Up Time Magazine- August/September 2008

(and production) departments in control of the machines. If you know that a fault is developing at an early stage, you can decide what action is most appropriate. Based on the criticality of the machine, the availability of spares, the demands on production, and the existing plans for maintenance, you can decide what action to take. Knowing the nature and severity of the fault condition puts you in control. And to clarify one point; the time to failure, and the nature of the failure is different for inner race, outer race, rolling element, and cage defects. Tough boots to ﬁll. Resonance At this stage, we need to introduce another term – resonance. When you strike a bell, it will vibrate at a specific frequency. In fact, in addition to the one dominant frequency you hear, it actually vibrates at a large number of frequencies. Bells of different sizes and shapes vibrate at different frequencies – it all depends on their mass and stiffness. The amount of vibration (or sound in this case) we hear is based on the amount of force used to strike the bell, and the amount of dampening. Machines and bearings (and the accelerometer we use to measure the vibration) act in the same way – they will all vibrate naturally when “excited”. Thanks to the “broadband energy” generated due to poor lubrication, the resonances will be excited. When metal-to-metal contact occurs, and when defects appear (subsurface, spalls, cracks, etc.) these forces will again excite the resonances. Now, at this point you may be wondering what resonances have to do with bearing faults. They are important for two reasons; the resonances amplify the vibration, making it easier to measure, and we can utilize the resonance in the accelerometer to further amplify the vibration. Remember, in the early stages of bearing wear the vibration amplitudes are very low, while in comparison, the vibration due to unbalance, misalignment and other sources is VERY high in comparison. Figure 1 – Vibration “spikes” that result from the rolling element coming into contact with the damaged area on the outer race. As the amount of damage increases, we witness more frequent contact between the metal surfaces. As cracks develop, or the subsurface defects grow and eventually break through to the surface, the vibration will change in three key ways: 1. The vibration will be periodic making it easier for us to understand the nature of the fault. Damage on the outer race has a different vibration pattern and frequency to damage on the inner race, rolling elements or cage. 2. The “broadband energy” witnessed will reduce in frequency. It will slowly move from the very high frequencies to the high frequencies, and eventually, to the low frequencies. When the bearing is quite badly damaged the vibration will be in your hearing range and it can be detected with conventional velocity spectra. When the bearing is very badly damaged we will see “hay stacks” in the spectrum and it will cause the “noise floor” of the velocity spectrum to lift up. 3. The forces involved will increase in strength and thus the vibration will increase in amplitude, making the fault easier to detect. As maintenance and reliability professionals retire, they often take the knowledge they possess with them. This loss comes at a cost to companies. That’s where Trico comes in. We can capture that knowledge. Trico can help you integrate proactive practices into your lubrication management program. Our approach–including assessments, audits, consulting, and training–will enable you to benchmark current practices, train employees and improve overall operations. If you have tough boots to ﬁll, give us a call. We’ve got the answers today, for today’s problems. Bearing Fault Detection Technologies There are a number of technologies that can be utilized to detect bearing faults. The following is a summary of those techniques: Airborne Ultrasound – Also known as acoustic emission, the high frequency (above our hearing range) vibration from the bearing can be monitored, either via a dB reading that can be trended, or by listening through headphones (the sound is ‘heterodyned’ so that it is in our audible range). If the bearing is in good condition, the bearing should make a muffled, smooth sound. If you hear a high-pitched rushing sound, or a crackly sound, the bearing may require lubrication, or it may be damaged. It is a simple method, so it can be used frequently in order to detect a fault that can then be further examined by one of the vibration analysis methods described below. This method can even be used during the lube rounds The Power of Knowledge There may be two questions on your mind right now: why do I need to know that the bearing has very slight damage if it still has a number of months of life left in it, and why do I need to know whether the damage is in the inner race, outer race, rolling elements or cage? These are good questions! There are two basic goals in vibration analysis: stop machines (bearings) from catastrophic failure, and provide the information that puts the maintenance www.uptimemagazine.com www.tricocorp.com 800-558-7008 63 http://www.tricocorp.com http://www.uptimemagazine.com

Table of Contents Feed for the Digital Edition of Up Time Magazine- August/September 2008

Up Time Magazine- August/September 2008 Contents Upfront Upclose Information Technology Lubrication Infrared Maintenance Management Motor Testing Precision Maintenance Reliability Ultrasound Vibration Upgrade

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.