{{Short description|Auxiliary engine to slowly rotate big engines}} 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.<ref>{{Cite book |last=Bloch |first=Heinz P |title=Steam Turbines - Design, Applications, and Rerating |last2=Singh |first2=Murari P |publisher=McGraw-Hill |isbn=978-0-07-164100-5 |edition=2nd |publication-date=2009 |pages=128 |orig-date=1996}}</ref><ref name=":0">{{Cite book |url=https://www.google.com/books/edition/Naval_Reserve_Multiple_Address_Letter/LVgnyPKixR4C?hl=en&gbpv=1&kptab=overview |title=Naval Machinery - Part II - Steam Turbines |date=1946 |publisher=Bureau of Naval Personnel |location=United States |pages=11 |chapter=Turbine Accessories}}</ref> Additionally, the jacking gear's assistance in rotation can be used when inspecting the shaft, reduction gears, bearings, and turbines.<ref name=":0" /> 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.{{CN|date=March 2026}}

==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&nbsp;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.{{CN|date=March 2026}}

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 {{nobr|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.{{CN|date=March 2026}}

== See also == * Barring engine

== References == <!--- See Wikipedia:Footnotes on how to create references using <ref></ref> tags which will then appear here automatically --> {{Reflist}}

Category:Marine propulsion Category:Steam turbines Category:Steam engine technology

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