What is the difference between a screw feeder and a screw conveyor?

At first glance, a screw feeder and a screw conveyor appear to be the same machine. They both utilize a rotating helical screw inside a trough or tube to move bulk materials. Visually, they share components like drive units, shafts, and flighting. However, confusing these two devices is a costly mistake in bulk material handling. They are engineered for diametrically opposite functions, and swapping them can lead to immediate operational failures.

The stakes of misapplication are high. Installing a standard conveyor where a feeder is required often results in motor overload, material bridging, and inconsistent metering. The equipment may stall immediately upon startup because it lacks the torque to handle the pressure of a full bin. Conversely, over-specifying a heavy-duty feeder for simple transport tasks wastes capital on oversized motors and specialized flighting that you do not need.

While the hardware looks similar, the engineering reality is distinct. The difference lies in the functional logic—Volumetric Control versus Mass Transport—and the internal loading states. Understanding whether your application requires a device to meter material out of a bin or simply move it between processes is the first step in successful system design. We will explore the technical distinctions to help you specify the right equipment.

Key Takeaways

• The Golden Rule: The primary difference is the inlet loading state. Screw Feeders are Flood Loaded (100%), while Screw Conveyors are Control Fed (<45%).
• Function: Feeders act as the "Valve" (controlling rate); Conveyors act as the "Train" (moving material at a constant speed).
• Design Cue: Feeders typically use variable or tapered pitch flighting at the inlet; Conveyors use constant full pitch flighting.
• Length Constraint: Feeders must remain short (<20 ft) to avoid internal bearings; Conveyors can span long distances using hanger bearings.

The Fundamental Distinction: Flood Loaded vs. Control Fed

The engineering boundary between these two machines is defined by how material enters the casing. This concept, known as the "loading state," dictates every subsequent design choice, from the horsepower of the motor to the geometry of the screw flighting.

The "Flood Loaded" State (Screw Feeder)

A screw feeder operates in a "flood loaded" condition. This means the inlet of the device is mounted directly underneath the discharge of a hopper, bin, or silo. Gravity forces material down into the screw, completely filling the flighting at the inlet.

In this state, the inlet is effectively 100% full. The screw flights are submerged in the product. Because the material is pressurized by the weight of the product in the bin above (head load), the feeder faces significant resistance. Its primary job is not just to move material, but to restrain it and meter it out at a specific volumetric rate. It must actively determine the flow rate, acting as the primary control valve for the system.

The "Control Fed" State (Screw Conveyor)

In contrast, a standard Screw Conveyor operates in a "control fed" state. Here, the material is metered into the conveyor by an upstream device, such as a rotary valve, a belt, or a separate screw feeder. The conveyor does not determine the rate of flow; it simply accepts whatever amount is fed into it.

Industry standards design these conveyors to operate with specific trough loading rates, typically 15%, 30%, or 45%. They are never designed to run 100% full. This intentional empty space, often called the "air gap," is critical. It allows material to tumble gently as it moves, reducing friction and power consumption. Because the screw is not fighting the weight of a full bin, the torque requirements are significantly lower than those of a feeder.

Hardware & Design Architecture: How to Spot the Difference

You can often identify whether a device is a feeder or a conveyor just by examining its internal geometry. The physical configuration changes to accommodate the stress of flood loading versus the efficiency of control feeding.

Screw Pitch Configuration (The Visible Tell)

The most visible difference lies in the flighting pitch—the distance between the flights.

Feeder Design: Screw feeders almost always utilize Variable Pitch or Tapered Outside Diameter (OD) flighting at the inlet section. In a variable pitch design, the flights are very close together at the back of the inlet and gradually widen toward the discharge.
Why? If a feeder used constant pitch, the first flight would fill up completely, preventing material from falling into the subsequent flights. This causes "rat-holing," where material only draws from the back of the hopper. Variable pitch creates a "live bottom" effect, drawing material evenly across the entire length of the inlet to ensure mass flow and prevent compaction.

Conveyor Design: A Screw Conveyor typically uses Full Pitch (where the pitch equals the screw diameter) throughout its entire length.
Why? Once material is moving, full pitch offers the most efficient transport. Since the inlet is not flooded, there is no need to regulate the draw; the screw simply pushes whatever falls into it.

Internal Obstructions & Bearings

No Hangers in Feeders: You will rarely see internal hanger bearings inside a screw feeder. In a flood-loaded environment (100% full), a hanger bearing acts as a dam. It impedes flow, causes material to compact, and creates a high-wear point that can lead to immediate blockage. This constraint limits the length of most feeders to under 20 feet, as the screw shaft must be supported entirely by the bearings at the ends (cantilevered or single-span).

Hangers in Conveyors: Because a screw conveyor runs partially empty (e.g., 30% full), there is plenty of room for material to flow under and around internal hanger bearings. This allows conveyors to span long distances—100 feet or more—by placing support hangers every 10 to 12 feet to prevent the shaft from sagging.

Drive Unit Placement

The placement of the motor also offers a clue. Engineers prefer to locate drives at the discharge end of the equipment. This places the screw shaft in tension (pulling the material) rather than compression (pushing it). While this is best practice for both, it is critical for feeders. Feeders require significantly higher "breakaway torque" to start turning under the weight of a full silo. A conveyor, starting with a relatively empty trough, demands much less initial force.

Operational Performance: Metering vs. Transporting

When the machine is running, the difference becomes one of control logic versus transport logic. Are you setting the pace, or are you just keeping up?

The Control Analogy

Think of a Screw Feeder as the Throttle. Because the inlet is always full, every revolution of the screw grabs a specific volume of material. If you double the RPM, you essentially double the output rate. The relationship is linear. It acts as a metering device, allowing operators to dial in a specific dosage in cubic feet per hour.

Think of a Screw Conveyor as the Transport Belt. It acts like a moving walkway or a train. If you increase the RPM of a conveyor that is being fed by a constant upstream source, you do not increase the throughput. You simply reduce the trough loading percentage. The material spreads out more, lowering the fill level from 45% to perhaps 20%, but the total amount of material exiting the discharge remains exactly what was fed into the inlet.

Accuracy Expectations

Because of its flood-loaded design, a screw feeder is capable of relatively high volumetric accuracy. With the integration of a Variable Frequency Drive (VFD), a well-designed feeder can achieve accuracies of ±1–2%. It serves as a reliable dosing mechanism for batching or blending processes.

A screw conveyor provides no inherent metering accuracy. It delivers material in pulses consistent with the screw rotation, but because the fill level varies based on the feed, it cannot be used to "measure" product. It is strictly a transfer device.

Flow Consistency and Surge Handling

Handling surges highlights another operational difference. If a surge of material hits a screw conveyor, the "air gap" in the trough acts as a buffer. The trough may temporarily fill from 30% to 60%, absorbing the surge without backing up, provided the motor has sufficient torque. A feeder, however, smooths out surges from the supply bin. It takes a chaotic, pressurized pile of material and converts it into a smooth, laminar output stream.

Critical Selection Criteria: When to Use Which?

To avoid the cost of misapplication, use this 6-point decision framework when specifying your equipment.

1. Position in Process: Look at the layout. Is the device located directly under a large silo, bin, or hopper? If yes, you need a Feeder. Is it connecting two pieces of machinery, like a mixer to a packaging line? You likely need a Conveyor.
2. Function Requirement: Define the goal. Do you need to set the rate (Dosing/Metering)? Use a Feeder. Do you need to simply match the rate of an upstream process (Transfer)? Use a Screw Conveyor.
3. Material Behavior: Consider flowability. Is the material free-flowing like dry grain, or cohesive and sticky like wet sludge? Cohesive materials often bridge over standard inlets. This requires feeders equipped with agitated hoppers or multi-screw designs (Twin or Quad screws) to actively break bridges and force material into the flighting.
4. Distance: Do you need to move material more than 20 feet? Feeders struggle with long distances due to the lack of internal support bearings. The solution is often a two-stage system: a short feeder metering into a long conveyor.
5. Incline: Do you need vertical lift? Feeders lose efficiency rapidly on steep inclines because the fluid-like behavior of flood-loaded material causes it to flow backward over the flights. Conveyors can be inclined, but efficiency drops as the angle increases. Vertical screw conveyors are a specialized subset that operates somewhat differently from both.
6. Sanitation & Safety: For food or chemical applications, consider Clean-in-Place (CIP) capabilities. Feeders are high-stress points; therefore, integrating sensor technology is critical. Zero-speed switches and anti-block sensors are essential for feeders to detect jams and prevent motor burnout if the discharge creates a bottleneck.

Advanced Configurations & TCO Considerations

Sometimes, a single standard device cannot solve the problem. Complex plant layouts often require hybrid approaches to balance accuracy with Total Cost of Ownership (TCO).

The "Hybrid" Problem

A common engineering challenge arises when you need to meter material out of a silo and transport it 50 feet away. A single screw feeder cannot span 50 feet without internal bearings, which are prohibited in feeder designs. A single screw conveyor cannot withstand the head load of the silo.

The solution is the "Feeder-Conveyor" combination. You install a short screw feeder (perhaps 6 feet long) directly under the bin to meter the material. This feeder discharges directly into the inlet of a longer, control-fed screw conveyor. The feeder handles the stress and metering; the conveyor handles the distance efficiently.

Wear & Maintenance (TCO)

When calculating TCO, recognize that feeders experience significantly higher wear. The pressure of the head load combined with the velocity of the material at the inlet creates an abrasive environment. The flighting in the inlet section of a feeder often requires hardened facing or abrasion-resistant alloys.

Power consumption also differs. Feeders require larger motors relative to their physical size. The "start-under-load" torque requirement means you might need a 10HP motor for a small feeder, whereas a much longer conveyor moving the same material might only need a 5HP motor because it starts empty or partially loaded.

Handling Difficult Materials

For materials that resist gravity flow, standard feeders may fail. This leads to the use of Mass Flow bins and Live Bottoms. A live bottom typically consists of multiple parallel screws (2, 4, or even 6) covering the entire bottom of a rectangular bin. This is essentially a complex, multi-shaft screw feeder designed to prevent bridging by keeping the entire floor of the material in motion.

Conclusion

While "Screw Conveyor" is often used as a catch-all term for any helical transport device, the distinction between transporting and feeding is absolute. A Screw Feeder is a specialized, high-stress application subset designed for volumetric control under flood-loaded conditions. A Screw Conveyor is a transfer device designed for efficiency under control-fed conditions.

Use this simple heuristic: If gravity fills the screw casing completely, it is a feeder. If another machine feeds the screw and leaves an air gap, it is a conveyor.

Before specifying your next system, ensure you calculate the required Volumetric Capacity (CFH) and assess the material's Bulk Density. These factors dictate the torque requirements for feeders far more critically than for conveyors. Getting the torque calculation wrong on a feeder usually results in a stalled machine on day one.

FAQ

Q: Can I use a standard screw conveyor as a feeder?

A: No. A standard conveyor lacks the variable pitch flighting required to draw material evenly. If installed under a hopper, it will draw only from the back, creating rat-holes. Furthermore, the motor and shaft are likely undersized for the "head load" pressure, leading to immediate stalls or mechanical failure.

Q: Why do screw feeders have variable pitch?

A: Variable pitch flighting starts short and gradually lengthens. This design ensures that every flight progressively opens up more space, drawing material evenly from the entire length of the inlet. This prevents compaction and ensures the feeder empties the hopper uniformly.

Q: What is the maximum length of a screw feeder?

A: Screw feeders are generally limited to approximately 20 feet. Since they run 100% full, they cannot use internal hanger bearings to support the shaft. Without these supports, the screw will deflect (sag) if it is too long, causing metal-on-metal contact with the trough.

Q: How do I calculate the capacity difference?

A: For a feeder, capacity is a direct calculation: RPM × Volumetric Pitch Capacity. For a conveyor, capacity is dependent on the upstream feed rate. You calculate the maximum capacity based on trough fill (e.g., 30%), but the actual throughput is determined by whatever device is feeding it.