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% ■

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? ■

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. ■

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. ■

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. ■

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. ■

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. ■

Can the formula for power requirement be applied to the circular screen machines ? ■

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

Hope Santa was good. ■

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. ■

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? ■