GB2428759A - Brake disc having two sets of brake pads associated therewith - Google Patents

Abstract

A disc brake system comprises a brake disc having independently operable first and second sets of brake pads associated therewith, wherein said first and second sets of brake pads respectively comprise different types of friction material. The brake pads may be fitted within separate independently, operable brake callipers, or a single brake calliper having independently operable pistons associated with each pad. One set of brake pads may be harder than the other. One set of brake pads may comprise organic friction material while the other made of inorganic materials (eg. carbon fibre reinforced ceramics). The second brake set may become operable when braking conditions exceed a predetermined intensity or duration, or information from a temperature sensor. A manual switch may select operability between the brake.

Description

86477.626 Improvements in or relating to disc brakes The present invention

relates to disc brake systems, more particularly to systems in which the disc is equipped with two independently operable sets of brake pads which respectively comprise different types of friction material.

As is well known, braking systems normally operate through dissipation of kinetic energy of a moving object, predominantly through conversion to heat. Disc brakes use the application of force, generally from hydraulic pressure, to bring a pair of brake pads within a caliper into contact with opposed surfaces of a rotating brake disc (also known as a brake rotor).

Disc brake calipers may be floating or fixed. Floating calipers generally have a single hydraulic piston which forces one brake pad onto one surface of the brake disc and generates a reaction which forces the other pad into contact with the opposite side of the disc. Fixed calipers are commonly used in performance cars, such calipers having at least one piston per pad to push that pad onto the brake disc.

A range of "multipot" caliper systems in which several (e.g. 4, 6, 8, 10 or 12) pistons act on each pad is manufactured and sold by Taroni & C. S. a.s. under the registered trademark TAR-OX.

Conventional disc brake systems have one caliper (whether floating or fixed) per disc. Dual caliper systems are also known, however. Thus, for example, US Patent No. 4,795,004 describes a safety brake system for a motor vehicle in which the front wheels are equipped with disc brakes each having a pair of floating calipers. The system incorporates multiple sources of pressurised hydraulic fluid and is designed to maintain braking power should one of these fail. Dual caliper braking systems have also been used in racing cars such as the Porsche 956 and 962, and are fitted to the current Maybach. Spring applied and electrically released dual caliper disc brakes for use with machinery such as mill motors are manufactured by Power Transmission Technology Inc. (www.pttech.com) and are said to minimise shaft bending which may otherwise occur if single caliper brakes are used.

It will be appreciated, however, that dual caliper disc brake systems such as those described above are specialist applications. The vast majority of existing disc brake systems employ single calipers and a single set of brake pads, since such systems are simpler and are considered adequate for most braking needs.

The present invention is based on the realisation that substantial benefits may be achieved by use of a disc brake system in which the disc is equipped with two independently operable sets of brake pads which comprise different types of friction material, in contrast to, for example, known dual caliper systems in which the same friction material is used in the pads for both calipers. In particular it is possible by appropriate selection of brake pad materials to optimise braking efficiency over a far wider range of operating conditions than is possible with existing disc brake systems. The invention thereby provides a simple but highly effective solution to the long recognised problem that there is no such thing as an ideal all round" disc brake pad.

Thus according to one embodiment of the invention there is provided a disc brake system comprising a brake disc having independently operable first and second sets of brake pads associated therewith, wherein said first and second sets of brake pads respectively comprise different types of friction material.

The two sets of brake pads may, for example, respectively be fitted within separate independently operable brake calipers which are appropriately spaced around the disc. Alternatively they may be fitted within a single multipot caliper having independently operable pistons associated with each pad.

The different types of friction material which may be used in the first and second sets of brake pads of disc brake systems in accordance with the invention may, for example, be selected from organic, semi-metallic, metallic and ceramic friction materials.

As is well known in the art, organic friction materials typically comprise an organic binder (e.g. a rubber, an epoxy resin or, more preferably, a phenolic resin, which last may if desired be derived from naturally occurring phenol sources such as cashew nut shell liquid) containing appropriate friction materials and additives.

Historically a high asbestos fibre content was employed in light of the excellent frictional properties of asbestos, but for environmental and health reasons it is now preferred to use non-asbestos based organic friction materials. Typically these contain reinforcing fibres of materials such as glass, mineral wool, mixed metal oxides such as silicalaluminalcalcialmagnesia, metals, ceramics such as potassium titanate, cotton, or organic polymers such as aramids (e.g. Keviar) or acrylics.

Other components may include abrasives intended to maintain surface cleanliness and control build-up of friction films (e.g. particles of aluminium oxide, iron oxides, quartz, silica or zirconium silicate); friction modifiers serving to lubricate, raise friction or react with oxygen to help control interfacial films (e.g. brass, graphite, copper, lead oxide, ceramic microspheres, cashew resin, iron sponge, magnetite, lead, antimony, bismuth, molybdenum, metal suiphides [e.g. cuprous sulphide, antimony sulphide, lead sulphide, molybdenum disulphide or zinc sulphide], mineral fillers [e.g. mullite, kyanite, sillimanite, alumina or crystalline silica] or petroleum coke); fillers (e.g. inorganic salts such as barium sulphate or calcium carbonate or low cost materials such as sea coal or rubber scrap); antioxidants (e.g. graphite); corrosion inhibitors for iron-containing systems (e.g. calcium hydroxide); or resilience modifiers (e.g. diene rubbers, nitrile rubbers or cashew nut shell liquid).

Semi-metallic friction materials typically comprise resin-bonded metallic materials, generally including at least a proportion of fibrous materials such as chopped steel fibres; the overall metal content is usually at least 40%, often 60% or more. The binder may, for example, be a phenolic resin. Other components as described above for organic friction materials may also be included, in particular friction modifiers/antioxidants such as graphite.

Metallic friction materials include copper- and iron-based materials sintered with inorganic additives. Examples include solid state sintered bronze with mullite, and sintered iron with graphite.

Ceramic friction materials include a variety of ceramic matrix composite materials, in particular carbon fibre-reinforced ceramics, also referred to as carbon- ceramics. Examples include carbon-carbon fibre composites and metal carbide- carbon fibre composites such as carbon fibre-reinforced silicon carbide or boron carbide structures. These latter structures may, for example, be manufactured by molten metal infiltration of carbon-carbon fibre composites (see e.g. EP- A- 1019339).

As noted above, the brake systems of the invention permit optimisation of braking efficiency over a far wider range of operating conditions than is possible using existing disc brake systems, by appropriate selection of the different types of friction material used in the first and second sets of brake pads. It is preferred that the friction material of the second set of pads is harder than the friction material of the first set so as to provide enhanced braking under-more extreme conditions, for example during emergency braking or high performance off-road or track use.

Thus, for example, the first set of pads may comprise organic friction materials, which are particularly suited to low temperature operation and which may be used to provide the main braking effort during light to normal braking duty. In such embodiments the second set of pads may comprise harder inorganic friction materials such as semi-metallic, metallic or ceramic materials, which are more effective at higher temperatures and which are therefore suited to provide efficient braking during high energy use such as high speed emergency stops or in racing conditions.

Such systems preferably include control means whereby the second set of pads are operable only under braking conditions which exceed a predetermined intensity or duration. Such control means may, for example, comprise a brake fluid pressure sensor, a torque sensor, a temperature sensor (e.g. which causes the second set of pads to operate once the temperature of the brake disc rises to a predetermined level, for example >500 C), an accelerometer (e.g. which causes the second set of pads to operate at decelerations exceeding I G) or an appropriate link into the anti- lock braking system. Operation of the second set of pads may advantageously be accompanied by partial or complete release of the first set of pads in order to minimise thermal degradation thereof.

In an alternative embodiment manual (i.e. driver operable) switching means such as an electrical flip switch may be provided in order to permit selection of whichever set of brake pads is to be operable. Such an arrangement permits simple switching between, for example, relatively soft (e.g. organic) pads for normal road use and harder pads, e.g. for competitive use.

Brake systems according to such embodiments of the invention make optimum use of the organic friction material of the first set of brake pads whilst avoiding the brake fade and excessive wear which would be expected at high operating temperatures (e.g. >500 C) such as would occur if high energy braking were not transferred at least in part to the second set of pads.

Where multipot calipers are used, either in single or dual caliper embodiments of the invention, it may be advantageous to employ systems in which the number of pistons acting on the pads of the second set exceeds the number of pistons acting on the pads of the first set, in order to maximise high intensity braking efficiency. Thus, for example, the first set of pads may be fitted within a 4- pot caliper, with the second set being fitted within a 6- or I 0-pot caliper.

Alternatively the two sets of pads may be fitted within a single 10- or 12-pot caliper in which 4 pistons act on each pad of the first set and 6 or 8 pistons act on each pad of the second set.

The brake systems of the invention advantageously use brake discs comprising carbon-ceramic materials. Such materials have major advantages compared to conventional cast iron discs, for example exhibiting substantially enhanced stopping force and wear resistance and permitting the disc weight to be reduced by up to 50-60%. This last property is important in improving performance and fuel economy; by reducing the unsprung weight of a vehicle it may also improve its road-holding, handling and comfort.

Existing brake systems using such carbon-ceramic discs together with a single set of carbon-ceramic pads, whilst effective under high energy braking conditions, tend to be noisy and relatively inefficient under light duties. The use of a brake system of the invention in which the first set of brake pads comprise organic friction material and the second set of pads comprise carbon-ceramic material provides a simple but effective solution to this problem.

More generally, however, the brake systems of the invention may include brake discs comprising any appropriate structural material including, for example, metals such as iron or steel and materials such as carbon, graphite, inorganic glasses, ceramics, carbon-carbon fibre composites (including carbon-graphite fibre composites), silicon carbide-carbon fibre composites, metal matrix composites (e.g. metal-containing carbon matrix composites or metal matrix composites reinforced with fibres or particles of ceramic, carbon or metal materials, for example as in aluminium reinforced with alumina fibres), carbon fibre composites, ceramic-carbon fibre composites, ceramic-ceramic fibre composites, polymer matrix composites (e.g. comprising an appropriate thermoplastic or thermoset polymer matrix reinforced with ceramic, carbon or metal fibres) or combinations thereof, optionally together with one or more additives serving to enhance friction properties, reduce wear and/or protect against oxidation.

The brake systems of the invention possess a number of significant advantages. Thus, for example, they may permit substantial weight reduction, both through allowing maximum use of relatively lightweight carbon-ceramic disc and pad materials and because it is possible in dual caliper embodiments to use two small calipers which are lighter (and which may also be cheaper) than a conventional large single caliper; the disc size may also be reduced compared to single caliper systems. Since the total pad area is significantly increased, overall pad lifetimes may likewise be increased; in this way the time and expense involved in pad changes may be encountered less frequently, further adding to cost effectiveness. Noise, vibration and harshness (NVH) levels may also be minimised, particularly at low braking energies, whilst extremely effective stopping power is available at high energies.

The systems provide markedly better performance than the variety of "dual purpose' disc brake pads currently on the market and avoid the need to manufacture complex (and therefore inherently expensive) materials having "broad spectrum" frictional properties for use in such pads. They are also clearly far more practical and convenient than the current practice of changing the brake pads of a vehicle for different usages, e.g. when going from ordinary road driving to rallying or other competitive driving.

Claims (10)

1. Claims: 1. A disc brake system comprising a brake disc having

   independently operable first and second sets of brake pads associated therewith, wherein said first and second sets of brake pads respectively comprise different types of friction material.
2. 2. A disc brake system as claimed in claim 1 wherein said first and second sets of brake pads are respectively fitted within separate independently operable brake calipers.
3. 3. A disc brake system as claimed in claim I wherein said first and second sets of brake pads are fitted within a single brake caliper having independently operable pistons associated with each pad.
4. 4. A disc brake system as claimed in any of claims ito 3 wherein said brake disc comprises a carbon fibre-reinforced ceramic material.
5. 5. A disc brake system as claimed in any of claims ito 4 wherein the friction material of the second set of brake pads is harder than the friction material of the first set of brake pads.
6. 6. A disc brake system as claimed in claim 5 wherein said first set of brake pads comprise organic friction material and said second set of brake pads comprise inorganic friction material.
7. 7. A disc brake system as claimed in claim 6 wherein said inorganic friction material comprises carbon fibre-reinforced ceramic material.
8. 8. A disc brake system as claimed in any of claims 5 to 7 further comprising control means whereby the second set of brake pads are operable only under braking conditions which exceed a predetermined intensity or duration.
9. 9. A disc brake system as claimed in claim 8 wherein said control means comprise a brake fluid pressure sensor, a torque sensor, a temperature sensor or an accelerometer.
10. 10. A disc brake system as claimed in any of claims I to 7 further comprising manual switching means adapted to select operability of the first or second sets of brake pads.