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Sizing

     MMD Sizer

 

In 1978 MMD created the original compact Twin Shaft Mineral Sizer. The Sizer has now progressed from a 500 series machine through to a 1500 series machine, with capacities in excess of 12 000 tonnes per hour.

 

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THE PRINCIPLES OF SIZER TECHNOLOGY

 

The basic concept of the Sizer is as follows: two rotors with large teeth on small diameter shafts, driven at low speed by a direct torque drive system. This design produces the three major principles which interact with each other when breaking materials using Sizer Technology. The three unique principles are:

  1. The Three-Stage Breaking Action
  2. The Rotating Screen Effect
  3. The Deep Scroll Tooth Pattern

 

1. THE THREE-STAGE BREAKING ACTION

 

 

1.1 The First Stage of Breaking

First Stage of Breaking

This first stage of breaking takes place when the large rotor teeth grip the material as it falls onto the shafts. These teeth subject the rock to multiple point loading, which induces stress into the lump. This causes any natural weakness present in the material to be exploited.

   

1.2 The Second Stage of Breaking  

Second Stage of Breaking

The second stage takes place close to the centre line of the rotor, where the three point loading induces tensile stress in the rock. The three point loading is created between the top face of the tooth on one rotor and the top of the two opposing teeth on the other rotor. As the tensile strength of most materials is approximately 10% of the compressive strength, it reduces the amount of power consumed at this stage.

   

1.3 The Third Stage of Breaking

Third Stage of Breaking

Fitting a breaker bar below the centre line of the rotors allows any over sized lumps carried by the scroll through the rotors to be broken. Again, this exploits the weaker tensile strength of the material. The breaker bar, which forms a comb which the rotor teeth have to pass through, can be adjusted up or down to modify the product size.

   
 

 2. THE ROTATING SCREEN EFFECT

Rotating Screen Effect  

The interlaced toothed rotor design allows free flowing undersize material to pass through the continuously changing gaps generated by the relatively slow moving shafts. Undersize material fed to the Sizer or produced during the first and second stages of breaking, flow through the rotors with the assistance of gravity and the constant agitation caused by the rotating shafts. If the machine is fed from one end, this feature gives the added benefit of the smaller material being deposited on the belt first, followed by the larger material. This forms a sealing layer above the dust, which has environmental benefits on conveyor systems.

   

3. THE DEEP SCROLL TOOTH PATTERN

Deep Scroll Tooth Pattern

The deep scroll conveys the larger material to one end of the machine and helps to spread the feed across the full length of the rotors. This feature can be used to reject oversize material from the machine.

 

 

There are obviously other contributing factors in the breaking and passing of material through the Sizer which have yet to be quantified. Most of the development work is done in the field by carefully selecting opportunities which widen the range of materials that are handled by MMD Sizers. This programme of application-based development has driven the rapid advance of Sizer Technology and produced a wide variation of proven designs which can be applied to future projects. This practical approach is complemented by sponsored university study and the application of the latest technology in design and analysis of the forces that the equipment has to withstand.  

 

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