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SAE J380-2017: Measuring Specific Gravity of Brake Friction Materials đ ī¸
1. Purpose and Scope
SAE J380-2017 establishes a uniform procedure for determining the specific gravity of brake friction materials. This nondestructive test serves as a quality control check to ensure consistency in formulation and processing. It is important to note that specific gravity alone does not indicate a material’s in-use performance; each formulation has its own acceptable range that manufacturers must establish.
Key Insight: Specific gravity is a quality control parameter, not a performance predictor. Acceptable values are unique to each formulation and must be defined by the manufacturer. The test is nondestructive, allowing reuse of parts.
For sintered metal powder friction materials, particularly those with steel backing members, the procedure may differ; refer to ASTM B 376.
2. Test Equipment and Procedure
Equipment Required
| Item | Requirement | Notes |
|---|---|---|
| Scale or balance | Accuracy of 0.1% of dry sample weight | e.g., a 100 g sample requires 0.1 g accuracy |
| Container | Clear water at 20–30 °C (68–86 °F), large enough to submerge sample without touching sides | |
| Immersion tray and monofilament cord | Attached to scale, holds sample underwater | Tray must be immersed when zeroing |
Step-by-Step Procedure
- If the specific gravity of the friction material alone is desired, remove all backing layers and plates. This is the preferred method.
- Zero the scale with the empty tray immersed in water.
- Place the sample on the scale and record the weight in air to an accuracy of 0.1% of the dry weight.
- Place the sample on the tray and completely immerse it. Record the weight in water to 0.1% of dry weight.
- Weigh in water as rapidly as possible (within 15 seconds) to minimize liquid absorption.
- Eliminate air bubbles from the specimen; adding up to 5 parts of a wetting agent to 1000 parts water can help.
⚠️ Caution: The weighing in water must be performed within 15 seconds to minimize water absorption. Ensure no air bubbles are attached to the sample. Use a wetting agent if necessary.
3. Calculation, Interpretation, and FAQs
Formula and Example
Calculate specific gravity using the formula:
Specific Gravity = A / (A - B)
Where A = weight in air (g), B = weight in water (g). Report the result to two decimal places.
Engineering Design Insight
The specific gravity range is unique to each formulation. This test is a quality control measure to verify consistency in manufacturing, but it should not be used to predict performance. Because the test is nondestructive, parts can be reused after testing if undamaged. The type of sample used (with or without backing) must be clearly reported for valid comparisons.
Frequently Asked Questions
Why is specific gravity measured for brake friction materials?
It serves as a quick, nondestructive quality control check to ensure the formulation and processing are consistent from batch to batch. It does not indicate performance but helps detect variations.
How is the specific gravity test performed?
The sample is weighed in air, then weighed while fully immersed in water. The specific gravity is calculated as the weight in air divided by the difference between the two weights. The test must be done quickly to avoid water absorption.
What is the significance of the 15-second immersion time?
To minimize the absorption of water by the friction material, which could alter the measured weight and lead to inaccurate results.
Can this test be used for all types of friction materials?
For sintered metal friction materials, the standard recommends referring to ASTM B 376, as the procedure may differ due to backing members or material characteristics.
