頂舉袋操作－第２部：頂舉的科學
Jonathan Hall
2012年10月16日
 
當頂舉袋被充氣時，形狀就像枕頭，它會減低與被頂舉物的接觸面積。
 
Jonathan Hall
 
頂舉袋的頂舉能力是一個簡單的物理學。頂舉袋的操作壓力，每平方英寸的磅數(PSI)乘以整個頂舉袋的表面積就等於頂舉力。本文就是來解釋如何計算充氣頂舉袋真正的頂舉力。為手邊的工作選擇適用的頂舉袋就要先得知頂舉袋的頂舉能力。
 
頂舉袋級別／動力曲線
 
每個充氣頂舉袋都有一個最大頂舉能力與最大頂舉高度。最大頂舉能力噸數會列印在頂舉袋上，而且與頂舉袋的表面積有關(參閱第1張照片)。整個系統操作壓力平均分布在頂舉袋整個表面並產生頂舉力。表面積愈大，頂舉力量愈大，頂舉能力也愈大。
 
每個頂舉袋也有一個最大頂舉高度。這是頂舉袋最大充氣的高度。首先要瞭解的是最大頂舉高度並不能產生最大的頂舉能力。這就是眾所皆知動力曲線的概念。動力曲線是頂舉高度與頂舉能力的負對比。頂舉袋頂舉高度增加時，頂舉能力就會下降。這是因為當被舉物被頂舉時表面積就會下降，只有在頂舉袋被充氣至大約1英吋高度時，才能在頂舉袋的中間點取得頂舉袋上面所列的最大頂舉能力。
 
舉重數字
 
每個舉重袋有一個理論頂舉能力與一個真正的頂舉能力。理論頂舉能力是頂舉袋整個尺寸置放在一個堅硬的表面，頂起被舉物的頂舉能力。頂舉袋與被頂舉物真正接觸面積才是頂舉袋真正的頂舉力。使用者必須瞭解兩種頂舉力，理論頂舉力比真正頂舉力要大很多。
 
理論頂舉能力數字公式：
．頂舉袋長×頂舉袋寬×操作壓力PSI=理論頂舉能力磅數
 
理論頂舉能力數字－案例1：
．使用一個20吋×20吋頂舉袋，操作壓力116 PSI
．20吋×20吋×116 PSI=46,400磅(23.2噸)
 
視製造廠與尺寸不同，每個頂舉袋在所有的邊都有一個1/4英寸到3/4英寸車縫。這些車縫會減少頂舉袋實際工作表面從開始的1/2英寸到1 1/2英寸。因此，一個20英寸×20英寸的舉重袋真正的工作表面可能只有19英寸×19英寸。
 
修正的理論頂舉能力數字－案例1：
 
．使用相同的20英寸×20英寸頂舉袋，操作壓力116PSI
．瞭解頂舉袋真正的工作面積為19英寸×19英寸
．19英寸×19英寸×116PSI=41,876磅(20.94噸)
．這個頂舉力是頂舉袋上面的列印頂舉力
 
下一個我們需要考慮的因素是頂舉袋與被頂舉物接觸的表面面積。當頂舉袋被充氣時會呈現枕頭狀，這會減少與被頂舉物接觸的表面積(照片2)使用者必須計算頂舉袋與被頂舉物接觸的表面積來取得頂舉袋真正的頂舉力。
 
真正頂舉力數字公式
 
真正長度×真正寬度×頂舉袋與被頂舉物接觸百分比×操作壓力PSI=真正頂舉力磅數
 
真正頂舉力數字－案例1：
 
．使用相同20英寸×20英寸頂舉袋，操作壓力116PSI
．瞭解頂舉袋真正的工作面積為19英寸×19英寸
．頂舉袋與被頂舉物的接觸百分比為75%
．19英寸×19英寸×75%×116PSI=31,407磅(15.7噸)
 
真正頂舉力數字－案例2
 
．與案例1相同，但頂舉袋與被頂舉物接觸百分比只有25%
．19英寸×19英寸×25%×116PSI=10,469磅(5.23噸)
 
因此修正的理論頂舉力(31,407磅)與只有25%接觸百分比的真正頂舉力(10,469磅)，有相當大的差距20,938磅。
使用者必須認知頂舉袋呈現枕頭狀與認知頂舉袋上列印的頂舉力與頂舉袋真正的頂舉力有相當大的差距。
堆疊頂舉袋對比並排頂舉袋
 
當在正常狀況下使用兩個頂舉袋，小的頂舉袋會放在大的頂舉袋上(照片3)。這樣做的原因是確保有足夠的頂舉高度來頂舉被頂舉物。當兩個頂舉袋堆疊時，最大頂舉力為小頂舉袋的最大頂舉力。頂舉袋的頂舉力並不是兩個頂舉袋相加。因此一個10噸頂舉力置放在20噸的頂舉袋，這個系統的最大理論頂舉力只有10噸。當然這個系統的真正頂舉力要視頂舉袋與被頂舉物的接觸表面積而定。
 
兩個頂舉袋並排，而且同時充氣時，兩個頂舉袋的頂舉力才能相加。兩個頂舉袋並排可以增加頂舉袋與被頂舉物的接觸表面積。
 
結論
 
非常重要的是，使用者必須瞭解充氣頂舉袋的頂舉力，明瞭理論與實際頂舉力的差距來確保系統不會失敗。如此才能快速決定使用何種頂舉袋來確保一個安全有效的救援操作。
Air Bag Operations – Part 2: The Science of Lifting
Jonathan Hall
Oct 16th, 2012
 
Photo 2. When the airbag is inflated it takes on a pillow shape, which reduces the surface area in contact with the object.
Jonathan Hall
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Air bag lifting capacity is based on simple physics. The operating pressure of the bag, in pounds per square inch (PSI), is multiplied over the entire surface area of the bag to equal lifting force. This article aims to explain how to calculate the air bag’s actual lifting capacity. It is essential that members have a firm grasp of lifting capacities to ensure that they select the proper bags for the task at hand.
Bag Ratings/Power Curve
Each air bag has a maximum lift capacity, as well as a maximum lift height. The maximum lift capacity is listed on the bag in tons, and it is based on the surface area of the bag (see Photo 1). The system’s operating pressure is spread equally across the bag’s entire surface and creates lift force. The greater the surface area, the greater the lift force, which results in increased lift capacity.
Each air bag also has a maximum lift height. This is the height of the bag at maximum inflation. It is important to realize that the maximum lift capacity cannot be achieved at the maximum lift height. This is the concept known as the power curve. The power curve is the negative correlation between lift height and lift capacity. The lifting capacity of the air bag decreases as the lifting height increases. This is due to less surface area being in contact with the object being lifted. The air bag’s maximum lift capacity listed on the bag can only be achieved until the center of the bag is inflated approximately one-inch.
Lift Capacity Math
Each air bag has a theoretical lift capacity and an actual lift capacity. The theoretical capacity is what the bag should be capable of lifting if the entire dimensions of the bag are in contact with a solid surface below and with the object to be lifted. The actual lift capacity is what the bag will be able to lift given the surface area of the bag that is actually in contact with the object being lifted. It is critical that members understand the difference between the two capacities, as the theoretical lift capacity can be significantly more than what the bag will actually lift.
Theoretical Lift Capacity Math Formula:

• Air Bag Length x Air Bag Width x Operating Pressure in PSI = Theoretical Lift Capacity in Pounds

Theoretical Lift Capacity Math – Example 1:

• Utilizing a 20-inch x 20-inch bag operating at 116 PSI
• 20-inch x 20-inch x 116 PSI = 46,400 pounds (23.2 tons)

Each air bag has approximately ¼-inch to ¾-inch of seams on all sides, depending on the bag size and the manufacturer. These seams reduce the actual working surface of the air bag by ½-inch to 1½-inch from the start. Therefore, a 20-inch x 20-inch bag may have an actual working surface area of 19 inches x 19 inches.
Modified Theoretical Lift Capacity Math – Example 1:

• Utilizing the same 20-inch x 20-inch bag operating at 116 PSI
• Realizing that the bag has an actual working surface of 19 inches x 19 inches
• 19 inches x 19 inches x 116 PSI = 41,876 pounds (20.94 tons)
• This is the capacity that is normally listed on the air bag

The next factor that we need to take into account is the surface area of the bag that is in contact with the object being lifted. The air bag takes on a pillow shape as it is inflated, which reduces the surface area in contact with the object (Photo 2). Members need to calculate the surface area that is in contact with the object in order to get the actual lift capacity of the bag.
Actual Lift Capacity Math Formulas:

• Actual Length x Actual Width x Percent of Bag in Contact with Object x Operation Pressure in PSI = Actual Lift Capacity in Pounds

Actual Lift Capacity Math – Example 1:

• Utilizing the same 20-inch by 20-inch bag operating at 116 PSI
• Realizing the actual working area of the bag is 19 inches x 19 inches
• With 75% of the bag in contact with the object being lifted
• 19 inches x 19 inches x .75 x 116 PSI = 31,407 pounds (15.7 tons)

Actual Lift Capacity Math – Example 2:

• Same scenario as Example 1, but with only 25% of the bag in contact with the object being lifted
• 19 inches x 19 inches x .25 x 116 PSI = 10,469 pounds (5.23 tons)

Notice the significant difference of 20,938 pounds between the modified theoretical lift capacity (31,407 pounds) and the actual lift capacity (10,469 pounds) when only 25% of the bag is making contact. Members need to recognize the pillowing of the air bag and realize the considerable difference between the stamped-rated capacity on the bag and what the air bag is actually capable of lifting.
Stacked Bags vs. Side-by-Side Bags
Under normal operating conditions two air bags, one smaller than the other, will be stacked on top of one another (Photo 3). This is done to ensure there will be enough lift height to achieve the objective. When two bags are stacked, the maximum lift capacity is that of the smaller bag. The capacities of the air bags are not added together. Therefore, if a 10-ton bag is placed on top of a 20-ton bag, the maximum theoretical capacity of the system is 10 tons. Of course the actual lift capacity of the system depends on the surface area in contact with the object to be lifted.
The only time the capacities of the two bags are added together is when they are placed side-by-side and inflated simultaneously. The bags being placed side-by-side increase the surface area in contact with the object to be lifted. Much more on bag placement will be covered in future articles.
Conclusion
It is necessary that members understand the load capacities of the air bag system. It is critical to be aware of the difference between the theoretical and actual lifting capacities to ensure that system failure does not occur. Quickly determining which air bags to use is essential for ensuring a safe and effective rescue operation.
For the full series of Air Bag Operations