Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1103301
20 | March - April 2019 | www . machinerylubrication.com Slip happens when t wo surfaces contact each other at different speeds without a fully developed lubricant film. Essentially, it means no metal- to-metal galling or adhesion is taking place during engage- ment. As a result, the coefficient of sliding friction is low (less than 0.1), surface damage is very slight and at least one of the surfaces is normally hard- ened steel. Therefore, there is negligible galling or severe sliding, although mild rubbing and polishing may occur. Under these conditions, the DNA12 score intermittently exceeds 25 percent of the DNA500 acceler- ation score. Sliding and impact contact is a damaging condition resulting from inadequate lubrication under compression. Surfaces are gouged and striated. Extremely high tempera- tures may smear, melt, and discolor wear debris and surface materials. Under these conditions, the DNA500 score is very high with impacts lasting several milliseconds. During these intervals, the DNA12 scores remain well below 25 percent of the DNA500 acceleration score. Cavitation, fluid turbulence and electrical arcing are occasional sources of ultrasonic vibration in industry. Cavitation results from bubbles imploding on impellers, setting off ultrasonic shockwaves. e fluid-structure interaction from leaking pressurized fluids can also stim- ulate ultrasonic stress waves. Electrical arcing, corona and discharges are another possible instigator of ultrasonic vibrations. Naturally, these uncommon sources of ultrasonic vibration will contribute to DNA12 far more than DNA500. e graph on page 19 includes an undecimated vibration waveform, a DNA500 waveform and a DNA12 waveform. In this example, prop- erly lubricated loading is evident, except from 49.7 to 50.5 milliseconds, when a slip occurred. During this interval, the DNA12 score exceeded 25 percent of the DNA500 score. is is when a brief speed adjustment took place. Negligible damage, if any, happened during this interval because it was a slip and not an impact or severe sliding event. is slip event was likely a rotation of a chain link. Measuring Forceful Events As mentioned previously, DNA500 is a peak-to-peak acceleration measurement waveform. It tracks forceful acceleration resulting from normal operation and process material movement from defect impacts and severe sliding contact. Fast Fourier Transform (FFT) analysis and correlation analysis of the DNA500 waveform can be useful for differentiating periodic and non-periodic events as well as for matching frequencies of occurrences with mechanical system movements. Monitoring More Faults, Covering More Equipment Intelligent decimation can extend the range of faults detected and expand the types of machines monitored. Traditional vibration analysis reveals problems associated with belt drives, bent shafts, cavitation, gear defects, looseness, misalignment, resonance, soft foot and imbalance. Oil analysis identifies problems related to lubricant chemistry, system contamination and component wear. With intelligent decimation, rubbing, severe sliding and slip friction faults can be detected while monitoring articulating, reciprocating and low-speed machines. Intelligent decimation can also be easy to implement. With simple computation, low power and narrow bandwidth requirements, it is suitable for long-life, battery-operated wireless sensors. ese sensors can be easily mounted and automatically cloud-connected and config- ured. is allows any internet- connected computer or mobile device to become a monitoring and analysis interface for reliability specialists at any time or place. e I-DNA installation shown on the left detected the following operating conditions: intermediate shaft vibration peaks and sidebands; possible cage faults for intermediate shaft bearings; and intermittent, strongly modulated vibration. As shown in this example, intelligent decimation is able to monitor low-speed machines by detecting occur- rence frequencies for high-frequency, metal-to-metal contacts. ML An I-DNA installation on a gearbox OIL ANALYSIS