When selecting a feeder, weighbelts are often forgotten. There are several reasons to use a weighbelt. This article takes a closer look at the most common weighbelt applications.
What Is Your Material?
When we first start talking to customers about their application and the type of feeder would be best, the first question we ask is “What material do you want to feed?”
If they mention the material is friable (can be easily broken) and that the end product’s physical shape cannot be damaged, one of the first feeders we recommend is the weighbelt. Cereals, pet food kibble, granola components, and raw vegetables are examples of materials we do not want to damage during the feeding process. Weighbelts are very gentle on friable materials. Other than the moving belt, they typically have no other moving parts (feed screws, internal agitators, etc.) coming into contact with the material.
If the material size is larger than a powder or a granule (e.g. wood strands going into a dryer in an OSB process or cement clinker going into a grinder), we would also recommend a weighbelt.
What Is Your Feed Rate Requirement?
Certain feed rate requirements are best suited for weighbelts. A gravimetric screw feeder with a 6-in. feed screw typically has a top-end feed rate of 1000 cu ft/hr. If the customer starts talking about standard or metric tn/hr we may need to look at using a weighbelt. Weighbelts are very flexible for high feed rate applications because we can use wider belts and increase the belt speed or bed depth of the material.
How Much Room Do You Have to Install a Feeder?
A continuous gravimetric screw feeder typically requires a dedicated refill hopper above it in order to refill as the feeder’s hopper becomes empty. The feed rate of the feeder will determine the size of the refill hopper needed. Often times there is not enough room to install both a feeder and a refill hopper. Que the weighbelt discussion. One of the unique features of a weighbelt is that the entire feeder does not get weighed. The weighing portion of the feeder is offset from the infeed. This feature equates to a low-profile piece of equipment that can be “shoe-horned” into tight spaces. The offset weighing area also allows us to flood feed and/or continuously refill a hopper attached to the weighbelt. You would not be able to continuously flood feed a gravimetric feeder, as the load cells would be in an equilibrium state as material is fed in as soon as it is fed out. This makes the weighbelt a great choice to place under a storage bin that might be continuously refilled during the process.
What Is Your Application?
Another example of a weighbelt application is when a material needs to be conveyed horizontally from point A to point B, while being weighed in the process. Weighbelts come in several standard lengths, providing many options for centerline of infeed to centerline of discharge requirements, whereas the centerline of the infeed to centerline of the discharge of a screw feeder is much more limited.
When talking about control philosophy, there are three ways to control a weighbelt:
1. Weigh Feeder. This is when a head load of material present on the weighbelt, for example a flood fed infeed hopper. The controller is given a setpoint by a PLC or an operator and controls the speed of the weighbelt to meet the required feed rate setpoint.
2. Pre-Feed Control. Here we run the weighbelt at a fixed speed and the controller will vary the speed of a prefeeder, such as an airlock, screw conveyor, or vibratory feeder based on the set point in the controller. The controller monitors the amount of weight of material going over the weigh area and then will vary the speed of the prefeeder to increase or decrease the bed depth of material on the weighbelt.
3. Wild Flow. With this control philosophy, the weighbelt will again run at a fixed speed. It does not, however, control to a setpoint; rather it simply totalizes the amount of material moving across the weighed area of the belt. We use this application in load-in and load-out stations.
When Should You Not Use a Weighbelt?
We need to take a closer look at the material and its characteristics. Is the material floodable, dusty, adhesive, or cohesive? Floodable materials can flood through the weighbelt without it even running. Weighbelts perform best when a uniform depth of material can be profiled across the belt. The material must be stable and not moving as it goes over the weighed area of the belt. Custom infeeds with special baffles and/or curtains may need to be designed to prevent flooding and assist with profiling the material.
Feeding dusty materials on a weighbelt will generally require more housekeeping. As the dust accumulates on the head and tail pulleys of the weighbelt, we may start to see belt tracking and slipping issues occur and hopefully shutting down the weighbelt before any damage can occur to the belt. If this is the case and a weighbelt is the only choice, make sure the weighbelt has features to assist with housekeeping. Items like self-cleaning pulleys or drag out conveyors can help. You also want to make sure that your weighbelt can tell you if there is a belt slip or off track scenario. Sensors and limit switches are available as options.
Certain adhesive materials can be self-cleaned off the belt via scrapers, but if the material is cohesive (i.e. it sticks to itself), then material testing should be done to determine if the weighbelt can positively extract and profile the material out of the infeed sheer gate. Always ask to test your material if you are unsure. Other than shipping of the material to and from the test lab, most companies will test your material for free.
If you are interested in a weighbelt, ask to speak with an applications engineer. They should be able to determine if a weighbelt is the right feeder for your application.
Todd D. Messmer has been an applications engineer for Schenck Process (Whitewater, WI) for 20 years. He is currently the applications engineering manager for the company’s weighing and feeding product lines. For more information, call 816-891-9300 or visit www.schenckprocess.com/us.
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