A construction hoist is a temporary elevator used on building sites to move workers, tools, and materials up the side of a structure. It matters because high-rise construction depends on safe, repeated vertical transport that is faster than stairs and more controlled than lifting everything by crane. A rack-and-pinion hoist is especially useful because it climbs on a fixed mast without needing a tall overhead machine room.
Its motion is easy to show with forces, gears, power, and safety systems.
Key Facts
- Weight of the loaded cage: W = mg
- Lifting power for steady motion: P = Fv, where F is lifting force and v is vertical speed
- For steady upward motion at constant speed, motor force must approximately balance weight plus friction: Fmotor = mg + Ffriction
- Rack-and-pinion motion converts gear rotation into straight vertical motion along the mast.
- Gear tooth force creates torque on the pinion: τ = Fr, where r is the pinion radius
- Safety brakes and overspeed governors stop the cage if it moves too fast or loses drive control.
Vocabulary
- Construction hoist
- A temporary elevator system used on construction sites to lift people, materials, or equipment vertically.
- Rack
- A straight toothed rail fixed to the mast that the rotating pinion gear climbs.
- Pinion
- A small gear driven by a motor that meshes with the rack to move the hoist cage up or down.
- Mast
- The vertical tower section attached to the building that guides and supports the hoist.
- Rated load
- The maximum safe weight of people and materials that the hoist is designed to carry.
Common Mistakes to Avoid
- Ignoring the mass of the cage, workers, and materials, which is wrong because the motor must lift the total loaded mass, not just the cargo.
- Treating the hoist like a free-flying crane load, which is wrong because the cage is guided by a mast and driven by gear teeth on a rack.
- Assuming constant speed means no force is needed, which is wrong because the motor must still balance weight and friction even when acceleration is zero.
- Overloading the hoist beyond its rated load, which is wrong because extra weight increases gear force, braking demand, and structural stress beyond safe limits.
Practice Questions
- 1 A construction hoist cage carries 900 kg of workers and materials. If the empty cage has a mass of 700 kg, what is the total weight of the loaded hoist? Use g = 9.8 m/s².
- 2 A loaded hoist has a total mass of 1800 kg and moves upward at a constant speed of 0.60 m/s. Ignoring friction, what power must the motor provide? Use g = 9.8 m/s².
- 3 Explain why a rack-and-pinion hoist can climb the side of a building without a cable pulling it from above, and describe one safety feature that helps prevent a fall.