Vgf Stepped Bars Xplained
Dear Mr. Baker,
VGFs' Angle of excitation is a compromise between a feeder and a screen.( Both Vibrating)
In a screen we want to work with low bed depth and high stratification. so the feeding rate is controlled and th angle of exitation steep. whereas in a feeder the angle of exitation is less steep and the lenght of the micro throw is high. This way it is possible to handle a large bed depth of material moving faster in a feeder.
In a vibrating grizzly feeder the exciter is fixed to give a throw angle which is larger than the vibrating feeder and less than a screen.
so the stratification of material is lesser: hence we step the deck to prevent (reduce) piggy backing fines being Carried over with the oversize material.
When you have flaky lumps ; there is a problem of flakes getting stuck in the bar gaps and remaining there till they are cleaned.
To prevent this you can weld some spoilers between the bars at intervals (across the direction of flow).
Regarding the length of blank portion being 12'; this is always decided after ploting the material flow line a the feeding point of the grizzly.
Just wanted to share these thoughts with you 
Thanks & trust it will be of help ■
Vibrating Grizzly feeder/step deck design
Let's suppose we are ordering a big brute force GEAR DRIVEN Heavy Duty Grizzly feeder.........to match up with a big jaw crusher.
Let's suppose the feeder is nominal 74" wide x 24' long.......this rig would typically have a big solid pan......say for the first 14' and say two 5' long grizzly bars segments following the pan.
The question is: do we put the pan and the grizzly sections on the same plane? OR do we STEP them down........IE: Pan on one plane....then say drop 4-6" to a grizzly bar area 5' long.......followed by another 4-6" drop to the next 5' long section of grizzly bars.
WHAT reason would we STEP the grizzly areas down? If we are scalping large BLAST ROCK at plus minus nominal 4" opening on the grizzly bars..... If we have large slabby pieces....with fines created from the blast.......the fines will piggyback on the large slabs and go into the crusher chamber.....as they simply carry over into the jaw on a straight in line pan and grizzly design.
On a stepdeck configuration, the large pieces TIP or ROLL over after tumbling over the 4-6" step or drop........tipping the fines off.....allowing them to PASS or BYPASS The crusher chamber........making the CRUSHER crush ONLY OVERSIZE MATERIAL ........and not UNDERSIZE. material .......which has no purpose in the crusher....except to add to high wear cost.....and reduced crushing capacity.
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Another TIP: most VGF grizzly feeders......are installed at zero degrees........I recommend most of my installations go in at 3 degrees install angle.
MUCH BETTER TONNAGE AND REDUCED MAINTENANCE.
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We can also apply this concept or design theory of step deck designs to heavy duty SCALPER SCREENS......for the same reasons. the more with tip and roll and expose all surfaces......the better more efficient scalping we do.
Wishing You happy feeding......George Baker MODERATOR...
Attachments
generic vgf pic 2 (ZIP)
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