Using Tribology to Evaluate Brake Pads


 

Made up of a variety of different ingredients, brake pads are composite materials that need to satisfy a large number of safety requirements. Ideal brake pads are designed to have a high coeffi­cient of friction (COF), low wear rate, as well as minimal noise. Moreover, they remain reliable under variations in the environment.

Tribology testing can be used to identify critical specifications, to ensure that the quality of brake pads can satisfy their requirements.

Importance of Evaluating Brake Pad Performance

The importance placed on the reliability of brake pads cannot be overstated: after all, they decide the safety of passengers, which should never be neglected. Thus, it becomes critical to replicate operating conditions and thereby identify potential points of failure.

Using the Nanovea Tribometer produces a constant load between a pin, ball, or ­flat as well as a counter material that is constantly in motion. The friction thereby caused between the two materials is collected using a stiff load cell, ensuring collection of material properties at different loads and speeds. Further, these are then tested in high temperature, corrosive, or liquid environments.

Equipment Featured

NANOVEA T50

  • Versatile Wear and Friction Tester
  • Multi-Module System
  • Speed Control ranging from 0.01-5000 RPM
  • Robust with Open Platform
  • Wide Range of Environmental Conditions

Measurement Objectives

This study analyzed the coeffi­cient of friction of the brake pads under an environment where temperatures were continuously increased from room temperature to 700 °C. Subsequently, the environmental temperature was raised in-situ until the brake pad noticeably failed. In order to measure the temperature near the sliding interface, a thermocouple was attached to the underside of the pin.

Sample of Brake Pad in Nanovea Hot Temperature Module (T50).

Sample of Brake Pad in Nanovea Hot Temperature Module (T50).

Tribology Results

Measurement Parameters

Table 1: Test parameters for the brake pads COF test.

Test parameters Value
Normal force 20 N
Rotational speed 300 rpm
Duration of test 45 min
Revolutions 13502
Distance traveled 847.9 m
Atmosphere Air
Temperature 24 °C (room) – 700 °C
Wear track radius 10 mm
Pin material SS440C
Pin diameter 6 mm

Samples

Sample of Brake Pad

Sample of Brake Pad

The main point of focus of this study was the temperature that causes brake pads to fail. The COF obtained do not represent real-life values; nor is the pin material the same as brake rotors. It also bears mentioning that the data collected regarding temperature represents the temperature of the pin and not the sliding interface temperature.

COF and Temperature vs Revolutions.

Figure 2: COF and Temperature vs Revolutions.

COF vs Temperature.

Figure 3: COF vs Temperature.

At the beginning of the test (i.e., at room temperature conditions), the COF between the SS440C pin and brake pad produced a consistent value of approximately 0.2. However, as the temperature started to increase, the COF also rose steadily, peaking at a value of 0.26 near 350 °C. Once the temperature crossed 390 °C, the COF quickly started to drop. However, the COF began to increase back to 0.2 at a temperature of 450 °C, but started to fall to a value of 0.05 shortly after.

Through this study, the failure temperature at which the brake pads consistently failed was identi­fied at temperatures above 500 °C. Beyond this level, the COF was no longer able to retain the starting point COF of 0.2.

Conclusion

As described above, the brake pads showed consistent failure at a temperature that was above 500 °C. The initial COF of 0.2 slowly rose to a value of 0.26 before it plummeted to 0.05 as the test ended (580 °C). A factor of 4 is the difference between 0.05 and 0.2. In other words, the normal force at 580 °C must be 4x greater than at room temperature to achieve similar results – i.e., the same stopping force.

However, what was not included in this study was the fact that the Nanovea Tribometer is also equipped to conduct testing to observe yet another vital property of brake pads: wear rate. The volume of the wear track can be analyzed to calculate how quickly samples wear, using 3D non-contact profilometers. To best simulate operating conditions, wear testing can be conducted with the Nanovea Tribometer under multiple diff­erent test conditions and environments.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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