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.
Fs = ms * W
Fs – Static Force in ounces (grams)
ms – 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