HELICOPTER PLANETARY GEARBOX FAULT DETECTION · FAULT DETECTION Dr. Wenyi Wang Aerospace Division...

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HELICOPTER PLANETARY GEARBOX FAULT DETECTION

Dr. Wenyi Wang

Aerospace Division

Defence Science and Technology Group (presented at ADF Propulsion Systems Symposium 2018)

UNCLASSIFIED

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Helicopter Planetary Gearbox Fault Detection UNCLASSIFIED

• Background

• Detection Method

• Test Data

• Results

• Summary

Planet Bearing

Planet Gear Bore Crack

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Background – 2009 accident of the Super Puma Helicopter

UNCLASSIFIED

AS-332L2 crash off Scottish coast (16 lives lost) – Caused by 2nd stage (upper) planet gear

fatigue failure (spall-induced) – One metal particle found (36 hours prior)

From Aviafora.com website

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Background – 2016 accident of the Super Puma Helicopter

UNCLASSIFIED

H-225 crash off Norwegian coast (13 lives lost) – Again caused by 2nd stage (upper) planet gear

fatigue failure (spall-induced) – metal particles trapped by oil cooler

In both cases, on-board Vibration Health Monitoring (VHM) didn’t detect the fatigue cracks

Extremely challenging to detect such fault & its progression – a worldwide challenge From LN-OJF Final Repot_05 July 2018

Figure 31: The fracture surface divided into three different zones. The macro marks/features observed are indicated: solid line – well defined, dashed line – less well defined. Photo:

AIBN/QinetiQ

Det

ecta

ble?

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Detection Method

UNCLASSIFIED

• Planetary rotation make detection very hard using fixed sensor

• Best candidate technique: US Patent US6298725-B1-2001: “Method for the

separation of epicyclic planet gear vibration signatures” by David Forrester of DST Group

Licensed to Honeywell (implementation ??) – certainly not in VHM system of Super Puma

Not mandatory for VHM

• Need to calculate Planet Gear Synchronous Signal Averaging

Web download: gear_fa_by_mcsoftware-d8bcjck.gif

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Detection Method UNCLASSIFIED

• SSA with planet carrier is commonly employed in VHM • SSA with planet gear is less commonly employed (if at all ??) • Composite Planet-Gear Synchronous Signal Averaging (CP-SSA) is employed here • ‘Composite’ means SSA is the combined vibration from every planet gear, and

there is no separation of the vibration from the individual planet gears Reasoning : • Planet gear deformation caused by a bore crack would distort its mesh with the

ring and sun gears, and thus distort the planet gear mesh signature in the CP-SSA • The distortion would produce extra modulations to the normal gear mesh

signature, which should appear as modulation sidebands in the CP-SSA spectra

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Detection Method

UNCLASSIFIED

Raw Vibration Signal

Composite Planet-SSA (CP-SSA)

remove the planet gear mesh harmonics

residual signal for the planet gear

RMS energy of the planet gear residual signal can be trended against time

In the envelope signal, we expect to observe double peaks when the defective section in the planet gear is in mesh with either the ring gear or the sun gear.

a squared envelope is then applied to the residual signal To further enhance the

diagnosis, we can obtain the CP-SSA over multiple revolutions of the planet gear.

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Test Data UNCLASSIFIED

• A simple planetary gear test rig at University of New South Wales (UNSW) in Australia

• Torques on output shaft: 45 Nm, 65 Nm and 85 Nm • Speed of input shaft: 293 rpm fixed • 3 planet gears

• Two accelerometers and one tachometer • Sampling rate of 131072 Hz, 100 sec of data length

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Results – Carrier-SSA & its spectrum

UNCLASSIFIED

• dominant spectral components are the planetary gear mesh harmonics indicated by multiples of 81 (ring gear tooth number)

• drive gear mesh harmonics by multiples of 55 (tooth number of the spur gear which meshes with the drive gear with 42 teeth)

0 160 320 480 640 800 960 1120 1280 1440

Carrier Shaft Order

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Am

plitu

de (

G)

UNSW Planetary (Baseline 85Nm) SSR & SSA Spectra (Carrier)

Carrier-SSR: Resampled

Carrier-SSA: Averaged

X: 81

Y: 0.8075

X: 55

Y: 0.1108

X: 110

Y: 0.4087

X: 165

Y: 0.213

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Results – CP-SSA

UNCLASSIFIED

• Could remove the two sets of dominant gear mesh harmonics at ×81 and ×55

• However, the removal of ×81 and ×55 harmonics from the carrier-SSA spectrum will still not be able to obtain the bore crack induced characteristics

• because the spectral content synchronous to planet gear rotation is averaged out in the Carrier-SSA

• This is why it is crucial to employ the SSA of the planet gears 0 30 60 90 120 150 180 210 240 270 300 330 360

Planet-gear Shaft Angle (deg)

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

Am

plitu

de (

g)

UNSW Planetary Gear Baseline at 85Nm CP-SSA

Obviously, there are 23 spikes in the CP-SSA.

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Results – residual signal from CP-SSA, healthy-state condition

UNCLASSIFIED

• removed the planet gear mesh harmonics at ×23 • a flat pattern (no peaks) with the planet gear residual signal across the whole revolution of the

planet gear. • However, the RMS under heavy load was increased slightly (from 0.0319 to 0.0353)

0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)

UNSW Planetary Gear Baseline at 45Nm CP-SSA(1000) residual signal

0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)

UNSW Planetary Gear Baseline at 85Nm CP-SSA(1000) residual signal

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Results – residual signal from CP-SSA, 1-mm slot faulty-state

UNCLASSIFIED

• some obvious spikes in the residual signals at certain rotation angles of the planet gear, • there is a monotonic increase of RMS values with the increasing load.

0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)

UNSW Planetary Gear Bore Slot 1.0mm at 45Nm: CP-SSA(1000) residual signal

0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)


0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)


45Nm torque where RMS = 0.0576



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Results – residual signal from CP-SSA, 1.5-mm slot faulty-state

UNCLASSIFIED

• Again, there is a monotonic increase of RMS values with the increasing load • the monotonic increase of the RMS values with the slot size • Under 65 & 85NM load, two abnormalities (double-peak) or fault features (separated by

roughly 180 degrees of shaft angle) are identifiable




0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)


0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)


0 30 60 90 120 150 180 210 240 270 300 330 360


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de (g

)


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Results – Trending of RMS values of residual signals derived from CP-SSA

UNCLASSIFIED

• The EDM notch lengths of 1.0 mm and 1.5 mm into the rim of planet gear are equivalent to 28.57% (i.e. 1 / 3.5) and 42.86% (i.e. 1.5 / 3.5) of the gear rim being cracked from the bore

• The RMS value of the residual signal seems to be a good index for trending the crack length (severity) development

• Monotonic trend with both load the slot size

• planet gear bore cracking can be detectable using the residual signal of the CP-SSA

0 0.5 1 1.5

Length of Bore Slot (mm)

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Am

plit

ude

SSA(1000) - yr RMS Values

45Nm

65Nm

85Nm

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Results – for diagnosis (1 mm slot) CP-SSA over 3 revs

UNCLASSIFIED

• With the 1.0 mm notch, the features with planet-sun mesh appear to be quite weak and the in-between feature is stronger, for which we do not have an explanation

0 120 240 360 480 600 720 840 960 1080

Planet-gear Shaft Angle

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Am

plitu

de

yr3 & ye3*5 for UNSW Planetary Gear Bore Slot: 1.0mm @45Nm

planet-ring mesh planet-sun mesh ??

0 120 240 360 480 600 720 840 960 1080


-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Am

plitu

de


0 120 240 360 480 600 720 840 960 1080


-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Am

plitu

de


45Nm torque – weak planet-sun mesh

65Nm torque – stronger planet-sun mesh

85Nm torque – stronger planet-sun mesh

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Results – for diagnosis (1.5 mm slot) CP-SSA over 3 revs

UNCLASSIFIED

• With the 1.5 mm notch, there are two main features associated with the planet-ring mesh and planet-sun mesh (roughly 180 degrees apart), and other smaller features can be seen in between the two main features

0 120 240 360 480 600 720 840 960 1080


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de


0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960 1020 1080


-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

Am

plitu

de


0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960 1020 1080


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Am

plitu

de


mesh with ring gear

mesh with sun gear

45Nm torque – strong planet-ring mesh

65Nm torque – stronger planet-ring mesh

85Nm torque – obvious planet-sun mesh

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Results – explained UNCLASSIFIED

• A diagram showing the opening on tensile stress (red) and the closing on compressive stress (blue) of the bore crack of a planet gear in mesh with the ring and sun gears, respectively

• spikes due to the planet-ring and planet-sun gear meshes.

• This is because the planet gear with a bore crack is more deformed when the crack is aligned with radial load from gear mesh, i.e. the crack opens up in the tensile stress zone, than the planet gear without a bore crack.

• When the crack is not aligned with radial load of gear mesh, i.e. the crack closes down in the compressive stress zone, the deformation of the cracked gear is similar to the un-cracked gear

Ring Gear mesh load

Sun Gear mesh load

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Summary

UNCLASSIFIED

• a technique to detect and diagnose a very challenging type of fault – planet gear bore cracking. • The fault detection can be achieved by trending the RMS values of the residual signal derived

from the composite planet-gear SSA (CP-SSA) signal. • When the RMS value exceeds a certain threshold, an anomaly of the planet gears is claimed to

be detected. • The diagnostic capability may be achievable using the squared envelope of the CP-SSA residual

signal over multiple revolutions of planet gear. • The meshing of the defective section in the planet gear with the ring and sun gears are

individually identifiable by the local spikes in the envelope of the residual signal, especially under high-load conditions.

• The technique has been validated using test data acquired from a small industrial planet gearbox test rig.

• Will be further validated by test data from a real helicopter gearbox

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Summary – planned testing using Bell-208B main gearbox

UNCLASSIFIED

• A full-scale Bell-206B helicopter main rotor gearbox will be tested with a very fine EDM notch defect inserted into the rim of the planetary gear

• Try to initiate a real fatigue crack from the EDM notch and propagate it for a certain length with sufficient margin of safety to avoid catastrophic failure

• Multiple types of sensors used during this test, such as an in-line oil wear debris sensor, a wireless vibration sensor placed on the planet carrier inside the planetary gearbox, and traditional accelerometers externally placed on the gearbox housing

• The data from the various sensors will be fused to produce more convincing results

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Question: Is this a unique problem for Super Puma? Answer: Not necessarily (but the solution can be unique for Super Puma)

UNCLASSIFIED

• If it would happen to any of the ADF Helicopters • we would be unable to detect it – safety & availability issue

HELICOPTER PLANETARY GEARBOX FAULT DETECTION · FAULT DETECTION Dr. Wenyi Wang Aerospace Division...

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