Crane Stability and Load Charts
Understand the physics of heavy lifting. Learn why a crane's capacity drops as the 'Radius' increases and how center-of-gravity affects tipping.
The Leverage Principle
A crane is essentially a giant lever. On one side, you have the heavy counterweights; on the other, you have the load. The pivot point is the crane's outriggers or tracks. As the load moves further away from the center (increasing the **Working Radius**), the leverage (Moment) increases. If the load's moment exceeds the counterweight's moment, the crane will tip over forward.
Net Capacity Equation
Common Deductions
- Hook Block: Even if nothing is attached, the weight of the massive hook block counts against the crane's capacity.
- Rigging Gear: Shackles, slings, and spreader bars.
- Jib Stowed: A jib tucked against the boom increases weight and reduces capacity even if not in use.
- Line Weight: For very high lifts, the weight of the wire rope itself must be deducted.
Environmental Risks
Crane boom behaves like a giant sail. High winds create lateral forces that can buckle the boom even if the vertical load is light. Similarly, lifting a load from the side (Side Loading) is the leading cause of structural failure in cranes. Cranes must always lift perfectly straight to maintain stability.
Frequently Asked Questions (FAQ)
What is 'Tipping Capacity'?
"Tipping Capacity" is the absolute weight that will cause the crane to lift off its rear outriggers. **Safe Working Load (SWL)** is usually set at $75\%$ or $85\%$ of the tipping capacity. You should never, ever lift based on when the crane starts to "feel light"; always follow the manufacturer's load chart.