Enjoy 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.

Vibrio

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.

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. 

Mach-scheme

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)

fig1

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.

Hand_probe2

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)

Limits

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.

1) The velocity decreases slowly along with the rpm drop. This is a true imbalance.

2) 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.

ElectricMech

Mechanical imbalance (left) & Electrical fault

Misalignment

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.

Resonance

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.

Those who do Nothing, Make no Mistakes

A lot of maintenance staff are unnecessarily nervous about carrying out vibration diagnostics. Simple devices are developed just for those who have no deep knowledge. If you take regular measurements, you will find that you have a much better overview of the condition of your machines. You will also certainly notice a decrease in the number of unexpected temporary shutdowns. 

 

Radomir-Sglunda

Radomir Sglunda,

Managing Director, Adash Ltd.

sglunda@adash.cz

Applications | 16.10.2017

Latest articles

AMS Device Manager software adds support for the Beamex MC6 documenting calibrator, simplifying compliance

Emerson has updated AMS Device Manager asset management software to support the Beamex MC6 documenting calibrator, adding electronic workflows that eliminate up to 50 percent of time spent on each calibration.

Applications | 16.5.2018

Caverion to be in Charge of the Monitoring and Maintenance of the Second-largest Open Data Centre in the Nordics

Caverion and Telia Finland have signed a contract covering the control room operator services and property maintenance of the new Telia Helsinki Data Center. The open data centre to be built in the Pitäjänmäki district in Helsinki will be the most secure, energy efficient and environment-friendly data centre in the Nordic countries.

Asset Management | 11.5.2018

EU Agency for Safety and Health at Work Launches Europe-Wide Awareness-Raising Campaign on Dangerous Substances

The European Agency for Safety and Health at Work (EU-OSHA) has launched its 2018-19 EU-wide campaign, Healthy Workplaces Manage Dangerous Substances.

HSE | 25.4.2018

Ensuring a Smooth Transition from OPC CLASSIC to OPC UA

Now, more than ever, industrial firms need to make sense of vast quantities of data having a critical impact on their performance. To support the variety of applications necessary today, information must be delivered with context so it can be understood and used in various ways by a variety of people. Growing adoption of the Industrial Internet of Things (IIoT) and Industrie 4.0 is also driving requirements for open and secure connectivity between devices and edge-to-cloud solutions.

Cmms | 30.3.2018

Is Your HMI/SCADA Network as Secure as You Think It Is?

Network security frequently makes the news, often when some new viral attack is discovered or, worse yet, is successful. HMI/SCADA networks can be as susceptible to these unlawful break-ins as any others, unless the proper precautions are taken. Many software and hardware vendors have made their own attempts to stay ahead of online criminals, while others have combined forces to thwart such attacks.

Partner Articles | 30.3.2018

Examples: Using Ultrasound and Infrared for Electrical Inspections

Ultrasound and infrared technologies are a perfect match when conducting inspections of electrical equipment. At any voltage, thermal anomalies and sources of ultrasound such as tracking and arcing can occur. Corona can also occur at 1000 volts and greater. Any of these conditions threaten the reliability of the equipment being inspected.

R&D | 29.3.2018

Maintenance: A Necessary and Important Function in the Future

Euromaintenance 2016 will take place in Athens at the end of May. It is the ideal moment to reflect on maintenance in a European context. Euromaintenance is known as the summit for all involved in maintenance across Europe, it’s the place to be. The conference, with the support of the EFNMS, is the only commercially independent conference covering the topics we deal with in the maintenance world.

EFNMS | 20.5.2016