Vibration Analysis

1. Vibration Analysis Adam Adgar School of Computing and Technology

2. Outline • What is Vibration? • Conventions • Vibration Characteristics • Amplitude • Frequency

3. Vibration • What is vibration ? • Vibration is a “back and forth” movement of a structure. • Also referred to as a “cyclical” movement. • What is vibration caused by ? • Imperfections in the machine: • Design • Assembly • Manufacture • Operation • Installation • Maintenance

4. Generate Vibration Misalignment Unbalance Worn belts & pulleys Bearing Defects Hydraulic Forces Aerodynamic Forces Reaction Forces Amplify Vibration Resonance Reciprocating Forces Bent Shafts Rubbing Gear Problems Housing Distortion Electrical Problems Frictional Forces Looseness Common machine problems

5. Why measure vibration ? • 1. Assess the condition • normally of the machine bearings • can eliminate catastrophic failures due to component degradation • 2. Diagnose root cause(s) of any excessive (destructive) vibration • can extend the life of bearings and other components • These are absorbing the stresses and fatiguing forces • Note: • Not useful for short-term event-based failures • E.g. loss of lubrication, sudden fracture of a component, etc. • no protection via any program that only collects data periodically • The time between onset and failure in these cases - which are rare - may take only minutes (in extreme cases), hours, days or weeks. • Many programs are based on monthly data collection.

6. Set standards that apply to every machine and application in the program Simplify training of new personnel and make sure everyone involved in the program is on the same page. There are at least three basic conventions that should be set up. They are: 1. Bearing Numbering 2. Position Naming 3. Direction Definitions Conventions 1 2 3 4 MOTOR PUMP

7. Vibration Characteristics Measured • Amplitude • How much movement occurs • Frequency • How often the movement occurs • How many “cycles” in a period of time (e.g. one second or one minute) • Phase • In what direction is the movement • Relative to other locations on the machine at a given moment in time

8. A measure of the amount of movement The amount of movement is related to the severity of the vibration. Several different ways to measure amplitude These are known as amplitude "units". Displacement measures the total distance the transducer (bearing) travels back and forth during one 'cycle' of movement Velocity measures the maximum speed the transducer achieves during a cycle Acceleration measures the force(s) that are causing the back and forth movement Amplitude

9. Peak (A) Peak to Peak (B) RMS (C) Average (D) Amplitude Measurements A B C D

10. Displacement Amplitude • Measures the amount a component is being bent back and forth • It is representative of the STRESS that the bearing structure is being subjected to • Hence it is an amplitude unit that is particularly sensitive to the likelihood of a stress failure occurring. • Stress failures are most likely to occur on slow speed equipment and are not a very common failure mode (hence displacement amplitudes not normally used as a primary monitoring tool).

11. Velocity Amplitude • Velocity measures how much the displacement is changing over a period of time • With sinusoidal motion the velocity constantly changes as the displacement changes • Velocity is a measure of the likelihood of FATIGUE being the mode of failure • Fatigue failures are by far the most common cause of general machinery failures • Velocity is the best monitoring tool for general machines.  • Speciality machines, components or specific problems may not be best monitored by velocity. 

12. Acceleration Amplitude • Acceleration measures the rate of change of velocity • With sinusoidal motion the acceleration constantly changes as the velocity changes • Acceleration is a measure of the likelihood of APPLIED FORCE being the mode of failure. • Applied force failures occur at higher frequencies - almost invariably 60,000 cycles per minute and higher. • There are a limited number of high frequency generating machinery problems. • e.g. rolling element bearing defects and gears.

13. Units

14. Pushing Action Causes sinusoidal motion Velocity changes steadily (creates a sine wave), the acceleration also changes steadily and plotting it generates a sine wave just as displacement and velocity do. Striking Action Causes instantaneous, transient motion Consider striking something with a hammer. The velocity undergoes a nearly instantaneous increase when the hammer strikes. Any movement then dampens out until the next impact. This type of acceleration must be measured differently and will be covered in a later section on "enveloping signals". Forces acting on machines

15. Frequency • A measure of How Often • the number of cycles occurring in a given period of time such as a second or minute • Example: • cycle time = 0.02 s • frequency = 50 cycles per second = 50 Hertz = 3000 cycles per minute = 3000 cpm = 3 kcpm

16. Measurement A - Stress (bending a component excessively causes it to fail) B - Fatigue (something simply wears out over time) C - Force ('pushing' and/or 'striking' actions applied to cause movement) A B C

17. Example - Gearbox 13.8 Hz

18. Further Work • Look at the ‘Introduction to Vibration’ section of the DLI Machine Condition Analysis website: • http://dliengineering.com/vibman/introductiontovibration.htm