Disc Harrow Requirements
reprinted from Farm Practices by Allis Chalmers Co.
We especially emphasize here the expected requirements of a disc harrow which should not be overlooked before taking a harrow to the field. One of the most important requirements is disc blade concavity, that is, correct concavity. Further along we set forth the purposes of disc concavity. We feel it is important enough to devote the extra time and words in a discussion of the subject, because seldom is disc concavity talked about, and very few know that there is difference enough to cause good and bad work. Almost everyone has the idea that discs are all alike, that curvatures and sizes are the same, and if one style disc does good work, that any other style should do likewise. Proper disc angling is also a needed requirement in order to get even depth of all discs for deep or shallow work, and to get maximum pulverization in all soils. Here are some of the requirements:
- Frame — Correct design very necessary; best materials should be used.
- Hitch — Must hold gangs securely; must also get quick depth.
- Gang Bolts — Should be large; durable; self-locking.
- Bearings & Bushings — Need to be close fitting and made of best quality material.
- Spacing Spools — Should be milled to fit the disc centers.
- Scrapers — To be easy on disc blades.
- Angling Controls — Quick and easy control is essential.
- Gang Angling — Must be correct for all depths and speeds.
- Follow Contours — Should be flexible as well as rigid.
- Discs — Correct concavity; properly treated for long life; extra thickness.
- Center Control — Should fill dead furrows and level back furrows.
- Compacting Soil — Correct angle and concavity will do so.
- Speed — Must be designed for faster tractor travel.
Discs today last longer than discs of 20 years ago and testing is done scientifically. Laboratory tests are made first, and then they are put in all field soils to determine their lasting qualities. Disc blades are heat-treated for long life, and seldom, if at all, will they chip while working. The angling of the gangs must conform to the disc concavity for holding penetration at any depth, and the disc concavity must also match speed at which disc harrows can travel and do quality work.
At the extreme angle the discs cut a clean furrow with no drag on the backside and with no chattering or jumping — that is, they should do so. If not, then there is either too much or too little angling. The concavity also gives the correct curvature in the ground for steady running when set with the least amount of angle. When discs notch the small furrows in washboard fashion working at the minimum angle, it is because the concavity does not follow through with the correct slant against the ground to take a cutting bite. It’s like trying to whittle with a pocketknife set too straight against the wood. With correct concavity, at all angles, and the disc edges set at a pocket knife pitch against the ground for clean cutting, smooth shavings result without jumping and notching the furrows. The correct concavity and angling eliminates the deep furrows.
The disc harrow, like the plow, must have a range of varied depths, or it would be of little value. The disc blade is to the harrow what the plow bottom is to the moldboard plow — it is the business or working end of the tool. Frames, bolts, spools, hitch and proper weight are needed and they do their share to cause good cutting. But the actual cutting, lifting and turning of the dirt is effected by the discs, and to do this work the way the users, soils and conditions require it done, demands discs that have correct concavity, made out of the best material, and then properly heat-treated for a lasting and uninterrupted life.
The subject of disc concavity — disc curvature in other words — seems to be misunderstood, therefore, it is our purpose to clarify the reasons for the disc concavity so as to set everyone right. But we also want to show that there is a difference in disc concavities, and we also want to show what happens when the different shapes are used.
The original harrow was merely a log rolled along to break clods, level surfaces and pack the seedbed to eliminate the air spaces between the turned furrow slice and the furrow sole. It did well as a surface packer and leveler, but it failed to reach down far enough, therefore, its use was limited. Very shortly, from experience in packing dirt around posts, foundations, planting trees and shrubbery by using the spade, discs were conceived — strung on a pole and rolled along — in order to reach down for packing the loose soil nearer the furrow bottom.
Tamping with the cutting edge of a spade in loose and lumpy dirt around posts or newly planted trees, packs to the depth of the hole by pulverizing the clods and compacting the pulverized dirt around them in order to eliminate air pockets, and avoids any great amount of settling later on when the dirt takes up moisture. The disc blade, likewise in a loose and lumpy seed bed, pulverizes the clods and compacts the loosened dirt down deep as it rolls along and, therefore, overcomes the dirt settling in spots over the field to effect water holes where clods were not pulverized — a condition often seen when the disc harrow has not been used.
The disc harrow is also used for seeding by mounting a hopper over each disc gang. It packs, levels, crushes clods, and drops the seed in the disc trenches. While the disc harrow proved the most versatile tool on the farm, it still presented problems of clogging, wearing quickly, disc breakage, buckling and heavy draft. The disc blades were responsible for most of these ills, consequently, much research followed to get proper disc curvature so as to have the best cutting curve for penetration and to lighten the draft. The proper concavity solved these problems, and that’s why correct concavity discs really dig deep when required, pull light and last so much longer.
A flat disc blade will cut just as deep as a concave blade if set at a cutting angle, the same as you would tilt a knife blade to cut a clean shaving. The trouble with the flat disc is: no control of the dirt, which merely slides on past to one side — all trash exposed — and more power is required to pull it. The flat disc will also leave deep furrows under the loosened surface and it is more sensitive to varying ground pressures. In hard ground it is jumpy, also true of a disc with too shallow concavity; and in loose ground, like the shallow discs, it digs too deep and pulls hard.