Abstract:
A vehicle braking system including a magnetically permeable rotatably mounted brake drum; an annularly distributed magnetic source generates a magnetic field within the brake drum; a control for selectively applying the magnetic field to the brake drum; a drive shaft rotatable during movement of the vehicle; and a rotational speed enhancing coupling between the brake drum and the drive shaft.

Description:
BACKGROUND OF THE INVENTION 
     The present invention principally relates to a magnet type, eddy current reduction braking system for assisting frictional brakes in large vehicles. 
     According to a generator type reduction apparatus (retarder) disclosed in Japanese Patent Application Laid-Open No. 60-255050 Publication, an annular stator body supports a field coil of an induction type generator and is rotated in a direction oppositely to an annular rotor body which supports an armature coil of the induction type generator. The resultant increase in the relative number of revolutions of the annular rotor body with respect to the annular stator body enhances braking performance. Braking energy is recovered as an electric power which is converted into and disposed of as heat in a load resistor or the like. Therefore, braking performance is not reduced by heat generation in the annular rotor body. 
     However, even if the above-described reversing mechanism is applied to a magnet type eddy current reduction braking system such that a magnet support tube is rotated in a direction reversed to that of a brake drum, cooling performance is not enhanced because the absolute number of brake drum revolutions remain unchanged. Furthermore, since the relative number of revolutions of the brake drum and the magnet support tube increases, braking performance is diminished substantially by increased heat generation in the brake drum. 
     In conventional magnet type eddy current reduction apparatus, rotational energy of a brake drum is converted into and disposed of as heat produced by eddy currents. However, at present, braking performance achieved is inferior to other reduction systems, particularly fluid type reduction systems. 
     The braking performance of a magnet type eddy current reduction apparatus is proportional to the magnetic force provided by the magnets, the diamater of the brake drum, and the rotational speed of the brake drum, and is different depending on the material used for the brake drum. However, the diameter of the brake drum is restricted by available space in the vehicle, and the rotational speed of the brake drum is controlled by the rotational speed of a drive shaft to which the brake drum is coupled. Braking performance is enhanced, therefore, only by increasing magnetic strength of the magnets or selection of a different material for the brake drum. However, when braking performance is enhanced, heat generation in the brake drum increases, and is not readily dissipated by cooling fins thereby compromising braking performance improvement. Thus, braking performance cannot be effectively enhanced unless heat transfer out of the brake drum is increased. 
     The object of the present invention, therefore, is to provide an improved magnet type eddy current reduction apparatus capable of enhanced braking performance. 
     SUMMARY OF THE INVENTION 
     The present invention is a braking apparatus in which a magnetic field generation system generates eddy currents in a brake drum to provide braking force. In addition, a coupling mechanism increases rotational speed of the brake drum relative to that of an associated drive shaft. The coupling can consist, for example, of a gear mechanism interposed between the drive shaft and the brake drum. For example, the peripheral speed of the brake drum is increased by about two times that provided by the prior art, and, therefore braking torque caused by eddy currents increased to two times the torque provided by conventional apparatus. Since braking torque generated by the brake drum is enhanced about two times, braking torque received by the drive shaft is increased about  4  times. Although heat generated in the brake drum also increases, rotational speed of cooling fins on the brake drum also is doubled to increase cooling efficiency. Consequently, the present invention produces greater braking torque than that provided by conventional magnet type eddy current reduction systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the invention will become more apparent upon a persual of the following description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a side sectional view of a magnet type eddy current reduction braking system according to a first embodiment of the present invention; 
     FIG. 2 is a front sectional view of the system shown in FIG. 1; 
     FIG. 3 is a front sectional view of a second embodiment of the invention; 
     FIG. 4 is a front sectional view of a third embodiment of the invention; 
     FIG. 5 is a side sectional view of a fourth embodiment of the invention; 
     FIG. 6 is a side sectional view of a fifth embodiment of the invention; and 
     FIG. 7 is a side sectional view of a sixth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A magnetically permeable, rotatably mounted brake drum  13  having a C-shape in section is coupled by a coupling mechanism  30  to a rotational drive shaft  4  of a vehicle (not shown). A guide tube  18  formed of a non-magnetically permeable material such as aluminum is disposed interiorly of the brake drum  13 , and magnet support tubes  19   a  and  19   b  are housed in a hollow portion of the guide tube  18 . 
     Formed on an inner peripheral wall of a fixed tube  7  is an annular plate  7   a  secured by bolts  8  to an end wall of a gearbox  2  of a speed change gear for the vehicle. An inner tubular wall portion  13   b  of the brake drum  13  is rotatably supported by a pair of bearings  6  on a left half portion of the fixed tube  7 . Integrally formed on the outer peripheral surface of a right half portion of the fixed tube  7  is a sun gear  7   b . The inner ends of the tube portion  13   b  and the fixed tube  7  are sealed by means of a seal member  5 . Projecting outwardly from an intermediate portion of a right end wall  13   c  of the brake drum  13  is an integrally formed cylindrical portion  23  defining, on an inner peripheral portion, a ring gear  23   a  which engages the sun gear  7   b . Preferably, a plurality of radial openings for cooling are provided in the outwardly extended cylindrical portion  23  of the brake drum  13 . 
     Boss portions of a plurality of radially extending arms  9  are coupled by keys  4   a  to an outer end of the output rotational shaft  4  which are supported by bearings  3  on an end wall of the gear box  2 . A planetary gear  31  is rotatably supported on support shafts  33  at outer ends of the arms  9 . The planetary gears  31  are meshed with the ring gear  23   a  and the sun gear  7   b  to thereby constitute the coupling mechanism  30 . Closing the coupling mechanism  30  is a cover plate  34  fixed to the cylindrical portion  23  by a plurality of bolts  32 . An inner edge of the cover plate  34  is sealed to the rotational shaft  4  by a seal member  35 . In response to rotation of the drive shaft  4  at a given speed, the coupling mechanism  30  rotates the brake drum  13  at a greater speed of, for example, twice the given speed. 
     A number of cooling fins  14  are provided at equal intervals on an outer peripheral wall of the brake drum  13  and a guide tube  18  having a hollow portion of rectangular shape in section is coaxially disposed within the brake drum  13 . The guide tube  18  is formed of a non-magnetically permeable material and could be constituted by bonding annular end wall plates to an outer peripheral wall portion  18   a  (FIG. 2) and an inner peripheral wall portion  18   b . However, the illustrated guide tube  18  is formed by fixing a tube portion in the form of a channel or C-shape in section to a left-hand end wall plate  22  with a number of bolts (not shown). A plurality of arms  22   a  projecting diametrically and inwardly from the end wall plate  22  of the guide tube  18  are secured to an end wall of the fixed tube  7  by a plurality of bolts  25 . 
     As shown in FIG. 2, a number of slots are provided at equal circumferentially spaced apart intervals along the outer peripheral wall portion  18   a  of the guide tube  18 . Juxtaposed to an inner peripheral surface  13   d  of the brake drum  13  are ferromagnetic pole pieces  21 , each fitted into and secured to one of the slots. Preferably, the ferromagnetic pole pieces  21  are bonded into the slots when the guide tube  18  is cast of a material such as aluminum. 
     A movable magnet support tube  19   a  and transversely adjacent immovable magnet support tube  19   b  are housed in the hollow portion of the guide tube  18 . The magnet support tube  19   a  is formed of magnetically permeable material and is rotatably supported in the hollow portion of the guide tube  18  while the magnet support tube  19   b  is fixed therein. Permanent magnets  20  facing the ferromagnetic plates  21  are coupled to the outer peripheral surfaces of each of magnet support tubes  19   a  and  19   b . The polarities of adjacent magnets  20  are opposite both circumferentially and axially. 
     A plurality of actuators  17  are supported at peripherally equal intervals on the end wall plate  22  of the guide tube  18 . Each actuator  17  is conventional with a piston fitted in a cylinder (not shown) to define a pair of fluid pressure chambers. The magnet support tube  19   a  is connected to an arm (not shown) extending from the piston into the hollow portion of the guide tube  18  via a slit in the end wall plate  22 . 
     In a braking condition shown in FIG. 2, the magnet support tube  19   a  is rotated by the actuators  17  into a position which radially aligns the magnets  20  with the pole pieces  21 . The magnets  20  on the stationary support tube also are radially aligned with the pole pieces  21 . In addition, the polarities of the magnets  20  on the magnet support tube  19   a  and those of transversely adjacent magnets  20  on the magnet support tube  19   b  are the same. Consequently, magnetic circuits  40  are formed between the magnet support tubes  19   a  and  19   b  and the brake drum  13  via the pole pieces  21 . As the rotating brake drum  13  crosses the magnetic fields produced by the magnets  20 , eddy currents are generated and a braking torque occurs in the brake drum  13 . Heat generated by the eddy currents is transferred to the open air through the cooling fins  14 . 
     In a non-braking condition, the magnet support tube  19   a  is rotated a full pitch into a position in which the polarities of transversely adjacent magnets  20  on the guide tubes  19   a  and  19   b  are opposite. Resultant magnetic short circuits are formed between the ferromagnetic plates  21  and the magnet support tubes  19   a  and  19   b . The magnets  20 , therefore, cease applying magnetic fields to the brake drum  13  which generates no braking torque. 
     In the aforementioned embodiment, a description has been made of a magnet type eddy current reduction braking system in which a movable magnet support tube  19   a  and an immovable magnet support tube  19   b  are disposed interiorly of a brake drum  13 . The movable magnet support tube  19   a  is rotated to switch between a braking position in which magnets  20  having the same polarities are axially aligned and also radially aligned with the ferromagnetic plates  21  and a non-braking position in which axially aligned magnets  20  on the guide tubes  19   a  and  19   b  are of opposite polarity and are radially aligned with the ferromagnetic plates  21 . However, the present invention is not limited thereto. For example, the rotational speed enhancing coupling mechanism  30  can be incorporated into other systems to improve braking performance. Examples of such other braking systems are illustrated in FIGS.  3 - 7 . 
     In an embodiment shown in FIG. 3, a single magnet support tube  19  is rotated by a half-pitch of magnets  20  to switch between braking and non-braking conditions. That is, in the braking state, when the magnets  20  of circumferentially alternating polarities are radially aligned with ferromagnetic plates  21 , the magnets  20  apply a magnetic field  40  on the brake drum  13  similar to that applied in the above described embodiment (see FIG.  2 ). Consequently, eddy currents in the rotating brake drum  13  generate a braking torque. Conversely, in a non-braking state shown in FIG. 3, the magnet support tube  19  is rotated a half pitch of the magnets  20  and transversely adjacent magnets  20  each are partly radially aligned with a ferromagnetic plate  21  to create a magnetic short circuit  40   a  between the magnet support tube  19  and the ferromagnetic plate  21 . Accordingly, the magnets  20  fail to apply a magnetic field on the brake drum  13  which generates no braking torque. 
     In the embodiment shown in FIG. 4, magnets  20  are distributed at equal intervals to the outer peripheral surface of a single magnetic support tube  19 . A pair of same polarity magnets  20  are provided for each ferromagnetic plate  21 , and the polarities of the pairs alternate circumferentially. In a braking condition, each pair of magnets  20  having the same polarity are radially aligned with a ferromagnetic plate  21 . Consequently, a magnetic circuit  40  is generated between the magnet support tube  19  and the brake drum  13  to produce a braking torque. Conversely, in a non-braking condition, the magnet support tube  19  is rotated by an arrangement pitch of magnets  20  and a pair of magnets  20  having different polarities are radially aligned with each of the ferromagnetic plates  21 . A magnetic short circuit therefore is created between the magnet support tube  19  and the ferromagnetic plates  21  and the magnets  20  do not apply a magnetic field to the brake drum  13  which generates no braking torque. 
     In an embodiment shown in FIG. 5, there is provided a magnet support tube  19  which can be moved axially of the brake drum  13  to switch between braking and non-braking conditions. A guide tube  18  formed of a non-magnetically permeable material and having a hollow portion in the form of a rectangle in section is fixed to a non-rotating portion of a vehicle (not shown) so as to face to an inner surface of a brake drum  13 . Disposed at equal intervals on an outer peripheral wall portion  18   a  of the guide tube  18  are a number of ferromagnetic plates  21 . The magnet support tube  19  is axially slidably mounted on an inner wall portion  18   b  of the guide tube  18 . Supported on an outer peripheral surface of the magnet support tube  19  and radially aligned with ferromagnetic plates  21  are magnets having circumferentially alternating opposite polarities. 
     In a braking condition, the magnet support tube  19  is moved into the brake drum  13 , as shown in FIG. 5, and the magnets  20  apply magnetic fields to the brake drum  13 . When the rotating brake drum  13  crosses the magnetic fields transmitted by the ferromagnetic plates  21 , eddy currents flow into the brake drum  13 , which generates a braking torque. In a non-braking condition, the magnet support tube  19  is moved leftward in FIG. 5 by a rod  17   a  actuated, for example, by a fluid pressure actuator (not shown). With the magnet support tube  19  drawn outside the brake drum  13 , the magnets  20  apply no magnetic field on the brake drum  13  which, therefore, generates no braking torque. 
     In an embodiment shown in FIG. 6, a tube body  41  formed of a non-magnetically permeable sheet  41  replaces the ferromagnetic plates in the embodiment of FIG.  5 . The sheet forms an outer peripheral wall portion of the guide tube  18  and is positioned closely adjacent to an inner peripheral surface of a brake drum  13 . If a wall-thickness of the sheet tube body  41  is, for example, not more than  1  mm, the strength of magnetic fields applied to the brake drum  13  by the magnets  20  during braking is reduced only slightly from that provided by the embodiment in FIG.  5 . Thus, when similarly operated, the embodiment of FIG. 6 exhibits similar braking performance. 
     An embodiment shown in FIG. 7 includes a brake drum  13  formed of magnetically permeable material and having a C-shape in section and a speed enhancing gear mechanism  30  rotatively coupling the brake drum  13  to a rotational drive shaft  4 , a magnet support tube  38  is disposed within the brake drum  13 , and a number of electromagnets  28  are supported at equal intervals on the outer peripheral surface of the magnet support tube  38 . Each electromagnet  28  is formed by winding an electromagnetic coil  37  on a magnetic core  36  secured to the magnet support tube  38 , and a magnetic-pole  36   a  of the magnetic core  36  is positioned to an inner peripheral surface  13   d  of an outer tube  13   a  of the brake drum  13 . Other components of the FIG. 7 embodiment are similar to those of the embodiment shown in FIG.  1 . 
     In a braking condition, the electromagnetic coils  37  are energized by an electrical source (not shown) to establish circumferentially alternating polarities for the magnetic-poles  36   a . Consequently, magnetic circuits are produced between the magnet support tube  38  and the brake drum  13  which generates a braking torque. In a non-braking condition, energization of the electromagnetic coils  37  is interrupted. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood, therefore, that the invention can be practiced otherwise than as specifically described.