MOLDED PISTON HAVING METALLIC COVER FOR DISC BRAKE ASSEMBLY

BACKGROUND OF THE INVENTION This invention relates in general to vehicle disc brake assemblies and in particular to an improved structure for a piston adapted for use in such a vehicle disc brake assembly. Most vehicles are equipped with a brake system for retarding or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. A typical brake system for a medium duty truck includes a disc brake assembly on all four wheels of the vehicle. The brake assemblies are typically actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefor, are well known in the art. There are two basic types of calipers for use in disc brake assemblies, namely, a "floating" caliper disc brake assembly, and a "fixed" caliper disc brake assembly. A floating caliper disc brake assembly is usually used on automobiles and light and medium duty trucks. A conventional floating caliper disc brake assembly includes a brake caliper which is supported by a pair of pins for sliding movement relative to an anchor plate which is secured to a fixed, non-rotatable component ofthe vehicle. A fixed caliper disc brake assembly is sometimes used on automobiles and light and medium duty trucks. A conventional fixed

caliper disc brake assembly includes a brake caliper which is secured to a fixed, non-rotatable component of the vehicle.

In both types of disc brake assemblies, a pair of brake shoes are supported by the disc brake assembly for sliding movement relative thereto. The brake shoes have respective friction pads which are disposed on opposite sides of a rotor. The rotor, in turn, is connected to the wheel of the vehicle for rotation therewith. To effect braking action, the brake shoes are moved inwardly toward one another so as to frictionally engage the opposed sides of the rotor. Such frictional engagement causes retarding or stopping of the rotational movement of the rotor and. therefore, the wheel ofthe vehicle in a controlled manner.

A considerable amount of heat is generated between the rotor and the brake shoes during braking. In a disc brake assembly having a piston constructed from a metallic material, the heat generated during braking will not usually damage the surface of the opened end of the piston. Unfortunately, a disc brake piston which is formed from a metallic material is relatively expensive. It is less expensive to manufacture a disc brake piston from a plastic mateπal than from a metallic material. U.S. Patent No. 4,928,579 to Emmett, U.S. Patent No. 4,449,447 to Yanagi, U.S. Patent No. 4,401 ,012 to Emmett, and Japanese Patent No. 5718857 disclose prior art disc brake pistons. However, in a disc brake assembly having a piston formed from plastic material, it has been found that the heat generated during braking can cause damage to the surface of the piston. Thus, it would be desirable to provide an improved structure for a piston adapted for use in a vehicle disc brake assembly which is durable, yet relatively inexpensive to manufacture.

SUMMARY OF THE INVENTION This invention relates to an improved structure for a piston adapted for use in a vehicle disc brake assembly which is durable, yet relatively inexpensive to manufacture. The piston includes a generally hollow cylindrical body having an opened end and a closed end. The body is formed from a plastic material and includes inner and outer cylindrical surfaces which extend axially between the opened end and the closed end. A generally annular steel cover is integrally molded to the cylindrical body of the piston. The cover extends over an annular end surface of the open end of the body and axially toward the closed end along portions of both the inner and outer cylindrical surfaces. The cover includes a plurality of tangs provided on an inner side wall thereof to assist in securing the cover to the piston when the piston in molded thereabout. The cover is formed from a metallic material so as to protectively shield the enclosed portions of the plastic body from the heat generated during braking. Also, the metallic cover uniformly distributes the load across the annular end surface of the piston to reduce the force concentrations thereon.

Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional elevational view of a portion of a floating caliper type of vehicle disc brake assembly including a first embodiment of an improved disc brake piston in accordance with this invention.

Fig. 2 is a plan view ofthe cover provided on the disc brake piston illustrated in Fig. 1.

Fig. 3 is a sectional view ofthe cover taken along line 3-3 of Fig. 2.

Fig. 4 is an enlarged sectional view ofthe disc brake piston illustrated in Fig. 1.

Fig. 5 is an enlarged sectional view of a second embodiment of an improved disc brake piston in accordance with this invention. Fig. 6 is an enlarged sectional view of a third embodiment of a disc brake piston in accordance with this invention.

Fig. 7 is an enlarged sectional view of a fourth embodiment of an improved disc brake piston in accordance with this invention.

Fig. 8 is a plan view of the cover provided on the disc brake piston illustrated in Fig. 7.

Fig. 9 is a sectional view of the cover taken along line 9-9 of Fig. 8.

Fig. 10 is a perspective view of a portion of a fixed caliper type of vehicle disc brake assembly including a fifth embodiment of an improved disc brake piston in accordance with this invention. Fig. 1 1 is a top plan view of the vehicle disc brake assembly illustrated in

Fig. 1 1.

Fig. 12 is an elevational view of the vehicle disc brake assembly taken along line 12-12 of Fig. 1 1.

Fig. 13 is an elevational view ofthe vehicle disc brake assembly taken along line 13-13 of Fig. 1 1.

Fig. 14 is a sectional view taken of the vehicle disc brake assembly taken along line 14-14 of Fig. 1 1, with a rotor being shown.

Fig. 15 is a sectional view of the disc brake piston illustrated in Fig. 14.

Fig. 16 is a sectional view of a sixth embodiment of an improved disc brake piston in accordance with this invention.

Fig. 17 is a sectional view of a portion of a seventh embodiment of an improved disc brake piston in accordance with this invention.

Fig. 18 is a sectional view of a portion of an eight embodiment of an improved disc brake piston in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, there is illustrated in Fig. 1 a portion of a sliding caliper type of vehicle disc brake assembly, indicated generally at 10. The general structure and operation of the disc brake assembly 10 is conventional in the art. Thus, only those portions of the disc brake assembly 10 which are necessary for a full understanding of this invention will be explained and illustrated. Although this invention will be described and illustrated in conjunction with the particular vehicle disc brake assemblies disclosed herein, it will be appreciated that this invention may be used in conjunction with other disc brake assemblies.

The disc brake assembly 10 includes a generally C-shaped caliper, indicated generally at 12. The caliper 12 includes an inboard leg portion 14 and an outboard leg portion 16 which are interconnected by an intermediate bridge portion 18. The caliper 12 is slidably supported on pins (not shown) extending outwardly from an anchor plate (not shown) which, in turn, is secured to a stationary component of the vehicle. The pins permit the caliper 12 to slide in both the outboard direction (left when viewing Fig. 1 ) and the inboard direction (right when viewing Fig. 1 ). Such sliding movement of the caliper 12 occurs when the disc brake assembly 10 is actuated, as will be explained below.

An actuation means, indicated generally at 20, is provided for effecting the operation of the disc brake assembly 10. The actuation means 20 includes a piston, indicated generally at 50, which is disposed in a counterbore or recess 24 formed in the outboard surface of the inboard leg 14 ofthe caliper 12. The actuation means 20, shown in this embodiment as being a hydraulic actuation

means, is operable to reciprocally move the piston 50 within the recess 24. However, other types of actuation means 20, such as for example, electrical and mechanical types, can be used. The structure ofthe piston 50 will be described in detail below. The disc brake assembly 10 also includes a dust boot seal 26 and an annular fluid seal 28. The dust boot seal 26 is formed from a flexible material and has a first end which engages an outboard end of the recess 24. A second end of the dust boot seal 26 engages an annular groove formed in an outer side wall of the piston 50. A plurality of flexible convolutions are provided in the dust boot seal 26 between the first and second ends thereof. The dust boot seal 26 is provided to prevent water, dirt, and other contaminants from entering into the recess 24. The fluid seal 28 is disposed in an annular groove formed in a side wall of the recess 24 and engages the outer side wall of the piston 50. The fluid seal 28 is provided to define a sealed hydraulic actuator chamber 30, within which the piston 50 is disposed for sliding movement.

The disc brake assembly 10 further includes a rotor 32. which is connected to a wheel (not shown) ofthe vehicle for rotation therewith. The rotor 32 extends radially outwardly between an inboard backing plate 34, which supports an inboard friction pad 36, and an outboard backing plate 38. which supports an outboard friction pad 40. The inboard and outboard backing plates 34 and 38, respectively, can be supported on guide rails (not shown) provided on the anchor plate. Alternatively, the inboard backing plate 34 can be supported on the piston 50, while the outboard backing plate 38 can be supported on the outboard leg portion 16 of the caliper 12. When it is desired to brake the rotation of the brake rotor 32 and the vehicle wheel associated therewith, pressurized hydraulic fluid is introduced into the chamber 30. Such pressurized hydraulic fluid urges the piston 50 in the

outboard direction (toward the left when viewing Fig. 1). As a result, the inboard friction pad 36 is moved into engagement with an inboard braking surface ofthe rotor 32. At the same time, the caliper 12 slides in the inboard direction (toward the right when viewing Fig. 1 ) such that the outboard friction pad 40 is moved into engagement with an outboard braking surface of the brake rotor 32. As a result, the friction pads 36 and 40 frictionally engage the opposed axial sides of the rotor 32 to retard relative rotational movement thereof. The structure and operation of the disc brake assembly 10 thus far described is conventional in the art. Referring now to Figs. 2 through 4, the structure of the piston 50 is illustrated in detail. As shown therein, the piston 50 includes a body 52 which is preferably molded from a plastic, and more preferably from a synthetic resin material such as a phenolic resin. However, the piston 50 can be formed from other materials, such as for example, ceramic. The body 52 of the piston 50 is generally hollow and cylindrical in shape, having a closed end 54 and an opened end 56. An axially extending inner cylindrical surface 58 and an axially extending outer cylindrical surface 60 are defined on the body 52 of the piston 50. The inner cylindrical surface 58 and the outer cylindrical surface 60 of the body 52 are preferably concentric with a longitudinal axis X of the piston 50. An annular groove 62 may be formed in the outer cylindrical surface 60 of the body 52 adjacent to the opened end 56 thereof. The groove 62 is adapted to receive the second end ofthe dust boot seal 26 therein, as described above.

A generally annular cover, indicated generally at 70, is integrally molded about the opened end 56 of the body 52 ofthe piston 50. The cover 70 includes an end wall 72, an inner cylindrical side wall 74, and an outer cylindrical side wall 76. The end wall 72 includes an inner edge 72a and an outer edge 72b, and defines an annular abutment surface which is oriented peφendicular to the

longitudinal axis X of the piston 50. The abutment surface 72 is adapted to engage the inboard backing plate 34 when the chamber 30 is pressurized to cause the piston 50 to slide toward the rotor 32, as described above. The inner cylindrical side wall 74 extends from the inner edge 72a ofthe end wall 72 a first predetermined distance Dl . The outer cylindrical side wall 76 extends from the outer edge 72b of the end wall 72 a second predetermined distance D2 which is greater than the first predetermined distance D 1. The cover 70 is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover 70 can be formed from other materials which are different from the material of the piston 50, such as for example, aluminum.

As best shown in Figs. 3 and 4, a plurality of angled "tangs" 78 are formed on the inner cylindrical side wall 74 of the cover 70. As used herein, a tang is defined as a projection having a portion which extends from the inner cylindrical side wall 74 toward the outer cylindrical side wall 76. In the illustrated embodiment, four equidistantly spaced angled tangs 78 which extend radially outwardly and axially inwardly are provided on the cover 70. However, the number and spacing ofthe tangs 78 may be varied as desired. In addition, the configuration ofthe tangs 78 may be other than illustrated. For example, the tangs 78 can be curved, stepped, or L-shaped.

Preferably, the tangs 78 are angled in the range of 30 degrees to 60 degrees relative to the axis X, with approximately 45 degrees being the preferred angle. However, the tangs 78 can be angled in the range of 5 to 85 degrees. When the plastic body 52 ofthe piston 50 is molded about the cover 50, the tangs 78 are embedded therein as best shown in Fig. 2. Thus, the tangs 78 assist in mechanically securing the cover 70 to the piston 50 when the body 52 of the piston 50 is molded thereabout.

In addition to the tangs 78, a plurality of flats 80 are formed on the inner side wall 74 of the cover 70. In the illustrated embodiment, two equidistantly spaced flats 80 are formed on the cover 70. However, the number and spacing of the flats 80 may be varied as desired. The flats 80 are provided for assisting in the accurate positioning of the cover 70 within the mold when the body 52 ofthe piston 50 is molded thereabout.

Fig. 5 illustrates a second embodiment of a piston, indicated generally at 90, in accordance with this invention. As shown therein, the piston 90 includes a body 92 which is preferably molded from a plastic, and more preferably from a synthetic resin material such as a phenolic resin. However, the piston 90 can be formed from other materials, such as for example, ceramic. The body 92 of the piston 90 is generally hollow and cylindrical in shape, having a closed end 94 and an opened end 96. An axially extending inner cylindrical surface 98 and an axially extending outer cylindrical surface 100 are defined on the body 92 of the piston 90. The inner cylindrical surface 98 and the outer cylindrical surface 100 of the body 92 are preferably concentric with a longitudinal axis XI of the piston 90. An annular groove 102 may be formed in the outer cylindrical surface 100 of the body 92 adjacent to the opened end 96 thereof.

A generally annular cover, indicated generally at 1 10, is integrally molded about the opened end 96 of the body 92 ofthe piston 90. The cover 1 10 includes an end wall 1 12, an inner cylindrical side wall 1 14, and an outer cylindrical side wall 1 16. The end wall 1 12 includes an inner edge 1 12a and an outer edge 112b and defines an annular abutment surface which is oriented perpendicular to the longitudinal axis X ofthe piston 90. The inner cylindrical side wall 1 14 extends from the inner edge 1 12a of the end wall 112 a first predetermined distance D3. The outer cylindrical side wall 1 16 extends from the outer edge 1 12b of the end wall 1 12 a second predetermined distance D4 which is greater than said first

predetermined distance D3. The cover 110 is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover 1 10 can be formed from other materials which are different from the material ofthe piston 90, such as for example, aluminum. The cover 1 10 further includes a plurality of angled tangs 1 18 to assist in mechanically securing the cover 1 10 to the piston 90 when the body 92 of the piston 90 is molded thereabout. Also, the cover 1 10 may include a plurality of flats (not shown) formed on the inner side wall 1 14 of the cover 1 10 to assist in the accurate positioning ofthe cover 110 within the mold when the body 92 of the piston 90 is molded thereabout.

Fig. 6 illustrates a third embodiment of a piston, indicated generally at 120, in accordance with this invention. As shown therein, the piston 120 includes a body 122 which is preferably molded from a plastic, and more preferably from a synthetic resin material such as a phenolic resin. However, the piston 120 can be formed from other materials, such as for example, ceramic. The body 122 of the piston 120 is generally hollow and cylindrical in shape, having a closed end 124 and an opened end 126. An axially extending inner cylindrical surface 128 and an axially extending outer cylindrical surface 130 are defined on the body 122 of the piston 120. The inner cylindrical surface 128 and the outer cylindrical surface 130 of the body 122 are preferably concentric with a longitudinal axis X2 ofthe piston 120. An annular groove 132 may be formed in the outer cylindrical surface 130 ofthe body 122 adjacent to the opened end 126 thereof.

A cover, indicated generally at 140, is integrally molded to the body 122 of the piston 120. The cover 140 is generally cylindrical in shape and includes an outer end wall 142, an inner cylindrical side wall 144, an outer cylindrical side wall 146, and an inner end wall 148. The outer end wall 142 of the cover

140 defines an annular abutment surface which is oriented peφendicular to the longitudinal axis X2 of the piston 120. The cover 140 is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover 140 can be formed from other materials which are different from the material of the piston 120, such as for example, aluminum. The cover 140 further includes a plurality of angled tangs 150 to assist in mechanically securing the cover 140 to the piston 120 when the body 122 of the piston 120 is molded thereabout. Also, the cover 140 may include a plurality of flats (not shown) formed on the inner side wall 144 ofthe cover 140 to assist in the accurate positioning of the cover 140 within the mold when the body 142 of the piston 140 is molded thereabout.

Referring now to Figs. 7 through 9, the structure of a fourth embodiment of a piston, indicated generally at 160, will be discussed. As shown therein, the piston 160 includes a body 162 which is preferably molded from a synthetic resin material. However, the piston 160 can be formed from other materials, such as for example, ceramic. The body 162 of the piston 160 includes a closed end 164, an opened end 166, and a center post 168 which extends from the closed end 164 toward the opened end 166. The center post 168 is coaxial with a longitudinal axis X3 of the piston 90, and includes an end wall surface 168a which, in the illustrated embodiment, is spaced from the opened end 166 of the piston 160.

A generally axially extending inner conical surface 170 is defined on the center post 168 of the piston 160, and an axially extending inner cylindrical surface 172 and an axially extending outer cylindrical surface 174 are defined on the body 162 of the piston 160. The inner cylindrical surface 172 and the outer cylindrical surface 174 of the body 92 are preferably concentric with the longitudinal axis X3 of the piston 90. The inner conical surface 170 of the center post 168 is preferably tapered radially from the end wall surface 168a toward the

closed end 164 of the piston 160. The piston 160 may also include an annular groove 176 formed in the outer cylindrical surface 174 of the body 162 adjacent to the opened end 166 thereof. The groove 176 is adapted to receive the second end of the dust boot seal 26 therein, as described above. A cover, indicated generally at 180, is integrally molded to the body 162 of the piston 160. The cover 180 is generally convoluted in shape and includes an outer end wall 182, an intermediate end wall 184, and an inner end wall 186. The cover 180 also includes an inner cylindrical side wall 188, an inner conical side wall 190, and an outer cylindrical side wall 192. The outer end wall 182 of the cover 180 defines an annular abutment surface which is oriented peφendicular to the longitudinal axis X3 of the piston 160. The cover 180 is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover 180 can be formed from other materials which are different from the material ofthe piston 160, such as for example, aluminum.

The cover 180 further includes a plurality of angled tangs 194 to assist in mechanically securing the cover 180 to the piston 160 when the body 162 of the piston 160 is molded thereabout. In the illustrated embodiment, the tangs 194 are provided on both the inner cylindrical side wall 188 and the inner conical side wall 190 of the cover 180. Alternatively, the tangs 194 may be provided on one of the inner cylindrical side wall 188 and the inner conical side wall 190 of the cover 180. Also, the cover 180 may include a plurality of flats (not shown) formed on the inner side wall 188 of the cover 180 to assist in the accurate positioning of the cover 180 within the mold when the body 162 of the piston 160 is molded thereabout.

Referring now to Figs. 10 through 14, there is illustrated a fixed caliper type of vehicle disc brake assembly, indicated generally at 210. The general

structure and operation of the vehicle disc brake assembly 210 is conventional in the art. Thus, only those portions of the vehicle disc brake assembly 210 which are necessary for a full understanding of this invention will be explained and illustrated. The illustrated disc brake assembly is an opposed four piston fixed caliper disc brake assembly 210, and is associated with a right wheel of a vehicle. The disc brake assembly 210 is a two-piece construction, and includes an inboard brake caliper assembly 21 1. best shown in Fig. 12. and an outboard brake caliper assembly 212, best shown in Fig. 13. The constructions of the inboard caliper assembly 21 1 and the outboard caliper assembly 212 are generally similar to one another, and like reference numbers will be used for corresponding parts. Alternatively, the inboard caliper assembly 21 1 and the outboard caliper assembly 212 may be integrally formed (not shown).

The inboard brake caliper assembly 21 1 and the outboard brake caliper assembly 212 are provided with respective pairs of aligned apertures formed therethrough. Threaded bolts 213 extend through the respective pairs of apertures, and washers 214 and nuts 215 are installed on threaded ends 213a of the bolts 213 to secure the inboard caliper assembly 21 1 to the outboard caliper assembly 12. The inboard caliper assembly 21 1 includes a pair of anchoring arms 216 and 217 provided at opposed ends thereof. Each ofthe arms 216 and 217 are provided with a pair of apertures 216a and 217a, respectively, formed therethrough. The apertures 216a and 217a are adapted to receive bolts (not shown) for securing the inboard caliper assembly 21 1, and therefore the assembled disc brake assembly 210, to a fixed, non-rotatable component of the vehicle. Such fixed component can be, for example, an axle flange when the disc brake assembly is installed for use on the rear of the vehicle, or a steering

knuckle when the disc brake assembly is installed for use on the front ofthe vehicle.

Referring now to Fig. 14, the inboard caliper assembly 21 1 includes a pair of cylindrical recesses 218 formed therein, one of which is shown. Similarly, the outboard caliper assembly 212 includes a pair of cylindrical recesses 218 formed therein, one of which is shown. A piston 219 is disposed in each of the cylindrical recesses 218 of the inboard caliper assembly 21 1 and the outboard caliper assembly 212. A fluid seal 220 is disposed in an annular groove 221 formed in a side wall 218a of the cylindrical recess 218. The fluid seal 220 engages an outer cylindrical surface 295 of the piston 219. The fluid seal 220 is provided to define a sealed hydraulic fluid chamber 222, within which the piston 219 is disposed for sliding movement. Also, the fluid seal 220 is designed to function as a roll-back seal which retracts the piston 19 within the cylindrical recess 18 when the disc brake assembly 10 is not actuated. The inboard caliper assembly 21 1 includes a pair of hydraulic fluid chambers 222 which are connected together by a fluid passageway (not shown) formed therein. Similarly, the outboard caliper assembly 212 includes a pair of hydraulic fluid chambers 222 which are connected together by a fluid passageway (not shown) formed therein. Thus, it will be appreciated that the illustrated brake actuating means is a hydraulic actuating means. However, other well known types of actuating means, such as pneumatic, electrical, and mechanical, can also be used.

A dust boot seal 224 is provided about the outboard end of the piston 219 to prevent water, dirt, and other contaminants from entering into the cylindrical recess 18. The dust boot seal 24 is formed from a flexible material and has a first end 224a which engages a shoulder 295c of the piston 219 and a second end 224b which engages an annular recess formed adjacent the open ends of the

associated cylindrical recesses 218. A plurality of flexible convolutions are provided in each dust boot seal 224 between the first and second ends thereof to accommodate movement of the pistons 219 within each of the respective cylindrical recesses 218. 5 The disc brake assembly 210 further includes a pair of arms 225 and 226 having respective guide rails 225a and 226a formed thereon. The guide rails 225a and 226a extend transverse to the arms 225 and 226, and extend parallel to one another. As will be discussed, a pair of brake shoes 227 and 228 are supported on the guide rails 225a and 226a for sliding movement relative thereto. i Preferably, a hardened replaceable insert 229 is secured to each of the guide rails 225a and 226a by a fastener 230. The inserts 229 are preferably formed from stainless steel, and provide a smooth, corrosion resistant sliding surface for the brake shoes 227 and 228.

As best shown in Fig. 14, the inboard brake shoe 227 includes a backing

15 plate 231 having a friction pad 232 secured thereto. Similarly, the outboard brake shoe 228 includes a backing plate 233 having a friction pad 234 secured thereto. In the illustrated embodiment, a brake shoe hold down clip 235 is releasably attached to the disc brake assembly 210 to bias the upper ends of the backing plates 231 and 233 of brake shoes 227 and 228 against the guide rails 0 225a and 226a. The hold down clip 235 is generally arcuate in shape, and is formed having a longitudinally extending central mounting portion 250. A first pair of opposed spaced apart arms 251 and 252 extend transverse to and outwardly from one side ofthe central mounting portion 250. The ends of the first pair of arms 251 and 252 terminate at and are interconnected by a generally 5 U-shaped first end arm 253 which extends generally parallel to the central mounting portion 250.

The hold down clip 235 further includes a second pair of spaced apart arms 254 and 255 which extend transverse to and outwardly from the opposite side ofthe central mounting portion 250. The ends ofthe second pair of arms 254 and 55 terminate at and are interconnected by a generally U-shaped second 5 end arm 256 which extends generally parallel to the central mounting portion 250.

The central mounting portion 250 further includes an aperture formed adjacent one end 250c thereof. A raised, generally inverted V-shaped strengthening rib 261 extends substantially the entire length of the central i o mounting portion 250 and terminates at an opposite end 250d thereof. The end 250d ofthe hold down clip 235 is disposed in an opening formed in a raised mounting pad 271 provided on the outboard caliper assembly 212. A bolt 272 having a threaded end extends through the aperture in the hold down clip 235 and is threadably received in a threaded aperture formed in a raised mounting 15 pad 270 provided on the inboard caliper assembly 21 1.

As shown in Fig. 14. the brake shoes 227 and 228 are disposed on opposite sides of a rotor 236. The rotor 236 is generally flat and circular in shape and is secured in a conventional manner to a rotatable wheel (not shown) ofthe vehicle. The illustrated rotor 236 includes a pair of opposed braking discs 20 237 and 238 which are spaced apart from one another by a plurality of intermediate vanes 239 in a known manner.

When it is desired to actuate the disc brake assembly 210 to retard or stop the rotation of the brake rotor 236 and the vehicle wheel associated therewith, the driver of the vehicle depresses the brake pedal (not shown). In a manner

25 which is well known in the art, the depression of the brake pedal causes pressurized hydraulic fluid to be introduced into the disc brake assembly 210 via a threaded inlet port 240 provided in the inboard caliper assembly 21 1. The inlet

port 240 is connected through a brake line (not shown) to a master cylinder (not shown) of the vehicle brake system. The brake line is attached to the inlet port 240 by a threaded fitting (not shown). The hydraulic fluid flows from the inlet port 240 into the inboard hydraulic fluid chambers 222 to urge the associated pistons 219 in the inboard caliper assembly 21 1 in the outboard direction (toward the left when viewing Fig. 14) into engagement with the backing plate 231 of the inboard brake shoe 227.

The hydraulic fluid chambers 222 ofthe inboard caliper assembly 21 1 are connected to the hydraulic fluid chambers 222 of the outboard caliper assembly 212 by a fluid supply assembly 241. Thus, at the same time, the hydraulic fluid flows through the fluid supply assembly 241 into the outboard hydraulic fluid chambers 222 to urge the associated pistons 219 in the outboard caliper assembly 212 in the inboard direction (toward the right when viewing Fig. 14) into engagement with the backing plate 233 of the outboard brake shoe 228. As a result, the friction pad 232 of the inboard brake shoe 227 is moved into frictional engagement with the inboard braking disc 237 of the rotor 236. and the friction pad 234 of the outboard brake shoe 228 is simultaneously moved into frictional engagement with the outboard braking disc 238 of the rotor 236. As a result, the rotor 236 is frictionally engaged by the friction pads 232 and 234 to retard relative rotational movement thereof.

The disc brake assembly 210 further includes a pair of bleeder screws 242 which are received in a threaded aperture provided in each of the inboard caliper assembly 211 and the outboard caliper assembly 212. The bleeder screws 242 are provided to bleed air from the associated hydraulic fluid chambers 222 when the disc brake assembly 210 is initially connected to the vehicle hydraulic brake system.

Referring now to Fig. 15, the structure of a fifth embodiment of a piston, indicated generally at 219, will be discussed. As shown therein, the piston 219 includes a piston body 291 which is preferably molded from a plastic, and more preferably from a synthetic resin material such as a phenolic resin. However, the piston 219 can be formed from other materials, such as for example, ceramic. The piston body 291 is generally hollow and cylindrical in shape, having an opened end 292 and a closed end 293. An inner cylindrical surface 294 and an outer cylindrical surface 295 extend axially between the opened end 292 and the closed end 293. The inner cylindrical surface 294 and the outer cylindrical surface 295 of the piston body 291 are preferably concentric with a longitudinal axis Y of the piston 219. Preferably, the piston body 291 is narrowed at its opened end 292, but this is not critical to the invention.

The outer cylindrical surface 295 of the piston body 291 includes a first outer cylindrical surface 295a and a reduced diameter second outer cylindrical surface 295b. The first and second outer surfaces 295a and 295b extend parallel to and concentric with the longitudinal axis Y of the piston 219. The transition between the first and second outer surfaces 295a and 295b defines a shoulder 295c. The shoulder 295c is adapted to receive the first end 224a of the dust boot seal 224. The outer cylindrical surface 295 of the piston body 291 is not resistant to moisture, because the surface 295 is ground after molding to achieve a precise tolerance. In contrast, the closed end 293 of the piston body 291 is an as-molded surface (it is not ground after molding), and therefore it is resin-rich and resistant to moisture.

A generally cylindrical first cover 296 is integrally molded about portions of the outer cylindrical surface 295 and the closed end 293 of the piston body 291. The first cover 296 includes an outer cylindrical side wall 297 and an end wall 298. The outer cylindrical side wall 297 of the first cover 296 extends

axially toward the opened end 292 of the piston body 291 along a portion of the outer cylindrical surface 295 of the piston body 291. The end wall 298 of the first cover 296 extends radially inwardly along a portion of the closed end 293 of the piston body 291. Thus, the outer cylindrical side wall 297 and the end wall 298 extend generally peφendicular to one another. Preferably the outer cylindrical side wall 297 of the first cover 296 extends over about 60% to about 80% of the outer cylindrical surface 295 of the piston body 291. However, the outer cylindrical side wall 297 of the first cover 296 does not extend all the way to the opened end 292 of the piston body 291 , in order to avoid heat transfer into the first cover 296 as will be discussed below. If a piston 219 having a radial groove were used (such as groove 62 shown in Fig. 4), the outer cylindrical side wall 297 of the first cover 296 would typically extend just short of such a groove.

As mentioned above, the outer cylindrical surface 295 of the piston body 291 is not resistant to moisture which can be picked up from the disc brake assembly. Thus, the outer cylindrical side wall 297 of the first cover 296 extends over the outer cylindrical surface 295 of the piston body 291 to protect against moisture absoφtion. Protecting the outer cylindrical surface 295 of the piston body 291 from moisture absoφtion protects the structural integrity and dimensional stability of the piston body 219, and also prevents the piston body 291 from swelling. Preferably, the outer cylindrical side wall 297 of the first cover 296 extends over that portion of the outer cylindrical surface 295 that comes into contact with the fluid seal 220. This further protects the outer cylindrical surface 295 of the piston body 291 from moisture.

The end wall 298 of the first cover 296 extends over a portion of the closed end 293 of the piston body 291. Extending the first cover 296 in this manner creates an excellent moisture seal between the first cover 296 and the piston body 291 so that moisture cannot leak under the first cover 296 around the

20

edges. This structure also creates smooth edges around the closed end 293 of the piston body 291 so that the dust boot seal 224 is not damaged during assembly of the disc brake assembly 210. The end wall 298 ofthe first cover 296 can extend further over the closed end 293 of the piston body 291 if desired, but it is not 5 necessary.

Preferably, the piston 219 includes a generally annular second cover 299 integrally molded about the opened end 292 of the piston body 291. The second cover 299 includes an end wall 300, an inner cylindrical side wall 301 , and an outer cylindrical side wall 302. The end wall 300 extends about the opened end i 292 of the piston body 291. The inner and outer side walls 301 and 302 extend axially toward the closed end 293 of the piston body 291 along portions of the inner and outer cylindrical surfaces 294 and 295 thereof. Thus, the second cover 299 has a generally U-shaped cross section. The end wall 300 of the second cover 299 defines an annular abutment surface which is oriented peφendicular to

15 the longitudinal axis Y of the piston 219. The abutment surface 300 is adapted to engage the respective backing plates 231 and 233 of the brake shoes 227 and 228, so that during braking the abutment surface urges the brake shoes 227 and 228 against the rotor 236. The second cover 299 protectively shields the enclosed portions of the piston body 291 from heat which is generated between 0 the rotor 236 and the brake shoes 227 and 228 during braking.

The first cover 296 and the second cover 299 are formed from materials which are different from the material of the piston body 291 Preferably, the first cover 296 and the second cover 299 are formed from a metallic material such as steel or aluminum. More preferably they are formed from stainless steel or 5 carbon steel, and can be electroplated with zinc for corrosion protection.

The first cover 296 and the second cover 299 are good thermal conductors compared to the piston body 291. Accordingly, it is important that the first cover

296 is spaced apart from the second cover 299 so that there is a discontinuity between the first and second covers. This discontinuity acts as a barrier against heat flow from the second cover 299 to the first cover 296 during a braking operation. It thereby helps to prevent heat transfer to the hydraulic brake fluid and/or heat damage to the fluid seal 220 or to the surface of the piston 219.

Standard molding procedures are sufficient to hold the first cover 296 and the second cover 299 to the piston body 291 , but if desired additional treatments or means could be used for this puφose. For example, a metal spray could be used to form an adhesive bond between the first and second covers 296 and 299 and the piston body 291 , or the inside of the first and second covers 296 and 299 could be coated and/or roughened to produce a rough surface for better adhesion. Also, projections or tangs (similar to the tangs 78 discussed above in connection with the cover 70) could be provided on the first and second covers 296 and 299. Referring now to Fig. 16, the structure of a sixth embodiment of a piston, indicated generally at 319, will be discussed. As shown therein, the piston includes a piston body 391 which is preferably molded from a plastic, and more preferably from a synthetic resin material such as phenolic resin. However, the piston 319 can be formed from other materials, such as for example, ceramic. The piston body 391 is generally hollow and cylindrical in shape, having an opened end 392 and a closed end 393. An inner cylindrical surface 394 and an outer cylindrical surface 395 extend axially between the opened end 392 and the closed end 393. The inner cylindrical surface 394 and the outer cylindrical surface 395 of the piston body 391 are preferably concentric with a longitudinal axis Z of the piston 319. A generally cylindrical cover 396 is integrally molded about the closed end 393 and the outer cylindrical surface 395 ofthe piston body 391. The cover 396 includes an end wall 397, an outer side wall 398 defining an outer diameter

399, and an opened end 400. The end wall 397 of the cover 396 extends about the closed end 393 of the piston body 391. The outer side wall 398 of the cover 396 extends axially along the outer cylindrical surface 395 of the piston body 391. In the illustrated embodiment, the opened end 400 ofthe cover 396 is castellated and defines a plurality of equidistantly spaced recessed sections 400a and a plurality of equidistantly spaced projections 400b. The castellations allow air to circulate to the interior of the piston 319 and provide help in cooling. The cover 396 is preferably formed from a metallic material, for example from steel or aluminum. More preferably the cover 396 is formed from stainless steel or carbon steel, and can be electroplated with zinc for corrosion protection. However, the cover 396 can also be formed from other materials which are different from the material of the piston body 391.

The outer side wall 398 of the cover 396 extends past the opened end 392 of the piston body 391 in the direction of the associated brake shoe. (In other words, the opened end 392 of the piston body 391 is recessed relative to the opened end 400 of the cover 396.) As a result, during the braking operation it is the opened end 400 of the cover 396 which engages and urges the associated brake shoe against the rotor, instead of the opened end 392 of the piston body 391 engaging and urging the brake shoe against the rotor. The opened end 392 of the piston body 391 does not come into contact with the brake shoe. This structure effectively protects the piston body 391 from the heat generated during braking and thereby improves the life ofthe piston 319. In more detail, the opened end 400 of the cover 396 defines an annular abutment surface which is oriented peφendicular to the longitudinal axis Z ofthe piston. The abutment surface 400 is adapted to engage the backing plate of the brake shoe, so that during braking the abutment surface 400 urges the brake shoe against the rotor.

Preferably, the piston 319 also includes means for supporting the outer side wall 398 of the cover 396 near the opened end 400 so that the outer side wall 398 does not collapse or crumple from the force of abutment against the brake shoe. This is generally important when the side wall 398 of the cover 396 extends more than a very small distance past the opened end 392 of the piston body 391. Fig. 16 shows a preferred means for providing this support: the piston body 391 and the cover 396 are reduced in diameter ("necked down") about their respective opened ends 392 and 400. Specifically, the outer cylindrical surface

395 of the piston body 391 includes a first outer surface 395a and a reduced diameter second outer surface 395b. Similarly, the outer side wall 398 of the cover 396 includes a first outer side wall 398a and a reduced diameter second outer side wall 398b. The first outer side wall 398a of the cover 396 defines a first cover outer diameter 399a, and the second outer side wall 398b of the cover

396 defines a second cover outer diameters 399b. The first and second outer surfaces 395a and 395b of the piston body 391 extend parallel to and concentric with the longitudinal axis Z of the piston 319. Similarly, the first and second outer side walls 398a and 398b of the cover 396 are concentric with the longitudinal axis Z. The transition between the first and second outer side walls 398a and 398b of the cover 396 define an inner cover shoulder 398c and an outer cover shoulder 398d. The outer cover shoulder 398d receives the end 224a of the dust boot seal 224. Preferably, the outer diameter 399b of the second outer side wall 398b of the cover 396 (i.e., the outer side wall of the cover 396 at the opened end 400) is not greater than the diameter of the first outer surface 395a of the piston body 391 (i.e., the outer surface of the piston body 391 at the closed end 393). However, the outer diameter 399b of the second outer side wall 398b of the cover 396 at the opened end 400 can also be greater than the diameter of the first outer surface 395a of the piston body 391 at the closed end 393. It is

seen in Fig. 16 that the piston body 391 behind the inner cover shoulder 398c provides support to the outer side wall 398 of the cover 396. Advantageously, this preferred structure also assists in retaining the piston body 391 inside the cover 396. A variety of other means can be used for supporting the outer side wall

398 of the cover 396 near its open end 400. An example is shown in Fig. 17 where an outer surface 495 of a piston body 491 and an outer side wall 498 of a cover 496 are straight, but where a piston 419 also includes a generally annular L-shaped piece 501 welded inside an opened end 500 of the cover 496, one leg of the L-shaped piece 501 being welded to the cover 496, and the other leg of the L-shaped piece 501 forming a flange 502 which extends radially inwardly. Again, this structure both supports the outer side wall 498 of the cover 496 and assists in retaining the piston body 491 inside the cover 496.

Another example is shown in Fig. 18, where a cover 596 is reduced in diameter about an opened end 600 thereof, but an outer surface 595 of a piston body 591 of a piston 519 is straight. In this embodiment, an inner diameter 603 of an outer side wall 598 of the cover 596 at the opened end 600 thereof is not greater than an inner diameter 594 of the piston body 591.

The thicknesses of the associated covers illustrated in Figs. 16 through 18 is not critical but generally a relatively thin cover is preferred. Of course, a thinner cover generally requires more support against the abutment force. The distance the cover extends past the opened end of the piston body also is not critical, but again a longer distance would generally require more support while a very small distance may not require any support. Standard molding procedures are sufficient to hold the cover to the body of the piston, but if desired additional treatments or means besides those mentioned above could be used for this puφose. For example, a metal spray could be used to form an adhesive bond

between the cover and the body, or the inside of the cover could be coated to produce a rough surface for better adhesion. Tabs or projections provided on the cover could also be used.

In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been described and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.