Thread: High pressure air bag question
07-12-2007, 10:24 AM #1
- Join Date
- Feb 2004
High pressure air bag question
If i put side by side two 40 tons high pressure air bag, how much tons i will be able to lift, 40 tons or 80 tons
07-12-2007, 11:23 AM #2
You can lift the ratings of the bag 2-3 " after that you're losing ground.Best policy is never max a bag.Crib as you go. T.C.
07-12-2007, 05:36 PM #3
- Join Date
- Dec 2003
Simple enough, the total of the lifting force is the sum of both bags. However, as 101 stated, that capacity is usually the initial 2".Developer and Sr. Presenter, Team Xtreme
BIG RIG RESCUE
07-13-2007, 02:52 PM #4
- Join Date
- Feb 2004
so the answer is 80 tons. If i have a 40 and a 20 tons, i will be able to lift 60 tons????
07-13-2007, 05:17 PM #5
Sise by side, 40 tons. But this will max out the 20 ton bag.
07-14-2007, 01:47 PM #6
Bags whenever possible should be matched.IE if your're using two twentys and two forties side by sidemput the forties dowm first with the twebties on top.Not the best plan to put a forty and a twenty side by side.It can be done but you want to be VERY careful. T.C.
07-14-2007, 11:19 PM #7
- Join Date
- Jan 2004
07-15-2007, 08:22 AM #8
If you figure out where to place the 20 ton bag on that same steel so that you only lift 33% of the weight (20 tons) and the other bag lifts the other 66% (40 tons) then it would work. But who figures that out on a scene?
So as to my original post, the max lift per the setup described would be 40 tons, which maxes the 20 ton bag and leaves 10 tons to spare on the 30 ton bag.
If I am wrong, I'd like to see it explained. But this is how I remember it working from my physics classes in college.
07-15-2007, 08:24 AM #9
07-15-2007, 09:33 AM #10
- Join Date
- Dec 2002
- Central NJ
An Uplifting Experience
By Mark Uttley
The call comes in. “Dispatch to all units responding—a man is trapped under a vehicle at a residence.”
Here’s the situation: A backyard mechanic (part of a vanishing breed, because cars are harder to tinker with now) is working under his car, but because he’s in a hurry, he chooses not to put safety stands underneath the car or chock the wheels. This guy figures that because the car is elevated on his homemade ramps, it must be safe to crawl beneath. Or, then again, maybe he doesn’t even stop to think about safety at all. (Let’s face it: Most people think safety is used only on the job and not around the house or garage. Thus the need for us—the “rescue folks”!)
The car suddenly rolls back down the ramps and ends up pinning the guy’s chest—and he stops breathing. (Not that 3,000 pounds of car weight has anything to do with it, of course.)
We arrive on-scene to find our customer non-responsive. His neighbor, meanwhile, is extremely responsive—he’s jumping up and down and screaming at us. At the same time, the guy from next door runs up and kicks the car hack in the feet, calling him a “big dummy.” (Obviously, these guys have spent way too much time watching old Sanford and Son reruns.)
It’s quite a sideshow we’re watching, but we have to push all that nonsense aside. Now it’s time for us to focus on the job at hand. We must react quickly and save the backyard mechanic’s sorry butt.
This scenario is all too familiar to most of us. Unfortunately, it is played out several times a year for many departments, causing them to re-evaluate their ability to perform and assess their available resources.
Benefits of Inflation
So how can we get this vehicle off the customer? What options do we have? I can sum up the answer in three words: pneumatic rescue bags. Air-powered lifting bags work well for this type of job. Some rescue companies carry with them a handy little tool called a floor jack (or a bottle jack) for such emergencies. A “high-lift jack,” which is basically a glorified bumper jack, is capable of lifting 7,000 pounds. That’s obviously more weight than you’ll find in any car on the road. Some rescue companies will carry a “Jimi-Jak,” an air-shore pneumatic vehicle stabilization device.
One incident I responded to several years ago involved a cyclist run over by a tandem gravel hauler. It wasn’t a pretty picture, but the victim turned out to be one lucky lady, considering her knee became wedged between the right front wheel rim and steering linkage and she was dragged about 100 feet. I say she was lucky, because if her leg had not been locked into that position, she would have almost certainly been more severely injured (the dual drive wheels that she was in line with were only about 12 inches from her head as she was dragged down the street.) Unfortunately, the driver of the rig didn’t even know she was there and had to be stopped and notified by a passerby.
We had a few options for mitigating this situation. We could wait for Rescue 2, which was responding from another station, but delayed by a traffic jam. The rescue unit carried high-pressure air bags and undoubtedly could have lifted the semi tractor without any problem. But we needed to act right then, because this girl was definitely not a happy camper. Looking at the minimal resources carried on our pumper, I could see that we really did not have anything sufficient for tackling the job. Talk about frustrating...
However, a short distance away, a backhoe crew was working on the roadway and came over to see what they could do to help. “Thank you for offering,” we told them. “Would you guys have something that could lift the rig?” Boy, did they! The heavy equipment operators had a 20-ton bottle jack which we quickly put to use, lifting the front axle. (Keep in mind, though, that as standard practice, cribbing is placed beside the jack and continually snugged up as the truck is raised, thus eliminating the chance of the rig dropping down on the patient should the bottle jack slip or lose pressure. Blocking, cribbing or shoring are safety techniques that must be used in every lifting operation.)
Once the tractor was lifted high enough for the wheel to be well above the ground, the wheel nuts were removed and the wheel rim was pulled away from the patient’s knee. The girl was in excruciating pain because the rim had cut into the bones around her knee, but she was rescued.
The remarkable part of this story is that none of the fire department vehicles had any tools to remove wheel nuts on such a large truck. If it had been a car, we could have utilized the vehicle’s own tire wrench, but on a big rig, these nuts are torqued anywhere from 250 to 450 foot-pounds. Because removing the wheel was really our only option, it was lucky that the nearby construction workers had a large adjustable wrench. They used that to loosen the wheel nuts, aided by a steel bar and hammer. Sure, it wasn’t the smoothest operation imaginable, but we made do with what was available and got the job done.
If the rescue unit had been able to make good time, we would have changed tack and used the high-pressure air bags for lifting, or we could have utilized the Hurst 32b spreaders to lift the corner of the truck. An even quicker and safer method for accomplishing the task would have been to utilize a heavy recovery unit for lifting. Should you decide to use a tow company with heavy recovery units, you should preplan and do some joint training before going into something like this cold. Insurance must also be in place and the tow cables and equipment need to be regularly tested and maintained so there won’t be any accidents or equipment failure.
Lifting Rescue Air Bags
There are three general types of lifting bag systems that are sold and used for rescue or heavy recovery work: high-pressure, medium-pressure and low-pressure systems.
• Low-pressure bag systems are essentially high-lift bags that operate at 7.25 psi maximum working pressure. These low-pressure cushions provide vertical lift over a large surface area and work especially well on thin-skinned, light-walled vehicles such as aluminum truck trailers, tankers, buses or aircraft. The construction of low-pressure bags utilizes seven-ply strong, reinforced fabric material for the top and bottom surfaces. The internal structure is designed with nylon strapping supports. The cushion itself is constructed of a simple canvas of Kevlar that’s impregnated and bonded to neoprene. The largest low-pressure lifting bags that Holmatro sells are 52 inches square and can lift 16 metric tons while being only 2-1/3 inches thick. Maximum lifting capacity is achieved only when the bag is fully inflated. (A word of caution: These low-pressure air bags are more susceptible to physical damage than high-pressure bags.)
• Medium-pressure bag systems are designed to operate at 15 psi and are not very common. Most tasks can be accomplished with 8 to 12 psi. These bags are designed to function at 15 psi, but register bursting pressures between 58 and 100 psi, depending on the size and style manufactured. Generally, medium-pressure bags have thicker sidewalls than low-pressure bags.
• High-pressure bag systems are without a doubt the most common type of bags found on rigs today. High-pressure air-lifting bags generally operate with inflation pressures of between 90 psi and 145 psi and can be supplied with air from SCBA bottles, 2216- or 4500-psi cylinders, truck or train air brake systems, cascade systems that may be mounted on a rig, or air compressors. And, if you’re really desperate for compressed air, an inflated truck tire could be put to use. With a high-pressure system, there is a direct relationship between lifting capacity and inflation height.
All high-pressure air bags carry ratings for both the maximum force in pounds that they can exert and the maximum height they can achieve when fully inflated. To calculate the theoretical lifting capacity of a high-pressure air bag, use the following formula. (In our example, we will calculate maximum force and maximum height for an air bag which has dimensions of 30 inches by 30 inches and an internal air pressure of 118 psi.)
Length x Width x Internal air pressure of bag
30 x 30 = 900 square inches of working area
900 square inches x 118 psi = 106,200 lb., which works out to a rating of 53 tons of lifting capacity.
The maximum inflation height would be about 18 inches, which would reduce the lifting capacity to 27 or 28 tons. A high-pressure air bag maintains a theoretical 100-percent capacity only until the center is approximately 2 inches in height. Continued inflation diminishes this capacity. Higher inflation, therefore, means lower capacity. Maximum capacity yields one-half maximum capacity.
You may be scratching your head over that last statement, but here’s why it’s so. As the bag continues to expand, the contact surface area begins to diminish, thereby reducing the total lifting capacity to the point that at full inflation, the height of the bag generally has only 50 percent of its original working surface area still in contact with the surface below.
As a high-pressure bag is inflated it tends to take the shape of a football, making it necessary to maintain stabilization on both sides of the bag. If two stacked bags were fully inflated, it would be like trying to balance one football on top of another. Full inflation would increase the risk of the bags kicking out of place, which would pose a serious threat to the safety of the personnel working around them.
A Grab Bag of Examples
In Photo 1, the car’s passenger has been ejected during rollover and the car has landed on top of him. The team of rescuers has placed two high-pressure bags on top of a solid box crib to prevent the car from crashing back down on the patient. Cribbing has been placed beside the patient’s head (out of the picture). As the bags are inflated, the car starts to lift, but—hold the phone!—the team discovers the center of the rear section of the roof is collapsing. The team must reposition the bags to a stronger area of support. But, because the necessary blocking was in place, the car will not fall on the patient when the pressure is released (and the bags are repositioned).
Something to keep in mind when performing a lift with any tools is that if the driver’s side of the car is being lifted, the passenger side must be lowering and it might move farther down and impinge upon a patient who’s in that location. To eliminate this problem, cribbing blocks must be set up on the side of the vehicle opposing the lift to prevent such downward movement.
Photo 2, taken at an extrication competition, shows a team that’s done a nice job of positioning cribbing around a vehicle to stabilize it and prevent any accidental vehicle movements. (Notice the “slick” air cascade cart the competition judge is standing by.) The controller operator maintains an ample distance back from the work area. Two high-pressure bags are stacked. One carries a green air line and another has a yellow line, so the rescuer calling the lift operations can identify which bag to lift and which bag to hold.
When using a two-bag stacked configuration (like that shown in Photo 2), the lifting capacities depend on required inflation heights. The usable height of both bags can be added together to help estimate the total lifting height.
When stacking air bags, always place the largest air bag on the bottom and the smaller air bag on top. Never stack more than two air bags on top of one another and never place anything between the bags or between the top bag and the object you’re lifting, such as a piece of plywood. Why? Because the bags could kick out and the plywood could become a projectile that could injure someone.
Stacking air bags (versus simultaneously lifting an object from two separate locations) does not allow you to add the lifting capacities of the bags together to get an estimate of their lifting capacity. This means that the smallest capacity bag that is inflated is the maximum that the stacked bags will lift. A 10-ton-capacity bag placed on top of a 26-ton-capacity bag will lift only a maximum of 10 tons.
Lifting capacity can be increased using two high-pressure bags side-by-side and simultaneously inflating the bags. In the diagram shown on page 17, two bags have been placed side-by-side on top of support cribs to get the bags as close to the object as possible, in order to maintain maximum contact and lift. One bag is rated for 21 tons of lift and the other is rated for 26 tons. Singularly, the bags cannot lift the 30-ton object, but together, these bags combine their potential lifting power to a hefty 47 tons and will thus be able to lift the object. Keep in mind that cribbing helps to maximize lifting height as well as lifting force.
The controller unit has gauges mounted on it which monitor the incoming pressure and air-bag pressures. All air-bag controllers should have “dead man”-type switches to protect against accidental inflation or deflation and require constant interaction to allow the air to flow. Should the flow-control valve be released, the airflow stops immediately. Once the air bag reaches its maximum inflation, the built-in relief valve will release air at 118 psi.
Photo 3 shows high-pressure air bags in use. In this example of vehicle override (in which the rear car has pinned a person between the cars), high-pressure air bags are used to raise the top car off the person. Air bags are used on both sides of the vehicle to maintain a level lift. A chain is wrapped around the lower car and its suspension to maintain tension and not allow the lower car to lift up when the top car is raised. The air in the rear tires of the lower car is removed to lower the car as well. This shows another style of double-bottle air cascade system in use, feeding air to the air bags, an impact gun and an air chisel set.
Regardless of bag types and sizes, a routine operational check should be done to ensure the safety of everyone involved in their use. Inspect bags for cuts, cracking, nicks or material separation before adding inflation pressure.
It’s really unnecessary to do daily inflation checks, which tend to weaken the equipment. Daily checks actually harm the bladder and wear down the system if a bag is filled to near-capacity level. Most manufacturers recommend a weekly test—inflating the bag to approximately 30 percent of its normal operating pressure. Once inflated, lock the bag off with the air line shut-off and then check for leaks, tears and separations in the bag materials. Leave bags inflated for five minutes and check for any noticeable pressure loss.
Any bubbles that show up in the outer skin of a high-pressure bag can indicate a leak in the interior bladder and outer protective skin. If you find bubbles in your inspection, immediately remove the unit from service and have it inspected by a factory-authorized technician. And make sure you inspect all the hoses and couplings for damage and cleanliness.
Because lifting bags are made of rubber, they have a limited shelf life—the manufacturer should be contacted for a recommendation of service time. It’s advisable to have your air bags inspected annually by a factory-authorized technician and keep records of all testing performed.
Training is the most important aspect of being competent as a rescuer. Air-bag systems probably get used less than spreaders, cutters, rams, hand tools, etc. However, you really should be familiar enough with them so you can put air-bag systems to work without hesitation, even in adverse conditions—say, on a pitch-black road during bad weather.
And, as we always urge you to remember whenever you’re performing rescue work,
B.E. S.A.F.E.R.—Before Everything, Stop And First Evaluate and then Re-evaluate. NF&R
Mark Uttley is the vice chairman of the Transportation Emergency Rescue Committee of the International Association of Fire Chiefs. A qualified captain and 20-year veteran firefighter with the Windsor Fire Rescue Service in Windsor, Ontario, Canada, he teaches transportation rescue to rescuers from around the world and at St. Clair College in Windsor. He has also participated in auto extrication competitions as both a competitor and judge since 1984. He can be reached via e-mail directed to firstname.lastname@example.org.
07-15-2007, 10:06 AM #11
As I sit and contemplate this idea of two air bags of different weights being used side by side, a thought occured.
If you are closely watching the inflation pressure of the bag I suppose it would be possible to take a percentage of the load off the larger bag by using the smaller bag. In essence, this would allow the lift of a heavier object. However, I would be nervous about overloading the smaller bag. I feel that the difference in lift heights from the larger bag to the smaller bag would actually make the smaller bag have to lift higher and thus reduce its capacity just to reach the height of the larger bag which will have less curvature of the surface because of the larger dimensions.
The idea of it from a physics standpoint would be more for a rigid support structure such as a wood block, or steel frame structure as opposed to a dynamic bag.
Did that make sense? I think I confused myself.
07-18-2007, 02:02 PM #12
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- Mar 2007
08-25-2007, 05:08 AM #13
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- Aug 2006
Wow!!! So many answers. Do the math, total surface area contact multiplied by PSI. As with all high pressure airbags, maximum lift is at the 1 inch inflation.
40 ton bag = surface area (27" X 27") X pressure 118 psi X 2 bags = 80 tons
Note - given complete surface contact at the start.
Right or Wrong?
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