Vibration Monitoring

1.Examples vibration monitoring & maintenance of machinery. 2.How to use vibration analysis for predictive maintenance. 3.Practical aspects & specific techniques. 4.Focus on rotating machinery. Unbalance Misalignment Gear tooth defects Bearing defects 5.Related techniques.

1. Examples

Definition of vibration monitoring:

Regular monitoring of machinery vibrations undertaken as part of a Predictive Maintenance Program. Readings are compared with past levels, with significant change as an indicator of developing machinery faults.The objective is to provide valuable lead-time for maintenance planning. A comprehensive monitoring program usually includes vibration analysis.

Condition Monitoring Techniques:

Vibration analysis Oil analysis Wear particle analysis Temperature monitoring. Ultrasonics Infrared thermography Performance evaluation

Examples:

Car suspension Crash Tests Aircraft wing tests Bridge vibration Machinery

2.How to use vibration analysis

Maintenance philosophies:

The main value of Predicted Maintenance is to allow convenient scheduling of corrective maintenance, and to prevent unexpected equipment failures. The key is "the right information in the right time". By knowing which equipment needs maintenance, maintenance work can be better planned (spare parts, people, etc.) and what would have been "unplanned stops" are transformed to shorter and fewer "planned stops", thus increasing plant availability. Other advantages include increased equipment lifetime, increased plant safety, fewer accidents with negative impact on environment, and optimized spare parts handling.Advantages of Predictive Maintenance:

1.Eliminate unnecessary disassembly. New bearings sometimes fail early. Poor assembly. 2.Reduce unscheduled downtime. 3.Avoid wrecks. 4.Reduce insurance costs.

Types of Vibration Monitoring:

1.Continuous monitoring Permanently installed transducers. Costly. 2.Periodic monitoring More cost-effective. 3.Trending Compare data with a baseline. Long-term trends for each machine more practical.

3.Practical aspects

Vibration monitoring entails the regular monitoring of the vibrations of machinery as part of a predictive maintenance program. Vibration readings are compared with past levels, with significant change as an indicator of developing machinery faults. The objective is to provide valuable lead-time for maintenance planning. A comprehensive monitoring program usually includes vibration analysis.

Classifications of Machines:

1.Critical: High safety, operational consequences. High cost to repair. Optimised operation save energy. 2.Essential: Moderate operational consequences and repair cost. Medium horsepower. 3.General: No criticality. Low repair cost, secondary damage minimal

Assigning Maintenance Strategies:

1. Critical Machinery: Proactive, Predictive

2. Essential Machinery: Preventive, Predictive (lesser sophistication)

3. General Purpose Machinery: Breakdown, Predictive (portable equipment)

How often?

Machines with a known history of problems should be monitored daily. Most machines monthly. Machines with a proven track record only quarterly. When problems arise, monitor more frequently.

Under what conditions?

1. Always the same 2. Try to monitor at full load. 3. The same speed, or implement advanced techniques to compensate for speed. 4. A 10% speed difference is acceptable in some cases such as fans and pumps.

Where to take readings?

Depends on failures that are monitored. Most often axial and radial readings on each bearing housing. Mark positions (washer for magnet, tape, pen). Transducer must be perpendicular to surface. Can use a tri-axial sensor for difficult to reach areas.

Accelerometer Mounting:

1.Order of preference: Stud mounting Adhesive Magnetic base Hand held probes or stingers 2.Stud mounted pickups provide maximum frequency response. 3.Sensor resonance is influenced by mounting method.

Fourier Transform:

An FFT consists of: 1. Amplitude 2. Phase

Parameters to choose when measuring:

1.Displacement: 1.Frequency range is below 10 Hz 2.Velocity: 1.Frequency range from 10 Hz to 1 kHz 3.Acceleration: 1.Above 1 kHz

4. Rotating Machinery

Common Machinery Faults:

Unbalance: Static

Amplitude due to unbalance will vary with the square of speed.

The FFT will show 1x rpm frequency of vibration. It will be predominant. Phase difference is as shown.

Bent Shaft:

> For a shaft bend near the centre: A frequency of 1xrpm is predominant.> Bend at ends: 2xrpm is predominant> No phase difference in radial direction at one location.> 180 phase difference in axial plane

Misalignment:

After unbalance, misalignment is the major cause for high vibrations.

Two kinds of misalignment: Angular - shaft ends meet an angle. Parallel - shaft ends are parallel but have an offset.

1. Angular Misalignment

1. Predominant peak in the frequency domain is at 1 rpm. 2. 1 x, 2 x, 3 x may be present. 3. High axial vibration with 1 and 2 . 4. Axial phase difference across the coupling is 180.

2. Parallel Misalignment

1. The parallel misalignment can be into horizontal and vertical misalignment. Horizontal misalignment is misalignment of the shafts in the horizontal plane: i.e. a motor shaft is moved horizontally away from the pump shaft, but both shafts are still in the same horizontal plane and parallel. Vertical misalignment is misalignment of the shafts in the vertical plane: i.e a motor shaft is moved vertically away from the pump shaft, but both shafts are still in the same vertical plane and parallel.

2. The phase difference in radial direction across the coupling is 180. 3. The predominant peak in the frequency domain is at 2 x rpm. 4. Vibrations in radial direction are higher than in the axial direction.

Gear tooth defects:

Gear tooth wear:1. Excitation of gear natural frequencies. 2. Sidebands are spaced at the running speed of the bad gear. 3. Sideband amplitudes rise with wear.

Bearing defects:

Cocked Bearing Symptoms: Vibration symptoms very similar to direct drive angular misalignment. High axial vibration @ 1x rpm, harmonics at 2x & 3x. 2x rpm radial component often as high or higher than 1x component. Axial phase shift around the face of the bearing equal to change in transducer location

Balancing:

1.Can be done with a vibration analyzer or a balancing machine. 2.Performed in a vacuum chamber. 3.Typically, rotors up to 8 tons, up 1.7 meters in diameter, at speeds of up to 60 000 rpm.

Alignment method: Laser

1.The laser alignment system comprises an analyser and two laser heads. 2.The fastened laser heads face each other. Each head has an emitter and a receiver for lasers. 3.As the shafts are turned, the receivers trace the shifts in laser beams. 4.These are communicated to the analyser and computations are done to correct the situation.

5. Related techniques

1.Vibration Analysis and Oil / Particle Analysis are the main PdM techniques. 2.There are also other standard techniques like: 1.Ultrasound 2.Thermography 3.These are useful tools in certain applications. 4.Complement the main techniques.

Thermography:

The use of thermograms to study heat distribution in structures or regions, for example in detecting tumours.

Thermography can be used to detect;

1.Misalignment. 2.Defective compressor valves. 3.Insufficient lubrication. 4.Bad bearings, gears, belts, belt slippage, clutches, chains. 5.Leaking valves, blocked pipes. 6.Tank levels.