Since this is factored in, my reference will always be heavy vehicle. I was asked to submit a proposal to do a class on heavy vehicle rescue at the South Carolina Fire Rescue Conference next year and if they go for it, I hope to see you there. There will be a limited number of students for the class, so register early if the class makes it.
Airbags for Lifting
We are going to look at the use of airbags for lifting in this article. But first I think we need to understand the physics behind airbags. Airbags are always advertised at the maximum lifting capability of each individual airbag, whether they are high pressure or low/medium pressure bags. First, the larger the bag the more it will lift. To understand this theory of lifting, let’s look at how this works. All bags lift on the formula of inlet pressure times the surface area of the bag. This means that we take the air pressure going into the bag and multiply it times the surface area. So, to keep this simple, we will look at a 10-inch by 10-inch bag, and an inlet pressure of 100 psi. The bag has 100 square inches of surface and it is being inflated with 100 psi of pressure. If we take 100 square inches, times 100 psi of pressure, we come up with 10,000 pounds of lifting capacity.
All airbag manufacturers can advertise their bags as having the capability to lift that amount of weight, based on size and pressure. Now let’s look at the hard facts. Capability is only good as long as the bag maintains that surface area. We all know as air bags increase with inlet pressure, the surface area decreases. This means that the lifting ability now goes down. The bag will lift that 10,000 pounds for the first inch or two at maximum.
So, if our airbag that is “rated” at 10,000 pounds of lift now has a surface area of say five inches, we have a total area of 25 square inches. We factor in the inlet pressure of 100 psi times the 25 inches, and we now know that the bag’s capability is reduced to 2500 pounds of lift, which is a quarter of what it started out to be. To give you a hard example, a square pillow bag that is rated at 70 tons, and a lifting height of 12 to 15 inches will be reduced to less than 4000 pounds at 12 to 15 inches, and you will have lost over 50 percent of its capability. The only way to overcome that is to increase the inlet pressure and airbag size.
Some companies have done a little of this, mainly by increasing the inlet pressure to increase the lift. However, to keep the bags safe and at a size that doesn’t become too large to work with, only minimal gains are accomplished. The other consideration when using square pillow bags is that when you stack two bags to gain in height, you lose the stability of the bags. The bags tend to rock on each other as their contact area decreases. There are some bags that do maintain a constant flat surface and lock together for better stability.
A perfect example of the loss of lifting of an airbag occurred when I was teaching a farm rescue class, and the evolution was to lift a combine. The department had a medium size pillow bag and was working on the lift with that bag. The patient was a hose dummy under the front wheel, which carried a considerable amount of weight. The bag was placed under the frame toward the front of the combine, the cribbing was set and the lift began. The tractor went up about two inches and then would not go up any further. They looked over and asked what was wrong with their airbag, assuming that there was a problem with the bag itself. Being that the bag was about a 20 ton bag, and the combine did weigh less than that, I could see why they were thinking what they were. What they failed to realize was the airbag lost almost half of its lift capacity after a little over two inches of lift. The front of the combine exceeded that amount. Their solution was going to be to lift several inches, crib, then lift several more, until they raised the tractor enough to get my hose buddy out. This is exactly what they had to do.
Next issue we will look more into airbags and discuss low and medium pressure bags as well. Until then, train hard; be safe, because we are training today for a safer tomorrow.