Hello Mark, excellent summary of your situation....

You are correct on many points:

1. You do have a 2 bearing, circle-throw inclined vibrating screen with adjustable stroke or otherwise known as CIRCLE SIZE.

2. the offsets of 3/16" x two equal 3/8" full stroke or circle size

3. The proper speed for at 3/8" circle size is nominal 840-860 rpm depending on the manufacturer of the screen box. As a guess, it sounds like a TYLER tyrocket 2 bearing screen to me.

4. You are absolutely dead on with you analogy of running faster......you are in fact at 5.57 G'S RUNNING over the design factor for which that screen was in fact DESIGNED. Generally, speaking a screen at 20 deg angle of decline screening at 2" over 1" is supposed to have a 3/8" stroke for that size of material...so all of that is good to go.

5. First things first, run the machine at the proper speed for the size of opening which in fact is 3/8" at 860rpm perfect....the vibration in the structure is being caused by something else for sure in my opinion.

6. SOME CHECKS:

- by operating somewhat faster, we can quite often quieten down the vibration to the structure. But, this is typically done by speeding up by only 50 RPM and normally done to negate sympathetic vibration of another screen running on the same structure close by on the same support beams.

- Is the screen mounted correctly? Look at the vertical supports under the springs. They must be directly in centre line beneath the two springs or three springs on the corner......NOT CANTI-LEVERED over the centre.

- The coil springs are supposed to eliminate 90% of the vibration inertia in a straight line down thru the springs upon shutdown.

If your coil springs are WORN OUT, BROKEN, CRACKED or otherwise not supporting their full weight EQUALLY on each corner.....you got problems with ERRANT or ERRATIC vibration vs a nice balanced equal loaded vibration. Maybe you need new springs. If springs are undersized........equal transmission of vibration and so on.

- Do the beams, I assume stationary operation, go right down to the ground into concrete or some other design?

- Must have plus minus 200% safety factor in the support beams to handled the dynamic and static loads from the screen.

- If you have a broken support member in the screen box......this will translate into structural vibration....shaking erratic and transferring to steel support

- Any historical problems with the support coil springs, or rubber shear mounts or other? If rubber, they could be COMPRESSED which means their load carrying characteristics AINT THERE....and more shake and bake.

- Is the line power clean and steady?

- Is the housekeeping around the corner springs good or are the coils filled with limestone fines......now we have a real problem...you have a mini crusher happenin inside the coils and no vibration transmission protection.

- SLOW it back down........quick.....or you will crack the sideplates out.

- ■

This is in the section CHAPTER 9 actually called PROBLEMS AND SOLUTIONS and it reads as follows:

Source: VSMA handbook

Excessive Structure Vibration:

PROBABLE CAUSE & POSSIBLE REMEDY

PC: Support structure too weak

PR: Add structural support or cross brace

...............

PC: Shipping blocks not removed

PR: Remove shipping blocks

..............

PC: Insufficient clearance between screen and structure or material buildup

PR: Provide adequate clearance or remove buildup

...............

PC: Fatigued suspension parts

PR: Replace with new and check other components

I carried VSMA with me all over N AMERICA as my little quick guide for troubleshooting and all in all .......it is pretty helpful in real life situations.

Hoping this helps a titch. ■

One fatal error people generally make when adjusting stroke size......is this.

People typically increase stroke by adding weights and that is it.

THIS of course, is a huge error.

We must always remember to do TWO THINGS not one. If we increase OFFSET weights to give bigger action (circle or straight line stroke) WE MUST slow the speed/RPM down.

Conversely, if we remove weights.......we MUST SPEED the RPM up......and both of these adjustments should be double checked with the MANUFACTURER as a secondary confirmation.

but, basically if we reduce stroke, we CAN by the VSMA rules speed up the RPM and still stay in the acceptable G FACTOR RANGE . If we increase stroke, we MUST reduce RPM to in fact stay in the acceptable operating G FACTOR range. Quite simple and logical actually in theory and in practice. IT WORKS. ■

On that screen you have a MAXIMUM 3/8" circle availability probably and usually.

If we take all the weights off, you would probably get a 1/4" circle which we would adjust speed up to 1050 rpm range.

If you took some of the weights off..........you could have 5/16" circle and run at 850-900 rpm range.

There is no adverse effect by going to 1/4" on your setup .......that is, of course as long as you adjust the weights on both sides at the same time to the same weight setup.

.................

If you have more vibration on the DISCHARGE end as you stated......then, I would say you are running in an unbalanced condition and this adversely submits the structure to erratic transmission of vibration.

A proper pencil STROKE CHECK would confirm this pretty quickly.

You should get a perfect 3/8" circle size on a piece of paper on each corner to indicate a balanced operating screen box....if you do not....YOU have a problem.

...............

Truly ....speeding up and slowing down and adjusting weights should not be what you are looking for at this point. Look for something at the discharge end......like interference fit with all structures and chutework. (Need minium 2" clearance all the way around) The box must operate free floating on the springs. Did we put EXTENDED DISCHARGE lips on this baby by chance? say to get to a crusher or something. This would offset the balance very quickly and happens all the time.

Or look for a flaw in the structure holding it. ■

doing what screening job. ■

Mark

If your springs are OK and the screen does not impact the scructure it looks like a typical natural frequency problem - get a dynamicist on site to determine the operating window (openings between the natural frrequencies through an impact test) and then adjust stroke through counterweights to get the "g" you need for the process. I presume you still have the screen in one piece.

Ziggy ■

THEORETICALLY, we assume the chutework and discharge hoppers are designed with enough ANGLE OF REPOSE to make sure we MOVE the material being screened. We confirm the natural angle of repose of the material and make the slopes steep enough to keep it moving.

One key point: You want to keep the VERTICAL beams supporting the coil spring baseplates directly under the centre of the coil springs and DIRECTLY down to the floor vs CANTILEVERED. ■

Michael,

The screen supporting beams should be designed to be safe in both bending and shear. Use the static screen mass plus material load. The actual material load acting upon the screen will vary depending upon the application (feed rate – conveying velocity – percentage passing through the lowermost deck), but commonly the material load is in the order of 10 to 15% of the unladen screen mass. Better to be conservative here, and common practice on small screens is to assume a total static load equal to twice the unladen screen mass.

To this static load the structural designer must add the reversal or dynamic loads produced as the screen moves up and down on its springs. The screen manufacturer should be able to supply dynamic loads for his machine, but otherwise the magnitude of these forces is equal to the number of springs x the spring rate x the amplitude (half the throw) of the screen. In any case the the dynamic loads are usually quite small when compared to the static load.

Formulas for calculating the safe beam size for a given loading as given in any engineering handbook are to be followed.

The beam sizes selected should then be checked for natural frequency so as to not to cause excessive vibration and stress levels due to operating to close to resonance.

Calculated natural frequencies of primary members should be at least 2.5 times the operating frequency of the screen. The similar ratio for secondary supports and floor panels should be between 1.5 and 1.8.

Where spans are long and the natural frequency is close to operating frequency, struts or braces can be used to reduce the effective span of supporting beams, thus increasing their natural frequency. ■

Dear Sirs,

Kindly advise address where I can get VSMA handbook. I am sure all of you must be fed up of this question, but please assist.

Thanks

Sajjad

Ocean Equipment ■

Don't you know how to Google?

http://store.aem.org/index.cfm?fusea...ndardsscreens ■