THEORY OF SCREENING

Welcome to Allied Mesh and Filters

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
  • 10/900's
  • 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

Machine Speed
Stroke 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450
3.00 0.94 1.07 1.20 1.35 1.50 1.67 1.84 2.02 2.20 2.40 2.60 2.82 3.04 3.27 3.50
4.00 1.25 1.42 1.61 1.80 2.01 2.22 2.45 2.69 2.94 3.20 3.47 3.76 4.05 4.36 4.67
5.00 1.56 1.78 2.01 2.25 2.51 2.78 3.06 3.36 3.67 4.00 4.34 4.69 5.06 5.44 5.84
6.00 1.88 2.13 2.41 2.70 3.01 3.33 3.65 4.03 4.41 4.80 5.21 5.63 5.98 6.53 7.01
7.00 2.19 2.49 2.81 3.15 3.51 3.89 4.29 4.71 5.14 5.60 6.08 6.57 7.09 7.62 8.18
8.00 2.50 2.84 3.21 3.60 4.01 4.44 4.90 5.38 5.88 6.40 6.94 7.51 8.10 8.71 9.34
9.00 2.81 3.20 3.61 4.05 4.51 5.00 5.51 6.05 6.61 7.20 7.81 8.45 9.11 9.80 10.51
10.00 3.13 3.56 4.01 4.59 5.01 5.56 6.13 6.72 7.35 8.00 8.68 9.39 10.13 10.89 11.63
11.00 3.44 3.91 4.42 4.95 5.52 6.11 6.74 7.39 8.08 8.80 9.55 10.33 11.14 11.93 12.85
12.00 3.76 4.27 4.82 5.40 6.02 6.67 7.35 8.07 8.82 9.60 10.42 11.27 12.15 13.07 14.02
13.00 4.06 4.62 5.22 5.85 6.52 7.22 7.96 8.74 9.55 10.40 11.28 12.21 13.16 14.16 15.18
14.00 4.38 4.98 5.62 6.30 7.02 7.78 8.58 9.41 10.29 11.20 12.15 13.14 14.18 15.24 16.35
15.00 4.69 5.33 6.02 6.75 7.52 8.33 9.19 10.08 11.02 12.00 13.02 14.08 15.19 16.33 17.52
16.00 5.00 5.69 6.42 7.20 8.02 8.89 9.90 10.76 11.76 12.80 13.89 15.02 16.20 17.42 18.09
17.00 5.31 6.04 6.82 7.65 8.52 9.44 10.41 1143 12.49 13.40 14.76 15.96 17.21 18.51 19.86
18.00 5.65 6.40 7.23 8.10 9.03 10.00 11.03 12.10 13.25 14.40 15.63 16.90 18.23 19.60 21.03
2: Rate of travel
  • 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.

Inclined Deck
Stroke
(mm)
Speed
(rpm)
Rate of Travel
(mm/sec)
"s"/stroke
(mm)
Dewatering 4.00 1400 66.00 2.83
General 10.00 900 106.07 7.07
Primary 15.00 750 132.58 10.61
Flat Deck
Stroke
(mm)
Speed
(rpm)
Rate of Travel
(mm/sec)
"s"/stroke
(mm)
Dewatering 4.00 1400 98.14 4.21
General 10.00 900 157.72 10.51
Primary 15.00 750 197.15 15.77
3: Aperture Type
  • 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.
3: Bed depth
  • 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.

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