CONDITION MONITORING

Sanjay B. Chari.
posted 6-5-06

Maintenance is a must for every equipment, in every factory and by every person.  It includes the three major processes namely predictive, preventive and breakdown maintenance.  These processes are adopted by almost all the factories and are going well with preventive maintenance, but could not totally implement the predictive maintenance to practice, specially the small and medium scale industries.  The most efficient way of doing predictive maintenance is by Condition Monitoring Techniques.  Predictive Maintenance by Condition Monitoring techniques will boost up the availability of the equipment, will increase the efficiency & industrial safety and will reduce the maintenance cost.
            Hence, this paper gives the brief idea of condition monitoring in a nutshell referring to vibration measurement.
            Condition Monitoring can be achieved by two major processes:
1. Vibration measurement and analysis: This will measure the vibration levels and then detect all the problems causing the same.
2. Oil and wear particles analysis: In this a sample of the lubricating oil of the running equipment is tested and the wear particles in it are analysed.  The report gives the quality of the lubricant tested as well as the condition of the equipment in which it is used.  Among the two processes in Condition Monitoring, the vibration measurement is considered to be the most informative and reliable.  As Art Crawford says, “Of all the parameters that can be measured  non-intrusively in industry today, the one containing the most information is vibration signature”.

VIBRATION MEASUREMENT AND ANALYSIS

The Parameter which can be measured without stopping an equipment and which will give the maximum information about its working condition is vibration.
            Vibration is measured at a bearing housing and in three directions i.e. horizontal, vertical and axial.  Usually when the rpm is upto 600, displacement is measured, between 600 & 20000 velocity is measured and above 20000 acceleration is measured.  The velocity is measured more often than displacement and acceleration because velocity is directly proportional to the stress and resulting wear on  a mechanical system.  The two limits are set for a particular machine depending upon various factors like motor kW, r.p.m., no. of starts, operating conditions, type of bearing etc.  These two limits are alarm limit and shutdown limit.  They are very important for running the equipment without any breakdowns.

Vibration Analysis:  The different  frequencies where there are peaks in the vibrations are noted down.  These frequencies are converted in term of orders i.e. simple multiples of rotating shaft frequency like 1 times rpm, 3 times rpm etc.   Depending on this simple multiples the different problems can be detected.  The few problems which can be detected by vibration monitoring are:

• For shaft and rotors: unbalance, bent shafts, misalignments etc.
• For gears: Tooth meshing faults, worn teeth, misalignment etc.
• For bearings: Distortion, loss of lubricants, damages to race and elements etc.

Trending: is a process of  keeping the track of all the readings and the period.  A graph is plotted between vibration readings and the time period.  A point can be found out from this graph when the equipment will cross the shutdown limit and when a breakdown is expected.  This is very much useful for the continuous running equipment.
            Hence, it is very much essential for all factories to go for predictive maintenance by condition monitoring techniques so that they can achieve their target easily and in an efficient way.