GB/T 17469-2024Evaluation of the friction characteristics of automotive brake linings - Small sample bench test method (English PDF)

1 Scope

China's national small-sample bench test for the friction characteristics of automotive brake linings - the test that assigns a friction coefficient grade to a brake material. It specifies the test equipment, the test preparation, the test procedure, the treatment of results, the classification and grade marking of the friction coefficient, and the test report, and it applies to disc and drum brake linings for automobiles. It does not apply to parking brake linings. The friction coefficient of a brake lining is not a constant. It falls as the material heats, recovers as it cools, changes with sliding speed and with contact pressure, and differs between the first stops of a new lining and the same lining after bedding. A material that gives a high coefficient cold and loses it at temperature is a fade risk; a material with a low coefficient that holds it is often the safer one. The purpose of the test is to characterise that behaviour on a small specimen rather than on a full brake assembly, which makes it fast enough to be used for development, for production control and for the grade code that appears on the edge of the lining. A small specimen is cut from the lining and rubbed against a rotating disc of specified material under controlled pressure and speed, through a prescribed sequence of bedding, heating, fade and recovery stages, with the coefficient recorded throughout. The classification then reduces that curve to two letters - the normal and the hot coefficient bands - which is the code a workshop reads when matching a replacement part. Because the answer depends entirely on how the specimen was run, the standard fixes the machine, the disc material and surface, the specimen dimensions and preparation, the bedding, the pressures and speeds, and the temperature sequence, and the informative annexes give the data recording sheet and the report format. Issued on 29 September 2024 and in force since 1 April 2025, it replaces GB/T 17469-2012.

This document specifies the test equipment, test preparation, test procedure, result processing, friction coefficient classification and friction coefficient grade marking, and test report for the small sample bench test method for friction performance evaluation of brake linings for automobile. This document applies to automotive disc brake linings and automotive drum brake linings (hereinafter referred to as "linings"). This document does not apply to parking brake linings.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 5620 Road vehicles - Vocabulary and definition for braking of automotive vehicles and their trailers

3 Terms and definitions

For the purpose of this document, the terms and definitions defined in GB/T 5620 apply.

4 Test equipment

4.1 Test machine The schematic diagram of small sample bench test machine for friction materials is shown in Figure

1.The test machine shall have the following functions.

5 Test preparation

5.1 Preparation of test pieces Installation position Brake drum's inner diameter phi/mm

5.1.1 Randomly select 5 linings from each batch of products. Prepare a test piece from each lining.

5.1.2 The test piece shall be cut from the middle part of the lining, and the distance from the sampling position to the periphery of the lining shall be roughly equal.

5.1.3 The length and width of the test piece is (25.4 ± 0.2) mm × (25.4 ± 0.2) mm; the test friction surface is arc-shaped, and the radius of the arc shall match the brake drum; the back is flat, and the thickest thickness (or the test piece plus the silencer) is (6.0 ± 0.5) mm.

6 Test procedure

6.1 First baseline At a brake drum speed of 417r/min, load (load 667 N) the test piece for 10 s and unload for 20 s, for a total of 20 times. The test starts from the brake drum temperature of 82 °C ~ 93 °C, and the cooling air is used to maintain the brake drum temperature at 82 °C ~ 104 °C during each loading. The cooling air is turned off for the 20th loading.

6.2 First decay Turn off the heating and air cooling, and let the brake drum rotate at 417 r/min to cool naturally. When the brake drum temperature drops to 82 °C, load the test piece and turn on the heater at the same time. Continuously drag-wear at a speed of 417 r/min and a loading force of 667 N.

6.3 First recovery After the first decay (6.2) is completed, turn off the heater and turn on the cooling air. At a test speed of 417 r/min, during the process of brake drum temperature dropping from 288 °C to 93 °C, load (load 667 N) the test piece for 10 s at 260 °C, 204 °C, 149 °C and 93 °C, and record the friction force and friction coefficient at each loading.

6.4 Second wear measurement Perform wear measurement according to 5.5.

6.5 Wear test At a brake drum speed of 417 r/min, load (load 667 N) the test piece for 20 s and unloaded for 10 s, for a total of 100 times. The test starts from the brake drum temperature of 193 °C ~ 204 °C, and the brake drum temperature shall be kept between 193 °C ~ 216 °C, which can be achieved by using a heater or air cooling.

6.6 Third wear measurement After the wear test (6.5) is completed, when it cools to 88 °C ~ 99 °C, perform wear measurement again according to 5.5.

6.7 Second decay After the third wear measurement (6.6) is completed, turn off the heater and cooling air immediately, and let the brake drum rotate at 417 r/min to cool naturally. When the brake drum temperature drops to 82 °C, turn on the heater, and continue to drag-wear at a speed of 417 r/min and a load of 667 N. When the temperature reaches 343 °C, the test is completed. If the temperature cannot reach 343 °C, the drag-wear time can reach 10 min, and the result at the end of 10 min shall prevail. Starting from 93 °C, record the friction force and friction coefficient every 28 °C, and record the time, friction force and friction coefficient to reach 343 °C.

6.8 Second recovery After the second decay (6.7) is completed, turn off the heater and turn on the cooling air. At a test speed of 417 r/min, during the process of brake drum temperature dropping from 343 °C to 93 °C, load (load 667 N) the test piece for 10 s at 316 °C, 260 °C, 204 °C, 149 °C and 93 °C, and record the friction force and friction coefficient at each loading.

6.9 Second baseline Repeat the steps in 6.1.

6.10 Final wear measurement Perform wear measurement according to 5.5.

6.11 Final thickness and mass measurement Take out the test piece after the test and measure the thickness and weigh the mass according to 5.4.

7 Result processing

7.1 Data collection and processing In the intermittent loading test, the friction coefficient is taken as the value at the end of loading. The measurement data retains three significant digits.

7.2 Test record Test data and curve graphs shall be recorded for each test piece. The test data record table and curve graph are shown in Annex A.

8 Friction coefficient classification and friction coefficient

8.1 Friction coefficient classification For ease of description, the friction coefficient is divided into six categories. C, D, E, F, G, and H, see Table 3.

8.2 Friction coefficient level marking The friction coefficient level of each batch of linings is marked according to the friction coefficient at room temperature and friction coefficient at high temperature calculated in

7.3.1 and 7.3.2, and the category code in Table 3.