A Short History of Screening
Sep 25, 2024
The beginning of the story of screening predates recorded history, but it almost certainly originated with mans early efforts to extract clays and minerals from the raw earth. The first recorded reference to screening dates back to 150 BC, when descriptions of Greek and Roman mining methods tell of crude sieves of woven horse hair, planks and hides punched full of holes being used for particle separation. The first use of woven wire screens is attributed to the Germans in the fifteenth century.
The first mention so far found of mechanically shaken screens is in the Englishman John Smeaton's diary of his "Journey to the Low Countries" in 1775. At Rotterdam in the Netherlands, he found the Dutch pulverising volcanic rock in stamp mills, and using screens in closed circuit. He called them sieves. By method of a cord tied to a stamp mill, the head of the end of a declined screen was raised 150mm or so when the stamper went up, and dropped back with a jog when it went down. The screen was hinged near the lower end. The undersized material dropped into a hopper below, while the oversize was shovelled back into the stamp mill.
This piece of history is worth recording here, because it shows that the fundamental idea of a vibrating screen, as well as the advantages of closed circuit crushing were known in Europe over two hundred years ago.
Following this, various attempts were made to shake or vibrate screens by methods such as impact with hammers, tappets or cams. Some screens of this type were put into operation in the late 1890's and early 1900's.
From 1900 on, many methods of screening were tried. Barrel or rotary drum screens, and slow speed shaker screens were among the popular units.
About 1910 the first truly modern vibrating screens (500 rev/min.) and faster started to make their appearance. These early vibrating screens took several forms.
The first and simplest of this type was the forerunner of today's two bearing circular motion screens. It was vibrated merely by a shaft with an off-centre weight. When the shaft was revolved rapidly, it shivered or vibrated whatever was attached to it. Hence when rigidly attached to the screen frame, the weights vibrated the screen. The simplest frame was a wooden box, one side open at the lower end, and with a screen cloth fastened to the bottom. About halfway down the top of the screen box was a plank with a shaft supported between two bearings, and driven by a belt and pulley. The whole rig was mounted on wagon springs top and bottom.
The next type of mechanical vibrating screen developed was the four bearing positive throw type. On this machine the screen itself takes the place of the off centre weights as it rides around the circle of its eccentric shaft. The underlying principle was that the screen travelled in an orbit or vibrating motion equal to the eccentricity of its shaft on which the screen body rode. Counterweights were employed either as part of the drive shaft or as unbalanced flyweights at the end of the shaft to absorb the vibration of the screen.
The third type of vibrating screen experimented with in the early part of the twentieth century was the electric vibrating screen. This unit depended upon electric magnets and moving armatures for its vibration. The screen cloth was held in tension and the reciprocating armature was attached to the screen surface not far from the centre, flexing it every cycle.
During the 1920's and 30's improvements were made in the design of these types of screens. The screen frame was improved and the vibrator simplified by better lubricating methods and bearings. At this time machines for hauling and crushing aggregates were relatively small, and so 0.9m to 1.5m wide screens were all that were necessary to handle the tonnages.
In the early 1930's experimentation began with the development of a screen that could do both sizing and de-watering jobs, and operate on the horizontal (or near horizontal) rather than relying on gravity to convey the material. It achieved material travel by a linear rather than circular vibratory motion.
These screens were promoted because of their great advantage in saving head room. Development of this type of machine was continued to a point where in the early 1940's, the low angle linear motion screen became an accepted piece of equipment for de-watering coal and in other mining preparation plants.
By the 1940's vibrating screens had become an important part of most processing plants, and were replacing the older less efficient screening machines such as shaking screens and trommel screens. As the size of machines for digging, hauling and crushing increased, and the techniques for building screens developed, 1800mm wide units became popular and necessary to handle the increased tonnages.
Through the 1940's and 50's all the basic types of vibrating screens mentioned continued to be improved and developed in design. By this time the four bearing positive throw screen had established itself as the industry standard for tough heavy duty applications - but during the 1960's there was again a demand for larger (2.4m wide) screens, and the four bearing machine was replaced by the two bearing screen which had previously been used for many years only on lighter duties. The two bearing units quickly proved that they were capable of performing any job that a four bearing machine could do, and at less initial cost.
In the late 1950's and early 60's the free resonance type of screen was introduced. Its great appeal was that it required very little power because it generated most of its vibratory motion by the interaction of two elastically connected actuated masses. However because of high maintenance, high first cost, and precise adjustments required, this type of screen has lost it popularity. It could however emerge again if electrical energy becomes a major operating cost.
The 1980's saw a further push for even larger screens, and today 3500mm and wider models are built. Another recent development has been the introduction of the 'banana' screen. It uses a linear motion similar to the low angle screen, but differs in that the initial section of the screening surface is steeply declined, and this is followed by a subsequent relatively flat section at the discharge end. Banana screens can offer considerable advantages for applications where there are large tonnages of fines to be separated from the feed, and can satisfactorily handle materials with high surface moisture and difficult to screen inclusions of clay and dirt.
While there has been an increase in screening application knowledge coupled with gradual improvements in the design of such things as anti-friction bearings, development of high quality steels, improvement in methods of manufacture such as welding and high strength bolted connections - the design of the mechanical vibrating screen remains pretty much the way it was at its early concept.
Today the manufacturers of vibrating screens continue to search for ways to better separate the product through such methods as higher speed, greater throw, altering the method of presenting particles to the apertures, changing the motion of the screen surfaces, and innovations such as bi and tri-sloped decks.
THE FUTURE
Looking to the future it is visualised that while the basic design concepts of vibrating screens will remain the same, more emphasis will be placed on a number of areas.
Larger Screens: A future trend towards even larger machines is probably inevitable. This does not imply however that it is possible today to build screen of any desired size. This is not feasible for a number of reasons:
(a) The larger the screen the greater will be its mass, and the mass must be accelerated by drive units. The present generation of large screens already require multiple connected drive units because large antifriction bearings suitable for the speed are not available in the sizes that would be required for a single drive unit.
(b) When large vibrating screens operate, they will displace large air volumes. For example a unit having 30 square metres will displace 0.15 cubic metres per revolution, or 150 cubic metres per minute. This is enough to produce air pulsations inside a building which would forbid the presence of workers in the surroundings and cause window panes to break.
(c) The wider the screen, the larger must be the components between the side plates. For instance, the larger the deck frame cross members, the smaller will be the free passage between the cross members and the undersize particles will then be hindered from passing through the apertures. The result is that a greater percentage of the screening area will be effectively blinded.
For these reasons screen size limitations are to some extent unknown at this stage (the largest screens todate are limited to around 5.5m wide), but will no doubt be subject to the normal progressive trends to meet the demands of larger outputs that will occur in the industry over the decades to come.
There are a number of reasons why fewer larger screens are more economical than a greater number of smaller screens, and these include a savings in space, less supporting structure and chute work, they are easier to feed and have less wearing parts.
Screening Media: Attention will continue to be given to the types and construction of screening media. A major change came some years ago with the introduction of rubber, and later polyurethane materials as options replacing the long time use of steel wire and perforated plate.
Originally the rubber and polyurethane screening media were of the cross tensioned type, employing large mats spanning the width of the screen. A recent innovation has been the introduction of modular panels of small easily handled dimensions. These are secured by a variety of methods - but all allow for the modules to be moved around into different positions on the deck to ensure that maximum life is achieved from the whole.
Developments will continue in this area. With correctly applied screening media a much longer life can be achieved, and there are subsequent cost reductions in labour brought about by the increased intervals between panel changes.
Screening Efficiency: It is envisaged that in the future higher efficiencies of vibrating screens will be a subject of some attention. This will particularly apply to large fixed installation plants where high capital costs are involved and the plants are fully automated.
Environmental Restrictions will continue to increasingly enforce limits on noise, dust emissions, and plant operating hours.
The manufacturer that can successfully bring about improvements in the above areas will enjoy a decided advantage when the final choice of screen is decided.
Read More
Factory Hot Sale Mobile Tracked Jaw Crusher BGJ1475E
Jul 23, 2024
