Screens exert forcing frequencies on their supports so the natural frequency of the spring is irrelevant and your building will resemble the Walls of Jericho (is it still there?).

Frequency beams are large because they have to be. Since you are using the antique avoirdupois system it might be well worth your getting hold of the Allis Chalmers design note for frequency beam sizing etc.

It is unwise to economise on safety matters which fly in the face of applicable Standards simply to please a tight fisted management. Your credibility, licence and, in some countries, you personal liberty will all suffer if the structure fails on you behalf. You have already found a benchmark viz. "We learned that natural frequency of supporting beam should be 2 to 3 times higher than operating frequency of screen. This means beam should have 1800 to 3600RPM." Other learned folk stated this and you should comply. Why? Because if the building collapses you, like Oscar Pistorius, won't have a leg to stand on.

"I also ignored SW of screen since it is on springs." You're pulling my leg? There you go again Oscar!

The only problem with beam depth is restricted headroom on the floor below but since the underpans will restrict the approach anyway it fades into a minor issue. Manufacturing cost is no obstacle. From a 1970's based recollection, the mass of the beam is the important feature so why not use shallower box girders? Sometimes you have to think beyond the box that the software came in. ■

You should heed Johns advice and do more work before this building ends up around your

ankles and you have noose around your neck.

If you want to see a crazy screen scenario read my posting about a simplicity screen and

the fools that installed it.

I would strongly suggest that you also contact Goodyear tire and rubber about air bag suspensions

for your screens I seriously doubt you only have four corner springs per screen. ■

i am not an engineer but, have installed 100's (yep) of vibrating screens over the years. John's advise is excellent, i would like to add a couple of other comments:

1. Purpose of the coil springs or rubber springs or airbag mounts or rosta mounts or shear rubber mounts: is to isolate the exciting force when mr vibrating screen is SHUT DOWN. Basically, they are there to isolate the vibration about +- 90% from the support structure.

2. at former company, the engineering lads always spoke of 200% saftey factor being built into the support beam for vibrating equipment.

3. if lining vibrating screens up 3 in a row and then 3 in a row across from each other, care should be taken. if all of them are the same 900RPM OR just the same RPM....you will most likely experience SYMPATHETIC VIBRATION between those machines especially if close. You will find the machines RAMP UP vibrations in a nice high pitch then, ramp back down and then up again....and create a LOT OF VIBRATIONS in the structure they sit on. A simple fix for this is to make every other machine run about 50 RPM slower and this should eliminate this little fun thing.

George Baker MODERATOR

>>>>>>>>>>>>>>>>>>>>>>>>>

Originally Posted by lzaharis

You should heed Johns advice and do more work before this building ends up around your

ankles and you have noose around your neck.

If you want to see a crazy screen scenario read my posting about a simplicity screen and

the fools that installed it.

I would strongly suggest that you also contact Goodyear tire and rubber about air bag suspensions

for your screens I seriously doubt you only have four corner springs per screen.

■

Hello everybody,

I am new in this forum so first of all let me say that it is a pleasure to partecipate such an appasionate and expert community.

My company manufactures vibrating motors and, as an application developer, I have been facing issues concerned to poor beam sizing many many times in the past.

Many times we have been claimed because of improper performance of the vibrating equipment where our motors where fitted (poor conveying performance, material flow concentrating on one side of the conveyor/screen, etc) and many times we visited the plant and discovered that it was all about poor sizing of the supporting elements (beams).

you could use the most powerful vibrating motor on earth but if you don't "drive" that energy properly you will only have 2 results:

1 you decrease the performance of the vibrating equipment,

2you overstress the machine without any reason to do that

since we don't have the drawings of the complete plant where the vibrating equipment will be installed most of the times it is very hard to identify the problem.

It takes days, or even weeks, to realize what the real problem is and most of the times it is possible only flying on site...with the expectable delay in the plant start up and related costs!!

best regards ■

Hi Davide,

I never encountered such conditions previously. As I understand it, your retrofitted motors are not identical to the originals, or else your Client is building the screen on his own recognizance. Either way you need not worry provided you have supplied what the customer required and he has accepted. It appears that the user is liable for your costs in rectification. A visit to the cleaners seems to be in order. On no account should you provide an acceptable frequency beam design for your customer. That will cause you trouble as I indicated previously because if you change a couple of beams the whole building must be checked and that is not what a vibromotor supplier does. ■

yes, in this case the customer built the machine according to his recognizance and accepted our motors "on paper".

since he never used our motors before (used some from a competitor) this was our first supply so when he saw the problem he immediately thought it was because of the different motors...

of course it was not...but we had to prove this.

of course this had been an issue in the beginning but it turned into a very close and strong relationship because of our quick reaction time. ■

Originally Posted by Cotta75

yes, in this case the customer built the machine according to his recognizance and accepted our motors "on paper".

since he never used our motors before (used some from a competitor) this was our first supply so when he saw the problem he immediately thought it was because of the different motors...

of course it was not...but we had to prove this.

of course this had been an issue in the beginning but it turned into a very close and strong relationship because of our quick reaction time.

You and your firm will go far and on the right track. ■

Below address maybe useful for designing support beam for vibrating screen:

http://www.ckit.co.za/secure/conveyo...ibraequip.html

Also pay attention that when a vibrating screen works with a frequency, the structure will be excited with the same frequency. The springs only reduce the force and can not do anything about the excitation frequency.

So when the screen works with 900rpm, the excitation frequency will be 900rpm=15Hz. So the natural frequency of the beam should be about 2 times of this number.

Meanwhile one of the way to reduce the size of the beam, is the installation of columns directly under the screen support points(spring points),

Originally Posted by rfireman

Hi,

I have fairly large single deck wet process screens. These are in a set of two feeding in middle. All 3 set of two screens (total six) on one floor( 120 ft. above ground) in a steel structure building (fairly large/complex building).

Each screen has 900RPM operating frequency but supported on a 4 springs.

I am a structural engineer and responsible to design beams for these screens. Support beam has 24 to 27 ft. span. We learned that natural frequency of supporting beam should be 2 to 3 times higher than operating frequency of screen. This means beam should have 1800 to 3600RPM. This results in a very large/deep beam-around 6 to 7 ft. deep (250lb/ft.). I also ignored SW of screen since it is on springs.

My question is since this is supported on springs, structure will feel only NF of springs not operating frequency of screen.

We should check our beam against NF of spring (natural frequency of spring is 100 to 200 RPM). Beam should have 200 to 400RPM.This will give us a reasonable beam.

Please advice me in this regard. Any practical example or article for help. Thanks

■

Mohandes,

Thank you for a very informative link and article., ■