HEAT TRANSFER IN BRAKING: THE SCIENCE OF FRICTION SELECTION AND PERFORMANCE

A cardinal rule when it comes to brakes is that heat is the enemy of braking performance. In today’s high-performance and severe-duty drum brakes, this is especially true as heat generation and its potential effects are strongly tied to the friction material that provides stopping power.

To better understand the connection between temperature and brake friction, it helps to look at some of the science behind heat transfer in braking, temperature ratings, and other factors that should be considered when choosing friction.

Temperature’s critical role in commercial vehicle drum brake applications are centered around three temperature points within the drum brake system (view the diagram below).

• Bulk Drum Temperature: overall average temperature of the drum
• Bulk Lining Temperature: overall average temperature of the friction material
• Engagement Temperature:temperature at the point of engagement where the friction makes contact with the drum surface (this can be roughly only thousandths of an inch in depth at both the drum surface and lining surface).

These three temperature points represent the full temperature range experienced by these major components within drum brake applications.It’s important to establish a point of reference when discussing friction and temperature.A brake system has a wide variety of components – each with unique thermal properties – which make it nearly impossible to identify one temperature as the most critical temperature for the entire system.

Understanding the brake system’s temperature during a brake application begins at the engagement point, where temperatures can briefly surpass 1,000°F. The large amount of heat that is generated at engagement begins to dissipate as heat is transferred into the drum, with the drum acting as a heat sink.Without that dissipation, the cast-iron drum would be red hot, creating an extreme environment difficult for any type of friction material to withstand.

In less frequent brake stop conditions, the typical temperature ranges for bulk drum and bulk lining temperatures tend to be at two different ranges since they are different materials.A typical bulk drum temperature would be in the range of 300-500°F, where the bulk lining temperature would be within a range of 200-300°F, depending on the friction.

As stops get more frequent, heat continues to transfer into the drum, raising the bulk drum and bulk lining temperatures.These temperatures can reach up to 600°F in bulk drum temperatures and approximately 300-400°F in bulk lining temperatures.

Braking temperatures can increase in some conditions such as severe-duty use, in applications requiring more frequent or harder stops, or in situations in which cooling air to the brake is restricted.Sometimes these two factors go hand-in-hand, where more frequent stops tends to mean less air flow or cooling.These severe-duty applications may require a higher grade friction to better withstand the increased levels of heat.

The goal of brake and friction design is to maintain the lowest possible braking temperatures.Reduced temperatures lower the risk of loss of stopping power, known as brake fade, while also extending the life of your brake friction.

It’s important to remember that the elevated temperatures at the engagement point of the friction and drum dissipate almost immediately. Although it may be acceptable to measure 1,000°F on the drum surface during engagement, prolonged exposure at this temperature will lead to the failure of any NAO (non-asbestos organic) friction.

Placing too much emphasis on keeping brake temperatures low, however, can lead to a misplaced reliance on brake-lining temperature ratings, particularly those that claim ratings of 950° to 1,000°F.

The most valuable and accepted industry tests in the OE and aftermarket friction certification process are the FMVSS 121 and the Society of Automotive Engineers (SAE) J2115 tests, used by Bendix and other leading brake and friction suppliers. These two tests – which all commercial vehicle drum brake friction materials must adhere to – establish the following bulk drum temperature guidelines:

• FMVSS 121 Dynamometer Test – Brake Power or Fade Section: 150° to 450°F
• SAE J2115 (Section 7 – Wear and Effectiveness at Temperature) – Measured at 250°, 350°, 450°, 550°, and 650°F

Neither of these tests references or makes a recommendation on “temperature ratings” for brake friction. Similarly, major truck manufacturers do not include temperature ratings in their friction material certification guidelines. Why? Because without specific information on what temperature is being measured – bulk drum or engagement – and where the readings are being taken, friction temperature ratings are numbers without context. Actual friction life and performance will be dictated by resin quality and performance in the SAE temperature wear section.

The results of both the FMVSS 121 and SAE J2115 tests should be requested from any potential supplier when considering friction for severe-duty applications – even if that supplier has provided “temperature ratings” far exceeding normal system operating temperatures. Regarding specific temperature claims, ask whether they’re measuring bulk drum or engagement heat, and where it’s being measured.

Both in the garage and on the road, it’s exceedingly difficult to accurately measure brake operating temperatures. The only reading that can be easily obtained without equipment such as thermocouples is the bulk drum temperature. And if the bulk drum temperature is anywhere near 1,000°F after brake operation, something is very wrong with the brake, and could even lead to a fire. Only brief in-stop drum brake temperatures should ever exceed 600°F.

Selecting the proper friction is vital for maintaining the performance, service life, and safe operation of drum brakes, especially in severe-duty applications. Familiarity with proven industry standards, as well as the science and reasons behind friction design and recommendations, goes a long way toward making the right choices to meet fleets’ needs and keep trucks on the road and operating safely.