Four Grease Tests for Centralized Lubrication Systems

When designing a centralized lubrication system, which comes first: the system or the grease selection? While many start by designing the system and then selecting a suitable grease, the optimal approach is to first choose a grease that meets the machine’s specific needs and then design the lubrication system around that lubricant.

Understanding Grease Composition

To begin, let’s clarify what grease is. According to ASTM D288, grease is defined as “a solid-to-semifluid product of dispersion of a thickening agent in a liquid lubricant, with other ingredients added to impart special properties.”

Grease essentially consists of three components:

1. Base Oil: Making up 70% to 95% of the grease, the base oil is the primary lubricant.
2. Additives: Comprising 0% to 10% of the grease, additives are included to enhance or suppress certain properties of the base oil, particularly for protection during the startup and shutdown phases.
3. Thickener: The thickener, which constitutes 3% to 30% of the grease, acts as a vessel to deliver the base oil and additives to the equipment.

Selecting the Right Grease: Four Essential Tests

Now that you understand the composition of grease, let’s explore four tests that are crucial for determining how the grease will perform in a centralized lubrication system.

1. Apparent Viscosity Test

Viscosity is the most critical property of any lubricant. To determine the required base oil viscosity, you need to identify the optimal viscosity for each component of your system. The apparent viscosity of grease, influenced by the base oil, additives, and thickener, is particularly important in centralized lubrication systems. The ASTM D1092 standard test measures this property and helps predict pressure drops in the system under steady flow and constant temperature, with results reported in centipoise.

2. Cone Penetration Test

To determine the required consistency or NLGI grade of the grease, consider factors like operating temperature, speed factor, bearing type, thickener type, and base oil viscosity. The cone penetration test (ASTM D217) gauges a grease’s consistency, both in storage (undisturbed) and during operation (worked). This test is crucial in centralized systems, where grease may sit in supply lines for extended periods before reaching the component.

3. Structural Stability Test

Assessing the grease’s stability under operating conditions is vital. The ASTM D1831 structural stability test involves using a penetrometer to measure the grease's consistency before and after it is subjected to rolling in a stability device for two hours. This test helps determine how the grease will perform under the intended loads and for how long before it begins to fail.

4. Ventability Test

The final test focuses on matching the grease to the lubrication system’s design and the tubing size. The ventability test helps identify the supply-line diameter required for a specific grease. It also determines whether the grease is suitable for use in a centralized lubrication system, ensuring that grease valves and injectors will function properly. The test pressurizes the grease in a 25-foot coil to 1,800 psi, and the pressure is measured after stabilization.

Designing Your Centralized Lubrication System

Armed with the results of these tests, you can now make an informed decision when designing your centralized lubrication system. Always design the system around the grease rather than compromising on essential grease properties just to fit the system. Avoid common mistakes such as using inappropriate supply-line diameters, applying incorrect pressure, or sacrificing crucial grease characteristics. Now, when asked which comes first—the system or the grease—you’ll have the answer.