Abstract:
A rotational element for a wheel comprises a cast iron segment and an anti-lock brake system exciter ring adhering to the cast iron segment as a cast-in insert. The anti-lock brake system exciter ring is made of a material having greater corrosion resistance and a higher melting point than the cast iron. The exciter ring includes at least a first tang penetrating the cast iron segment. The exciter ring is preferably made from ferritic stainless steel. For disk brake systems the rotational element serves as a disk brake rotor. In vehicles using drum brakes the rotational element serves as a wheel hub.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to motor vehicle brake systems and more particularly to a corrosion resistant cast-in insert ABS exciter ring integrated with either a disk rotor or, for drum brake systems, a wheel hub. 
     2. Description of the Problem 
     Brake units for motor vehicles should provide smooth braking with reasonable service life. This need has been met with disk brake rotors made from relatively inexpensive gray iron castings. Gray iron is however highly susceptible to corrosive attack, particularly in the operating environment of vehicles where brake components are open to the air, subject to substantial transient heating and exposed to water and salt water spray. In regular use, the working surfaces of the disks are rubbed clean by contact with the disk pads, which are typically made of a composite material and which rub off corroded areas. However other areas of the brake disks are not swept by the brake pads and thereby cleaned of corrosion. Prior to anti-lock braking systems, such concerns were not paramount with brakes which were frequently in use, since the rotor is a regularly replaced part and the remaining areas subject to attack were non-critical. 
     With the advent of anti-lock braking systems other sections of the disk brake rotor can take on importance. Among other sections of a typical disk rotor of mechanical importance is an anti-lock brake system exciter ring which has typically been cast as one piece with the rotor. The exciter ring is a cylindrical section of the rotor having a common axis of rotation with the rotor. A plurality of teeth are positioned in a ring, flat in the plane of rotation of the rotor and outwardly oriented on the exterior of the ring to pass closely by a stationary sensor. The stationary sensor is a variable reluctance sensor which generates an electrical pulse train as a function of the varying magnetic flux leakage between the sensor head and the exciter ring. The frequency of the resulting electrical pulse train indicates the rotational speed of the wheel on which the rotor is mounted. The generation of clean pulse train is greatly aided by having teeth of uniform shape, size and spacing. Corrosion can greatly compromise all of these factors, resulting in difficulty in detecting the passage of teeth and gaps and resulting in a corrupted pulse train. 
     It is known that coating parts suppresses corrosion. Corrosion protection coatings can be used such as that described in U.S. Pat. No. 5,569,543 and those supplied by Magni Corp., including the Magni 109 and Magni 111 coatings. Such coatings can be easily compromised when applied to exciter rings since the sensor usually needs to pass within very close proximity to the teeth and, consequently, the chance exists for removal of the coating from the teeth, which again leaves the teeth exposed to corrosive agents. In addition, such coatings are relatively expensive and their long term durability under all of the widely varying conditions of vehicle use is not well known. 
     It is known to make disk rotors out of more than one material, although no application of such an approach to solving the problems of ABS exciter rings is known to the inventor. German Laid Open Application 42 37 655 describes a brake disk for a motor vehicle disk brake system. The rotor comprises two abrasion rings, including an inner ring made of iron and an outer ring made of a composite fiber material. The rings are bonded to one another, preferably using rivets. The application mentions the possibility of casting the iron disk onto the composite fiber disk. The application is not directed primarily to corrosion problems but rather to improving weight balance, reducing the potential for cracking and reducing brake vibration. 
     The problem of corrosion of exciter rings for anti-lock brake systems is not limited to disk brake systems, but is also an issue with drum brakes. In drum brakes the exciter ring has not been an integral part of any part of the working brake, but rather has been a separate part press fitted on the end of a wheel hub. Such hubs are frequently made from nodular iron, which like gray iron is highly susceptible to corrosive attack. Improper control of the fitting process can result in exciter rings becoming displaced from an optimal position. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a rotational element for a wheel comprising a cast iron segment and an anti-lock brake system exciter ring segment, which adheres to the cast iron segment as a cast-in insert. The anti-lock brake system exciter ring segment is made of a corrosion resistant, magnetic material having a higher melting point than the cast iron segment. The exciter ring includes at least one tang penetrating the cast iron segment. The exciter ring is preferably made from ferritic stainless steel where magnetic anti-lock brake sensors are to be used. For disk brake systems the rotational element serves as a disk brake rotor. In vehicles using drum brakes the rotational element serves as a wheel hub. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of a vehicle equipped with anti-lock brake systems. 
     FIG. 2 is a perspective view of disk rotor incorporating the cast-in insert exciter ring of the preferred embodiment. 
     FIG. 3 is a plan view of the disk rotor. 
     FIG. 4 is a plan view of an exciter ring for a disk rotor. 
     FIG. 5 is a cross sectional view of the disk rotor of FIG. 3 taken along section line A—A. 
     FIGS. 6A and 6B are alternate cross sectional views of the disk rotor of FIG. 3 taken along section line B—B. 
     FIG. 7 is a cross sectional view of the disk rotor of FIG. 3 taken along section line C—C. 
     FIG. 8 is a cross sectional view of a wheel hub used with a drum brake equipped vehicle. 
     FIG. 9 is a plan view of a cast-in insert exciter ring from the wheel hub of FIG.  8 . 
     FIG. 10 is a perspective view of a section of a corrugated exciter ring shown in FIG.  6 B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, system overview of a vehicle  101  equipped with an antilock brake systems (ABS) is shown without vehicle bodies. Vehicle  101  is illustrative of disk or drum brakes and has rear wheels  105  mounted on opposite outside ends of a rear axle  108 . Front wheels  106  are similarly mounted from the opposite outside ends of a front axle  107 . A wheel  106  or  105  may comprise one or two tires. Wheels  105  and  106  are mounted for rotation on axles  108  and  107 , respectively. 
     Vehicle  101  in the FIG. 1 sketch shows disk brakes, which in turn include a disk rotor  110  and a caliper  112  on each of the four wheels. The ABS further includes exciter rings  114  associated with each disk rotor  110 , ABS wheel speed sensors  103  positioned along the exciter rings  114 , ABS modulators controlling the calipers  112  and an ABS electronic controller  102 . 
     ABS wheel speed sensors  103  may sense wheel rotation in a variety of ways. In the subject embodiment this is accomplished by positioning a variable reluctance sensor so that it is stationary with respect to the exciter ring, which rotates as part of the disk rotor mounted to the wheel. The variable reluctance sensor has a cylindrical body, the central longitudinal axis of which is aligned with, and normal to, a sensing circle, that is the target for alignment of the sensor on the exciter ring. The moving magnetic material, which in the preferred embodiment resemble gear teeth laid out in a circle on a plane, pass in front of the stationary sensor, inducing a varying, cyclical voltage, the frequency of which is proportional to the angular velocity of the wheel. 
     The amplitude, or signal strength produced by the sensor system is a function in several variables, the most important of which is the gap between the tip of the sensor and the exciter ring. Larger gaps produce weaker signals. Corrosion has deleterious effects on both the amplitude and consistency of the electrical signal. Since corrosion acts to destroy the dimensional consistency of the exciter ring teeth, the electronic consistency of the signal is likewise compromised. In addition, corrosion increases the gap between the tip of the sensor and the exciter ring teeth, while simultaneously reducing the gap between the sensor tip and the bottoms of the gaps between the teeth, resulting in reduced amplitude of the resulting electrical signal. As corrosion progresses the amplitude and signal quality can be reduced such that they drop below the thresholds necessary for interpretation by ABS systems. This problem is particularly bad with exciter rings cast as one piece with the disk rotor. With drum brakes the exciter ring has been press fitted to a cast iron wheel hub. Such rings have been made of materials other than cast iron, but have been susceptible to loosening and falling off due to the nature of press-fit process. 
     Referring now to FIG. 2 a disk brake rotor  110  comprising a brake rotor body  120 , made from cast iron, and a cast-in insert exciter ring  114 , which is made from a corrosion resistant magnetic material, preferably ferritic stainless steel. The material forming exciter ring  114  has a substantially higher melting point than iron so as to retain its integrity when molten iron comes into contact with the exciter ring during casting of the disk rotor  110 . This allows the exciter ring  114  to be placed in a mold and the disk brake rotor  110  formed on it by casting the iron to form brake rotor body  120  into the mold after positioning of the exciter ring. The teeth  116  of exciter ring  114  lie in the plane of rotation of disk rotor  110 . 
     Referring to FIG. 4 a disk brake exciter ring  114  is illustrated in isolation. Exciter ring  114  is preferably flat on one side or corrugated with at least one tang  122 , and preferably four tangs  122 , extending outwardly from the outer circumference of the ring. For a flat ring the exciter ring teeth  116  comprise alternating raised and lowered areas on one major surface of the ring  114 . For a corrugated ring the teeth  116  are the protruding portions of the ring side intended to face the sensor head. Teeth  116  extend radially from the center point of the ring on the intended axis of rotation of the ring and oriented to lie in the plane of rotation. 
     Referring now to FIG. 3, a plan view of disk rotor  110  is used to indicate the locations of a series of section views taken along section lines A—A, B—B, and C—C, which are presented in FIGS. 5,  6 A,  6 B and  7 , respectively. Exciter ring  114  is positioned between the braking surface  120  and an inner mounting flange  126 . Holes  124  for mounting rotor  110  are provided through inner mounting flange  126 . Tangs  122  penetrate the body of abrasion ring section  120  in a locking arrangement. 
     Referring to FIGS. 5 and 7, exciter ring  114  is presented in side elevation, first in situ on rotor body  120  and second in isolation. Tangs  122  extend outwardly from the ring of teeth  116  into the rotor body  120 . Rotor body  120  and the ferritic stainless steel have an interface along the circumferential edge and one major surface of ring  114 . 
     FIGS. 6A-B illustrate alternative profiles for an exciter ring taken along section line B—B, with FIG. 6A illustrating a flat sided ring  114 A and FIG. 6B illustrating a corrugated profile ring  114 B. For ring  114 A teeth  116  alternate with grooves  134  on one face of the ring. The opposite major surface is flat along an interface  119 A with rotor body  120 . Ring  119 A can be made by a variety of ways such as sintered metal, machining or casting. For corrugated ring  114 B ridges  136  on the inner surface and  118  on the outer surface of the ring relative to the rotor body  120  form recesses and teeth, respectively. Ring  114 B can be stamped from sheet metal to form the alternating ridges. Referring to FIG. 10, a section of ring  114 B including a tang  122  is presented in perspective view. 
     FIGS. 8 and 9 illustrate application of the invention to a wheel hub assembly  210  usable with a drum brake equipped vehicle employing an anti-lock brake system. The inner surface cylindrical surface  208  of exciter ring  238  includes ring mounting tangs  214 , which point inwardly toward the intend axis of rotation, which penetrate an end portion  226  of a cast iron hub member  212 . Exciter ring  238  is corrosion resistant, magnetic material, preferably ferritic stainless steel and hub member  212  is generally made from ductile iron. In the prior art exciter rings were typically press fit to the wheel hubs, which, if not carefully done, can result in misalignment of the exciter rings or even eventual detachment of the rings. The positive mounting provided by the tangs  214  prevents such detachment. 
     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.