Construction machinery
Each machine has one main job.
- Excavator: digs and loads. Its arm has a boom, a stick and a bucket moved by hydraulic cylinders (oil pushes a piston).
- Bulldozer: pushes and levels soil with a wide blade.
- Dump truck: carries soil and stone.
- Roller: compacts soil and asphalt with heavy drums.
- Crane: lifts heavy loads up and across.
- Concrete mixer and pump: make and deliver concrete.
Choosing a machine depends on the job size, soil type, space and cost per hour.
How a crane stays balanced
A crane is a lever. The turning effect of a force is its moment = weight × distance from the tower. The load moment must be smaller than the counterweight moment, with a safety margin. If load × distance is bigger, the crane tips forward. That is why a crane can lift less when the load is far out. Operators always read the load chart before lifting.
Hydraulics work the same way in many machines: a small push on oil makes a large force in a big cylinder.
Construction electrical facilities
A site needs temporary electricity for tools, lights and pumps. It comes from the grid through a meter, or from a generator. Power goes to a distribution board, which feeds separate circuits. Each circuit has a circuit breaker that switches off if the current is too high.
Current in amps is power ÷ voltage (I = P ÷ V). Add the currents of all tools on one circuit. If the total goes past the breaker rating, it trips.
Electrical safety on site
Sites are wet and dusty, so electric shock is a real danger. Rules: use an earth wire (a safe path to the ground if a fault happens), a residual current device (RCD) that cuts power in a split second if current leaks, cables lifted off wet ground, plugs and sockets made for outdoor use, and checks by a trained electrician. Never repair live wires. Keep cranes and long poles away from overhead power lines.
Try it: add up the amps
Look at the labels on three home appliances (they show watts). Divide watts by 230 V to get amps. Add them up. Would they run safely together on a 16 A line? Check your answer with the breaker in the 3D.
Key formulas and definitions
- Moment = weight × distance
- Crane safe if load moment < counterweight moment
- I = P ÷ V (amps = watts ÷ volts)
- Total current = sum of all tool currents
- Breaker trips if total current > its rating
Worked examples
1. A crane has a counterweight of 4 t at 3 m. What is the largest load it can lift at 3 m?
Counter moment = 4 × 3 = 12 t·m. At 3 m, load × 3 must be less than 12, so the load must be below 4 t.
2. The same crane lifts a load at 6 m. What is the largest load now?
Load × 6 < 12, so load < 2 t. Far out, it can lift only half as much.
3. A drill uses 1150 W on a 230 V supply. What current does it draw?
I = P ÷ V = 1150 ÷ 230 = 5 A.
4. A circuit has a 16 A breaker. A 5 A drill and an 8 A mixer are on. Can a 6 A grinder also be switched on?
5 + 8 = 13 A. Adding 6 gives 19 A, which is above 16 A, so the breaker would trip. No.
5. A roller makes a 20 cm layer into a 14 cm layer. By what percentage did the thickness fall?
Fall = 6 cm. 6 ÷ 20 × 100 = 30 %.
6. A site needs 3 lamps of 100 W each on 230 V. Find the total current.
Total power = 300 W. I = 300 ÷ 230 ≈ 1.3 A.
Common mistakes
- Looking only at the load weight and forgetting the distance. Moment = weight × distance.
- Adding too many tools to one circuit. Add up the amps first.
- Using indoor sockets or damaged cables outdoors in the rain.
- Removing the earth wire or bypassing the breaker "because it keeps tripping". The trip is a warning.