A rough-terrain crane is a compact mobile crane designed to lift heavy loads on muddy, rocky, sloped, or crowded construction sites. Its oversized tires, short wheelbase, and powerful drivetrain help it move where ordinary truck cranes cannot. The machine matters because many lifts happen before roads, pads, or smooth surfaces are finished.
It combines mobility, lifting power, and stability in one rugged vehicle.
The crane lifts by using a telescopic boom that extends outward and upward, while a hoist line and hook block support the load. Outriggers spread the crane’s support points to create a wider base and reduce the chance of tipping. Operators must consider load weight, boom length, boom angle, ground condition, and the load radius, which is the horizontal distance from the crane’s center of rotation to the load.
Safe lifting depends on keeping the load moment within the crane’s rated capacity.
Key Facts
- Load moment = load weight x load radius
- A larger load radius reduces the maximum safe lifting capacity.
- Outriggers increase the support base and improve stability during lifts.
- Telescopic booms use nested sections that extend to reach higher or farther locations.
- Ground pressure = force / contact area, so larger tires help reduce pressure on soft soil.
- Static equilibrium requires total clockwise torque = total counterclockwise torque.
Vocabulary
- Rough-terrain crane
- A mobile crane with large tires and a compact chassis designed to travel and lift on uneven off-road job sites.
- Telescopic boom
- A crane boom made of nested sections that slide in and out to change the crane’s reach.
- Outrigger
- An extendable support leg that presses against the ground to widen the crane’s base during lifting.
- Load radius
- The horizontal distance from the crane’s center of rotation to the vertical line through the load.
- Hook block
- The pulley and hook assembly that connects the hoist line to the load and can multiply lifting force.
Common Mistakes to Avoid
- Ignoring the load radius, which is wrong because the same weight becomes more likely to tip the crane when it is lifted farther from the center of rotation.
- Assuming big tires make any ground safe, which is wrong because soft soil can still fail if the ground pressure is too high.
- Lifting without fully setting outriggers, which is wrong because the crane’s base is much narrower and less stable on tires alone.
- Using boom length as the only limit, which is wrong because safe capacity also depends on boom angle, load radius, counterweight, and ground conditions.
Practice Questions
- 1 A rough-terrain crane lifts a 6000 kg load at a load radius of 5 m. What is the load moment in kg·m?
- 2 A tire supports 80,000 N of force and has a ground contact area of 0.40 m². What ground pressure does it apply in pascals?
- 3 Explain why a crane may be able to lift a load close to the machine but not the same load when the boom is extended farther outward.