A Fundamental Approach to Belt Feeder Loads

Belt Feeder Design

A Fundamental Approach to Belt Feeder Loads

How to assess loads on Feeders, (practically)
Feeders are widely used for metering bulk solids and discharging the contents of hoppers and silos. Numerous attempts have been made to describe the process of feeding but quite often they only cover certain products and hopper construction. In this article the reader will find a more general approach to this field of problems.
(ed. WoMaMarcel - 01/9/2015)
<Blank Space>

Reason why outlets are larger than critical span should be critically examined.

  • Predictive theories are undoubtedly conservative.
  • To accommodate exceptional conditions that may arise rarely, if ever.
  • To provide margin to guarantee flow.
  • To guarantee the flow rate required.
  • To secure progressive extraction from slot outlets.
  • Accommodate possible change or uncertainty of product condition.
  • Provide extra storage capacity.
  • Save storage headroom.
  • Poor design confidence.
     

The feeder must accommodate both initial start-up and running loads. The forces required to initiate discharge can be many times that to sustain running, but they only occur for a short time, are usually infrequent and, most importantly. It is therefore good practice to take steps to mitigate these. By contrast, running loads continue through the total operating life of the equipment and determine the essential energy consumption. This paper therefore concentrates on the dynamic condition of operation and means to reduce starting loads. Excess feeder loads increases both capital and operating costs. (Construction, maintenance and power).

The shear stress generated by a feeder extracting material from the hopper outlet is dependent on the normal pressure acting, but is also sensitive to whether the failure plane has been previously sheared. This feature greatly emphasises the benefits of initiating discharge under conditions of light normal stress at an early stage of initial filling of the hopper. Very limited belt travel is needed to develop a shear plane. In most cases this can be accomplished before extracted material reaches the end drum of an empty belt.

The maximum transfer capacity that can be handled with a flat belt is a triangular cross section with a slope at the angle of repose, θ, of the material, so the width and height of the sides of the exit and the product repose angle determines the belt width required. For a firm, granular product that is likely to be free flowing, the hopper outlet width should be small in relation to the height, to secure the largest slope possible for the shear plane to expand along the hopper length.

The hopper outlet width is likely to be dictated by the span necessary to avoid the formation of a structural arch of large granular materials, whereas a wide span needed for a cohesive product may have lower heights because the expansion along the shear plane is less for fine products and a shallower shear plane may be utilised.

An interface construction on these lines is shown in Fig. 5 with the shape of the exit from the hopper aligning with the stressed arch and repose conditions on the bulk material. There will be a slight collapse of the arch shape at exit as the inclination at the base of the stressed arch will almost certainly be steeper than the repose condition of the bulk.


Fig. 5: Optimised belt feeder interface.

Upcoming Events

Facebook