Hello Canuck1,

The subject pertains to screening efficiency. Certain value of screening efficiency is considered while designing / selection of the screen. The quantity of passing size retained on the screen deck will be according to this screening efficiency (in reverse order).

Suppose, you are feeding 100 tph of 100% (-) 19 mm size on screen deck having opening corresponding to 19 mm passing size and if the screening efficiency is 75%, then you will have around 75 tph passing through the screen and 25 tph will be on screen deck for discharge as oversize.

The screening efficiency affects the size and price of the screen. Therefore, if this issue is not taken care of during choosing the screen then although screen may appear to be economical, but it will have more proportion of the screen ‘passing size’ getting discharged as ‘oversize’.

The screening efficiency arises due to reason that all the material will not have opportunity to pass through the deck during course of its movement on the deck. Hence, bigger the screen, the material will have more opportunity to get through the screen deck. Also, more quantum of oversize in the feed material tends to reduce the screen efficiency because such oversize material will tend to cover the available area of the screen and thereby the passing size will have less opportunity to pass through.

The subject matter is more suited to the consultancy / advisory services from the professionals in this field.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

You are experiencing a TYPICAL carryover problem of fines in your overs. The question is WHY?

Primary Reason: SLOPE is INCORRECT = TOO LOW:

The SLOPE angle of decline should be minimum 20 degree decline by VSMA rules. If you apply this assumptive rule......the expected efficiency should be 95% efficient. This is pretty normal by math calculation and by practical in the field experience. 95% efficiency is also assuming your material is relatively DRY, FREE FLOWING vs WET or non-free flowing.

In theory: you should only be seeing 5% of minus material in your overs by the rules....assuming we are operating at 95 percent efficiency.

YOUR SLOPE: is 10.5 degrees.

Here is what happens or DOESN'T happen more correctly:

The foot travel which at 20 degree angle decline would be around 60-75 FPM is now TOO SLOW. The material gangs up on the screencloth, BED DEPTH or thickness of the material mat becomes too deep. The FINES or minus material you want to pass thru the opening CAN NOT. Why?

BED DEPTH too deep: the fine material can not be separated or thrown out of the bed because it is too DEEP. So...it stays trapped inside the bed thickness....and carries over the deck when it should be passing.

VSMA: WILL give you size of screen deck area needed......but, you must also double check the BED DEPTH calculation for efficiency to accomplish 95% EFFICIENCY.

I took the liberty to recheck your application in the VSMA book:

3/8" at 850 RPM is OK:

running directional flow/ assuming the weight per cubic foot is 100 lbs/ and material is relatively free flowing to ensure the bed depth does not gain up too deeply

GENERAL RULE THUMB: We must have enough STROKE or circle size to clear the top deck opening of wirecloth firstly...or we don't get to the bottom deck.

FOR YOUR APPLICATION: The SLOPE ANGLE is defined in VSMA chart at 18 - 25 degree slope range as the pick.

CONCLUSION:

You are meeting most of the rules for efficiency BUT, your slope is TOO LOW.

Troubleshooting: This is the problem.

ACTION REQUIRED:

RAISE the feed end of the machine up to 20 degrees and recheck your testing sieve results.....you will find this should correct your problem.

Also: when you raise the angle....you can actually go 1/16" bigger opening on the wirecloth and still stay on your product spec...try one section and you will see. This is because at 20 degree angle the material will NOT SEE as big a hole due to foreshortening of the opening due to 20 deg slope.

LASTLY: VSMA also recommends 5/16" circle or stroke at 900RPM

which you could try if raising the angle is NOT AN OPTION.

Sorry...for late reply........I have been out of country.

HOPE THIS HELPS.

PS: If you need on site help in ONTARIO CANADA......I will come to you. No problem

George Baker - VIBRATING SCREEN CONSULTANT & MODERATOR ■

CANUCK 1

You may also want to check:

WIRECLOTH wire diameters are not too thick

WIRECLOTH is NOT pitting which will slow down material

George ■

Hi canuck 1

I would gladly work out your problem, but would need the following information.

1. Density of material.

2. Wet or dry screening.

3. Wire or poly decking.

4. Most important is a gradiation analysis of your material.

5. Cut point sizes of your deck, square or long slot.

Forward this imformation when you have time.

I will be in Canada next week, and could probably arrange a meeting if you stay in Ontario.

Regards

Ian McCallum ■

can you confirm if you raised the angle?

and if so............What actual slope are you at now?

If bed depth is not a problem: can you actually tach the speed of the screen and advise that?

and we still have the question: ARE YOU BLINDING or plugging the hole openings on the 1/2" openings?

You percentage of NEAR SIZE material is consecutive sieves is very high and this will also create a problem of passing or splitting out the NEAR SIZE from the total feed mix.

YOU MAY have to actually really tighten up the top deck to HELP LOAD RELIEVE prior to getting to the second deck.

WE SHALL RECTIFY THIS !!!

Regards, Are we having fun yet? George Baker MODERATOR ■

Thankyou for the update.

HONESTLY: you problem is mainly WRONG INCLINE or decline or slope (you pick).

ANGLE is the demon here....in actual fact the foot travel is too slow....if you were to flourescent paint a stone and put on right at the feed end and time with your watch til it drops off the end of the screen deck discharge lip....this will confirm FOOT TRAVEL rate is too slow due to low angle.

So you are landlocked so to speak....tight within the area you are installed and no room to raise up. A very typical problem unfortunately.

Is the machine grease lube? If it is easily raised without a detremental effect on bearings. If splash oil or oil in the end covers......we must adjust and be careful we do not GANG UP oil on low side and starve bearings of oil on the high side...IF WE RAISE THE SCREEN BOX AT THE FEED END...... MORE LATER.......on this little prob.

IS it two bearing or four bearing........whose mfr.......?

Reading the CIRCLE: A tip or trick for you. Most guys have a magnetic plate with actual circle sizes on it. They show 1/2" 3/8" 1/4" 3/16" actual circles on them. When you put it on the side of the machine when vibrating..........HERE IS WHAT YOU DO.

You look at all circles.....and ONE of them.......and only one of them will show a nice little DOT in the dead centre when vibrating. THIS IS THE "Actual" circle or stroke size you are operating that box at.

The other circles will show.......BIGGG shaded areas in the middle and these are NOT the correct or actual circle size operating at.

TRICK: We have sticky back paper we put on magnetic card - use a sharp pencil - touch ever so lightly........like real light touch and just one revolution needed not ten......just messes up the stroke or circle and makes it unable to read. OR just tape a piece of white paper onto magnetic card.......mark FEED END RIGHT, FEED END LEFT, DISCH LEFT RIGHT ETC, CENTRE. and do stroke check in all those areas.

NOW, take a tape measure and carefully measure across width of actual circle....and determine if it is 1/4" or 3/8" line to line and now you know how to do it.

IF too small........check with factory and increase offset weights of the wheel to increase to proper size.......or decrease weight and increase speed........BUT, always advisable to check with factory first.....THEY KNOW the limits.

LAST again: You really should try.....some FLEXMAT self cleaning wirecloth for the crushed material....more open area and better efficiency. THIS STUFF IS by MAJOR WIRE......PQ

George Baker Vibrating Screen Consultant ■

The 6" drop may be for clearance but, it is quite often built into the deck to allow the material to TUMBLE.....over the drop and this allows the material to be exposed at all angles. On a feeder for example the drop is to make the fines tip off the big pieces of slabby rock prior to the crusher.

Removing all the weights will probably give you a 1/4" circle and you can check by stroke check to confirm.

IF grease lube...raising is easy.....no concerns......

THANKS for the INFORMATION...........GEORGE ■

With a screen surface of 8.92 m2 (6' x 16 ' 1.83 m x 4.88 m) an empiric efficiency of 96.77 % could be expected, if used wire screen of 12.5 mm (1/2") aperture has an open area around 61 %. With this efficiency a 22.53 % unclassified fine particles should be reported to coarse fraction, so if feeding 170 t/h you shoud get 21.94 t/h coarse fraction, formed by 17 t/h coarse particles > 12.5 mm, plus 4.94 t/h missplaced particles finer than 12.5 mm.

Note: To get efficiencies values over 94 % a good vibrating screen is needed with amplitude and frecuency in agreement with feed tonnage and particle size distribution ■