In a post a few weeks ago 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.
Click Here for the supplemental page containing the information.
Nice job.
Alright! More math equations to memorize! Im still getting used to those darn hydraulics equations….lol. Anyhow, this is another great tool to keep under that leather lid. Ive had this explained to me before, but this post actually answered the how and why. Not just another instructor saying “Thats just how these things work.” Good job.
Thank you for taking the time to explain this, very informative. Ill keep this in mind.
This is awesome! It’s amazing how a detail like the rated capacity to the actual can be so far apart. I mean there was over 80% loss in effective lifting capacity. That’s something that could really have you scratching your head on scene. Thanks for keeping us informed!
Always learning something new….
I’ve always used the rule of thumb that an air bag will only lift 30%-50% of its rated capacity at maximum height. I also don’t rely on a bag lifting any where near its maximum rated height for the same reasons it will not lift its max rated wt more than an inch.
Not to plu a product, but most air bags are square or slight rectangle. Paratech Maxiforce bags come in a 17X35 size that is essentially two parallel bags. This bag has less crowning by its nature but also takes up cabinet space.
Great topic.
Great info. !!! The best description I’ve seen that explains why you have max. capacity for the first inch of lift only. We always stack two bags to get the most lift possible out of our fist lift.
Good Stuff. Just posted a copy at the station. Thanks.
as long as 1+1=2 i think i get it..
this is the easiest explanation i have ever heard.
thanks guys
mike
Really!!! Jeez guys! Thank you, Jimm. I agree completely. (NOTE: This is a comment about the last post, however Jimm apparently closed out the comment section. Remember a good truckie always finds a way in!)PS Please dont ban me! 🙁
I wrote the max lift for 1″ on the bags in a white paint pen. I always teach the FNG’s that the regulator should be set at 135 and the bags will blow off at 118psi. Also as it has been said before, when you are finished remember to back the regulator spring out so you do not destroy it.
Jeez Jimm, you’re a nerd!! No surprise to me that this info is given. Working with Jimm for a while, this topic is something we talked and trained about when showing new guys the air bags.
Could you crib the bag as you go to keep the bag close to what is being lifted? There by only lifting say 1″ to 3″s at a time. That way you keep the a good surface under the cribing and object being lifted.
Very helpfull , thanks guys!!
Nice job Jimm, I have been teaching heavy lifting and moving for a while and airbags are always the sticking point in the class.. Fill the void, cover the sharps, lift a inch crib a inch and like you said When in doubt use a bigger bag.
Math? Thank you for the warning on the main page… that would have hurt without the warning.
I like writing it on the bag with a white paint pen. That helps to prevent math errors at 3 AM.
Stay safe.
~Egan
ummmmmmmmmmmm carry the 2. OK great article thanks. We also have it wrote on bags in white so we know at a glance.
This is important information and anyone who thinks otherwise should reconsider their career. Knowing and undersanding basic stuff like frictions loss and airbag operations is what seperates firefighters from the public. Any blo-joe can stick a bag and lift, but a true firefighter understands the concept behind his actions. Good info!!
Steve,
You could crib under the bag as you go, but just make sure your cribbing is capable of supporting the load your lifting.
I also like the white paint pen idea. We currently have plywood base pads cut to the size of the individual bags with the info. written on them. It doesn’t wear off, but also doesn’t help if someone grabs the wrong pad.
Anyone else ever wonder why the Paratech didn’t just round up the pressure to 120 psi?
I guess knuckleheads will be knuckleheads. I can’t imagine anybody lifting an object 8″ with a lift bag without cribbing at much smaller increments.
Nice explanation though with adequate visual aids (pictures for the knuckleheads).
Nate999, Ok now the real geek in me comes out… The 118psi pressure is due to the conversion from bar to psi. Remember, Paratech is a European company, and the bags were designed over there… The actual operating pressure in the bag is 8bar, or roughly 118psi (give or take.) That still doesn’t explain not rounding up, but at least we know where the 118 came from. Who would have thought that I could have possible added more math to this post? -Jimm-
Thanks, Jimm. I knew there had to be a reason. Those crazy Europeans, how dare they not comform to our system of measurements. Instead they one-up us by making everything divisible by ten…I guess they don’t like rounding either.
Jimm,
Thanks, I knew something was missing from this post.. MORE MATH was the answer.
On a serious note, thank you for this information and the webpage. Keep it up.
Stay safe.
~Egan
Parateck has information on their web page for height vs. weight in charts that can be placed on the rig or in the case where you keep the regulators. NO MATH at 3 am! Knowing the principles behind the charts is a good idea.
More math, remember that when lifting an object (such as a trailer from an 18 wheeler) that you are not lifting the whole trailer. If the trailer weighs 10,000 lbs and you are lifting the front you can calculate the weight as being around 5000 lbs. You should still go with the biggest bag you have but this can help when you must use bags at two different locations and you have limited bags. Just a little more info.
Another thing to keep in mind . . . when you stack bags (one on top of the other) to minimize the pillow effect your lifting capacity is limited by the smaller bag. For example if you put a 15 ton bag on top of a 30 ton bag your lifting capacity is 15 tons. However, if you put a 15 ton bag immediately adjacent (end to end) to a 30 ton bag you have a lifting capacity of 45 tons. Just some more food for thought . . .
Great info for the airbags. Well laid out and easy to understand. As a rule of thumb I was always taught that the bag can lift its rated capacity 1 inch, and half the rated capacity the entire lift height. For example a 20ton bag can lift 10tons 11 inches. The math in your example does not support this. I am guessing that when under load of an object the surface area is in greater contact due to the weight of the load. Is this an accurate assumption.
And this is why I LOVE this website!!
this website put out by army corps of engineers has some great material for air bag in all applications plus structural collapse info. AND YOU CAN PRINT IT FOR FREE. Has pocket guides sold on other sites for up to $20.
http://www.disasterengineer.org/Library/tabid/57/Default.aspx
this is the page with all the pocket guides. Be safe brothers.
Another important factor is that the true capacity of the lift bag is governed by the surface area of the object that is being lifted. If the bag is 15″ x 15″ and the object is 7″ wide and 3 ft long, the surface area of the object that is in contact with the bag will be 7″ x about 14″(subtracting the 1″ edge around the bag, as previously mentioned). This is the limiting factor in the lift capacity because this is the dimension of the load that is in contact with the bag. Multiplying this surface by the bag pressure will determine the approximate total lift capacity.
In summary, you can place a 73 ton bag (146,000 Lb capacity) under a load, but if the load only contacts 100 square inches of the bag, the capacity of the bag is limited to about 11,800 Lbs. It doesn’t matter what bag you place under that 100 square inch area (lets call it a 10×10 plate), the capacity of any bag will be 11,800. The only difference as you increase the size of the bag under that same object is the height at which the bags will lift the object. Bigger bag = more lift, same capacity.
The advantage of using the bigger bag whenever possible is greater lifting height. Stick with the rule of always using the biggest bag that will fit into the space every time for the best results.
When using a rectangular bag, always use it with the load across the widest dimension of the bag, maximizing the bag contact with the load.
You can use a square bag diagonally to maximize its surface contact with an object that is long and narrow, such as an axle or I-Beam.
Using bags in side by side adds capacity to a load, as long as they are inflated as equally as possible. In theory, if you place 4 73 ton bags next to each other on a load that has full surface contact with each bag, the capacity of the lift is about 292 tons (at about 1-2 inches of height).
All figures are based on a Paratech system operating at 118 PSI. Hope this helps a bit.
Nobody like doing all the math. For a nice general rule of thumb without the complicated numbers just multiply the area of the bag (length x width) times 100. Nice round number and helps you choose a bigger bag faster if it’s close to maximum load.
The Paratech website has a link to download the manual for Paratech bags — http://www.paratech.us/#/downloads/
On page 14 of the pdf, there is a chart with max.height vs load. Kind of small and hard to read, but you could blow it up on a photocopier. Note that the grid on the graph is in metric (but there is in./tons on the scale). I made some photocopies and put them in our airbag box in a plastic sleeve for easy reference.