Increasing Speed
Thanks for the reply, George
We are going to run a trial with a spare 950RPM drive available. If we reduce the stroke down to 10mm, this is a g--factor of 5.1. We currently runa t a G of 4.75.
Do you see this as a major problem?
1. Where should I be looking for a failure here- sideplates? bearings? exciter shaft? bolts? How can I ensure we will not run into a problem.
2. By increasing speed, will the stroke remain unchanged until counterweights are removed? My colleagues and I disagree on this point. I am unsure as to whether stroke will be reduced because of an increased damping effect.
Any thouights with these?
Thanks again, mate
Philip ■
Re: Matching Stroke To Apertures
1. Generally, if we run faster RPM (slightly faster say 50 rpm) basically is no biggie...but, still should be double checked against the actual mfr's specs. BUT, say we boost by 100 plus rpm we should in fact reduce the stroke to maintain reasonable G Factor ratio.
2. If operating too high of accelerating force - typically will CRACK a sideplate as a result.....and/or bearing failure.
3. INCREASING the speed is increasing the speed only. The stroke will NOT change unless you reduce the counterweights to affect a reduction of stroke or throw. Increasing speed and not touching stroke WILL increase operating or accelerating G FORCES.
4. If you are carrying OVER fines that should pass thru the openings.....reduce stroke length and increase speed. OR correct overfeed or BED depth problem if any.
"Stroking your way to success" Good luck. GB ■
Re: Matching Stroke To Apertures
thelaz
You sure you have different speeds on the same screen.
Ziggy
ww.vibfem.com.au ■
Re: Matching Stroke To Apertures
Ziggy
Thanks for the reply
No- the three decks I refer to are 3 separate decks, on three levels of the plant, with the undersize of each screen feeding the one below. They operate totally independently of each other in tems of vibration.
My question is specifically regarding the sroke/amplitude match for the aperture size.
I am hesitant to recommend that the client increase the aperture size on the 5mm deck, as literature suggests that a smaller aperture requires a smaller stroke and a higher RPM.
Any comments on this? ■
Re: Matching Stroke To Apertures
thelaz
There are different theories - I prefer stroke to speed even when it comes to dewatering screens. Changing aperture is up to the process engineer - he has to get the right product. Try to increase the stroke as that would help you to prevent blinding and pplugging and help with screening and stratification.
Ziggy ■
Counter Intuitive Ponderings........
Your counter intuitive notion is quite correct.
Those strokes seem oversize for the openings being screened.
THE VSMA RULE OF THUMB:
is the smaller the opening in the wirecloth the smaller the stroke that is needed and the FASTER the RPM should and can be while still not operating over the design G FACTOR.
Yeah though that is NOT CAST in GRANITE...... as many things in life are not.
.........................
I have experienced a new VIBRATING screening concept design which applies for example: a 5/8" stroke at a low angle 12 deg to screen tough clay/silt layden sand (40%) and pounds the SNOT out of this material and keeps the apertures clean with a BALL TRAY deck.....and works like a charm! All these variables contradict VSMA charts.
So.....proof in real life sometimes (NOT ALWAYS) general rules of thumbs of VSMA can be challenged with real life operating conditions. We must allow ourselves to think outside the box sometimes in practical instances. Generally though VSMA works quite nicely. ■
Re: Matching Stroke To Apertures
Thanks George + ziggy
I will let you know how it goes.
A few questions.-
1. The calc for g-facotr does not include a mass term ( i.e I would expect that the mass of a vibrating screen would be a factor in calculating the g-forces and not simply speed and stroke.... Any thoughts here??)
2. We do not have specs for the g-factor tolerances on our screen decks (at least not ones that can be easily found), A supplier has given us the rule of thumb of running between 5 to 5.5 g's. Is this a safe enough range to work in, or are we walking a tightrope.
3. I have heard a lot about natural frequency being an issue. Is it worth investigating this? How is the theory behind natural frequency best applied?? DO we need to change the springs on the screens with our new drives?
Thanks for the feedback
Thelaz ■
Re: Matching Stroke To Apertures
Answer to ITEM 2:
g force, general rule of thumb.
INCLINED CIRCLE THROW.....ie 20 deg decline on the deck.
Typically operate between 3.5 and 4 g's max.
HORIZONTAL twin shaft gear driven screeners: ie Zero degree angle on the deck. Typically operate between 5.5 and 6 g's.
Higher g forces as we DO NOT have mother nature or SLOPE to help us here typically.
George ■
Matching stroke to apertures
I am faced with a triple deck of screens with the following strokes and apertures.
Top deck - 18mm slot apertures at 9 mm stroke, 980 RPM
Middle deck - 10 mm slot apertures at 12.5 mm stroke, 750 RPM
Bottom deck - 5 mm slot apertures at 15 mm stroke, 750 PRM
The logic here seems to be counter intuitive to what is suggested by VSMA. From my understanding, a smaller slot size needs a lower stroke and a higher speed. Is my assumption here correct?
Any help would be great.
Any words of wisdom, George? ■