1. #1

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    Jun 2007
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    Default Are there any airbag gurus out there?

    Hey everyone, I've done some searches but haven't come up with anything solid. I've had a few experiences with airbags lately that have me re-evaluating my knowledge of them.
    Please correct me if I'm wrong - I understand that a bag's ability to lift is a function of the pressure in the bag being applied over the surface area to create lift - hence a 20"x20", 118 psi bag theoretically being rated to about 47000 pounds. As the bag inflates, its contact with the object and its base decreases, which is why we lose lifting capacity the higher we go. This leads me to believe that even though a bag may be rated for 47000 pounds this is only true if we're lifting off a spot where the whole bag is in contact with the object. For example if the bag is only in contact with a 6"x6" plate is our maximum capacity really diminished to 4200 pounds? That number seems way too low for me to believe.
    Does the bag's capacity come from it's contact with the base or the object or some more complicated function of it's contact with the base and the object? All I can come up with that makes a little bit of sense is 47200 pounds being exerted downward and 4200 up for a total of 51400 pounds divided by 2 for a capacity of somewhere in the area of 25700 pounds, which sounds much more reasonable to me and jives a lot more with what I've experienced. Am I just completely over-thinking this?
    Thanks a lot!

  2. #2
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    rmoore's Avatar
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    Default

    Lifting airbags such as you are mentioning, the high-pressure kind for example, have their lifting capacity calculated by their internal psi multiplied by the square inches of contact area. A 20 inch x 20 inch airbag has 400 square inches of surface area on each side.

    If inflated to 100psi and all surface area remains in contact with the ground on one side and the object being lifted on the top side, then the rated capacity is calculated at 40,000 lbs or 20 tons.

    If this same bag is again inflated to 100psi but due to its pillow shape, only a very small 6 inch x 6 inch surface area remains in contact with the ground and the object being lifted, then the capacity is reduced to 3,600 lbs.

    Generally, a high-pressure airbag will lift at capacity to only one inch of center height. As a rule of thumb, at full inflation, consider high-pressure airbags reduced to approximately 50% of their maximum rating due to reduced surface area contact.

    It's not your math... it's physics that determines the reality of lifting heavy loads with air bags.
    Ron Moore, Forum Moderator
    www.universityofextrication.com

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