1. Technical Field
The present invention relates to an eddy current type brake system adapted for a vehicle, and more particularly to an eddy current type brake system which produces eddy current in a rotor by use of permanent magnets.
2. Background Art
An eddy current type brake system is known as a vehicle retarder ensuring stable continuous braking in combination with a main brake (foot brake) and preventing burning of the main brake.
The eddy current type brake system generally includes a rotor mounted on a shaft, such as a propeller shaft, drivingly connected to wheels of a vehicle and a magnetic power source, such as an electro-magnet or a permanent magnet, mounted on a fixed portion, such as a frame of the vehicle, so as to be located near the rotor. The eddy current is produced in the rotor due to a relative velocity difference between the rotor (a rotary portion) and the magnetic power source (the stationary portion). The eddy current serves as the brake force against the rotation of the rotor, whereby the vehicle is decelerated.
In order to design a compact, light weight eddy current type retarder, a compact permanent magnet which possesses very strong magnetism may be employed as the magnetic power source.
FIG. 6 of the accompanying drawings shows an eddy current type retarder which uses permanent magnets as the magnetic power source. This arrangement was developed by the assignee of the present application. A rotor (b) of the eddy current type retarder is mounted on an output shaft (a) of a transmission of a vehicle. The rotor (b) is made from conductive, ferrogmagnetic material and shaped like a drum having a bottom. The rotor (b) extends coaxially with the output shaft (a). A stator (d) is supported by a transmission casing (c) and positioned inside the rotor (b) in a manner such that the stator (d) can move reciprocatively in the axial direction of the rotor (b). The stator (d) includes a support ring (e) and permanent magnets (f). The support ring (e) is shaped to be annular and coaxial with the output shaft (a). The permanent magnets (f) are mounted on the support ring (e). The stator (d) is mounted on the transmission casing (c) with a mounting element (g). The permanent magnet (f) is made from rare earth meatal such as neodymium, light in weight and compact in size. The magnets (f) are arranged in a circle to face the inner wall of the rotor (b), with the pole (S or N) of one magnet being opposite the opposite pole (N or S) or a next magnet in the circumferential direction of the rotor (b). The number of the magnets (f) is an even number such as eight, ten or twelve. The stator (d) includes permanent magnets (f) and the support ring (e) and sealingly housed in a casing (h). The stator (d) moves reciprocatively in the casing (h). The casing (h) includes a brake member (i) and a brake releasing part (j). The brake member (i) and the brake releasing member (j) extend between the rotor (b) and the stator (d). The brake member (i) magnetically connects the permanent magnets (f) of the stator (d) with the rotor (b) so as to apply the brake force to the rotor (b). The brake releasing member (j) disconnects the magnetic connection between the magnets (f) and the rotor (b) by preventing the magnetic flux of the magnets (f) from reaching the rotor (b).
Referring to FIG. 7, the brake member (i) includes a ferromagnetic element (oblique lines) and a non-magnetic element (dots). That part (k) of the permanent magnet (f) which faces the rotor (b) (called "pole piece") is made from the feromagnetic material having a large magnetic permeability and another part is made from non-magnetic material having a small magnetic permeability. The pole pieces (k), in order to magnetically connect the permanent magnets (f) with the rotor (b), are arranged with the same intervals as the permanent magnets (f) and in the same direction as the permanent magnets (f) so that one pole piece (k) makes a pair with one permanent magnet (f). The brake releasing member (j) is made from ferromagnetic material to magnetically directly connect one pole (N) of a magnet with a pole (S) of a next magnet.
When the brake force is applied to the vehicle by the eddy current type retarder having the construction described above, the stator (d) is moved under the brake member (i) by an actuator (1) as indicated by the solid line in FIG. 6. In other words, the stator (d) is moved to the right in the drawing. Since the pole pices (k), the rotor (b) and the support ring (e) are made from ferromagnetic material, a magnetic circuit is made between each two adjacent magnets, namely the magnet circuit extending from the N pole of one magnet (f), the brake member (i), the rotor (b), the brake member (i), the S pole of a next magnet (f), the stator (d) and the N pole of the just-mentioned next magnet (f) and the S pole of the above-mentioned one magnet (f). Thereupon, the eddy current flows through the magnetic circuit and the brake force is applied to the rotor, whereby the output shaft of the transmission is decelerated and then the vehicle is decelerated.
When the brake of the eddy current type retarder is released from the rotor, the stator (d) is moved under the brake releasing member (j), as indicatd by the dashed line in FIG. 6. In other words, the stator (d) is moved to the left in the drawing. Under the brake releasing member (j), another magnetic circuit is established. Specifically, this magnetic circuit dies not enter the rotor (b) so that the eddy current no longer flows in the rotor (b). Thus, no brake force is applied to the rotor (b). The magnetism of the permanent magnets (f) is stopped by the brake releasing member (j) and does not reach the rotor (b).
If the brake releasing member is designed to be relatively thin in order to reduce the total weight of the retarder, adequate stopage of the magnetism of the magnets (f) cannot be expected. Then, part of the magnetic flux leaks into the rotor (b) and the eddy current flows through the rotor (b). Thereupon, the brake force is applied to the rotor (b) even though the stator (d) is located under the brake releasing member (j).
The thickness of the brake releasing member (j) is designed thicker in order to assure adequate brake release. Then, the thickness of the brake member (j) has to be designed thicker if balance is to be maintained. This means that the thickness of the pole pieces will become thicker and that the resistance against the magnetic flux will become large. Therefore, a strong eddy current is not produced and a strong brake force is not produced. In addition, the weight of the retarder becomes large.
Another way to avoid an inadequate stopage of the magnetic flux at the brake releasing member is to reduce the width D of the rotor (b), i.e., to reduce the width D of the rotor (b) to the width (D1), as indicated by (m) in FIG. 6. In this case, the rotor (b) covers the brake member (i) only and does not cover the brake releasing member (j). With this arrangement, when the stator (d) is moved to the left in the drawing, the magnetic flux from the permanent magnets (f) penetrates the brake releasing member (j) and then leaks to the atmoshpere. This means that the eddy current does not flow in the rotor (b) when the stator (d) is under the brake releasing member (j) and that the brake force is no longer applied to the rotor (b) at that time.
However, the heat releasing surface area of the rotor (b) becomes small as the width of the rotor (b) is reduced. Therefore, the temperature of the rotor (b) having a width D1 becomes higher than that at the rotor (b) having a width D. The electrical resistance of the rotor (b) becomes larger as the temperature of the rotor (b) becomes higher. Accordingly, the eddy current becomes smaller and the brake force applied to the rotor (b) becomes smaller. In addition, deformation and/or crackign will occur in the rotor (b) due to the heat. The rotational balance of the rotor will be deteriorated upon deformation and/or the cracking of the rotor.