GB2257213A - Lightweight and high thermal conductivity brake rotor - Google Patents

Abstract

A lightweight and high thermal conductivity rotor (12), for use with a brake caliper, comprises: a hub (26); a plurality of openings (25, 25', 25"...) for attachment of the rotor to an axle of a vehicle to rotate with a wheel; spokes (29, 29', 29"...) radially extending from said hub; and an annular head portion attached to said spokes, said head portion having first and second friction surfaces (16) thereon engageable by brake pads on actuation of said caliper to effect a brake application, wherein said rotor is made from a composition comprising from 50-85 percent by volume of silicon carbide, from 0-15 percent by volume of graphite and 50-15 percent by volume of copper, said composition having a theoretical thermal conductivity of 160-310 W/mK. <IMAGE>

Description

LIGHTWEIGHT AND HIGH THERMAL 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 high thermal conductivity characteristics to carry away thermal energy generated between first and second friction surfaces and brake pads located in a caliper during a brake application.

In an effort to increase the overall fuel efficiency of vehicles, the overall weight of vehicles has been decreasing in recent years. One of the ways in which the weight can be reduced is to replace cast iron brake rotors with brake rotors made from aluminium or other lightweight metal. Unfortunately, aluminium is not normally resistant to abrasion. As a result, when aluminium is used, a wear resistant surface coating of the type disclosed in U.S.

Patent No. 4,290,510 must be applied to the friction engagement surfaces. This type of protection for aluminium rotors is adequate for most applications, as long as the thermal energy generated during a brake application produces a temperature below 9000F (4820C).

However, in some instances, the temperature generated may approach the melting point of aluminium, 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 from a wear surface, while maintaining good mechanical properties, such as hardness and strength, at high temperatures during a brake application.

A rotor made from a chromium copper alloy has exhibited a thermal conductivity approximately six times greater than that of cast iron and has exhibited satisfactory performance. Unfortunately, the density of such chromium copper rotors is also more than corresponding cast iron rotors, and the resulting increase in the overall weight of a vehicle would not improve the fuel efficiency as desired.

After evaluating many compositions, silicon carbidecopper alloy composites have been developed for use as brake rotors which have high thermal conductivity and a relative density of approximately two-thirds that of cast iron.

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 compositions of high thermal conductivity and relative light weight for use in a brake rotor to withstand 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 70 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.

This invention relates to a rotor for use with a caliper braking means which comprises: a hub having a plurality of openings therein for attachment of an axle of a vehicle to rotate with a wheel; spokes radially extending from said hub; and an annular head portion attached to said spokes, said head portion having first and second friction surfaces thereon for engagement with brake pads on actuation of said caliper to effect a brake application, wherein said rotor is made from a composition comprising from 50-85 percent by volume of silicon carbide, from 0-15 percent by volume of graphite and 50-15 percent by volume of copper, said composition having a theoretical thermal conductivity of from 160-310 W/mK.

In one preferred embodiment, the rotor is made from a composition which comprises from 50-85 percent by volume of silicon carbide and 50-15 percent by volume of copper, and has a theoretical thermal conductivity of from 160-250 W/mK.

In another preferred embodiment, the rotor is made from a composition which comprises from 50-70 percent by volume of silicon carbide, from 10-15 percent by volume of graphite and 40-15 percent by volume of copper, and has a theoretical thermal conductivity of 280-310 W/mK and a density of 4.0 to 6.0 g/cm3.

The objects and advantages of the invention 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; and Figure 2 is a side view of the rotor of Figure 1.

In the brake system shown in Figure 1 for a wheel of a vehicle, a caliper 28 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'...25a located therein for attachment to an axle 27 of a vehicle. The rotor 12 rotates with a wheel and has spokes 29, 29'...29" which radially extend from said hub to the annular head portion 14. The head portion 14 has a pair of friction faces 16 and 18 formed thereon, which are connected together by a plurality of webs 24 having radially extending spaces 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'...29" allows a certain amount of air flow to cool the rotor 12.

The caliper 28 located on a 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 26, which include a friction lining 38 and a backing plate 40, are positioned on caliper 28 to move axially in a direction generally perpendicular to the plane of 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 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 towards face 18 on rotor 12, while at the same time leg 32 acts through web 31 and leg 30 to pull brake pad 36 towards face 16 on rotor 12. As the friction material 38 of brake pads 34 and 36 engages friction faces 16 and 18, thermal energy is generated. At temperatures below 4000F (2040C), the wear rate of the friction material is primarily controlled by the selection of friction modifiers in the friction material, while at temperatures above 4000F (2040C) 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 rotors 12 may be manufactured and their particular physical properties are identified in Table 1 below.

TABLE 1 Therm. Heat Density Therm. Diff. Cap.

(Kg/m3) Cond. (m2/sec.) (J/m3-K) Material x10-3 (W/m.K) x104 x104 Cast iron 7.1 48 14 3.4 AL MMC (20 SiC) 2.8 177 74 2.4 Copper Alloy 8.9 324 93 3.5 A Cu (50%), SiC (50%) 6.0 304 93 3.3 B Cu (15%), SiC (85%) 4.0 290 110 2.6 C Cu (15%), SiC (55%), 4.9 296 110 2.9 C(15%) * For Comparison 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/K of thermal energy away from the friction pads 34 and 36 at a rate of 14 x 10-6 M2/sec. As long as the temperature generated during a brake application is below 16000F (8710C), 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 aluminium such as aluminium metal matrix composite which includes 20 percent of silicon carbide. A typical rotor 12 made from this composition would have a weight of approximately 4.6 pounds or 2.1 Kg. The use of an aluminium 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 fraction material. As long as the temperature produced by thermal energy generated during a brake application is below 9000F (4820C), a rotor made from this type of aluminium composition performs in a satisfactory manner.Unfortunately in meeting the current standard for braking, the temperature generated may exceed 9000F (4820C) which can result in degradation of the brake lining and braking surfaces on aluminium 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 a rate of thermal conductivity and rate of diffusion approximately six times that of cast iron, and the chromium copper rotors performed satisfactorily in vehicle tests, but unfortunately the weight of the rotor increased to approximately 15.2 pounds or 6.9 Kg. Thus, the use of this type copper base alloy would increase the overall weight of a vehicle. In order to utilize the high conductive property of copper the following compositions identified as A, B and C in Table 1 were developed.

A brake rotor 12 made from composition A having 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 that of a cast iron rotor, and both the conductivity and rate of thermal diffusion as shown in Table 1, would remain approximately that of the chromium copper alloy.

A brake rotor 12 made from composition B having 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 that of a cast iron rotor and both the conductivity and rate of thermal diffusion as shown in Table 1, 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 silicon carbide and copper composition, to produce composition C. A brake rotor 12 made from composition C having 55% by volume of silicon carbide, 15% by volume of graphite fibers and 30% by volume of copper would have a weight less than that of a cast iron rotor, and both the conductivity and rate of thermal diffusion remain approximately that of the chromium copper alloy.

During the manufacture of a rotor 12 from composition A, B or C, silicon carbide and optional graphite fiber are infiltrated by molten copper or copper alloy, such as chromium coper at approximately 1100-14000C. 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.

Claims (7)

CLAIMS:

1. A rotor (12) for use with a caliper braking means (28) which comprises: a hub (26) having a plurality of openings (25, 25', 25"...) therein for attachment of an axle (27) of a vehicle to rotate with a wheel; spokes (29, 29', 29"...) radially extending from said hub; and an annular head portion (14) attached to said spokes, said head portion having first and second friction surfaces (16, 18) thereon for engagement with brake pads (34) on actuation of said caliper to effect a brake application, wherein said rotor is made from a composition comprising from 50-85 percent by volume of silicon carbide, from 0-15 percent by volume of graphite and 50-15 percent by volume of copper, said composition having a theoretical thermal conductivity of from 160-310 W/mK.

2. A rotor according to Claim 1 wherein the copper forms a matrix for uniformly conducting thermal energy away from said first and second friction surfaces (16, 18) on engagement with said brake pads (34).

3. A rotor according to Claim 1 or 2 wherein the composition comprises from 50-85 percent by volume of silicon carbide and 50-15 percent by volume of copper, and has a theoretical thermal conductivity of from 160-250 W/mK.

4. A rotor according to Claim 3 wherein the composition comprises 70 percent by volume of silicon carbide and 30 volume percent of copper and has a density of 4.9 g/cm3.

5. A rotor according to Claim 3 wherein the composition comprises 85 percent by volume of silicon carbide and 15 volume percent of copper and has a density of 4.0 g/cm3.

6. A rotor according to Claim 1 or 2 wherein the composition comprises from 50-70 percent by volume of silicon carbide, from 10-15 percent by volume of graphite and 40-15 percent by volume of copper, and has a theoretical thermal conductivity of 280-310 W/mK and a density of 4.0 to 6.0 g/cm3.

7. A rotor according to Claim 1 and substantially as hereinbefore described, with reference to the accompanying Drawings.