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Gear Failures

November 8, 2010 Page 1

Gears

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Advantages of using gears• high power to size ratio, rigid, no slip, accurate• low expense for the amount of torque transmitted• can run at high speeds

Disadvantages• require lubrication• require precise alignment• can be quite noisy

Example Gearbox Monitoring Application

• Helicopter Gearbox Health Monitoring– These gearboxes operate at near peak

capacity– Critical application and failure mode– Significant amount of industry interest

Gears

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Automotive TransmissionContains planetary gear arrangement

Gears

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Planetary Reduction Gearbox

Gears

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Gears

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Gears

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Parallel Shaft ArrangementGears

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Gears

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Types of Gears

Spur gears:• used in parallel shaft boxes• straight teeth parallel to gear shaft axis• contact along entire length of tooth• two pairs in contact for 1/2 the time, one pair in contact for

1/2 the time• maximum stress capability limited by capability of

individual teeth• variation in tooth profile (poor design, manufacturing,

deflection) occurs across the entire tooth• this may cause very high tooth mesh frequency vibration

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Helical gears:• cylindrical gears with spiral (helical) teeth• teeth cut on parallel axis• line of contact is a slanting line• contact starts at one end of the tooth and goes to the other• smooth running due to averaging effect on tooth profile

errors• higher stress capacity (higher loads)• axial force developed due to slanting line of contact (may

cause high axial vibration)• lower radial vibration.• double helix (herring bone) axial thrusts cancel

Types of Gears

Types of Gears

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Worm gears:• screw thread teeth• lots of sliding wear• non-intersecting (right angle) shafts• high gear ratio

Types of Gears

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Bevel gears:• conical in shape• intersecting shaft axes• straight, axially aligned teeth• low ratio right angle drives• spiral bevel - equivalent to helical gears

• Pitting• Scuffing• Tooth breakage• Tooth damage• Cracking • General wearHowever, the most common failure mode in a

gearbox is associated with bearing failure• Other problems: Alignment, eccentricity,

manufacturing defects…

Gear Failures

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Gear Failures

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Local high spot causes uneven wear

Gear Failures

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Gear Failures

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– age– overloading of the gearbox– lack of proper lubrication and – contamination of lubricants (all problems which can be mitigated using

established methods such as oil particle analysis, proactive lubricant changes and other preventative methods).

– Material and manufacturing defects can also lead to premature gear failures

Gears Failures

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• Oil Analysis– Ferrographic analysis– Different types of faults result in specific

types of particles – Size or shape can reveal the fault type

Condition Monitoring of Gears

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• Vibration Analysis– Torsional Vibration

• Oscillation of shaft relative to casing at input &/or output shafts

• Can be a better detection and diagnostic tool• Difficult to instrument

– Bump Test• Mechanical changes in gear shape can

influence resonant frequencies– Gearbox Casing Vibration

• Most common method

Condition Monitoring of Gears

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– Obtain Drawing or Draw Sketch of Gearbox– Calculate

• Gear-meshing Frequencies• Shaft Speed Frequencies

plt passF −

6.3 HzFswing-gmSwing Pinion Gear Mesh

1.5 HzPlanet Passing Frequency

38.8 HzFsun-gmSun Gear Mesh Frequency

19 HzFplanet-gmPlanet Gear Mesh Frequency

269.2 HzFinputpin-gmInput Pinion Gear Mesh Frequency

FrequencySymbolCharacteristic

Selected Gearbox Frequencies at 950 RPM Input

Condition Monitoring of Gears

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Schematic diagram of a double-reduction gearbox

101T

26T3585 RPM

GM 476.8 Hz

GM 1553.5 Hz

923 RPM

295 RPM

97T

31T

Condition Monitoring of Gears

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• Meshing frequency calculations – Given input shaft speed = 3,585 RPM– Intermediate shaft speed

= (3,585 RPM) [(26 T)/101 T] = 923 RPM– Output shaft speed = 923 x 31T/97T=295 RPM– High-speed gear mesh = 3,585 RPM x 26T =

93,210 CPM (1,553.5 Hz)– Low-speed gear mesh = 922.87 RPM x 31 T =

28,609 CPM (476.8 Hz)

Condition Monitoring of Gears

– Frequency Range• Choose accelerometer suitable for expected

Gear-mesh Frequency – Measurement Direction

• Radial for Spur Gears• Axial for gears that are loaded in axial direction

– Measurement Location• As close to gear of interest as possible• Consider transmission path of vibration• Typically near bearing housing

Condition Monitoring of Gears

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Load zone measurement locations

Vibration Transducer Location

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Gears

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Difficulties Interpreting Gear Vibrations• Often difficult to measure close to the gear of interest• Poor signal to noise ratio due to other vibration sources:

– Other gears– Bearings, adjacent machines, couplings etc.– Debris passing through gears

• noise may not indicate a faulty gearbox• noise increases when:

transmission error increases, frequency of operation increases, tooth load increases, # of gears increases

Gears

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Factors (other than faults) Influencing Gear Vibrations

• Speed– Influences amplitude– Influences frequency

• Loading– Influences amplitude– Gears must be loaded to transmit

vibration– Beware of backlash condition

Ideal condition for measurement:– Steady load and steady speed

Must be considered when analyzing signals

• Duty plays a major role in the vibration signal.• Ex: Excavator Swing Transmission Vibration

No loadLow torque / backlash

Transition between reversing Light Load / Unloaded

High torqueHigh torqueEmpty Bucket

Nominal or No load

Low torque / backlash

Transition between reversing Light Load / Unloaded

Highest torqueHighest torqueFull Bucket

Digging Dumping Idling

CoastBacklashFull Deceleration(Braking)

Full Acceleration

Gears

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Influence of SpeedGears

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Gear-meshing Orders During Speed Ramp Up

Gears

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Influence of Load on Gear SignalLoad applied here

Gears

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Duty Influence on Amplitude

Steady Acceleration Backlash Deceleration Coasting

In ideal situation, monitor during steady load and speed

Gears

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Gear Vibration Analysis

• Time Domain• Frequency Domain• Time-Frequency Domain

Gears

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Gears

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Principal vibration frequencies:• gear shaft bearing characteristic defect frequencies (and

harmonics)• rotational speed and harmonics (for both gear shafts)• gear mesh frequency (# of teeth times shaft rotational

frequency)• harmonics of gear mesh• side bands of gear mesh or harmonics (primary frequency +

or - shaft speeds)• wobble of the gear (disk resonance)• tooth / shaft resonance• bearing deflections due to loading on teeth

Time Domain Analysis

• Individual tooth faults result in peaks in the time waveform– At location of defect– Kurtosis is a good indicator

Gears

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Time domain averaging for Gear Vibration

• Allows defect to be accentuated

• efficient data reduction method (N segments down to 1)

• reduces random noise

• suitable for periodic/repetitive signals

• a trigger is necessary to mark the start of each segment

Vibration Signal Frequency Analysis (FFT)

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Gears

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gear mesh and orders in spectrum; varying gear-mesh amplitude in time waveform –shaft frequency plus low-amplitude orders

gear mesh and/or natural frequencies

gearbox distortion

pulses in time waveform; natural frequencies in spectrum

natural frequencies

broken, cracked, or chipped gear teeth

gear mesh with orders and sidebands at frequency of pinion or gear

gear meshimproper backlash of end float

gear mesh with sidebands at frequency or worn, scored, or pitted gear(s); sometimes ½, 1/3, ¼ harmonics of gear mesh

gear meshgear-mesh wear

gear mesh with sidebands at frequency of eccentric gear

gear mesheccentric gearsSpectrum Time WaveformFrequencyFault

Gearbox faults and symptoms

Gears

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• Gear-mesh problems are attributed to uneven wear, improper backlash, scoring, and eccentricity.

• The characteristics in the spectrum are the appearance of gear-mesh with sidebands at the frequency of the speed of the faulty shaft.

• Badly worn gears will show multiples of gear-mesh frequency with sidebands.

Gears

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Gear Spectrum

Gears

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Gears

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Gear spectra for specific faults: Normal

GearsGear spectra for specific faults: Normal

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Gear Failures

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Normal gear set frequency spectra – peaks are symmetrical (paired and equal)

Gears

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Gear spectra for specific faults: Normal (changing load)

Unloaded gear sets have much higher vibration levels than loaded gear sets

Gears

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Gear spectra for specific faults: Normal (changing load)

Gears

Excessive Tooth Load

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Gears

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Tooth wear

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Gears

Tooth wear

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Gears

Tooth wear

Gear tooth wear or excessive clearance changes sideband spacing

Gears

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Gear eccentricity and backlash

Gears

Gear Eccentricity and Backlash

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Gears

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Gear misalignment

Gears

Gear Misalignment

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Gears

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Cracked or broken gear tooth

Cracked or Broken Teeth

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Gears

Cracked or Broken Teeth

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Gears

A broken tooth may produce an asymmetrical sideband profile

Cracked or Broken Teeth

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Gears

Gear Loose Fit

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Gears

Gear Assembly Problem

GAPF – Gear Assembly Phase Frequency (defines groups of teeth that come into contact during meshing)

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Gears

Special tooth repeat problem on gear sets.

A mating flaw on one faulty gear tooth and one faulty pinion tooth match up once every few revolutions.

NA = Assembly Phase Factor – defines the timing when given sets of teeth will come into repeated contact with one another.

Gear Hunting Tooth Problem

FHT – frequency of Hunting Tooth = GMF x NATGear x TPinion

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Gears

Gear Hunting Tooth Problem

FHT – frequency of Hunting Tooth = GMF x NANA = Assembly Phase Factor TGear x TPinion

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Gears

Gear Hunting Tooth Problem

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Gears

Gears (Background)

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• adjacent teeth on the same gear should share the same normal to common tangent

• a single line is normal to the common tangent at two adjacent contact points and this line passes through the pitch point

• all pitch points are in the centre of teeth• when joined they form the pitch circle

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Geartooth shape• involute curve• curve traced by the end of a tight string as it is

unwound from the circumference of a circle• small errors in centre to centre distance do not

violate meshing action• low noise and vibration levels can be expected

from gearboxes that have been well designed and manufactured

Gears (Background)

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Some definitions:• Pitch circle diameter: diameter of pitch circle• Diametrical pitch: # of gear teeth divided by the

pitch circle diameter.• Circular pitch: distance between teeth on the

circumference of the pitch circle• Normal pitch: distance along the normal to the

common tangent between successive tooth surfaces

• Base circle diameter: diameter of circle from which involute curve is generated.

Gears (Background)

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• Gear ratio: ratio of # of teeth on each gear.

gear ratio =

• Lead: distance of travel axially along a helical gear for one tooth to rotate through 360°.

• Line of action: distance along the normal to the common tangent during which one tooth is in contact with one tooth on the other gear

• Backlash: the clearance between the adjacent teeth when two teeth are in contact

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Gears (Background)

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• Working depth: radial distance from the point of tip contact on one tooth to the tip of the contacted tooth

• Contact ratio: average number of pairs of teeth that are theoretically in contact

• Addendum: difference between the pitch circle radius and the radius of the outside diameter

• Dedendum: difference between the radius of the root circle and the radius of the pitch circle

Gears (Background)

Gears (Background)

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Gear teeth in contact

Gears (Background)

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Gears (Background)

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Pitch circle

Gears (Background)

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• Gearboxes generate high-frequency vibrations as a result of the gear-meshing function of the gear.

• The greater the number of gear teeth the smoother is the performance of the box.

• Gear-mesh frequencies with sidebands at operating speeds identify wear and gearbox distortion.

• Gear-mesh problems are attributed to uneven wear, scoring and eccentricity.

• Both axial and radial measurements can be used.

• Load zone measurement locations

Vibration Transducer Location

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Belt Drives

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Belt Drives

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Belt Drives

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Belt Drives

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Frequency spectra showing resonance excited by a belt defect frequency

Belt Drives

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Belt Drives

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Next Time

• Machinery Vibration Testing and Trouble Shooting

• Fault Diagnostics Based on Forcing Functions

• Fault Diagnostics Based on Specific MachineComponents

• Fault Diagnostics Based on Specific Machine Types

• Automatic Diagnostics Techniques

• Non-Vibration Based Machine Condition Monitoring and Fault Diagnosis Methods

November 8, 2010 Page 79