Thread: side by side air bag lifting

04132014, 05:44 PM #1
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side by side air bag lifting
Been arguing this for years  two lifting bags in tandem (not stacked) say a 20 and a 40  is the max the weakest one ? so maxed out at 20  or can you lift 60  I always averaged them  so a 40 plus 20 would max out at 30. But that is completely un scientific .Help ?
?

04152014, 03:50 PM #2
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There really shouldn't be an argument. Its really just simple math with a few considerations.
The short answer is, if you have a 40 ton bag and a 20 ton bag, you can lift 60 tons 1 inch. The max height of the lifting system though, is only as high as the smallest bag. So if the 40 ton bag is 13" and the 20 ton is 6", you can only lift up to 6"
The above answer only applies to when the bags are in tandem.
If the bags are stacked however, people generally like to say that the lifting potential is only as high as the smaller bag. Which in simple terms and no other variables is true, but really it isnt. That rule only applies if you lift the smallest bag its full inflation. Its just a simple means of teaching without the science behind how the lift works.Last edited by BrooklynBravest; 04152014 at 03:58 PM.

04152014, 05:46 PM #3
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04152014, 08:45 PM #4
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If you take a 10" x 20" bag and a 10"x10" bag, and place them next to each other. It is the same as having a 10" x 30" bag.
Not sure if that clarifies anything.
Basically the bags can only apply lifting force when there is full contact on both sides of the bag. That's why a bag can lift it's rated load 1" and half the full distance. Because it's an inverse relation. As the bag gets bigger the lifting potential diminishes due to the fact that the bag pillows into a ball and less of the bag is able to make contact with the ground and load.
The bag exerts 116118 pounds of lift per square inch. Bags are 1 inch thick. So if 10x10" of bag are touching both the ground and load, you can lift 11,600lbs. If you inflate the bag and now only 5x5" of bag are touching the load and ground, you can only lift 2900lbs.
That said let's take an example.
You have a frame rail of a truck to lift. The rail is 6" wide.
You put the 10x20" bag along side the 10x10" bag.
So you have a 30x10" span of lifting surface.
We lay that 6" frame rail across it the 30" span. That leaves a 2" overhang of bag space on the sides because the bag is 10" wide and the rail is 6".
So you have 6x30" of contact with the bag. The area is 180 square inches. 180x116=20,880 lbs.
If the frame rail had been 10" wide or you got creative with your cribbing, you would be able to theoretically lift the full 10x30 span resulting in 34,800lbs of
lift.
Now let's change it slightly. You have a 10x30 bag and a 20x40 bag. Two bags that just don't match up in any direction.
Put them along side and it's all over the place in size. But let's say we put it so it spans the longest way possible. 70inches long. (30+40)
We put that 6" rail across. We have a 7" of overhang per side on the one bag and 2" of overhang on the other. The rail is still only touching 6x70". So the area is 420, lift is 48,720. Because any noncontacting bag space regardless how big the bag is, is wasted entirely.

04192014, 01:55 PM #5
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Hi
That is a good question, I would like to hear the right answer for this.
My thinking is that if you use a 40 t bag with a 20 t bag next to each other, you are limited to the 40t bag, the weight you are trying to lift will be the deciding factor.
If you are trying to lift more than 40 t then the 20 t bag will not be sufficient as it will be overloaded, the 40 t will lift to its max and the 20t will already be overloaded? Is this correct?
if they are the same loading I agree that a 40+40t will apply, but 40 + 20 im not so sure.
Please let me know the math on this one

04192014, 03:26 PM #6
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Read my post that you obviously skipped right over.
I explained all the math. The answer is bags side by side will lift their combined weight.

04192014, 10:19 PM #7
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maybe im what iffing this too much, notice I said tandem , not necessarily "side by side" (close proximity) , does your formula still apply if you a lifting a large truck lets say, and you have to separate them several feet apart ?
?

04192014, 11:55 PM #8
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Your overcomplicating this.
Realistically, it only applies if the bags are next to one another in close proximity.
Once you spread them out theres tons of other forces at work. If one set of bags is lifting the empty bed of a truck, and another is lifting the engine compartment on the opposite end, its not going to be the same because the actual load you are lifting is different.

04202014, 01:18 PM #9
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04202014, 01:40 PM #10
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The lowest bag only creates the limit when the bags are stacked, and only if the smaller bag is inflated to begin with.
In any lift, the lifting force of the system is definite defined by the area of bag space contacting BOTH the ground and load.
A 30x30" bag with a 10" square load on it won't lift any more than a 20x20" bag with the same load. (Not considering fancy cribwork)

04202014, 01:41 PM #11
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The bigger bag will however lift the same load ultimately higher. Because there is less pillowing.

04202014, 01:51 PM #12
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Im pretty clear on that and the limit on stacking  my concern was having to spread the bags out to get an even lift (or just access problems preventing putting them side by side)  especially if the you get to far ahead with the biggest bag , will you load up the smaller?  guess we just need to buy some more bags problem solved.
?

04202014, 03:54 PM #13
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It's good practice to have two of the same bag for a medium sized bag for these situations. This eliminates any potential uneven distribution or question. Also a reason why we usually stack the bags. Realistically most loads aren't 40,000lbs. Even a 70,000lb tower ladder isn't a 70,000lb lift. You are only lifting one portion you may only need 10,000lbs of lift. It's only a 70,000lb lift if you are fully raising the entire vehicle off the ground. It is pretty difficult to encounter something THAT heavy that the bags can't lift it as long as you know the science to maximizing how that bag works. Proper cribbing will often make a lift easier than multiple bags.
As far as spacing the load out, almost never is a load going to be a constant weight. Like a car or truck, one end is massively heavier than the other it's not as clear cut as it is on paper when the stars align.

04202014, 06:28 PM #14
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The lifting ability is defined by the ability of the bag to lift. This is designed into the bag. Yes, the rating decreases as contact decreases. The lighter rated bag can only do what it can do. Doesn't matter if it's stacked or not.
Original question was how to handle lifting with two bags at two separate points. The answer is not averaging out the bags. It is really like two separate lifts. You would have to somehow estimate the weight at each point and go from there. And as has been pointed out, that weight would probably change due to shifting of load.
As Brooklyn pointed out:
Use highest rated bag to maximize contact
Don't over inflate
Use cribbing
We usually only lift a small portion of overall weight of involved object.
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