Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-12-06
2004-02-24
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S579000, C073S599000, C073S602000
Reexamination Certificate
active
06694816
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and a method for evaluating a radial vibration of a rotating body and a rotation unit with the rotating body evaluated by the radial vibration evaluation method, and an apparatus and a method for evaluating a rotational accuracy of a rolling bearing and the rolling bearing evaluated by the rotational accuracy evaluation method, in which the radial vibration asynchronous with the rotational speed of the rotating body for example non repeatable round out of such as a bearing or a spindle with a bearing incorporated therein is evaluated on the basis of frequency analysis. In particular, the present invention relates to the evaluation of a maximum (minimum) amplitude value and a maximum (minimum) azimuth of a specific frequency component of which the amplitude varies depending on a radial azimuth.
2. Description of the Related Art
A rotating body of such as rolling bearing or a spindle with a rolling bearing incorporated therein generates radial vibration due to the circularity of a bearing portion, or the like. Therefore, the rolling bearing and spindle including such a rotating body may be a vibration source generating serious vibration of a structure such as a machine tool including them.
Such a rotating body generates radial vibration also at the time of constant-speed rotation. The radial vibration contains radial vibration components asynchronous with the rotational speed of the rotating body. The asynchronous radial vibration components are called “NRRO (Non Repeatable Round Out) vibration components”. The NRRO vibration component is constituted by a plurality of frequencies. The frequencies of the NRRO vibration component are determined in accordance with predetermined calculation expressions on the basis of the geometric sizes of inner and outer rings, and rolling bodies such as balls in a bearing incorporated in a rotation unit, the accuracy of form thereof and the constant-speed rotational speed of the rotating body. For the NRRO vibration components constituted by the plurality of frequencies, it is known that there is the NRRO vibration component constituted by a part of frequencies wherein the magnitude (amplitude) of vibration changes depending on the azimuth in the direction of rotation of the rotating body (hereinafter called “depending on the azimuth”).
On the other hand, particularly in a hard disk device, vibration of a disk due to vibration of a ball bearing used in a rotational shaft of the hard disk becomes a main cause of error in positioning a magnetic head. Therefore, the ball bearing needs to have strict rotational accuracy.
For the provision of such a ball bearing satisfying the rotational accuracy required for the hard disk device, it is necessary to evaluate the NRRO vibration components quantitatively to thereby remove a ball bearing having NRRO vibration components unsuitable for positioning the magnetic head. It is further necessary to mainly evaluate the NRRO vibration components constituted by frequencies (hereinafter referred to as “specific frequencies”) near to the resonance frequency of the hard disk device particularly selectively to thereby remove a ball bearing which may otherwise cause resonance of the hard disk device. Here, “specific frequencies” is referred in “Vibration and Noise” that is described in PP. 919-963, Chapter 25, in “Rolling Bearing Analysis (Third Edition) ” written by T. A. Harris and published by John Wiley & sons, inc. (1991), for example, f
c
, f
ci
, Zf
c
, Zf
ci
, f
R
defined by expressions (25.14)-(25.18) in pp. 950-951. The “specific frequency” is also described in REFERENCES 25.4 and 25.10 in pp. 962-963 of the same, that is, 25.4) O. Gustafsson, T. Tallian et al., “Final Report on the Study of Vibration Characteristics of Bearings”, U.S. Navy Contract NObs-78552, U.S. Navy Index No. NE071 200 (Dec. 6, 1963) and 25.10) O. Custaisson and U. Rimrott, “Measurement of Surface Waviness of Rolling-Element Bearing Parts”, SAE Paper 195C (June 1960).
Generally, the NRRO vibration components of the specific frequency includes an NRRO vibration component dependent on the azimuth and an NRRO vibration component independent of the azimuth. Both NRRO vibration components are respectively constituted by a plurality of frequencies. That is, the specific frequency of the NRRO vibration component dependent on the azimuth has a plurality of frequencies, and the specific frequency of the NRRO vibration component independent of the azimuth has a plurality of frequencies. Accordingly, for example, if a NRRO vibration component of the specific frequency depends on the azimuth, the evaluation of such NRRO vibration component is carried out in such manner that magnitude (amplitude) of the NRRO vibration component constituted by the plurality of frequencies dependent on the azimuth is measured in accordance with each azimuth over all the azimuths in the direction of rotation of the rotating body; and a maximum amplitude value and an azimuth exhibiting the maximum amplitude value are determined among the magnitudes measured for NRRO vibration component constituted by the plurality of frequencies.
As one of methods for evaluating the radial vibration of the rotating body at all azimuths in the direction of rotation of the rotating body, there is known a method in which displacement sensors such as displacement measuring units for measuring the radial vibration are disposed in two places near the outer circumference of the rotating body so that the azimuths of the displacement sensors are different from each other, and radial vibration at a third azimuth different from the two azimuths is evaluated by use of vibration values measured at the two azimuths. In the method for evaluating the radial vibration, two displacement sensors are disposed so that the radial vibration may be measured in two directions (x- and y-directions) perpendicular to a rotational shaft (or rotating body) of the rotation unit and perpendicular to each other (FIG.
12
). When t is the time for measuring vibration and x(t) and y(t) are measured x- and y-direction vibration components respectively, radial vibration f(t, &thgr;) at an azimuth &thgr; wherein &thgr; is an angle rotated in the direction of rotation of the rotating body from the x axis on which one of the displacement sensors is disposed is given by the following expression:
f
(
t
, &thgr;)=
x
(
t
)cos &thgr;+
y
(
t
)sin &thgr;
The amplitude of the NRRO vibration component is generally evaluated by the maximum value of the fluctuation width in the case where the NRRO vibration components are extracted every rotating period of the rotating body and superposed on each other. Here, a NRRO evaluation value means the maximum amplitude selected from the amplitudes of the NRRO vibration components at respective azimuths, and the maximum azimuth &thgr;
max
means an azimuth exhibiting the NRRO evaluation value.
Generally, the radial vibration f(t, &thgr;) includes other vibration components than the NRRO vibration component dependent on the azimuth. For this reason, the amplitude of the NRRO vibration component of the specific frequency dependent on the azimuth must be selectively obtained from the radial vibration f(t, &thgr;) by frequency analysis in order to evaluate the amplitude of the NRRO vibration component of the specific frequency dependent on the azimuth.
Generally, frequency analysis using Fourier transform is performed to selectively obtain the amplitude of the specific frequency from vibration constituted by the plurality of frequencies. In the aforementioned method of evaluating the radial vibration, in the case where radial vibration f (t, &thgr;
max
) expressing the NRRO evaluation value and the maximum azimuth &thgr;
max
are obtained, even if the amplitude of the NRRO vibration component of the specific frequency dependent on the azimuth is to be evaluated by use of frequency spectra {F
k
(&thgr;
max
), k=0, 1, . . . N−1} obtained by Fourier transform of a seque
NSK Ltd.
Saint-Surin Jacques
Williams Hezron
LandOfFree
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