If you take Sieve Shakers for granted, you may be surprised to know that the selection of the right shaker can have a profound effect on your sieving results.
To begin with the basics:
The primary purpose of a sieve shaker is to provide motion to a sample
A combination of rotary and vertical motion will usually achieve that objective.
This concept of sieve shaking began with early Egyptians when they sieved grain. Later instructions for manually sieving become more detailed. The following process prescribed by ASTM.
A sieve or stack of several sieves is cradled loosely in a slightly inclined position in the crook of the arm. The sieves are tapped at the rate of approximately 120 times per minute with the flat of the hand. After about 30 taps, the sieves are put into a horizontal positioned and given a sharp vertical shake and a hard tap.
If This sounds to you like a tedious process --- You’re right. That’s why mechanical sieve shakers were invented.
The first me
In the beginning sample sizes were about what can fit into a large coffee mug. Manual sieving ,though tiresome was possible using 8” (200mm) sieves. This was the size adopted for the early shakers.
The application of sieving for particle size analysis expanded bringing extended requirements for:
These requirements set the stage for the development of many specialized shakers. With the advent of these new shakers, the need to evaluate performance characteristics was a development. Selection of the optimum shaker was a necessary consideration to meet the new criteria and specifications of changing samples.
Along with these requirements designers of the next generation of shakers began to deal with other problems. Among these are noise, agglomeration, changing sample size patterns and special characteristics of small micron size particles..
Electromechanical designs, rather than mechanical rotating and banging, were introduced to deal with appalling noise problems. Noise so deafening it cleared a la
Other solutions included jolting the sample through sudden starting and stopping, using ultra sonic agitators on the sieves, and applying air to break up the clumps.
A small sample in too large a diameter sieve leads to inconsistent results. The 3” (100mm) diameter sieve solved some of these problems. Of course modification of shakers were needed to accommodate these small sieves.
At the other end of the requirement is the need, in several industries, to do test sieving with large samples. Some material and some standards require 20 lb. (10kg) to 40 lb. (20kg) of sample. A robust design is called for that must still create a motion that exposes all the particles to all the apertures in the sieve.
Consideration of the special needs of each test sample is essential when selecting a sieve shaker. Because we handle shakers that cover the full gamut of these variables, we are sensitive to these special needs when evaluating a sieve shaker. The optimum shaker ensures good, reliable and repeatable sieve test results.
To more easily delineate the main issues in determining the best shaker, we have divided the units into three categories.
This category includes shakers that handle the simpler product and require 3” (100 mm) and 8” (200 mm) sieves and up to 6 lb.. of sample. The category also includes items that have capability to break up agglomerates and perform the wet sieving process. These shakers are quiet and can comfortably run sieve tests while not disturbing other lab functions.
Heavy duty shakers include the noisy mechanical Rotate and Tap machines that operate with smaller samples and a mechanical cam and motor device that can deliver good results with 12” sieves and
There are two other machines in this category that
These modern designs effectively deal with most sample problems such as agglomeration, electro-static clumping and blinding for all but the very small particle separations.
For some test samples a modern design, general purpose shaker can deliver consistent results down to 20 Microns. However, when the test sample for these very small particle exhibit the special problems we described, different techniques are needed.
Another methods is use a combination of a stream of high frequency air and a mechanical shock to get the desired result. These units can do up to six separations per test run.
The lower limit of particle size analysis with techniques that push the particles through hole is a mesh is about 3 Microns. Other techniques are used below that.
One of the commonly used methods for small particle analysis is Laser Diffraction. In this technique a laser beam is deflected by the particles. This deflection is measured and particle size dispersion is calculated. There are also several other method of small particle analysis.
I hope that this rant about sieve shakers does two things. First, you will get a sense that the selection of the best sieve shaker for your application needs to consider several factors. Second,that it further demonstrates why I am frequently sand bagged by unexpected detail/complications of the test equipment business.
Please share this with any colleagues who might find it interesting and useful.
I remain as confused as ever.
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