The construction sand in its natural status, contains organic matters, pebbles, and gravels of different shapes and sizes. When we need the sand of required specification for the particular construction work, we are left with no option but to sieve the sand. To make this process speedy, and according to the type of sand and site conditions, various models of sand screening machines are available in the market.
Sand should be poured in the hopper and the sieved sand gets collected beneath the cylinder, or in front of it. In some regions, the cylindrical drum is also called a trommel drum and hence the machinery as a trommel drum screening machine.
Sand is poured over the vibratory table and the sieved sand gets collected in the sloppy tray provided at the bottom, which separates them in different directions. In these two types, circular vibratory is more efficient in screening wet sand.
In this type, after screening the sand, washing will be done in the second layer simultaneously to remove the silt, dirt, and unwanted chemicals from the sieved sand. This machine saves power and labor costs when the sand brought from the supplier needs washing. Otherwise, you have to install a separate sand washing machine to carry out this washing process.
All these types of machines are available in various models, with the different screening capabilities according to the person's needs. However, the basic working principle remains the same as described above.
For the characterization of bulk goods of different forms and sizes, the knowledge of their particle size distributions is essential. The particle size distribution, i.e. the number of particles of different sizes, is responsible for important physical and chemical properties such as solubility, flowability and surface reaction.
In many industries such as food, pharmaceutics and chemistry traditional sieve analysis is the standard for production and quality control of powders and granules. Advantages of the sieve analysis include easy handling, low investment costs, precise and reproducible results in a comparably short time and the possibility to separate the particle size fractions. Therefore, this method is an accepted alternative to analysis methods using laser light or image processing. To guarantee a high degree of reproducibility and reliability, sieve shakers and accessories have to fulfill the requirements of national and international standards. This means that test sieves, sieve shakers and all other measurement instruments (e.g. scales) which are used for the characterization of particle distributions have to be calibrated and subjected to test agent monitoring as part of the quality management system. Apart from that, it is absolutely necessary to carry out the sample preparation with great care. Only then is it possible to achieve sieving results which allow a reliable characterization of a product.
During sieving the sample is subjected to vertical movement (vibratory sieving) or horizontal motion (horizontal sieving). With tap sieve shakers both movements are superimposed. During this process the particles are compared with the apertures of every single sieve. The probability of a particle passing through the sieve mesh is determined by the ratio of the particle size to the sieve openings, the orientation of the particle and the number of encounters between the particle and the mesh openings. The appropriate sieving method depends on the degree of fineness of the sample material (fig. 1). Dry sieving is the preferred method for the size range between 40 m and 125 mm. However, the measurement range is limited by properties of the sample such as a tendency to agglomerate, density or electrostatic charging.
The sample is thrown upwards by the vibrations of the sieve bottom and falls back down due to gravitation forces. The amplitude indicates the vertical oscillation height of the sieve bottom. Due to this combined motion, the sample material is spread uniformly across the whole sieve area. The particles are accelerated in vertical direction, rotate freely and then fall back statistically oriented. In RETSCH sieve shakers, an electromagnetic drive sets a spring/mass system in motion and transfers the oscillations to the sieve stack. The amplitude can be adjusted continuously to a few millimeters.
In a horizontal sieve shaker the sieves move in horizontal circles in a plane. Horizontal sieve shakers are preferably used for needle-shaped, flat, long or fibrous samples. Due to the horizontal sieving motion, hardly any particles change their orientation on the sieve.
In a tap sieve shaker a horizontal, circular movement is superimposed by a vertical motion generated by a tapping impulse. Tap sieve shakers are specified in various standards for particle size analysis. The number of comparisons between particles and sieve apertures is substantially lower in tap sieve shakers than in vibratory sieve shakers (2.5 s-1 as compared to ~50 s-1) which results in longer sieving times. On the other hand, the tapping motion gives the particles a greater impulse, therefore, with some materials, such as abrasives, the fraction of fine particles is usually higher. With light materials such as talcum or flour however, the fraction of fine particles is lower.
The air jet sieve is a sieving machine for single sieving, i.e. for each sieving process only one sieve is used. The sieve itself is not moved during the process. The material on the sieve is moved by a rotating jet of air: A vacuum cleaner which is connected to the sieving machine generates a vacuum inside the sieving chamber and sucks in fresh air through a rotating slit nozzle. When passing the narrow slit of the nozzle the air stream is accelerated and blown against the sieve mesh, dispersing the particles. Above the mesh, the air jet is distributed over the complete sieve surface and is sucked in with low speed through the sieve mesh. Thus the finer particles are transported through the mesh openings into the vacuum cleaner or, optionally, into a cyclone.
We all know the term quality. It is widely used to describe a product of particularly high value. However, the exact definition of quality is as follows: Quality is the compliance of defined properties with the detected properties of a product as determined by performing tests. A product can be described as high-quality if a test measurement ascertains that the desired properties lie within a given tolerance. If the measured values deviate too much, the quality is lower. Many materials, whether natural or artificial, occur in dispersed form (material which does not form a consistent unity but is divided into elements which can be separated from each other, e.g. a pile of sand). The particle sizes and their distribution within a material quantity - i.e. the fractions of particles of different sizes have a crucial influence on physical and chemical properties.
These examples clearly show how important it is to know the particle size distribution, particularly within the context of quality assurance of bulk goods for production processes. If the particle size distribution changes during the production process, the quality of the product will change as well.
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