THEORY OF SCREENING
The objective of Screening is to achieve SEPERATION and the basic principal for this is STRATIFICATION and the PROBABILITY OF SEPERATION.
In order for STRATIFIACTION to take place the distribution of particle size in feed to screening surface must be suitable for screening application, the correct amount of G force is required in order to allow the larger particle sizes to rise to the top and the smaller ones to the bottom in order to find a hole to fall through.
In order for a particle to find a hole to fall through the following must be taken into consideration,1: "g" Force
- Critical to ensure particles are vibrated to force stratification quickly and effectively
- Adversely affected by high installed mass
- Adversely affected by excessive bed depth
- Directly affects stroke and speed
- Rate of travel across screening surface
- G force = (amplitude x rpm2)/900 000 where amplitude = stroke/2
Surveys have revealed a large percentage of machines are set to factory test standard of 4.5 "g's" therefore not set for specific application (gravel , coal etc).Blinding and pegging can be a result of 10/900 machines.
Use the below table as a guide should your machine not be set up for a specific application
- Direct effect on both speed and stroke
- A flat deck or Inclined deck will also affect the rate of travel
Please see the below table as a guide.
- All things equal aperture does not affect stratification but low Open Area% will affect bed depth and have an impact on screening efficiency.Correct aperture type may assist with blinding and pegging.
- A direct correlation between feed to deck and Open Area% , if your feed rate is to high the bed depth of the screen will be incorrect and this in turn will throw out all other settings (g force ,rate of travel) making stratification impossible and therefore screening efficiency ineffective and this will have a negative effect on the life of your screens due to high wear as a result of poor Open area.
In summary , STRATIFICATION is essential to ensure cost efficient screening and all of the above has an impact on Stratification , if you can tune your machine to the correct settings for your application you will enjoy HAPPY COST EFFECTIVE SCREENING.
For any assistance with the above please contact us for some advice and solutions.