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COOLING TOWER FAN GEARBOX HIGH VIBRATION

CASE STUDY # 2

This Photo by Unknown Author is licensed under CC BY-SA

Muhammad Uzair Siddiqui

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Problem Statement

  • Machine: B-4420 C
  • Location: EPCL Utilities II
  • Gearbox reduced motor 1500 rpm to fan 118 rpm
  • High vibrations were observed in gearbox after 11 years of trouble-free operation.
  • When vibration data collected and analyzed through CSI 2140 analyzer, it was revealed that gearbox first set that is bevel gear set was not running at right angled to each other due to non linearity of pinion (input) shaft thereby impacting tooth contact and forcing uneven load.
  • This uneven tooth contact resulted in high vibration of gearbox around 18 mm/sec against baseline value of 5 mm/sec.
  • As a result, gearbox had to be replaced.
  • The question is how did the centerlines of first set of gears were changed from right angle (misaligned) which resulted in high vibration?

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MISALIGNMENT

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Thermal Growth or Expansion

Input shaft and output shaft temperature difference

Uneven materials heating

Lack of lubrication

Causes of shaft misalignment

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seizures causing high load

High moment of inertia

Torsional Movement

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Poor levelling

Corrosion of the base or foot

Anchor bolts incorrectly torqued

Soft Foot

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Machines

Methods

Environment

Materials

Man

Measurements

SHAFT MISALIGNMENT

Poor maintenance planning

Improper gearbox oil level checks

Improper analysis of vibration data

High moisture content

Improper re-alignment

Assembly error

Thermal expansion

Internal and external corrosion

Airborne abrasive materials

Soft foot of foundation or base plate

High initial torques during start-up

High or low ambient temperatures

Material quality decay

Inexperienced operator

Gear tooth backlash

Worn coupling insert

High vibrations observed

Faulty vibration sensor

Improper supports below shaft

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Conclusion

  • To dig in deep to the real root cause of the failure, some questions should be answered like was the vibration data properly analysed? Did the personnel do the scheduled preventive maintenance? How was the environmental conditions and if the gearbox was disassembled in the past, were the parts re-assembled properly with proper looseness checks? There are so many questions yet to be answered yet we can still doubt some causes that can be the contributing factors to shaft misalignment:
    • soft foot
    • high torsional movements
    • lack of lubrication which may lead to high thermal growth or expansion

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A proactive, periodically scheduled preventative maintenance plan should be set up and proper oil servicing should be carried out to avoid the dangerous consequences of failure.

A soft foot is a leading factor to shaft misalignment and it should be avoided by using only flat shims, a dent-free baseplate should be used and laser system can be used for proper alignment

Looseness should be avoided of the bolts and other main components like coupling and properly working vibration sensors should be used to detect any abrupt changes

For machine parts to work effectively, an oil film thickness should be maintained which prevents wear and provide cooling, and hence reducing chances of high torque problem

Effective strategies should be implemented to tackle with moisture content, dry air and thermal growth and abnormal expansion

Suggestions for better tackling of shaft misalignment and hence gearbox failure

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