Predictive Maintenance (PDM)
Questions and Answers

Question: What is Predictive Maintenance (PDM) ?
Answer:
  • Maintenance actions based on machine Condition Data
  • Vibration, Ultrasonic, and other relative monitoring during normal operations
  • Repairs when required with advanced notice prior to unscheduled downtime

Question: How can PDM save money ?
Answer:
  • Reduce repair costs - Early warning
  • Reduce quantity of unnecessary inspections
  • Eliminate catastrophic failure costs
  • Reduce unscheduled downtime - Increase machinery Reliability

Question: Why use Vibration for PDM ?
Answer:
  • Adaptable to a variety of machines
  • Indicates Overall machine condition
  • Indicates problem severity
  • Analysis indicates specific faults

Question: What Is Vibration ?
Answer:
  • Motion of machine components
  • Caused by dynamic forces

Question: What types of Vibration are there ?
Answer:
  • Periodic - (i.e., Machinery Shaft Speed)
  • Random - (i.e., Varying)
  • Transient - (i.e., Pump cavitation due to improper system line-up)

Question: How is Vibration described ?
Answer:
  • Amplitude - (i.e., Normally Velocity)
  • Time (sec. or msec.)
  • Frequency (Hertz or CPM)
  • Phase (degrees)

Question: How are Vibrations measured ?
Answer:
  • Transducer - Converts Vibration - Motion into electrical signal for processing
  • Vibration Meter - Detects only amplitude (i.e., No frequency components)
  • Vibration Analyzer - Performs conversion of Amplitude vs. Time to Amplitude vs. Freq. (Spectrum Analysis)

Question: What units are used to describe Vibration ?
Answer:
  • Displacement = Mils [Peak-to-Peak]
    (1 mil = 0.001")
  • Velocity = In/Sec. [Peak]
  • Acceleration = g's [Peak]
    (1 g = 32.2 ft/sec-sec)

Question: Where are machinery Vibrations measured ?
Answer:
  • Bearing Housings
  • Machine Casing
  • Support Structure
Examples
Typical Horizontal Machine


Typical Vertical Machine


Question: What are acceptable Vibration Levels ?
Answer:
  • ISO & Rathbone Charts
  • Machinery Comparison - History
  • Manufacturer's Guidelines
  • Trending - Relative Levels

Question: How are Vibrations analyzed ?
Answer:
  • Spectrum Analysis (Amplitude vs. Freq.)
  • Parameter Trends (i.e. 1 x SS, 2 x SS, HFD - High Freq. Bearing Wear Detector)
  • Waveform & Phase Analysis

Question: How can faults be detected early ?
Answer:
  • Full Signature (Spectrum) Analysis
  • Frequency Analysis Parameter Sets
    (i.e., 1 x SS, 2 x SS, HFD) Used for detailed trending
  • Overall Vibration Levels (i.e., Meters) used for initial trending. No specific machinery faults can be identified with this method.

Question: What types of faults can be detected ?
Answer:
  • Mis-alignment
  • Looseness
  • Bearing Defects / Wear
  • Unbalance
  • Internal Component Rubbing
  • Structural Resonant Conditions

Question: Why is Ultrasonic (UT) analysis now added to the PDM Process ?
Answer:
  • Enhances roller element bearing and coupling fault detection
  • Determines if correct amount of Lubricant is present in roller element bearings prior to creating faults
  • Used to detect outer race spinning condition in roller element bearing housings

Question: How long has UT been used for bearing Condition analysis ?
Answer:
  • Evaluation period ten (10) years
  • Simultaneous with Vibration Analysis
  • IMPACT Programs' acquire Over 80,000 UT
    Measurements / Year.

Question: What are the frequency ranges of normal sound vs. ultrasound ?
Answer:
  • Audible: 20 Hz - 10 kHertz
  • Ultrasonic: 20 kHertz - 200 kHertz
  • Acoustic Emission: 200 kHz - 2 MHertz

Question: Why has Ultrasound now an integral part of Condition Monitoring ?
Answer:
  • Simple and quick measurement
  • Indicates high friction condition - so bearings can be greased and saved !
  • Provides early warning of developing faults so that planned repairs can eliminate catastrophic failures

Question: What are the Details of Ultrasound Test Equipment ?
Answer:
  • Ultrasound frequencies are shifted into audible range with detailed audible characteristics
  • 39 kHerz (fault detection proven by NASA and others)
  • 4 kHertz Bandwidth (Narrow Band)
  • Fixed frequency and filter (no tuning)
  • Short wavelength (source location)
  • No special sensor mounting (hand held)
  • Precision Measurement (Digital Meter)

Question: Where do you measure Ultrasound ?
Answer:
  • With contact probe:
    • One measurement per bearing or on rigid structure to bearing
    • Radial and/or Axial (load) direction
    • Consistent locations
    • Radial and/or Axial (load) direction
  • With non-contact microphone:
    • Listen to coupling with extension tube

Question: How is Bearing ultrasound severity determined ?
Answer:
  • Good bearings and Couplings produce smooth / mono-tone sounds without distinct characteristics
  • Under lubricated bearings produce clicking sounds that indicate excessive friction associated with a dry condition
  • Bad / defective bearings produce distinctive popping / excessive clicking sounds that typically do not reduce / disappear with greasing
  • Dry/worn/damaged Couplings produce excessive popping / clicking
  • In general, increases in dynamic loading or speed increase digital amplitude values measured

Question: What is the typical Condition of bearings analyzed with UT ?
Answer:
  • Bearing OK
  • High friction / Lack of lubricant - Grease !
  • Developing fault - Greasing reduces UT and popping sounds but return after time
  • Significant / Severe fault present - Greasing does not reduce UT levels nor does severe popping reduce

Question: What are the Advantages of the Ultrasound Technology ?
Answer:
  • Low cost with high benefits
  • Less time to get results
  • Requires less skill and training than vibration equipment
  • Locates specific bearing with fault
  • Non-contact measurement for Coupling analysis in now available
  • Early warning to save bearing or couplings with lubrication, coupling maintenance, or shaft alignment

Question: How Is Machinery Problem Severity Determined ?
Answer:
  • By Machine type
  • Fault type
  • Fault severity
  • Spare machine availability
  • Critical machine for operation
  • Past experience with the type of fault
  • Maintenance required and time to make repairs

Question: How Is Problem Severity Rated ?
Answer:
  • Severe
  • Significant
  • Developing
  • None, continue to monitor

Question: What Special Tests Can Be Conducted ?
Answer:
  • Variable Speed / Load
  • Impact Testing
  • Relative Motion Analysis
  • Driver un-coupled
  • Shaft alignment check
  • Balance sensitivity

Question: Machinery Problems Are Caused By ?
Answer:
  • Normal wear
  • Poor installation - design
  • Operational problems
  • Improper maintenance procedures

Question: What Problem Feedback Info. Is Useful ?
Answer:
  • Operational actions taken
  • Details of faults found
  • Possible cause of the fault
  • Details of repair work done
  • Actual repair costs
  • Estimated costs if problem was undetected and had failed catastrophically


IMPACT Engineering Copyright 2003. All rights reserved.
14209 29th Street East, Ste 105 - Sumner, WA 98390
Tel: (253) 826-9003 - FAX (253) 826-9004
Email Address: ops@impactengineering.com