Jacking gear
From Wikipedia, the free encyclopedia
A jacking gear (also known as a turning gear or barring 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 steam or gas turbines), to ensure uniform cool-down. Without turning, hogging or sagging can occur.[1][2] Additionally, the jacking gear's assistance in rotation can be used when inspecting the shaft, reduction gears, bearings, and turbines.[2] 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.[citation needed]
Motor and drivetrain
The jacking gear motor is generally designed to rotate the shaft at approximately 1/10 rpm. Most jacking gear motors are rated at 5 hp. 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.[citation needed]
But some industrial gas turbines (GE Frame 5 and Frame 6 types) use a jacking system in which two hydraulic cylinders rotates the shaft about 1/4 turn in one or two seconds every 30 seconds. GE Frame 9B gas turbines, in which accessories were designed in beginning of 1970's by Alstom Belfort engineering, use a high pressure hydraulic motor which connects to the main shaft through an automatic SSS Clutch. Turning speed was chosen about 6 RPM, because the resisting torque is the smallest at this speed, then requiring a very low power.[citation needed]
