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News Release from: Ajax Equipment | Subject: Screw Feeder Book
Edited by the Manufacturingtalk Editorial
Team on 18 April 2007
Guide to screw feeders
Lyn Bates, Managing Director of Ajax Equipment has written a book entitled 'Guide to the Design, Selection and Application of Screw Feeders', to improve the performance of screw conveyors.
Screw conveyors have been with us since antiquity but, as Lyn Bates points out in the introduction to the 'Guide to the Design, Selection and Application of Screw Feeders', a lack of knowledge of the way bulk materials behave continues to impede progress towards radical improvements in industrial performance This guide will go some way towards helping companies to achieve this radical improvement
This article was originally published on Manufacturingtalk on 8 Jun 2007 at 8.00am (UK)
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The 'Guide to the Design, Selection and Application of Screw Feeders' is in seven parts: classes of screw equipment (conveyors, elevators and feeders), screw feeder types, screw construction, interfacing screw feeders with hoppers, screw selection criteria, special forms of screw feeders where feeders are used in a direct processing role and case studies.
In many respects, the 'Interfacing screw feeders with hoppers' chapter represents the guide's key to understanding screw feeders as most practical screw feeder problems occur here.
This chapter introduces the many variables that influence screw feeder performance, such as the design of the hopper, the material's flow properties, mass flow from hopper to feeder, avoiding arching and preferential flow and, particularly, the design of the screw.
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Readers are advised to look beyond just the hopper capacity and consider the flow characteristics from the hopper outlet into the screw feeder.
Screw feeders need a flood-feed supply of bulk material in order to control the rate of discharge.
It follows that getting the flow right has a large bearing on the efficiency of the screw feeder.
So what are the types of flow best suited to our feeder? You may not be surprised to hear that mass flow has many advantages.
As Mr Bates point out, however, understanding what mass flow isn't, is as important as understanding what it is.
For example, mass flow is a flow pattern and not the hopper geometry.
It describes a condition whereby there is a total slip of the hopper contents on all wall contact surfaces during flow, no region of storage remains static during discharge.
And the flow must take place over the entire cross- section of the outlet; mass flow does not mean a flow has uniform velocity across the outlet cross section as you might expect.
A subtle but nonetheless important difference.
In an ideal world every material would exhibit good flow characteristics and, as many readers will know, materials handling equipment designs often assume mass flow.
This is why the section on the flow patterns from bulk containers is probably the most important in the book.
There is a spectrum of flow patterns.
Knowing where you are on that spectrum has a large influence on the interface with the screw feeder and its ultimate efficiency.
Essential to the working of the screw feeder is that the whole cross section of the screw is filled with material, replenishing material transferred along the screw axis with material from the hopper.
Factors such as screw geometry, the volume forwarded by the screw and effective axial transfer velocity of the material are influenced by the flow characteristics of the material.
A fine free-flowing powder will need a different treatment to that of a cohesive powder.
The engineer has further options in the selection of screw forms to better suit the application.
Conventional feeder screws have full- face flights welded to a centre shaft are acceptable for simple applications.
However, where a differential intake of material is needed, a stepped pitch, variable pitch, stepped or taper centre shaft, variable screw diameter, part ribbon or shaftless construction can be used.
Add to this selection criteria a range of surface finishes such as milled finish, blasting and mechanical and electropolishing, and you may appreciate why the BRMB guide is an valuable addition to the engineer's reference library.
Many of the principles given in the guide are applied in the chapter on case studies, which offers a problem / solution approach to the operation of an agitated feeder, loss in weight feeder make-up system and inclined screw feeder with agitator.
Most engineers will come to the 'Guide to the Design, Selection and Application of Screw Feeders' with both good and bad experiences of screw feeders.
For some, the guide will provide an important refresher on screw feeders with the opportunity to go into more detail.
However, for most, it will be a handy reference when specifying a new screw feeder or looking for ways of overcoming a feeder problem.
The Guide to the Design, Selection and Application of Screw Feeders is available from Ajax Equipment and costs GBP50.00.
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