Circle-throw vibrating machine

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Template:Copyedit A circle-throw vibrating machine is a screening machine employed in processes involving particle separation. In particle processes screening refers to separation of larger particles from smaller in a given feed stream, using only the physical properties of the material. Circle throw machines have simple structure with high screening efficiency and volume of output however holds limitations to the types of feed that can be processed smoothly. Some characteristics of circle-throw machines, such as frequency, amplitude of vibration and angle of incline deck also affect the unit output. When designing a circle throw vibrating screen, aside from the rotation inducing motor, specific consideration must be devoted to control of the vibration principles employed in the separation, in terms of structural stability and bearing stress and how suitable the feed is to use a circle-throw vibrating screen.

Range of application

Circle-throw vibrating screens are relatively new, multilayer, high efficiency mechanical screening equipment for material classification. They are widely used for screening stone stock of quarry, and classifying products in mining, sand, gold, energy and chemical industrial processes.[1] The targeted substance of screening is predominantly the finer particles, which can then be directed into a separation unit capable of further extraction e.g. hydrocyclones or are materials which can be removed and used. Removed materials are often formed intentionally and are classified into various categories by their shape, size and physical properties. For example, construction wastes are sorted and sieved of the by circular vibrating screen into coarse and fine particles, then the particles are taken to make concrete, architectural bricks and road base materials.[2]

Advantages and Disadvantages over competitive processes

Circle-throw vibrating screens operate on an inclined surface using a deck moving in circular motion presents an advantage over the typical flat vibrating screens as feed can be fed into the screen in a continuous matter rather than batch, leading to much greater product output. Having an incline overcomes the transport issue within the unit. A use of both circular motion and vibration is to separate particles delivers very high sieving efficiency.

Disadvantages are that circle-throw machines have large physical dimensions, and may require larger plots to place than other screening units. However as circle–throw screens are predominantly used in heavy-duty separations, large unit sizes are expected. Furthermore, the machine is less efficient towards fine wet and sticky materials unless a water spraying method is used. A water spraying process is to wash fine materials under spray bars.[3] Another drawback is to have a large stroke and allow heavy component to circulate and throw up in the screen box, a high horsepower motor is needed, which is a limitation over separators not requiring large motor action.

Circle throwing vibrating screens are usually used after a grinding process. There is no waste stream obtained from this unit, rather the feed is separated into ‘upper deck’ and ‘lower deck’ streams, the number of exit streams correspond to the number of decks. Quantity of product exiting from the unit is dependent on size distribution of the particles. Upper deck streams are can be immediately re-fed into the grinding units due to the continuous nature, thus greatly reducing transport time and costs.

Designs available

The standard circle-throw vibrating is a single shaft, double bearing unit constructed by sieving box, mesh, vibration exciter and damper spring. The screen framing is steel side plates and cross-members that brace static and dynamic forces produced when the machine operates. At the center of the side plates, two roller bearings with counterweights are connected for running the drive. Four sets of springs are fixed on the base of the unit to overcome the lengthwise or crosswise tensioned from sieves and panels and dampen movements.[2] An external vibration exciter (motor) is mounted on the lateral (side) plate of the screen box with a cylindrical eccentric shaft and stroke adjustment unit. At the screen outlet, the flows are changed in direction, usually to 90 degrees or alternate directions, which reduces the stream speed upon exiting.[4] High strength bolts and ring-grooved lock bolts are used to connect components.[5]

Circle-throw Vibrating Screen T-class[6]

Variations available in this design regard the positioning of the vibration components. One of them is top mounting vibration, which the vibrators are attached to the top of the unit frame and produces an elliptical stroke, this decreases the efficiency of the unit but will increase capacity by increasing the rotational speed, which is required for rough screening procedures where a high flow rate must be maintained.[7]

This can be varied to a counter-flow top mounting vibration, in which the sieving is more efficient as the material bed is deeper and feed is held on the screen for a longer time and would be employed in processes where a higher separation efficiency per pass is required.

If particularly loose particles are present in the feed, a dust hood or enclosure can be mounted. Water sprays may be attached above the top deck and the separation can be converted into a wet screening process.[2]

Characteristics and assessment

Screen deck inclination angle

The circular-throw vibrating screen generates a rotating acceleration vector and the screen must maintain a steep throwing angle to prevent transportation along the screen deck.[8] The screen deck is commonly constructed to have an angle within the range of 10° to 18°, in order to develop adequate particle transportation. An Increase of deck angle speeds the motion of particle with proportional relationship to particles size.[9] This decreases ranges of residence time and fine particles place in bottom and rough particles set at top (stratification) along the mesh screen faster.[7] However, if the angle is bigger than 20°, efficiency would decrease due to reduction of effective mesh area. Effect of deck angle on efficiency is also influenced by particle density. In the mining industry where the optimal screen deck inclination angle is investigated to be about 15°. [8] Exceptions are the dewatering screens at 3° to 5° and steep screens at 20° to 40°.

Short distribution time

On average, 1.5 seconds is required for the screen process reach a steady state and particles to cover the entire screen.[8] This is induced by the circular motion, the rotary acceleration has a loosening effect on the particles on the deck. Centrifugal forces spread particles across the screen. With the combination of the gravitational component, the efficiency of small particle passing through aperture is improved, and large size particles are carried forward to discharge end.[8]

Vibration separation

Under vibration, particle mixtures of different sizes segregate (Brazil nut effect). Vibration is an important characteristic of this type of screen as it lifts and segregates particles on the inclined screen It is suggested that when vibration amplitude is within the range of 3 to 3.5mm, equipment segregates the large and small particles with best efficiency.[8] If the amplitude is too high, the contact area between particles and screen surface is reduced and energy is wasted; if too low, particles will block the aperture, causing poor separation.[7]

Higher frequency of vibration improves component stratification along the screen and leads to better separation efficiency.[7] Most of circle throw screening equipment are designed with 750 ~to 1050 rpm, which is used to screen large materials. However, too high frequency vibrates particles excessively; therefore the effective contact area of mesh surface to particles will again decrease.[9]

Characteristics of feed

Presence of moisture in the feed cause formation of larger particles by coagulating small particles onto oversized particles, and this effect will reduce efficiency of sieving.[7] However the centrifugal force and vibration and acts to prevent blockage of aperture and formation of agglomerated particles. Sizes of feed particles are distinguished into fine, near-sized and oversized particles; most near-sized and fines particles pass through aperture rapidly. Therefore ratio of fine and near-size particles to oversize should be maximized to obtain high screening rates.[7]

Rate of feed is proportional to efficiency and capacity of screen; high feed rate reaches steady state and results better screening rates. However, there is an optimum bed thickness that should be maintained for consistent high efficiency of sieving.[7]

Stable efficiency

The steady state screening efficiency is greatly affected by the vibration amplitude. A good screening performance usually occurs when the amplitude is 3-3.5mm. The particle velocity should be no bigger than 0.389 m/s, if the speed is too big, poor segregation will occur, consequently low efficiency. Eo shows the efficiency of undersize removal from the oversize at steady state.

Where F is stph(short ton per hour) of feed ore, O is stph of oversize solids discharging as screen oversize, fx is cumulative weight fraction of feed finer than ‘x’ and ox is cumulative weight fraction of oversize finer than ‘x’. Eu shows the efficiency of undersize recovery. U is mass rate of solids in the undersize stream.

Thus

Heuristics/design considerations

Vibration design

Circle-throw vibrating units rely on the screen component being operated at a resonant frequency in order to achieve efficient sieving of particles. While properly selected vibration frequencies drastically improve the filtration abilities of the screen, a static deflection factor in introduced. A deflection factor occurs as the vibrations causes smaller particles to displace thus not properly passing through the screen due to excess movement and is a property of the system natural frequency. The natural frequency that a system will preferably vibrate at Fn is 188(1/d)2 (cycles per min) where d = (188/Fn)2 (inches). Static deflection corresponding to this frequency. Vibration isolation is a control principle employed to mitigate the transmission of vibrations. On circle-throw vibrating screens, passive vibration isolation in the form of mechanical springs and suspension are employed at the base of the unit, which provides stability for the unit and control of the vibration generated by the motor. A general rule of thumb regarding the amount of static deflection minimization that should be targeted with respect to operating RPM is provided in the table below.[10]

Critical vibration isolation installations[10]

Critical installations refer to roof mounted whilst non-critical refers to base mounted installations. Weight, loading and weight distribution are all elements which must be considered.

Common rule of thumb table for vibration isolation design[10]

Heuristic on the roller bearing design

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Whilst designing a circle-throw vibrating screen with a shaft and bearing system, consideration must be made regarding the loading the unit will undergo as well as extra loading to the screen box created by the centrifugal force, due to the circular motion of the load as it passes through the unit. Bearings must be designed to accommodate for the extra stress. Bearing load due to screen box centrifugal force (Fr) is

Supplementary factor of Fz = ~1.2 is used to account for unfavorable dynamic stressing:

Index of dynamic stressing FL, speed factor Fn are used to calculate minimum required dynamic loading (kN)

FL is taken between 2.5-3 generally as to correspond to a nominal fatigue life of 11,000-20,000 hours as part of a usual design.[11]

Structural support of vibrating equipment

As the processing ability of the unit is related to the vibration, a high level of care must extend to the design of the structural and support aspects. If the structural design does not meet the requirements to stabilize the unit (e.g. too resonant will lead to extreme vibrations) then excessive vibrations may be introduced, again leading to higher deflection or reducing the effectiveness of installed isolation.

Unit loading with total spring stiffness 2k [12]

Taking in total static force applied to the unit and stiffness of the springs used in the design:

When the dynamic forces of the loading is taken into account,an amplitude magnification factor (MF) must be considered:

An estimation of the magnification factor for a system with one degree of freedom may be gained using:[12]

Most structural mechanical systems are lightly damped. If the damping term is neglected:

where fd/fn represents frequency ratio (frequency due to dynamic force, fd, and natural frequency of the unit, fn).

the relationship between amplitude magnification factor of vibrations to the amount the unit has been dampened to resist natural frequencies (lower denominator in ratio thus higher ratio, and lower MF) [12]


Length/width for the circle-throw vibrating screen

Once the area of the unit is known, as a general vibrating screen design rule the length and width must be calculated so that a ratio of 2-3 width(W) to 1 length (L) is maintained. Capacity is controlled by width adjustment and efficiency by width.[13]

The bed depth D must be lesser or equal to

Xs is desired cut size.

(ft)

Starting deck angles can be estimated from

F= ideal oversize flowrate, standard widths for circle-throw machines are 24,36,48,60, 72, 84, 96 inches. Measurements should be matched to available "on-shelf" units to reduce capital cost.

Aperture size and shape

At a fixed screen capacity, efficiency is more likely to decrease with as the aperture size decreases. The aperture shape strongly influences screen performance, in general, particles are not required to be separated precisely at their aperture size when using circle-throw vibrating screen. However efficiency is improved if the screen is designed to filter at as close to the intended cut size as possible. The selection of aperture type is generalized by the table below:

Rules of thumb equations for screen aperture design[7]

New developments

In previous years most processes have employed two-bearing circular vibrating screens. Two- bearing circular vibrating screens with a screen box weight of 35 kN and speed of 1200RPM. The centroid axis of the screen box and unbalanced load does not change during rotation. After adopting traditional screens advantages and absorbing technology from abroad, a four-bearing vibrating screen (F- Class) was developed.[14] This development is meets demands especially for iron ore, phosphate and limestone production industries. F-Class features a HUCK-bolted screen body that is connected for extra strength and rigidity and carbon steel is used for building the side plates to give a high strength. The shaft is strengthened with a reinforcing plate, which attaches to the slide plate and screen panels.

Four-bearing screen provide much greater unit stability thus higher vibration amplitudes and/or frequencies may be used without excess isolation or dampening; overall plant noise emission. The new design gives an accurate, fast sizing classification with materials ranging in cut size from 0.15 to 9.76 inches and high tonnage output where it can process up to 5000 tons per hour.[6]

References

  1. Interval Equipment, ‘Separating Different Materials’, U.S.A., 2013 [Accessed: 13 October 2013] http://www.intervalequipment.com/contact.html
  2. 2.0 2.1 2.2 Siebtechnik Gmbh, ‘Screening Machines Process Equipment – Circular and elliptical motion screens, Double counterweight screens’, Germany, 3rd 2013
  3. Crushing Plant Grinding Mill, ‘Circular Vibrating Screen’, China, 15 February 2012 [Accessed : 9 October 2013] http://www.coal-crusher.net/equipment/circular-vibrating-screen.html
  4. PT. Rutraindo Perkasa, ‘Round (Circular) Vibrating Screen/ Single Eccentric vibrating screen’, Indonesia, 2000 [Accessed: 9 October 2013] http://www.rutraindo.com/stonecrusher/round-circular-vibrating-screensingle-eccentric-vibrating-screen
  5. ZhengZhou YiFan Machinery Co. LTD, ‘YK Series Inclined Vibrating Screen’, China, 2009
  6. 6.0 6.1 W.S. Tyler, ‘Technical Specification F-Class Four Bearing Vibrating Screen’, 2012
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 FUERSTENAU, M.C. and HAN, K.N., Principles of Mineral Processing. Society for Mining, Metallurgy, and Exploration (SME).
  8. 8.0 8.1 8.2 8.3 8.4 L. Zhao, Y. Zhao, C. Liu, J. Li and H. Dong, ‘Mining Science and Technology (China)’, Vol 21, pp. 677-680, 2011
  9. 9.0 9.1 STANDISH, N., BHARADWAJ, A.K. and HARIRI-AKBARI, G., 1986. A study of the effect of operating variables on the efficiency of a vibrating screen. Powder Technology, 48(2), pp. 161-172.
  10. 10.0 10.1 10.2 Engineering Cookbook- A handbook for the Mechanical Designer. 1999. 2nd edn. Springfield, MO: Loren Cook Company
  11. The Design of Rolling Bearing Mountings [Homepage of FAP Group], [Online]. Available: http://www.m3.tuc.gr/ANAGNWSTHRIO/STOIXEIA%20MHXANWN/PDF%20APO%20FAG/WL_00200_5_T6-8_de_en.pdf[10/12, 2013]
  12. 12.0 12.1 12.2 SAYER, R.J., The effect of structural support conditions on the vibration characteristics of machinery. Medina, Ohio: Sayer & Associates Inc
  13. MULAR, A.L., 2003. Size Separation. In: M.C. FUERSTENAU and K.N. HAN, eds, Principles of Mineral Processing. Littleton, Colorado, USA: Society for Mining, Metallurgy and Exploration;, pp. 119
  14. Canada’s National Equipment Newspaper Equipment Journals, No. 4, pp. B9 25 March 2013