Couplings & Turning Gear
Joining the Shaft Line and Keeping It Straight
Module 2.1 described the full shaft line as separate rotor forgings coupled end-to-end. This module covers exactly how those connections are made, and closes with turning gear — the system that keeps a hot, stationary rotor from bowing between shutdown and the next startup.
Rigid vs. Flexible Couplings
A rigid (solid) coupling bolts two adjacent rotor flanges directly together, effectively creating one continuous shaft with no relative motion allowed between the two halves. Because a rigid coupling permits essentially no misalignment, the rotors on either side must already be precisely aligned before bolt-up — this is exactly where Module 2.4's shaft alignment discussion matters most, since a rigid coupling has zero tolerance for correcting misalignment after the fact.
A flexible coupling, by contrast, transmits torque while tolerating a small amount of angular or parallel misalignment — through designs like gear couplings, diaphragm couplings, or flexible disc packs. Flexible couplings are often used specifically to connect the turbine shaft line to the generator, since they can absorb small differences in thermal growth or minor residual misalignment between two large, independently supported rotating systems without transmitting damaging bending stress across the joint.
Flexible couplings accommodate a specific, small, engineered tolerance of misalignment — they're a precision solution to a real mechanical problem, not a substitute for doing alignment work reasonably well in the first place.
Coupling Guards and Bolting Integrity
Coupling bolts are torqued to precise specifications, often using fitted (dowelled) bolts to maintain exact alignment as well as clamping force — the same principle covered for casing joints in Module 2.2. A physical coupling guard covers the rotating coupling to prevent personnel contact, but it's a secondary protection layer, not a substitute for correct bolt condition. Coupling bolt inspection and torque verification is a standard, non-negotiable outage task, because a failed coupling bolt at operating speed is a serious mechanical event, not a minor issue.
Turning Gear — Why a Stopped Turbine Keeps Turning
The turning gear is a small electric motor and gearbox that slowly rotates the entire shaft line — typically just a few RPM — whenever the turbine is stopped but still hot. It engages automatically once the shaft coasts down below a set speed threshold after a trip or normal shutdown, and stays engaged until the rotor has cooled sufficiently, or until the next startup roll-up disengages it automatically past turning gear speed.
Turning gear exists to prevent rotor bow. If a hot rotor were simply left stationary, uneven cooling — the top of the shaft exposed to ambient air differently than the bottom, which may sit closer to residual heat sources — would cause the shaft to cool asymmetrically, developing a real, measurable curvature. Continuous slow rotation ensures every point around the shaft's circumference spends equal time in whatever thermal environment surrounds it, cooling (or reheating, on the next startup) evenly and preventing that asymmetric bow from developing.
Turning gear operation is a required procedural step, not an optional precaution. A rotor left stationary while hot for even a few hours can develop enough bow to cause significant vibration on the next startup — this is a real, well-documented failure mode, not a theoretical concern.
Turning Gear Speed and Lube Oil
Turning gear speed is deliberately far below what's needed to establish a full hydrodynamic oil film, as described in Module 2.4. This means extended turning gear operation still depends on continuous, adequate lube oil supply even though the shaft is barely moving — the bearings aren't riding on a fully developed oil wedge at turning gear speed the way they are at full operating speed, making oil supply reliability just as important during this slow-speed condition as during normal running.