What is the principle of moments?

The principle of moments states that when a body is balanced (in equilibrium), the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot. A moment is force multiplied by its perpendicular distance from the pivot, so moment equals force times distance. This principle lets you calculate the effort needed on a lever, or the load it can balance, by setting the two moments equal.

How is mechanical advantage calculated?

Mechanical advantage (MA) is the load divided by the effort, MA equals load over effort. It tells you how many times a machine multiplies the input force. A lever, gear train or pulley system with a mechanical advantage greater than 1 lets a small effort move a larger load, which is the main reason these mechanisms are used.

How is a gear ratio calculated, and what does an idler gear do?

Gear ratio is the number of teeth on the driven gear divided by the number on the driver gear. A ratio greater than 1 (a reduction) makes the output turn more slowly but with greater torque, and output speed equals input speed divided by the gear ratio. Two meshing gears turn in opposite directions; an idler gear placed between the driver and driven reverses the output direction and bridges distance without changing the overall ratio.

How is the velocity ratio of a belt-and-pulley drive found?

The velocity ratio of a simple belt drive is the diameter of the driven pulley divided by the diameter of the driver pulley. A larger driven pulley slows the output and increases torque; a smaller driven pulley speeds it up. An open belt drives both pulleys the same way, while a crossed belt reverses the direction. Belts are quieter and can slip to protect a mechanism, an advantage over gears.

What is the difference between a strut and a tie in a structure?

A strut is a structural member in compression, being squeezed or pushed, so it resists being shortened. A tie is a member in tension, being stretched or pulled, so it resists being lengthened. Identifying which members are struts and which are ties tells a designer where to use stiff, thick parts (struts, which can buckle) and where a thin strong part (a tie) will do.

SingaporeDesign and Technology

Mechanisms and Structures: how Singapore O-Level Design and Technology students use levers, linkages, gears, pulleys and structural members to transmit force and motion and carry loads, with the key calculations

A Singapore O-Level Design and Technology (SEAB 7059) module overview of mechanisms and structures. Levers and the principle of moments, linkages, gears and gear ratios, pulleys and belt drives, and structures with struts, ties and stability, including the core force and ratio calculations, with links to every dot point.

Generated by Claude Opus 4.88 min readSEAB-7059Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module covers
Levers and moments
Linkages
Gears
Pulleys and belt drives
Structures, struts and ties
How this module is examined
Check your knowledge

What this module covers

Mechanisms and Structures is the most calculation-heavy part of O-Level Design and Technology (SEAB 7059), and it rewards students who can both explain how a mechanism works and compute the numbers. Levers, linkages, gears and pulleys transmit and change force and motion, while structures carry loads safely. These topics are a reliable source of structured questions in the written paper, and they often appear inside Design Project artefacts that move or support something. This overview links the five dot points and gathers the key formulae.

See the full set of dot points for this module under /sg-o-level/design-and-technology/syllabus/mechanisms-and-structures.

Levers and moments

The foundation is the lever. The levers and the principle of moments dot point classifies the three orders of lever and applies the principle of moments (clockwise moment equals anticlockwise moment, where a moment is force times perpendicular distance) to calculate effort, load and mechanical advantage. Mechanical advantage, load divided by effort, is the idea that underlies every force-multiplying mechanism in this module.

Linkages

Linkages change the direction or nature of motion. The dot point covers reverse-motion, push-pull, bell-crank and parallel-motion linkages, each redirecting an input to a useful output, for example turning a push into a pull or changing motion through a right angle. Linkages are common in everyday products from folding mechanisms to control levers.

Gears

Gears and gear ratios transmit rotary motion and trade speed for torque. The gear ratio is driven teeth over driver teeth, output speed is input speed divided by the ratio, meshing gears turn opposite ways, and an idler reverses direction and bridges distance without changing the ratio. A reduction (ratio above 1) slows the output but increases torque.

Pulleys and belt drives

Pulleys and belt drives transmit rotary motion between shafts using a belt. The velocity ratio is the driven pulley diameter over the driver pulley diameter, an open belt keeps direction while a crossed belt reverses it, and belts offer advantages over gears such as quiet running and slip that can protect a mechanism from overload.

Structures, struts and ties

Structures, struts, ties and stability covers how structures carry loads: struts are in compression, ties are in tension, triangulation stiffens a frame, and a low centre of gravity with a wide base gives stability. Reinforcement methods (gussets, webs, lamination) make a structure stronger without simply adding bulk.

Calculating effort and mechanical advantage on a lever

A first-order lever (like a crowbar) is used to lift a load. The load is $600\ \text{N}$ at $0.2\ \text{m}$ from the pivot, and the effort is applied at $1.2\ \text{m}$ from the pivot on the other side. Find the effort needed and the mechanical advantage.

Step 1: Write the principle of moments

For balance, the anticlockwise moment equals the clockwise moment about the pivot:

\text{effort} \times \text{effort distance} = \text{load} \times \text{load distance}

Step 2: Substitute the values

\text{effort} \times 1.2 = 600 \times 0.2

\text{effort} \times 1.2 = 120\ \text{N m}

Step 3: Solve for the effort

\text{effort} = \frac{120}{1.2} = 100\ \text{N}

So an effort of just $100\ \text{N}$ balances a $600\ \text{N}$ load, because the effort acts at a longer distance from the pivot.

Step 4: Find the mechanical advantage

\text{MA} = \frac{\text{load}}{\text{effort}} = \frac{600}{100} = 6

The mechanical advantage is $6$ , meaning the lever multiplies the input force six times. A mechanical advantage has no unit because it is a ratio of two forces.

Mechanisms and Structures (SEAB 7059) is the calculation core of the subject. Levers obey the principle of moments (clockwise moment equals anticlockwise moment, moment = force x distance) and give a mechanical advantage of load divided by effort. Linkages (reverse-motion, push-pull, bell-crank, parallel) change the direction or nature of motion. Gears trade speed for torque: gear ratio = driven teeth / driver teeth, output speed = input speed / ratio, and an idler reverses direction without changing the ratio. Belt-and-pulley drives have a velocity ratio of driven diameter / driver diameter, with belts offering quiet running and protective slip. Structures use struts in compression and ties in tension, with triangulation, a low centre of gravity and reinforcement giving strength and stability.

How this module is examined

State the formula, then substitute. Moments, mechanical advantage, gear ratio and velocity ratio all reward showing the formula before the numbers.
Get direction right. Meshing gears reverse; an idler restores direction; open belts keep direction, crossed belts reverse it.
Classify members and levers. Identify struts (compression) and ties (tension), and name the order of a lever.
Explain trade-offs. A reduction gains torque but loses speed; mechanical advantage above 1 multiplies force at the cost of distance moved.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

State the principle of moments. (2 marks)
A lever has a load of $400\ \text{N}$ at $0.3\ \text{m}$ from the pivot. An effort acts at $1.2\ \text{m}$ from the pivot on the other side. Calculate the effort needed to balance the load. (2 marks)
A gear train has a driver gear with 20 teeth and a driven gear with 60 teeth. Calculate the gear ratio. (2 marks)
State two functions of an idler gear. (2 marks)
State the difference between a strut and a tie. (2 marks)
A belt drive has a driver pulley of diameter $40\ \text{mm}$ and a driven pulley of diameter $120\ \text{mm}$ . Calculate the velocity ratio. (2 marks)

Solutions

Q1: For a body in equilibrium, the total clockwise moment about a pivot equals the total anticlockwise moment about that same pivot.
Q2: $\text{effort} \times 1.2 = 400 \times 0.3$ , so $\text{effort} = \dfrac{120}{1.2} = 100\ \text{N}$ .
Q3: $\text{gear ratio} = \dfrac{\text{driven teeth}}{\text{driver teeth}} = \dfrac{60}{20} = 3$ (a 3:1 reduction).
Q4: It reverses the direction of the driven gear so it turns the same way as the driver, and it bridges the distance between two shafts, without changing the overall gear ratio.
Q5: A strut is a member in compression (being pushed or squeezed), while a tie is a member in tension (being pulled or stretched).
Q6: $\text{velocity ratio} = \dfrac{\text{driven diameter}}{\text{driver diameter}} = \dfrac{120}{40} = 3$ .

Sources & how we know this

Singapore-Cambridge GCE O-Level Design and Technology (Syllabus 7059) — Singapore Examinations and Assessment Board (2026)