General Grease Overview & Bearing Lubrication
These influences go beyond just grease formulation and are associated with factors such as bearing designs, load, vibration and oscillation, operating temperatures and service environment.
Greases are typically applied in areas where a continuous supply of oil cannot be retained. Greases work by releasing small quantities of oil to the bearing elements over time. Oil films are very thin, smaller than the volumes that would be required in a fully flooded oil lubricated system. The main advantages of using grease rather than oil lubrication is the ease of application and the ability of the lubricant to stay in place where oil is not feasible. The main disadvantages of using grease is its limited service life and the propensity to hold contamination.
Lubricating grease formulations are a blend of three basic components; base oil, thickener and additives. The proportions of these components generally break down as follows:
- Base oil 70%-95%
- Thickener 3% - 30%
- Additives <1% - 10%
The viscosity of the base oil plays a major role in determining what applications the grease will be useful for. The base oil viscosity must meet the film thickness requirements for the component that is being lubricated. The base oil can be a refined mineral oil, synthetic oil or a combination of both. Base oil viscosity in greases used to lubricate rolling bearings typically range from 15 cSt (ISO VG 15) to 500 cSt (ISO VG 460) at 40 °C. The base oil viscosity used in grease formulation is determined by the maximum temperature, speed and load for a bearing.
Thickeners are commonly derived from a metallic soap, of which there is a variety; lithium, calcium, barium, sodium, etc. Complex soap thickeners are derived from adding an organic acid as a complexing agent for greater temperature resistance. Non-soap thickeners are also commonly used in grease formulations, such as bentonite clay or polyuria, which provide excellent heat, and water resistance and shear stability. The type of thickener will determine the ability of the grease to resist water washout if water is present. Where base oil viscosity is the prime determiner of the grade selection of grease, grease types are classified primarily by thickener type.
Greases thickness or consistency is primarily classified by NLGI grade. NLGI or National Lubricating Grease Institute has an established scale for grease consistency based on the ASTM D217 Worked Penetration range. Grease consistencies can range from a 000 grade to a 6 grade, where 000 is the thinnest consistency and 6 is the thickest consistency. The most common NLGI grade is Number 2; however, lower grades are also commonly used in cooler operating environments and where grease pumpability is of concern. Base oil viscosity plays no role in the consistency grade of the grease. Consistency is determined by the ratio of thickener to base oil content.
Additives are used to enhance useful properties such as antioxidants, rust and corrosion inhibitors, extreme pressure properties, antiwear and friction-reducing agents. Special boundary lubricants to decrease friction can be suspended in grease, which perform without a chemical reaction that other additives require.
Grease application provides several functions:
- Sealant to minimize leakage while keeping out contaminants
- Simpler lubrication application for some components
- Retain particle contaminants in suspension
- Improved stop/start performance
The majority of rolling element bearings are grease lubricated since grease is easily retained in the bearing assembly and raceways. More than 90% of all rolling element bearings are grease lubricated. The correct quantity of grease applied to a bearing is very important. Too little grease leads to metal-to-metal contact and premature bearing failure. Too much grease can cause the operating temperature within the bearing to rise rapidly, particularly when running at high speeds. Excessive amounts of grease can also cause seal failure when there is no place for the grease to go within the bearing assembly. Too much grease leads to excessive churning, high temperatures, and accelerated grease degradation. If the bearing is properly filled, excessive grease will be pushed to the shoulders of the bearing and onto the seals during normal churning.
When adding grease to a bearing assembly the free volume in the housing should be partly filled. Excess grease in a bearing must be given time to settle or escape during a running-in period before operating at full speed. At the end of the running-in period, if the grease has been distributed evenly in the bearing arrangement the operating temperature will drop considerably. It is best to check with the bearing manufacturer regarding volume fill requirements. One rule of thumb states that 30% of the free volume of a bearing should be initially filled with grease for the running-in period, but this can vary due to bearing speeds. Some slow speed bearings may require up to a 90% volume fill, while medium to high speeds may require 30–50% based on manufacturer requirements.
While it seems that greasing a bearing is one of the simple tasks in maintenance, there is actually a precise methodology that can and should be followed to ensure bearings are lubricated properly. Base oil viscosity, grease thickener type and grease consistency should all be considered when selecting a grease for a particular application. Volume quantities and relubrication intervals should be determined based on the bearing size, operating temperature, loads, and working environment. Establishing a precise grease lubricating methodology can lower maintenance costs and increase equipment reliability.
David Doyle, CLS, OMA I, OMA II