Lifting and Jacking Systems, Rollers, and Skidding
Core concepts
When a crane can't reach, riggers jack, roll, and skid. Candidates must understand: hydraulic jack capacity and tonnage per point, cribbing and load spreading, the difference between rolling (low friction, but dynamic) and skidding (higher friction, controlled), and the use of rollers, skates, and slide plates. Also: center-of-gravity control during jacking, the danger of jacking one point at a time without load sharing, and the need for a jack-and-hold plan with secondary support.
Traps in questions
- Assuming a 100-ton jack can lift 100 tons at any point - capacity is per jack, and load sharing matters.
- Forgetting that cribbing must spread load to the ground bearing capacity.
- Rolling loads can accelerate; skidding is more controlled but requires more force.
- Jacking without secondary support (blocks, stands) - never rely on the jack alone.
Rule of thumb
Jack capacity must exceed the calculated point load with margin; always place secondary support before working under a load. Ground bearing pressure = load ÷ cribbing contact area.
Worked example
A 40,000 lb machine on four jacks: nominal 10,000 lb per jack, but the CG is off-center, so one corner may see 15,000 lb. Select jacks rated well above 15,000 lb, crib to spread the load, and set blocks before reaching under.