A crane jib is the long arm that reaches out from a crane to place heavy loads where workers need them. It acts like a beam and lever at the same time, giving the crane horizontal reach but also creating large turning effects on the crane structure. Understanding the jib matters because a crane can only lift safely when the load, radius, and boom or jib angle stay within its rated limits.
On a construction site, the jib is often the most visible part of the lifting system and the part most directly linked to reach and capacity.
Key Facts
- Load moment = load weight x lifting radius, or M = W x r.
- As lifting radius r increases, the maximum safe load usually decreases.
- A fixed jib stays at a set angle and gives extra reach beyond the main boom or tower crane mast.
- A luffing jib can raise or lower its angle, changing radius without moving the crane base.
- For rotational balance, clockwise moment and counterclockwise moment must be controlled: Wload x rload <= Wcounter x rcounter plus structural limits.
- The hook load includes the lifted object plus rigging, spreader bars, hook block, and any lifting attachments.
Vocabulary
- Jib
- A jib is the projecting arm of a crane that supports the hoist line and positions the load away from the crane body.
- Load radius
- Load radius is the horizontal distance from the crane center of rotation to the load's center of gravity.
- Luffing jib
- A luffing jib is a crane arm that pivots up and down to change reach while keeping the crane in place.
- Fixed jib
- A fixed jib is a crane extension held at a constant angle to increase reach or height for certain lifts.
- Load chart
- A load chart is the manufacturer provided table that tells the maximum allowed lift for a given radius, jib length, and configuration.
Common Mistakes to Avoid
- Using the load weight alone to judge safety. This is wrong because crane stability depends strongly on load moment, so the same load becomes more dangerous as radius increases.
- Measuring radius along the slanted jib. This is wrong because load radius is the horizontal distance from the crane's rotation center to the load, not the length of the jib or boom.
- Ignoring rigging and hook block weight. This is wrong because every item hanging from the crane adds to the total hook load used in the load chart.
- Assuming a longer jib always means a stronger lift. This is wrong because a longer reach usually lowers capacity by increasing bending forces and overturning moment.
Practice Questions
- 1 A crane lifts a 4,000 kg load at a horizontal radius of 12 m. Calculate the load moment in kg m.
- 2 A crane configuration allows a maximum load moment of 60,000 kg m. What is the maximum load it can lift at a radius of 15 m, ignoring rigging weight?
- 3 A site has two possible crane setups: a fixed jib with long horizontal reach and a luffing jib that can raise steeply. Explain which setup is better for a crowded city site with nearby buildings and why.