Preventive Maintenance and Vibration Analysis in Machinery
Preventive Maintenance to Reduce Machine Damage
The purpose of preventive or planned maintenance is to reduce the likelihood of damage or loss of performance of a machine or installation. The goal is to plan interventions that conform fully to the life of the element.
Piezoelectric Transducer (Accelerometer) Operation
It generates a voltage proportional to acceleration by the pressure of an inertial mass on a piezoelectric crystal, usually quartz or ceramic.
Forms of Fixing the Sensor to the Machine
- Mounting screw
- Mounting Paste
- Magnetic Mounting
- Manual Securing
The Bathtub Curve and its Three Stages
The bathtub curve represents the reliability or risk of component failure or equipment throughout its life. It sets out three areas or stages of intervention:
- Premature or infant mortality failures
- Maturity and planned maintenance
- Accelerated aging
Appointing the Measuring Points in the Machine
- From the free part of the motor to the free part of the driven element.
- Trigram: letter + number + letter
Typical Defects in a Bearing
- BPFO: Frequency of outer bearing track
- BPFI: Frequency of the bearing inner ring
- BSF: Frequency of passing ball or roller
- FTF: Fundamental frequency train
Stages of Deterioration of a Bearing
- The first symptoms appear in the band 250 – 350 kHz. As wear increases, it moves outside of 20 – 60 KHz. These margins are outside the range of recruitment of general-purpose accelerometers.
- High-frequency vibration readings appear. They are the first indication of deterioration of the bearing. The impacts of these small defects excite the natural frequencies of the tracks.
- The characteristic frequencies of the defects and their harmonics appear. By increasing the damage, the magnitude and number of harmonics increase, as well as the presence of sidebands and the total value of vibration.
- Clearances and general background noise. The bearing is very damaged and symptoms are similar to clearances and friction. 1xRPM and its harmonics increase. Failure frequencies diminish or disappear, masked in background noise.
Equipment for Predictive Maintenance of Electrical Machines
- Clamp Meter
- Flow Sensor
- Thermography
Hertz Contact Pressure Distribution
Hertz explains the mechanical equilibrium in a point-contact area at the expense of elastic deformation and the resulting mechanical stress, which causes cracks and propagates toward the surface to the detachment of particles.
Peak, Peak-to-Peak, and RMS Value in a Vibration Wave
- Peak value: Maximum ordinate from the point of balance.
- RMS value: Also called root mean square. From the position of equilibrium to the reference line.
- Peak-to-Peak Value: From the positive ordinate to the negative ordinate.
Different Types of Transducers
A transducer converts mechanical vibration into an electrical signal to be processed, measured, and analyzed. Following the constructive principle, there are displacement, velocity, and acceleration transducers.
Expressing the Amplitude of a Vibration Wave
- Displacement (micron PP / Mils PP): It is the most appropriate scale for low frequencies (typically up to 10 Hz), where the accelerations are low.
- Velocity (mm/s RMS / mils/s RMS): It is the most appropriate size for a mid-range of frequencies (typically between 10 and 1000 Hz), where most mechanical problems often occur.
- Acceleration (Gs RMS): This quantity is used for measurement at high frequencies (typically above 1000 Hz), where speed changes are very large.