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D3337-91 – Standard Test Method Technical Guide
📐 Test Apparatus and Bearing Specifications
ASTM D3337-91 (Reapproved 2002) provides a standardized procedure for evaluating the life and running torque of lubricating greases in small ball bearings. The test utilizes a specifically defined apparatus and a new R-4 bearing for each individual test to ensure consistency and reproducibility across different laboratories and grease formulations.
The specification for the R-4 test bearing is rigorous, ensuring that variations in the bearing itself do not influence the test results. The bearing is run under a constant 0.5 lbf (2.2 N) radial load and a 5 lbf (22 N) axial load at the specified speeds.
| 🟦 Parameter | 📏 Specification (inch-pound) | 📐 Specification (SI) |
|---|---|---|
| Bearing Size | R-4 | R-4 |
| Bore | 0.2500 in | 6.350 mm |
| Outer Diameter (OD) | 0.6250 in | 15.875 mm |
| Width | 0.1960 in | 4.978 mm |
| Material | Type 440C Stainless Steel (Heat Stabilized) | |
| Precision Class | AFBMA Class 7 | |
| Radial Clearance | 0.0003 – 0.0005 in | 0.007 – 0.013 mm |
| Retainer | Stainless Steel Ribbon Type | |
| Shields | Removable with Snap Rings | |
⚙️ Test Procedure and Sample Preparation
Proper sample preparation is critical to the validity of the test. The grease sample must be procured from below the surface of the container to avoid contamination from any separated oil. If separated oil is present, it must be poured off. The grease is then screened through a 40-µm retention filter to remove any large particles or crystallized components.
💡 Technical Note: After screening, the test bearing must be packed in a clean environment. The correct charge of grease is exactly one-third of the free space within the bearing. Over or under packing can significantly alter torque readings and apparent grease life.
The test bearing is rotated at 12,000 r/min under the specified loads and at a selected elevated temperature to determine grease life. The total running time is recorded as the measure of durability. For torque evaluation, the apparatus measures running torque at both a low speed of 1 r/min and the high speed of 12,000 r/min. Torque values are reported in g·cm.
| ⚡ Test Parameter | 🎯 Value | 📝 Purpose / Notes |
|---|---|---|
| Life / High Speed Torque | 12,000 r/min | Primary speed for durability and viscous shear torque |
| Low Speed Torque | 1 r/min | Evaluates break-away and low-speed friction |
| Radial Load | 0.5 lbf (2.2 N) | Constant load applied during the test |
| Axial Load | 5 lbf (22 N) | Constant load applied during the test |
| Grease Screening | 40 µm retention filter | Mandatory step prior to packing the bearing |
📊 Key Measured Properties and Significance
This standard serves as a critical screening tool for differentiating the expected performance of greases in ball bearings operating at high temperatures. It defines two key measurements:
- Grease Life: The total running time until failure, reflecting the durability of the grease at elevated temperatures. It is a measure of how long the grease can effectively lubricate the bearing under the defined conditions.
- Running Torque: A measure of friction in the rotating bearing due to load, speed, and viscous shear of the grease. This can be further isolated as Grease Torque, which represents the friction specifically caused by the viscous shear of the grease itself.
⚠️ Important Limitation: This test method is intended to provide a relative ranking of greases and predict relative life at high temperature. It is clearly stated in the standard that this is not the equivalent of a long-time field-service test.
❓ Frequently Asked Questions
🔍 What is the difference between grease life and grease torque as measured by this standard?
Grease life measures the total running time until bearing failure, indicating the grease’s durability. Grease torque specifically measures the friction resulting from the viscous shear of the grease, while running torque is the total friction due to load, speed, and the grease combined.
💡 How much grease should be packed into the test bearing?
The standard explicitly requires the test bearing to be filled with a quantity of test grease that occupies exactly one-third of the free space inside the bearing. Packing must always be done in a clean environment to prevent contamination.
⚡ Why are two different speeds specified for torque measurement?
The low speed of 1 r/min is intended to evaluate the break-away torque or start-up friction of the grease. The high speed of 12,000 r/min evaluates the viscous shear resistance of the grease under operational conditions, which is the same speed used for the grease life test.
📌 What are the specific dimensional requirements of the R-4 test bearing?
The R-4 bearing required by the standard has a bore of 0.2500 in (6.350 mm), an outer diameter of 0.6250 in (15.875 mm), and a width of 0.1960 in (4.978 mm). It must be made of heat-stabilized Type 440C stainless steel to AFBMA Class 7 precision tolerances.