Re: Vibrating Screen

Posted on 11. Dec. 2006 - 11:26

The following formula can be used in order to to determining minimum electric motor sizes for circular motion vibrating screens

kW = (mR x n x 1.25) / (97,400 x LRT)

where

kW = design power

mR = total static moment of counterweight assemblies. i.e.: mass x radius value (kgcm)

N = screen speed (rpm)

1.25= a reserve factor for cold weather starts, possible unanticipated field machine speed increases, shortcomings in line voltage, wire sizes etc.

LRT = motor locked rotor torque in a decimal form ie: 250% would be 2.5 in the formula. For vibrating equipment the general recommendation is that the motor should have a min LRT of 200%

John McKenzie

Re: Vibrating Screen

Posted on 11. Dec. 2006 - 02:48

Hi John,

Thanks for the formula ...

But how do you relate the total weight of screen ( empty wt of screen box + wt. of moving material ) w.r.t total static moment of counterweights in terms of formula.

is it m * r * w2 = M * g

where m = mass of eccentric weights,

r = eccentricity ie distance from cg tocentre of shaft.

w = angilar velocity,

M = total mass of screen,

g = acceleration due to gravity.

Please help me out..

Thanks once again,

regards,

sujeet

Re: Vibrating Screen

Posted on 12. Dec. 2006 - 10:59

Sujeet,

I am not exactly clear as to your question, but the static moment mR (kg.cm.) in the formula is simply the total mass of the eccentric weights (kg) x the distance from the centre of the shaft to the centre of gravity of the eccentric weights (cm).

Another way to determine mR is to multiply the total vibrating weight of the screen (kg) by its vibrating radius (cm). This will give the same answer.

Does this answer your question?

John McKenzie

Re: Vibrating Screen

Posted on 12. Dec. 2006 - 12:26

Hi John,

Thanks for the answer....

Can i take the vibrating radius as maximum amplitude to be developed with full load on the screen?

very eager to hear from you...

regards,

sujeet

Re: Vibrating Screen

Posted on 13. Dec. 2006 - 05:05

Sujeet,

Yes, for the vibrating radius you can use the amplitude of the screen when fully laden – provided that when determining mR you use the weight of the fully laden screen.

Alternatively you can use the unladen amplitude multiplied by the unladen screen mass – theoretically both will give the same answer.

John McKenzie

Re: Vibrating Screen

Posted on 13. Dec. 2006 - 01:34

Hello John!

I am quite happy with your answer and realy greatful to you....

So now is the controversy..when manufacturer specifies stroke and rpm...is the stroke value with fully loaded screen or empty screen....? to my understanding it should be with full load but people while doing the testing they do it with empty screen and decides the stroke...

Please advise on this..

meanwhile i remain..

regards,

sujeet

Re: Vibrating Screen

Posted on 14. Dec. 2006 - 08:44

Sujeet,

From my experience most screen manufacturers will state the unladen stroke.

In practice the original stroke is selected to withstand a reduction due to material load (typically perhaps up to 10%), and still achieve good screening efficiency.

However there are always exceptions where the reduction in stroke can be substantial and cause problems. Say for example we have a relatively light screen with a reasonably high feed rate, and with only a small proportion of the feed passing thru the lowermost screen cloth. In such a case the material load can be great enough to bog the screen down, but if the throw is increased to overcome this there can be danger when running the screen in its unladen condition because the high acceleration can be enough to decrease the life of the bearings and screen frame components.

In selecting a stroke set up I always like to consider both unladen and laden conditions, and compromise if necessary. Of course to be able to do this the manufacturer needs to know the full application details (feed rate, material feed grade analysis, mesh specifications etc.etc.), and this is often difficult to obtain.

John McKenzie

Re: Vibrating Screen

Posted on 18. Dec. 2006 - 06:49

Hi John!

Sorry for my late response as i was out of station...thank you once again for details...

John ...can u help me solving my problem?..I have a screen for scalping limestone material which has to be upgraded for higher TPH coupled with narrow gradation of material...

These are the details of screen..

Make : Peytec, Austria. circuler motion.

TPH : 280 tons/hr ( rated )

rpm : 850.

stroke : 10mm

mesh details : top - 50 mm.

middle - 30 mm.- releaving deck.

bottom - 20 mm.

motor kw : 22 kw.

gradation of i/p material :

-160+50 = 20% ie 140tph

-50+20 = 10%. ie 70 tph

-20 = 10%. ie 70 tph

size of screen = 1600 * 2400 length...

this has to be upgraded for..

tph : 370 tons/hr.

gradation of i/p material :

-160+50 = 28% ie 200 tph

-50+20 = 14.5%. ie 100 tph

-20 = 10%. ie 70 tph

is the configuration ok? or should i replace top deck with 75 mm mesh, middle with 50 mm mesh and bottom with 20 mm mesh..

very eager to know from you,

thanks,

regards,

sujeet

Scalping Screen

Posted on 22. Dec. 2006 - 04:33

Hi Sujeet:

1. The screen is typically setup for the application data received from customer. Yes, it is tested empty typically for 4 hrs to ensure all components are tight, bearings are not running hot etc prior to shipping.

2. Your stroke and speed is actually in line with the openings you are running. Just over 3/8" in size in 850 range is good.

3. Going from 280 to 370 is plus minus a 32% increase in feed to the screen box. You are only dealing with nominal 42.4 sq ft of box (5.3 feet wide by 8 feet long) SO...... a question is this.

4. Before you make a lot of changes to openings, DOES the present setup operate efficiently or no. Do you have carryover problems or no.

5. Lastly, a typical scalper screen setup needs only to be plus minus 85% efficient vs 95% in a real sizing application.

Good luck.

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Motor Sizing Screens

Posted on 22. Dec. 2006 - 05:16

John gave you the math for calculating which is excellent.

Generally:

1. We need enough motor to turn the shaft and start to move the body over the 12 o'clock position from a dead stop. Once their we are good to go.

2. We typically go to a high torque motor for startup as one option, but normally just double the standard hi eff motor pick from the calculated to ensure we can get it started. Also, availability of std motors is readily available vs Hi torque.

3. On startup we see high amps and once started we run at 1/2 amps. We really just need the bigger motor to get er going.

Hope this helps.

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Re: Vibrating Screen

Posted on 22. Dec. 2006 - 06:28

Hello George!

Wishing you a ...MERRY CHRISMAS...!

and thanks for your help and interest shown in my issue..

Ya..the screen works well in normal operation of 280 TPH with hardly some 2 - 3 % undersize carry over in +20 mm material...

Initially there was more carry over but later we reduced the inclination from 14 degree to 12 degree and now it's fine..

Thanks,

regards,

sujeet

Carryover

Posted on 23. Dec. 2006 - 03:10

Sujeet:

thanks for your answer. Interesting comment: you reduced the incline from14 to 12 deg (actually flatter) and corrected the fines carryover on the 20mm opg.

I would predict you would increase fines carryover by making the inclination flatter.

For more tph: a wider screen (width equals more TPH) and more length equals more retention time equals more efficient equals less fines carryover.

Lastly, if you want more TPH on this scalper and stay with the same size, raise the machine to somewhere between 12 deg and 20 deg and you will see a HUGE difference in increased TPH.

Hoping Santa is shaking his way towards your house now.

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Re: Vibrating Screen

Posted on 26. Dec. 2006 - 10:55

Hi George!

In fact after reducing the inclination and increasing the stroke value compared to initial setting we could reduce the fine carry over.

Actualy the big lumps were just carrying the fine powder on top of it hence we just reduced the inclination to boost the retention time...

ya...it was trial and erroe method but we could achieve the result..

thanks,

regards,

sujeet

Trial And Error Method Works Fine....

Posted on 26. Dec. 2006 - 06:38

Now that you tell me that, increasing the retention time makes sense.

Hope Santa was good.

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Step Deck Design

Posted on 26. Dec. 2006 - 06:42

One other point on scalping screens: (on your next project)

You could put a STEP DECK design in, which incorporates a 5" step in it, which causes the big lumps or slabs to TIP OVER the step or drop and cascade the fines off.

If you want you could even retrofit your deck with a TRELLSTEP urethane panel setup that SNAPS into the support frame and actually has a small step in it to accomplish the same thing vs WIRECLOTH that is inline on a parallel inline deck.

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Screen Calculations

Posted on 27. Feb. 2008 - 02:47

Originally posted by John McKenzie

The following formula can be used in order to to determining minimum electric motor sizes for circular motion vibrating screens

kW = (mR x n x 1.25) / (97,400 x LRT)

where

kW = design power

mR = total static moment of counterweight assemblies. i.e.: mass x radius value (kgcm)

N = screen speed (rpm)

1.25= a reserve factor for cold weather starts, possible unanticipated field machine speed increases, shortcomings in line voltage, wire sizes etc.

LRT = motor locked rotor torque in a decimal form ie: 250% would be 2.5 in the formula. For vibrating equipment the general recommendation is that the motor should have a min LRT of 200%

Hello Everyone

I got a little confused about the above mentioned power formula. Dividing the power(kW) by locked rotor torque will decrease the final power in kw.

ex: mr=420 kgcm

N=1000 rpm

LRT=%200

than the final consumption will be = 420x1000x1,25/97400x2=2.7 kW, say a 3 kW motor.

The problem here is that the motor will give the LRT during start-up and will consumpt 7~8 times of nominal current from the network, and as the system reaches to nominal speed the motor will provide rated torque at rated current.

Without dividing the final power to LRT( in decimals), we get 5,4 kW from calculation, but if we use a 3 kW motor than it will consumpt more current to produce more torque and finally it will blow.

Isn't this confusing?