Bulk Engineering, Vibrators

Good vibrations for mass flow

David Wilkinson compares the fundamental physical conditions necessary to produce mass flow of bulk solid feed material for gravity flow and vibratory pan type systems through a hopper typically located under a storage vessel.

To eliminate flow impediments, consider the hopper and feeder trough as one integrated unit, as per figure 7, and adopt the ideal hopper/trough geometry recommended by Syntron in “Working with Hoppers”[1] as the starting point of a design.

Figure 7 - integrated vibe feeder and hopper.
Figure 7 – integrated vibe feeder and hopper.

Bulk solid flow control

Feed rate is accurately controlled by adjusting the trough amplitude of a two-mass vibe feeder or the frequency of a single-mass vibe feeder. Changing the gate position is not recommended for rate control as it can create geometry that may cause problems with the flow. The gate can be adjusted slightly to cater for variations in bulk density and flowability of the material being handled, however it is best used for isolation to help facilitate maintenance.

The control of gravity flow systems by belt and screw feeders is far less accurate and reliable due to the areas of flow stagnation as explained below. This can lead to material build up and possible blockages.

Jenike direct shear test cell

Bulk material’s physical properties are measured using a standardised Jenike Shear Test Cell. In “Influence of Vibrations on the Strength and Boundary Friction Characteristics of Bulk Solids and the Effect on Bin Design and Performance” Roberts, Oom & Scott’s [3], the effect of vibrating halves of the shear tester for a pyrophyllite bulk solid is investigated. 

Fig 1 – Vibe and no vibe shear cell results comparison.

Similarly fine limestone and titania powders investigated by Kollman and Tomas in “Effect of Applied Vibration on Silo Hopper Design”[4] produced similar results which are consolidated into the general diagrams below where “the angle of internal friction remains constant and the yield loci is shifted parallel towards smaller shear stresses.”

The relation between vibe and no vibe yield loci and flow functions shown in figure 1 and 2 can also be expected in real integrated hopper-feeder applications as introducing vibrations moves the flow function down. This enables mass flow at lower values of unconfined yield stress and major consolidation pressure.

Fig 2 – Flow functions comparison.

The flow function is determined from the instantaneous yield loci. It simulates the bulk solid failure states. Note however Roberts, Kollman and Tomas have only investigated devices that vibrate the hopper wall or bin activators and have not considered a vibrating feeder pan. Accurate data on feeder installations is scarce so a qualitative approach is taken here.

It can be difficult, although not impossible to retrofit a vibe feeder once a large silo is built so consideration for a vibe feeder under bulk storage early in a project is recommended so as to maximise control, output, hygiene and safety.

Bin hopper flow systems

Installations where a circular bin is added onto a hopper will increase the fill pressure onto the consolidation pressure already present in the hopper. As bulk granular materials having cohesive properties, moisture, and very fine particles (typical in powdered bulk solids, coal and ore fines) tend to stagnate flow and so they commonly require vibration or air cannons to break up lumps. Fine powdered bulk solids will require careful evaluation considering particle size and density to achieve a workable solution.

It is more common for flow to partially or totally stagnate (funnel flow, ratholing) when belt or apron feeders are used under a hopper without vibrations to reduce the bulk material internal friction.

Bin and hopper vibrators can help but their effects are minor compared to that of a vibrating feeder trough pan. 

Jenike’s sacred hand sketches in figures 3 and 4 show the flow profile that can occur when a large vibrating surface of a feeder is absent. A meaningful comparison between the feeding methods is illustrated by the amount of dead feed. 

Figure 3 (left) – Effective area of an apron and belt feeder hardware. Figure 4 (right) – Effective area of outlet in a screw feeder.

As the effective area of a vibe feeder pan in contact with feed is much larger than the effective area of a belt or apron feeder’s chain and bar gear, the capability to move bulk feed volumes is vastly different.

Discrete Element Modelling (DEM) in figure 5 shows excitation forces inducing particle movement up into the hopper, breaking up any stagnation or dead zones.

Figure 5: DEM of iron ore feed in a vibe feeder.

Beware of rock box add-ons. Intended to reduce wear, they can compromise hopper throat and gate geometry and can introduce severe throttling of flow due to the increased internal friction reducing bulk material transport velocity.

Eliminating coal fines arching

Fines and other cohesive material can still support an arch over a hopper opening even with a vibe feeder if it falls short of the criteria defined in “Flow Properties and Design Procedures for Coal Storage Bins” by Brian A. Moore.[7] 

This was the case at Clarence Colliery NSW where an existing coal fines hopper discharge opening continually blocked preventing coal blending. Opening the hopper transition output shown in figure 6, above the critical arching dimension coupled with the effect of the vibe feeder pan enabled mass flow.

Quantifying vibration effects

Syntechtron uses a simple vibration factor Vf shown in the equation below, derived from “Effect of Applied Vibration on Silo Hopper Design” Th. Kollman and J. Tomas [4] to adjust unconfined yield stress from a static to the dynamic application.

unconfined yield strength = Vf x function (major consolidating pressure) x scaling factor [13]

where Vf = constant + Fn (gate)

Belt and apron feeders

Apron feeders are effective in handling large lumps and cope well with damp fines, however they are expensive and have higher levels of maintenance. Belt feeders are effective as conveyors transporting bulk materials over relatively long distances.

Good vibrations

For a wide range of feed applications vibratory feeders conserve plant height and footprint. They also reliably deliver feed at an easily controlled rate. The only maintenance required on resonant electromagnetic type feeders is the replacement of wear liners in the trough over an expected 40 year service life.

Similarly for bulk solid material stockpiles, mass flow and larger capacities can be expected by installing vibratory stockpile reclaim equipment [11]. 


  1. Working with Hoppers – Syntron Materials Handling, LLC.
  2. Syntechtron is the Australian licencee of Syntron MH Mississippi USA.
  3. Influence of Vibrations on the Strength and Boundary Friction Characteristics of Bulk Solids and the Effect on Bin Design and Performance – A.W. Roberts, M Ooms and O.J. Scott, Australia.
  4. Effect of Applied Vibration on Silo Hopper Design – Th. Kollmann and J. Tomas 
  5. Storage and Flow of Solids – Andrew W Jenike
  6. Flow Properties Testing of Limestone Samples Mincore – Andrew Grima & Jon Roberts, Bulk Materials Engineering Australia, University of Wollongong
  7. Flow Properties and Design Procedures for Coal Storage Bins – Brian A Moore, University of Wollongong.
  8. DEM Feed Hopper Transition Bluescope Ship Unloader No3 – Brad Glass Avesta
  9. Feeders for Bins and Hoppers – H.Colijn and P.J. Carroll- Materials Handling Magazine Article August, 1970 Vol. 15, No8
  10. New Design Criteria for Hoppers and Bins – Dr J.R.Johanson and H.Coljn US Steel Corp October 1964
  11. Vibratory Stockpile Reclaim Flow Stimulation and Control ICBMH 1998 – K. Bagust, Syntechtron
  12. Performance Characteristics of Gravity Reclaim Stockpiles of Concical Form – A.W.Roberts Dean, Faculty of Engineering, The University of Newcastle, NSW
  13. Vibratory feeder calculations – Robert Rudd, Syntechtron
  14. Flow Properties of Powders and Bulk Solids – Dr.Ing. Dietmar Schulze, University of Applied Sciences Braunschweig/Wolfenbuttel, Germany
  15. Case Study Clarence Colliery – Eliminating Feed Arching Syntechtron
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