Jacking gear
A jacking gear (also known as a turning gear) is a device placed on the main shaft of an engine or the rotor of a turbine. The jacking gear rotates the shaft or rotor and associated machinery (such as reduction gears and main turbines), to ensure uniform cool-down. Without turning, hogging or sagging can occur. Additionally, the jacking gear's assistance in rotation can be used when inspecting the shaft, reduction gears, bearings, and turbines. As an auxiliary function, the jacking gear also helps to maintain a protective oil membrane at all shaft journal bearings.
Hogging is when the shaft bows upwards due to thermal stratification.
On the engine shaft of a marine vessel, this process also prevents the shaft from warping when a ship is preparing to achieve maneuvering status.
Motor and drivetrain
The jacking gear motor is designed to rotate the shaft at approximately 1/10rpm. Most jacking gear motors are rated at 5hp. The jacking gear motor assembly applies power and torque to the reduction gear by a flexible coupling or clutch that can freely engage and disengage to the high-pressure pinion (driving gear). Engaging is accomplished by means of a simple lever. Some newer propulsion arrangements utilize an automatic control system located in the engine room. Jacking gears often feature a lock to prevent the shaft from turning during current or tide changes or when being towed.