DISC HARROW - Optimal disc diameter? 16"-18"-20"?

[jeff9366]

I am contemplating purchase of a 3-Pt., 60" (+/-) box frame disc harrow, new.

I do NOT own a plow. Pretty much decided I want notched discs on the harrow, to cut Bermuda and native Bahia grass. Disc wear rate is not an issue.

Soil is Florida sandy-loam. Very few rocks, just infrequent clumps of soft limerock.

Primary use: level, smooth "clean up" recently cleared areas. I have removed 60% of Oaks, Pines, Myrtles, Palmettos by the roots so I can 'hog in the future unobstructed.

Still plagued by <3/4" residual Rotundfolia wild grape roots running through top 3"-5" of soil. I pulled the "mother" vines
---> attached roots up to 60 feet long!

Secondary use: food plot preparation prior to seeding. I have a 60" Cultipacker.

What factors should I consider relative to disc diameter? I see 14"-16"-18"-20" disc diameters, with 18" seemingly most common.

What other factors should I consider?

Tractor is a Kubota B3300SU / 33-hp / 1,800 lbs / industrial tires.

T H A N K S

[sweettractors]

18" would be a good all round diameter. Ken Sweet

[jeff9366]

18" would be a good all round diameter. Ken Sweet

But why?

I want to learn what factors work into selecting disc diameter.

Is it primarily a question of determining weight of implement?

Cutting depth?

What is the reasoning?

[sweettractors]

But why?

I want to learn what factors work into selecting disc diameter.

Is it primarily a question of determining weight of implement?

Cutting depth?

What is the theory?

Your depth is 1/2 the diameter of the blade minus 1/2 the disc spool spacer diameter. Probably 7-8" max working depth. That is more than you can pull in most soils, however, it is available if needed. Ken Sweet

[flusher]

But why?

I want to learn what factors work into selecting disc diameter.

Is it primarily a question of determining weight of implement?

Cutting depth?

What is the theory?

Read this

http://www.caes.uga.edu/departments/...%20tractor.pdf

[art]

Depending on the amount of use most farmers will wear one to two inches off in a year! So a larger diameter not only means the depth that they can go but how long they can go before they are worn out and need to be replaced.

[Wyobuckaroo]

I would suspect the usual compaction dept of soil in your situation would be a determining value to have in mind as you do 1/2 disk diameter and other calculations..........

Good luck

[npalen]

Something to keep in mind is that the smaller the disc diameter the better it will cut in hard ground. The disc blades are acting as wheels albeit very thin wheels but there are lots of them. The larger "wheel" has more flotation and does not penetrate as easily. Weight of the disc frame is, of course, a big factor also as more weight per foot or weight per plate translates into more cutting power requirement as the blades will cut deeper if needed. I did scan thru the University of Georgia paper mentioned above and it appears to be a great resource for horsepower requirements.

[jeff9366]

FLUSHER:

I looked at the interesting caes.uga link you kindly provided yesterday.

I see a problem, at least for my purpose, in the formula treats draft of a harrow as given: i.e.: 92 pounds draft per foot of width, per inch of depth.

However, surely draft will vary depending on 1.) number of discs = disc spacing (16 discs or 20 discs in my case) and
2.) diameter + width of the discs, as discs are not flat but dished 3.) slotted or smooth discs X 16 or 20 discs.

Harrow weight may not be a variable because weight is indirectly included in disc depth.

Also, you have to look closely at the calculation. Author talks about 8" depth but numbers used in calculation are for 4" depth.

So I am uncertain if 16" or 18" disc diameter is best for my purpose. 20" size is OUT of consideration.

I know there are ag types reading T-B-N with wide ranging disc harrow experience.
I have "0".
Lend me your experience.

[npalen]

Jeff,
You mentioned "discs are not flat but dished" which is referred to as "concavity" in the industry. Concavity is usually measured by laying a straight edge across the cutting face of the blade noting the depth to the center of the blade. This measurement is a little misleading, however, as it depends on the diameter of the blade. A better indicator might be the spherical radius of the blades contour. A high concavity blade will move much more soil laterally than a low concavity blade so, yes, draft is affected. The angle of the disc gangs also has a great effect on draft with a larger angle increasing the amount of soil moved laterally. The concavity of the blade correlates to the gang angle in achieving the optimum attack angle of the blade to the soil.
You also mention draft per foot per inch of depth. This should probably be non-linear as a change in depth at, say, 6" will affect draft much more than a change in depth at, say, 3". Cutting slightly above or slightly below the hardpan would change the draft exponentially also. The formulas for draft should be taken for what they are, a guide. I would be much more inclined to take the advice of someone who has experienced a very similar disc application as yours.
Regarding the number of blades or "blade spacing" is another large factor. Discs with blades spaced relatively close to together are normally referred to as disc harrows while wide spacing is sometimes referred to as a disc plow. The disc harrow, with it's larger number of blades for a given width, could be said to draft lighter, however, because it is normally used in previously tilled soil.