GB2221312A

GB2221312A - Method of testing fan impellers

Info

Publication number: GB2221312A
Application number: GB8818148A
Authority: GB
Inventors: Alexandr Evgenievich Bozhko; Alexandr Ivanovich Fedorov; Anatoly Anatolievich Pavlov; Vadim Vladislavovich Tripolsky
Original assignee: INST MASH AKADEMII NAUK UK SSR
Current assignee: INST MASH AKADEMII NAUK UK SSR
Priority date: 1988-07-29
Filing date: 1988-07-29
Publication date: 1990-01-31
Also published as: FR2635186B1; FR2635186A1; SE8802767D0; SE461479B; SE8802767L; GB8818148D0

Abstract

A method of testing fan impellers for strength in which dynamic loads are repeatedly applied to the impeller by inducing torsional vibrations of 5 &cirf& 10<-3> to 5 &cirf& 10<-1>s in period and 2 to 10 DEG in amplitude. The impellers are subjected preferably to a continuous torque to more nearly simulate their loading during normal operation. The test is an improvement on the test in which the impellers are repeatedly started and stopped.

Description

METHOD OF TESTING FAN IMPELLERS This invention relates to testing structural elements of rotary action machines and, more particularly, concerns a method of testing fan impellers.

The proposed method can be used for testing impellers of axial-flow and centrifugal fans for strength. In addition, the method can also be used for testing rotating elements of other rotary action machines, such as blade wheels of a range of compressors.

During operation rotating elements of fans are subject ed to loads varying in character, origin and degree of wear affecting their service life. These loads are normally caused by forces of inertia and vibration. The loads are exerted predominantly on the fan impeller. High dynamic loads are- imparted to the fan impeller (blades, disks) in the course of fan starting periods mostly due to the forces of inertia. As a result, elements of the fan impeller are subject to cyclic loads amplified by residual unbalance of the impeller, as well as by radial and end plays. At high levels of static stresses in the elements of the fan impeller these elements are susceptible to fatigue failure due to the action of loads at transient operating conditions, particularly when the fan starts or stops.

Evaluation of the structural reliability of fans within the rated service life necessitates testing the fan impellers due to that they are most subject to high loads. Data on operating reliability of fan impellers is essential both during mass production and at the stage of designing and improving pilot models.

There is widely known a method of testing fan impellers for strength residing in repeatedly exerting dynamic loads on the fan impeller. Such repeated action is exerted by multiple starts of the fan. Therefore, the fan has to be put through the number of starts equal to the number of starts envisaged by its rated operation life. Each starting of the fan is followed by accelerating its impeller to the rated rotational speed, and by a subsequent stop. The time required for accelerating the impeller to stable-state operating conditions even at minimized operation time at t stable state conditions is normally 30 to 500 5, depending on the construction and dimensions of the fan, which amounts to 1000 - 1500 hrs of test procedures.This method fails to allow expeditious evaluation of the structural reliability of fan impellers. Another disadvantage of the method is failure to increase the dynamic loads arising due to starting and stopping the fan.

It is an object of the present invention to reduce the time consumed for testing fan impellers for strength.

The essence of the invention resides in that in a method of testing a fan impeller for strength the impeller is repeatedly acted upon by dynamic loads through providing torsional vibrations of 5'10 3 to 5.10'1 5 8 in period and 2 to 100 in amplitude at the fan impeller.

The proposed method of testing fan impellers for strength allows to reduce the time of test procedures by a factor oi 20 to 50 through increasing the dynamic loads exerted on the fan impeller and through shortening the time periods between each preceding and succeeding application or dynamic loads. The character of the dynamic loads thus applied to the fan impeller during the test is equivalent to the operational dynamic loads.

The invention will now be described in greater detail with reference to a specific embodiment thereof and example of carrying it out.

The proposed method of testing fan impellers for strength involves exerting repeated action of dynamic loads on the impeller by producing torsional vibrations 5.10- to T'1o'l 5 in period and 2 to 10 in amplitude at this impeller.

Specific values of the period (frequency) and amplitude of torsional vibrations are preselected so that the centrifugal forces of inertia acting on the fan impeller in the position of the highest amplitude of torsional vibrations would be equal to the centrifugal forces imparted in the fan impeller under normal operation conditions, and in each specific case these values are determined by the structural arrsngement of the impeller.Due to the fact that the natural frequency of vibration of the elements of assembled fan impellers have been found to range from 10 to 200 Hz, it is desirable to carry out tests in the resonance operating conditions for testing the most loaded elements of the impeller requires that the period of torsional vibrations be from 5.10- to 5.io1 8, and that the value of the period be controlled in the course of testing depending on the natural frequency of vibration of the element. Therewith, for pro viding a dynamic load equivalent to that experienced by the impeller during regular operation the torsional vibrations are provided both as the impeller rotates and as it remains stationary.Of decisive importance is obtaining the maximum magnitude of centrifugal force of inertia in the position of the highest amplitude of torsional vibrations.

Fan impellers are tested by the proposed method in the following sequence.

Initially, the natural frequency of vibration of the impeller or separate elements thereof (such as blades, disks and hub) is determined by calculations or experimentally.

Test duration is then determinedq The time of repeated action by dynamic loads on the impeller is preselected taking into consideration the aim and type of tests.

For example, this duration can be predetermined wit the aim of covering a preset number of cycles equal to the corresponding number af starts through the operating time of the impeller, or with the aim testing the impeller to failure.

After this the fan impeller ie mounted on a drive shaft.

Any fan drive can be used as the drive. Then the natural frequency of vibration of the element of the fan impeller subsected to testing in this cycle of teste is determined, This element can be either the most loaded element of the fan impeller, or another element of interest for the test operator. Torsional moments periodically varying in magni trade are produced at the drive shaft to cause in the fan impeller torsional vibrations of 5 10~3 to 5s10~ 5 in peri od and 2 to 100 in amplitude0 The frequency of torsional vibrations of the drive shaft is set so as to equal the natural frequency of vibrations of the fan impeller or an element thereof.The amplitude of torsional vibrations is preselected so that the amplitude of vibrations of the impeller be within 2 and 100, and stresses imposed on the impeller be sudi that the elements thereof be subjected te resilient deformations. The magnitude of deformatIon in the stressed state of the element being tested is determined by any known suitable method, such as strain measurements. The amplitude of vibrations of the impeller is measured by any known suitable method, such as strain measurements. The amplitude of vibrations of the impeller is measured by any knon suitable method, such as by a measuring microscope. Having set the predetermined frequency and amplitude of torsional vibrations of the fan impeller, the impeller is subjected to repeated action of dynamic loads for a predetermined period of time.

Because the frequency of induced tersional vibrations of the drive shaft coincides with the natural frequency of vibration of the element under test the thus stepped up le vel of dynamic loads substantially accelerates the testing procedure.

Example t Six impellers of centrifugal fans having an outside diameter 800 mm were tested for strength. By amplitude and frequency measurements carried out in a known manner the natural frequency of vibration of the impeller blade at the point where it is atteched to the rear disk were measured, which amounted to 16 Hz. Then the amplitude of torsional vibrations of the impeller was preset.With this aim in view the fan impeller was preliminarily mounted on a drive shaft, a resistance strain gauge was applied to the blade and wired to a strain measurement circuit, and torsional vibrations of 16 Hz in frequency were exerted on the shaft gradually varying the amplitude of such vibrations from 2 to 4 . It has been found that at an amplitude of torsional vibrations of 40 strains monitored by the resistance strain gauge attained their peak value. However, since the level of these strains was below the strains arising due to the action of total loads in regular operating conditions, an additional continuous torque M was provided at the drive shaft of a magnitude ensuring that the stresses induced in the blades be equal to the stresses arising under the action of loads during regular operation. Then each fan impeller was tested. The drive shaft was rotated to produce a continuous torque X and torsional vibrations of 6,25.10 2 s in period and 40 in amplitude.

The average time of testing sufficient for estimating the service life of the fan impellers was t6 hrs.

Claims

WHAT WE CLAIM IS:

1. A method of testing fan impellers for strength in which dynamic loads are repeatedly applied to the impeller by inducing torsional vibrations of 5.10- to 5.10- s in period and 2 to 100 in amplitude.

2. A method of testing fan impellers for strength carried out substantially as claimed in the preceding claim and described in the description.