Grease is most commonly used for the lubrication of roller bearings. Lubrication grease is defined as a semi fluid to solid dispersion of a thickener in a liquid (base oil). It consists of a mixture of up to 90% mineral or synthetic oil and a thickener. In almost 90% of all greases the thickener is a metallic soap, which is formed by a metal hydroxide in order to react with a carboxylic acid. An example of this is called lithiumstearate also called lithium soap. By varying in the proportions of soap, oil and additions, it is possible to produce different greases for a wide range of application.
The applied grease should have qualities that provide sufficient lubrication under the operating conditions, in which the bearing is applied.
The consistency, the degree of thickness of a grease, is mainly defined by the type and amount of thickener. The consistency is subdivided according to one by the American NLGI (National Lubricating Grease Institute) developed scale. This scale is based on the degree to which a standard cone drops in the grease in a time of 5 seconds measured in tenths of millimeters. The result is the WP-index (Worked Penetration, mechanical intrusion). The softer the grease, the less the NLGI-index. For roller bearings usually greases are recommended with a NLGI-index of 2 or 3. Greases with a NLGI-index of 0 or 1 can also occur in cases where the operating temperatures are very low or in which automatic lubrication systems are used.
Range of temperature
It is important to choose a grease that functions well at the operating temperatures of the bearing.
Types of grease and additives
In most lubricating greases calcium (Ca), sodium (Na), or lithium (Li) are applied as a thickening. Greases on the basis of lithium soap are very suitable for the lubrication of roller bearings.
This term is used for greases that contain both a salt as metal soap, usually of the same metal. Examples are lithium, sodium, barium and aluminum complexes. These greases are resistant to higher temperatures than the corresponding normal greases.
Calcium soap greases (Ca)
Calcium soap greases have a smooth, buttery texture and a good mechanical stability. They do not dissolve in water and may not be used at temperatures above 60° C. Calcium soap greases are recommended for installations that are exposed to water and where the temperature does not rise above 60° C such as the wet part of paper machines. Some calcium soap greases also offer good protection against salt water and are therefore used when contact with sea water occurs. Calcium soap greases that are stabilized with media other than water can be used at temperatures up to 120° C. An example of these are the calcium complex greases.
Sodium soap greases (Na)
Sodium soap greases, also called soda greases, can be used in a wider temperature range than normal calcium soap greases. These fats have good adhesive and sealing properties. They also have good anti-corrosive properties because they absorb water, though at the expense of the lubricating ability. When too much water penetrates into the bearing, there is a chance that the grease is flushed out. These greases can therefore not be used in “wet” applications. Synthetic sodium soap grease for high temperatures are resistant to temperatures up to 120° C.
Lithium soap greases (Li)
The structure of these greases is similar to that of calcium soap greases. They have the positive qualities of calcium and sodium soap greases and practically none of the disadvantages. They adhere well to metal surfaces. They are very stable at higher temperatures. Most lithium soap greases can be used over a wide temperature range. Lithium soap greases practically don’t solve in water. They can be used in “wet” applications, such as when the temperature is too high for calcium soap greases.
This group includes greases based on synthetic lubricating oils such as polyalfaolefinen (POA’s), esters and silicone, which are not as quickly oxidizing as mineral oils. Synthetic greases generally have a wider scope than other greases. There are various thickeners, including lithium soap, bentonite and PTFE. Synthetic greases are manufactured for different special applications that, for example, must meet military specifications, and for applications in, for example, instruments and operating systems, aircrafts, robots and satellites. These greases often keep a low frictional resistance at temperatures up to -70° C.
To lubricating greases different means are added to obtain additional qualities. Some of the most commonly used are: corrosion mediums that improve the protective effect of the grease. These additives are useful for applications of bearings in a wet environment. They protect the bearings also during transport and storage. Antioxidans slow down the degradation of the base oil at high temperatures. This allows prolongation of the lubrication periods and the costs can be limited. EP-additives (EP= Extreme Pressure) contain sulphur, chlorine and phosphorus compounds. (Some EP-additives are harmful to bearings so choosing must happen carefully.) Solid lubricants such as molybdenum disulphide and graphite.
Grease that is used in a bearing should always provide protection against corrosion. The anti-corrosion component should preferably be insoluble in water. The grease should demonstrate good adhesion so that steel surfaces are always covered with a layer of grease. This should also be the case if the grease is saturated with water.
Some greases tend to be softer under the influence of mechanical processing, which can lead to leakage. If the bearing is subject to vibrations, the grease is thrown away from the bearing house into the bearing. When the grease is mechanically not stable, it will be grinded in a continuous process by the bearing and worked out again. This leads to breaking down the soap structure and it destroys the grease. It is important to choose a grease with a good, mechanical stability.
A seal must both protect the bearing and the lubricant against the environment. There must not be any dirt particles or moisture penetrate into the bearing where they could cause harm. Seals must also ensure that the grease leaking is minimized. The effectiveness of a sealing is decisive for the lifetime of a bearing. It is important that the right kind of seal is chosen. Never continue using a bearing whose seal is damaged or in poor condition.
Never mix fats that do not fit together. When that does happen, the result is usually a softer mixture, which ultimately causes bearing failure caused by leakage. If you do not know with what type of grease the bearing was originally lubricated the old grease remainders must be completely removed from the bearing and the space around it before the bearing is lubricated again.
Classifying lubricating greases
On the basis of their suitability of temperature and operating conditions, lubricating greases can be subdivided in groups. The consistency and lubricating qualities of a grease are influenced by the operating temperature. A bearing that is running at a certain temperature must be lubricated with a grease that posseses the correct consistency and lubricating effect at the same temperature. Greases are manufactured in compositions for different operating temperatures and are classified as low temperature greases (LT), normal temperature greases (MT) or high temperature greases (HT). There is also a group of greases that are referred to as EP (Extreme Pressure) or EM (Extreme pressure Molybdenum disulphide, molybdenum disulphide); these contain additions that enhance the lubricating film.
Normal temperature greases (MT)
The so called “multi purpose” greases are recommended for bearings that run at temperatures from -30 to + 120° C. They can be used for the vast majority of applications that need grease lubrication. The base oil viscosity should be between 75 and 200 mm2/s at 40° C. The consistency is usually 2 (or 3) according to the NLGI scale. For applications that run consistently above 80° C a high temperature greases is recommended.
Greases with the indication EM contain molybdenum disulphide (MoS2) that produces a layer of MoS2 molecules on the metal surfaces that slide over each other. As a result, these are temporarily lubricated. Sometimes also graphite or similar solid lubricants are be used.
EP-greases contain sulfur, chlorine or phosphorus compounds. They have the property to strengthen the lubricating film, i.e. they increase the load-carrying capacity of the lubricating film. This is important when greases are used for heavily loaded medium and large bearings. When the contacts between the peaks of the metal contact surfaces of the bearing are hot enough, a chemical reaction occurs whereby scuffing of the two surfaces is prevented. The viscosity of the base oil is about 200 mm2/s at 40° C. The consistency is similar to NLGI class 2. Generally these greases cannot be used at temperatures below –30° C or above + 110° C.
Some EP-additives are harmful to bearings and can shorten its lifetime considerably. For applications that run constantly at temperatures above 80° C a high temperature grease is recommended.
Choosing a grease
All precautions to prevent bearing failure have little meaning if the wrong grease is chosen. It is important that the chosen grease contains a base oil with the right viscosity for a good lubrication at the prevailing operating temperature. The viscosity strongly depends on temperature; it decreases at increasing temperature and increases with decreasing temperature. It is therefore important to know the viscosity of the base oil at the operating temperature. Manufacturers of machines usually prescribe a particular type of grease. Most standard greases are suitable for a range of applications. The most important considerations in choosing a grease are: type of machine, bearing type and size, operating temperature, load conditions, including the functioning of the bearing, rotational speed, operating conditions (vibrations and deposition of the shaft horizontally or vertically), cooling conditions, effectiveness of the sealing, environmental conditions. Most users of bearings choose a series of greases, which are suitable for almost every application. There are greases developed for effective working in 95% of all bearing applications. LGHT-grease should be used when the temperature is above 80°C; LGLT-grease when the temperature is below 0° C. For bearings that are exposed to extremely high loads and bearings that rotate at very low rotational speeds, LGEM 2-grease can be used; medium-high loads require LGEP 2-grease, small to medium-sized bearings can be lubricated with LGMT 2, larger bearings with LGMT 3.
In the table on the following page you will find a choice of several bearing greases.
The life expectancy of grease depends on several factors such as the type of grease, the rotational speed and the operating temperature of the bearing. The environment and seals also play a role. At small ball bearings the lifetime is sometimes so long that there is no need to be relubricated. In such cases, “for life lubricated” bearings with protecting tags or seals, are a good choice.
A groove ball bearing with a diameter (d) 100 mm runs at 1000 rpm. The operating temperature ranges from 60 to 70° C. What is the correct lubrication interval? Follow the line from 1000 on the X-axis of the chart till this cuts the line for d= 100 mm. Now draw an imaginary horizontal line (parallel to the X-axis) from this point of intersection to the “scale a” (radial ball bearings) indicated column.
|BEARING GREASES||LGMT 1||LGMT 2||LGEP 2||LGLT 2||LGHQ 3||LGFP 2||LGWA 2|
|> 120° C High temperature||+||+|
|Very high speeds||O||O||O||+||O||O|
|Very low speeds or
|Low torque and friction||+||O||+||O||O||O|
|Moisture resistant properties||O||O||+||+||O||+||+|
|Description||All industrial and automotive applications||All industrial and auto-mobile applications||Extreme pressures||Low temperatures||High temperatures||Food
|Lithium soap /
|Lithium soap /
|Lithium soap /
|Lithium soap /
|Lithium complex soap /
|Aluminium complex /
medical white oil
|Viscosity of base oil||110||120-130||200||15||110||130||185|
|LGLC 2||LGMB 2||LGHB 2||LGEM 2||LGEV 2||LGWM 1|
|> 120° C High temperature||+|
|Very high speeds||+||–||O||–||–|
|Very low speeds or
|Low torque and friction||+||–||–||–||O|
|Moisture resistant properties||+||+||+||O||+||+|
|Description||Low temperature and high rotational speeds||High viscosity EP||High viscosity EP and
|High viscosity EP with
|Extreme high viscosity with
|Extreme pressures and
|Calcium complex soap / ester mineral oil||Lithium / calcium soap / mineral oil||Calcium sulfate complex / mineral oil||Lithium soap / mineral oil||Lithium / calcium soap / mineral oil||Lithium soap / mineral oil|
|Viscosity of base oil||24||780||400-450||500||1020||200|