A spider crane is a compact lifting machine designed to move heavy loads in places where a full size crane cannot fit. It uses crawler tracks to travel through tight doors, corridors, and indoor construction zones. When it is ready to lift, four long outriggers spread outward like spider legs to create a wide, stable base.
This makes spider cranes useful for installing steel beams, glass panels, machinery, and roof components inside partially finished buildings.
The main physics idea is balance of torque around the crane's support points. The load creates a tipping moment, while the crane's weight and outrigger reaction forces create stabilizing moments. Operators must consider boom angle, load radius, floor strength, and the rated load chart before every lift.
Even though the machine is small, it can safely lift large loads when the outriggers are properly placed and the load stays within the rated capacity.
Key Facts
- Torque is the turning effect of a force: τ = Fd, where d is the perpendicular distance from the pivot.
- A crane is stable when the stabilizing moment is greater than the tipping moment.
- Load moment is often estimated by M = W × r, where W is load weight and r is horizontal load radius.
- Increasing the boom radius lowers the safe lifting capacity because the same load creates more tipping torque.
- Outriggers spread the support points outward, increasing the base of stability and reducing the chance of tipping.
- Pressure on the floor can be estimated by P = F/A, so outrigger pads reduce pressure by increasing contact area.
Vocabulary
- Spider crane
- A compact mobile crane with crawler tracks and four extendable outriggers used for lifting in tight spaces.
- Outrigger
- A support leg that extends away from the crane to widen its base and improve stability during lifting.
- Load radius
- The horizontal distance from the crane's rotation center to the vertical line through the lifted load.
- Tipping moment
- The torque caused by a load that tends to rotate the crane toward overturning.
- Rated capacity
- The maximum load a crane can safely lift under specific conditions such as boom length, boom angle, and load radius.
Common Mistakes to Avoid
- Ignoring the load radius, which is wrong because a farther load creates a larger tipping moment even if its weight stays the same.
- Lifting before fully setting the outriggers, which is wrong because the crane may not have a wide enough support base to resist tipping.
- Assuming a strong crane means the floor is safe, which is wrong because indoor floors can fail if outrigger pressure is too high.
- Using the same capacity for every boom angle, which is wrong because crane capacity changes with boom length, boom angle, and horizontal reach.
Practice Questions
- 1 A spider crane lifts a 1200 kg steel beam at a load radius of 2.5 m. Using W = mg with g = 9.8 m/s², calculate the load moment in N m.
- 2 An outrigger carries a downward force of 18,000 N. If its pad area is 0.30 m², calculate the pressure on the floor using P = F/A.
- 3 A spider crane can lift a beam safely at a 1.8 m load radius, but the operator wants to move the same beam to a 3.2 m radius. Explain why the lift may become unsafe even though the beam's weight has not changed.