physics

Jacking gear

Jacking gear is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Jacking gear rather than just read about it. In short: 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.

Key takeaways

Jacking gear belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Jacking gear to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Jacking gear from memory before moving on to harder problems.

Reference excerpt

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. 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 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.

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. 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.

Worked examples

Example 1 — a first encounter with Jacking gear

Start with the simplest possible case. Write down what Jacking gear claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Jacking gear before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Jacking gear ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Jacking gear

In research: Jacking gear appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Jacking gear in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Jacking gear is common in secondary-school and first-year university syllabi. It links to neighbouring topics Marine propulsion, Mechanical engineering stubs, Steam engine technology, so understanding it makes those chapters shorter.
In everyday life: Look for Jacking gear outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Jacking gear in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Jacking gear means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Jacking gear out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Jacking gear in simple terms?

Why does Jacking gear matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Jacking gear?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Jacking gear.