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
Twitter: @EXAIR_jb

Photo: Yogurt by BUMIPUTRAPixabay Licence

Super Ion Air Knife Removes Dust From Yogurt Cups

A yogurt cup

Part of the due diligence in discussing applications with our customers is gathering an understanding of their application process and needs.  In emails, phone calls, or online chats, we determine the parameters and constraints of the application to be sure that we have a proper solution to offer.  And, if we don’t, we explore custom solutions or help find something available outside of EXAIR.

In a recent email exchange with a yogurt manufacturer I received the photo below showing the filling process for their yogurt cups.  The adage goes that “a picture is worth a thousand words”, and it certainly holds true here.  The full process is shown, with labels and dimensions.  What more could you ask for to understand the flow of the application?

machine setup
The filling process for yogurt cups in this application

In this system, small dust particles were adhering to the internal walls of the yogurt cups.  When the cups would pass through the vacuum unit, the station designed to remove any dust or debris, static cling would prevent the system from removing all the dust.  Then, in the filling station, yogurt would be fed into insufficiently cleaned cups, resulting in defects and wasted product.

This customer contacted EXAIR in search of a solution to remove the static on the inside of the cups, and to help facilitate removal of the dust, if possible.  What they found was a perfect solution in our Super Ion Air Knife.

When the cups exit the feeder and enter the conveyor, they are placed in the same orientation with (6) cups across the conveyor over a width of 580mm (22.8”).  By installing a static eliminating solution over the conveyor at this point, we can remove the static before the cups enter into the vacuum cleaning station.  And, if we can provide a blowoff source as well, we can remove the dust particles before the cups are cleaned again via the vacuuming system.

The solution for this application was the stock model 111024 24” Super Ion Air Knife along with model 7907 Power Supply.  This Super Ion Air Knife provides a fully laminar and consistent sheet of static eliminating air, removing both the static and debris from inside the yogurt cups.  Operating at a low pressure (~20 psig), these units are also quiet and consume low volumes of compressed air.

By installing the Super Ion Air Knife into this application, this customer found a solution to remove both static and debris from the product.  In doing so, defects were eliminated and output was increased.

If you have a similar application in need of a similar solution, give us a call.  We’ll be happy to help.

Lee Evans
Application Engineer