A mini crane project shows how simple machines and electric motors can work together to lift a load. In this build, a base supports a vertical tower, a rotating jib arm, a string cable, pulleys, gears, and a small motor. The goal is to lift the greatest safe test weight while keeping the crane balanced and stable.
This project matters because it connects classroom physics to real engineering used in construction, shipping, and robotics.
The crane works by trading force, distance, and speed through pulleys and gears. A fixed pulley changes the direction of the pulling force, while a movable or compound pulley can reduce the force needed to lift the load. The motor provides rotational motion, gears can increase torque, and the jib arm creates torque around the tower or base.
By changing variables such as pulley count, gear ratio, jib length, and load mass, students can test which design lifts best without tipping or breaking.
Key Facts
- Weight is the force of gravity on a mass: W = mg, where g is about 9.8 m/s^2.
- Torque measures turning effect: tau = rF, where r is lever arm distance and F is force.
- A longer jib creates more tipping torque for the same load: tau_load = r_load W.
- Ideal mechanical advantage for a pulley system equals the number of rope segments supporting the load.
- Gear ratio = teeth on driven gear / teeth on driving gear, and a larger gear ratio increases output torque while reducing speed.
- Motor power relates force and lifting speed: P = Fv, ignoring losses from friction and slipping.
Vocabulary
- Jib arm
- The horizontal or angled beam of a crane that reaches outward to hold the lifting cable and load.
- Pulley
- A wheel with a groove that guides a rope or string to change force direction or increase mechanical advantage.
- Mechanical advantage
- The factor by which a machine multiplies the input force applied by a person, motor, or other source.
- Torque
- A turning effect caused by a force acting at a distance from a pivot point.
- Gear ratio
- The ratio that compares the sizes or tooth counts of two gears and determines how speed and torque change.
Common Mistakes to Avoid
- Using only the motor force to judge lifting ability is wrong because the crane can fail by tipping even if the motor is strong enough. Always compare lifting torque with stabilizing torque at the base.
- Counting every pulley as added mechanical advantage is wrong because only rope segments that directly support the moving load reduce the required lifting force. A fixed pulley may only change direction.
- Making the jib longer without adding counterweight or a wider base is wrong because the same load creates more torque as distance from the tower increases. Longer reach usually lowers the safe maximum load.
- Ignoring friction in pulleys, gears, and string is wrong because real systems need more force than ideal formulas predict. Test data should be compared with theory and used to explain efficiency losses.
Practice Questions
- 1 A crane lifts a 0.80 kg test mass at the end of a 0.30 m jib. What is the load torque about the tower? Use g = 9.8 m/s^2.
- 2 A compound pulley has 4 rope segments supporting the load. If the load weighs 24 N and friction is ignored, what input force is needed to lift it slowly?
- 3 A team can choose a short jib with fewer pulleys or a long jib with more pulleys. Explain which design might lift a heavier load safely and what measurements would support your conclusion.