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 gear A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...

s and main steam or

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...

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 An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

membrane at all shaft

journal bearing A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...

s. 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 An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power g ...

is generally designed to rotate the shaft at approximately 1/10 rpm. 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. 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 Alsthom 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