In short. The engine, flywheel, coupling and alternator rotor form one rotating shaft system that behaves as a spring with masses on it. Bearings carry the rotor, alignment and foundation keep the bearing loads within design, and a torsional vibration analysis confirms that the shaft system’s natural frequencies fall clear of the engine’s firing pulses. Most bearing failures trace back to misalignment, lubrication or a moved foundation, and most coupling failures to a torsional check never done.
Single-bearing and two-bearing alternators
A single-bearing alternator has one bearing at the non-drive end. The drive end of its rotor is bolted to the engine flywheel through a set of flexible steel discs, so the engine crankshaft bearing carries the drive end of the alternator rotor. The stator frame bolts to the flywheel housing through a standard SAE adaptor, and alignment is fixed by the machined spigots. It is compact, needs no field alignment and is the standard arrangement on packaged sets up to around 1 MVA.
A two-bearing alternator has a bearing at each end and is connected to the engine through a flexible coupling, often a rubber element or a gear-type coupling, and may sit on a separate baseplate. It is used on larger machines, on engines without a suitable flywheel housing, and where the alternator must be removable without disturbing the engine. It requires alignment at installation and after any move, and its coupling is a wear item. The choice between the two is normally made by the set builder, but it matters when an alternator is being replaced or when an engine and alternator from different sources are being matched, as explained under generator alternator architecture.
Bearings: types, lubrication and life
Generator alternators use rolling-element bearings, usually deep-groove ball bearings, with a cylindrical roller bearing at the drive end of some larger two-bearing machines. Up to a few hundred kVA the bearings are often sealed and greased for life, which in practice means replacement at a stated interval or at major overhaul. Larger frames have regreasable bearings, and the manufacturer specifies grease type, quantity and interval, commonly a few grams every 1,000 to 4,000 hours, confirmed from the data sheet. Rated life follows the L10 basis, meaning 90 percent of bearings reach it, and it falls steeply with load above design and with temperature above about 70 degrees.
| Observation | Points towards | First check |
|---|---|---|
| Bearing temperature rising over weeks at the same load | Grease degradation, over-greasing, early fatigue | Grease log, housing temperature trend, noise with a stethoscope |
| Vibration peak at one times shaft speed | Unbalance, bent shaft, loose coupling disc | Phase measurement, coupling bolt torque, fan condition |
| Vibration peak at two times shaft speed, axial component | Misalignment (two-bearing sets), soft foot | Alignment readings, hold-down bolt check |
| High-frequency noise, metallic rasp | Bearing race damage, contamination | Vibration envelope spectrum, grease sample |
| Fretting marks or pitting on races | Shaft currents, standing vibration while stopped | Insulated non-drive-end bearing, shaft earthing brush |
| Cracked coupling discs or sheared flywheel bolts | Torsional resonance, repeated hard load steps | Torsional vibration analysis, operating history |
Over-greasing is as damaging as neglect: excess grease churns, overheats and is forced past the seals into the windings. Grease should be added with the machine running and the relief plug removed, in the quantity stated, and the old grease allowed to purge. Replacement bearings should be the manufacturer’s specified type and clearance class, fitted with an induction heater rather than a hammer.
Alignment
On a single-bearing set, alignment is set by the spigots, but it can still be wrong if the flywheel housing face runs out, a coupling disc is bent or a shim was left out of the adaptor. The check is a dial gauge on the flywheel housing face and bore before the alternator is fitted, within the engine manufacturer’s limits. On a two-bearing set, shaft alignment is done with dial gauges or a laser system to the coupling manufacturer’s tolerance, typically a few hundredths of a millimetre of offset and angular misalignment, after the set has reached operating temperature, because thermal growth moves the engine more than the alternator. Soft foot, where one mounting foot does not sit flat, distorts the frame and must be corrected before any alignment reading means anything.
Foundation and vibration limits
The set sits on anti-vibration mounts between the base frame and the foundation, or on a rigid base frame bolted to a concrete block. Where rigid mounting is used, the block mass is commonly taken as one and a half to two times the wet weight of the set, isolated from the building floor so that engine vibration does not travel into the structure. Vibration at the bearings should remain within ISO 8528-9 limits for the set class, and a reading taken at commissioning is the baseline for every later comparison. Installation practice is covered under generator installation and the building-side issues under acoustic engineering.
Torsional vibration: the check that is easy to skip
A diesel engine does not deliver smooth torque. Each cylinder firing gives the crankshaft a pulse, so the shaft system is excited at multiples of running speed, called engine orders. The crankshaft, flywheel, coupling discs and alternator rotor form a torsional spring-mass system with its own natural frequencies. Where a natural frequency coincides with a strong engine order at running speed, the shaft twists far more than the mean torque would suggest, and the result is fatigue cracking of coupling discs, broken flywheel bolts, crankshaft damage at the rear journal, or a fan or exciter hub that works loose. Nothing is visible from outside until something breaks, which is why the analysis is done on paper before the set is built.
Set builders run a torsional vibration analysis for every standard engine and alternator pairing, and alternator manufacturers publish rotor inertia and torsional stiffness for the purpose. The analysis is needed again whenever the pairing changes: a replacement alternator of a different make or frame, a change from single-bearing to two-bearing, a different coupling, an engine rebuilt with a different flywheel, or an alternator reused with a different engine. The answer to “will this alternator fit this engine” is therefore not only about the SAE housing size; it is about the shaft system’s natural frequencies, and the engine and alternator manufacturers are the people who can confirm it.
Common mistakes
- Fitting a replacement alternator of a different make on the strength of a matching SAE housing; the torsional system is new and untested, and the coupling discs crack within months.
- Greasing on a calendar without removing the relief plug; the bearing housing fills, overheats and grease is pushed into the exciter.
- Aligning a two-bearing set cold and never re-checking; thermal growth puts it out of tolerance at every full-load run.
- Diagnosing a vibration complaint as a bearing fault without a spectrum; the real cause is unbalance from a damaged fan or a loose coupling disc, and the new bearing fails in the same way.
What to send us
Send photographs of the alternator and engine nameplates, the coupling or flywheel adaptor arrangement, and the mounting and foundation, together with the hours run, the greasing history, and a description of the vibration or noise and when it started. If you have vibration readings or a spectrum, attach them. Send a service enquiry and we will advise whether the problem is bearing, alignment, foundation or torsional before any part is replaced; our alternator service includes vibration measurement.
Questions engineers ask
How long should alternator bearings last?
Within rated load, temperature and lubrication, tens of thousands of hours, which for a standby set is longer than the life of the installation. In practice they are changed at major overhaul or when temperature, noise or vibration trend upward. Life is shortened most by misalignment, over-greasing, contamination and shaft currents.
Can I replace a single-bearing alternator with a two-bearing one?
Mechanically yes, with a new coupling, baseplate arrangement and alignment procedure. It also changes the torsional system and the engine crankshaft loading, so the engine and alternator manufacturers should confirm the pairing. It is usually simpler to replace like with like.
What vibration level is acceptable on a generating set?
ISO 8528-9 gives limits for generating sets by size and application, measured as velocity in mm/s RMS at defined points on the engine and alternator. The commissioning reading is the most useful reference: a set that doubles its baseline vibration has changed, whatever the absolute number.
Why do coupling discs crack on a set that runs smoothly?
Torsional vibration is invisible from outside; the set can feel smooth while the shaft twists at a resonant frequency every revolution. Repeated cracking of discs or flywheel bolts, without obvious misalignment, is the classic sign that the engine and alternator pairing was never analysed, or that something in the pairing has changed.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.