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Lubrication
Lubrication in bearings
We can apply a lubricant to separate the peaks and valleys in surfaces from each other. In this way we reduce friction between the surfaces.
a. In this process or lubrication, the properties of adhesion and cohesion in the lubricant play an important part in the formation of a suitably thick oil film. Let us consider these properties in more detail:
i. Adhesion
Adhesion is the ability of one material or substance to cling to another. For example, oil adheres strongly to steel, while water does not adhere to steel at all. So the adhesive strength of oil to steel is higher that the adhesive strength of water to steel.
ii. Cohesion
Cohesion is the force in a substance or material which holds or binds it together. For example, thick grease has a much higher cohesive strength than oil or water. To see how the properties of adhesion and cohesion affect lubrication, let us consider figures 1, 2 and 2, which shows the action taking place when two materials slide across each other, with lubricant between them.
Look at figure 1 in the previous page, which shows two metal surfaces which are about to have a load applied to them. Now look at figure 2 on the previous page which shows that when we apply a lubricant to the two surfaces, layers of oil molecules labeled as layers 1 and 4 in the figure, will cling to the two metal surfaces because of the strong adhesion of the lubricant to the other material. Now look at figure 3, which shows that the low cohesive strength of the lubricant allows the middle layers of molecules, that is later 2 and 3 in the lubricant to move easily against one another or slide away from one another.
b. Methods of lubrication in bearings
We shall discuss types of lubricants later in this
Fig 1 Two surfaces about to have a load appliedFig 2 Oil molecules in layers 1 and 4 cling to the surfacesFig 3 Oil molecules in layers 2 and 3 moves easily against each other
learning unit, but regardless of the type of lubricant we use, there are basically three methods of lubrication in bearings. In order of effectiveness, they are;
■ Hydrostatic lubrication
■ Hydrodynamic lubrication
■ Boundary lubrication. Let us consider each in more detail:
i. Hydrostatic lubrication
This is the most effective method of lubrication. We use it when reliable and prolonged functioning of the bearing is vital. The advantages of this type of lubrication justify the expense.
The success of this method depends on a continuous supply of the lubricant, which an external source such as a pump, provides under pressure. The pump continuously forces the oil at high pressure between the surfaces. With hydrostatic lubrication, the lubricant is introduced at a point where the pressure is highest. Look at figure 4 on the previous page, which shows the working of a journal bearing and the areas of high and low pressure.
We use hydrostatic lubrication in steam turbines, ship engines, motor engines and machinery requiring a constant supply of oil.
ii. Hydrodynamic Lubrication
With hydrodynamic lubrication, the adhesive and cohesive strength of lubricants contribute to the building up of hydrodynamic pressure in the lubricant. No external source such as oil pump is necessary.
In contract to hydrostatic lubrication, with hydrodynamic lubrication the lubricant is introduced at a point where pressure is lowest. Look again at figure 4 which shows low pressure area. Hydrodynamic pressure builds up in the oil film as a result of the movement of the work pieces relative to each other. There must be sufficient clearance between the surfaces into which the lubricant can be drawn. When the shaft starts rotating, the lubricant will be drawn into the existing wedge-shaped clearance. As this wedge shape becomes narrower, the pressure in the oil increases, and this way the oil separates the two friction surfaces from each other.
This process happens in four stages. Let is discuss these. Look at figure 5, which shows the four stages.
Fig 4 Rotating shaft in a bearing with areas of high and low pressure
- They cool the bearings surfaces
- They wash out grit and other impurities.
- They prevent the bearings from rusting.
- They act as a seal to keep impurities away from bearings.
An effective lubricant is one which will provide a lasting oil film between the friction surfaces, and which will not fail when pressure is applied to the bearing or when temperature changes occur.
1.3.1 Choice of lubricant
We should consider the following factors when choosing a lubricant.
- The temperature range to which we will expose when choosing a lubricant.
- The tolerances between the friction surfaces.
- The environment and conditions under which the lubricant is to function.
1.3.2 Classification of lubricants
We may classify lubricants into the following three groups. These are:
- Liquid lubricants
- Semi-solid lubricants
- Solid lubricants
We shall consider each more closely:
a. Liquid lubricants
Liquid lubricants for example oil have different grades and viscosities. They are particularly suitable for high speeds and light loads.
b. Semi-solid lubricants
We use semi-solid lubricants for example grease in moderately slow and slow moving bearings. The viscosity of grease is an advantage in dusty conditions since it enables grease to form a seal, which prevents the entry of dirt or impurities between the friction surfaces. Grease is commonly used in rolling bearings since it is able to form a lubricating and protective film, which prevents the lubricant from escaping and dirt or impurities from entering.
c. Solid lubricants
We can mix solid lubricants for example white lead, talc and mica with lubricants such as oil or grease to improve the properties of these lubricants. Graphite is one of the most common of the solid lubricants, because it has excellent frictional properties.
1.3.3 Methods of lubrication and lubricating devices in general
We use lubricating devises to provide a regulated quantity of a lubricant to machine parts. There are five genera methods of lubrication. These are:
- Gravity-feed lubrication
- Grease lubrication
- Splash lubrication
- Forced lubrication
- Pressure-feed lubrication
Let us consider each in further detail:
a. Gravity- feed lubrication
The four most widely used gravity-feed methods are the needle lubricator, the siphon wick lubricator; the sight feed lubricator and the mechanically operated sight-feed lubricator. We shall look more closely at each of these.
i. The needle lubricator
The needle lubricator consists of a glass container filled with oil, mounted upside with a stopper or plug inserted in the neck. Look at figure 6 which shows this process.
A thin needle is pushed through the plug. It is long enough for one end to rest in the container and the other end to rest on the moving shaft. When the shaft starts turning, the vibrations cause the oil to flow down the needle onto the shaft. As soon as the shaft stops rotating, the downward flow of the oil stops. The amount of oil, which flows down, is decided by the viscosity of the oil, the intensity of the vibration and the tolerance between the needle and the opening in the plug through which it passes.
ii. The siphon-wick lubricator
The siphon-wick lubricator consists of a central tube sticking out above the oil in the container. Look at figure 7 on the next page, which shows this.
Fig 6 Needle lubricator
i. The screw gap grease cup or Stauffer grease cup
This grease cup has a fluted cap, which is screwed down once the cap has been filled with grease. Look at figure 9 which shows this.
When the cap is screwed down, if forces the grease through the hole to the friction surfaces which need lubrication. To replenish the grease, we unscrew the cap and refill the cup with grease.
ii. The tell-tale or spring loaded grease cup
This grease cup has a spring loaded piston in the cap, which is pushed up when we fill the cup with grease and screw the cap on. Look at figure 10, which shows this.
The piston pushes the grease down through the hole and onto the friction surfaces. The knob mounted at the top of the piston rod shows when we need to refill the grease cup. This is why this is called the tell-tale grease cup.
c. Splash lubrication
Splash lubrication happens when the lubricant in splashed onto the moving part or friction surfaces. Many forms of splash lubrication are used in industry. Two examples are ring lubrication and cylinder wall lubrication. Let us look at these more closely:
i. Ring lubrication
The rings are loose and roll on the shaft when it rotates. The lower part of the rings is dipped into an oil sump. The oil clings to the rings and then the revolving actions of the rings throws it onto the shaft. The oil moves along the shaft to the bearings. Look at figure 11, which shows the process of ring lubrication.
Fig 9 Screw-cap grease cup or Stauffer grease cup
Fig 10 Tell-tale or spring-loaded grease cup
ii. Cylinder wall lubrication With cylinder wall lubrication, oil is splashed onto the different moving parts. The movement of the piston rod through the oil causes an oil vapour which fills the inside of the engine. In this way the various parts are lubricated. Look at figure 12, which shows cylinder wall lubrication.
d. Forced lubrication
With forced lubrication, we sue hand oil pumps or hand grease pumps. These pumps force oil or grease into holes or nipples and onto the friction surfaces.
e. Pressure-feed lubrication
Pressure-feed lubrication refers to lubrication which we apply with oil pumps like the oil pump of a motor car, which distributes oil under pressure to all parts of the engine.
Fig 11 Ring lubricationFig 12 Cylinder-wall lubrication
