Michael NealeOBE, FREng, FIMechE

Neale Consulting Engineers Ltdwww.tribology.co.uk

Learning From Rotating Machinery Failures

Around The World

Table 1. 107 Design related

causes of failure

Description of the cause of failure Number of cases

• Unexpected interaction between components 20

• Errors in detail design 19

• Loss of operating clearance from thermal instability 17

• Errors in design layout 15

• Errors in material choice 12

• Errors in lubrication system design 9

• Errors in lubricant selection 8

• Unexpected system resonances 7

Table 2. 83 Working related

causes of failure

• Description of the cause of failure Number of cases

• Manufacturing errors 18

• Installation errors 17

• Insufficient lubrication 12

• Lubricant contamination 11

• Machine overload 7

• Maintenance and monitoring errors 7

• Operating errors 6

• Environmental effects 5

Most failures occur at component

surfaces carrying loads with

relative movement.

Bearings, Gears, Pistons, Seals

and Couplings.

Common Causes of Failure are:-

1. Unexpected Interactions between Components

2. Clearance Losses due to Thermal Instabilities

3. General Design errors

4. Installation and Maintenance errors

1

Unexpected

Interactions between

Components

• Clutch teeth failed by fretting due to

eccentricity of shaft operating in plain bearings

Example: Circulator Drive

Eccentricity

The shaft of a

plain bearing

needs to

operate

eccentrically

to develop

hydrodynamic

pressure

Gear Coupling Loads:

• Gear couplings, when operating with a ‘Z’

shaped pattern of misalignment, generate

high lateral loads on the adjacent

machines.

• This can be sufficient to overload adjacent

bearings or gears

The moments generated at a gear coupling mesh

MT = 0.2T MF = 0.13T MR = 0.25T

All Forces shown

are those on the

female gear

Gear Couplings - forces applied to

connected machines

Resultant

bearing load (0.3T/ L approx)

C Pattern

Z Pattern

Torque = TL

Moments on the

sleeve balance out

Moments on the sleeve add-up.

Additional lateral forces F arise

F

The angle between the direction of

offset and the direction of the bearing loadis ! where:

! = "an-1 MF / MT = 35o typically

F

!Direction of relative

offset of far end coupling

• A power station

main coolant pump

with a epicyclic

gearbox driven by a

gear coupling failed

its sun gear teeth

from the lateral loads.

• Cured by replacing

the gear coupling with

a flexible spline shaft

Tim Jones Principal Engineer Aircontrol Technologies Ltd. Hawthorne Road Staines Middlesex TW18 3AY 4th June 2001 Dear Tim, I have now examined all the gears and studied the various papers relating to your gear pump test programme. I also expext to have a copy of the book by Braithwaite within a couple of days.

Motor Coupling

Points of

Articulation

Gear

Coupling

Spacer

Planet

Wheel

Sun Wheel

Example: Gear coupling drive to

epicyclic sun gear

• Steam turbine

shaft on left

side lifted by

gear coupling

reaction.

• The lower

bearing load

produced half

speed vibration

of the turbine

rotor.

• Cured by

altering the

vertical

alignment.

Example: Steam turbine half-speed vibration

• Roller bearing

failures in the

motor caused by

rotor resonance

at its critical

speed, lowered

by overhung shaft

mass and flexible

stator mounting

• Cured by

stiffening the

frame and

reducing the drive

length

300 kW

3.3 kV Motor

735 RPM

Example:

Bearing

failures from

rotor

vibration

• Cylindrical rolling wheels only move at right angles to their axes, and can

overload any installed lateral location.

• To avoid this the rollers must be free to steer and follow the required track.

To achieve this the outside of the rollers must be part spherical and the axle

bearings self-aligning.

Example: Guide wheel

overloading

2

Clearance Losses due

to Thermal Instabilities

- when warming up

• Ram air turbine on civil aircraft for emergency hydraulicpower. In its stowed position has a temperature of -10oC.

• When lowered into the airsteam it speeds up to a fewthousand RPM in 5 seconds. The light weight shaft warmsup more rapidly than the rigid housing. Bearings loseclearance and fail.

• Cured by increasing the clearance in the bearings andmounting them in a thin-walled housing

Example: Ram air turbine

• On cold starting the large 3rd stage gearwheel does not warm up as rapidly

as the shaft, and the bearing inside it fails due to loss of clearance

• Cured by increasing the bearing clearance and changing its axial position

3rd Stage Gearwheel

Example: Wind generator gearbox

• Spherical roller

bearing outer

race could not

slide in its cold

rigid housing, and

generated high

shaft thermal

expansion loads

against the thrust

bearing

• Cured by

replacing the

spherical roller

bearing with

cylindrical roller

bearing

Example: Thruster unit below a ship

Example: Coal Mill

• A power station coal mill in the open air failed itsbearings on a very cold start. A spherical bearing wasrequired to slide in its housing which lined up with aheavy external web. As a result, when the bearingwarmed up, it lost its sliding clearance in the housing,and was overloaded axially to failure

• Cured by using a cylindrical roller bearing instead, whichallowed axial movement between its race and rollers

• 35Mw alternator with a substantial bearing housing, which warmed

up from low temperature more slowly than the shaft and the plain

bearing lost its clearance

• Cured by increasing the bearing clearance

Example: Large alternator in low ambient temperature

3

General Design Errors

Example: Steam turbine

• A new small steam turbine was modelled

on a larger machine. It suffered half speed

rotor vibration because its bearing loads

were too low.

• The loads ! D3.

The bearing area ! D2.

• The seal location bearing wore out rapidly due to contamination

from dirt centrifugally trapped when the original oil drain was from

the inside

• By changing the feed to the inside, and the drain from the outside,

it was made self-flushing, which solved the problem

Example: Alternator hydrogen seal

Example: Fan• A reliable fan, turbine driven via a gearbox was

duplicated with another close to it.

• To match the pattern of the air ducts, it wasarranged to rotate in the opposite direction.

• The loads on plain journal bearings in thegearbox were then in the direction of the oilinlet grooves, and the bearings failed.

• Cured by fitting the journal bearings in adifferent angular position.

• A very large roller bearing had its rollers made from steel bar stock.

Axial inclusions in the steel caused the rollers to crack in half.

• Cured by using individually forged rollers

Example: Very large conveyor roller bearing

• A large tilting pad thrust bearing was designed by computer, to give

maximum operating film thickness. The computer programme did not

recognise the need for large gaps between the pads to allow hot exit

oil to be replaced by new cold oil feed.

• Result: the bearing overheated, and required redesign

Example:

Very large

thrust

bearing

4

Installation and

Maintenance Errors

• Large buoy to

load and unload

oil from tankers.

• Rotary top to

allow pipes to

follow tanker

movements

Example: Large

rotating top

buoy

• Bearing housing was

too large to machine so

bearing was mounted in

resin.

• Supported on 4 jacks

during resin casting. It

sagged between them,

giving 4 areas of

tightness and fatigue.

• Cured by using 16

jacks to provide

adequate support

3945 mm diam.

Example: Helicopter Gearbox

• A helicopter gearbox failed in flight when itsroller bearings failed by fatigue. It had magneticplugs which collected fatigue debris, to giveadvanced warning of failure. The gearbox wasto be removed for repair when the area of debriscollected was 50 sq mm i.e. 7mm x 7mm

• The overseas maintenance crew regarded 50sq mm as a square with 50mm sides. It hadreached 25mm x 25mm when the accidentoccurred.

Summary

• There is great scope for learning by experiencefrom plant failures and using this as a basis fordesign audits

• The operating experience is spread amongcompeting companies, and therefore needs tobe collected anonymously, and correlated by anindependent professional body, who can thenpublish design guidance.

• This could be a role for the IMechE.

Cylinder liner wear data

collected from a wide

range of companies

around the world

The typical wear performance of the

cylinders of internal combustion

engines

Example:

Cylinder Liner

Wear

1.0

0.1

Motor cyclesand portableequipment

Motor cars

Commercialvehicles

Railwaylocomotives

Largestationaryengines

2 strokelarge marine

engines

Band of

performance

for 4 stroke

engines

.01

1 5 10 20 30 30

10

.001

100 1000

.010

.001

.0001

.001

.0005

.0002

5

.0001

.00005

.000025

Dia

metr

al w

ea

r ra

te m

m / 1

00

0 h

rsBore diameter inches

Bore diameter mm

Diam

etral w

ear

rate

ins/in

ch/1000 h

rs