An electric motor-driven gear unit that slowly rotates the main engine crankshaft for maintenance, cooling-down, or warming-through.
In practice
The turning gear circuit is interlocked with the engine start control to prevent the engine being started under full pneumatic pressure while the gear remains engaged, which would cause severe mechanical damage. SOLAS II-1 and classification society rules require that this safety interlock be functional at all times. Engineers confirm the turning gear is disengaged and the interlock is working before every engine starting sequence, and record the status in the engine room log. Turning gear motors are routinely tested during port stays and their current draw and gear engagement checked for signs of tooth wear or coupling damage.
Regulatory detail & full definition
Turning gear is an electrically driven reduction gearbox permanently connected to the main engine crankshaft via a pinion, used to rotate the engine slowly — typically below one revolution per minute — for maintenance, pre-departure warming-through, and post-arrival slow cooling-down. By slowly rotating the engine, any hydraulic lock from lubricating oil drainage into a lower cylinder can be detected before pneumatic starting, preventing a catastrophic hydraulic ram failure of a connecting rod. It is also essential during liner honing, valve grinding, and crankshaft deflection measurements.
The turning gear circuit is interlocked with the engine start control to prevent the engine being started under full pneumatic pressure while the gear remains engaged, which would cause severe mechanical damage. SOLAS II-1 and classification society rules require that this safety interlock be functional at all times. Engineers confirm the turning gear is disengaged and the interlock is working before every engine starting sequence, and record the status in the engine room log. Turning gear motors are routinely tested during port stays and their current draw and gear engagement checked for signs of tooth wear or coupling damage.