Condition monitoring should always start with a list of machine faults, specific for each machine. Only if you know exactly what you expect from the monitoring method, can you apply it efficiently and correctly. Otherwise, there is a danger that you will simply be collecting data. And data is of no use unless it is converted to useful information that you can act upon to realize your true goal of maintaining plant equipment in good working order.
When we look at the rotating component that gives us the most concern, it usually comes down to the bearing. I think it’s fair to say that 70-80% of rotational problems are bearing related. Whether the causes are due to under or over lubrication, contamination, installation faults, secondary forces or just plain fatigue, we need to know the operating condition of bearings most frequently. So it’s very important to determine the best technique for identifying your particular bearing problems. The other rotational problems certainly need to be identified as well, so again, choose the most cost effective, efficient technique to accomplish that.
How do you run a cost efficient, effective condition based monitoring program? Start by selecting the appropriate technique for the application and for the type of answers needed. As a general rule, you can apply the 80/20 rule in many facilities. That is, around 80% of equipment needs to be monitored without the need of spectral data and large amounts of data collection. You could then utilize spectrum analyzing only on the equipment that needs it. For those pieces of equipment that are so critical that periodic monitoring is not enough, then continuous monitoring needs to be considered.
The Shock Pulse Method (SPM) is the front line technique the Hallsta Mill in Sweden chose to quickly manage input from its 800 rolls, with 4000 machines and 6,000 measurement points. With eight inspectors, they need a quick method to know whether bearings need to be greased or not, or that damage is present and needs to be monitored more frequently.
What is Shock Pulse?
What we loosely call ‘machine vibration’ is a very complex form of movement that has many different causes and that can be described and measured in many different ways. Vibration exists in all machines with moving parts, because some of the force, which makes the machine work, is directed against the machine structure and tries to shift it from its position. Thus, vibration is normal up to a degree, and all machines are constructed to withstand a certain amount of vibration without malfunctions. In order to use vibration monitoring to diagnose machine condition, we have to:
- Find a suitable way of measuring vibration, and
- Decide what normal vibration is and what excessive vibration is for any particular machine.
All vibration measurement starts with a time record, a registration of vibration over a length of time. A transducer converts the movement into an electric signal, which an instrument quantifies, displays and stores. The signal can then be evaluated in terms of ‘good’ or ‘bad’.
One way of looking at vibrations is to define the type of force, which causes it. Most industrial machines are rotating, so the main force is rotational, operating on masses which are imperfectly balanced. This accounts for approximately 99% of the total vibration energy. Rotational forces are continuous and cyclic – the force does not stop (while the machine is running under power) and the movement is repeated once per revolution of a part. About % of machine vibration is due to shock. Shock forces are not continuous but can be repeated, either at regular or irregular intervals. The remaining small amount of vibration, about 0.1%, is attributed to frictional forces.
Even bearing damage can be detected through vibration analysis. A bearing produces a group of peaks in the vibration spectrum, caused by the rolling elements passing, at different speeds, over the inner race and the outer race, and by spinning around their axis. A further peak is caused by cage rotation. Given the small mass of the bearing in relation to the large mass of the machine, these peaks normally have very low amplitudes and many times are difficult to pick up with a spectrum before there is severe damage.
A shock pulse transducer contains a reference mass (m) and responds with a dampened oscillation when hit by a shock wave. Attached to the reference mass is a piezoelectric crystal which produces a voltage when compressed by the movement of the reference mass. This voltage is proportional to the amplitude of the oscillation and thus to the energy of the shock wave. The principle is the same as used in accelerometers for vibration measurement. There is, however, an important difference.
When a mass is excited at its resonance frequency, it will oscillate with much greater amplitude than at any other frequency. For vibration measurement, one normally stops measuring far below the resonance frequency of the transducer. On the other hand, shock pulse meters are mechanically and electrically tuned to operate exclusively at their resonance frequency of 32 kHz (fm), where the resulting signal is strongest. This gives us a very sensitive transducer for shocks only, but which will not react to "normal" machine vibration frequencies.