Method and apparatus for evaluating vibrations of a rotary body while maintaining the rotary body in a static or non-rotational state

A method and apparatus for evaluating the vibration of a rotary body in a static or non-rotational state to determine a profile of critical speeds including profiles of critical speeds which exist beyond a rated revolution velocity of the rotary body. This invention comprises: a plurality of bearings for maintaining the rotary body in a non-rotating or static condition; a rotary vibrator for applying an exciting force which is rotated about a shaft to the rotary body; a vibration sensor for detecting the vibration of the rotary body; a vibration meter for measuring an output from the vibration sensor; and a vibration power source for supplying AC power to the rotary vibrator and for sweeping a frequency of the exciting force applied from the rotary vibrator to the rotary body from a low-frequency region to a high-frequency region. Thus, the present invention obtains the vibrational characteristics of the rotary body in a simulated rotational or dynamic state in which the rotating body is actually maintained in a static or non-rotational state.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for evaluating the 
vibration of a rotary body in a static or non-rotational state, and more 
particularly relates to a vibration evaluation method and apparatus 
adapted to obtain a vibrational response which is very similar to that 
which appears in a rotational or dynamic state from which a gyroscopic 
effect due to the rotation of a rotary body is eliminated, by applying an 
exciting force which is rotated about a shaft to the rotary body with the 
rotary body maintained in a static or non-rotational state. 
A rotary machine has a critical speed at which an amplitude of vibration 
increases suddenly when the revolution velocity of the rotary body 
increases to coincide with a natural frequency. When a rotary body having 
a plurality of critical speeds is rotated to reach a rated revolutionary 
velocity, the rotational speed of the rotary body passes critical speeds 
which constitute vibration resonance points. If a vibration damper cannot 
be set at an optimum level for a particular critical speed, the unbalance 
occurring during the manufacture of the rotary machine appears as a 
violent vibrational response at the particular critical speed, and the 
rotary machine will enter an unstable vibrational condition. Consequently, 
the breakage of bearing occurs, and further rotation of the rotary body 
becomes impossible. Therefore, the vibration damper is set at an optimum 
level to minimize vibration during the designing and testing of the rotary 
machine. Accordingly, in order to design and manufacture a rotary machine, 
it is first necessary to obtain a vibrational response of a rotary body 
first. 
When the length and revolution velocity of a rotary body in a rotary 
machine increase, the vibration resonance points at critical speeds of the 
rotary body exhibit a complicated mode of occurrence. Consequently, a 
vibration design analysis of the rotary body is performed, and a critical 
speed at which the rotary body forms a vibration-stable system is set. A 
satisfactory rotary body just as described, however, is not necessarily 
obtained due to a difference in physical properties of the rotary body. 
When a critical speed error is large, the vibration of the rotary body 
becomes unstable, and the rotary body becomes unable to be rotated. 
Therefore, it is necessary to subject a rotary body to a vibration test in 
a static or non-rotational state, obtain a mode of occurrence of vibration 
at a critical speed and secure the rotational performance prior to actual 
rotation of the rotary body. 
The conventional techniques for evaluating the vibration of a rotary body 
by vibrating the rotary body in a static or non-rotational state include 
the following: 
(1) A method of striking a rotary body with an impulse hammer. 
(2) A method of vibrating a rotary body by using sound waves via an 
electrodynamic loudspeaker. 
(3) A method of vibrating a rotary body directly by using an electrodynamic 
system. 
(4) A method of vibrating a rotary body directly by using an 
electrohydraulic system. 
(5) A method of vibrating a rotary body by attracting an iron shaft of the 
rotary body to an electromagnet. 
All of these methods are basically one-dimensional vibrating methods which 
are divided into methods of vibrating a rotary body in a suspended state, 
and methods of vibrating a rotary body inclusive of a static or 
non-rotational body. 
Among these conventional techniques for vibrating a rotary body in a static 
or non-rotational state, method (1) of striking a rotary body with an 
impulse hammer is simple but an exciting force obtained is small such that 
conducting a highly accurate measurement is difficult; method (2) of 
vibrating a rotary body by using a loudspeaker has an advantage in that 
the vibrating of the rotary body can be performed in a non-contacting 
manner but an exciting force obtained is small. Furthermore, since an 
air-borne exciting force is utilized, an exciting force transmission delay 
occurs; method (3) of the electrodynamic type direct vibrating brings 
about a result which is different from an originally expected result since 
the vibrating object is fixed directly to a rotary body; method (4) of the 
electrohydraulic system has a problem in that vibrating a rotary body with 
a high frequency is difficult to perform; and method (5) of the 
electromagnetic attraction system cannot be applied to a non-magnetic 
rotary body, and furthermore, it provides only a small exciting force to a 
magnetic rotary body. The results of the one-dimensional vibrating 
operations in all of these conventional methods show that vibration 
attributed to the rotary body itself, which does not occur in a rotating 
or dynamic state, occurs in addition to critical speed resonance 
vibration, that inconsistent data is obtained due to the variation of the 
suspended state of the rotary body, and that the accuracy of the data is 
unreliable since the resonance of solely a static or non-rotational body 
occurs as well when such a static body is included in a rotary machine. 
When the accuracy of the data is unreliable, a critical speed cannot be 
set accurately, and this has great influence upon the operation efficiency 
of a subsequent practical rotation test. 
According to the conventional techniques, a rough test is conducted by the 
above-mentioned methods. However, when a rotary body is designed and 
manufactured, it is necessary to install the rotary body in an operable or 
functioning rotary machine, measure the unbalanced vibration with the 
machine in operation and determine a profile of a critical speed. For 
example, in order to measure the vibration at an N-th order critical speed 
in a rotary machine, it is necessary first to dampen the first to (N-1)th 
order vibrations in order to exceed the N-th order critical speed. 
According to the balancing techniques, the vibration is suppressed by 
adding a balance weight to a rotary body or, conversely, by cutting off a 
part of the rotary body on the basis of the measured unbalanced vibration. 
Utilizing the data for designing a rotary body, which is obtained by thus 
balancing a plurality of critical speeds, requires many process steps and 
requires extensive time and labor. 
If a profile (the characteristics of vibrational amplitude with respect to 
rotational frequency) of a critical speed can be obtained accurately by 
conducting a test without rotating a rotary body during the designing of a 
rotary body, it becomes possible to set frequencies at critical speeds 
suitably and attain a rated revolution velocity while maintaining stable 
operation of the rotary machine. The operational efficiency of the rotary 
machine therefore would be improved greatly. The development of such a 
novel method and apparatus for performing the method has strongly been 
demanded. 
In a rotary machine, a critical speed also exists in a region which exceeds 
a rated revolution velocity. For example, when a critical speed exists in 
a rotational frequency region that is somewhat higher than a rated 
revolution velocity, the rated rotational vibration becomes unstable. 
However, a profile of a critical speed greater than a rated revolution 
velocity cannot be obtained accurately since a rotary body cannot be 
rotated practically at such a critical speed. According to the 
conventional techniques, a profile of a critical speed greater than a 
rated revolution velocity is determined simply be estimation in view of 
the influence upon an analysis value and a rated rotational vibration. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a vibration evaluation 
method and apparatus which is capable of determining a profile of a 
critical speed of a rotary body in a static or non-rotating condition 
within a short period of time under safe conditions and with high 
accuracy, thereby improving the work efficiency in the designing and 
manufacturing of a rotary machine. 
Another object of the present invention is to provide a method and 
apparatus capable of accurately determining a profile of a critical speed 
of a rotary body and that of a critical speed of a rotary body in a region 
which exceeds a rated revolution velocity. 
According to the present invention, there is provided a vibration 
evaluation method and apparatus for a rotary body in a static or 
non-rotational state comprising a plurality of bearings for maintaining a 
rotary body in a non-rotating condition; a rotary vibrator for applying an 
exciting force to the rotary body which is rotated about a shaft; a 
vibration sensor for detecting the vibration of the rotary body; a 
vibration meter for measuring an output from the vibration sensor; and a 
vibration power source for supplying AC power to the rotary vibrator and 
for sweeping a frequency of current supplied to the rotary body from a 
low-frequency region to a high-frequency region, whereby the vibrational 
characteristics of the rotary body in a simulated rotational or dynamic 
state are obtained. 
The rotary vibrator used in the method of the present invention is 
preferably provided with a structure comprising a ring-shaped magnet 
bipolarly magnetized and mounted coaxially on a shaft of the rotary body, 
a ring-shaped stator, mounted adjacent to the magnet in an axial direction 
of the shaft, which has the same construction as a regular induction 
motor, having coils and adapted to generate a rotating magnetic field by 
controlling an electric current supplied to the coil, and a 
frequency-variable vibration power source adapted to supply an AC power to 
the coils of the stator. The magnet and the stator have different magnetic 
radii and are disposed so as to be opposed to each other in a direction 
identical with the magnetization direction of the magnet to thereby apply 
the exciting force which is rotated about the shaft to the rotary body by 
an interaction of the rotating magnetic field generated by the stator with 
the magnet. The vibration evaluation is carried out by sweeping a 
frequency of the rotating magnetic field generated by the stator with the 
rotary body maintained in a static or non-rotating state by bearings. The 
magnet my be a permanent magnet or an electromagnet. 
When AC power of a predetermined frequency is supplied to the coils on the 
stator, the rotating magnetic field corresponding thereto is generated. 
Due to an interaction of the magnetic field generated by the stator with 
the magnet, a lateral or transverse force is exerted on the rotary body, 
and the application of this lateral or transverse exciting force is 
rotated in response to the rotating magnetic field of the stator. This 
results in rotational vibration of the rotary body, which then shows a 
vibrational response very similar to the critical speed vibration of a 
rotating or dynamic state. This enables the determination of a critical 
speed which is difficult to ascertain by using the previously described 
conventional one-dimensional vibrating methods. The presence of a critical 
speed in a region which exceeds a rated revolution velocity can also be 
determined accurately by merely increasing the frequency of the AC power 
supplied to the stator.

PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 is a schematic diagram showing an embodiment of the vibration 
evaluation method and apparatus for a rotary body in a static or 
non-rotational state according to the present invention, and FIG. 2 a 
detailed view of a rotary vibrator which is a principal part of the 
present invention. The vibration evaluation method for a rotary body in a 
static or non-rotational state shown in FIG. 1 comprises the steps of 
maintaining a rotary body 10 in a static or non-rotating state; providing 
a rotary vibrator 12 for applying an exciting force which is rotated about 
a shaft 11 to the rotary body 10, providing vibration sensors 14 for 
detecting the vibration of the rotary body 10, and providing a vibration 
meter 16 for measuring outputs from the vibration sensors 14. A 
vibrational response is measured by sweeping a rotational frequency of the 
exciting force which is rotated about the shaft (11). Thus the vibrational 
characteristics of the rotary body 10 in a simulated rotational or dynamic 
state are obtained. 
The rotary body 10 is supported in the same manner as it is supported while 
in a rotating or dynamic state of a rotary machine in which the rotary 
body 10 is to be installed so as to allow operability of the rotating 
machine. In this embodiment, the rotary body 10 is supported in a 
horizontal plane (XY direction) by upper and lower radial bearings 18, 20, 
respectively, each having a spring and a damper, and the rotary body 10 is 
supported in a vertical direction (Z direction) by a thrust bearing 22 
also having a spring and a damper. 
As shown in detail in FIG. 2, the rotary vibrator 12 is adapted to apply an 
exciting force which is rotated about the shaft 11 to the rotary body 10, 
and comprises a ring shaped permanent magnet 30, a ring-shaped stator 32 
having coils and a vibration power source 34. The permanent magnet 30 is 
bipolarly magnetized in the axial direction, and mounted on the rotary 
body 10 (shaft 11 of the rotary body in this embodiment) coaxially. The 
stator 32 may have the same construction as a regular induction motor, and 
is adapted to generate a substantially bipolar rotating magnetic field 
with an electric current which is controllably supplied to the coil. The 
stator 32 is mounted adjacent to the magnet 30 in an axial direction of 
the shaft. The permanent magnet 30 and the stator 32 have different 
magnetic radii (distance between a position in which a magnetic pole is 
formed and the axis of the rotary body shaft 11, the magnetic radii of the 
permanent magnet and the stator being represented by symbols R1 and R2 
respectively), and are disposed so as to be opposed to each other in the 
axial direction. The vibration power source 34 is an inverter for 
supplying alternating current to coils of the stator 32. Furthermore, an 
inverter in which the frequency of the alternating current can be varied 
is preferably used. 
In the rotary vibrator 12 structure of FIG. 2, the ring-shaped permanent 
magnet 30 is magnetized such that, for example, the upper and lower 
surfaces have an N-pole and an S-pole respectively. Assume that an S-pole 
and an N-pole occur at a certain instant in the right and left side 
portions respectively in the drawing of the stator 32 due to a current 
supplied to the coils. Consequently, a left-upward electromagnetic force 
due to the repulsive force of the S-poles occurs in the right side portion 
in the drawing, while a left-downward electromagnetic force due to an 
attractive force between the N-pole and S-pole occurs in the left side 
portion in the drawing. If they are synthesized, the components in the 
axial direction of the permanent magnet 10 are in opposite directions and 
therefore these components offset each other but the components F1 and F2 
in the direction (in a horizontal plane in the drawing) perpendicular to 
the axial direction are in the same direction and are therefore summed up 
to provide a lateral or transverse force against the shaft 11. The 
direction of a magnetic field occurring in the stator 32 changes 
circularly in accordance with the frequency of the alternating current 
supplied from the vibration power source 34, such that the direction of a 
lateral or transverse electromagnetic force supplied to the permanent 
magnet 30 also changes circularly. This lateral or transverse force can be 
controlled on the basis of a value of the alternating current supplied to 
the stator 32. Namely, due to the interaction of the rotating magnetic 
field generated in the stator 32 with the permanent magnet 30, a desired 
level of an exciting force which is rotated about the shaft 11 can be 
applied to the rotary body 36, and an electric input generates, according 
to a desired level, an appropriate exciting force of a rotational 
vibration vector. 
The inverter used as the vibration power source 34 is generally set to 
produce a rectangular waveform having a variable voltage and frequency by 
temporarily turning commercial electric current into a direct current, and 
carrying out positive-negative voltage switching in accordance with the 
frequency. Such a rectangular wave inverter is commercially available and 
is very economical. The waveform of the vibrating AC power may 
alternatively be a sine waveform. However, a sine wave inverter is 
expensive and is not as commercially available as a rectangular wave 
inverter. 
Returning to FIG. 1, either a single or a plurality of vibration sensors 14 
are provided for detecting the vibration of the rotary body 10 in the 
vicinity of the rotary body 10. A vibration meter 16, such as an 
oscilloscope or a FFT (high-speed Fourier transformer) for determining a 
vibrational amplitude is connected to the vibration sensors 14. 
The vibration power source 34 is adapted to gradually sweep the frequency 
of the current supplied to the stator 32 from a low frequency region to a 
high-frequency region thereby exceeding a critical speed. In accordance 
with this sweeping operation, the frequency of the rotating magnetic field 
generated from the stator 32 is also swept, and a vibrational response 
produced during this operation is measured with the vibration sensors 14 
and the vibration meter 16. As a result, the vibration characteristics in 
a simulated rotational or dynamic state are obtained. Since the 
vibrational response of the rotary body includes the influence of the 
ring-shaped permanent magnet 30 mounted on the rotary body, the 
ring-shaped permanent magnet preferably has the same shape and mass as the 
rotor of a motor which operates to rotate the rotary body. When the 
ring-shaped permanent magnet 30 used for rotationally vibrating the rotary 
body is removed from a rotary machine, it is replaced by the rotor of the 
driving motor. When the mass of the ring-shaped permanent magnet 30 is 
different from that of the rotor of the driving motor, the obtained 
vibration characteristics are subjected to an appropriate correction. In 
the structure of FIG. 1, the stator of the driving motor can be utilized 
similarly for the stator 32 for rotationally vibrating the rotary body. 
FIG. 3 is a graph showing an example of the relation between the rotational 
frequency and the vibrational amplitude of a test rotary body (in order to 
simplify a description, a part of a vibrational response is omitted). This 
graph enables the profiles (vibration damping characteristics and 
arrangement) of critical speeds to be read clearly. As shown in the 
drawing, even the critical speeds at rotational frequencies higher than 
the rated revolution velocity can be measured according to the present 
invention. If a critical speed shown by the letter "a" shifts to a 
position "b" shown by a broken line, it can be realized that a critical 
speed shown by the letter "c" in turn violently vibrates in response as 
shown by the letter "d" (broken line) and diverges (breakage occurs in an 
actual rotating or dynamic state). Since such profiles of critical speeds 
can be determined simply in a static or non-rotational state, the 
designing and manufacturing of the rotary body can be done safely and 
easily. When a gyromagnetic effect is large, correction is require. 
The waveform of the AC power supplied from the vibration power source 34 
may be a rectangular waveform as mentioned earlier but, in order to obtain 
data having a high-degree of accuracy, a sine waveform of a single 
frequency is preferable. Vibrating a rotary body with AC power using a 
rectangular waveform, or any other waveform other than a sine waveform, 
yields a basic frequency and additional frequencies which are obtained by 
multiplying the basic frequency by odd numbers (odd number-multiplied 
frequencies). Thus, when the rotary body is vibrated using AC power having 
such a rectangular waveform, or any other waveform other than a sine 
waveform, vibrations due to the basic frequency and the odd 
number-multiplied frequencies are compounded. As a result, a vibrational 
response occurring when a critical speed exists in a basic frequency may 
be compounded if an additional critical speed exists in an odd 
number-multiplied frequency. This produces data having a lower degree of 
accuracy than data generated when using a sine waveform. Furthermore, the 
levels of exciting forces which are rotated about the shaft are inversely 
proportional to the frequency, thus the vibrational response of the rotary 
body decreases as the odd number-multiplied frequencies increase. 
Nonetheless, vibration evaluation can be performed for a greater frequency 
range when a waveform other than a sine wave is used since the frequency 
range which is swept from a low level to a high level is enlarged as a 
result of odd number-multiplied frequencies. 
The above-described example of the rotary vibrator for a rotary body used 
in the present invention is preferred, however, a structure for applying a 
rotational exciting force to a rotary body is not limited to this example. 
Although a permanent magnet is used as the ring-shaped magnet, it may be 
replaced by an electromagnet. Since the rotary body is placed in a static 
or non-rotational state, the supply of an electric current to an 
electromagnet can be performed easily. 
It is also possible that a magnet and a stator can be disposed in the same 
plane. An example of such an arrangement is shown in FIG. 4. A ring-shaped 
permanent magnet 40 is radially bipolarly magnetized (the outer 
circumferential surface has an S-pole, and the inner circumferential 
surface has an N-pole) and mounted on a rotary body 42. The rotary body 42 
is supported by a bearing 44. A stator 46 is ring-shaped, and adapted to 
generate a rotating magnetic field with an electric current which is 
controllably supplied to a coil. The permanent magnet 40 and stator 46 are 
disposed in the same plane so as to be opposite to each other in the 
radial direction (i.e., the permanent magnet 40 is positioned on the inner 
side of the stator 46). If a magnetic field generated by the stator 46 in 
this structure is directed, for example, as shown in FIG. 4, an attractive 
force of different poles occurs in a left portion of the rotary body 42, 
while a repulsive force of the same poles occurs in a right portion of the 
rotary body. Consequently, a leftward force is exerted on the rotary body 
42. Since this force rotates with the rotating magnetic field generated by 
the stator 46, the rotary body 42 receives an exciting force which is 
rotated about the rotary body 42. 
Although the magnet is mounted directly on the rotary body in both of the 
above-described examples as shown in FIGS. 2 and 4, it may be mounted via 
a bearing. Such an example is shown in FIG. 5. The inner surfaces of ball 
bearings 52 are fixed to a rotary body 50, and a ring-shaped permanent 
magnet 54 is mounted on the outer circumferential surfaces of the ball 
bearings 52. In this example, the permanent magnet 54 is axially bipolarly 
magnetized similar to the permanent magnet 30 as shown in FIG. 2. A 
ring-shaped stator 56 is disposed so as to be opposed axially to the 
permanent magnet 54. Such an arrangement of the magnet mounted via the 
bearings has the advantage in that a rotary body need not be provided with 
any additional parts. Namely, the rotary body is left mounted with a rotor 
of a motor which operates to rotate the rotary body 50. The ball bearings 
52 may be considered as parts corresponding to the bearing 20 as shown in 
FIG. 1, to which an exciting force which is rotated about the rotary body 
50 is applied to measure a vibrational response. 
According to the present invention, a high-quality exciting force which is 
rotated about a shaft can be applied to a rotary body with high efficiency 
by exposing a rotating magnetic field onto the rotary body in the same 
condition in which the rotary body is disposed as if it were rotating. 
Therefore, the rotary body produces a vibrational response, though the 
rotary body is in a static or non-rotational state, which is very similar 
to the response obtained when the rotary body is in a rotating or dynamic 
state in which the gyroscopic effect due to the rotation of the rotary 
body is eliminated at a critical speed. Accordingly, a profile of a 
critical speed can be obtained simply and with a high degree of accuracy, 
and the evaluation of vibration can be carried out easily, by merely 
sweeping the frequency of a rotating magnetic field. 
When anisotropy exists in a rotary body, the charged position of the rotary 
body appears as a vibration according to the method of the present 
invention, such that the symmetry of the rotary body can be obtained. 
Since the rotary body is not actually rotated, the arrangement of critical 
speeds and the vibrational amplitude of the rotary body, even for a 
velocity exceeding a rated revolution velocity, can be determined 
reliably. 
The present invention enables greater efficiency in the designing and 
manufacturing of a rotary machine and allows for an easier and accurate 
determination of critical speeds. Since a critical speed and a vibration 
response can be obtained before an actual rotation test is conducted, the 
accuracy and efficiency of a vibration balancing operation can be 
improved. Moreover, the present invention can be applied to a performance 
evaluation test for vibration elements such as bearings.