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Interesting facts about Oil?
Interesting facts about Oil?
Primary goal of lubrication is to minimize friction as friction generates heat, promotes wear, wastes power and increases maintenance cost.
The term minimize is used because friction cannot be eliminated.
Friction – Two Types
There are two types of friction; solid friction and fluid friction.
First we will consider solid friction.
When two surfaces, such as two metal blocks, move against each other, they experience friction. The rougher the surface of the metal blocks, the greater the friction. This is called solid friction. In fact if you look at the metal surface microscopically, you will see metal peaks rather than a smooth surface. These peaks are called asperities
It is estimated that due to this surface roughness, only around 20% of the surface between two metal blocks are in contact.
What is the effect of solid friction? It can cause extremely high pressures and temperatures to build up, such that the contacting high points of the surface asperities actually weld together and then tear apart. This surface damage is called “wear”.
In order to reduce the friction between the metal blocks, we need to introduce a substance of lower friction which separates the blocks. The substance used can include air, water, mineral oil, grease, soft metal, fats, waxes, etc. and is referred to as a lubricant. The lubricant forms a film between the two surfaces and allows them to slide easily over one another, without the surfaces touching.
What do we mean by the term fluid friction? This is friction within the lubricant itself. This is due to the particles/molecules rubbing against each other and, so, creates what is termed internal friction or fluid friction. It is a measure of the fluid’s internal resistance to flow.
Why is Friction Reduced with Lubricants
This diagram depicts the fluid molecules as balls rolling over each other. If you were to stand on a mass of ball bearings you would soon understand the concept that is being proposed here.
Friction & Viscosity
One way of minimizing friction is to have an oil of suitable thickness to separate the metal surfaces.
Viscosity is a key term which represents the thickness of an oil and it provides a measure of an oils internal resistance to flow. The higher the internal resistance to flow, the higher the viscosity.
To give you an example: What has a higher viscosity at room temperature, water or honey? Honey – it is thicker and so more viscous.
Note that the question specified the temperature conditions, because at 0oC water would be substantially thicker.
Measuring Viscosity
Viscosity is measured by two primary methods. You can either measure the resistance to turning an object in the oil – called dynamic viscosity or measure the rate of flow of the oil down a tube called Kinematic viscosity.
Rotational viscometers measure dynamic viscosity.
A rotor is immersed in the oil. The torque required to rotate it at a given speed, or the speed achieved for a given torque is measured. This provides an indication of an oils viscosity. The thicker the oil, the greater the torque required to turn the rotor or the slower the rotor turns.
The Kinematic viscometer is normally used to measure viscosity at 40°C and 100°C. The viscosity is measured by timing an oil to pass two points of a narrow capillary tube. This time is then multiplied by a constant (based on the tube size) to give the viscosity.
Effect of Temperature on Viscosity
The one most important variable affecting viscosity is temperature.
When an oil is heated up, it thins out and when an oil is cooled down, it thickens up.
3 Factors Influencing Viscosity Selection
There are essentially three factors that influence the choice of viscosity. They are the temperature that the oil is subjected to; the load or pressure on the oil: the speed of the application.
Temperature v Viscosity Selection
The ambient and operating temperature of an application will affect the choice of oil viscosity.
The picture shows clearly the difference in operating temperature of a motor, a coupling and a driven component. Looking at the high temperature that the motor is operating at you would think that it needs a high viscosity oil – but temperature alone does not tell the full story. There is also load and speed to consider.
Load v Viscosity Selection
The load refers to the weight or pressure on the surfaces. Effective lubrication means being able to separate the load carrying surfaces and, if the load changes, then the optimum viscosity of the oil required to separate the surfaces may change.
If the load is too high, the oil film may be squeezed too thin to protect the metal surfaces from making contact. This results in solid friction, meaning an increase in heat (and wear). Because of the relationship between temperature and viscosity, the oil will thin out, resulting in an increase in solid friction, resulting in more and more heat generated and ultimately machine failure.
Speed v Viscosity Choice
The faster a shaft rotates in a bearing, the thicker the developing oil wedge will become as oil is readily dragged into the area between the metal surfaces. Therefore, for high speed applications, a low viscosity oil is required.
Conversely, for low speed applications, a high viscosity oil is required to maintain the oil wedge and separate the surfaces.
Factors Effecting Film Thickness
The relationship between film thickness and viscosity, speed and load is given by the equation:
Film Thickness = Viscosity x Speed
Load
Viscosity is related to temperature so the temperature part of the equation is captured within the viscosity measurement.
Effect of Viscosity/Speed/Load on Friction
This relationship can also be represented by this chart called a Stribeck curve. It provides a relationship between friction and viscosity, speed and load for an oil in a plain bearing.
Viscosity Index – An Oil Viscosity Rate of Change
We know that oils change viscosity with change in temperature. This rate of change is called Viscosity Index. As the rate of change in viscosity with temperature is different between oils, we need to know this value in order to compare oils and choose suitable oil. The higher the number, the less the viscosity change. A typical number for an oil is around 100.
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