LUBRICATING GREASE

E. F. Jones*

Grease is often thought of as the poor cousin of lubricants. In this series of articles we hope to make some progress in dispelling this idea. The series will include articles on the manufacture of grease; its composition and additives; the usefulness of laboratory tests; grease lubrication of bearings; lubrication in strange environments; and new developments in grease lubrication.

WHAT IS GREASE?

In general terms a lubricating grease may be defined as ‘a liquid lubricant thickened to form a solid or semi-fluid pro- duct by means of a thickening agent.’ The thickening agent most commonly used is a metallic soap although modified clays and a whole range of organic products may be used. The liquid lubricant used in the vast majority of greases is mineral oil although once again other lubricants such as animal or vegetable oils and synthetic esters are used in order to impart specific properties to the finished grease.

Since soaps play such an important part in the manufacture of greases it is desirable to explain what is meant by the term soap. When a mineral acid reacts with an alkali, a salt is formed. When a fatty acid reacts with an alkali the salt pro- duced is called a soap. This can be illustrated by the reac- tion between stearic acid and sodium hydroxide which takes place in the preparation of toilet soap.

C,,H,, COGH + NaOH + C,,H,s COONa i. Ha0

stearic sodium sodium water acid hydroxide stearate

When a soap is dispersed in a lubricant under certain con- ditions a grease is formed. By using different alkalis and different fatty acids a whole range of soaps can be produced. The most common soaps in use for the manufacture of lubri- cating greases in the United Kingdom are calcium, sodium, lithium and to a lesser extent aluminium. The greases form- ed from these soaps are referred to as say calcium soap based grease or sodium soap based grease.

Greases vary considerably in consistency. Consistency is the term used to describe the ‘thickness’ of a grease. In much the same way as lubricating oils are classified by means of viscosity in the SAE system, so the consistency of a grease iS classified by means of penetration in the NLGI system (National Lubricating Grease Institute of America). Penetra- tion is measured by means of determining the depth to which a standard cone penetrates a grease at 25°C during a period of five seconds. A later article in this series will be dealing with testing and it is not therefore proposed to go into this test in any detail at this stage.

The NLGI classification is as follows:

NLGI number

000 00 0 1 2 3 4 5 6

g;yOpenetration

445-475 400-430 355-385 310-340 265-295 220-250 175-205 130-160

85-115

* Research Scientist, Esso Research Centre, Abingdon, Berkshire

AS cati be seen, the products with a high penetration are classified as 0 grades and the products with a low penetra- tion are classified by a higher number on the NLGI scale. Examples of the 0 grades are semi-fluid greases, of number 2 grades are the automotive-hub greases and of number 6 grades are the hard block-like greases.

Although the type of lubricant that has been thickened natural- ly affects the rheological properties of the grease, the consis- tency of a grease at ambient temperature is determined very largely by the amount of soap present. Soft greases can be made with high viscosity lubricants and vice versa. Semi- fluidgreases are often made from high viscosity oils and block greases are sometimes made from low viscosity oils. Nevertheless, the base oil characteristics are of importance. Greases do not behave as Newtonian liquids. As the rate of shear is increased so the grease approaches the viscosity of the lubricant from which it was made. The particular appli- cation for which a grease is being considered therefore plays a part in deciding the viscosity of the base lubricant.

PROPERTIES REQUIRED BY A GREASE

The essential property that a grease must possess is that of forming a film of lubricant between surfaces which will minimise, as far as possible, surface to surface contact. In order to do this successfully, under certain circumstances, it must also possess other properties. These properties which are nowadays often taken for granted can be illustra- ted by considering their use in an application where con- ditions for the lubricant are relatively severe and varied. Such an example is the steel industry. Grease is often the preferred lubricant for use in the majority of sections such as the rolling mills, blast furnace, sinter plant and melting shop. Most of the grease is used for the lubrication of plain and anti-friction bearings but throughout the works grease is also to be found in linkages and gear boxes.

In many instances the bearings are subjected to heavy, con- tinuous and sometimes shock loadings. The film must there- fore sometimes possess a high resistance to breakdown.

The surrounding atmosphere may be heavily contaminated with water, scale and abrasive particles. Figure 1 illustra- tes this point by showing a steel mill in which wet conditions are caused by high pressure descaling of white-hot plate. The grease must therefore also act as a bearing seal and possess good anti-corrosion properties. The effect of cor- rosion on a ball bearing is shown in Fig 2. The surfaces have become pitted causing rough running which would re- sult in eventual failure.

Throughout the works, temperatures fluctuate greatly. In the hot rolling mills high temperatures will be experienced and in other applications cold temperatures may be experienced in winter. The grease must therefore be able to operate in cold conditions and resist temperature rises without becom- ing softened in the process.

To achieve the most economic form of handling and to elim- inate the possibility of contamination or the use of +he wrong grease, modern practice is to use centralised lubri- cation systems. Further requirements are therefore im- posed on the grease. Mechanical lubricators are used to

TRIBOLOGY August 1988 161

feed long lines which may contain as many as 1,000 grease points. The grease must be capable of being pumped long distances and remaining stable under various pressures and temperatures in the lines.

Greases therefore have to possess certain properties in or- der to meet a considerable number of exacting requirements. Should the grease fail to meet any of these, a shutdown could result in a critical section of the works. At first glance it would appear that many greases would be required. The fact that the vast majority of applications in a works can nowa- days be met by only two or three greases is a tribute to the progress that has been made in grease technology.

TYPES OF GREASE AND THEIR CHARACTERISTICS

During the consideration of the requirements that greases must possess for use as satisfactory lubricants it was ap- parent that water resistance and the ability to withstand high temperatures were of importance. These properties are imparted to a grease very largely by the type of soap used to thicken the grease. The properties of the simple soap greases discussed so far can be tabulated as follows:

Soap base

Appearance

Calcium (lime) Sodium

Smooth Usually fibrous

Lithium

Smooth

Water resistance

Good Poor Good

Drop point 95°C (203°F) 150°C (302°F) 180°C (356°F) (change to the fluid state)

As can be seen, calcium-soap based greases have good water resistance but poor ability to withstand high temperatures. This is due to the fact that calcium greases are stabilised by the presence of a small amount of water. As the temperature approaches 100°C so this water is driven from the grease and eventually breakdown of the grease will occur yielding soap and oil. These greases are however very extensively used where conditions require a grease to have good water resistance and where temperatures during use are not high.

Sodium and lithium soap based greases do not require water for stabilisation and can therefore be used at very much higher temperatures. Sodium soap is soluble in water and the soda-soap based greases should not therefore be used where the washing action of water could cause the grease to form an emulsion. They do however possess good natural anti- corrosion properties and have been used successfully in humid atmospheres.

Lithium-soap based greases possess good water resistant properties and together with the ability to withstand relative- ly high temperatures and an excellent stability have gained a high degree of popularity of multi-purpose greases. They are now used widely for automotive as well as industrial pur- poses and have replaced the necessity for several different types of grease to be used and stocked.

So far only the conventional or simple soap based greases have been discussed. Other types of soap greases may be encountered. By mixing different soaps, some of the proper- ties of each type can be imparted to a grease. These greases are known as mixed soap based greases. For ex- ample a calcium grease thickened with some sodium soap would possess some of the high temperature properties of the sodium soap grease and some of the water resistance of the calcium grease. The mixtures do not exhibit to the same extent the most desirable characteristics of either of the single soap greases used.

As has been mentioned calcium soaps are stabilised by the presence of water which restricts their use for applications where high temperatures are encountered. If the water is replaced by a salt of a low molecular weight acid, such as calcium acetate, then the resultant grease will have very much improved high temperature properties. By this means the drop point of the grease can be raised to 250°C. Other pro- perties such as load carrying are very often improved at the same time and the natural water resistance of calcium

Fig 1 A steel mill where wet conditions are caused by high pressure descaling of white-hot plate

Fig 2 The effects of corrosion on a ball bearing

greases is retained. Such products are known as ‘complex greases’ and should not be confused with the normal or con- ventional calcium greases. A further type of complex grease is the aluminium complex grease. An example is the alu- minium benzoate stearate grease which has a drop point of about 250°C.

Before leaving soap based greases some mention must be made of the products in which lubricants other than mineral oils have been thickened. One example of this ty-pe of pro- duct is the way in which soaps have been used to thicken or-

162 TRIBOLGGY August 1968

ganic di-esters. Because of the good low temperature pro- perties of organic di-esters the greases made also have very good low temperature properties. Lithium soap based or- ganic di-ester greases have been used successfully for many years at operating temperatures within the range of -50°C to 120°C. Soaps have also been used to produce greases from vegetable oils. An example of this type of pro- duct is the soap based grease made using castor oil. The advantage of this type of product is that the grease posses- ses good resistance to the washing action of petroleum pro- ducts.

For most conditions soap based greases are able to provide adequate lubrication. Of recent years however, conditions in certain fields have become increasingly severe and special- ised, particularly in relation to high temperatures. This has resulted in the production of the ‘non-soap’ type of grease. Thickeners such as carbon black, finely divided silica and modified clays have been used in order to produce greases. Probably the best known of these is the grease made from an organophilic clay named ‘bentone’, which gives rise to the popularly termed ‘bentonite’ greases. When a soap is heated in mineral oil a point will eventually be reached where the soap becomes partially soluble in the oil. At this point a soap based grease would tend to become fluid. When bentone is used as thickener this will not happen and hence the product when heated will not reach a point at which it will flow. For this reason the drop point of a ben- tonite grease cannot be determined. A point will obviously be reached when the lubricant or the treated clay which has been used will start to break down and non-soap thickened products do therefore have a temperature limitation.

The major types of lubricating greases have now been covered. Their general characteristics can be summarised as in Table 1.

Table 1 General characteristics of the major types of lubricating grease.

Tee Calcium soap

Appear - ante

Smooth

Drop Water Relative point resistance cost

95°C (203°F) Good Least expensive

150°C (3 02°F) Poor I Sodium Fibrous soap

Lithium Smooth soap

Calcium Smooth complex soap

180°C (356°F) Good

250°C (482°F) Good

may be impractical and under these circumstances it is usual to keep the new grease to the same soap type as that already in use.

Incompatibility may not necessarily be a problem. If the grease in use is not working near its limitations a slight lowering in one or other of the properties resulting from mixing may not prove to be a problem. The performance of the mixture may still prove to be perfectly adequate for the particular application. It is therefore difficult to generalise and each case should be considered on its own merits.

GREASE VERSUS OIL

As yet no direct comparison has been made between lubri- catinn oil and crease as a lubricant. The major advantage of ” 0~

oil lies in the fact that it is a very much better heat trans- fer medium. For this reason oil is generally considered to be the better lubricant. However, because of its flow proper- ties expensive sealing devices would often be necessary. The use of grease will overcome this problem. Dusty conditions may also exist in which grease will not only act as lubricant but also serve as an effective seal to prevent the ingress of contaminants. Grease is less easily thrown from machinery and the risk of contamination by lubricant of the article being manufactured is therefore reduced. This property is illustrated by Fig 3 which shows the ‘wet-end’ of a large box-board-making mill in which the bearings are grease lubricated.

Clay Smooth Not Good Most determinable expensive

Fig 3 The ‘wet end’ of a large box-board-making mill

Table 1 can only be used as a guide. Some sodium-soap based greases have a smooth texture and also drop points around 250°C. It should also be appreciated that the drop point, which can be considered as the change to the fluid state is not the maximum temperature at which a grease can be used. This is many degrees below the drop point and will depend on the individual grease. This matter will be dealt with in a later article in this series.

It is convenient at this stage to consider the problems as- sociated with the mixing of greases in practice. When two greases are mixed it often happens that the properties of mixture lie in the middle of the properties possessed by the individual greases. If however the properties of a product obtained by mixing two greases fall outside the properties ex- pected from a knowledge of the individual properties then the greases are considered to be incompatible. It happens some- times that if a sodium soap based grease is mixed with a lithium soap based grease the drop point of the resultant mixture is depressed considerably below the expected drop point. Other properties such as penetration could well be found to be intermediate. Compatibility can also be affected by the proportions in which the greases are mixed.

It is usual when changing from one grease to another to re- move the old grease as completely as possible. This, however,

SUMMARY OF GREASE PROPERTIES

The necessary properties possessed by greases can now be summarised as follows:

1 The ability to form a film of lubricant which is not easily removed and which may be required to resist shock loadings

2 The ability to form a large mass which will not be easily removed, enabling a seal to be made which will prevent contamination by water, scale and dust

3 The ability to prevent corrosion

4 The ability to resist temperature rises without becoming softened unduly in the process

5 The ability to withstand shear without breakdown of the structure

The way in which the thickener and the lubricant affect some of the above properties has now been briefly covered. The other properties possessed by greases are imparted by the method of manufacture and the additives which are incorpor- ated. This aspect will be covered in the next article in this series.

TRIBOLGGY August 1968 163