Control of Screen Supporting Structure

Tafakor
(not verified)
Posted in: , on 9. Jun. 2010 - 14:09

Dynamic Control of Screen Supporting Structure

Dear visitors

I’m designing a support structure for a Vibrating Screen with 1000 RPM Excitation Frequency. The structures bay in the direction of Horizontal Screen Vibration (given X Direction) is about 8 meters and in the other (suppose Y Direction), nearly 6 meters, the structures height is approximately 4 meters in the first level and 6 meters in the Screen level (totally 10 meters).

As we all know the fundamental natural frequency of such a structure might be about 2.5Hz and due to the case (its stiffness and mass) in the range of 1.5-4 Hz, thus it’s natural frequency is lesser than 12Hz (25% away from the Excitation Frequency, 1000/60 = 16.7Hz) and in the next modes it can reach to 12Hz, because of the fourier expansion. I wonder that how could I prevent the resonance phenomenon and check the structure for this.

I’ve made the structure so stiff in the X direction because of avoiding its vibration. hence, I’m afraid the first mode shape in this direction occurs in about mode number 10 of the whole structure (at a frequency about 8 or 9). Besides the mass participation percent is about 55% in that mode and in order to reach 90% mass participation, we will reach to mode number 25 or 30 (i.e. about 12Hz frequency), and I’m worried about this. Although I don’t know that if the critical frequency occurs in high modes (because of stiffening the structure in that direction), is it still hazardous and serious?

I also have performed a dynamic time history analysis with Sap2000 V11, applying Force Vibration with the following function (Given by the constructor) to the 4 Supports of the Screen:

F = F0 Sin (wt) = 0.6 Sin (104.7 t) (KN) --> for Z direction

= 0.3 Sin (104.7 t) (KN) -->for X direction

These are forces under each set of springs settled on the Screens supports. The maximum displacement of the beam under supports (HE-B 300), in the Z direction (due to Vertical Vibration), is about 0.2 mm. I think with 1000 RPM Excitation Frequency, this displacement is undesirable.

I’d be so thankful if you could present me a scale for finding out how much should the displacement be, and if I would not want to perform dynamic time history analysis on the structure, how could I know that is the mode shape critical or not.

Sincerely yours,

N. SH. Majd

Wow.......Nice And Technical....

Erstellt am 10. Jun. 2010 - 04:58

a good question for our member JOHN MACKENZIE.......

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email]gbaker@telsmith.com[/email] E: (home) [email] gggman353@gmail.com[/email] website: [url]www.telsmith.com[/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Screens

Erstellt am 10. Jun. 2010 - 05:15

Ah,

resonance and its all encompassing coat tail issues when a screen is started.

The VSMA.org web site may be of some help to you.

MY past expereice with a Simplicity screen 8' by 16' installed in an underground screen plant was disappointing for a lot of reasons.

After the spring supported screen was installed and the screen plant was started in its Shake down cruise(literally) it was creating so much shaking and resonance it vibratewd through the steeel structure assembly of the underground screen plant so badly it had to be stopped.

the structural steel support the screen plant weighs 700 tons and the ROM belt and other tertiary equipment being an infeed belt, flight conveyor, secondary screeners, dust suppression filter cabinets spokane crusher, finished ore belt weighed another one hundred tons +-

The manufacturer was called and thier collective response was:

So what!!!!!!!???

Just weld more steel under the screener steel work to stop the resonance and add mass to the weight supporting the screener cancelling out the elliptical orbits effect on the screen plant steel work.

I was very disappointed personally in this response as it was incorrect in my opinion having dealt with Tyler and Derrick screeners.

The response given by them told me the screen was insufficiently supported and overloaded as it was spring mounted rather than suspended with wire rope.

I suggested remounting it with air bag springs which is quite common for large spring mounted screens to cancel out vibration as we had compressed air service in the screening plant for air tools and blast dust cleaning.

I was told I did not know what I was talking about and they being Simplicitry, he canadian designers, and we know they know what they are doing, you do not.

If you have used stroke cards to determine the proper orbits, george and I both will tell you thats half the battle.

Have you considered making it a horizontal installation and reduce its operating speed?

Tafakor
(not verified)

Untitled

Erstellt am 11. Jun. 2010 - 01:42
Quote Originally Posted by lzaharisView Post
Ah,

resonance and its all encompassing coat tail issues when a screen is started.

The VSMA.org web site may be of some help to you.

MY past expereice with a Simplicity screen 8' by 16' installed in an underground screen plant was disappointing for a lot of reasons.

After the spring supported screen was installed and the screen plant was started in its Shake down cruise(literally) it was creating so much shaking and resonance it vibratewd through the steeel structure assembly of the underground screen plant so badly it had to be stopped.

the structural steel support the screen plant weighs 700 tons and the ROM belt and other tertiary equipment being an infeed belt, flight conveyor, secondary screeners, dust suppression filter cabinets spokane crusher, finished ore belt weighed another one hundred tons +-

The manufacturer was called and thier collective response was:

So what!!!!!!!???

Just weld more steel under the screener steel work to stop the resonance and add mass to the weight supporting the screener cancelling out the elliptical orbits effect on the screen plant steel work.

I was very disappointed personally in this response as it was incorrect in my opinion having dealt with Tyler and Derrick screeners.

The response given by them told me the screen was insufficiently supported and overloaded as it was spring mounted rather than suspended with wire rope.

I suggested remounting it with air bag springs which is quite common for large spring mounted screens to cancel out vibration as we had compressed air service in the screening plant for air tools and blast dust cleaning.

I was told I did not know what I was talking about and they being Simplicitry, he canadian designers, and we know they know what they are doing, you do not.

If you have used stroke cards to determine the proper orbits, george and I both will tell you thats half the battle.

Have you considered making it a horizontal installation and reduce its operating speed?



Mr. Izaharis

I appreciate your opinion.

In fact, I’ve acquired the VSMA Package, and unfortunately didn’t find something functional for my case.

The Vibrating Screen Machine, previously mentioned, has not been placed and its structure has not been constructed yet. Therefore I do not need to change the Machines state and I can design the structure however I insist, but I’m worried about its Dynamic Control under Screens Vibration.

About stiffening the structure and adding mass to it concurrently, I think these are opposing concepts. I can’t understand how could it work?

Another question, the structure I’m working on, has very smaller mass than the case you stated, and the structure of mine may have so bigger frequency, in fact this bothers me.

I again say thanks for your attention.

Re: Control Of Screen Supporting Structure

Erstellt am 11. Jun. 2010 - 02:50
Quote Originally Posted by TafakorView Post
Mr. Izaharis

I appreciate your opinion.

In fact, I’ve acquired the VSMA Package, and unfortunately didn’t find something functional for my case.

The Vibrating Screen Machine, previously mentioned, has not been placed and its structure has not been constructed yet. Therefore I do not need to change the Machines state and I can design the structure however I insist, but I’m worried about its Dynamic Control under Screens Vibration.

About stiffening the structure and adding mass to it concurrently, I think these are opposing concepts. I can’t understand how could it work?

Think of it this way many sky scrapers in Japan and several in San Franciso and The Petronas tower have weight(s) suspended in the steel framework to cancel the buildings sway in high winds or in earth quakes to some degree.

what your doing is creating a mass that counters any movement by cancelling its ability to move with more mass than the actual weight of the structure If I rememebr the chain of events properly adn reduces the resonance due to the trotal mass as the wieght hold the structure with a very strong load within its self.

I think think The Birgh Dubai has a sway canceling pendulum weight also?

Another question, the structure I’m working on, has very smaller mass than the case you stated, and the structure of mine may have so bigger frequency, in fact this bothers me.

think of the little monster this way-

You have an end table near a love seat or hide a bed

And you unfortunately live near the train depot in Kolcatta on the third floor flat.

The end table is the lightest piece of furniture in the room as it is small and has less mass than the dining room table or the overstuffed chair by the reading lamp stand-which has a heavy base and does not move.

Every afternoon you come home from work and you find the end table

has moved a meter away from the love seat and after a week of this you know there have been no minor quakes or S wave tremors streaking along the indian ocean floor and past Kolcatta to the eastern lowlands.

The end table is the smallest and lightest piece of furniture in the 3 floor flat that you own and you have a hard wood mahogany floor of which the building used to be a malting house, warehouse or millinery.

And you know the building is solid- But you find the end table halfway between the wall and the love seat, you also know that X number of trans pass your home every day while you are gone with fewer trains after evening tea.

So you try an experiment by hanging a a very large hollow core building block under the coffee table use a good blanket or woolie to cover the table top and avoid maring the teak :^) with two very good peices of manilla rope that are tied over the end table top and through both hollow cores of the building block.

After this you go to work and when you come home you find the end table still at the left side if the love seat or sofa and it did not move.

WHY? every time a rake or commuter train rolled by no matter the tonnnage the ground vibrated due to the alluvial soil and the vibration was transferred to your flat and the other flats below but today is different because the cinder block tied under the coffee table cancelled the harmonic resonance created by the rakes and commuter trains byswaying everso slightly wth its heavy weight.

the frame is the carrier of the screen and also recieves any excess energy reacting to it by vibrating or being heavier and canceling the elliptical forcce generated by it as the greater weight lessens the effect of the energy transitted as it is heavier and able to absorb the energy created by the screener.

BUUT all wagers on the muddy soccer field are cancellled if the screen is NOT BALANCED to start with by using the stroke cards to check for proper orbits.

I again say thanks for your attention.

your welcome much

lzaharis

Re: Control Of Screen Supporting Structure

Erstellt am 12. Jun. 2010 - 02:47

Thanks for the referral George - but the specific questions are outside my competence, and need review from a structural engineer who is experienced in this field.

From practical experience this seems a relatively low structure, and a simple rigid design which is adequately braced in all directions would normally prove adequate. Problems usually arise when walkways and related conveying equipment require omission of part of the bracing system.

The usual rule of thumb is that structural members should be selected employing two different criteria. First, the member must be chosen for structural adequacy based upon its performance as a beam, column or brace, and secondly for stiffness and resistance to vibration by calculating its natural frequency. The calculated natural frequencies of primary members should be at least 2.5 times the operating frequency of the screen and the similar ratio for secondary supports should be 1.5 to 1.8 times.

John McKenzie

Re: Control Of Screen Supporting Structure

Erstellt am 15. Jun. 2010 - 10:18

Screen structure has to be checked for natural frequencies but you have to take damping into account and then if the isolation is done correctly using the isolating frame if necessary it is not a big problem. With steel springs only the isolation could be about 96%, if you add isolating frame you end up with maybe with 2% of the signal transmitted to the structure. It will be enough to excite some braces but not the main structure.

Regards

Ziggy Gregory

Ziggy Gregory www.vibfem.com.au