Michael Neale OBE, FREng, FIMechE Neale Consulting Engineers Ltd - - PowerPoint PPT Presentation

Michael Neale

OBE, FREng, FIMechE

Neale Consulting Engineers Ltd

www.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

• perate

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 = T L

Moments on the sleeve balance out Moments on the sleeve add-up. Additional lateral forces F arise

F

The angle between the direction of

• ffset and the direction of the bearing load

is where: = = an-1 MF / MT = 35o typically

F

• Direction of relative
• ffset 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

• f 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
• verload 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

• verloading

2 Clearance Losses due to Thermal Instabilities

• when warming up

• Ram air turbine on civil aircraft for emergency hydraulic
• power. In its stowed position has a temperature of -10oC.
• When lowered into the airsteam it speeds up to a few

thousand RPM in 5 seconds. The light weight shaft warms up more rapidly than the rigid housing. Bearings lose clearance and fail.

• Cured by increasing the clearance in the bearings and

mounting 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 its

bearings on a very cold start. A spherical bearing was required to slide in its housing which lined up with a heavy external web. As a result, when the bearing warmed up, it lost its sliding clearance in the housing, and was overloaded axially to failure

• Cured by using a cylindrical roller bearing instead, which

allowed 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
• n 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 was

arranged to rotate in the opposite direction.

• The loads on plain journal bearings in the

gearbox were then in the direction of the oil inlet grooves, and the bearings failed.

• Cured by fitting the journal bearings in a

different 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

• il 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

• il 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 its

roller bearings failed by fatigue. It had magnetic plugs which collected fatigue debris, to give advanced warning of failure. The gearbox was to be removed for repair when the area of debris collected was 50 sq mm i.e. 7mm x 7mm

• The overseas maintenance crew regarded 50

sq mm as a square with 50mm sides. It had reached 25mm x 25mm when the accident

• ccurred.

Summary

• There is great scope for learning by experience

from plant failures and using this as a basis for design audits

• The operating experience is spread among

competing companies, and therefore needs to be collected anonymously, and correlated by an independent professional body, who can then publish 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 cycles and portable equipment Motor cars Commercial vehicles Railway locomotives Large stationary engines 2 stroke large marine engines

Band of performance for 4 stroke engines

.01

1 5 10 20 30 30

10 .001 100 1000

.010 .001 .0001

. 1 . 5 . 2 5 . 1 . 5 . 2 5

Diametral wear rate mm / 1000 hrs Bore diameter inches Bore diameter mm

Diametral wear rate ins/inch/1000 hrs