conditions. If at all any wear takes place, it could be due to the viscosity of the oil being too low or due to insufficient bearing clearances.