# Jacking gear

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{{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 gear](/source/reduction_gear)s and main steam or [gas turbine](/source/gas_turbine)s), 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](/source/oil) membrane at all shaft [journal bearing](/source/journal_bearing)s.

Hogging is when the shaft bows upwards due to thermal stratification.

On the engine shaft of a [marine vessel](/source/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](/source/Electric_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](/source/clutch) that can freely engage and disengage to the high-pressure [pinion](/source/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](/source/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

{{Mech-engineering-stub}}

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Adapted from the Wikipedia article [Jacking gear](https://en.wikipedia.org/wiki/Jacking_gear) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Jacking_gear?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.