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Vibration CH 7
Bearing and Gear Fault Analysis
| Question | Answer |
|---|---|
| Studies have shown that _____% of bearings reach their design life. | 10 |
| Faults in rolling element bearings generate what type of peaks? | Non-synchronous peaks. |
| Name some other physical information of bearings that may be helpful in identifying forcing frequencies. | Number of balls, ball diameter, pitch diameter, contact angle. |
| Rule of thumb for cage frequency or FT is? | .4 orders |
| Describe some symptoms of stage 1 bearing faults? | Frequencies generated in the ultrasonic range (usually 20-60 KHZ range). |
| What technologies are available for detecting stage 1 bearing faults? | Acoustic emissions(ultrasonic), Shock Pulse, HFD, spike energy, SEE, Envelope and Peak Vue. |
| What are some characteristics of a stage 1 bearing condition? | Not detectable using vibration analysis, L10 life = approx 10-20%. |
| Describe some characteristics of stage two bearing faults. | defects cause the bearing to ring like a bell. May be a raised noise floor in the upper regions of the the spectrum. 5-10% of L10 life left. |
| Describe some characteristics of stage three bearing faults. | Bearing defect frequencies begin to appear. First is bearing frequencies then harmonics, sidebands, noise floor, etc.Amplitudes increase. |
| True or False: Bearing failure patterns always follow the classic pattern? | False. The signs of wear may appear to come and go. One day it will appear that a problem exists, and the next data may reveal nothing. |
| Describe some characteristics of stage four bearing faults. | Very high frequency vibration may trend downwards. Smoothing of metal reduces sharp impacts. Looseness increases, Noise increases.Defect frequencies swallowed by noise. |
| What is one common mechanical cause of uneven air gap in an induction motor? | Soft Foot |
| Name some common induction motor faults as it pertains to the rotor. | Eccentric or loose on shaft, Broken or open rotor bars, Bowed rotor,Uneven air gap |
| Name some common induction motor faults as it pertains to the stator. | Eccentric stator, Loose or shorted laminations. |
| Name some common induction motor faults as it pertains to the mechanical. | Bearings, rubs, soft foot, insulation |
| Name some common induction motor faults as it pertains to the electrical. | Unbalanced phase, partial discharge within the stator bar insulation, slot discharge between the stator bar insulation and the stator core, surface discharge over the end winding, discharge between broken conductors. |
| A recent study showed that for AC motors ____% failed due to bearing problems, and ____% due to winding and insulation problems. | 30, 40 |
| Why do we sometimes see vibration patterns at twice line frequencies as it pertains to an AC induction motor? | The magnetic attraction between the stator and the rotor varies at this rate, and the iron itself changes dimension a little in the presence of the varying magnetic field due to "magnetostriction." |
| Define "magnetostriction." | It is the deformation of a magnetic material in the presence of a magnetic field. Causes vibration at twice line frequency in all electric devices such as motors, generators, xformers, etc. |
| When is it difficult to distinguish the twice line frequency peak? | When the AC motor being tested has two poles. Synchronous speed at the 2X will show up the same as twice line frequency. |
| Define slip frequency. | The difference between the actual RPM and the synchronous speed. |
| Define pole pass sidebands. | Pole pass frequencies is the number of poles times the slip frequency. 2 poles = 3600 or 3000, 4 pole = 1800 or 1500, 6 pole = 1200 or 1000. |
| Define rotor bar pass frequency. | RBPF= Running speed X # of rotor bars. |
| List some causes of high amplitudes at blade pass frequencies in pumps or fans. | Rotor or housing eccentricities, Non uniform blades, loose, bent, misaligned housing diffuser vanes, blade or vane wear, Improper operation, Improper damping, dirty, missing filters, inlet or discharge restriction. |
| True or False: When the impeller is loose on the shaft, there will be vane pass frequency with sidebands of turning speed. | True. |
| Pump starvation can look like imbalance, are some symptoms that help distinguish it? | The distorted time waveform produces harmonics of turning speed. |
| Random, higher frequency vibration or noise which is often observed as a hump in the vibration spectrum is called? | cavitation |
| Belt misalignment generates what type of vibration pattern? | 1x vibration in both the radial and axial direction. |
| What is the first frequency called in belt drive problems? | The first forcing frequency is known as the belt rate or fundamental belt pass frequency or simply belt frequency. |
| If a belt is worn or loose there will be what? | Peak at the belt rate, and harmonics with twice this frequency the highest when there are two sheaves. Belt frequency will always be sub synchronous to either component. |
| What are some reasons gearboxes may fail? | Tooth wear, tooth load, gear eccentricity, blacklash, gear misalignment, broken or cracked teeth, etc. |
| What % of failures can be attributed to lubrication skills and practical issues. | 60 |
| In reference to gearboxes, what are the three key frequencies involved? | The input speed, the frequency of the gearmesh,which is the numberof teeth multiplied by the speed of the shaft, and the output speed. |
| Gearmesh forcing frequency =? | Number of teeth X shaft speed |
| Gearbox output speed = ? | Input speed X input teeth/Output teeth |
| Name some other forcing frequencies in a gearbox? | Hunting tooth frequency and gear assembly phase frequency. |
| Peaks that occur at shaft speeds and gear mesh frequencies that are low level refer to what type of component? | Gearboxes |
| gear mesh center frequencies and shaft speed sidebands are most prominent in which axes? | radially for spur gears, axial for helical gears. |
| Most faults with gearboxes occur at which gear mesh frequencies? | 2X and 3X gear mesh frequencies along with their sidebands. |
| Names some faults that are occur in gearboxes. | Tooth wear, tooth load, gear eccentricity, backlash, gear misalignment, broken or cracked teeth. |
| Where can natural frequencies be found? | pipes, foundations, and rotating machinery. |
| Resonances amplify ___________. | vibration |
| The excitation of natural frequencies causes the structure to what? | resonate |
| what % of machine failures can be affected by resonance? | 50% |
| True/False: Every single structure in your plant has natural frequencies. | True |
| The forces which cause vibration are called? | Excitation forces |
| What is a natural frequency? | The frequency at which a part likes to vibrate. |
| When does resonant amplification occur? | Whenever the forced vibration of the defect coincide with the natural frequency of the system. |
| The amount of resonant amplification depends on what? | System damping characteristics |
| The natural frequency is affected by three variables. What are they? | Mass, stiffness, damping. |
| Increasing mass moves the natural frequency to a ______ frequency. | Lower |
| Increasing stiffness moves the natural frequency to a ___________ frequency. | Higher |
| Increasing damping doesn't increase or decrease the frequency but simply ____________________. | Decrease the amplitude |
| When a machine's RPM coincides with the first mode of vibration it is called? | Its critical speed. |
| What happens at critical speed? | Vibration increases dramatically. |
| A machine should not operate within __% of a critical speed? | 20% |
| True/False: Vibration will be lower in amplitude in the frequency range affected by resonance, up to 50 times lower. | False. It is actually higher, and it can be up to 50 times higher. |
| There are a number of special tests one can conduct in order to detect natural frequencies or resonant conditions. What are some of them? | Change the speed of the system and retest. Bump test or impact test, Transient(run up, coast down), Operating deflection and modal analysis. |
| If the base of a system is not stiff enough what can this cause? | This can affect the alignment and create problems with resonance. |
| There are 3 ways to move or reduce the resonant frequencies. What are they? | Change the stiffness. Change the mass. Change the damping. |
Created by:
davidkintner