A repeat customer recently asked for a price quote for an in-motion checkweigher. After reviewing the pre-quote questionnaire, I mentioned that I would include an option that his previous checkweighers did not include. This new quote would specify shroud covers for the stand in addition to the draft cover that we include as standard. Draft covers are placed above the conveyor scale. Shroud covers are placed around the stands to guard against moving air from all other directions.
My customer asked why I thought shroud covers were important since they had not been included on two previous in-motion checkweighers his company had purchased.
I pointed out that the other checkweighers his company had purchased were weighing boxes of 30 to 40 pounds to a division size of 0.05 lbs. The draft cover that we included as a standard item was all that was needed for those situations.
This time the application was to checkweigh baked goods and they needed to be diverted if they were 20 grams too heavy or 20 grams too light. In addition to the greater sensitivity required, the scale itself had a lot of surface area in order to accommodate the size of the baked goods.
The required sensitivity combined with the number of square inches represented by the conveyor scale section meant that air movements from various directions could easily have an impact on the scale’s performance.
Here’s a great way to see the impact that even slight moving air can have on sensitive weighing equipment.
Take a basic lab balance and set the units to grams. Position yourself about a foot or so from the balance and exhale toward the weigh platter with the amount of air that might cause a birthday candle to flicker. You will most likely see a three-to-five gram read out on the display. That’s how easy it is for air currents to have an impact on the weighing performance of a very sensitive scale.
We recommend shroud covers for the stand when the application allows and we see a possibility that air movement could negatively impact a checkweigher’s performance.
A conveyor scale is a live scale supporting a section of conveyor. It provides weight data of the items which cross it for various activities. This data may be utilized and incorporated with label printing and application.
In-motion conveyor scales function by averaging multiple weight measurements as an item passes across the scale section. Photo eyes are positioned so as to initiate and terminate the averaging while an item is solely on the scale section. This of course indicates the need for product spacing. There can be no accuracy if more than one item is on the live scale section at the same time. For this reason, there are often methods introduced to induce the spacing required for only one item crossing the scale at a time.
Sometimes conveyor scales simply accumulate total items and weight volume over a period of time for accountability and record keeping. Some conveyor scales accomplishing this activity are referred to as flow scales, as they monitor the weight volume of materials flowing across the scale in a continuous stream for a measured segment of time per revolution of the conveyor belt as determined by an encoder device. Obviously this is a specific type of conveyor scale.
In contrast to the scale activity mentioned above, a checkweigher scale provides the functionality of setting weight specifications for rejecting or diverting items accordingly. As individual items cross the live scale section, the weight of the individual item is checked against the established set points for respective action after leaving the scale. There may be several different points forward of the scale at which a product may be diverted according to program activities in response to the weight set points. Items might be diverted for reworking because of over or under weights according to targeted values. Thus the items crossing a checkweigher scale are manipulated according to the set values for respective activity.
In conclusion, a conveyor scale provides weight information for utilization and application, where as a checkweigher results in material manipulation according to variable weigh data and set points established by the user.
What are the differences between conveyor scales and checkweighers?
Let’s first describe a checkweigher; a checkweigher is a scale that allows you to set a target weight for a product often referred to as the “Accept range”. Other ranges can include “Over,” “Under,” “Accept Over” and “Accept Under.”
The accept range is generally a very small range where the target weight was almost perfectly hit. The accept over and accept under ranges refer to a product whose weight just slightly missed the target weight but is still acceptable enough to pass inspection. The over and under ranges are set to where a product is too far off of the target range to pass inspection at all, and will need to be reworked.
Checkweighers are available in two very different styles; the first is a “static” scale where an operator will have to place each individual product on a scale, the scales controller will then indicate to the scale operator what range the product fell into, often times with the use of a series of red, yellow, and green lights.
The second type is an “automatic checkweigher.” With an automatic checkweigher the products are conveyed across a series of conveyor belts and the weight is automatically collected with
An automatic checkweigher offers distinct advantages over the static checkweigher. Mainly it can be very cost effective, it also reduces the chance for unintentional human error, as well as removing the possibility of an operator having a bad day and letting their attitude enter into their decision making process.
Additionally, this type of automated system can also keep track of production data by keeping track of the total numbers of packages that fell into each of the ranges.
Now that you have a good understanding of what a checkweigher is we can look at what a conveyor scale is.
A conveyor scale is a component to be used in some sort of system, it’s essentially a scale with a conveyor mounted to it, and some sort of a controller to gather the weighing information.
The only thing the conveyor scale really does with this weight information is to send it to a controller that controls a larger system.
A checkweigher is a good example of one of these systems; another would be a sortation system where products need to be sorted according to weight so that a decision can be made as to how to process each particular product that goes across the scale based on its weight. A final example would be a box labeling system that would apply a net weight label.
In-motion weighing is a growing trend in many industries, and fewer places is this more apparent than in the various food industries. The big problem encountered with food is that the large variety of products requires a large variety of solutions. For example, in beef plants where chubs are being packaged, a specialized approach is needed in order to checkweigh various sizes coming from the same machine.
Ground beef chubs typically come in 1 lb, 5 lb, and 10 lb varieties and weighing each type is challenging to accomplish with one in-motion system. A customer came to us with the intent of weighing all of these varieties with one scale. They also asked that it meet NTEP standards, not because they had to meet the requirement, but because they wanted that assured accuracy that is promised by NTEP qualification.
Eventually, they decided to focus on just the 1 lb and 5 lb varieties with this system, as the 10 lb chubs would require a much more complex solution, and they wanted something simple and cost-effective.
Obviously, the two varied in size quite greatly. The 1 lb chub was around 6 inches long, while the 5 lb was about twice that at 12.” The scale had to be the right size to accommodate both of them, so it had to be longer than the ideal size for the 1 lb. This was a fairly straightforward constraint; the more difficult part would be being able to account for the difference between the weight of the two products and the required throughput frequency.
The biggest issue with the weight difference is making sure that the precision is on target for both varieties. To meet NTEP standards, the division size required had to be small enough to encompass both products. In this case, the smaller product defined the precision. At 1 lb, coming up with the appropriate amount of divisions for a 1 lb product can be very difficult, but we were able to accomplish it thanks to our scale expertise.
The rates of throughput presented another tough challenge. The machines that output the beef for the chubs are able to output much faster for the smaller chub, naturally. It wouldn’t make sense to run our checkweigher at one speed because that would mean slowing down the output when 1 lb chubs were being produced.
To make a checkweigher that lived up to plant capabilities, we created a high accuracy variable speed checkweigher. It allowed the customer to easily switch between different speeds for their products, and we were still able to meet NTEP approval specs for both modes of operation without having multiple scales. This kept the simplicity intact and the cost down.
You would have a tough time finding many other companies that make a NTEP-approved checkweigher like we do, and especially not with the level of custom-tailoring that we are capable of.
Are you thinking about getting an in-motion checkweigher for your production facility to weigh your product and reject any items that are over or under weight?
Once a product is weighed on the conveyor, a reject device of some sort will be needed to remove the out-of-weight (or out-of-tolerance) items from the line. Most of these devices will need a power source of some sort to activate the reject device. The most common power sources that come to mind are pneumatic, electrical, and hydraulic. For this article, the focus is pneumatically operated devices.
Many types of pneumatically operated devices can be manufactured and installed on an in-motion checkweigher.
Here are five types of pneumatic reject devices:
The first option is a simple air knife or air comb device. These devices simply blow a puff of air at the product and blow it off the line. These are typically used on very light-weight products since they do not physically push or pull the product.
A second option is a linear thruster.
A linear thruster mounts to the side of the conveyor and pushes the product straight off the other side of the conveyor.
A third option is either a pull-off or push-off arm. An arm of this type has an arm tied to the pneumatic cylinder with a pivot point on the side of the conveyor. When activated, the cylinder pushes the arm open while the arm pivots about the pivot point. It either pushes product off the opposite side of the belt or pulls it off to the same side the divert is mounted on. As it pushes, it also guides the product to the desired side.
A fourth reject device is an overhead mounted guided cylinder. A cylinder is mounted between guided rods with a block mounted on the guided rods. The cylinder moves the block along the guided rods. A plate of some sort is mounted to the block and hangs down along the side of the conveyor. This plate moves with the block to push product off the side of the conveyor.
A fifth type of reject is called a drop-down reject. With this type of divert, the conveyor itself actually has a pivot point located at the beginning of the conveyor and drops down to direct product off the end of the lowered conveyor.
Each of these types of pneumatic reject devices work well when used for the proper applications. If you have any questions about in-motion checkweighers, in-motion sortation, grading, classifiers, etc or whatever reject device would work well for your application, please contact us!