Considerations for Ejecting Parts with an Air Nozzle: Weight and Friction

I had a customer wanting to reject a container off a conveyor belt.  The container held yogurt, and when an optic detected a reject, they wanted to operate a solenoid to have a nozzle blow the container into a reject bin.  They had a range that went from 4 oz. (113 grams) for the small containers to 27 oz (766 grams) for the large.  He wanted me to suggest one nozzle for all sizes, as they would automatically regulate the pressure for the full range of products.  In looking at the largest size, this container will need the most force to blow off the conveyor.  The two factors that affects the force in this type of application is weight and friction.  When it comes to friction, it is generally an unknown for customers.  So, I was able to help with a couple of things to determine the friction force.

Friction is a dimensionless number that represents the resistance created between two surfaces.  We have two types; static friction, ms, and kinetic friction, mk.  Static friction is the maximum amount of resistance before the object begins to move or slide.  Kinetic friction is the amount of resistance that is created when the object is moving or sliding.  So, Static friction is always greater than kinetic friction, ms > mk.  For this application, we will use an air nozzle to “shoot” horizontally to hit the rejected product.

Let’s take look at our customer’s application.  We have a system to reject a non-conforming part with air.  The conveyor has a urethane belt.  The container is plastic.  For the largest container, they have a weight of 27 oz. (766 grams).  Being that the conveyor belt is only 12” (30.5 cm) wide, we can determine that if we get the part moving, it will continue off the belt and into the reject bin.  The equation for the maximum amount of force required to move a container is below as Equation 1.

Equation 1

Fs = ms * W

Fs – Static Force in ounces (grams)

m– Static Friction

W – Weight in ounces (grams)

One way to determine the amount of force is to use a scale similar to a fish scale.  The scale should have a maximum indicator to help capture the maximum amount of force.  You will have to place the object on the same belt material because different types of materials will create different static forces. Keep the scale perpendicular to the object, and slowly pull on the scale.  Once the part begins to move, record the scale reading.  For the exercise above, it showed 9.6 oz. (271 grams) of force to move the 27 oz. (766 gram) object.

Another way would be to calculate the static friction, ms.  Static friction can be found by the angle at which an object starts to move.  By placing the container on a section of supported urethane conveyor belt, you can lift one end until the object starts to slide.  The height of the lift can be measured as an angle.  As an example, we take 3 feet (0.9 meter) of supported urethane conveyor belt, and we lifted one end to a height of 1 foot (0.3 meters) before the 27 oz (766 gram) container moved.  To determine static friction, it is the tangent of that angle that you lifted.  With some right triangle trigonometry equations, we get an angle of 19.5o.  Thus, ms = tanq or ms = tan(19.5o) = 0.354.  If we plug this into Equation 1, we get the following:

Imperial Units                                                    SI Units

Fs = ms * W                                                         Fs = ms * W

= 0.354 * 27 oz.                                                = 0.354 * 766 grams

= 9.6 oz. of force                                              = 271 grams of force

1″ Flat Super Air Nozzles

Now that we have the static force, we want to be slightly higher than that.  In looking at the force requirements that are published in the EXAIR catalog, it shows that the model 1126 1” Super Flat Air Nozzle has a 9.8 oz. (278 grams) of force at 80 PSIG (5.5 Bar).  This force is measured at a 12” (30.5 cm) distance with a patented .015” (0.38mm) shim.  So, this nozzle will be able to slide the largest container into the reject bin.

1″ Flat Super Air Nozzle shims

To expand on the benefits in using the EXAIR Flat Super Air Nozzles, the force can be changed easily with a regulator or with a Shim Set.  This is a unique feature as most competitive flat nozzles do not allow you to do this.  The patented shims control the force rating in a wide range with lower air consumption and lower noise levels; making them safe and efficient.  So, if this manufacturer decided to produce other sizes in the future, then they could change the shim to target even larger containers.  The flexibility of using the EXAIR Flat Super Air Nozzles allow you to increase or decrease the force by just removing two screws and changing the thickness of the shim inside.  EXAIR does offer a pack of shims with different thicknesses which are called a Shim Set.

With air pressure or shim manipulation, the customer could use the same nozzle for the yogurt containers.  If you have any applications that need products to be rejected quickly, an Application Engineers at EXAIR will be happy to help you with a solution.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Photo: Yogurt by BUMIPUTRAPixabay Licence

Calculating Static Friction To Eject Parts with Air

2″ Flat Super Air Nozzle

In today’s fast-paced world, companies are always looking for ways to do things faster, cheaper, more efficiently without sacrificing safety.

A cereal company had a high-speed system to check the quality of each box of cereal.  When a box did not meet the quality criteria for visual and/or weight, the box would be rejected.  The rejection system that they used was a quick blast of compressed air to remove the box from the conveyor line into a non-conforming bin.  For their first attempt, they tried to use a ¼” copper tube with a solenoid valve attached to a reservoir tank.  When a “bad” box was detected, the solenoid would be triggered, and compressed air would “shoot” the box off the rubber conveyor belt.  The ¼” copper tube can be an inexpensive, common, and easy-to-use device; but they found that the copper tube was very loud (above OSHA limits for noise exposure) and not very effective.  As a note, this company had a safety committee, and they wanted to keep all blowing devices below 80 dBA in this department.  The ¼” copper tube was around 100 dBA.  So, they contacted EXAIR to get our expertise on this type of application.

The cereal company gave me some additional details of the operation.  The box weighed 26 oz. (740 grams) with a dimension of 7.5″  wide by  11″ tall by  2 3/4″ deep (19 cm X 28 cm X 7 cm respectively).  The issue with the ¼” copper tube was the small target area compared to the area of the box.  With any slight variation in the timing sequence, the force would miss the center of mass of the box.  The box could then spin and remain on the conveyor belt.  This would cause stoppage and disruption in the system.  They asked if EXAIR had a better way to remove the defective boxes.

I recommended a model 1122, 2” Flat Super Air Nozzle.  The reason for this style of nozzle was for a variety of reasons.  First, we needed a larger area to “hit” the box.  This Flat Super Air Nozzle has a width of 2” versus the ¼” copper tube.  This increased the target area by 8 times.  So, any small variations in time, we could still hit the center of mass and remove the box.  The second reason was the force rating.  The model 1122 has a force of 22 oz. (624 grams) at 80 PSIG (5.5 bar).  This is slightly under the 26 oz. (740 grams) weight of the cereal box, but we are just sliding the box and not lifting it.  If we can overcome the static friction, then the box can be easily removed.  With Equation 1, we can calculate the required force.

Equation 1:

Fs = ms * W

Fs – Static Force (grams)

m– Static Friction

W – Weight (grams)

From the “Engineering Toolbox”, the static friction between rubber and cardboard is between 0.5 to 0.8.  If I take the worse case condition, I can calculate the static force between the belt and cereal box using Equation 1:

Fs = 0.8 * 740 grams

Fs = 592 grams

The model 1122 has a force of 22 oz. (624 grams), so plenty enough force to move the box from the rubber conveyor belt.

The third reason for this nozzle is the noise level.  The noise level of the model 1122 is 77 dBA, well below the safety requirement for this company.  Noise levels are very important in industries to protect operators from hearing loss, and the model 1122 was able to easily meet that requirement.  I added an additional reason for recommending the 2” Flat Super Air Nozzle; compressed air savings.  Companies sometimes overlook the cost when using compressed air for blow-off devices.  In this comparison, the ¼” copper tube will use 33 SCFM (934 SLPM) at 80 PSIG (5.5 bar) while the model 1122 will only use 21.8 SCFM (622 SLPM).  This is a 33% reduction in compressed air; saving them money.

At the intro, I mentioned that companies are looking to do things faster, cheaper, more effective without sacrificing safety.  For this company, we were able to increase production rates by removing every cereal box from the conveyor belt.  We also saved them money by reducing the compressed air requirement as well as keeping it safe by reducing noise.

If you have an application that needs products to be moved by air, you can contact an Application Engineer at EXAIR to help you with a solution.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

How Much Force Does It Take?

In case you weren’t aware, the answer to “How much force does it take?” is always going to be, ALL OF IT.   At least that is what we generally think when trying to blow product off a conveyor belt or diverting parts into bin, etc. Speed and efficiency play a direct role in to what nozzle or blow off device you should use in order to get the job done and be able to repeat the process.

The question we are often asked by customers is, “How much force to I need to move this?”  That is a question that we cannot often answer without asking more questions.  The good part of this is, there is a formula to calculate just how much force you need to move an object.   A good video explaining friction is shown below.

In order to answer the question of how much force do I need, we really need to know all of the following:

Weight of the object
Distance from target
Is it on an incline or level
Distance needed to move
Then, the usually unknown variable, the coefficient of friction between the target and what it is sitting on.

Often times it is the thought process of, my target weighs 5 pounds, I need 5 pounds of force in order to move it from the center of this conveyor belt to the edge, this is not the case.   If you wanted to lift the object over a break between two conveyors then you would need slightly more than 5 pounds in order to ensure you are lifting the front edge of the unit high enough to meet the other conveyor.

Whether you know all of the variables or only a few, if you need to get an object moved and you want to try using compressed air to do so, give us a call and we will help you find the best engineered solution for your application.  Then, we’ll back all stock products with a 30 day guarantee if you don’t like how the system performs – but rest assured, we get it right almost every time.

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Brian Farno
Application Engineer Manager
BrianFarno@EXAIR.com
@EXAIR_BF