|Tips on greases include the following topics:|
|Grease Lubrication Schedule - Spherical Roller Bearings|
Grease may be defined as a solid to semi-solid material produced by the dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may be included.
Complex grease is similar to a regular grease except that the thickener contains two dissimilar fatty acids, one of which is the complexing agent. This imparts good high temperature characteristics to the final product.
The high percentage of oil by weight (75-95%) in a grease necessitates that it be of high quality and proper viscosity for the intended application. A light viscosity oil is normally used for low temperature, low load and high speed applications. Conversely a heavy viscosity oil is generally used for high temperature, high load and slow speed applications.
The most common additives found
in grease are as follows:
The most important factors affecting the properties and characteristics of a grease are:
A grease is expected to:
Consistency - is the degree of hardness of a grease and may vary considerably with temperature. This has been classified by the National Lubricating Grease Institute (NLGI) into the following categories:
Shear Stability - is the ability of a grease to resist a change in consistency during mechanical working. Under high rates of shear, grease structures tend to change in consistency (usually softer). Oil Separation - is the percentage of oil which separates from the grease under static (e.g. storage) conditions. It cannot predict separation tendencies in use under dynamic conditions. High Temperature Stability - is the ability of a grease to retain its consistency, structure and performance at temperatures in excess of 125°C.
The following five (5) categories for Automotive Service Greases have been developed by the NLGI. The scope of this classification (ASTM D 4950) covers greases designed for the lubrication of chassis components and wheel bearings of passenger cars, trucks and other vehicles. The NLGI classifies automotive service greases into two (2) main groups. Chassis greases, designed by the prefix L and Wheel Bearing greases designated by the prefix G. The following table outlines the five (5) categories:
NLGI Automotive Service Grease Categories
Occasionally, grease substitution in an application may be necessary to correct problems arising from the original product in service. If the thickeners are incompatible, the mixture will fall short of the properties of the individual greases. It is strongly advised that, in all cases, the old grease be purged or cleaned out from the system before a new one is introduced.
However, compatibility between greases is temperature dependent. As the temperature rises, the problems associated with incompatibility also increase. The following chart indicates the compatibilities of major Petro-Canada greases. With competitors' products, it is strongly advised to treat them as incompatible unless strong evidence exists to the contrary.
The following chart is designed to help you select a type of grease that will satisfy the intended application.
*These temperatures refer to continuous operation. They may be exceeded temporarily in the case of complex greases, and where rigorous lubrication practice is followed.
The over-packing and over-greasing of bearings accounts for more failures than any other factor. Excess grease in a bearing cavity increases internal friction, which in turn raises the bearing temperature above the dropping point of the grease. This causes oil separation and eventually lack of lubrication. When packing a split housing pillow block bearing ensure that the grease cavity is only one-third full.
Re-greasing intervals are determined from:
The following chart provides a guide to re-greasing intervals and the amount of grease to be applied.
Spherical Roller Bearings
Temperature 90C (200F)
Horizontal Shaft Equipment