Eddy current braking system

[0002] The present invention principally relates to a magnet type, eddy current reduction braking system for assisting frictional brakes in large vehicles.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] The object of the present invention, therefore, is to provide an improved magnet type eddy current reduction apparatus capable of enhanced braking performance.

[0008] 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.

[0009] 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:

[0010] 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;

[0011] FIG. 2 is a front sectional view of the system shown in FIG. 1;

[0012] FIG. 3 is a front sectional view of a second embodiment of the invention;

[0013] FIG. 4 is a front sectional view of a third embodiment of the invention;

[0014] FIG. 5 is a side sectional view of a fourth embodiment of the invention;

[0015] FIG. 6 is a side sectional view of a fifth embodiment of the invention; and

[0016] FIG. 7 is a side sectional view of a sixth embodiment of the invention.

[0017] 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 19aand 19bare housed in a hollow portion of the guide tube 18.

[0018] Formed on an inner peripheral wall of a fixed tube 7 is an annular plate 7asecured by bolts 8 to an end wall of a gearbox 2 of a speed change gear for the vehicle. An inner tubular wall portion 13bof 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 7b. The inner ends of the tube portion 13band the fixed tube 7 are sealed by means of a seal member 5. Projecting outwardly from an intermediate portion of a right end wall 13cof the brake drum 13 is an integrally formed cylindrical portion 23 defining, on an inner peripheral portion, a ring gear 23awhich engages the sun gear 7b. Preferably, a plurality of radial openings for cooling are provided in the outwardly extended cylindrical portion 23 of the brake drum 13.

[0019] Boss portions of a plurality of radially extending arms 9 are coupled by keys 4ato 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 23aand the sun gear 7bto 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.

[0020] 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 18a(FIG. 2) and an inner peripheral wall portion 18b. 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 22aprojecting 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.

[0021] As shown in FIG. 2, a number of slots are provided at equal circumferentially spaced apart intervals along the outer peripheral wall portion 18aof the guide tube 18. Juxtaposed to an inner peripheral surface 13dof 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.

[0022] A movable magnet support tube 19aand transversely adjacent immovable magnet support tube 19bare housed in the hollow portion of the guide tube 18. The magnet support tube 19ais formed of magnetically permeable material and is rotatably supported in the hollow portion of the guide tube 18 while the magnet support tube 19bis fixed therein. Permanent magnets 20 facing the ferromagnetic plates 21 are coupled to the outer peripheral surfaces of each of magnet support tubes 19aand 19b. The polarities of adjacent magnets 20 are opposite both circumferentially and axially.

[0023] 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 19ais 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.

[0024] In a braking condition shown in FIG. 2, the magnet support tube 19ais 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 19aand those of transversely adjacent magnets 20 on the magnet support tube 19bare the same. Consequently, magnetic circuits 40 are formed between the magnet support tubes 19aand 19band 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.

[0025] In a non-braking condition, the magnet support tube 19ais rotated a full pitch into a position in which the polarities of transversely adjacent magnets 20 on the guide tubes 19aand 19bare opposite. Resultant magnetic short circuits are formed between the ferromagnetic plates 21 and the magnet support tubes 19aand 19b. The magnets 20, therefore, cease applying magnetic fields to the brake drum 13 which generates no braking torque.

[0026] 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 19aand an immovable magnet support tube 19bare disposed interiorly of a brake drum 13. The movable magnet support tube 19ais 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 19aand 19bare 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.

[0027] 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 40abetween 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.

[0028] 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.

[0029] 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 18aof 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 18bof 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.

[0030] 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 17aactuated, 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.

[0031] 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.

[0032] 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 36aof the magnetic core 36 is positioned to an inner peripheral surface 13dof an outer tube 13aof the brake drum 13. Other components of the FIG. 7 embodiment are similar to those of the embodiment shown in FIG. 1.

[0033] 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 36a. 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.

[0034] 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.