Table of Contents
3.5.2.2 Sliding bearings/journal bearings
In industrial (stationary) gas turbines of the heavy frame design ( "Ill. 2.1-7") as in industrial compressors and sationary gears (Chapter 3.7.2) traditional different to aer-engine derivatives sliding bearings are used as main bearings (on the main rotor shaft) in axial and radial configuration ( "Ill. 3.5-14"). In the following typical failure types and their causes are discussed. For vibration analysis there are norms like DIN 31692-3 and specifications.
Sliding bearings function with the build-up of a hydrodynamic lubricating film ( "Ill. 3.5-14") and/or hydrostatic (oilpressure from outside of the bearing). They have specific failure types to which particularly cavitation ( "Ill. 3.5-19") counts. Anyway sliding bearings proved itself excellent. Their operation behavior has, compared with antifriction bearings, as well advantages as disadvantages.
Advantages:
- Damping of rotor vibrations, excited by unavoidable rest unbalances.
- Small installation space.
- Large diameter (shafts) usable.
- High load capacity.
Disadvantages:
- Triggering self exciting vibrations.
- Limited detectability of vibrations with outside positioned acceleration probes.
- In the case of hydrodynamic lubrication at lower sliding speeds (start, shut down) it is to anticipate with an unsufficient building-up of the lubricating film . So it will come without hydrostatic assistance to mixed friction and increased abrasion.
"Illustration 3.5-14": (Lit. 3.5-12 and Lit. 3.5-13): We distinguish, according to the direction of the load axial bearings and radial bearings. Tereby different design principles are used. Axial bearings have normally adjustable segments (tilting pad bearing above left and middle right). Radial bearings use a zylindric sleeve (journal bearing/ sleeve bearing, lower scetch) or axial split bearing shells e.g., segments (sketch obove right). During operation the slide faces are separated by a dynamic build up oil film. This develops by friction forces between the oil and the slide faces in the oil itself. Thereby the oil pulled into the gap by the rotating part (shaft). So a pressure builds up and the shaft is lifted about the size of the lubrication gap, swimming on the oil film. Every operation which influences this condition, so that it comes to a metallic contact of the slide faces at the roughness points (mixed friction), is potential damaging and absolutely to avoid. This can be prevented, e.g., during start and shut down by a sufficient high static pressure of the oil feeded at a suitable position on the sleeve.
To guarantee in every case at least over a short time enough emergency property one side of the slide face (normally the static side, e.g., the casing) furnished with a soft metallic slide coating. This can consist of several layers ( "Ill. 3.5-19"). The other, mostly rotating slide face (shaft), normally consists of hardened steel.
Typical failures of sliding bearings are
- Dynamic fatigue (‘paving stone breakouts’, "Ill. 3.5-17").
- Corrosion ( "Ill. 3.5-16").
- Abrasion by foreign particles in the oil ( "Ill. 3.5-15").
- Erosion because of electrical continuity (Electro erosion, "Ill. 3.5-18").
- Cavitation ( "Ill. 3.5-19").
"Illustration 3.5-15": (Lit. 3.5-8, Lit. 3.5-10 and Lit. 3.5- 14): Mechanical abrasion primarily has two main causes: Galling (‘cold fusings’, seizure; sketch left) develops under dry running conditions (churning) or mixed friction (contact of the roughness points). First signs are groove-shaped damages with microxcopic signs of material, torn and pushed together. This failure is self-aggravating. In very short time (seconds) the end stadium with extreme heat production can be reached. This leads to plastic deformation of the slide surface up to extensive luting, melting and ablation.
There are some reasons for a dangerous contact of sliding surfaces. To those belong:
- Low rotation speed (start, shut down, compressor washing), not enough for a sufficient pressure buildup in the lubrication gap.
- Local overload in the lubrication gap as a result of onesided loads, geometric imperfections, unbalances/vibrations, misalignments and deformations.
- Insufficient lifting capacity of the oil film due to bad oil properties. Causes are too high operation temperature, oil aging/deterioration, oil contamination (water) and not suitable oil.
- Oil deficiency by a blocked fresh oil line or the failure of a component of the oil system (pump, filter etc.).
A further cause are inadequate emergency characteristics of the sliding surface/coating. Wear by abrasive particles in the oil (right scetch). Such particles are dust from the environment, abrasion vom labyrinths, blasting grit, core remainders from castings, machining chips and oil coke. Particles that pass the lubrication gap with the oil produce in the soft sliding surface/coating grooves similar to grinding (sketch obove right) that show microscopic typical features of a cutting process (detail below left).
Remain the abrasive particles sticking in the soft slide coating (detail below right) it comes to wear by chipping of the rotating hard steel shaft. Thereby further abrasive chips develop which accelerate the process.
"Illustration 3.5-16": (Lit. 3.5-8 and Lit. 3.5-10): Corrosion can occur also at sliding coatings. The macroscopic damage pattern reaches from a dark discoloration (left sketch) up to a prous, rough (etched) surface (detail in the middle). In special cases the sliding coat (e.g., leaded bronze) can be totally removed (right detail). selective corrosion occurs during stand still and is supported by the electrolytic influence of a weak electrical continuity.
Corrosion appears primarily in connection with oil:
- Aged/deteriorated oil, e.g., after long time intervals of oil change.
- Contaminated oil (water, acids, brines). Water can originate from a defect oil cooler ( "Ill. 3.7.2-2") and/or from condensation during frequent stand stills.
- Oil with aggressive additives.
"Illustration 3.5-17": (Lit. 3.5-10 and Lit. 3.5-11): The soft sliding coating is subject to intensive dynamic load. It’s to be about pressure fluctuations that are tranferred by the oil film. They release high frequency shear stresses in the slide coating. These produce small fatigue cracks that form with the time a mesh (damage phases at the right). In the advanced stage edged particles breake out (‘flagging’ , sketch left, detail) and accelerate as foreigen objects secondary damages.
"Illustration 3.5-18": (Lit. 3.5-11 and Lit. 3.5-14): Electrical continuity can lead in the lubrication gap to electric micro arcs and local initial fusing of the sliding surface/coating (detail in the middle). In the initial phase single craters with rounded melting structures and elevated edges can be seen. Pronounced failures have a clifty surface with distinct initial fusings (SEM-indication, right detail). They cover larger areas of the sliding surface which look in the shown case of a tilting pad bearing darker (left sketch).
"Illustration 3.5-19": (Lit. 3.5-8, Lit. 3.5-11 and Lit. 3.5- 14): Cavitation develops by implosion of vapor bubbles (sketch middle left) in the lubrication gap. Thereby an ‘oil spike’ hits the surface. The many bubbles produce a high frequent load with plastic deformations of the surface. The result is vibration fatigue with outbreaks (detail middle right). So at first micro craters with diameters in the region of 0,01 mm are formed in the sliding coating. They increase to small holes/pittings (cavities).
Multi phase sliding coatings of sliding bearings (e.g., ternary bearings) show according to H. Klingele a special failure mechanism (sketches below). At first begin fatigue cracks advance from the surface right into the sliding coating.
The Cracks are oriented parallel to the Ni-coating. Such a crack becomes widened, a hutch develops.
The soft sliding coating gets on the ground of the hutch plastified under the pressure shocks and is pushed to the surface. At the edge of the hutch a ridge arises.
Becomes the Ni-coating breatched under the cavitation load the deeper lying bearing layer (leaded bronze) will be damaged. Fragments of the Ni-coating can be pressed in a way into the bronze that it protrudes between (sketch below right). Cavitation failures are characterized by plane discolored, micro rough zones in certain areas of the sliding surface. Its position is dictated by low pressure zones in the oil flow. They are located at oil grooves (sketch above left) or at the edge of the sliding surface (sketches above right). Those typical positions can identify the damage as cavitation.
Cavitation conditions develop under
- too low oil viscosity as result of high oil temperatures or improper oil,
- Vibrations of the shaft,
- Water in the oil,
- Sliding coating with too low fatigue strength.
Literature of chapter 3.5
3.5-1 W.R.Loomis,NASA Lewis Research Center,“Aircraft Engine Sump-Fire Studies“, Page 443-456.
3.5-2 FAG, Kugellagerzeitschrift 242,Page 22-25.
3.5-3 M.J.Kroes,T.W.Wild,“Aircraft Powerplants“,Seventh Edition,Glencoe Aviation Technology Series, Mc Graw Hill Page 352-356.
3.5-4 S.Jung,“Technische Diagnoseverfahren kurz vorgestellt“,TIZL 20 (1984) Heft 4, Page 114-116.
3.5-5 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 2, ISBN 3-00-008429-0, 2001, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.
3.5-6 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 5, 2008, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.
3.5-7 S.Verstege, F.Böckel ,“Lagerung- Grundlagen und konstruktive Gestaltung“, in „Stationä- re Gasturbinen“, Herausgeber C.Lechner, J.Seume, ISBN 3-549-42831-3, Springer Verlag Berlin Heidelberg, 2003, Page 699-723.
3.5-8 „Handbuch der Schadensverhütung“, Allianz Versicherungs-AG München und Berlin 1972, Kapitel „Stationäre Getriebe“, Page 361-375.
3.5-9 P.Lynwander ,“Gear Drives for Turbomachinery“, in „Sawyer’s Turbomachinery Maintenance Handbook, Volume III“, Turbomachinery International Publications ISBN 0-937506-02-8, 1980, Page 11-19.
3.5-10 E.Greuter ,“Motorschäden: Schäden an Verbrennungsmotoren und deren Ursachen“, Vogel Buchverlag, ISBN 3-8023-1515-4, 1994, Page 143-200.
3.5-11 L.Engel, H.Klingele, “Rasterelektronenmikroskopische Untersuchungen von Metall- schäden“, 1974 Gerling Institut für Schadensforschung und Schadensverhütung GmbH Köln, ISBN 3-9800043-0-9, Pages 76, 85-89, 109.
3.5-12 G.Niemann, “Maschinenelemente, Erster Band“, Springer-Verlag, Berlin/Göttingen/ Heidelberg, 5.Neudruck, 1961, Page 239-258.
3.5-13 W.L.Sapp ,“Centrifugal Compressors“, in „Sawyer’s Turbomachinery Maintenance Handbook, Volume I“, Turbomachinery International Publications ISBN 0-937506-03-6, 1980, Page 11-1bis 11-11.
3.5-14 K.C.Ludema ,“Failures of Sliding Bearings“, in „Metals Handbook, Ninth Edition, Volume 11, Failure Analysis and Prevention“, American Society for Metals (ASM), November 1986, ISBN 0-87170-007-7, Page 483-489.