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Success with Small Vibrometers
How can a simple vibrometer successfully detect defects? This article contains a guide for reading the measurement results and how to reliably determine machine condition.
Experimental articles on vibration diagnostics are almost always written about using a powerful analyzer. From my long experience in this area, I have only exceptionally met with a description of using a simple vibrometer. The usual opinion among maintenance people is that a powerful analyzer is needed for a real diagnosis; this is a myth. In my opinion even with a simple vibrometer, 90 percent of defects can be accurately determined, and for the remaining 10 percent it will at least point you in the right direction.
A Simple Vibrometer - What Is It?
A simple vibrometer must be able to carry out at least two basic measurements:
RMS vibration velocity measurement in the 10-1000 Hz band (referred to as velocity in this article)
RMS vibration acceleration measurement in the 500-15000 Hz band (referred to as acceleration)
If band frequency ranges are slightly different, this is not a defect. It is important that when measuring acceleration, speed frequency and its harmonics have been removed. The aim of measuring velocity is to detect mechanical defects such as an imbalance, misalignment, looseness and soft-foot. The purpose of acceleration is to determine the condition of the roller bearings and gears.
If the vibrometer can also measure TRUE PEAK values, display time signal and evaluate the signal spectrum, then these measurement types will make the analysis even more reliable.
The Basic Scheme of the Machine
Simple machines have a drive part, usually an electric motor, and a driven part such as a fan, or pump. Both parts are usually connected by a shaft coupling and both shafts are mounted on rolling bearings. From now on we will refer to the driven part as “a fan”.
A measurement point is a place on the machine where the vibration sensor is placed.
There are standards that determine where the machine needs to be measured, but we will not be dealing with them. They typically need many more measurement points than the five points that will be enough for our measurement. The machine has four roller bearings and we should select four measuring points as close as possible to these bearings. These four points must be radial, i.e. perpendicular to the shaft. Do not worry about whether to measure vertically or horizontally. You can choose any direction between these two directions. The last fifth point will be axial, i.e. parallel to the shaft. Put it on the coupling and it doesn’t matter whether it is on the engine or the fan. This fifth point is therefore perpendicular to the previous four.
Once the measuring points have been chosen, they need to be prepared for the measurement. It is not possible to simply take the sensor with a magnetic base and put it on the uneven surface of the machine. Measuring pads must be stuck on the selected locations before measurements are taken. They have a flat surface. In addition, they guarantee that you will always measure at the same machine location. The basic rule for taking measurements is to make sure the measurement conditions are 100 percent repeatable. That is exactly what the measuring pads guarantee. Let’s try, for example, 10 repeated measurements in one place i.e. put the sensor on the pad, measure it and then remove it from the pad. You will find that the measurements are almost identical. They will vary by approximately +/- 5%. If the same test is carried out without a pad, the results will vary by +/- 50%. (See Figure 1)
Figure 1. Measurement pad glued onto the machine surface (1), magnetic base (2), acceleration sensor (3).
We deliberately did not mention measuring with a sensor that has no magnetic base, and that is just pushed onto the machine by hand. This method is unrepeatable. Unfortunately, it is sometimes used in maintenance and so the results are disappointing. Sometimes the whole vibration diagnostics programme is rejected, the only reason being unprepared measuring points.
How to Find Warning and Alert Vibration Levels
The first measurement has already been taken and the results obtained. But what do the numbers mean? Are the vibration readings low or high? With what should the results be compared? The easiest way is to use ISO 10816, but the limits given here have one significant defect. They apply to machines with speeds of 600-3000 RPM. Let’s suppose the fan is unbalanced. The centrifugal force that causes vibration will vary significantly for 600 RPM and 3000 RPM. The dependence of the force on the speed is quadratic, i.e. 2x higher speed means 4x higher force. Therefore, the weight of the heavy point on the rotor may not create a problem at 600 RPM but will cause the fan to destruct at 3000 RPM. The warning and danger limit values should depend on the speed. (See Figure 2)
If several similar machines are measured, then the situation is simpler because we can compare the values from all machines. If we get results equal to 1.8, 2.1, 1.9 and 4.5 from the same point, then it is obvious that 2.0 means good machine condition. A machine condition with a value of 4.5 should be investigated further.
The first step deals with regular measurements and monitoring the vibration trend. If it is stable and has a permissible value, then the machine is in good condition and there is nothing else to do. If the value gradually increases and the warning threshold is exceeded, the second step of the evaluation must be carried out.
The aim of the second step of the evaluation is to find the cause of the increased vibration. I will now describe the procedure of deeper analysis.
Bearing Condition Needs Acceleration
Increased acceleration value
If the acceleration value has increased and the increase is only in one radial location, then it is easy. The problem is the poor condition of the roller bearing at this point. If the gears are measured, then the acceleration values can be increased in more places and it shows a problem with the gearing.
Imbalance, misalignment and looseness need velocity
Imbalance or looseness
The values are significantly increased on only one part (either the motor OR fan). If the increases in both radial directions are similar, then it is most probably an imbalance. If you have a signal spectrum at your disposal, you can find the significant value only on the speed frequency.
If the increase differs significantly in both radial directions, or there is only a vertical increase, then it is most probably due to looseness. You should measure each machine foot. You would probably find significantly higher values on one of them.
Electrical defect generates vibrations
Electrical defect of the motor
When the electric motor vibration looks like imbalance is the problem, then you should always also consider electrical defect. The electric motor may have winding defects, and despite this the vibration behaviour indicates an imbalance. Therefore a switch-off test on the motors should always be carried out. After switching off the power, one of two situations will occur.
- The velocity decreases slowly along with the rpm drop. This is a true imbalance.
- Immediately after switching off, the velocity increases for a very short time (1 sec), by a multiple and then drops to a very low value where it remains until the machine stops. This is an electromagnetic problem. The force field is not uniform and shifts the rotor off its mechanical centre of gravity. In the vibrations it will manifest as an imbalance. After switching off, the force is instantly lost and the rotor jumps back to the mechanical centre of gravity. This shock causes an increase in the value. Then the rotor starts spinning normally and the vibrations disappear.
Mechanical imbalance (left) & Electrical fault
If the velocity in the axial direction increases (usually it is higher than in the radial), always check the coupling and the alignment. It is misalignment that causes vibrations in the axial direction. If you have a spectrum, you can find higher values on the speed frequency and several harmonics.
The velocity significantly increases on both parts (motor and fan) and only in the vertical directions. Take measurements across the frame below the machine. If there is a low value where the frame is supported, and high values between the supports then there is a resonance problem.
A coast down measurement or gradual reduction of speed (frequency changer) will help. In case of resonance, the vibrations will decrease dramatically with a small speed change. If the standard operating speed cannot be changed, the frame must be additionally reinforced.
*blogpost from mainworld written by Radomir Sglunda, Adash Ltd