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 steam or

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

s), 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 bearing Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad A sliding table with four cylindrical bearings A wheelset from a Great Western Railway (GWR) wagon showing a plain, or journal, bearing end A pl ...

s. Hogging is when the shaft bows upwards due to thermal stratification. On the engine shaft of a

marine vessel A watercraft or waterborne vessel is any vehicle designed for travel across or through water bodies, such as a boat, ship, hovercraft, submersible or submarine. Types Historically, watercraft have been divided into two main categories. *Raf ...

, 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 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 A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...

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. 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 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.

References

Marine propulsion Steam turbines Steam engine technology {{Mech-engineering-stub

Motor and drivetrain

See also

References