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Thread: Vibrating Screen Design

  1. #1
    Join Date
    Jul 03
    Posts
    8

    Unhappy Vibrating Screen Design

    Dear All,

    Could anybody advise me as to how should I carry out calculations for a screening system.. What I mean is :

    A vibrating screen ( 2 bearing design) has an eccentric shaft and 2 counter weights at the shaft ends. I would like to know what should be the amount of eccentricity on the shaft and how much should be the weights at the end...

  2. Hello Jaspal,

    Well I am not an actual designer of screen, however, following hints may be of some help.
    1) Decide amplitude and frequency, which can suit screening operation. This depends upon material and its size.
    2) In general, more amplitude and lesser frequency for coarse screening. Contrary lesser amplitude and more frequency for finer screening.
    3) Decide screen type based on 1 & 2 above. The type could be circular-throw, elliptical-throw, liner-throw, unbalanced mass, opposite revolving unbalanced masses, electromagnetic, etc. These can be intermixed.
    4) Decide screen deck size in conjunction with desired screening efficiency.
    5) Decide material flow (movement) speed on deck, and corresponding material mass that would be resting on the deck.
    6) Vibrating screen is a vibrating system. Thus, screen deck mass plus material mass on deck is total mass which is resting on springs. This is vibrated by exciting force. Mass, spring stiffness, damping factor (hysteresis losses) and exciting force are complementary; and together will create desired vibration. Proper design will result into desired screening, least efforts and least vibration to supporting-floor / house

    It is better to start with some reference existing model rather than from scratch. The knowledge of vibration engineering is must. The empirical reference data / information will be needed so that your design is in tune with present time, rather than at odd with well evolved present day design, since very long time development.

    Above are only the main (brief) guidelines.

    Regards,
    Ishwar G Mulani.
    Author of Book : Engineering Science and Application Design for Belt Conveyor.
    Email : parimul@pn2.vsnl.net.in
    Tel.: 0091 (0)20 5882916

  3. #3
    George Baker

    George Baker

    Regional Sales Manager

    TELSMITH Inc

    TELSMITH Inc

    Professional Experience 36 Years / 10 Month George Baker has 36 Years and 10 Month professional experience

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    Eccentricity Calculations

    Dear Sir: I am NOT an engineer, see my experience in the forum information area. I have been applicating, sizing, installing, commissioning and trouble shooting vibrating equipment since 1977. All those years in the field always, hands on.

    Contact with an actual vibrating screen engineer would be most ideal in this conversation but,

    Three key elements need to be addressed when designing a vibrating screen:

    SPEED
    STROKE
    SLOPE (or inclination)

    These guidelines are nicely described in the Vibrating Screen Manufacturers Association handbook. Describes the exact reasons why for all of the important elements required to be taken into consideration when designing a vibrating screen.

    The basic steps are many and here are but a few:

    1. Determine what needs to be screened: material, wgt cu ft, tons per hour, max infeed size, product size wanted.

    2. This will give you base data to do a mathematic vibrating screen sizing (standard formula) to determine the size of the vibrating screen needed in square feet to do the job.

    3. After determinig this factor, you must cross check the BED DEPTH to decide if it meets the guidelines of VSMA. This will either leave the size the same or require you to UPSIZE the machine footage to meet the bed depth needs. In other words, we DO NOT want to have 6" of material bed depth or mat at the end of the machine length for example on a 1/2" opening. This would result in carryover. RULE of thumb: WIDTH = More TPH and LENGTH = more clean or efficient screening

    4. I have a chart of SPEED STROKE SLOPE ideal combinations for different screencloth openings for both INCLINED circle throw type machines that use 20 degree incline or gravity to help the foot travel of the material on the deck. This chart also covers FLAT or horizontal screens to identify the proper setup to screen at various openings. Rule of thumb: The stroke or eccentricity must be large enough always to CLEAR the TOP DECK openings first or nothing gets thru and nothing happens or in other words = ZERO SCREENING EFFICIENCY. The assumed efficiency of screening calculations is always 95% efficiency on top deck, and declines for the lower decks on the same machine.

    5. Some typical SPEED STROKE SLOPE COMBOS for a 2 bearing free floating circle throw vibrating screen. We basically want to stay within 3.5 to 4.g's accelerating force to allow for efficient vibrating forces to actually screen good and not have the machine fly apart by operating at too high an accelerating force. This range of accelerating force basically refers to INCLINED CIRCLE THROW MACHINES working in the -3" +10mesh ranges.

    OPG: +1" -3": need 3/8" circle, 850rpm, 18-25 deg range
    - 1"+4mesh: need 5/16" circle, 900rpm, 18-25 deg range
    -1/2"+10mesh need 1/4" circle, 1000rpm 18-25 deg range

    Typical install angle is 20 degree and if material is non free flowing then, we make the angle either variable if on a portable screening plant or steeper if on a stationary plant. Typical FPM IN THE 60-75 FPM range.

    6. The finer the screening the smaller the eccentric slope needed to stratify the coarse fractions and the faster the rpm is needed to make more introductions to the cloth openings. For very fine screening we go to 45 deg slope availability with a pin prick or very tiny 1/32" or smaller eccentric stroke with many different types of vibrating equipment specially designed to specifically handle these fine cuts.

    7. HORIZONTAL SCREENS: Run at higher G forces due to the fact we do not have MOTHER nature or gravity to help us with the FOOT TRAVEL rate OR FOOT PER MIN. (FPM) On a horizontal the typical fpm is 45 FPM.

    8. Once we build the vibrating screen, with all the bells and whistles on it..........ie.....thicker liner plates, heavier than normal wirecloth, extended lips, heavy rubber vs wirecloth and take all the static weight into consideration - we now design the shaft and the eccentric offset to accomodate moving that static weight at the desired stroke or eccentric offset. The counterweights are typically a solid engineered mass that would have bolted on flywheel weights. These weights differ by manufacturer model.
    But, for example: It is not unusual for the flywheel to have three 1/2" plate weights bolted onto each wheel on the heavy side or six oclock position of the wheel. This would basically give us the largest stroke or circle you could get on the vibrating body. IE: 3/8" circle at 840 rpm. If i took 1 of plate weights off both wheels - it may reduce the stroke or circle size to 5/16" circle and then I would and could increase the speed of the motor sheave to run at 900 rpm, and if I took all the weights off the wheel completely - it may give me 1/4" circle which i could then speed up the machine to 1000 rpm by the rules of VSMA. RULE OF THUMB: always do 2 things..........not one. If you change stroke you should normally always change the speed. If i just sped up a unit that had a 3/8" circle size to 1000 RPM.......the accelerating forces would be way over the design factor and the sideplates and bearings would crack out or self destruct the machine.

    Hope this helps........for now........

    George Baker - MODERATOR

  4. #4
    Join Date
    Jul 03
    Posts
    8
    Thank you sir for your help

  5. #5
    George Baker

    George Baker

    Regional Sales Manager

    TELSMITH Inc

    TELSMITH Inc

    Professional Experience 36 Years / 10 Month George Baker has 36 Years and 10 Month professional experience

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    Thankyou Received

    Glad we could help you in this forum.

    and.........You are most welcome.

    George Baker - MODERATOR

  6. #6
    Join Date
    Jul 03
    Posts
    8

    flywheel on screen

    Dear All,
    I was just wondering what would happen if I put a flywheel on a vibrating screen??

  7. #7
    George Baker

    George Baker

    Regional Sales Manager

    TELSMITH Inc

    TELSMITH Inc

    Professional Experience 36 Years / 10 Month George Baker has 36 Years and 10 Month professional experience

    Discussions 872 George Baker acceded to 872 discussions, Articles 0 George Baker wrote 0 articles, Publications 0 George Baker Baker released 0 publications

    Searching nothing specified

    Know-How Screening (353) George Baker used this tag 353 times, Manufacturer (69) George Baker used this tag 69 times, Conveying (24) George Baker used this tag 24 times

    Flywheel

    Most conventional circle throw vibrating screens in fact operate with a big steel flywheel on both sides. These flywheels are either fixed weight flywheels or counter-weighted flywheels. Both fixed and counter weighted give the machine its "Stroke or Throw". The size, weight of the flywheel must be determined by figuring out the "static weight" and dynamic loading forces of the vibrating body.

    Counter-weighted flywheels will allow the user to add or subtract plate weights equally to make the machine stroke larger or smaller.

    George Baker, MODERATOR

  8. #8
    Join Date
    Jul 03
    Posts
    8

    Flywheel

    Dear Sir,
    Plz clarify these doubts. We use eccentric weights on both sides of our inclined vibrating screens and not flywheels. Would the flywheel arrangement result in any better performance than just semi circular eccentric weights?

    Also what do you mean by counter weighted flywheels?

  9. #9
    George Baker

    George Baker

    Regional Sales Manager

    TELSMITH Inc

    TELSMITH Inc

    Professional Experience 36 Years / 10 Month George Baker has 36 Years and 10 Month professional experience

    Discussions 872 George Baker acceded to 872 discussions, Articles 0 George Baker wrote 0 articles, Publications 0 George Baker Baker released 0 publications

    Searching nothing specified

    Know-How Screening (353) George Baker used this tag 353 times, Manufacturer (69) George Baker used this tag 69 times, Conveying (24) George Baker used this tag 24 times

    flywheels

    Are you using an INVICTA motor type drive system? IE: Half pie weights offset and adjustable from a setting of one to 10?

    On big screeners, circle throw type, we use a 7,8,9" diameter steel shaft on screeners that are 6,7,8 feet wide x 20 or 24 feet long. The flywheels are quite large and heavy to throw this mass.
    The counter weight aspect is..........the wheels have bolted on plate weights say 1/4" thick to 1/2" thick (1/2 circle size or pie shape) typically in the SIX O'CLOCK POSITION on the flywheel.
    If you have all the weights on the wheel - you would get the full stroke or eccentric offset and run correspondingly at the slowest RPM that cross relates to that circle size. If you took all the weights off the wheel and say ......ran a circle size of 1/4" or even 3/16" circle size ......you could run at the corresponding faster RPM for that size of circle (Higher or lower g force or accelerating force)

    You DO NOT WANT to run a heavy or large circle at a faster RPM as the accelerating forces would be too much or too high and cause the screen box to CRACK UP as it would be operating over the design limit of the box static weight.

    CHECK OUT THE VSMA GUIDELINES FOR full details.

    George Baker

  10. #10
    Lyndell Fuller Guest

    Symons Screen Co.

    Symons Screen Co. has recently developed a New Horizontal Shaker Screen for the mineral processing industry that can be adjusted to meet your screening requirements.
    Take a look at the New Symons H-Screen and all of Symons Screens at http://WWW.SymonsScreens.com
    There are several interesting facts and information concerning screen design and calculations on our web site. Take a look and give me your comments.

    WWW.SymonsScreens.com

    Symons has been manufacturing shaker screens for over 70 years for the aggregate, asphalt and coal industry.

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