Steam Turbine Journal Bearings: Babbitt, Tilting Pads, and Why They Wipe

A journal bearing on a steam turbine spends almost its entire working life doing something nobody can see. While the machine runs normally, the shaft never touches the bearing metal. It floats on a film of oil a few hundredths of a millimetre thick, and as long as that film holds, the bearing can run for decades with very little wear.

When the film fails, it fails quickly. The soft white lining, the babbitt, smears, melts or cracks, and the machine trips or keeps running with damage nobody has found. Babbitt is designed to be the part that gives way, which is why a wiped bearing is usually a warning rather than a disaster, provided somebody reads the warning correctly.

What follows covers how the bearing carries load, the designs in use, how babbitt fails, what catches it early, and when to repair rather than replace.

How a Journal Bearing Carries the Rotor

A plain journal bearing is a bored shell slightly larger than the shaft journal it supports. The difference in diameter, the clearance, is small but deliberate. At rest the journal sits in the bottom of the bore, supported by little more than a residue of oil.

When the shaft turns, it drags oil round with it. Because the journal sits off centre, the gap between journal and bore narrows in the direction of rotation, and oil dragged into that narrowing gap has nowhere to go. Pressure builds in this converging wedge until it lifts the journal and carries the weight of the rotor. The journal then runs at an eccentric position inside the bore, riding on a film whose thinnest point, the minimum film thickness, decides whether the bearing survives.

Film thickness rises with speed and oil viscosity and falls with load, and that relationship explains most of what goes wrong. Low speed under heavy load, such as a large rotor on slow roll, gives a thin film. Hot oil is thin oil. A misaligned journal concentrates load on one edge of the bearing, where the film is thinnest of all. Large turbines deal with the low speed problem using jacking oil: high pressure oil pumped under the journal to lift it hydrostatically before the shaft starts to turn and while it runs on turning gear.

In a healthy journal bearing, metal contact happens only at start and stop. A bearing worn after years of steady running has been reporting a film problem for some time.

Bearing Types, and Why Tilting Pads Exist

A cylindrical sleeve is the simplest design and carries load well, but it has a weakness at high speed and light load. The oil film that supports the journal also pushes it sideways, at right angles to the load, and under some conditions that sideways force drives the journal round the bore in a whirling orbit. This is oil whirl, and it shows in the vibration spectrum as a component a little below half running speed. If the machine runs above roughly twice its first critical speed, whirl can lock onto that critical and become oil whip, a large self-sustaining instability that no amount of balancing will cure.

Lemon bore or elliptical bearings. The bore is machined with two lobes, so the clearance is tighter top and bottom than at the sides. The upper lobe generates a second film that loads the journal downwards and stiffens it against whirl. This is common on older and medium sized machines.

Pressure dam bearings. A step is cut into the upper half of the bore. Oil carried into the step builds pressure that pushes the journal down into the lower half, raising the effective load and the stability that comes with it.

Multi-lobe and offset bearings. Three or more lobes, or halves offset from each other, each generating its own converging film. They give up some load capacity in exchange for stability.

Tilting pad bearings. The bore is replaced by a set of separate pads, each on a pivot that lets it tilt freely. Every pad forms its own oil wedge and aligns itself with the journal. Because each pad can only push towards the shaft centre, the sideways force that drives whirl is largely removed. That is why tilting pad journal bearings are the usual choice on high speed turbines and compressors. They cost more, have more parts to inspect, and their pivots wear, but for a rotor running well above its first critical speed they are the stable option.

Converting an older fixed geometry bearing to tilting pads is a legitimate cure for recurring subsynchronous vibration, but it changes the rotor dynamics and needs analysis first. Our guide to vibration analysis for rotating equipment covers how to separate whirl from the faults that resemble it.

The main journal bearing designs on steam turbines and how each deals with oil whirl. Tilting pads are the stable option for rotors running well above their first critical speed. Source: MIMAH engineering practice.

Babbitt: the Layer Designed to Fail First

Babbitt, also called white metal, is a soft alloy cast onto a steel backing shell. Turbine bearings almost always use a tin based grade, mostly tin with antimony and copper. The commercial alloys are specified in ASTM B23, which covers the typical white metal bearing alloys sold as babbitt metal.

Its properties are chosen so that it loses to the shaft every time. It conforms, deforming slightly to accommodate small misalignment. It embeds, taking small hard particles into its surface instead of letting them score the journal. It is compatible with steel, so a brief contact at start up smears the babbitt rather than welding to or tearing the journal. And it melts far below any steel, so when the film does collapse, the lining sacrifices itself and the expensive rotor journal usually survives.

The price is weakness. Babbitt loses fatigue strength as it gets hotter, and a thick layer is weaker in fatigue than a thin one, so a bearing running hot has less margin in the material keeping it alive.

Why Babbitt Fails

Most babbitt damage falls into seven patterns, and each points to a different cause.

Wiping. Surface babbitt smeared or melted and dragged in the direction of rotation. The film collapsed, even if only briefly: loss of oil supply, an oil pump that failed to change over, overload, a misaligned journal loading one edge, or low speed running without jacking oil. Wiping is the headline failure and is almost always a symptom of something else.

Fatigue cracking. A network of cracks in the loaded zone, sometimes with pieces of babbitt missing. It comes from cyclic loading, usually dynamic load from vibration, and high metal temperature accelerates it. A cracked bearing on a machine with a vibration problem needs the vibration fixed before the new bearing goes in.

Electrical pitting and spark erosion. Current passing from shaft to bearing through the oil film discharges as tiny sparks, leaving a frosted or finely pitted surface and eventually opening the clearance. Shaft voltage on steam turbines comes from static charge generated by wet steam in the low pressure stages and from magnetic asymmetry in the generator. The defences are shaft grounding brushes and insulated bearings on the generator. A worn or disconnected grounding brush is a cheap and frequently overlooked cause.

Contamination scoring. Circumferential scratches and embedded particles, from dirt too large for the babbitt to absorb, often left after an overhaul or admitted by a breather or failed filter.

Varnish. A brown or amber lacquer on the bearing surface or pads, formed from oxidised oil. It reduces clearance and heat transfer, so metal temperature creeps up with no obvious fault. It is an oil condition problem showing up in the bearing.

Overheating. Babbitt that has crept, flowed or discoloured under sustained high temperature without the sudden collapse of a wipe. Causes include high oil inlet temperature, a fouled oil cooler, a restricted supply orifice to one bearing, and excessive load.

Loss of bond. The babbitt separates from the steel shell. It can start at manufacture or at a poor rebabbitting job where the shell was not properly cleaned and tinned, and it grows with thermal cycling. A debonded patch cannot pass heat to the shell, so it runs hot and eventually cracks or flakes out.

Several of these can appear on one bearing. Wiping and fatigue cracking at one edge of the same shell often point to misalignment, and our shaft alignment and balancing guide sets out how to confirm it. The wider list of turbine faults that end in bearing damage is in our article on common steam turbine failures.

Seven ways babbitt fails, each pointing to a different cause. The pattern on the failed shell is the start of the investigation, not the end of it. Source: MIMAH engineering practice.

The Monitoring That Catches It Early

Three measurements between them catch most bearing problems before the machine trips.

Bearing metal temperature. Resistance temperature detectors or thermocouples embedded in the shell, close to the babbitt in the loaded zone, give the most direct view of film health. On a tilting pad bearing the sensor belongs in the most heavily loaded pad. Oil drain temperature is a weaker substitute because it averages the whole bearing and lags behind the metal. Trend metal temperature rather than only alarming on it: a slow rise over months at constant load is varnish, fouling or wear until proven otherwise.

Shaft vibration and position. A pair of proximity probes at right angles measures the journal's movement relative to the bearing. From them come the shaft orbit, which shows whirl and rubs, and the average shaft centreline position, which shows where the journal sits in the bore. A journal that sinks or drifts over time is wearing its bearing or changing its load. API Standard 670 sets minimum requirements for machinery protection systems, covering radial shaft vibration, axial position and bearing metal temperature among others. For evaluating vibration, ISO 20816-1 now covers what used to be split between ISO 10816 and ISO 7919, the old shaft vibration standard that many specifications still quote. Our note on ISO 10816 versus ISO 20816 explains the change.

Oil analysis. Tin in a spectrometric analysis, and lead on older bearings, points at babbitt, and a rising particle count shows that something is shedding. Oil analysis will not say which bearing, but it will tell you to start looking. Water, oxidation reserve and varnish potential in the same report show whether the oil itself is part of the problem.

Repair or Replace

A damaged bearing leaves three routes: dress it and return it to service, rebabbitt the existing shell, or fit a new bearing.

Light wiping on a bearing with a sound bond, and clearance still inside the manufacturer's limits, can sometimes be dressed by hand scraping and put back. That is a judgement made on inspection, and it only makes sense once the cause of the wipe has been found.

Rebabbitting is the standard repair for a shell that is dimensionally sound. The old babbitt is melted out, the shell is cleaned and tinned so the new metal bonds to it, new babbitt is cast, often centrifugally for cylindrical shells, and the bore is machined to the specified clearance and profile. The quality of the job depends almost entirely on the bond, and the bond is tested rather than assumed. ISO 4386-1 specifies ultrasonic pulse echo testing for bond defects between the bearing metal and the backing, with penetrant testing used on the visible bond line at the joint faces and ends. A rebabbitted bearing should come back with a bond test report and a dimensional record of bore and clearance, and for tilting pads, a check of every pad's thickness and pivot.

Replacement is the right answer when the shell is cracked or distorted, its fit in the housing is lost, tilting pad pivots are worn beyond repair, or the bearing design itself is the problem. On machines whose original manufacturer no longer supplies bearings, replacement often means reverse engineering: measuring the old bearing, establishing clearances and profile from the rotor and whatever original data survives, and manufacturing to that. MIMAH reverse engineers critical spares for exactly this situation, and it is often faster than waiting on a supply route that has quietly closed.

Whichever route is taken, clearance, shell crush and contact pattern are measured on assembly and recorded, because they become the baseline for the next investigation.

The three routes for a damaged bearing. Whichever is chosen, clearances are measured on assembly and the cause is found first. Source: MIMAH engineering practice; bond testing per ISO 4386-1.

Root Cause First, Then the Bearing

The most expensive thing to do with a wiped bearing is replace it and restart. The new bearing inherits the cause and usually fails the same way, sooner.

An investigation starts with the evidence on the failed shell, where the damage sits and what pattern it has, then moves to the machine's history: metal temperature and vibration trends before the event, the oil analysis record, alignment readings, when the grounding brushes were last checked, and what changed in the weeks beforehand. Our steam turbine inspection checklist sets out what to record when the bearing caps come off.

MIMAH's rotating equipment team inspects and refurbishes turbine bearings, reverse engineers replacements where original supply has lapsed, and investigates the failures behind them. If you have a bearing running hot, a wiped shell on the bench, or a machine that keeps eating bearings, get in touch.