1. Field of the Invention
This invention relates to brushless motors of the coreless type.
2. Description of the Related Art
Brushless motors, consist of the plane-opposed type and the circumference-opposed type. In the plane-opposed type, a coreless form having air-core coils is used. In the circumference-opposed type, the salient-pole-equipped core motors are primarily used.
The motor of the plane-opposed type, however, has the disadvantage that the motor is becomes large. Also, the attempt to reduce the motor diameter by increasing the strength of the magnet leads to an increased loss because the force of magnetic attraction becomes large.
Meanwhile, in the salient pole form of the circumference-opposed type motor, as the strength of the magnet increases, the cogging torque gets larger. Also, the decrease of the motor diameter makes it impossible to assure creation of an space adequate for coils to occupy.
Again, in the coreless form of the circumference-opposed type motor, there are drawbacks; for example, to maintain a hold of the coils is difficult, and, if the motor diameter is small, the use of flat air-core coils, because it causes the gap to become larger, lowers the magnetic efficiency.
FIG. 1 and FIG. 2 are schematic sectional views of conventional motors. These motors have a feature suited to be used as the optical spindle motor.
In FIG. 1, a motor housing 1 has its inner diameter portion hold two bearings 5A and 5B in a spaced relation from each other by a predetermined distance. These bearings 5A and 5B form a shaft receptor portion, of which the central hole rotatably contains a shaft 2.
The shaft 2 is for centering an optical medium or like record bearing body (not shown), on which a turntable 3 for carrying the optical medium to rotate is fixedly fitted at its upper portion.
A bush 7 is fitted on the lower portion of the shaft 2 in an adjusted position and then fixedly secured to the shaft 2 by adhesive agent or the like.
By this fixture of the bush 7, a state that an inward impelling pressure (pre-load) acts on the two bearings 5A and 5B in between the turntable 3 and the bush 7, is maintained.
Under the condition that such a pre-load functions, the inner or outer races and bearing balls of the bearings 5A and 5B are biased to one direction. Therefore, the shaft receptor portion can be constructed without looseness.
The above-described way of sealing the pre-load in the bearings 5A and 5B by regulating the positions of the bearings 5A and 5B is, in general sense, called the "constant-position pre-load".
A rotor 8 is fixedly mounted on the aforesaid bush 7 by screw fasteners 10, and fixedly carries magnets 9 on the inner surface thereof.
On the lower end of the aforesaid motor housing 1, a PCB (printed circuit board) 12 for coil arrangement on which coils 11 are fixedly mounted and a stator yoke 13 constituting a magnetic circuit together with that PCB 12 adhered thereto are fixedly mounted by screw fasteners 14.
The coils 11 and the magnets 9 are arranged in plane-opposed relation.
When the coils 11 are supplied with current, an electromagnetic action takes place as is known in the art, so that the magnets 9 receive a rotative force. This force is transmitted to the rotor 8, the bush 7, the shaft 2 and the turntable 8 successively to drive the record bearing body (not shown) such as the optical medium.
In another conventional example of the bearing mechanism shown in FIG. 2, a pre-load spring 15 is used as arranged between one of the bearings 5B and the shoulder of a projected step portion 1b.
A collar 16 is positioned between the inner races of the bearings 5A and 5B.
Then, the bush 7 is fitted on the shaft 2 and fixedly secured thereto by adhesive agent or the like in the condition that the bearings 5A, the collar 16 and the bearing 5B abut against the turntable 3. With this, the outer race of the bearing 5B is pressed by the action of the aforesaid pre-load spring 15. Thus, removal of the looseness of the bearings 5A and 5B can be attained.
The structure of the shaft bearing mechanism of FIG. 2 is different from that of FIG. 1 in the above-described manner. The other parts have the same structure. So, the corresponding parts are denoted by the same reference numerals and their detailed explanation is omitted here.
This second example of the conventional mechanism of bearing the shaft is to use the pre-load spring 15 for removing the looseness of the bearings 5A and 5B. Such a method is called the "constant-pressure pre-load".
But, the constant-position pre-load type of shaft bearing mechanism has a tendency that if, as the motor housing thermally expands, or the bearings 5A and 5B are worn out, the pre-load is lost, looseness is apt to occur.
In more detail, as is seen from the graph of FIG.3A the thrust displacement .delta. for the thrust load P of the bearing amounts to only a few microns. As the thermal expansion or abrasion increases, therefore, the amount of pre-load rapidly decreases. Hence, the mechanism of FIG. 1 has a property of allowing the looseness to take place.
For the constant-pressure type shaft bearing mechanism of FIG. 2, on the other hand, because the assembly of the outer race of the bearing 5B and the motor housing 1 is free fitting, a position error of the bearing 5B is liable to occur.
From this reason, it was sometimes practiced in the high-precision motor to fixedly secure the bearing 5B to the motor housing 1. In this case, similar to the constant-position pre-load type of FIG. 1, a decrease in the pre-load is apt to occur.
Another problem with the structure of FIG. 2 is that when the aforesaid fixedly securing means is not in use, the shaft bearing portion has its rigidity lowered.