How is the vacuum suction cup calculated?

- Jul 28, 2019-

After reading the technical article on the working principle of vacuum suction cups, you must understand that the vacuum suction of the suction cup is actually caused by the fact that the atmospheric pressure on the opposite sides of the object is no longer balanced after the pair of atmospheric pressures are broken. So how is this vacuum suction calculated?

Take a nap, let's start with a concept that is easy to confuse!




Popular science 1, what we usually call atmospheric pressure units, whether it is "bar", "Pa (Pascal)", or "Hg (mercury)", etc. are actually pressure units, so it should be It is called atmospheric pressure, and should not be called atmospheric pressure, because it is not a unit of force, and the unit of force is N (cattle)! Remember the formula for calculating the pressure and force between our junior high school physics, F=PS, F is pressure, P is pressure, S is the area, that is, the pressure multiplied by the area of action is the pressure! The unit of pressure is N, and the unit of area is m2, so the unit of P is N/m2, and N/m2 is Pa!


* Popular Science 2, we often say that our environment is normal temperature and pressure, normal pressure refers to a standard atmospheric pressure, or 1bar, or 105Pa, we also usually call our normal pressure pressure 1 kg. Wait, here is not right, kilograms is a quality unit. So called 1 kilogram force? This has become atmospheric pressure, and it is not what we call atmospheric pressure. Where is the problem? Think about the pressure formula we just had in science. In the formula we have atmospheric pressure (1 kg force), and we know exactly the atmospheric pressure under normal pressure (1 standard atmospheric pressure), then switch between pressure and pressure. What are we missing when we are? correct! Missing area! So here, the conclusion comes out, if you don't know it, just go back! "Under normal pressure, the atmospheric pressure per square centimeter is 1 kilogram force!" Remember? This area that is always forgotten by us refers to square centimeters! Later, we have to use it repeatedly when calculating the suction force of the vacuum chuck, but don't forget it!


For example, if we are using a circular suction cup with a diameter of 300mm, what is the maximum vacuum suction that can theoretically produce? Maximum suction? Then we need to pump all the air inside this suction cup! Is that 100% vacuum? is it possible? (To understand, we are talking about theory, but in fact the high vacuum can only be 99.9999...%, infinitely close to 100%, and never reach it!) Ok, then we will follow In theory, 100% vacuum will continue! I changed the line and let you see more clearly!


The top surface of the object is subjected to atmospheric pressure calculations. Since the air in the suction cup is completely exhausted, there is no air, and the atmospheric pressure becomes 0. Then think about the calculation formula of our F=PS pressure. Use this 0 pressure and multiply the area of the suction cup. Is the suction cup because of the current atmospheric pressure in the area covered by the suction cup after being vacuumed! What is the value now? It is 0, it is right!


The bottom surface of the object is calculated by atmospheric pressure. There is no suction cup under the object, so the atmospheric pressure received by the bottom surface does not change. It is still a standard atmospheric pressure, but it exists in pairs according to atmospheric pressure, and the balance is offset. Therefore, the atmospheric pressure in the suction cup on the top surface of the object changes. For the existence, the law of balance offset is broken! Then what is the atmospheric pressure of the bottom surface that cannot be balanced? Still use the formula F = PS to calculate, P is still a standard atmospheric pressure, then S? Yes, it is the area of the sucker! Because the atmospheric pressure on the top surface of the object only covers the area covered by the suction cup space, only the atmospheric pressure of this part of the area and the bottom surface of the object is no longer balanced, and the atmospheric pressure outside the suction cup space can still be a standard atmospheric pressure! Ok, then let's use the 1 standard atmospheric pressure on the bottom to multiply the area of the suction cup. The circular suction cup with a diameter of 300mm has an area of 70,650 mm2, and is converted into square centimeters. The unit is 706.5 cm2. Why should it be converted into square centimeters? Remember the conclusion of our previous Science 2? “The atmospheric pressure per square centimeter of area is 1 kilogram force!” Knowing the area of the suction cup is how many square centimeters, of course, knowing how much kilogram force the atmospheric pressure on the suction cup area is! Well, the calculation results came out. The 706.5cm2 suction cup area is theoretically 100% vacuum, and the atmospheric pressure is 706.5 kg force! If it is 50% vacuum in reality? Ok, it’s ok to make a fold! Simple? This is not even the trapezoidal suction cup you will calculate its suction.


To conclude, the downward atmospheric pressure inside the suction cup on the top surface of the object is 0, and the upward atmospheric pressure on the bottom surface of the object is 706.5 kg force. The difference between the two is that the object is finally subjected to an upward force of 706.5 kg. That is to say, the suction force of the 300mm diameter suction cup can provide 706.5kg force under the condition of 100% vacuum. No, the suction force of the suction cup is a difference of atmospheric pressure, which is a very very important conclusion. Let me remember it!


Then according to T=G, we can know that this 300mm diameter suction cup can theoretically suck up a 706.5kg object! But is this feasible in practical applications? is it safe? Of course not! Because this is both theoretical and critical, if the object is subjected to any external force that causes it to fall under this critical state, or if the object is moving at a certain acceleration rather than at a constant speed, then there is a suction cup. The danger of falling off, this is what we usually say is not sucking! How to do it? The safety factor must be improved! There are two ways to do this, let's change one line and say it!


1. Because the 100% vacuum is theoretical, it is a bit high, so we routinely calculate the vacuum of the suction cup to 60%, and hit a 60%, the suction of the suction cup becomes smaller, the safety factor increased. In terms of professional terms, "in general practical applications, the suction of the suction cup is measured at 60% vacuum."


2. Internationally, we have set the safety factor of this suction cup in the left picture to 2, that is to say, we have made this theoretical maximum 706.5 kg force suction in half, and the suction of the lower suction cup is smaller. And the safety factor has doubled again.


Now let's take a look at the circular suction cup with a diameter of 300mm. In the conventional practical application, we will determine its suction force. The vacuum degree will be a 60% discount, 706.5x60%=423.9 kg force, and the safety factor will be In a double fold, 423.9/2 = 211.95 kg force. Ok, this is safe. In the future, you should not use this suction cup to absorb more than 211.95 kg of heavy objects.

 

The suction cup is sucked from the side of the object. This application is also very common, and it is not clear. The suction of the suction cup is already calculated, but the direction of the suction can be different. Just now, the suction is upward, and now the suction is right! Let's take another analysis of the force, or take this 300mm diameter suction cup as an example, the old rules, new line first!


In the horizontal direction, the suction force T of the suction cup (that is, the difference in atmospheric pressure summarized in the previous section) is to the right. If the vacuum is applied, the atmosphere on the left side of the object pushes the object to the right with a force of 706.5 kg. Then, in order to maintain balance in the horizontal direction, the object is naturally subjected to a leftward equivalent reaction force given to the object by the suction cup support surface, that is, the supporting force N, then in the horizontal direction, T=N, balanced.


In the vertical direction, the object is subjected to the downward gravity G. If it is desired to maintain the balance in the vertical direction without falling, another upward force f is necessary to balance the gravity G, that is, f=G is required. So what is the power of this f? That's right! It is friction! The calculation formula of f=μN is still remembered! μ is the coefficient of friction, and N is the support force the object receives. We know that the friction coefficient μ is between 0 and 1. The smoothness of the surface like glass is smaller, the friction coefficient of the surface like concrete is larger, and the average value is taken as the standard internationally. μ=0.5. Change another line, let's take a look at a few formulas.


T=N, f=μN, μ=0.5, and then convert it clearly. Let's look at the relationship between f and T, f=0.5T, is that so? Seeing no, the friction is equivalent to half of the suction. In the application of sucking the object on the side of the suction cup, in order to ensure that the object does not slip, the suction of the suction cup seems to have been hit in half! Then use this 300mm diameter suction cup to quantify it. We have just determined that the suction force of this suction cup is 211.95 kg force when considering the actual vacuum degree and the double safety factor, then the side suction object is used with this suction cup. When it can provide the maximum frictional force to ensure that the object does not slip down is 0.5x211.95=105.975 kg force, that is to say, we use this 300mm diameter suction cup, which can only be used for side suction of heavy objects below 105.975kg. do you understand!


Pro tips:


1. There are two factors that affect the suction force of the vacuum suction cup. The degree of vacuum and the area of the suction cup are a very, very important concept.


2, the diameter of the 200mm diameter suction cup suction is 94.2 kg force (including 2 times the safety factor), the length of 300mm, the width of 100mm rectangular suction cup suction is 90 kg force (including 2 times the safety factor). If your calculations are the same as me, then I really don't have so many words in white code! Thank you for your attention!