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"Soft" Bag Protection
Airbags are all vulnerable to physical damage or destruction; their surfaces are essentially a rubber material. High heat such as from a catalytic converter can melt an airbag. Sharp, pointed objects such as bolts or brackets can puncture into and through the airbag. The top surface of an airbag that contacts the object to be lifted should be protected from such damage. The bottom of the bottom airbag should likewise be protected. The most appropriate protective covers for airbags are soft and pliable materials; not something rigid or stiff. Old truck mud flaps, sections of rubber conveyor belts, treads from snow machines, or specially designed protective pads called Extrication Guards work well for bag surface protection. Hard, brittle or slippery materials such as plastic cribbing blocks or plywood when placed between the load and the airbag introduce a high probability of slipping out under load; a dangerous potential. Make your bag protection the soft stuff!
A fundamental element of lifting heavy loads with rescue airbags is knowledge of the lifting capacity of the airbags you have in your inventory. Strictly on paper, the airbag rating is square inches of airbag surface multiplied by the pounds per square inch air pressure. A 30-inch-by-30-inch airbag would theoretically have 900 square inches. This number is inaccurate, though, because measuring the outside dimensions of the bag includes the edges which are not part of the usable working surface. Anyway, if you multiply 900 square inches by, let's say, 100 psi, the airbag would theoretically lift 90,000 pounds of weight (45 tons), but that doesn't happen in the real world.
The reality is that the rated capacity of your airbag is achieved only when the entire lower surface of the airbag is in contact with the base or ground and the entire upper surface of the airbag is in contact with the object being lifted. This is almost an impossible situation. We rarely lift something that covers or contacts the entire upper surface of the airbag. In addition, once we start to inflate the airbag, the bottom edges curl and we lose square inches of contact with our base so we never really achieve 100% upper and lower surface contact. We are more likely to be operating at 50% of theoretical capacity or less in actual incidents.
Field experience shows that real-world rescuers place the largest airbag that will fit into position and hope that its real-world lifting force will be sufficient to lift the object, whether that turns out to be 10% of its capacity, 25% of the rating, 50% or more. We need what it takes to get the job done and the bigger bags have a greater chance of giving us that necessary lift. The victims don't care; they just want out.
The lifting capacity of stacked bags is limited by the capacity of the weakest inflated bag of the stack. The 20-ton capacity bag on top of a 30-ton bag will be the weak link; allowing the stack to lift only what the smaller bag will support. If lifting power is important, then a multi-point lift might be preferred over a stacked bag, single-point lift.
Bags Only Lift; Cribbing Stabilizes
It is crucial that rescuers realize that when a heavy object is lifted by one or more airbags, the object is literally floating on air. Without proper cribbing, the lifted load can actually be more unstable now than it was before the airbag lift started. Remember, bags only lift. They do not stabilize! Cribbing or struts are what stabilize the lifted load. Equipment to stabilize the lifted load must be in place before and during the entire lifting process. "Lift an Inch...Crib an Inch" is a common recommendation for crews to adhere to. With box cribs, wedge blocks help take up the small distances until a two-inch by four-inch or four-inch by four-inch cribbing block will fit under the load.