Machine sieving by the 911MPESS200/300 and is carried out by stacking the sieves in ascending order of aperture size and placing the powder on the top sieve. A closed pan, a receiver, is placed at the bottom of the stack to collect the fines and a lid is placed at the top to prevent loss of powder. A stack usually consists of five or six sieves in a root two progression of aperture size. The stack of sieves is clamped on to a test sieve shaker that is vibrated for a fixed time and the residual weight of powder on each sieve is determined. Results are usually expressed in the form of a cumulative percentage of the nominal sieve aperture.
The three essentials required in a test sieve shaker are: An effective sieving action in order that an end-point is reached this end-point to be reached in a short time and reproducible results.
For the vibratory test sieve shaker a rapid vertical movement is needed to help keep the apertures clear and prevent blinding.
For routine control purposes it is usual to machine sieve for 20 min., after which time the sieving operation is deemed to be complete. It is recommended that sieving be continued until less than 0.2% of the sample passes through in any 2-minute sieving period.
For coarse aggregates, sieving is deemed complete when the rate falls below 1% per min.
It is generally recommended that if losses during sieving exceed 0.5% of the total feed, the test should be discarded. Preliminary hand sieving on the finest sieve should be carried out for the removal of dust. This dust would otherw ise pass through the whole nest of sieves and greatly prolong the sieving time; it would also percolate between sieves in the nest and increase powder loss. Extreme care needs to be taken when weighing the sieve residue. The powder retained on each sieve should be emptied on to a sieving paper and the underside of the sieve brushed lightly to remove any particles adhering to the sieve.
Sieving time is selected based on the time period after which no significant difference in mass change is observed. Sieving for a fixed period of time is more convenient than sieving until less than a certain fraction of the total mass passes through the sieve in a given period of time.
Weights 90 Kg