In a post titled “Set It Don't Forget It” there seemed to be some misunderstandings on the capabilities of lift bags that we felt should be cleared up. We apologize in advance for the heavy use of math that is contained in this post.
It’s extremely important to know that lift bags can only lift their capacity for one inch of height. Anything taller than one inch and the bag’s capacity is decreased. It all has to do with the surface area of the bag. The less surface area of the bag is touching the ground and the load, the less the bag can lift. As a bag inflates, it begins to have the pillow effect, therefore decreasing the surface contact, and its capacity.
To figure out the capacity of a bag take the surface area in inches (length x width) and multiply that by the operating pressure of the bag (generally 118psi.) So a 1ft square bag has a capacity of 16,992lbs (12x12x118=16,992) as that 1ft square bag inflates it has less than 1ft square of contact, so the actual capacity is less.
In simple terms, the more a bag is inflated, the less it can lift. If you needed to lift 17,000lbs more than one inch you’ll need a bigger bag.
Take for example the Paratech Maxiforce KPI-12 Lift Bag (bag used for these pictures.) The bag is rated as a 12 ton bag, and has a lifting height of 8”. But as we mentioned before it will not lift 12 tons 8” high. It can only lift 12 tons for 1”. If you need to move 12 tons more than 1” you will need a bigger bag.
The KPI-12 measures 15”x15” so the assumption would be that 15*15*118=26550 (approx 13 tons.) However, we must point out that the 15” X 15” measurements are the outside dimensions of the bag. Because of the way the bag is made there is approximately ½” to ¾” of un-inflatable area around the bag (as shown below.)
So the 15” x 15” bag actually only has a usable surface area of 14.5” x 14.5”. Now by doing the math with the usable area of the bag we can see why the 15” square bag is rated at 12 tons: 14.5*14.5*118=24,809.5lbs (rounded down to 12 tons.)
As you can see in the photo above this is the KPI-12 fully inflated. It has in fact inflated to the height of 8” but let’s look at what the natural pillow effect has done to the surface (contact) area of the bag. Based on our observations we’ll say that the bag has approximately a 6” x 6” area of contact at full inflation. So if we apply our formula to those numbers we get 4228lbs (6x6x118=4228.) So that “12 ton bag” only has the capacity of 4,228lbs or just over 2 tons at full 8” inflation.
It is absolutely imperative that we know the capabilities and limitations of our equipment, so we’ll simply sum it up with this…
When in doubt use a bigger bag.