1. Field of the Invention
The present invention relates to a magnetic disc apparatus, particularly to an improvement of a fixed portion of a head holder thereof.
2. Description of Related Art
FIG. 1 is a perspective view showing a schematic construction of a fixed portion of a head holder of a magnetic disc apparatus similar to the one disclosed in, for example, Japanese Patent Application Laid-Open No. 63-866 (1988).
In this FIG., 1A and 1B designate magnetic discs, 2 servo information written in the upper surface of the magnetic disc 1A, and 3 a servo head provided at the upper surface of the magnetic disc 1A to read the servo information written therein.
Reference numeral 4 designates a data head provided at a lower surface of the magnetic disc 1A, writing information to and reading information from the magnetic disc 1A. In addition, only one data head 4 is shown in the figure, however, it is also provided at the magnetic disc 1B.
Symbols 5A, 5B and 5C designate head holders to support the servo head 3 or the data head 4, 5A for the servo head 3, 5B for the data head 4, and 5C for the other data head not shown.
Numeral 6 designates a carriage, 7 a screw being a fixing member for fixing the aforesaid respective head holders to the carriage, thereby, respective head holders 5A, 5B and 5C are stacked and fixed to the carriage 6.
In such a construction, the magnetic discs 1A and 1B rotate at a constant speed, thereby information is written to and read from it through the data head 4 fixed to the head holders 5B and 5C.
By obtaining information of position by means of the servo information 2 read by the servo head 3 which is written in the upper surface of one magnetic disc 1A, closed-loop control system (not shown) for positioning is operated to rotate the carriage 6 for controlling the position of the servo head.
Since the servo head 3 and the data head 4 are fixed to the carriage 6 through the head holders 5A, 5B and 5C, and they are respectively held along a line parallel with the axis of rotation of the magnetic discs 1A and 1B, the data head 4 is also positioned properly when the servo head 3 is positioned as above.
The magnetic disc apparatus is thus constructed, and the head holders 5A, 5B and 5C are fixed to the carriage 6 by screws 7, 7, 7. FIG. 3 depicts an exploded view of this construction. The head holders 5A and 5B have respectively three through holes for the screws 7, 7, 7, and the head holder 5C has two through holes and a center screw hole. The three head holders 5A, 5B, 5C are combined with the center screw 7 and the combined holders are fixed to the carriage 6 with the other two screws 7, 7. Howver, in the case where only one screw is used, there is a problem in that the head holders 5A, 5B and 5C can easily vibrate. In the case where a plurality of screws are used but linear expansion coefficients are different between the head holders 5A, 5B and 5C and the carriage 6, there is also a problem in that the head holders 5A, 5B and 5C deform due to bimetal effect responsive to temperature change.
In addition, in an article from the January 1986 issue of "Machine Design", for example, reports that in the case where the linear expansion coefficient of the head holders 5A, 5B and 5C is larger than that of the screws 7, 7, 7 compressive load from the screws 7, 7, 7 against the head holders is increased as the temperature rises. Compressive load more than the stress-strain proportional limit or more than proof stress of the head holders 5A, 5B and 5C is loaded on the head holders 5A, 5B and 5C. Permanent deformation occurs even though the temperature becomes normal, and the head holders 5A, 5B and 5C are released from the compressive load. On the contrary, when temperature is lowered, axial tension of the screws are reduced, thereby, the head holders 5A, 5B and 5C easily vibrate.
When thermal deformation occurs in the head holders, and there are variations in thermal deformation, the servo head 3 and all the data head 4 are not positioned along the line parallel with the axis of rotation of the magnetic discs 1A and 1B. Therefore, the data head 4 deviates even if the servo head 3 is positioned to cause a problem that signals are not written and read properly. In addition, vibration of the head holders 5A, 5B and 5C adversely affects closed-loop control for positioning, thereby, positioning without stabilization results in the data head also deviating, whereby signals not being written and read properly.