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.)
Figure 1 (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)
Figure 2 (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.
Article by W. S. Scholtz
issued by Safeline: Oct. 2001, rev. May 2002
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