Condition Monitoring Techniques – What is Shock Pulse Monitoring (SPM)?

Shock Pulse Monitoring also known as SPM is a patented technique of predictive maintenance by measuring vibration and shock pulses of bearing in motors and to identify their condition and operating life before the next overhaul procedure.

It was introduced in 1969 and is now a known Condition Monitoring method for monitoring of machines like electric motors using roller bearing.

Difference between SPM and other Vibration Measurement Techniques

Vibration analysis has been used for motor predictive maintenance on ships for many years. Traditionally marine engineers have been using a listening rod to listen to the sound and ascertain the condition of rotating machinery. Nowadays to avoid premature overhaul of motors many companies are supplying vibration analysis pens and SPM instruments on board ships. However there is a difference between vibration analysis and Shock Pulse Measurement.

When two metal surfaces contact each other while in motion, an impact occurs and a shock wave develops, which travels through the metal. The shock wave is in ultrasonic range and is around 36 KHz. This shock wave is utilized in the SPM.

As impact continues the metal flexes and is compressed and recoils. This second phase is called vibration. The frequency of this vibration is dependent on stiffness, shape, mass and the dampening property of the material.  In SPM this phase of collision i.e. vibration is filtered out as it is depending on the structure and the material of the machine. Thus the inaccuracies that are frequently encountered by hand held vibration analysis pens is not here, especially in machinery with flexible mountings and working in vicinity of other vibration prone machinery.

Another difference between SPM and other vibration measurement techniques is that in SPM the transducers respond and resonate to a frequency of 36 KHz only, which ensures a calibrated response and accurate measurement to the shock pulses.

How does SPM Works?

Whether new or old, any bearing generates shocks in the interface between the loaded roller element and the race way. Initially these shocks or vibrations are subtle and hardly felt till already damage is done, but these are captured routinely by the SPM machine which tells about the condition of the bearings, the state of lubrication and the maintenance interval required. This type of monitoring and maintenance based on this evaluation is called as Condition Monitoring System or Condition Based Monitoring.

The shock pulses are measured by accelerometers with filters. The accelerometers have piezoelectric crystals so designed that they resonate at a frequency of 36 KHz which corresponds to the frequency of the pure shock pulses. This helps the accelerometers to measure both shock pulse as well as vibration making other vibration pens only measuring vibration obsolete.

The amplitude of shock pulses measured by the SPM meter is due to the following factors:

• Rolling velocity which is a function of speed or rotation and size of bearing.
• The thickness of the oil film, which in turn depends on preload and the quantity of oil supplied as well as the viscosity of the oil.
• The alignment of the system. That means between the prime mover and the load.
• Other mechanical factors like roughness of the raceways, the stress and damages.

The shocks are received by the SPM transducer which then gives an output signal proportional to the magnitude of the shock felt.  The SPM meter measures the shock pulses per second and then lowers its threshold so that two amplitude levels are discovered, first the decibel carpet value of 200 shocks per second and secondly, the maximum level of incoming shock under 2 seconds.

The decibel carpet value gives an indication of the condition of the lubrication and the peak value gives the extent of bearing damage.

The peak value can be ascertained by increasing the threshold value till no signal is received.  In this equipment the noise generated due to the rolling velocity is negated by entering the shaft diameter and RPM of the motor. This gives an accurate condition assessment of the machine being monitored.

The amplitude of the shock is a function of the rolling element and the instrument measures the absolute value and subtracts from it the expected shock value from a good bearing at similar speed. This gives us an indication of the bearing operating condition.

There are three condition zones namely Green for Good Condition, Yellow for Caution and Red for Damaged condition. The peak value measured by the operator gives the status of the machine and the zone it belongs to.

Uniqueness of Shock Pulse Monitoring

• SPM specializes in determining accurate information on the mechanical condition of the bearing surfaces as well as the state of lubrication on the bearing throughout the life of the bearing.
• SPM instrument can measure both the vibration as well as shock pulses.
• One additional advantage of SPM over other vibration measurement methods is that analysis of the SPM spectrum enables to pin point the source of the trouble.  This is because in case the bearings are damaged, the shocks generated by the damaged bearings have a pattern that corresponds to the frequency of the balls passing over the damaged area.  In case the shocks generated are due to gears they will have a different shock pattern, while random shocks will not have a pattern and not be investigated.
• As both the RPM as well as the shaft diameter are entered into the instrument evaluation can be done independent of speed in variable speed drives.
• Advanced warning of lubrication problems can be given.
• As any machine with metal to metal contact will give shock pulses it is useful for analysis of other machines like screw compressors, gear boxes, lobe compressors, centrifuges, purifiers, centrifugal pumps apart from motors.
• SPM also gives an indication of faulty installation and alignment.
• In vibration analysis advanced stage of damage occurs before the vibration levels exceed above the noise carpet. In SPM the noise carpet is eliminated and the bearing as well as the lubrication condition is continuously monitored after filtering out the vibration as well as the background noise, giving sufficient warning to avoid breakdown and plan maintenance.
• SPM is not influenced by the size, speed, design, background noise or installation of the machine.

References: spminstrument, maintenanceonline, conditionmonitoringsystem

Image Credits: wikimedia, vibrationsound.webs.com