SCREW AUGER FEEDERS

Screw Feeder Benefits & Features

Screw Feeders are volumetric devices, consisting of single, or multiple, rotating screw 'helixes' installed within an enclosed casing. When designed correctly, they are a cost effective and reliable solution for feeding a range of materials - from free flowing grain or medium sized lumps ores, to cohesive filter cakes.

The mass flow rate Discharged will be dependent on a range of variable factors, including:

Screw Flight Geometry, Diameter and Rotational Speed
Material Bulk Density
Flight Filling %
Conveying Efficiency

SCREW FEEDERS

With more than 25 years of experience in the design and manufacture of standard and bespoke screw feeders, with an emphasis on heavy duty mining and industrial applications, our engineers can help develop the best solution for your requirements.

BHT Screw Feeders are designed and manufactured to ensure long life and reliable service in the harshest of operating environments. Using only quality components, our designs meet or exceed Australian Standards, and are proven to be reliable and hard wearing.

By working with BHT for the supply of your next screw feeder you will enjoy the following benefits:

A tailored solution, engineered to suit customer specific site requirements,
Compliance to customer engineering and manufacturing specifications,
A consultative design process utilising the latest industry standards and guidelines for material flow properties, Australian Standards and BHT proprietary design systems,
The confidence of Finite Element Analysis for major structural and mechanical components,
Provision of 3D models to allow convenient integration into overall plant layouts to ensure accurate interface and tie-in with connecting chutes, hoppers, and infrastructure.

OPTIONS

The Screw Geometry and Size are determined to suit the characteristics of the material. Conveying efficiency varies between different materials and different material conditions (e.g. moisture content, particle size distribution, temperature), although as a general rule, the more cohesive a product is, the lower the conveying efficiency becomes. Additionally, as the angle of inclination of a screw conveyor increases, conveying efficiency will also reduce unless steps are taken to modify the design. Flight designs can also affect conveying efficiency.

For a screw feeder (screw auger), filling efficiency depends on the ability of the material to truly ‘flood’ the screw shaft around the full circumference of the feed flights. There may be some ‘empty space’ between large lumps or cohesive materials in some cases and this can affect the actual filling efficiency.

Options and features typically include the following:

Fully enclosed tube or trough style configurations for dust and spillage containment
Single, twin, or triple screw shafts for wider hopper discharge openings to avoid material bridging at hopper outlets
Slow speed screw feeders to improve wear life
Standard full or ribbon flights
Mild steel, quenched and tempered, or hard faced flights can be used to improve life
In order to promote even draw down of material in the hopper tapered shafts, tapering flight outer diameter, and increasing flight pitch can used individually or in combination together
High temperature materials can be handled or even cooled during conveying
Can be manufactured from stainless steels or specialty materials for food handling or corrosion protection
Bearings are mounted out board from casing seal to provide additional protection from material contaminants
Can be configured with multiple inlets and outlets
Guarding to Australian Standards AS4024.3610

DESIGN FOCUS

SCREW FEEDER FLIGHTS

Selecting the correct flights for a screw feeder is one the most important factors in a successful design. Speed, installed power, material draw down, build-up of material and wear life are all considered when selecting the style of screw feeder flights.

Screw flights are typically made in segments and cut from a plate disc with split to suit the required OD and ID, before being pressed to produce a right-hand or left-hand 'pitch'. The pitch is simply the axial distance of a full 360 degree flight and represents the distance material will move along the casing with one full rotation of the flight shaft. The pitch of a flight can vary from nearly zero, up to a 'standard' full pitch which is the diameter of the screw flight. Increasing the pitch increases the conveyed volume per revolution, allowing a reduction in shaft rotation without reducing capacity.

The majority of all flights manufactured are either full face flights or ribbon flights. Full face flights are the simplest and most common, while ribbon flights are used when handling cohesive materials or in applications which can benefit from mixing as the product is conveyed. There are many other flight variations available including cut, cut and folded, shaftless etc and the final decision must be made to suit the application.

Screw feeder flights can be manufactured from standard carbon steel, quenched and tempered steels, chromium carbide faced steel, and numerous other stainless and specialty steels. As with most decisions to use specialty materials, improvements in wear life and corrosion resistance need to be balanced with additional costs.

BEARINGS AND SEALS

There are numerous methods for mounting bearings and for sealing conveyor casings used throughout the industry, but our preference is for a robust solution involving outboard mounted bearings and a separate, adjustable, triple barrier casing seal.

Our preferred bearing assembly incorporates spherical roller bearings mounted in solid, flange mounted housings with taconite or labyrinth seals. These housings have been specifically designed for efficient transfer of radial and axial loads through the mounting flange. Axial loading on plummer block style housings is not ideal, as the axial load generates a moment about the mounting bolts.

The BHT casing seal incorporates a multiple barrier solution, with a grease purged lantern ring packed into a multi-strand packing gland. This separate casing seal provides a space barrier between the bearing and casing seal, protecting the bearing housing seal from direct material pressure. Many low-cost, agricultural designs involve mounting the bearing direct to the screw conveyor casing, using the bearing housing seal as the casing seal.

This does not provide any opportunity to monitor the condition of the seal, or allow adjustment. Eventually the bearing housing seal will fail and, when it does, material will enter the bearing and shorten its life considerably. This will result in a catastrophic bearing failure with little opportunity for prior warning.

If required, third party proprietary mechanical seals, with or without air or gas purging, can also be installed.

Any exposed rotating shafting is guarded in accordance with Australian Standards.

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