Tons per hour is a function of screen width. In vibrating screen nomenclature (width equals tons per hour and length of the screen equates to screening efficiency)

First of all ........a mathematical equation must be done, basically from the vibrating screen manufacturing association guidelines (VSMA). This is the science part of the procedure, basically assigning plus minus factors of (1) to eventually arrive at the number of square feet of vibrating screen needed to do your particular job. For example: if the formula says you need 96 sq feet of screen, you would rightly assume you would buy a 6'wide by 16' long vibrating screen box.

That is "partially" correct and this is quite often where the average user goes wrong with the size to actually purchase.

As screening aggregate is not an "exact science" we must then apply a few more factors to arrive at the size needed.

BED DEPTH: We must do a bed depth calculation which basically determines whether the mat or bed of material coming down the screen is too deep at the discharge end of each screening deck.

VSMA rules state generally: you are allowed up to 4 times MAXIMUM BED DEPTH in relationship to the size of the opening in the wirecloth, rubber or urethane media. Example: if your deck has a 1/2" opening - then you should NOT be deeper than 2" deep at the discharge end of the screening deck or if for example you were 4" deep by tape measure at the discharge end of a 16' long screen - one could assume rightfully - the machine would be carrying fines over the 1/2" that should be through the 1/2" opening. We know this from field experience.

Basically, the stroke in the machine which lifts the larger particles up in the air - allows the fines to go down through the bed depth and try to pass the opening in the wirecloth media. If the bed is too deep....the fines are captured and carried over into the overs pile. This most likely contaminates your spec from passing the lab testing.

The other priority is the "ART" of screen sizing must be applied before finalizing the size of screen to buy. This is getting the heads together of the operators and the vibrating screen manufacturer to disucss material characteristics etc and make the screen a little bigger for the "just in case scenario". These could include factors like: more of less moisture, non free flowing aspects of the material and numerous other factors.

We may in fact buy a vibrating screen such as a 6x20 or even a 7x20 after adding in the practical experience of the operators to the VSMA mathemical equation. REMEMBER: the math said we should buy a 6x16 size deck.

Three basic elements are absolutely critical to be setup in unison to each other: SPEED/STROKE/SLOPE.

Some quick examples:

S&G 1"-2" opg 840 rpm 3/8" stroke 20 deg slope

sand 1/4" opg 1050 rpm 1/4" stroke 20-30 deg slope

sand 5/32" opg 1200 rpm 3/16" stroke 20-45 deg slope

The variation of speed stroke slope combinations is endless and care must be taken to keep the speed/stroke especially within the guidelines of screen manufacturer"s recommendation. We want to screen efficiently but, NOT break the machine up by running over the recommended acceleration forces.

Your technical questions are welcomed.

George Baker - Moderator ■