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SAE J661-2021: Brake Lining Quality Test Procedure – An Engineering Guide
1. Purpose and Scope of SAE J661-2021
The SAE J661 Recommended Practice establishes a uniform laboratory procedure for securing and reporting the friction and wear characteristics of brake linings. Developed in the 1950s and revised in 2021, this standard is essential for in-plant quality control by friction material manufacturers and for quality assessment of incoming shipments by purchasers. The procedure focuses on ensuring consistent and reproducible results, addressing long-standing issues with test variability.
🛠️ Engineering Insight: The 2021 revision specifies permissible variation for recertification testing compared to original certification, helping to standardize friction coefficient identification per SAE J866.
2. Test Equipment and Specimen Preparation
The friction materials test machine must meet strict specifications. The drum must have an inside diameter between 277.4 mm and 279.9 mm, with proper thermocouple placement as per the table below. Temperature, force, and speed measuring systems require ±2% full-scale accuracy.
| Drum Inside Diameter | Location in Drum |
|---|---|
| 277–278 mm | 1 |
| 278–279 mm | 2 |
| 279–280 mm | 3 |
Test specimens must be taken from the center of the friction material, with dimensions 25.4 × 25.4 mm and a working surface radius matching the drum. Proper grinding and polishing of the drum surface (final grit 320) are critical. The working surface of the specimen should never be handled or contaminated.
⚠️ Caution: Failure to achieve at least 95% contact during burnishing can lead to inconsistent test results. Always verify contact before proceeding.
3. Test Sequence and Key Parameters
The test procedure consists of multiple runs performed without interruption: baseline, fade, recovery, and wear sequences. Each run has specific load, speed, and temperature conditions. Wear measurements are taken at defined intervals.
| Run | Type | Load (N) | Speed (rpm) | Temperature Condition |
|---|---|---|---|---|
| Baseline | 20 applications (10 s on, 20 s off) | 667 | 417 | 82–104°C |
| First Fade | Continuous drag (max 10 min) | 667 | 417 | Up to 288°C or 10 min |
| First Recovery | 10 s applications at specified temps | 667 | 417 | Cool from 260°C to 93°C |
| Wear Run | 100 applications (20 s on, 10 s off) | 667 | 417 | 193–216°C |
| Second Fade | Continuous drag (max 10 min) | 667 | 417 | Up to 343°C or 10 min |
| Second Recovery | 10 s applications at specified temps | 667 | 417 | Cool from 316°C to 93°C |
| Baseline Rerun | Repeat baseline | 667 | 417 | 82–104°C |
The fade and recovery runs simulate thermal stress and cooling behavior, offering insight into how the lining performs under extreme conditions. Accurate temperature measurement via proper thermocouple placement is essential for reliable results.
Frequently Asked Questions
What is the purpose of burnishing?
Burnishing ensures that at least 95% of the specimen surface is in contact with the drum, which is crucial for repeatable friction and wear measurements.
Why must the test proceed without interruption?
Interruptions can alter the thermal and mechanical history of the specimen, affecting the validity and comparability of results. The standard mandates uninterrupted execution.
How is wear measured in the SAE J661 test?
Wear is assessed using thickness and mass measurements taken before and after the wear run, as well as indicator readings of specimen holder height at intermediate stages.
What are the consequences of incorrect thermocouple placement?
Using the wrong thermocouple location for the drum diameter can lead to inaccurate temperature readings, compromising the accuracy of fade, recovery, and friction coefficient data.
