Technical and Training Technical Articles Burnishing of Brake Linings

Burnishing of Brake Linings

Background

It is a known in the friction industry, specifically when dealing with brake pads / linings, that the newly installed units need to be bedded in properly before being used in full service.

Although often neglected and not explained well to the user, bedding-in or conditioning of brake pads is critical to initial performance. Bad conditioning / bedding-in allowance often causes not only the product to fail, but can damage the user’s perception of a brand. It is important to realize that all brake pads, regardless of brand or manufacturer, to a greater or lesser degree, needs to be bedded in before full service. Improper bedding-in or conditioning leads to a phenomenon called green fade.

Green Fade is a term used to describe a condition where the brake lining fails to perform after a series of heavy-duty brake applications soon after installation and where proper bedding-in of the brake linings has been ignored. Another way to describe green fade would be to state that in the event of a rapid rise in braking temperature a rapid decrease of breaking force (friction) is experienced. (See Figure 1 below.)

Portion of competitor dynamometer test graph

Manufacturers have employed various strategies or combinations thereof to deal with this problem. These strategies mostly deal with the design of the friction compounds but inclusion of certain methods in the manufacturing process has also been successfully implemented. One such process is burnishing, or often called scorching, of the friction material.

Introduction to Burnishing / Conditioning

During the bedding-in and subsequent use of the brake lining, the top layers of the friction material in contact with the rotor or disk is exposed to temperature and wear as a result of the energy being converted during the braking applications.

During a braking application the kinetic energy generated is converted to thermal energy. The heat generated (thermal energy) during these applications causes the new resin (glue) which binds the compound together to form gasses. The gasses generated causes pressures to be high enough to force the brake lining to part from the rotor/disk and in turn reduces the frictional properties of the brake lining and in this way causes green fade. This phenomenon is also known as hydroplaning. There are other factors, inherent in the compound, contributing towards fade but resin by far has a major influence under these conditions.

Bedding-in and proper conditioning of the brake linings allows these gasses to be extracted from the working surface of the friction compound over a longer and controlled period of time. Once this conditioning has taken place the brake lining can be used for normal to heavy-duty use without the harmful effects of green fade.

This is possible as the conditioned layer of the friction face is constantly transferred deeper into the lining during use.

Burnishing – Scorching

Burnishing is a method of conditioning the top layer of the friction compound during the manufacturing process removing the need for the bedding-in. This effect is achieved by process equipment that subjects the friction surface, in a controlled environment, to a short burst of heat exposure in the temperature range where green fade occurs. (See Figure 2 below)

Safeline Dynamometer Fade Test Graph

Burnishing is therefore a method of pre-conditioning the brake lining rendering it ready for full service upon installation. The user is in a position to install and drive without having to follow complicated bedding in procedures, although a certain amount of "self-bedding-in" is still present. This method of brake lining preparation is not common in the after-market environment and is mostly used by the Original Equipment Manufacturers.

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