Relationship Between Suction Force & Suction Distance

  • #1

Summary:

Suction force weakens as you move your suction source away from the object. I need a method to quantify the relationship between the distance and suction force.

Main Question or Discussion Point

Hi All,

We all have experiences of using vacuum cleaner to clean carpet, floor, etc. and might probably have noticed a super intuitive phenomenon that as you move your vacuum cleaner suction hose closer to the floor, the suction force increases which helps you pull some of large trash off the floor. On the other hand, as you move it away from the flow, the suction force weakens.

The same analogy could be applied to a suction cup and the object you wanna lift. Assume there is no gap between the suction cup and the plate, and contact surfaces of both objects are perfectly flat. In that case, the perfect vacuum should be able to be generated within certain time and suction force should reach its maximum value for the given settings. However, as you move the suction cup just SLIGHTLY away from the object, the percentage drops significantly.

I am pretty sure this has to do with the percentage of vacuum generated in that specific volume (diameter of the hose times the distance between the object and the hose). The more air particles you could remove per unit time from that volume, the higher percentage of the vacuum you can generate. However, the tricky part is how would you be able to quantify the relationship between the suction force and suction distance

I have been thinking this question for over a week but still can't find a hint. I was hoping if some of you could give me some inputs or thoughts or resources I could consult with.

Thank you
Stanley
 

Answers and Replies

  • #2
[Mentor Note -- two duplicate threads merged into one]

Summary:
Suction force drops dramatically as the gap between the object and suction force is created and increases. How to find a method to quantify this relationship?

Summary: Suction force drops dramatically as the gap between the object and suction force is created and increases. How to find a method to quantify this relationship?

Hi All,

We all have experiences of using vacuum cleaner to clean carpet, floor, etc. and might probably have noticed a super intuitive phenomenon that as you move your vacuum cleaner suction hose closer to the floor, the suction force increases which helps you pull some of large trash off the floor. On the other hand, as you move it away from the flow, the suction force weakens.

The same analogy could be applied to a suction cup and the object you wanna lift. Assume there is no gap between the suction cup and the plate, and contact surfaces of both objects are perfectly flat. In that case, the perfect vacuum should be able to be generated within certain time and suction force should reach its maximum value for the given settings. However, as you move the suction cup just SLIGHTLY away from the object, the percentage drops significantly.

I am pretty sure this has to do with the percentage of vacuum generated in that specific volume (diameter of the hose times the distance between the object and the hose). The more air particles you could remove per unit time from that volume, the higher percentage of the vacuum you can generate. However, the tricky part is how would you be able to quantify the relationship between the suction force and suction distance

I have been thinking this question for over a week but still can't find a hint. I was hoping if some of you could give me some inputs or thoughts or resources I could consult with.

Thank you
Stanley
 
Last edited by a moderator:
  • #3
167
42
The force imparted on a plate, relative to the suction cup, is caused by the pressure difference between the face of the plate inside the cup and the face of the plate outside the cup.

Atmospheric pressure is assumed to be 1 bar, or about 15 psi.

if you have a suction cup with an area of 10 square inches, which is sealed to the flat plate and has achieved a perfect vacuum, IE 0psi inside, then the plate will be "sucked" onto the cup with 15psi of pressure (15-0). 15psi (pounds per square inch) on 10 square inches, 150lbs of suction.

As you pull the cup away, the suction will reduce rapidly. This is down to the volume of air which is sucked in through the gap. I don't know the exact calculations, but I believe that you will need to calculate the air flow through the gap between the tiles, and establish the effect this has on the pressure at the face of the plate, which will be complicated to work out and a bit beyond my knowledge.

As the suction will drop off greatly with very little gap, I think that the best method for lifting a plate is to ensure full contact with the suction cup. it would be very difficult to achieve any form of hovering with a suction system, as the object will close the gap as it lifts and thus increase the force, essentially "snapping" it to the cup once it has achieved lift.

What is it you're trying to accomplish with these equations? or is it just for the thought exercise?
 
  • #4
DrClaude
Mentor
7,339
3,520
I am pretty sure this has to do with the percentage of vacuum generated in that specific volume
There is "vacuum" generated. What you have is a difference of pressure.

In the case of the vacuum cleaner picking up an object, what is more important is the air flow created by that difference of pressure. This will be very sensitive to the particular geometry of the nozzle, the floor, etc. I don't know if there is a simple formula to express suction as a function of distance.
 
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  • #5
jrmichler
Mentor
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1,120
If you are sucking up an object large enough to plug the hose, then total vacuum is the controlling variable. Total vacuum, in this case, is the air pressure difference across the object. The suction force is the pressure difference times the area being sucked on.

If you are sucking up dirt particles with a vacuum cleaner, then air velocity across the particles is doing the work. The air velocity knocks the particles loose and carries them away. A rule of thumb for sucking into a simple hose (no nozzle) is that the velocity one hose diameter from the hose entrance is 0.1 times the velocity inside the hose. And since the "knocking loose" force is proportional to the velocity pressure, which is proportional the air velocity squared, the "knocking loose" force is only 0.1 squared, or 1% at one hose diameter. I think (too lazy to check) there is something about this in the ASHRAE Handbook of Fundamentals.

When the hose end is very close to a surface, the velocity gets high. Here you can calculate the velocity from the pressure difference and Bernoulli's equation. A nozzle just spreads out the area of high velocity to suck dirt over a larger area.

Suction cups for lifting depend on a seal between the cup and the surface because they use relatively small vacuum pumps. They use "high" vacuum to get lifting force. If the cup lifts off the surface, the air flow gets very high very quickly. A suction cup designed to lift objects with an air gap would need a huge vacuum pump.

I once designed a vacuum conveyor to lift and carry objects with a corrugated surface. Since there was a large leakage air flow, the conveyor was designed with large area, large vacuum holes, large flow area, and a variable speed centrifugal blower.

To understand what is going on, you need to really get into Bernoulli's equation. Look at where pressure drops occur, look at velocities, look at flow rates, look at the flow vs pressure curve for the vacuum source, and look at the effect on the objects. Start by doing some system flow calculations for a suction cup with a tight seal, then a small gap (calculate the total air flow through the gap), a slightly larger gap, and a very large gap.

There are companies that specialize in making suction cups for lifting. Study one of their catalogs. Have fun, this is a topic that a person can make a career of.
 
  • #6
203
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First we need to understand what the word vacuum means. The word ‘vacuum’ is a convenience term invented to represent the lack of pressure.
A vacuum is not a thing...it is a condition. It is the absence of something.
It is most often seen as the absence of pressure. Since we live on earth, in an atmosphere that applies 14.7 pounds/sq. inch of pressure at sea level at all times, we accept this as being ‘normal’.

This atmospheric pressure is caused by the number and movement of gas molecules contained in each cubic inch of this soup (atmosphere) we live in. Pressure is simply the indication of the average kinetic energy in the gas molecules contained within a given space.
If you remove gas molecules from a confined space.....you lower this pressure. If you remove all the molecules of gas from a given confined space you will have zero pressure; there will be nothing within the space to cause the indication of molecular movement, hence, no indication of pressure.
The word ‘vacuum’ was invented to represent the lack, or reduction, of pressure.

Another misnomer is the term ‘suction force’. It implies, when you have a reduction of pressure (vacuum), there is a pulling force; when in fact the force is a pushing force, caused by a pressure differential. If there was no atmospheric pressure available, a vacuum could never cause a suction force! That is why a vacuum cleaner will not work in outer space.

A vacuum cleaner, or vacuum pump, moves gas molecules from one place to another; thus reducing the indication of pressure. The movement of molecules per unit of time is a function of the design and precision of the device trying to move the molecules. If the design of the device is efficient in sweeping out molecules and leaks are minimized, then a greater reduction in pressure can be achieved per unit of time. It is this differential in pressure that causes clamping forces to occur between the lower pressure areas and the higher pressure areas of the system.

Any leakage between these two areas, which allow gas molecules to pass from the high pressure area to the low pressure area will reduce this effective clamping force accordingly. In any event, the clamping force can never be greater than about 14.7 lbs/sq. in., in the open atmosphere at sea level. The movement of the gas in the leakage areas follow the principals of gas flow dynamics as related to pressures, flow rates, temperatures and gas densities.
 

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