When purchasing an in motion checkweigher, one of the main things that has to be considered is product throughput. How many weighments an hour will have to be done to keep up with production? And how long are the products being weighed?
Let’s say you have a product that is 7 inches long and production is turning out 75 products per minute. Simple math will tell you that is 525 inches of product per minute or 43.75 feet per minute.
Now, that would be running the packages across the scale nose to tail with no space between the packages at all. This would result in two different packages being on the scale at the same time
, and this will not give accurate weighments.
To correct for this, we will need to know how long the weighing section of the scale is. For the purpose of this blog, we are not going to go into how to determine the scale length that will work best.
We are going to say that we are working with a weighing section that is 22 inches long. The weighing process for a product will stop just prior to the product’s leading edge reaching the end of the weighing section. At this point, the next product can begin to enter the weighing section of the checkweigher. Note that the first product does not have to clear the weighing section before the next product can begin to enter the scale as the weighing process does not begin until the next product is all the way on the scale and the scale has had a little time to settle.
So, with a 22 inch scale, the product should be spaced 22 inches from leading edge of product to the leading edge of the next product. Let’s also give ourselves a little cushion to be sure the weighing process has ended before we send the next product across by making that spacing 22.7 inches.
Now, this would all mean that we have 22.7 inches for each product that goes across the scale, and we are doing 75 products per minute. 75 x 22.7 = 1702.5 inches of belt speed per minute. Divide that by 12, and we know that we need 141.875 feet per minute.
We could use a shorter scale and go with a much slower belt speed, and that would be great because a slower belt speed would give us more accurate weights. For instance, an 18 inch weighing section would result in a minimum belt speed of about 117 feet per minute.
So, if your only product was 7 inches long, by all means an 18 inch weighing section would be for you. But, if you also wanted the ability to run a 13.75 inch-long product across the checkweigher, the longer 22 inch weighing section would be needed.
Another consideration is whether this 13.75 inch product also runs at 75 products per minute. If so, an even longer weighing section would be needed to allow for the product to be on the scale for enough time for the scale to determine an accurate weight. The most throughput you could expect on a 22 inch weighing section with a 13.75 inch product would be about 45 products per minute.
As you can see there are many things to take into consideration when selecting an in motion checkweigher.