Lubricants and lubrication systems

Lubricants and lubrication systems

Purpose and action of lubricants

A lubricant separates the sliding and rolling surfaces so that there is little or no metal to metal contact. In effect, it forms a cushion

between the surfaces and if properly chosen it will keep the surfaces apart even when squeezed by high contact pressures. As the surfaces move over each other the lubricant is subjected to shearing

and offers some resistance to the motion. This, however, is far less than that when the surfaces are in dry contact. In the case of liquid lubricants such as oils, the resistance depends on a property

known as dynamic viscosity. This will be described in more detail in Chapter 5. It is a measure of the internal resistance of the liquid to being stirred, poured or, in the case of a lubricant, to being sheared. The SI units of dynamic viscosity are Nsm 2 and some typical values for different grades of lubricating oil are given in Table 3.1.

A lubricant fulfils purposes other than reducing frictional resistance and wear. A steady flow of lubricant can carry away heat energy and solid particles from the contact area. This is an important function of the cutting lubricants used in the machining of metal components. They reduce the friction between the cutting tool face and the metal being removed and they also carry away the heat energy and the metal swarf which is produced by the cutting tool Another function that a lubricant performs is to prevent corrosion which might occur from the presence of moisture or steam. It

is important that a lubricant has good adhesion with the contact surfaces and does not drain away when they are at rest. It is then able to protect them from corrosion and maintain surface separation

in readiness for start-up. A lubricant must not react with any other substances in the working environment. Its function is alsoto prevent contamination of the contact surfaces and prevent the

ingress of solid particles that would damage them

Lubricant types and applications

Lubricants may be broadly divided into mineral oils, synthetic oils vegetable oils, greases, solid lubricants and compressed gas. Mineral oils and greases are perhaps the most widely used but each has its own particular characteristics and field of application. We will now examine some of these

Mineral oils

Mineral oils are hydrocarbons. That is to say that they are principally made up of complex molecules composed of hydrogen and carbon. The chemists who specialise in this field refer to the

hydrocarbon types as paraffins, naphthenes and aromatics. Mineral oils for different applications contain different proportions of these compounds. This affects their viscosity and the way that it changes under the effects of temperature and pressure

Traditionally, lubricating oils have been grouped according to their different uses and viscosity values as shown in Table 3.1. Within the different types shown in Table 3.1, lubricating oils are graded according to the change in viscosity which takes place with a rise in temperature. This is indicated by the viscosity index number that has been allocated to the oil. It can range from zero to over one hundred as shown in Table 3.2. The higher the value, the less is the effect of temperature rise. A value of 100+indicates that the viscosity of the oil is not much affected by temperature change

Table 3.3 shows the typical viscosity and viscosity index values for the different types of lubricating together with the likely percentage composition of paraffins, naphthenes and aromatic hydrocarbon

molecules which might be expected. You might notice that within each group, the high viscosity index oils, which are least affected by temperature change, have the highest percentages

of paraffin compounds. Also within each group, the oils with the lowest viscosity have the highest percentages of aromatic compounds. It might thus be stated as a general rule, that aromatic compounds lower the viscosity of an oil whilst paraffin compounds help it to retain its viscosity as the temperature rises There are other ways of measuring viscosity and if you go to buy oil for a motor car or motor cycle, the service manual might recommend SAE 10W-40 lubricating oil for the engine and gearbox. The letters stand for the Society of Automotive Engineers of America and the numbers indicate the viscosity and a measure of its variation with temperature. The system is widely used by the motor industry, but is not entirely suitable for other industrial applications where wider ranges of working temperatures, pressures, running speeds and service environments are to be found

Additives and synthetic oils

Plain mineral oils are suitable for applications such as the lubrication of bearings, gears and slide-ways where the operating temperature is relatively low and the service environment does not contain substances that will readily contaminate or degrade the lubricant. Typical contaminants are air (which is unavoidable), ammonia, water, oil of another grade, soot, dust and wear particles from the lubricated components. Special chemicals are often added to plain mineral oils to improve their properties and prolong their life. Table 3.4 lists some of the more common additive types

Polymers are sometimes added to mineral oils to enhance their properties. These are often referred to as synthetic oils although it is the mineral oil that forms the bulk of the lubricant. Various types of ester, silicone and other polymers are added, mainly to increase the viscosity index and wear resistance of the oil

Some of the more expensive multi-grade motor oils contain polymers. They appear to be much thinner than the less expensive engine oils at normal temperature but undergo a much smaller

change of viscosity as the temperature rises. This makes cold starting easier and gives improved circulation of the lubricant during the warming-up period. At running temperatures their  viscosity is similar to that of the less expensive multi-grades but they are purported to have better wear resistant properties