When properly designed for the material and application, short screw conveyors are a great way to accurately meter volume flow. They can also work well as volume feeder devices in the bottom of bins. But they aren’t ideal for transporting coarse, fibrous materials over long distances. Such has proven the case time and again for manufacturers who employ screws to convey woodchips and other sized biomass.
Still, there are ways to compensate for their shortcomings, although some of the fixes add as many problems as they solve. For this reason, it’s often prudent to replace screw conveyors with a different style conveyor altogether. (For biomass, woodchips, and other fibrous products, we recommend BE&E’s SMART Conveyors™.) Still, all options are worth considering when you’re troubleshooting.
Below is a short list of problems manufacturers commonly encounter when they use screw conveyors to transport (not simply meter or feed) sized wood products and other fibrous materials. Each problem is followed by possible causes and solutions. Keep in mind that the screw conveyor’s general design lends itself to wear because it slides material against the flights and trough. There’s friction everywhere, and it’s unavoidable.
Problem: Material is catching on the hanger bearings
- Important! We advise against long screw conveyors that require a hanger bearing when handling fibrous materials. Hanger bearings are highly problematic in such applications.
- The material has poor flow characteristics. Fibrous materials catch easily and weave together, forming obstructions. Alternatively, if the material is stringy, it may be wrapping around the bearings.
- Material is adhering to the bearings. Sticky materials like resin will adhere to components inside a conveyor, including the hanger bearings. Other materials that tend to cake can have the same problem.
- Get rid of hanger bearings. Note that your screw may require hanger bearings, thereby precluding this solution. Also, removing the bearings will allow the screw to rest on and wear through the trough.
- Enact vigorous cleanout procedures as needed to eliminate residue. Alternatively, you may employ a shaftless screw, which has no center shaft around which material can build up. If you must use a shaft, polishing the flight weld to a fine or mirror finish may deter residue from sticking. You may also try a nonstick coating on the flights and other interior surfaces.
Problem: The screw is bowing out of the conveyor
- The screw is deflecting because it is too light for the application.
- The screw is deflecting due to compression stress.
- The screw is deflecting because it is overloaded.
- Replace the screw. It is not uncommon for plant operators to install lightweight grain conveyors in biomass applications. Such conveyors are not robust enough for the material, though, and bend due to the stresses involved. Biomass requires heavy duty components.
- Drive the screw from the head of the conveyor. We’ve seen instances where people drive the screw at the tail (the infeed end) of the conveyor rather than the head (the discharge end). Driving the screw at the tail results in the system pushing the material rather than pulling it. Pushing the material adds compression stress to the screw, which magnifies the deflection. Pulling, on the other hand, helps with deflection by keeping the center tube in tension. Note that deflection may have more than one cause; the screw may be too light for the application and driven at the wrong end of the conveyor.
- Adjust the material volume passing through the conveyor. Conveyor capacity is measured in two ways: volumetric and mass flow. Volumetric flow refers to the physical space material takes up. Screw conveyors are designed to be loaded to 45 percent of less of the system’s volumetric capacity. The poorer the flow characteristics and denser the material, the lower that percentage is. For biomass and other fibrous materials, the load is typically 30 percent. The capacity also decreases as it approaches an angle of 45° and increases thereafter. Mass flow refers to the density of the material. Denser materials typically require more power and can put more stress on components. All this being said, overloading the conveyor in any way may result in damage.
Problem: The bottom of the trough has worn through
- The screw is resting on bottom of the trough.
- Material is wedging under the flights.
- The flights are grinding abrasive material into the trough.
- Sorry, bub. While you could equip your conveyor with hanger bearings, which support the screw so it doesn’t rest on the trough’s floor, you shouldn’t. Materials with poor flow characteristics will catch on the bearings and obstruct the flow through the conveyor. It’s for this reason that many screw conveyors in the biomass sector lack hanger bearings. Also, this solution is not possible with shaftless/ribbon screws.
- Nothing to be done. Unfortunately, fibrous material is going to wedge under the flights.
- Install a liner. Most screw conveyor manufacturers offer wear liners for their equipment to protect it against wear. Liners may be constructed of plastic, such as UHMW, or a hard alloy like AR steel. AR is expensive, however, and limited to conveyors in which the screw doesn’t rest on the trough. Plastic liners will decrease wear for a short time in such applications, but workers must remain diligent to keep up with liner replacements to meaningfully extend the conveyor’s life.
Problem: The trough is packed
- Material is building up around the hanger bearings.
- The material flow is obstructed upstream.
- Carryover is building up in the trough after the discharge area.
- The conveyor is overloaded.
- A foreign or oversized object has entered the conveyor.
- See Problem: Material is catching on the hanger bearings.
- Remove the obstruction and conduct an analysis to determine the root cause. If material is building up inside the discharge chute, the chute may require a steeper angle. Material may also be catching on a ledge. In this case, rework the chute so it has smooth sidewalls.
- Extend or move the discharge to the end of the trough. Otherwise, the screw may carry material past the exit, where it will collect.
- See Problem: The screw is bowing out of the conveyor, Solution C.
- Remove the obstruction and screen for oversized objects.
Problem: Flights are bent on the conveyor
- The conveyor is packed.
- See Problem: The screw is bowing out of the conveyor.
- See Problem: The trough is packed.
- See Problem: The screw is bowing out of the conveyor.
Screw conveyors can work well for dosing or metering fibrous materials over short distances, but they are not optimal for transporting materials over long distances for the reasons outlined above. For these applications, you’re better off with a different conveyance system altogether.
BE&E offers premium drag conveyors capable of spanning long runs over which screw conveyors perform poorly. Our conveyors boast best-in-class reliability and much lower wear than typical drag chain systems. Learn more about our conveyance solutions at https://www.biomassengineeringequipment.com/equipment/smart-conveyors/.