Why use machines?
A person with a spade moves about 0.01 m³ of soil per scoop. An excavator bucket holds about 1 m³, so one scoop does the work of about 100 spadefuls. Machines also lift loads no person can lift. They save time, effort and cost, and they reduce injuries, if they are used safely.
Earthmoving machines
- Excavator: arm and bucket on a rotating body. Digs pits, trenches and loads trucks.
- Bulldozer: a wide front blade on crawler tracks. Pushes soil and levels ground.
- Loader: a wheeled machine with a front bucket. Scoops and loads loose material.
- Dump truck: carries soil, stones and debris and tips them out.
- Grader: a long blade between the wheels that makes a smooth, sloped surface.
- Roller (compactor): a heavy drum that presses soil, gravel or asphalt so it becomes dense and strong.
Lifting machines
A tower crane has a tall mast and a long horizontal jib. A mobile crane drives to the site. A hoist lifts materials or people up a building. A crane can lift less when the load is farther from the mast, because the load times its distance (the turning effect) must stay under a limit. So capacity = limit ÷ radius. Operators use a load chart, a trained signaller and a clear zone below.
Concrete machines
The concrete mixer turns a drum to blend cement, sand, stone and water evenly. A transit mixer truck keeps it turning on the road. A concrete pump pushes concrete through a pipe to high or far places. A vibrator (a poker pushed into the wet concrete) shakes out air bubbles so the concrete is dense and strong. Skipping the vibrator leaves honeycomb holes.
Hand tools, power tools and measuring tools
- Hand tools: trowel (spreads mortar), hammer, spade, saw, chisel, wrench.
- Power tools: drill, grinder, circular saw, jack-hammer. They need guards and eye protection.
- Measuring and checking tools: tape for length, spirit level (bubble in the middle means level), plumb bob (a weight on a string that hangs exactly vertical), set square (right angles), laser level and total station for large layouts.
Safe use of machines
Before starting any machine: trained operator, clear zone around it, machine checked (brakes, hoses, ropes, guards), and a signal person if the operator cannot see. Never stand under a lifted load. Never carry people on a machine not made for it. Switch off and lock before repair.
Try it
Make a plumb bob: tie a nut to a string and hang it from a nail. Check a door frame: the string should run parallel to it. Then check a table with a phone level app or a bottle of water: tilt it and see how the water level stays flat.
Key formulas and definitions
- Crane capacity ≈ limit ÷ radius (in this lesson 120 t·m ÷ radius)
- Number of scoops = volume ÷ bucket volume
- Number of truck trips = volume ÷ truck volume
- Spirit level: bubble in the middle = level
Worked examples
1. A pit has 100 m³ of soil. The excavator bucket is 1 m³. How many scoops?
100 ÷ 1 = 100 scoops.
2. The soil must be taken away by trucks that carry 10 m³ each. How many trips?
100 ÷ 10 = 10 trips.
3. A crane has a limit of 120 t·m. At a radius of 6 m, what load can it lift?
120 ÷ 6 = 20 t.
4. Can the same crane lift 20 t at a radius of 10 m?
At 10 m the capacity is 120 ÷ 10 = 12 t. 20 t is too heavy: bring the load closer or use a bigger crane.
Common mistakes
- Thinking a crane lifts the same load at every radius. Capacity falls as the load goes farther out.
- Skipping the vibrator because the concrete "looks full". Trapped air weakens it.
- Mixing up excavator and bulldozer. The excavator digs with a bucket; the bulldozer pushes with a blade.
- Standing or walking under a lifted load.