GB2217391A

GB2217391A - Apparatus for testing fan blades for vibration survival strength

Info

Publication number: GB2217391A
Application number: GB8808416A
Authority: GB
Inventors: Alexandr Evgenievich Bozhko; Alexandr Ivanovich Fedorov; Vladimir Petrovich Shpachuk; Anatoly Anatolievich Pavlov
Original assignee: INST MASH AKADEMII NAUK UK SSR
Current assignee: INST MASH AKADEMII NAUK UK SSR
Priority date: 1988-04-11
Filing date: 1988-04-11
Publication date: 1989-10-25
Also published as: FR2630213B1; GB8808416D0; FR2630213A1

Abstract

The apparatus has an electric motor 1 with a shaft carrying an impeller of a fan 4 being tested. An input of the electric motor 1 is connected to an output of a control unit (8, Fig 2). A strain gauge 10 is mounted on a blade of the fan and an output of the strain gauge is connected to an input of a first matching amplifier (12). Connected to an output of the first matching amplifier is an input of a unit (13) for determining the resonance frequency of the fan blade having its outputs connected to inputs of a unit (14) for maintaining the resonance frequency of the fan blade. An output of the unit (14) for maintaining the resonance frequency of the fan blade is connected to an input of the control unit (8). <IMAGE>

Description

APPARATUS FOR TESTING FAN BLADES FOR VIBRATION SURVIVAL STRENGTH The invention realtes to testing of structures and installations for vibration and, in particular, it deals with apparatuses for testing fan blades for vibration survival strength.

The invention will find application in ventilation systems built around axial and centrifugal fans in the mechanical engineering, aviation and space technology, shipbuilding, coal and oil industries.

The invention resides in that an apparatus for testing fan blades for vibration survival, comprising an electric motor having a shaft to which an impeller of a fan being tested is secured and an input connected to an output of a control unit, and a strain gauge mounted on a fan blade having an output connected to an input of a first matching amplifier, according to the invention, also comprises a unit for determining the resonance frequency of the fan blade having an input connected to an output of the first matching amplifier, a unit for maintaining the resonance frequency of the fan blade having inputs connected to outputs of the unit for determining the resonance frequency of the fan blade and an output connected to an input of the control unit.

The unit for determining the resonance frequency of the fan blade preferably comprises a series circuit including a full-wave rectifier converter having an input connected to the output of the first matching amplifier, a rectifier filter, a differentiator, a null detector, and a first switch.

In addition, it is preferred that the unit for maintaining the resonance frequency of the fan blade comprise a series circuit including an air stream pressure pickup mounted on the fan blade, a second matching amplifier, a first square pulse former, a phase detector having an output connected to a first input of a comparison member and, via a second switch having its control input to which is connected an output of the null detector of the unit for determining the resonance frequency of the fan blade, to an input of a memory unit having an output thereof connected to a second input of the comparison member, an output of the comparison member being connected, via a third switch having its control input connected the output of the null detector of the unit for determining the resonance frequency of the fan blade, to a first input of an integrator having a second input thereof connected to an output of the first switch of the unit for determining the resonance frequency of the fan blade and an output connected to the input of the control unit, and also comprise a second square pulse former having an input connected to an output of the first matching amplifier and an output connected to a second input of the phase detector.

The use of the invention will make it possible to improve reliability of fan blades: vibration survival strength and vibration resistance of fan blades as a result of fan blade testing in the resonance oscillation modes and under an automatic maintenance of the resonance oscillation mode of the fan blade with high accuracy.

The invention will now be described in detail with re ference to a specific embodiment of an apparatus for testing fan blades for vibration survival strength illustrated in the accompanying drawings, in which: Figure 1 is a general view of an electric motor having a shaft to which is secured an impeller of a fan, according to the invention; Figure 2 shows a block-diagram of an apparatus, according to the invention.

An apparatus for testing fan blades for vibration survival strength comprises an electric motor 1 (Figure 1) having a shaft 2 to which is secured an impeller 3 of an axial fan 4 being tested. Installed in an inlet opening 5 of a casing 6 of the fan 4 is an aerodynamic exciter 7 in the form of a planar disc with apertures.

An embodiment of the apparatus for testing a centrifugal fan may have an aerodynamic exciter in the form of a perforated cylinder.

Connected to input terminals of the electric motor 1 is an output of a control unit 8. The control unit 8 is of a conventional type (cf. Handbook of Transducer Technology (in Russian). I.M. Chizhenko, 1978. Tekhnika Publishing House, Kiev, pp. 377-389).

A blade 9 of the fan 4 carries a strain gauge 10 and an air stream pressure pickup 11. An output of the strain gauge 10 is connected to an input of a first matching amplifier 12 (Figure 2). An output of the first matching amplifier 12 is connected to an output of a unit for determining the resonance frequency of the fan blade. The apparatus also has a unit 14 for maintaining the resonance frequency of the fan blade having inputs thereof connected to outputs of the unit 13 for determining the resonance frequency of the fan blade and an output connected to an input of the control unit 8.

The unit 13 for determining the resonance frequency of the fan blade comprises a series circuit including a fullwave rectifier converter 15 having an input connected to the output of the first matching amplifier 12, a rectifier filter 16, a differentiator 17, a null detector 18, and a first switch 19.

The unit 14 for maintaining the resonance frequency of the fan blade comprises a series circuit including the air stream pressure pickup 11, a second matching amplifier 20, a first square pulse former 21, a phase detector 22 having an output connected to a first input of a comparison member 23 and to a first control input of a second switch 24. A second control input of the second switch 24 is connected to an output of the nall detector 18 of the unit 13. An output of the second switch 24 is connected to an input of a memory unit 25. An output of the memory unit 25 is connected to second input of the comparison member 23.An output of the comparison member 23 is connected to a first control input of a third switch 26. A second control input of the third switch 26 is connected to the output of the null detector 13 of the unit 13. An output of the third switch 26 is connected to a first input of an integrator 27. A second input of the integrator 27 is connected to an output of the first switch 19 of the unit 13. An output of the integrator 27 is connected to Qhe input of the control unit 8. The unit 14 for maintaining the resonance frequency of the fan blade also comprises a second square pulse former 28 having an input connected to the output of the first matching amplifier 12 and an output connected to a second input of the phase detector 22.

The apparatus for testing fan blades for vibration survival strength functions in the following manner.

In the initial position the switches 19, 24 and 26 (Figure 2) are deactuated. Upon the arrival of a starting signal "P" at the first control input, the switches 19 and 24 are actuated. A reference voltage UO is fed to the input of the integrator 27. An output electric signal of the integrator 27 acts, via the control unit 8, upon the electric motor 1 (Figure 1) to cause the shaft 2 to rotate. The aerodynamic air stream formed by the fan 4 moves through the apertures of the exciter 7 to act upon the blades 9 to induce their forced oscillations which are converted by the strain gauge 10 into an electric signal. The air stream pressure pickup 11 converts pressure of the air stream acting upon the fan blade 9 into an electric signal. The signal from the pickup 11 gores via the second matching amplifier 20 (Figure 2) of the unit 14 for determining the resonance frequency of the fan blade to the input of the first square pulse former 21. The signal from the strain gauge 10 goes via the first matching amplifier 12 to the full-wave rectifier converter 15 of the unit 13 for determining the resonance frequency of the fan blade. The rectified signal is fed, via the rectifier filter 16 filtering-off high-frequency signal components, to the differentiator 17.

If the oscillations frequency of the fan blade 9 (Figure 1) is lower than the resonance frequency, the output signal of the differentiator 17 (Figure 2) is positive as the amplitude of the forced oscillations of the blade 9 and the output signal of the rectifier filter 16 (Figure 2) continuously increase with an increase in the oscillations frequency of the blade 9 (Figure 1), the output signal of the differentiator 17 decreasing.

The output signal of the integrator 27 and speed of the shaft 2 (Figure 1) of the electric motor 1 will increase until the signal at the output of the differentiator 17 (Figure 2) becomes zero which corresponds to the resonance os- cillations of the blade 9 (Figure 1). This results in actuation of the null detector 18 (Figure 2), and its output signal will deactuate the switch 19 (the reference voltage UO is switched off), deactuate the switch 24 (the coupling between the units 22 and 25 is broken) and actuates the switch 26. The switch 26 will connect the comparison member 23 of the unit 14 to the input of the integrator 27. The switch 24 is deactuated when oscillations of the blade 9 become resonant.Consequently, the memory unit 25 memorizes the value of the output signal Ur of the phase detector 22 at the resonance oscillations of the blades 9 (Figure 1) of the fan 4 the value of which is proportional to the phase difference between the output signals of the square pulse formers 21 and 23 (Figure 2) correspondin to the phase difference between the action of the aerodynamic stream flowing through the apertures of the exciter 7 (Figure 1) and the forced oscillations of the blade 9 of the fan 4.

At the moment of actuation of the switches 19, 24 and 26 (Figure 2), an output signal of the comparison member is equal to zero as signals from the phase detector 22 and memory unit 25 fed to the inputs thereof are proportional to the resonance phase difference between the output signals of the square pulse formers 21 and 28.

During the tests, characteristics of rigidity and damping of the blade 9 (Figure 1) continually vary. This calls for a continuous speed readjustment of the shaft 2 of the electric motor 1, i.e. for retuning frequency of the aerodynamic stream acting upon the blade 9 for the testing of the blade 9 to be conducted at the resonance frequency. The resonance value of the phase difference (the phase difference at the resonance frequency of the blade 9) of the sig nals from the square pulse formers 21 and 28 (Figure 2), which characterizes a given blade 9 of the fan 4 (Figure 1) and which does not substantially depend on changes in rigidity and damping of the blade 9, have been determined by the unit 13 (Figure 2) and recorded by the memory unit 25 and is automatically maintained by the unit 14.For example, if as a result of the action of certain destabilizing factors upon the testing conditions oscillations frequency of the blade 9 (Figure 1) decreasest$ < çr speed of the shaft 2 of the electric motor 1 will be automatically raised since the level of the output signal of the integrator 27 (Figure 2) has increased because a positive signal has been formed at the output of the comparison member 23. The phase v.

frequency characteristic of the blade 9 (Figure 1) of the fan 4 is continuously monotonous so that upon a change in the oscillations frequency of the blade 9 the phase difference between the output signals of the square pulse formers 21 and 28 (Figure 2) will be lower than the phase difference between these signals at the resonance frequency of the blade 9 of the fan 4 (Figure 1) S C g r. Consequently, the r output signal of the phase detector 22 (Figure 2) becomes lower than the value Ur, and a positive signal is formed at the output of the comparison member 23 so as to result in.

an increase in the level of the output signals of the integrator 27 and control unit 8 to cause an increase in speed of the shaft 2 of the electric motor 1 (Figure 1). The speed of the shaft 2 of the electric motor 1 will increase until the phase difference between the output signals of the square pulse formers 21 and 28 (Figure 2) reaches its resonance value and the output signal of the phase detector 22 reaches the value of Ur In this case the output signal of the comparison member 23 is again equal to zero, and the output signals of the integrator 27 and control unit 8 cease to change.

Therefore, the unit 14 ensures an automatic maintenance of the resonance frequency of oscillations of the fan blade with a high accuracy.

As a result of oscillations of the fan blade at the resonance frequency, the tests for vibration survival strength are carried out with maximum stresses in the fan blade material so as to cut short the tests0 The employment of the apparatus makes it possible to prolong fan life.

Claims

WE CLAIM:

1. An apparatus for testing fan blades for vibration survival strength, comprising an electric motor having a shaft to which is secured an impeller of a fan being tested and an input to which is connected to an output of a control unit; a strain gauge mounted on a fan blade, having an output connected to an input of a first matching amplifier; a unit for determining the resonance frequency of the fan blade, having an input connected to an output of the first matching amplifier; a unit for maintaining the resonance frequency of the fan blade, having inputs connected to outputs of the unit for determining the resonance frequency of the fan blade and an output connected to an input of the control unit.

2. An apparatus as claimed in claim 1, wherein the unit for determining the resonance frequency of the fan blade comprises a series circuit including a full-wave rectifier converter having an input connected to an output of the first matching amplifier, a rectifier filter, a differentiator, a null detector, and a first switch.

3. An apparatus as claimed in claims 1,2, wherein the unit for maintaining the resonance frequency of the fan blade comprises a series circuit including an air stream pressure pickup mounted on the fan blade, a second matching amplifier, a first square pulse former, a phase detector having an output connected to a first input of a comparison member and, via a second switch having its control input connected to an output of the zero detector of the unit for determining the resonance frequency of the fan blade, to an input of a memory unit having an output connected to a second input of the comparison member, an output of the comparison member being connected, via a third switch having its control input connected to the output of the null detec- tor of the unit for determining the resonance frequency of the fan blade, to a first input of an integrator having a second input connected to an output of the first switch of the unit for determining the resonance frequency of the fan blade and an output connected to an input of the control unit, and wherein there is provided a second square pulse former having an input connected to the output of the first matching amplifier and an output connected to a second input of the phase detector.

4. An apparatus for testing fan blades for vibration survival strength as claimed in claims 1 through 3 substantially as hereinabove described and as shown in the accompanying drawings.