Vibration Analysis of Unbalance on Axial Fan Engine 5.5 KW

The unbalance on an axial fan engine causes vibration and noises in a small open wind tunnel construction circuit. The unbalance is not only speeding up the damage to the engine but also disrupt the convenience surrounding the engine, especially for the engine users. In this research, measurement analysis is done to find the cause of the unbalance. Initial vibration measurement results show the amplitude exceed[s] the ISO 10816-3 standard limit (0.292 G-S equal to 15.45 mm/s). After going through the balancing process, the unbalance of the axial fan decreases by 84% with the amplitude of 0.047 G-S equal to 2.5 mm/s, so the engine vibration and the vibrations of its surrounding rope.

. Main causes of engine damage [2] The axial fan engine construction that is installed in the open type wind tunnel has a direct connection between the axial fans with the engine through a coupling like a rotor connected to the stator. According to reference data the damage to direct connection machines is usually caused by an unbalance, bearing damage, and excessive aerodynamic load (see Figure 1). [2] Referring to the figure 1, excessive vibration on the axial fan engine 5.5 KW is estimated by the occurrences of a type of static unbalance, resulting from an engine misdeeds (such as the un-similarity in an angle between blades) or due to the imperfection in fabrications of engine components. [2] The measurements of vibration of the axial fan engine owned by PT. Chroma has been carried out to determines the static unbalance on the engine, and further to reduces the vibration, so it may work in a safe operation range. This may improve the health and safety of machine users. Fig.1: Dominant vibration in horizontal direction. [2] International Journal of Advanced Engineering Research and Science (IJAERS)   The main goal of measuring the unbalance carried out by PT Chroma on axial fan engines is to meet the standards of the regulations of the minister of environment of the Republic of Indonesia to keep up human health especially who works in Indonesia.
To protect Indonesia's health and safety from the effects of high vibration, the minister of environment of the Republic of Indonesia has made two rules that can be used as a law of human protection (see table 2 and table  3). [6] Table 3 shows the categories of vibration limits as regulated by the minister of environment of the Republic of Indonesia. The limit associates with damage may cause, shown in four categories (A = no damage, B = causing cracks, C = causing structural damage, D = causing damage to load-bearing walls).  [6] II.
THEORETICAL BASIS Vibration is an alternating motion that passes through the same trajectory. This kind of movement is called periodic motions. Between these periodic motions there is a movement called a harmonic motion. So the vibration of an object is said to be a simple harmonic function, with the following deviation equation below. Y = A cosω t ………………………….…...(1) [3] Where A is the amplitude, and ω is the angular frequency. The existences of the angular frequency at harmonic function can be used to calculate the period or time of vibration of back and forth with this Equation : [5] The frequency that occurs can also be approached by this The speed of a simple harmonic moving object can be obtained from the first derivative of this deviation Equation (1). vy = -A ω sin ω t ……….…………………..(4) [3] While the accelerations of simple harmonic moving objects comes from the second derivative of Equations (1). ay = -A ω 2 cos ω t = -ω 2 Y ……………..…(5) [3] The above formula is usually used to measure vibrations that occur on a machine, s o that the unbalance in the engine due to vibration can be investigated. The decrease in the unbalance correction ratio in a single plane balancing can be approximated using the following equation: [4] Where U1 initials value unbalance and U2 is the value of unbalance after correction.

III.
MEASUREMENT METHOD Usually to measure vibration on a machine, a measurement acquisition system is needed which consists of a vibration machine, trigger, and accelerometer, where the system will change the vibration signal data into electrical signal data. The measurement acquisition system can measure vibration amplitude of a stationary or moving specimen or engine. The amplitude can be decomposed into some parameters of velocity and acceleration according to the directions of the required axis, so the placements of an accelerometer on the specimen or machine shows the direction axis of vibration measured of the specimen or machine. Velocity parameters is one of the best indicators to know the problem of vibration at medium velocity engine. The method of placing the accelerometer measuring instrument is determined according to the direction of the axis of the specimen or machine to be measured. To measure a vibration amplitude in the directions of the specimen or machine axis, the accelerometer is placed parallel to the x axis, and to measure a vibration amplitude in the horizontal directions of the specimen or machine axis, the accelerometer is placed parallel to the y axis, while to measurevibration amplitude in the vertical direction of the specimen or machine the accelerometer is placed parallel to the z axis.
For thisaxial fan engine case, the vibration data required is in the vertical direction, so the accelerometer is placed parallel to the z axis in the engine's front bearing area. While the trigger is placed on the front of anaxial fan engine hub (see figure 3). Then the device is connected to the CSI-2130 analyzer as a vibration counter.
The acquisition data process of an axial fan engine is started while the accelerometer sensor receives the vibration amplitude signal data from the engine, then the vibration signal data is converted into electrical signal data, then the electrical signal data is sent to the CSI-2130 analyzer to be calculated iterative. Furthermore the CSI-2130 analyzer engine provides information simultaneously about the phase angle of the placements of the correction load positions and the value of the correction load, so that the unbalance value on an axial fan engine can be known and reduced. Tabel    The value of unbalance obtained from the measurements of 15.45 mm/sec is quite large beyond the allowable vibration limit for the axial fan engine, therefore the value must be reduced to the allowable limit. The second step is to provides a trial weight as shown in fig. 5.This trial weight will be used to calculate the correction weight, at this moment the trial weight is used 20.1 grams at an angle of 0° on the axial fan engine hub (see first positions/ no. blade 14).Then turn on the engine again. While the engine is running, the initials value of unbalanceis measured by using a trial weight. The measurement locations is also at the front bearing positions of the axial fan engine on vertical direction.

SPECIFICATIONS OF SPECIMEN
From the measurement, it is obtained the value of initials unbalance velocity of the axial fan engine, that is 0.479 G-S (25.34 mm/s) 326 o .Besides that, it is obtained the correction load phase angle and the correction load value as explained in step three, Afterward, the engine is turned off.   According to the ISO 10816-3, the vibration velocity value on axial fan engine with 2.5 mm/s is included in group 4 with flexible foundation classification with the allowed vibration velocity range of (0 -4.5 mm/s),so the fan operating area of the axial engine is in a region B, (see table 6). [7] Meanwhile, according to the decree of the minister of environment KEP-49/ MENLH/11/1996 that the value of vibration velocity on the axial fan engine 2.5 mm/s is included in category A and does not cause damage (see table 3). [6] Evaluation of measurement of vibration bump test to the axial fan engine buffer, showed the increase rapidly in vibration and frequency from 7.81 Hz to 19.70 Hzwith rapid increase of the velocity from 0.0030 in/s to 0.0060 in/s. This shows the effect of natural frequencies on the less rigid axial fan engine buffer (see table 5 and figure  9). Based on the results, it comes to consideration that the axial fan engine buffer needs to be strengthened so that the natural frequency value of the axial fan engine buffer occurs outside the engine's operation frequency (see figure 9 and 11).