Selecting the correct lubricant is a critical factor in ensuring the functional reliability and optimum operating life of a rolling bearing. Failure statistics show that a significant proportion of premature rolling bearing failures are directly or indirectly related to the lubricant used. The main causes of failure here are unsuitable lubricants (20%), aged lubricants (20%) and insufficient lubrication (15%).
A typical cylindrical roller bearing from Schaeffler.
Although lubricating oils (e.g. mineral oils and synthetic oils) are sometimes recommended for use with rolling bearings in extreme operating conditions (e.g. high temperatures or radiation), most bearing manufacturers recommend the use of greases. This article will therefore focus on selecting suitable greases for rolling bearings.
When selecting a suitable grease for a rolling bearing, a number of application-related factors need to be considered. These include the type of bearing, operating speed, temperature and load. Other factors such as mounting position, sealing, shock and vibration, and legal/environmental regulations may also need to be considered.
Grease Characteristics and Classification
The characteristics of grease fundamentally depend on the following three properties:
Base oil type & viscosity
The viscosity of the base oil is responsible for the formation of the lubricant film. As a base oil, mineral oils or synthetic oils are commonly used. It is important that synthetic oils are differentiated according to their type (polyalphaolefin, polyglycol, ester, fluoro oil, etc.), as these possess very different characteristics.
Typical thickeners used include metal soaps or metal complex soaps. Organic or polymer thickeners such as polycarbamide are becoming increasingly important.
All greases contain additives. A distinction is made between additives that have an effect on the oil itself (oxidation inhibitors, viscosity index improvers, detergents, etc.) and additives that have an effect on the bearing or the metal surface (e.g. anti-wear additives, corrosion inhibitors, friction value modifiers).
Greases are classified in terms of their principal components: thickener and base oil. Greases are produced in various consistencies, which are defined as NLGI grades. These are determined by the ‘worked penetration’ of the grease according to ISO 2137. The higher the NGLI grade, the harder the grease. Preferred greases for rolling bearings are those with NGLI grades of 1, 2 or 3.
Causes of failure for rolling bearings [source: Antriebstechnik, 93]: (1) Lubricant starvation, (2) unsuitable lubricant, (3) aged lubricant, (4) material and production defects, (5) unsuitable bearing selection, (6) secondary damage, (7) mounting defects, (8) liquid contaminants, (9) solid contaminants.
Factors influencing grease selection
A distinction needs to be made between point contact (ball bearings) and line contact (needle roller bearings and cylindrical roller bearings).
In ball bearings, each overrolling motion at the rolling contact places strain on only a relatively small volume of grease. In addition, the rolling kinematics of ball bearings exhibit only relatively small proportions of sliding motion. The specific mechanical strain placed on greases in bearings with point contact is therefore significantly less than in bearings with line contact. Typically, greases with a base oil viscosity ISO VG 68 to 100 are used.
In rolling bearings with line contact, higher requirements are placed on the grease. Not only is a larger grease quantity at the contact subjected to strain, but sliding and rib friction is also to be expected. This prevents the formation of a lubricant film and would therefore lead to wear. As a countermeasure, greases should be selected that exhibit a higher base oil viscosity (ISO VG 150 to 460 or higher). Anti-wear additives may also be required and consistency is normally NLGI 2.
The speed parameter of the bearing should always be a good match for the speed parameter of the grease. This depends on the type and proportion of the thickener, the base oil type and the proportion of base oil.
The speed parameter of a grease is not a material parameter but depends on the bearing type and the required minimum running time.
As a general guide, for rolling bearings rotating at high speeds or with a low requisite starting torque, greases with a high speed parameter should be selected. For rolling bearings rotating at low speeds, grease with a low speed parameter is recommended.
The temperature range of the grease must correspond to the range of possible operating temperatures in the rolling bearing. The operating temperature range is dependent on the type and proportion of thickener, the type and proportion of base oil, the production quality and production process. The stability of the grease at high temperature also depends primarily on production quality and production process.
In order to achieve reliable lubrication and an acceptable grease operating life, it is generally recommended that greases should be selected according to the bearing temperature that normally occurs in the standard operating range.
Other factors to consider include the upper operating temperature of the grease, the dropping point (i.e. the temperature at which slowly heated grease passes from a semi-solid to a liquid state and the first drop of grease falls from the standardised dropping point nipple), and the lower operating temperature.
For a load ratio C/P <10 or P/C > 0.1, greases are recommended that have higher base oil viscosity and anti-wear additives. These additives form a reaction layer on the metal surface that provides protection against wear. These greases are also recommended for bearings with an increased proportion of sliding motion (including slow running) or line contact, as well as under combined radial and axial loads.
Selecting the most suitable grease for a rolling bearing can prevent premature bearing failure and ensure high reliability and optimum operating life.
Water & Moisture
If the application is in a damp environment, moisture can enter the bearing. Water may condense within the bearing if there are rapid temperature fluctuations between warm and cold. This is a particular problem if large cavities exist in the bearing or housing.
Water can cause severe damage to the grease or bearing and is often due to ageing or hydrolysis, interruption of the lubricant film and corrosion. Barium and calcium complex soap greases have proved favourable in these conditions as they provide good water resistance and act to repel water. The anti-corrosion effect of a grease is also influenced by additives.
Oscillations, shocks and vibrations
Oscillation loads can have a considerable effect on the structure of thickeners in greases. If mechanical stability is not sufficient, changes in consistency may occur. This leads to softening, de-oiling on an isolated basis, but also hardening of the grease with a corresponding reduction in lubrication capability. It is therefore recommended that a grease should be selected whose mechanical stability has been tested accordingly. Options here include the expanded worked penetration, the Shell Roller Test in accordance with ASTM D 1831 and a test run on the FAG AN42 test rig.
If hard contaminant particles penetrate the bearing, this will not only lead to increased noise but also to wear. Appropriate sealing of the bearing should prevent this. The grease can assist this sealing effect by forming a stable collar on the seal. In this case, more solid type greases are more suitable, as greases that are too soft tend to favour the escape of grease.
Mounting position & adjacent components
Even where an axis of rotation is vertical or inclined, lubricant must remain at the lubrication point. In addition to appropriate seals, flowing away of the grease can be prevented by using a more viscous grease. If several lubrication points are located close together, unintentional contact can occur. Attention must therefore be paid to compatibility of the lubricants with each other. However, where possible, the optimum solution is to use only one grease, which should also be compatible with the cage and seal material.
Legal & Environmental
Depending on the application and the industry sector, legal and environmental factors must be considered when selecting a suitable grease. In the food processing industry, for example, the use of greases with appropriate authorisation is specified. A worldwide standard that can be used is approval in accordance with the NSF (National Sanitary Foundation) H1 or H2, listed in the so-called White Book™.
A lubricant with the code H1 (food-grade lubricant) may be used where occasional, technically unavoidable contact with foodstuffs cannot be eliminated. This means that the grease must be non-toxic, rapidly broken down by the organism and neutral in terms of both odour and taste. Such lubricants often comprise aluminium complex soap thickeners and polyalphaolefins or medicinal white oils as a base oil.
H2 lubricants are intended for general use within the food processing industry where no contact with foodstuffs occurs. Greases with biological degradability must be provided where the lubricant can pass directly into the environment.