## 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

## Remembering & Honoring This Day

Today I had the honor to participate in a local memorial event to recognize 9/11/2001.  This was the fifth year for the event in which participants climb 2,071 stairs within Nippert Statium at The University of Cincinnati.  This number symbolizes the stairs of the 110 floors each of the World Trade Center towers had.  The amount of time to complete the event is 56 minutes.  This was my first time attending the event and I must say, I was awestruck.

The number of people that attended the event was amazing.  The event started at 6:34 this morning, I arrived around 5:30 and met with a local group that were going to ruck the stairs rather than simply running / walking.   We each carried a pack, ruck sack, with us with a 30 lb weight plate.  The goal was to complete the 4 laps that the event required.

This slideshow requires JavaScript.

As I was going through the repetitions up and down the stairs, they were making announcements of the events that transpired on that day in history, and the names of those that lost their lives scrolled across an electronic screen I began to recount where I was on that day.  I was actually on that exact campus just a few hundred yards away.   I walked right through that stadium on 9/11 to go to class.

The stairs began to wear on me quickly and I was only halfway through my first lap.  Then I saw a group of young ROTC students with Xavier University’s Air Force ROTC program.  That picked my hopes up for a bit  and I went on.  The harder it got for me the more details I remembered about that day. The more flights I did the more I thought about those that lost their lives, seeing the names I began to take a step for each one just to push on to the next.  At the end of the time I did not complete the 4 laps needed to commemorate the total number of stairs.  I did complete two full laps of stairs knowing that after that event was done I still get to go home and hug my family.

This day is always going to be a day of remembrance in my family.  Today, I was lucky enough to commemorate it among some amazing veterans, active duty, and future military and first responders.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF