A FEW WORDS ABOUT LUBRICANTS

We all know that a drop of oil or a little dab of grease can make wonders when things jam, run booly or squeak. Suddenly things run smoothly again and the noise stops. If we have chosen the correct lubricant and continue to supply it in a suitable way and at the right time, we will not have any problems with jams or noises in the future.

Lubricants, primarily, reduce the friction between the machine elements which move relative to each other and therefore decrease the wear. They are thus a basis for the functioning of our society with all its different types of machines.

Tribology is the name of the science concerning friction, lubrication and wear. Below we will give a short description of the two most common lubricants for industrial use, vehicles and mobile machines: oils and greases.

OILS

Oils should function in different machines, for different purposes, in different industries, in different vehicles and machines under different operating conditions concerning temperature, pressure, load, speed and material. In order to meet the large and differing demands, the oil industry has developed a multitude of oils for different functions, e.g. machine and engine oils, oils for transmission and circulation, hydraulic oils and oils for turbines and cylinders etc.

The oils are primarily classified according to their viscosity, which is a measurement of their resistance to movement. The viscosity is usually measured in centistokes, abbreviated cSt. One cSt = 1 mm2/S. The viscosity of an oil differs with the temperature, and the viscosity is therefore specified at certain temperatures, normally 40°C and 100°C.

The American SAE system (SAE = Society of Automotive Engineers) is used to classify engine and transmission oils for vehicles and machines. It comprises 10 different viscosity classes for engine oils and six different classes for transmission oils. Each viscosity class is designated with a number, wich for the thinnest grades is followed by the letter W. This letter indicates that the oil is suitable for winter use.

Since 1975 industrial oils are divided into 18 different viscosity classes in accordance with ISO 3448 (ISO = International Standard Organization). These classes are also documented by SIS 155441 (SIS=Sveriges Standardiserings Kommission).

Each viscosity class is designated with ISO VG (viscosity grade) and a number that corresponds to the viscosity in mm2/S at 40°C (a small deviation for Nos. 2,3,5 and 7) within a viscosity range with ±10% deviation.

The figure below gives a good survey of the difference between the viscosity classes of SAE and ISO. Drawing a horizontal line from one class to the other makes it obvious that most of the SAE classes cover a much larger viscosity range than the corresponding ISO classes.

The SAE/ISO classifications cover only the viscosity grades and are independent of the qualities of the oils in other respects.

The other qualities of the oils are dependent on their chemical composition and which additives they contain in order to obtain specific qualities. Oils can be divided into mineral oils, vegetable oils and synthetic oils. There are also oils which are composed of one or more of these base oils with or without additives for different purposes.

GREASES

A grease can be defined as a solid, semi-solid or semi-fluid product, which is obtained when a thickening agent has been added to an oil. The thickening agent (often called a gel-maker) works as a sponge which, during operation, emits small quantities of the lubricating oil.

The “gel-maker” is usually a metallic soap, which is a combination of a metal and a fatty acid – usually animal or vegetable fat.

The difference between oil and grease is, first of all, the consistency. The consistency of the grease depends on the oil and its viscosity, the thickening agent and the quantities of these that the grease contains. The consistency is also dependent on the temperature to some extent. In order to classify the consistency a special method is used, in accordance with ASTM D 217 (ASTM=American Society for Testing Materials) and corresponding to SIS 155111. The depth to which a specially designed cone sinks during 5 seconds in the homogenized grease sample at 25°C is measured. The consistency is specified as penetration in 1/10 mm. According to the National Lubricating Grease Institute in the USA the consistency of a grease is classified in the following NLGI numbers.

In order to suit the different fields of application, temperatures, pressures, water contacts and corrosion risks there are several types of grease. No international rules, other than those from NLGI, have been compiled. SKF, on the other hand, have drawn up important requirements and methods of testing. In Germany and, to some extent, other countries the grease classes classified and specified by DIN (Deutsche Industrie Normen) are used.

The most important functions of a grease are:
1. To lubricate at different temperatures and pressures.
2. To seal against dirt, dust and water.
3. To protect against corrosion.
4. To reduce vibration and noise.
5. To function for a long time with the same consistency.

The most common types of grease, when it comes to metal soaps, are: calcium, sodium and lithium greases.

CALCIUM GREASES

are water resistant but can only stand temperatures up to approx. 50°C.
They are used in wet installations such as paper machines and water turbines.

SODIUM GREASES

are used up to 120°C and are suitable for high bearing speeds. Sodium greases are not recommended for wet lubrication points, as large quantities of water decompose the sodium soap and thus the ability to lubricate disappears.

LITHIUM GREASES

have the same temperature properties as sodium greases and almost the same properties regarding water resistance as the calcium greases. They have also a high mechanical stability even at very high r.p.m. Most lithium greases are today used as universal greases, for vehicles and in industry.

SYNTHETIC GREASES

are made of different types of synthetic oils and usually thickened with lithium soaps.

They are used both at very low and high temperatures (from about 70°C to about +200°C). Their main applications are for aircraft, missiles and satellites but their use is rapidly increasing in other fields as their special qualities ensure reliable lubrication under extreme conditions. The synthetic lubricants are then well worth their price. Today, synthetic lubricants cost many times the price for common lubricants based on mineral oils.

Two important qualities of a grease are their ability to stand pressure and filtration stresses, which can cause the oil and thickening agent to separate. This is particularly important with greases, which are to be distributed by centralised lubrication systems. These expose the grease to filtration stresses more or less all the time. During pumping through the tubing system and distribution elements like distributors and dosers, the grease is under pressure for shorter or longer periods and is also forced to pass trough narrow passages. If the grease cannot cope with these stresses the oil and the thickening agent may separate. This can, in turn, cause plugging of channels and drilled holes in the distributors, dosers and bearings and even clog the tubing. The lubricant supply will then cease and if this is not quickly discovered by the monitoring and alarm systems, serious stoppages and break-downs might occur. It is therefore important that the lubrication systems are being installed and operated in such a way that the filtration stresses on the grease will be at a minimum. It is equally important to choose the correct grease and supply the installations with suitable monitoring and alarm systems.

When choosing a lubricant, one should always follow the machine manufacturer’s recommendations and ask for advice from the lubricant manufacturer’s experts. The quality demands should always provide the primary guidance and serve as a standard. In order to use the properties of the lubricants most efficiently and ensure proper lubrication at all times it goes without saying that the lubricants must be supplied in the correct quantity at the right time and in the most reliable way possible. Assalub has the right products and systems to do that. You will find them listed and described in this catalogue. Our experts are always at your disposal to recommend economic and reliable solutions of your lubrication problems.