Mechanical Properties Testing for Greases

Greases are vital lubricants used extensively across various industrial applications to reduce friction, protect surfaces from corrosion, and provide a barrier against contaminants. 

Unlike lubricating oils, greases have a semi-solid or solid consistency, resulting from the presence of a thickening agent suspended in a base oil matrix. 

The mechanical properties of greases—such as shear stability, consistency, and penetration—are crucial for ensuring their performance and reliability under different operating conditions. 

This article provides a comprehensive overview of the mechanical properties of greases, the importance of these properties, and the standardized methods used to evaluate them.

Greases are formulated by thickening a base oil with a thickener and adding various additives to enhance specific properties. 

The base oil, which could be mineral, synthetic, or bio-based, makes up the majority of the grease composition (70-90%). 

The thickener, which is often a soap (such as lithium, calcium) or a complex thickener (like lithium complex or calcium sulfonate), provides the structure needed to trap the base oil within the grease matrix. 

Additives, including anti-wear agents, rust inhibitors, and extreme pressure (EP) additives, are incorporated to enhance properties like oxidation stability, load-carrying capacity, and corrosion resistance.

Significance of Mechanical Properties in Greases

The mechanical properties of greases are paramount for their performance in real-world applications. 

These properties determine a grease’s ability to maintain its structure, provide effective lubrication, and protect surfaces under varying levels of stress, load, and environmental conditions. 

Key mechanical properties that significantly influence grease performance include consistency and penetration, shear stability, mechanical stability, dropping point, oil separation, rheological properties (such as viscosity), and low-temperature behavior. 

Understanding these properties and their testing methodologies is essential for selecting the most suitable grease for specific applications and ensuring optimal performance.

Consistency and Penetration

Consistency is defined as the degree of firmness or softness of a grease, which affects its pumpability, retention within bearings, and resistance to leakage. It is largely influenced by the type and concentration of the thickener, as well as the viscosity of the base oil. 

Penetration is the most common measure of a grease's consistency and is defined as the depth, in tenths of a millimeter, that a standard cone penetrates a grease sample under specific conditions. Greases are classified according to the National Lubricating Grease Institute (NLGI) grades, which range from 000 (fluid-like) to 6 (block-like), with the most common being NLGI 2.

The standardized method for determining grease penetration is outlined in ASTM D217, known as the Cone Penetration of Lubricating Grease test. 

In this test, a sample of grease is placed in a cylindrical cup and leveled to remove air pockets. A standard cone, weighing 150 grams, is allowed to drop freely into the grease for five seconds at a controlled temperature of 25°C (77°F). 

The depth of penetration is then measured, with the value indicating the grease’s consistency. A penetration value between 220 and 250 corresponds to an NLGI grade 3. Greases with higher penetration values (e.g., NLGI 000 or 00) are softer and more suitable for centralized lubrication systems or low-speed applications, whereas lower penetration values (e.g., NLGI 3 or 4) indicate firmer greases ideal for high-speed or high-load applications where leakage could be an issue.

Shear Stability

Shear stability refers to a grease's ability to maintain its consistency when subjected to mechanical shear forces, such as those encountered in bearings or gear systems. 

Under shear conditions, the thickener network within the grease can break down, leading to a change in consistency that can affect the grease's performance, such as increased leakage or reduced re-lubrication intervals. 

The ASTM D1831 (Roll Stability Test) is commonly used to measure the shear stability of greases. In this test, a specific amount of grease is placed in a rolling cylinder, which is then rotated at a constant speed of 165 rpm for two hours. 

After the test, the penetration of the worked grease is measured and compared to its original penetration value. 

A small change in penetration (less than 30 units) suggests good shear stability, while a significant change indicates that the grease may lose its lubricating properties under shear conditions. 

This is particularly critical for applications like electric motor bearings and high-speed gears, where maintaining a consistent lubricant film is crucial for reliability and longevity.

Mechanical Stability

Mechanical stability is the ability of a grease to withstand repetitive deformation without undergoing significant physical changes, such as bleeding, softening, or hardening. 

This property is influenced by both the thickener type and the additive package used in the grease formulation. 

The extended worked penetration test, as described in ASTM D217, is a common method for evaluating mechanical stability. 

This test measures the change in consistency of a grease after it is worked for 100,000 strokes, providing an indication of its ability to maintain its consistency under prolonged mechanical stress. 

Another test specific to railway applications is ASTM D8022, also known as the V2F Test Method for Railway Greases. 

This test involves subjecting the grease to repetitive mechanical shocks to determine its mechanical stability under high-impact loads. 

Mechanically stable greases are essential for applications with frequent mechanical stresses, such as automotive chassis points, ball joints, and railway bearings, where resistance to mechanical degradation is crucial for maintaining lubrication integrity.

Dropping Point

The dropping point of a grease is the temperature at which it transitions from a semi-solid to a liquid state, signifying the upper-temperature limit for its serviceability. 

The dropping point provides an indication of the thermal stability of the thickener used in the grease formulation. 

The ASTM D2265 standard outlines the test procedure for determining the dropping point of lubricating grease. 

During the test, a grease sample is heated in a controlled environment until the first drop of the melted grease falls from the sample cup, and the dropping point is recorded as the temperature at which this occurs. 

Greases with a higher dropping point are suitable for high-temperature applications, such as oven conveyors, electric motor bearings, and automotive wheel bearings, where the lubricant must remain stable and retain its properties at elevated temperatures.

Oil Separation

Oil separation is a measure of a grease's tendency to release its base oil under static or dynamic conditions. 

Excessive oil separation can lead to inadequate lubrication and potential failure of the lubricated component. 

The ASTM D6184 standard, which tests oil separation from lubricating greases, evaluates a grease's tendency to separate oil at elevated temperatures. 

In this test, a specific amount of grease is placed on a mesh screen and subjected to a constant temperature. The separated oil is collected and weighed to determine the percentage of oil separation. 

A low percentage of oil separation is desirable for applications such as sealed bearings and gearboxes, where maintaining the lubricant’s integrity is critical to prevent leaks and ensure a long service life.

Rheological Properties 

Rheological properties, such as apparent viscosity, reflect a grease's flow characteristics under various shear rates and temperatures. 

Viscosity affects a grease's ability to form a lubricating film, especially in start-up and high-speed applications. 

The ASTM D1092 test method involves using a rotational viscometer to measure the apparent viscosity of greases under controlled shear rates and temperatures, with the viscosity typically reported in centipoise (cP) or Pascal-seconds (Pa·s). 

Understanding rheological properties is crucial for selecting greases for high-speed applications, such as electric motors and turbine bearings, where low apparent viscosity ensures minimal energy loss and heat generation.

Low-Temperature Behavior

Low-temperature behavior indicates a grease's ability to remain pumpable and provide adequate lubrication at sub-zero temperatures. 

This property is critical for outdoor applications, particularly in cold climate regions, automotive wheel bearings, and aerospace applications. 

The ASTM D1478 standard, which measures the low-temperature torque of ball bearing grease, provides an assessment of the starting and running torque of ball bearings filled with grease at low temperatures. 

The torque values obtained from this test provide an indication of the grease's performance under cold conditions. 

Greases with low starting and running torque values at low temperatures are ideal for applications such as automotive components, aeronautics, and wind turbines, where reliable lubrication is required even under freezing conditions.