The grease shear tester is a specialized equipment that simulates the long-term shear action of lubricating grease in actual working conditions, used to evaluate its structural stability, shear resistance, and long-term lubrication performance. The accuracy of test results is affected by various factors. The following provides a detailed analysis from the aspects of instrument design, testing conditions, sample characteristics, and operating procedures.

1、 Instrument design and structural factors

1. Shear mode and mechanical simulation

The cutting mode of the instrument (such as rotary cutting, reciprocating cutting, or cone plate cutting) directly affects the test results. Rotating shear is closer to the actual working conditions of bearings, while reciprocating shear is suitable for simulating linear moving components. The differences in stress distribution among different modes may lead to varying degrees of structural damage in lubricating grease.

-Key point: The cutting mode should be selected according to the actual application scenario, for example, rolling bearings should be simulated using rotational cutting.

2. Shear rate and frequency

The shear rate (number of shear cycles per unit time) and frequency (cycle period) determine the mechanical load strength that the grease can withstand. High shear rates will accelerate the fracture of the thickener fiber structure, leading to intensified oil separation; Low rates may prolong testing time and affect efficiency.

-Typical problem: If the rate is too high, it may mask the gradual decay process of the lubricating grease, while if the rate is too low, it is difficult to simulate high load conditions.

3. Contact interface material and surface roughness

The material properties (hardness, surface energy) and roughness of cutting components such as rotors and bearing steel balls can affect the friction and wear behavior of lubricating grease. For example, high hardness metal surfaces may accelerate the wear of thickeners, while surface scratches may induce local stress concentration.

-Solution: Use the same material as the actual working conditions (such as GCr15 bearing steel) and control the surface roughness (Ra 0.2-0.8 μ m).

2、 Test condition control

1. Temperature control

Temperature is a key factor affecting the rheological properties of lubricating grease. High temperature will reduce the yield stress of thickeners and accelerate oil separation; Low temperature may cause lipid hardening and increase shear resistance.

-Typical phenomenon: Lithium based grease is prone to soap fiber thermal degradation above 100 ℃, while testing errors may occur due to insufficient oil separation at low temperatures.

2. Pressure and load

Normal pressure (load perpendicular to the shear direction) affects the extrusion effect of lubricating grease. Excessive pressure may cause lipids to be squeezed out of the contact area, altering the actual amount of lipids involved in shearing; If the pressure is too low, it is impossible to simulate the squeezing effect of actual working conditions.

-Control method: The load should be set according to the NLGI grade (such as 1-2 MPa pressure commonly used for grade 2 grease).

3. Environmental media

The humidity, oxygen concentration, and corrosive gases (such as H ₂ S) in the testing environment may accelerate the oxidation or corrosion of lubricating grease. For example, lithium based grease is prone to water absorption and softening in humid environments, leading to a decrease in shear performance.

-Response measures: Use inert gas protection or a constant temperature and humidity chamber to assist in testing.

3、 Characteristics of lubricating grease samples

1. Type of base oil

The viscosity, volatility, and polarity of base oil directly affect the shear stability of lubricating grease. High viscosity base oils (such as PAO) can enhance cohesion, but volatile mineral oils are prone to loss during high-temperature shear.

-Case comparison: Ester oil, due to its strong polarity, binds more firmly with thickeners and has better shear resistance than low viscosity mineral oil.

2. Types and contents of thickeners

The structure of thickeners (such as metal soaps and organic colloids) determines the thixotropic properties of lubricating grease. Lithium based grease has a denser soap fiber network and better shear resistance than sodium based grease; Organic colloids, such as urea, are prone to deconstruction during long-term shearing.

-Optimization direction: Composite thickeners (such as lithium calcium mixed soap) can balance high temperature stability and shear recovery ability.

3. Compatibility of additives

Additives such as antioxidants and rust inhibitors may affect the shear stability of lubricating grease. For example, polar additives such as ZDDP may weaken the interactions between soap fibers, leading to structural relaxation.

-Attention: It is necessary to evaluate the compatibility of additives and thickeners to avoid excessively reducing the mechanical stability of fats.

4、 Operation and Data Processing

1. Uniformity of sample loading

If there are bubbles or uneven density during loading, it will cause abnormal distribution of local shear forces. For example, the bubble aggregation area may experience oil separation in advance, misleading the overall performance evaluation.

-Standardized operation: Adopt vacuum degassing treatment and layer by layer compaction for sample loading.

2. Testing cycle and downtime interval

Continuous operation during 10000 shear tests may cause overheating or grease accumulation damage to the friction pair. Properly setting intermittent shutdowns (such as cooling to room temperature every thousand times) can simulate actual start stop conditions, but it is necessary to balance the testing efficiency.

-Controversy point: Intermittent shutdown may mask the fatigue failure mechanism under continuous shear.

3. Data Sampling and Analysis

The sampling frequency for real-time monitoring parameters such as torque, temperature, and oil separation rate needs to be high enough to capture key decay nodes. For example, the shear stability of lithium based grease may rapidly decay in a short period of time and then stabilize.

-Analysis method: Use a dynamic viscosity model (such as Herschel Bulkley equation) to fit the shear stress-strain curve.