The “run to failure” maintenance strategy is still prevalent among many plants nowadays. This means that no maintenance will be done for machinery until it fails while the maintenance staff will run from one disaster to another one. This can induce a much higher maintenance cost and lesser productivity.
Some companies have transitioned to preventive, or calendar-based, maintenance. Actions are scheduled regardless of the actual condition of the equipment. With this approach, fault-free machines can be repaired unnecessarily, leading to higher program costs.
Over the past 30 years, the US Navy and many Fortune 500 companies transitioned from preventive maintenance to condition-based maintenance. With condition-based maintenance, machines are measured with methods such as vibration analysis, which don’t require tearing a machine down to find out its condition. When a machine condition fault comes up, a repair is scheduled when it’s needed – not before and not too late.
Early indicators of machine health
Several technologies are used to measure and diagnose machine health. Two of the most important are vibration testing and infrared thermography. The graph shows how you can detect changes first with vibration testing, then with infrared thermography. Only later on – shortly before machine failure – can you hear audible noise and feel the heat.
Benefits of early vibration testing include:
- Give maintenance staff time to schedule the required repairs and acquire needed parts.
- Take faulty equipment offline before a hazardous condition occurs.
- Incur fewer unexpected and serious failures, helping to prevent production stoppages that cut into the bottom line.
- Increased maintenance intervals. Extend the life of equipment and schedule maintenance by need.
- Incur fewer unexpected or catastrophic failures because problem areas can be anticipated before failure.
- Peace of mind. Build confidence in maintenance schedules, budgeting, and productivity estimates.
Mechanics of vibration testing
A transducer picks up vibration signals from bearing locations and transmits these signals to a data collection device. Here are a few important things to note about the mechanics of vibration testing:
- All rotating equipment generates a unique vibration signal or signature.
- These unique signals are usually captured in series, with the signal’s amplitude (y-axis) depicted over time (x-axis). This is called a time waveform.
- The waveform contains information about the machine at the point of measurement. Vibration comes from the rotating shaft, adjacent machines, foundation, noise, rotating components, structural resonances, flow turbulences, and other sources.
- However, the patterns of different events are overlapped and jumbled together. Separating and isolating one vibration signal from another is complicated.
- Frequency analysis performed in the data collector simplifies the waveform into certain repetitive patterns. Fast Fourier Transform (FFT) is a mathematical algorithm performed by the vibration testing tool to separate individual vibration signals.
- Spectrum is the plot of each of these individual signals on a simple plot of amplitude (y axis) against frequency (x axis).
We can simplify it down to a three-step process.
- Identify vibration peaks as they relate to a source component on the machine.
- Look for patterns in the data based on vibration rules.
- Measure the amplitude of the vibration peak to determine the severity of the fault.
Once the fault and severity are determined, you can recommend a repair and generate a work order.
Bearing faults and failures
A study conducted by the SKF Group tracked the life of 30 identical bearings and found that there is a wide variation in bearing life. This precludes the use of an effective calendar-based maintenance program.
Another study found that bearing faults can account for over 60 percent of mechanical faults. Although bearings are a major contributor to mechanical problems, sometimes bearing faults are the result of a separate underlying problem, such as unbalance. Some customers replace bearings every few months until they learn to balance and align the machine – then bearings will last for years. Bearings fail because of:
- poor insulation
- poor lubrication
- wear fatigue
- other faults
A roller bearing – also called a rolling-element bearing – carries a load by placing round elements between the two pieces. Most machines today have roller bearings.
Analyzing roller bearing faults
Bearing frequencies are non-synchronous. The geometry of the balls, cage, and races show up at different speeds; these speeds are not a multiple of shaft speed. In most cases, non-synchronous peaks are roller bearings. Most vibration programs use the following bearing frequencies:
- inner race
- outer race
- ball spin
Vibration pens, meters, and testers
When you move up to a vibration meter, you have the capability to measure overall vibration in addition to specific variables. The Fluke 805 Vibration Meter has a combination vibration and force sensor tip that compensates for user variance (force or angle) – yielding accurate, repeatable readings. This meter has a four-level severity scale and an onboard processor that calculate bearing condition and overall vibration using easy-to-understand textual alerts (Good, Satisfactory, Unsatisfactory, Unacceptable). Its sensors can read a wide range of frequencies (10 to 1,000 Hz and 4,000 to 20,000 Hz), covering most machine and component types. The 805’s straightforward user interface minimizes user inputs to RPM range and equipment type. This gives frontline maintenance personnel and operators a screening tool to determine which equipment is healthy and which needs further troubleshooting.
As described previously, an advanced vibration testing tool, the Fluke 810 Vibration Tester, has a diagnostic engine that combines algorithms with a database of real-world measurement experience. You can also get this product for A SPECIAL DISCOUNT so make sure to CONTACT US NOW or email us at firstname.lastname@example.org!
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