A gearbox is being lifted into position. Two slings (strong lifting straps or cables) connect it to a hook above. The gearbox is hanging still while the installation team prepares to place it on its mounting points.
Before writing an equation, we need to answer one question:
What forces are acting on the gearbox, so that we can later understand how the two slings are supporting it?
This question comes before the free-body diagram. We do not draw an FBD because a chapter asks for one. We draw it because it helps us answer an engineering question.
Start with the problem, not the arrows
Picture the lifting arrangement. The gearbox is below the hook. One inclined sling connects the hook to the left lifting point. A second inclined sling connects it to the right lifting point.
The gearbox also has a centre of gravity. This is the point through which we represent its weight. It may not be at the geometric centre of the gearbox.
For now, assume that the gearbox is hanging still. Treat it as one rigid body. Treat each sling as a light, flexible member that can pull only along its own direction.
We are not calculating the sling forces yet. We are also not checking the lifting lugs or proving that the lift is safe. Our first task is smaller:
Identify every external force acting on the gearbox.
Choose the body that can answer the question
The complete lifting arrangement contains the gearbox, two slings, the hook and the crane. Which part should we study?
Return to the question. We want to identify the forces acting on the gearbox. Therefore, the gearbox is the body we choose.
Imagine drawing a closed line around the gearbox. Everything inside the line belongs to the chosen body. Everything outside it belongs to the surroundings. We call this line the system boundary.
Take a moment here. Choosing the body correctly is often the hardest part. A neat diagram of the wrong body will not answer the intended question.
What is physically acting on the gearbox?
Now ask: What outside objects or influences are acting on the chosen body?
There are three. The left sling pulls at the left lifting point. The right sling pulls at the right lifting point. Earth attracts the mass of the gearbox. We represent this last interaction as the gearbox weight acting vertically downward through its centre of gravity.
We are not adding these arrows because a lifting FBD usually looks this way. Each arrow appears because something outside the chosen boundary is acting on the gearbox.
The left sling acts on the gearbox, so its effect must appear. The right sling acts on the gearbox, so its effect must appear. Earth acts on the gearbox through gravity, so the weight must appear.
Because these effects come from outside the chosen boundary, we call them external forces. The arrows are the result of physical interactions. They are not the starting point.
Remove the surroundings, but keep their effects
Now remove the hook and both slings from the drawing. Only the gearbox remains.
The slings have disappeared from the picture. Their physical effects have not.
Replace the left sling with a force at the left lifting point. Draw the force along the direction of the left sling. A light, flexible sling pulls along its own length; it does not push sideways on the gearbox.
Replace the right sling in the same way. Then add the weight, acting vertically downward through the centre of gravity.
You now have the free-body diagram of the gearbox. In simple terms, you have separated one component from the assembly and kept the interactions that its surroundings have with it.
That is the central idea behind an FBD.
Use one check for every arrow
A short label such as T1 may be useful later. At the beginning, fuller labels make the physics clearer:
- force of the left sling on the gearbox;
- force of the right sling on the gearbox;
- force of Earth on the gearbox, also called the gearbox weight.
Each label answers two questions: Who is exerting the force? On which body is it acting?
ONE-ARROW CHECK
Who is exerting this force, and on which body?
Use this check for every arrow. An arrow that cannot answer both parts needs more thought.
For example, do not add a separate force from the crane hook directly on the gearbox. The hook does not touch the gearbox. It acts through the two slings. The forces crossing the gearbox boundary are therefore the two sling forces and the weight.
What does the FBD help us do?
The FBD does not complete the lifting analysis. It prepares the next step.
Because the gearbox is hanging still, its acceleration is zero. The external forces must balance. We can later use equilibrium equations to relate the gearbox weight, the sling directions and the sling forces.
The diagram also helps us ask better questions:
- Is the centre of gravity between the two lifting points?
- Are the sling angles the same?
- Should the two sling forces be equal?
- What information is still missing before calculation?
A useful FBD makes the next engineering step clearer. It does not add arrows merely to make the page look complete.
For readers who want to go one step deeper
Suppose the centre of gravity is not midway between the lifting points. The FBD still contains the same three forces, but the two sling forces may not be equal. Their values depend on the geometry, the sling directions and the location of the centre of gravity.
This shows why the position and direction of each force matter. Counting the arrows is not enough.
Now change the engineering question. Suppose we want to understand the total load transferred to the crane hook. We could enlarge the boundary to include the gearbox and both slings.
The forces between the gearbox and the slings would then lie inside the new boundary. They become internal interactions for the combined system, so they do not appear as external forces on that system. The hook interactions at the top of the slings and the gearbox weight remain external.
The same lifting arrangement can therefore produce different valid FBDs. The useful boundary depends on the problem being solved.
A correct rigid-body FBD still does not prove that the lift is safe. A complete assessment may also require sling-capacity checks, lifting-lug strength, accurate centre-of-gravity information, dynamic effects, connection details and an approved lifting procedure.
The FBD answers one defined part of the problem. Good engineering also recognises what the model does not answer.
Try it yourself
Cover the completed diagram and rebuild it. Use the same order followed in the article:
1. State the engineering question: What forces act on the hanging gearbox?
2. Choose the body: the gearbox.
3. List what is outside the boundary but acts on it: Earth, the left sling and the right sling.
4. Draw only the gearbox.
5. Replace each outside influence with the force it applies.
6. Label every force by its source and receiving body.
Your drawing is complete when:
- only the gearbox is the selected body;
- each sling force acts at its lifting point;
- each sling force follows the direction of its sling;
- the weight acts vertically downward through the centre of gravity;
- no direct force from the hook or crane appears on the gearbox.
An extra or missing arrow does not mean that you cannot understand FBDs. Return to one question: What is physically acting on the chosen body?
The arrows are the result
A free-body diagram begins with an engineering question. You then choose the body that helps answer it, remove the surroundings and keep the effects those surroundings have on the body.
At Struxinova, we use this body-first reasoning to help learners move from familiar textbook diagrams to unfamiliar engineering systems.
About the author
Avinash S is the CEO and Partner at InnoventEdutec, leading the Struxinova and Mathinova learning initiatives. He has more than 16 years of experience spanning engineering skill development, application engineering, technical-content development, project leadership and learning-product strategy. His work includes university- and industry-aligned learning programmes, academic and OEM engineering projects, engineering simulation programmes and technical training. Through Struxinova, he focuses on scientific thinking, engineering judgement, applied structural-mechanics fundamentals and physics-based simulation validation.