sounds like a good question for JOHN.

Too engineering type question for me. ■

Maybe it's only practical to do it 'practically'.

Take the screen, add an adjustable vibrator, run the vibrator over it's range and see at what frequency the screen bounces about most.

Any better ideas out there ????? ■

“How to theoretically calculate the natural frequency of a screen?” is a BIG subject for such a small space, and any comment must be as general as the question. If you want to calculate the natural frequency of a screen – employ a qualified design engineer with experience in this field.

These days most reputable screen manufacturers spend considerable design time (and money) to ensure that standard equipment (particularly big screens) do not have any inherent natural frequency problems. For smaller “one off” machines, or where similar machines have been built without problem before, they probably would not bother – but would test run the screen in the workshop with a variable speed drive to at least say plus and minus 10% of the nominated speed to ensure that no spurious vibrations occur. ■

Before you start building a screen you should do a FEA model and run different options to check not only for natural frequency but also static stresses. In the past most of the manufacturers buit prototypes but it takes a lot of time and money and it was worth doing only when big orders where placed.

I have built FEA for few hundred of screens and I would not try to build a screen without FEA analysis - when the proposal dwg is ready it can take about a week to check - you are building a virtual prototype. You can do hand calcs but they are rather indicative and with any change you have to start from the beginning. ■

Hello: When we manufacture the DILLON vibrating screens, we mount every one on a TEST STAND for minimal 4 hours check prior to ship.

My previous company did the same for every machine that was built and for 4 hours also prior to ship.

This allows us to put a magnet with a paper sticky back STROKE CHECK CARD on it, do a pencil check for stroke indication and then REVIEW all strokes at the top right hand, top left hand and over the centre of the shaft assembly to MAKE SURE all circles are mirror images of each other. (They look similar and true circles for inclined screen or straight line strokes for gear driven or linear screens)

WHAT DOES this prove? It allows the manufacturer to ensure the machines is BALANCED all around. If balanced all around the stroke checks will MATCH if UNBALANCED weight ....stroke check will be NON Matching all around indicating an off balance distribution of weight situation on the body or running too fast over the recommended G FACTOR or accelerating force.

Quite often: when this test indicates a problem, the machine will be taken apart or inspected to fix the problem prior to shipping far away from the factory, then chasing it in the field.

WE also check the operating temperatures of the oil or grease and the temperature of the bearing housings TO ensure the bearings were not installed a TAD TOO TIGHTLY. If the heat is higher than recommended for the bearing size......we disassemble and correct the problem.

WE TEST all machines prior to ship on a test stand then, put the paper stroke check into a file at time of ship. Then, if the unit goes into the field and a problem happens....we can say....it was balanced when it left the factory. What did you do or add?

A nice double check system and it works.

People add thicker liners, bells and whistles to the body before installing all the time and this is a problem.

NORMALLY: for circle throw inclined screens at 20 degrees the accelerating G FORCES are in the 3.5 - 3.8 G ranges.

for horizontal gear driven flat screens: 5.5 - 6 G range. ■

George

Testing on the shop floor is next stage and you do to confirm your design. Another step is to do a proper commissioning with checking the structure and plant vibrations and natural frequencies (in and out). Many companies spend a million on screens but than try to save few dollars on commissioning.

Problem is what if you try a completely new design how you predict or rather calculate natural frequencies? - that is a reason not many screen manufacturers would do one-off job without knowing how to do because the risk is too big. FEA is not foolproof but it is a very good tool when you know how to use it and with proper procedures for manufacturing and quality control I can not see a problem to design any screen.

Regards

Ziggy Gregory ■

Ziggy: that is the other part of the deal............as you say field commissioning. We also do this as common practice.

I had a call once on grain cleaner machines.......10 decks. When we got there the BUILDING was shaking mercilessly....WILD VIBRATIONS impeded to the structures, handrails, steps, columns and 10 stories ontop of huge silos....on open grate flooring.

Very interesting, exciting, LOUD commissioning call. We had the people who installed their sensitive electronic scales BITCHING pretty good at us......the vibrating screen mfr. We had the owners of the CO-OP site FLIPPING out because of the vibrations and worry we could crash the building down. Lots of excitement.

They actually brought the people who designed the CN TOWER in Toronto to try to figure out the problem. The assumption was....it was all the vibrating screen mfr fault.....putting this darn thing that vibrates in here!

PROBLEM FOUND: When the steel workers installed the steel support structures.......they actually installed the support beams for the base of these 2 ten deck circle throw vibrating screens in the wrong place. They were too far to the left for some reason, maybe a drawing was READ WRONG.....WHO knows.

Doing my commissioning investigation, I asked the question....why are these two BEAMS which the screens were mounted on........HALF SIZE....of all the other beams around the floor area? And why are there 2 beams twice the size sitting beside each of these screens with ZIP ALL nothing on them?

It was determined the units were installed on the wrong size support beams and this was the cause of the MAJOR VIBRATIONS. They had to jack up and support these units.....pull the undersize beams and install the PROPER BEAMS. Problem SOLVED.

Onsite commissioning is wild important. ■