The word grease is derived from an old French word “Graisse” meaning thick or fat. A grease is a solid to semi-solid product of suspension of thickener into a liquid lubricant.
Grease is made by adding base oil, thickener and additives. The percentage of all the three ingredients may vary depending upon application. Two typical grease compositions are given below:
Mineral oil and synthetic oil are commonly used base oils. Thickeners or thickening agents are metal soaps which can be simple metal soaps, complex metal soaps or non-soap.
The main function of additives is to enhance and add desirable properties to grease. Main additives used are oxidation inhibitors, extreme pressure, adhesion promoters, anti-wear and friction modifiers. The most popular greases worldwide are lithium-based greases (derived from lithium soap) with a market share of more than 75 percent.
- Should have good consistency
- Should have high dripping point
- Should repel water or moisture
- Should withstand low temperature and high temperature environments
- Should resist oxidation
- Stays put
- Excellent sealing capability
- Can withstand heavy shock loads
- Requires less frequent application
- Low heat transfer capability
- Lower resistance to oxidation
- Under-greasing and over-greasing results in loss of grease quantity
- Difficulty in used grease sample analysis
- No filtration hence higher risk of contamination
When to use grease
Grease is generally preferred for low temperature, low speed and high load applications. It is also the most preferred lubricant by maintenance professionals for lubricating inaccessible machine parts.
Grease NLGI consistency number
The NLGI consistency number expresses a measure of the relative hardness of a grease used for lubrication. The table provides NLGI classification :
Grease compatibility chart
Proper attention and care should be taken when mixing two different greases or switching from one grease to another as thickeners used in manufacturing of both the greases can react with each other jeopardizing the physical and chemical structure of grease. This can lead to excessive oil bleed or base oil separation resulting in depletion of grease characteristics.
A grease compatibility chart helps in choosing a compatible grease. A typical representation of grease compatibility chart is given below.
Grease application methods
Grease application methods can be divided into two main categories: Manual and Automated
Grease guns are used to apply grease through an aperture to a specific point, usually on a grease fitting or ‘nipple’. 1 shot of grease gun is equivalent to 1 ounce or 2-3 grams. Grease guns are of three main types – lever, piston and battery operated.
Single point lubricator is mounted directly over the system and it dispenses precise amounts of lubricants at regular intervals.
Centralized lubrication systems is used to deliver or direct precise amounts of grease to multiple locations on a machine while the machine is operating. They can be categorized into single line, dual line and multiline systems.
Analysis of grease is as important as oil analysis. Testing and analysis of used grease on a regular basis helps in keeping a track on its condition, properties and level of contamination. Routing in-lab and on-field testing of grease helps in keeping the machine healthy. Some of the grease analysis tests that are often used are given below:
- Consistency: Cone penetration test
- Wear debris analysis: Fourier transform infrared spectroscopy (FTIR) & Analytical ferrography
- Viscosity and rheological properties: Rheometer & Viscometer
- A grease container should never be kept half opened or uncovered.
- Grease containers should be properly labelled
- Grease containers should be stored in cool, clean, dry and contaminant free room.
- Always follow FIRST IN FIRST OUT policy which means using the first received container first
- Grease should never be heated above 75°F
Two main bearing grease calculations are given below:
- En.wikipedia.org. Grease (lubricant) – Wikipedia.
- Efficient Plant. What’s in a Lubricant: Characteristics of Grease – Efficient Plant.
- Skf.com. SKF.
- Thelubricantstore.com. Grease Compatibility Chart.
- White, P., Oil and Grease Application Methods.
- Machinerylubrication.com. Grease Guns – Learning the Basics.
- G. Ludwig, L., Storing Grease to Avoid Bleed and Separation.
- Herguth, B., Grease Analysis – Monitoring Grease Serviceability and Bearing Condition.
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