A crane hoist is the lifting system that raises and lowers the hook so heavy materials can be placed safely and accurately. It turns electrical or hydraulic power into controlled motion through a motor, gearbox, winch drum, wire rope, brake, and hook block. Hoists matter because a small error in load, speed, or braking can create very large forces in the rope and structure.
Understanding the hoist helps explain how cranes lift tons of material with precision on busy construction sites.
The motor supplies torque, the gearbox changes speed and torque, and the winch drum reels wire rope in or out. As the drum rotates, the rope either wraps onto the drum to lift the hook or unwraps to lower it. The brake holds the load when the motor stops and helps control motion during lowering.
A hook block may use multiple rope segments, called parts of line, to increase lifting capacity by sharing the load across several rope sections.
Key Facts
- Torque is a twisting effect: τ = F r, where F is force and r is drum radius.
- For an ideal single rope on a drum, lifting force is related to drum torque by F = τ / r.
- Mechanical power can be calculated as P = F v, where v is lifting speed.
- Load weight is W = mg, where m is mass and g ≈ 9.8 m/s^2.
- In an ideal block system with n supporting rope parts, rope tension is T = W / n.
- A gearbox trades speed for torque, so reducing drum speed usually increases available lifting torque.
Vocabulary
- Hoist
- A machine that raises and lowers a load using a powered drum, rope or chain, and a hook.
- Winch drum
- A rotating cylinder that winds and unwinds wire rope to move the hook up or down.
- Wire rope
- A strong flexible cable made from many steel wires twisted into strands around a core.
- Gearbox
- A set of gears that changes the motor's speed and torque before the motion reaches the drum.
- Hook block
- The pulley and hook assembly that supports the load and may divide the load among several rope segments.
Common Mistakes to Avoid
- Ignoring the hook block arrangement is wrong because multiple rope parts can reduce the tension in each rope segment while also changing the hook speed.
- Using mass as weight is wrong because the load force is W = mg, so kilograms must be converted to newtons before force calculations.
- Assuming the brake only stops the load is wrong because it also holds a suspended load and helps control lowering under load.
- Forgetting the drum radius is wrong because the same torque produces less lifting force on a larger drum according to F = τ / r.
Practice Questions
- 1 A hoist lifts a 1200 kg load straight upward. What is the load's weight in newtons using g = 9.8 m/s^2?
- 2 A winch drum has a radius of 0.25 m and applies 6000 N of rope tension. What torque must the drum provide, ignoring losses?
- 3 A crane uses a hook block with four supporting rope parts instead of one. Explain how this changes rope tension, hook speed, and the amount of rope the drum must wind for the same lift height.