Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LIGHTWEIGHT AND HIGH T~T~'RMAT CONDUCTIVITY BRAKE ROTOR
This invention relates to a brake rotor made from
composites of from 50-85 percent by volume of silicon carbide
and from 50-15 percent by volume of copper. The copper in the
composite imparts a high thermal conductivity characteristic
to carry away thermal energy generated between first and
second friction surfaces on the brake rotor and brake pads
located in a caliper during a brake application.
In an effort to increase the overall fuel efficiency for
a vehicle, the overall weight of vehicles has been decreasing
for a period of time. One of the ways that the weight can be
reduced is to replace a typical cast iron brake rotor with a
brake rotor made from aluminum or another light weight metal.
Unfortunately, aluminum is not normally resistant to abrasion,
and as a result, when aluminum is used, a wear resistant
surface coating of the type disclosed in U.S. Patent 4,290,510
must be applied to the friction engagement surfaces. This
type of protection for aluminum rotors is adequate for most
applications as long as the thermal energy generated during a
brake application is below 900~F. However, in some instances,
the thermal energy generated may approach the melting point of
aluminum and as a result the rotors will become too soft.
Therefore it is imperative to develop a rotor having the
capability~of conducting thermal energy away from a wear
surface while maintaining good mechanical properties such as
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hardness and strength at high temperatures during a brake
application.
A rotor made from a chromium copper alloy which was
developed, exhibited a thermal conductivity of approximately
six times greater than cast iron, and exhibited satisfactory
performance. Unfortunately, the density of such chromium
copper rotors is also more than corresponding cast iron rotors
and as a result an increase in the overall weight of a vehicle
would not improve a desired fuel efficiency.
After evaluating many compositions, silicon
carbide-copper alloy composites were developed for use as a
brake rotor which has high thermal conductivity and a relative
density of approximately two-thirds of cast iron. The alloy
was selected from a composition having from 50-85 percent by
volume of silicon carbide, from 0-15 percent by volume of
graphite (optional), and from 50-15 percent by volume of
copper. The composition is heated in a mold to form a unitary
brake rotor. The brake rotor has a hub with a plurality of
openings therein for attachment to an axle of a vehicle which
rotates with a wheel and spokes which radially extending from
said hub to an integral annular head portion. The head
portion has first and second friction surfaces thereon for
engagement with brake pads during a brake actuation. The
brake rotor has a density of 4.0 to 6.0 g/cm3 and a resultant
thermal conductivity of from 280-310 W/mK. Therefor maintains
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a substantially uniform structural strength above 900~F.
It is an object of this invention to provide compositions
of silicon carbide and copper or copper alloys for use in a
brake rotor.
It is a further object of this invention to provide high
thermal conductivity and relative light weight compositions
for use in a brake rotor, capable of withstanding the
generation of thermal energy during a brake application
without degradation.
It is a still further object of this invention to provide
an alloy for use in a brake rotor having a silicon carbide,
graphite fiber and copper composition with a density of
approximately seventy percent of cast iron but with a six
times greater thermal conductivity to maintain the
effectiveness of a brake system over a wider range of
operation.
These objects and advantages should be apparent from
reading this application while viewing the drawings wherein:
Figure 1 is a schematic illustration of a brake system
wherein a rotor made according to this invention is located
between friction pads carried by a caliper;
Figure 2 is a side view of the rotor of Figure 1; and
Figure 3 is a table illustrating physical characteristics
of various compositions of the rotor of Figure 1.
In the brake system shown in Figure 1 for a wheel of a
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vehicle, a caliper 10 retains brake pads 34 and 36 for
engagement with a rotor 12 made from an alloy selected from a
composition shown in Figure 3.
Rotor 12 has a hub 26 with a plurality of openings 25,
25 ~ . . .25n located therein for attachment to an axle 27 of a
vehicle. The rotor 12 rotates with a wheel and has spokes 29,
29'...29n which radially extend from the hub 26 to an integral
annular head portion 14. The head portion 14 has a pair of
friction faces 16 and 18 formed thereon which are separated
from each other by a plurality of webs 24 to define a radially
extending space therebetween. The webs 24 hold the engaging
faces 16 and 18 parallel while the spaces therebetween allow
the flow of cooling air between the webs to promote cooling of
the rotor 12. In addition the space between the spokes 29,
29 ' . . .29n also allows a certain amount of air flow to cool the
rotor 12.
A caliper 28 located on the vehicle has a pair of legs 30
and 32 which are located in a spaced parallel relationship
with faces 16 and 18 on rotor 12. Brake pads 34 and 36, which
include a friction lining 38 and a backing plate 40, are
positioned on caliper 28 to axially move in a direction
generally perpendicular to the planar rotation of the rotor 12
in response to hydraulic fluid being supplied to chamber 41 of
fluid motor 42.
The fluid motor 42 is carried by leg 32 of caliper 28 and
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includes a piston 44 located in cylinder bore 46. A flexible
boot or seal 48 has one end fixed to the caliper and the other
end fixed to piston 44 to seal chamber 41 and prevent dirt,
water and other contaminants from entering bore 46.
During a brake application, hydraulic fluid is supplied
to chamber 41 to move piston 44 and brake pad 34 toward face
18 on rotor 12 while at the same time leg 32 acts through web
31 and leg 30 to pull brake pad 36 toward face 16 on rotor 12.
As the friction material 38 of brake pads 34 and 36 engage
friction faces 16 and 18 thermal energy is generated. At
temperatures below 400~F, the wear rate of the friction
material is primarily controlled by the selection of friction
modifiers in the friction material while at temperatures above
400~F the wear rate increases exponentially with increasing
temperature due to thermal degradation of the binder in the
friction material. Thus, it is important that thermal energy
generated during braking be conducted away from the friction
material as quickly as possible.
Various materials from which such rotors 12 may be
manufactured and their particular physical and thermal
characteristics are identified in Figure 3.
From experimentation it has been determined that a
typical rotor 12 made from gray cast iron weighs about 12
pounds or approximately 5.5 Kg. A rotor of this type could be
expected to conduct 48 W/mK of thermal energy away from the
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friction pads 34 and 36 at a rate of 14 M2/sec x 10-6. As long
as the temperature generated during a brake application is
below 1600~F this type of rotor performs in a satisfactory
manner. However, in order to reduce the overall weight of a
vehicle, it has been suggested to replace the cast iron with
aluminum, such as aluminum metal matrix composite, which
includes 20 percent silicon carbide. A typical rotor 12 made
from this composition (Al MMC) would have a weight of
approximately 4.6 pounds or 2.1 Kg. The use of such an
aluminum alloy composition provides a considerable reduction
in weight, has a three and one-half increase in the
conductivity of thermal energy with an approximate five fold
rate of diffusion away from the friction material. As long as
the thermal energy generated during a brake application is
below 900~F, a rotor made from this type of aluminum
composition performs in a satisfactory manner. Unfortunately
in meeting the current standard for braking, the thermal
energy generated may exceed 900~F which can result in a
degradation of the brake lining and braking surfaces on
aluminum composite rotors. Thus, a need exists to increase
the thermal capability of the brake rotor.
A brake rotor 12 was made from a chromium copper alloy.
The chromium copper alloy has approximately a six times rate
of thermal conductivity and rate of diffusion and the chromium
copper rotors performed satisfactory in vehicle tests, but
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unfortunately the weight of the rotor also increased to
approximately 15.2 pounds or 6.9 Kg. Thus, the use of this
type of copper base alloy would increase the overall weight of
a vehicle to an unacceptable level. In order to utilize the
high conductive property of copper the following compositions
identified in Figure 3 as A, B and C were developed.
A brake rotor 12 made from composition A having about 50~
by volume of silicon carbide and 50~ by volume of copper would
have a weight of approximately 10.2 pounds or 4.7 Kg which
would be less than a cast iron rotor, and both the
conductivity and rate of thermal diffusion as illustrated in
Figure 3 would remain approximately that of the chromium
copper alloy.
A brake rotor 12 made from composition B having about 85~
by volume of silicon carbide and 15~ by volume of copper would
have a weight of approximately 6.8 pounds or 3.1 Kg which
would be less than a cast iron rotor, and both the
conductivity and rate of thermal diffusion as illustrated in
Figure 3 would remain approximately that of the chromium
copper alloy.
In order to provide additional structural strength to a
rotor it was suggested that graphite fibers could be added to
the basic silicon carbide and copper composition to produce
composition C shown in Figure 3. A brake rotor 12 made from
composition C having about 55~ by volume of silicon carbide,
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15% by volume of graphite fibers and 30% by volume of copper
would have a weight less than a cast iron rotor, and both the
conductivity and rate of thermal diffusion as illustrated in
Figure 3 would remain approximately that of the chromium
copper alloy.
During the manufacture of a rotor 12 from composition A,
B or C, silicon carbide and graphite fiber are infiltrated by
molten copper or copper alloy such as chromium copper at
approximately 1100-1400~C. This temperature which is below
the melting point of silicon carbide and graphite fibers is
sufficient to cause the copper to flow and create an
interconnected matrix for the resulting rotor 12.
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