## 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

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.

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

Photo: Yogurt by BUMIPUTRAPixabay Licence

## Friction Measurement

I had a customer wanting to reject a container off a conveyor belt.  The container held fruit, and when an optic detected a reject, they wanted to operate a solenoid to have a nozzle blow the container into the reject bin.  They had a range of containers that went from 6 oz. (170 grams) to 5 lbs (2,270 grams).  He wanted me to suggest one nozzle for all sizes, as they would automatically regulate the pressure for the full range of container sizes.  In looking at the largest size, this container will need the most force to remove.  The two factors that affects the force in this application is weight and friction.  When it comes to friction, it is generally an unknown for customers.  Here are a couple of things to help in determining the friction in your application.

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 slide.  Kinetic friction is the amount of resistance that is created when the object is sliding.  So, Static friction is always greater than kinetic friction, ms > mk.  For this application, we will have the air nozzle shoot horizontally to hit the target.  This is the most common and efficient way.

Let’s take a look our customer’s application.  We have a system to reject a non-conforming part with air.  The conveyor is a urethane belt.  The container is plastic.  We need to determine the correct nozzle to reject the 5 lb (2,270 gram) container.

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 the container is Fs = ms * W(Equation 1).

Fs – Static Force – lbs (grams)

m– Static Friction

W – Weight lbs (grams)

One way to determine the amount of force is to use a spring scale.  The spring scale should have a maximum indicator to help tell you the maximum amount of force.  You will have to attach the scale to the container on the conveyor belt. Static friction is the resistance between two surfaces; so, you will have to use the same conditions as required for the operation.  Keep the scale parallel to the conveyor.  While slowly pulling on the scale, watch the dial.  Once the part begins to move, record the weight.  For the exercise above, it showed 1.82 lbs (826 grams) of force to move the 5 lb (2,270 gram) object.

Another way would be to determine 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 and lifting one end of the conveyor belt until the object starts to slide, you can measure the angle or the height of the lift.  As an example, we take 3 foot (0.9 meter) of supported urethane conveyor belt and we lifted one end to a height of 1 foot (0.3 meters) before the 5 lb (2,270 gram) container moved.  To determine static friction, it is the tangent of the angle that you lifted, ms = tan(B) (Equation 2 below).  In this example, B = 20o.  Therefore Equation 2 gives us, ms = tan(20o) = 0.364.  If we plug this into Equation 1, we get the following:

Imperial Units                                                    SI Units

Fs = ms * W                                                         Fs = ms * W

= 0.364 * 5 lbs                                                    = 0.364 * 2,270 grams

= 1.82 lbs of force                                               = 826 grams of force
Now that we have the static force, we want to be slightly higher than that.  In looking at the force requirements that are in the EXAIR catalog, it shows that a model 1104 nozzle has a 1.9 lb (850 grams) of force.  This is at a 12” (30.5 cm) distance with a pressure of 80 psig (5.5 bar).  This nozzle will be able to slide the largest containers into a reject bin. With pressure manipulation, the customer can also use this same nozzle for the smaller containers.  If you have any applications that need products to be moved, you can always contact the application engineers at EXAIR to help you with a solution.

John Ball
Application Engineer
Email: johnball@exair.com

Image courtesy of Chobist, Creative Commons License

## Super Air Knife Used For Product Sorting

We recently announced the launch of the new 1126 1” Flat Super Air Nozzle, a marvel of its kind – compact, fully laminar, adjustable, and quiet.

Another laminar airflow product we manufacture is nothing new.  The Super Air Knife has been tried and tested for decades with new applications coming to light every day.  For example, in the sketch below we worked to integrate an EXAIR Super Air Knife (Aluminum) into a conveyor application in order to laterally move a low weight item from one belt to another.

The air knife solution provides a vital function for multiple conditions in this application.  It allows for product movement in a fully lateral plane with little to no product disorientation in the event of a defect, or an overage on the main conveyor line.  Rejected items and workflow backup were causing unnecessary costs for this producer, and we were pleased to offer a solution.

When coupled with a PLC (similar to the EXAIR EFC – Electronic Flow Controller), the application was integrated with a time delay so that maximum energy efficiency was achieved.  No compressed air was wasted, and instantaneous blow off force didn’t have to be sacrificed.

This is an excellent example of how a disciplined and educated approach benefited an application.  If you know of an application with which EXAIR may be able to help, give us a call, email, or tweet.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE