With very small feed rates screening efficiency actually increases with increased tonnage. With too small a bed depth there will be little particle interaction, individual particles will tend to skip down the screen with increased velocity, and a high percentage of the 'near size' will report with the 'overs'.

At the optimum feed rate the material bed stratifies into layers ranging from fine to coarse, with the fines next to the screening surface, and few fines reporting with the overs.

Beyond the optimum feed rate, efficiency rapidly drops off with increased tonnage. i.e.: the screen is not large enough to pass all the undersize material. ■

Yes a screen can be unloaded, John eluded to it in his answer.

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Couple of things kinda happen or don't happen when underloaded.

1. On fine screening decks, screen openings will plug sometimes due to lack of TONS going across and thru FROM above. We need a nice mix of feed to help make bigger stones or particles pass top and go down and SCRUB the finer openings...to help keep clear of plugging.

2. If you read the VSMA data on this subject...you will find if you do not have enough TONNAGE or even MIX or split of tonnage to a vibrating screen you will basically be INEFFICIENT by the rules and in practice.

3. LOW TONS causes the bed depth to be very THIN and the stroke in the screen actually makes the small tonnages be OVER ACTIVE, stratify wildly, not allow material to come back down and try to pass......all of which makes the screen inefficient.

4. Sometimes we can hang a old piece of conveyor belt rubber across the width, inside the sideplates and almost touch the wirecloth.....and this will KEEP the dancing particles DOWN and help them to settle and pass.

Running UNDER the radar as usual. Hope this helps. ■

thelaz: The rubber curtains are very practical and used fairly frequently actually....typically to control out of control material bouncing or erratic jumping down the deck.

- The rubber can be any old 3ply or 2ply 3/16"x1/16" covers type deal PIW 330. The rubber is usually hung above the screen top deck on an 3"x3" angle across the top of the screen typically welded stationary on flooring. The rubber is generally cut to fit say 1" inside the sideplates both sides, and the length is basically to the wirecloth surface....not laying on the deck normally.....some folks lay in down on the deck 2-3" " no big deal. PURPOSE: it settles the JUMPING material down, controls the feed going down the deck and this actually makes the screen...SCREEN BETTER. It controls the foot travel rate down the screen also....that is kinda IMPORTANT. If too fast....inefficient screening. The rubber actually does work nicely....for erratic bouncing due to whatever.

- Increasing TONNAGE in your instance is GOING TO HELP ...you will be less erratic bouncing with small load on it....control the bed depth better and give the vibrating screen a better chance to do it job EFFICIENTLY.

- VSMA is an assumptive compilation of about 7 big manufacturer's of VIBRATING EQUIPMENT over the course of 1,000's of actual screener applications in actual variable conditions in the field and the practical various probable causes and probable solutions. IT IS NOT MEANT to be thee ABSOLUTE definitive answer...it is meant to guide to a solution but, must be coupled with SOME one with practical experience in the field like MR CUSTOMER who is there all the time and knows his own conditions.

Thankyou very much for your questions...... ■

Here the customer feed had lots of round potato shaped rocks in the natural sand and gravel FEED.

the curtains contained to Foot travel of the round rock kinda RACING down the deck, settled the material speed down and stopped rocks from bouncing over the sideplates and possibly hurting someone nearby.

Attachments

5x12 2 dk screen it material on top deck (JPG)

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Underloading is fairly obvious when watching the machine....if the flow of material is kinda sporadic or just LOW TPH......you can see that it is underloaded by eye.....no problem

Look for material being OVER ACTIVE...kinda erratic moving down the deck when unloaded. ■