Learning From Rotating Machinery Failures Around The World Michael Neale OBE, FREng, FIMechE Neale Consulting Engineers Ltd www.tribology.co.uk
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
Example: Circulator Drive • Clutch teeth failed by fretting due to eccentricity of shaft operating in plain bearings
The shaft of a plain bearing needs to Eccentricity 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 All Forces shown are those on the female gear M T = 0.2T M F = 0.13T M R = 0.25T
Gear Couplings - forces applied to connected machines C Pattern Moments on the sleeve balance out F Z Pattern F Moments on the sleeve add-up. Additional lateral forces F arise L Torque = T The angle between the direction of Direction of relative offset and the direction of the bearing load � offset of far end coupling is � where: Resultant = � an -1 M F / M T = 35 o typically � = bearing load (0.3T/ L approx)
Example: Gear coupling drive to epicyclic sun gear Motor Coupling • A power station main coolant pump with a epicyclic Tim Jones Principal Engineer gearbox driven by a Aircontrol Technologies Ltd. Points of gear coupling failed Articulation Hawthorne Road Staines Middlesex TW18 3AY Gear its sun gear teeth 4 th June 2001 Coupling Spacer from the lateral loads. Dear Tim, • Cured by replacing I have now examined all the gears and Planet the gear coupling with studied the various papers relating to Wheel your gear pump test programme. I also a flexible spline shaft expext to have a copy of the book by Braithwaite within a couple of days. Sun Wheel
Example: Steam turbine half-speed vibration • 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: • Roller bearing Bearing 300 kW failures in the failures from 3.3 kV Motor motor caused by rotor 735 RPM rotor resonance vibration at its critical speed, lowered by overhung shaft mass and flexible stator mounting • Cured by stiffening the frame and reducing the drive length
Example: Guide wheel overloading • 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.
2 Clearance Losses due to Thermal Instabilities - when warming up
Example: Ram air turbine • Ram air turbine on civil aircraft for emergency hydraulic power. In its stowed position has a temperature of -10 o C. • 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: Wind generator gearbox 3 rd Stage Gearwheel • 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
Example: Thruster unit below a ship • 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: 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
Example: Large alternator in low ambient temperature • 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
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 � D 3. The bearing area � D 2.
Example: Alternator hydrogen seal • 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: 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.
Example: Very large conveyor roller bearing • 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 thrust 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
4 Installation and Maintenance Errors
Example: Large rotating top buoy • Large buoy to load and unload oil from tankers. • Rotary top to allow pipes to follow tanker movements
• Bearing housing was too large to machine so bearing was mounted in resin. • Supported on 4 jacks during resin casting. It sagged between them, 3945 mm diam. giving 4 areas of tightness and fatigue. • Cured by using 16 jacks to provide adequate support
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 occurred.
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.
Example: Bore diameter inches 1 5 10 20 30 30 1.0 Cylinder Liner rate ins/inch/1000 hrs Band of Diametral wear performance for 4 stroke Wear engines 2 stroke .010 1 large marine 0 0 . Motor cycles engines Cylinder liner wear data 5 and portable 0 0 0 equipment . Diametral wear rate mm / 1000 hrs 0.1 5 2 0 collected from a wide 0 0 Motor cars . 1 0 0 0 . range of companies 5 0 .001 0 0 0 5 . 2 0 Commercial 0 0 around the world 0 vehicles . .01 Railway locomotives The typical wear performance of the Large .0001 stationary cylinders of internal combustion engines engines .001 10 1000 100 Bore diameter mm
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