In most spindle applications with super-precision bearings, grease with a mineral base oil and lithium thickener is suitable. These greases adhere well to the bearing surfaces and can be used in applications where temperatures range from –30 to +110 °C (–20 to +230 °F). For applications with high speeds and high temperatures or where long service life is required, grease with a synthetic base oil e.g. SKF diester oil based grease
For angular contact thrust ball bearings for screw drives, grease with an ester or mineral base oil and calcium complex thickener can be used under most operating conditions.
Alternative greases may be required under any of the following conditions:
- operating temperatures < 10 °C (50 °F) or > 100 °C (210 °F)
- bearing speed is very high or very low
- static operation, infrequent rotation or oscillation
- bearings are subjected to vibration
- bearings are subjected to heavy loads or shock loads
- water resistance is important
- screw drive bearings at low speeds, under heavy loads or exposed to vibration should be lubricated with a lithium soap grease with a mineral base oil and EP additives like SKF LGEP 2
An appropriate grease selection process comprises four steps.
1. Select the consistency grade
Greases are divided into various consistency grades in accordance with the National Lubricating Grease Institute (NLGI). Greases with a high consistency, i.e. stiff greases, are assigned high NLGI grades, while those with low consistency, i.e. soft greases, are assigned low NLGI grades. In rolling bearing applications, three consistency grades are recommended:
- The most common greases, used in normal bearing applications, have an NLGI grade of 2.
- Low consistency rolling bearing greases, classified as NLGI 1 greases, are preferred for low ambient temperatures and oscillating applications.
- NLGI 3 greases are recommended for large bearings, vertical shaft arrangements, high ambient temperatures or the presence of vibration.
2. Determine the required base oil viscosity
For detailed information about calculating the required base oil viscosity, refer to Lubrication condition – the viscosity ratio, κ. The graphs are based on the elasto-hydrodynamic theory of lubrication (EHL) with full-film conditions.
It has been found, however, that when using greases containing very low or very high viscosity base oils, a thinner oil film than that predicted by EHL theories results. Therefore, when using the graphs to determine the required base oil viscosity for grease lubricated super-precision bearings, corrections may be necessary. From practical experience, determine the required viscosity ν at reference temperature 40 °C (150 °F) and then adjust as follows:
- ν ≤ 20 mm2/s → multiply the viscosity by a factor of 1 to 2 In this low range, the viscosity of the oil is too thin to form a sufficiently thick oil film.
- 20 mm2/s < ν ≤ 250 mm2/s → no correction factor is used
- ν > 250 mm2/s → contact the SKF application engineering service
Calculations can also be made using the SKF program Viscosity.
High viscosity greases increase friction and heat generated by the bearing but may be necessary, for example, for ball screw support bearings in low-speed applications or in applications where there is a risk of false brinelling.
3. Verify the presence of EP additives
Grease with EP additives may be appropriate if super-precision bearings are subjected to any of the following conditions:
- very heavy loads (P > 0,15 C)
- shock loads
- low speeds
- periods of static loading
- frequent starts and stops during a work cycle
Lubricants with EP additives should only be used when necessary and always within their operating temperature range. Some EP additives are not compatible with bearing materials particularly at higher temperatures. For additional information, contact the SKF application engineering service.
4. Check additional requirements
In some applications, operating conditions may put additional requirements on the grease, requiring it to have unique characteristics. The following recommendations are provided as guidelines:
- For superior resistance to water wash-out, consider grease with a calcium thickener over a lithium thickener.
- For good rust protection, select an appropriate additive.
- If there are high vibration levels, choose grease with a high mechanical stability.
To select the appropriate grease for a specific bearing type and application, the grease selection program SKF LubeSelect for SKF greases can be used.