Patent Application
20060289255
Not Applicable
Not Applicable
Not Applicable
1. Field of Invention
This invention relates to disc brake systems, specifically to the types that are commonly used on road vehicles.
2. Prior Art
Disc brake systems are well known, having been used extensively in the automotive industry for many years. The prior art concerning disc brakes includes systems that are produced in a wide variety of sizes, types, and configurations, but the basic purpose of all such friction devices is the same—to convert kinetic energy (the energy of motion) into thermal energy (heat). By design, the friction between disc brake pads and brake rotors during braking generates tremendous quantities of heat as brake systems work to slow or stop moving vehicles.
However, although the prior art includes many methods and devices for dissipating and/or insulating brake-generated heat, there is a problematic aspect of brake heat generation which has been essentially unrecognized. This aspect has to do with the generation of excessive heat at friction interfaces (pad/rotor interfaces) during braking, a phenomenon which has a significant, adverse effect on braking efficiency and brake system longevity.
The Unrecognized Problem
It is not only natural but also unavoidable that brake rotors distort during use because of thermal expansion in the presence of the intense pressures, frictional drag, and other dimensional factors involved in braking action. As rotors repeatedly heat up and cool down, permanent irregularities develop on their friction faces. These irregularities may be referred to as “high spots”. High spots begin to develop as soon as rotors are put into service, and although they tend to develop gradually during normal brake use, they nonetheless begin to affect braking dynamics as soon as they begin to form—long before their presence becomes noticeable to drivers as a “pulsating pedal”.
It is well known that the “column of fluid” which presses brake pads against rotor faces during braking is typically pressurized to several hundred psi. Thus, because of the incompressible nature of liquids, disc brake pads are pressed against their corresponding rotor faces by a highly pressurized but incompressible column of fluid.
Therefore, when brakes are applied, and brake pads are pressed against rotating rotor faces by highly pressurized, incompressible fluid, high spots on the rotor faces are forced to “squeeze” between the pads as the rotors rotate. The brake pads, of course, resist any backward movement because of the incompressible fluid behind them (“backward movement” refers to movement of the pads perpendicular to and away from the face of the rotor, and will hereafter be termed “lateral” movement). Thus, as high spots on the rotor faces pass between applied pads during braking, severe and excessive frictional drag is produced. This generates much more heat than would be the case if there were no high spots on the rotors.
The problem which has been unrecognized in the prior art then is that the severe frictional drag between rotor high spots and brake pads during braking generates excessive heat. Perhaps the fact that brake systems are designed to generate tremendous quantities of heat has obscured recognition of the role that rotor high spots play in the generation of excessive friction and heat.
Common Problems Caused or Aggravated by High Spot Friction
It is known that brake systems operate at varying degrees of efficiency until friction interface temperatures reach the 205-232° C. (400-450° F.) range, and that as interface temperatures continue to climb above this “efficiency threshold”, there is a corresponding drop in brake efficiency. The hotter brakes get (above the efficiency threshold), the less efficient they become.
It is also known that the rate of wear of brake system friction components (pads and rotors) increases as braking temperatures increase.
Thus, the unrecognized problem (excessive heat generated by high-spot friction) is the underlying cause of reduced braking efficiency, accelerated brake wear, and other commonly recognized problems. These problems include:
Further problems caused by severe frictional drag at high spots (related more directly to frictional drag itself than to the excessive heat it generates) are these:
Brake system engineers have long recognized the problems caused by excessive brake-generated heat, and, as mentioned above, many methods and devices have been proposed to insulate and/or dissipate brake-generated heat in the attempt to improve brake efficiency and longevity. For instance, U.S. Pat. No. 6,796,405 (Ruiz, 2004), U.S. Pat. No. 6,119,820 (Steptoe, et al., 2000), U.S. Pat. No. 5,878,848 (Zhang, 1999), and U.S. Pat. No. 5,161,652 (Suzuki, 1992), among others, describe various ways of ventilating rotors to help dissipate brake-generated heat into the air.
U.S. Pat. No. 6,508,340 (Bunker, 2003) and U.S. Pat. No. 5,330,034 (Rancourt, et al., 1994) describe vanes and air scoops, respectively, to improve the cooling of brake assemblies by forcing air over and through them. U.S. Pat. No. 6,315,091 (Nilsen, et al., 2001) describes a method for channeling “ram” air to brake assemblies for cooling, and U.S. Pat. No. 6,578,678 (Lee, 2003) provides a splash shield to divert air to rotors for cooling.
U.S. Pat. No. 6,722,476 (Cornolti, 2004) and U.S. Pat. No. 6,491,139 (Budica, 2002) describe methods for dissipating brake heat with cooling liquids.
U.S. Pat. No. 6,419,054 (Schulba, 2002) describes a system of nozzles that spray liquefied nitrogen and carbon dioxide on brake assemblies, for cooling.
U.S. Pat. No. 6,085,636 (Ruckert, et al., 2000) describes a caliper piston with improved heat conductivity which helps reduce pad degradation caused by excessive braking temperatures.
U.S. Pat. No. 4,373,615 (Melinat, 1983) describes a laminated disc brake pad assembly with ceramic and rubber-like layers designed to provide a heat barrier for the prevention of brake fluid boiling and to suppress brake-generated noise.
U.S. Pat. No. 6,446,770 (Qian, et al., 2002) describes a rotor with grooved faces which minimize thermal distortions (high spots). However, the grooved rotor of Qian's invention, although it slows the formation of high spots, does not eliminate their formation, and no provision is made for reducing high-spot friction once the high spots begin to form.
Also, to combat the problem of wheel lockup (problem #6, above), many versions of electronic anti-lock brake systems have been invented. These systems, however, are designed to engage only during “lockup” stops for the purpose of providing steering control, and do nothing to improve brake efficiency during normal stops, which make up the vast majority of stops made during the life of any vehicle.
Additionally and in general, there have been other attempts to reduce the harmful effects of excessive brake-generated heat. For example, friction material (brake pad) composition has evolved from asbestos-laden recipes to semi-metallic and non-organic recipes which not only withstand excessive heat more effectively, but also absorb and dissipate heat more effectively. Brake fluids with higher boiling points have been developed. Low-drag calipers and step-bore master cylinders have been developed to reduce the heat generated by non-braking wear.
However, although each of the above methods/devices provides some measure of improvement in overall brake system efficiency, and/or helps to solve the commonly recognized problems caused by excessive braking heat, none of them address the underlying problems of (a) excessive heat generated by high-spot friction, and (b) the wheel lockup and torque transfer problems caused by severe frictional drag at high spots.
Solving the Problem
One method for solving the problems described above would involve increasing the size of disc brake systems—using larger (and thicker) rotors and larger-area brake pads (which would require larger calipers and redesigned master cylinders, et cetera). This method would practically eliminate the formation of high spots on rotor faces by improving the ratio of friction material area to vehicle weight and by increasing the heat-sink capabilities of brake assemblies, resulting in lower brake operating temperatures. However, this method would add undesired weight to vehicles and would be prohibitively expensive to implement.
Another method would involve providing a cushion (such as a hydraulic accumulator) inside the brake hydraulic system, which, if designed and calibrated correctly, could reduce high-spot friction and provide more efficient braking. Actually, several devices of this general type have been proposed (U.S. Pat. No. 5,265,942, issued to Jones in 1993; U.S. Pat. No. 5,820,227, issued to Spero in 1998; and U.S. Pat. No. 6,322,160, issued to Loh, et al. in 2001, for instance), but these (and similar) devices are designed to absorb hydraulic shock, and do not recognize the role that high spots play in the generation of excessive heat during braking. However, even if such a device were designed and calibrated specifically to provide optimum high-spot friction compensation, it would nonetheless be difficult to market because of consumer cost and liability issues.
So, there remains a need for a simple and cost-effective method for reducing high spot friction and heat—which brings us to the method of this invention.
The method of this invention is to provide a mechanical cushion between brake pads and their corresponding caliper members with such characteristics of resilience that brake pads are allowed to follow the irregularities (ride over the high spots) of their corresponding rotor faces while under pressure during braking. Such cushioning significantly reduces high-spot friction, which lowers operating temperatures and increases brake system efficiency.
Resilient Shims in the Prior Art
The use of resilient brake pad shims (cushions) is not unknown in the prior art. For instance, U.S. Pat. No. 3,885,651 (Odier, 1975), U.S. Pat. No. 4,093,045 (Kawamura, 1978), and U.S. Pat. No. 4,660,685 (Thacker, et al., 1987), among others, describe resilient shims which are placed in disc brake systems in generally the same location that my invention occupies—between outboard brake pads and outboard caliper fingers, and/or between inboard brake pads and caliper pistons. However, the shims of these patents are designed specifically to attenuate high-frequency vibrations and eliminate brake “squeal”, and do not possess the special characteristics of resilience required to solve high-spot friction and heat generation problems (the vibrations caused by frictional drag at rotor high spots occur entirely within a much lower frequency range, and at a much higher amplitude, than do audible vibrations).
Similarly, the vibration (squeal) attenuator disclosed in U.S. Pat. No. 5,014,827 (Wang, et al., 1991), although it is resilient and is placed on the face of a caliper piston, does not possess the special characteristics of resilience required to solve high-spot friction and heat-generation problems.
The prior art, then, although it contains methods and devices designed to compensate for excessive braking heat after it is generated, does not contain a method or device designed to prevent the generation of excessive heat by severe frictional drag at rotor high spots. The prior art also contains methods and devices which, although similar to my invention in structure and in their placement in disc brake assemblies, are nonetheless designed to solve an entirely different problem (brake squeal), and do not possess the characteristics of resilience necessary to solve the problem of excessive heat generation during braking.
It should be mentioned here that, although the prior art solutions for brake squeal do not solve the problem of excessive heat generation at rotor high spots, the reverse is not true; that is, solving the problem of excessive heat generation at rotor high spots by the method of this invention also effectively attenuates high-pitched brake squeal.
Accordingly, several objects and advantages of the present invention are:
Further objects and advantages are to provide a cushion for disc brake pads which is compatible with any disc brake system and with any electronic ABS, which shortens stopping time (helping to compensate for slowed reaction time of elderly drivers, poor visibility, or surprises in traffic), which is inexpensive for consumers, which is easy to install, which effectively attenuates brake squeal, and which never requires any maintenance or adjustment. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
In accordance with the present invention, disc brake pad cushions comprise: (a) resilient devices with such characteristics of resilience that, when the resilient devices are secured in place between disc brake pads and their corresponding caliper members, the brake pads are allowed to follow the irregularities (ride over the high spots) on their corresponding rotor friction faces while under pressure during braking, and (b) means for securing the resilient devices in place.
A preferred embodiment of the brake pad cushions of the present invention is illustrated in
Cushion Material—Preferred Embodiment
In the preferred embodiment, the cushions are cut from a sheet (or molded from a mixture) of elastomeric material which is a blend of natural rubber (NR) and polybutadiene rubber (BR) in an approximate ratio of 75% NR to 25% BR. Small amounts of aromatic amines approximately 1-4 parts per hundred rubber) are also incorporated in the blend to prevent the possibility of metal-catalyzed oxidation, which could otherwise occur when the cushions are mounted on brake systems using cast-iron calipers. Such a compound can be obtained from Goodyear Tire and Rubber Company of Akron, Ohio, USA, or from other rubber mixing companies.
Cushions made from this particular elastomeric compound provide very high rebound resilience with very low heat buildup from cyclic (high spot) deformation. They exhibit very low compression-set and stress-relaxation characteristics, and very good resistance to abrasion, tear, and fatigue. However, although cushions made from this particular compound perform very well, maintain their integrity in service, and provide the intended benefits (including squeal damping) for all typical applications, the cushions can, of course, be made from other compounds. There are many elastomeric compounds—natural and synthetic—which could be used to make cushions which could satisfy the performance objectives set forth in this specification in a generally acceptable way. In fact, with further research and testing, application-specific cushions can easily be developed and produced. For instance, disc brake systems in severe-service applications such as racing cars or aircraft, which encounter extremely high braking temperatures, can benefit from cushions which have been designed specifically to operate in their particularly rigorous environments.
Cushion Shape and Size—Preferred Embodiment
In the preferred embodiment, the inboard brake pad cushion 12 (
In the preferred embodiment, the outboard brake pad cushions 20 (
Brake systems which incorporate “fixed” calipers with opposed pistons use the above inboard cushion 12 design for each piston. This includes those pistons which apply an outboard brake pad.
Cushion Thickness—Preferred Embodiment
Cushions 12 and 20 (
Cushion Mounting and Placement—Preferred Embodiment
In the preferred embodiment, one side of inboard cushion 12 is provided with peel-and-stick adhesive 26, and one side of each outboard cushion 20 is provided with peel-and-stick adhesive 28 (FIGS. 3A and 3B)—available from 3M Company—to secure the brake pad cushions in place. However, although the use of peel-and-stick adhesive to secure the cushions in place is very simple, convenient, and inexpensive for “do-it yourself” consumers, professional mechanics, and brake manufacturers alike, there are, of course, various ways to secure the cushions in place. This includes (but is not limited to) methods such as using peel-and-stick adhesive on both sides of the cushions, using other appropriate types of adhesive, using clips of various designs, using rivets, screws, or bolts, or providing mating physical features on the cushions and/or on their corresponding caliper members or brake pad backing plates which serve to “lock” them together. Still further methods for attaching or securing cushions in place during brake pad or caliper manufacture (such that caliper assemblies or sets of brake pads could be sold with cushions attached or included in packaging) can easily be devised—the important consideration here being that the cushions remain securely in place throughout the service life of the brake pads.
In the preferred embodiment, the inboard cushion 12 may be adhered either to the face 47 of its caliper piston 46 or to the backing plate 34 of its corresponding brake pad 32, and the outboard cushions 20 may be adhered either to the caliper fingers 48 or to the backing plate 40 of their corresponding brake pad 38 (
Operation—Preferred Embodiment
Making use of the brake pad cushions 12 and 20 (
Installation involves following the vehicle manufacturer's instructions for replacing disc brake pads, with the following simple addition: after making sure that brake pad backing plates 34 and 40, caliper piston face 47, and caliper fingers 48 are clean and dry, peel-and-stick the brake pad cushions 12 and 20 in place (
Peel-and-stick adhesive may be provided on one or both sides of the cushions.
Brake systems which incorporate fixed calipers with opposed pistons use the above inboard cushion 12 design for each piston. This includes those pistons which apply an outboard brake pad.
Once the installation is complete, the cushions are ready to provide their benefits—there is no break-in period required. Every time the brakes are applied, the cushions instantly and automatically absorb any backward (lateral) movement of the brake pads, which is caused by high spots on the rotating rotor faces. Because of their unique characteristics of resilience, the cushions are able to absorb the lateral pad movement caused by rotor high spots, thereby practically eliminating the transfer of this motion to the caliper piston and caliper fingers. The result of this cushioning is that the brake pads are allowed to follow the irregularities (ride over the high spots) on the rotor friction faces during braking, which significantly reduces the severe frictional drag at rotor high spots. Reduced frictional drag means lower braking temperature, and lower braking temperature translates directly into increased braking efficiency and brake system longevity.
The principle of the present invention (providing resilient cushioning for disc brake pads to accomplish the performance objectives described above) can be practiced using a variety of structures and/or materials. The important consideration concerning cushion manufacture is not so much what they are made of but whether they are capable of providing adequate and appropriate resilience for brake pads during braking. The preferred embodiment described above has been chosen because of performance, cost, and convenience considerations, but is not the only way to practice the invention.
Advantages
The advantages provided by the brake pad cushions described above can be divided into three categories: Safety, Savings, and Other Advantages.
Safety Advantages
Shorter stopping distance is perhaps the most important safety-related improvement provided by the brake pad cushions. However, there are several other benefits derived from improved braking efficiency which are also critically important to driving safety. These additional safety benefits include:
The action of the pad cushions during braking reduces the frictional drag at rotor high spots, which keeps friction interface temperatures much lower. Lower temperatures equal lower rates of wear for friction components (pads and rotors). As average braking temperatures increase, brake pad life expectancy correspondingly decreases. Reducing average braking temperatures from 800° F. to 400° F., for instance, more than doubles brake pad life expectancy. The reduction of braking temperatures which the present invention provides can generate substantial savings on brake pad replacement costs, as well as on replacement of other brake system components (rotors and calipers, most notably).
Another important effect of reducing the severe frictional drag at rotor high spots is the smoothing of the transfer of braking torque through the tires to the roadway. This effect lessens tire strain during braking, practically eliminating tire cupping and helping to reduce internal tire problems such as cord separation. Over the lifetime of any vehicle, the resulting savings from increased tire life can be quite significant.
Other Advantages
Accordingly, the reader will see that the brake pad cushions of this invention are inexpensive, easy to install, and trouble-free during use. It is also evident that the cushions provide a profound benefit to personal safety (because of significant increases in braking efficiency) and remarkable savings (because of increased brake system longevity).
The description above contains many specificities, but these should not be construed as limits to the scope of the invention, but only as illustrations of some of the presently preferred embodiments. There are many “variations on the theme” which can accomplish the performance objectives described for this invention. For instance, elastomers other than rubber blends can be used, and any elastomeric cushion can be produced in a variety of shapes and sizes. Elastomeric cushions may also include a variety of surface protrusions, slits, or voids to compensate for shear forces (caused by worn brake pad abutments, for instance); shear forces may also be compensated by providing an anti-friction coating on one side of each cushion. Cushions may be caged or housed by stiffer materials to control “spread” of softer elastomers. Elastomeric cushions may also be impregnated with a variety of natural or synthetic materials (including but not limited to metallic springs or fiberglass fabric), to change or enhance physical properties or performance characteristics.
Metals or other materials can be used to make a variety of springs (including wave, leaf, or coil springs) which can satisfy the performance objectives set forth in this specification; any such spring may be combined with an elastomer or other material to change or enhance physical properties or performance characteristics.
Also, pneumatic or hydraulic dampers of various designs can be manufactured and secured in place between brake pads and their corresponding caliper members to provide adequate and appropriate resilience for brake pads during braking.
Clearly, the principle of this invention (to provide cushioning for brake pads which allows the pads to follow the irregularities (ride over the high spots) of their corresponding rotor faces during braking) can be practiced in a variety of ways. Accordingly, the scope of this invention should be determined not by the embodiments illustrated or described, but by the appended claims and their legal equivalents.
