3.2. Ground Driven Cultivators

Rotary cultivators driven by ground contact have been in production for many years. A twin rotor machine (see Figure 11.) with the rear rotor chain driven by the front rotor is preferred by some farmers for seedbed prepara­tion and stubble cleaning. Working depth is controlled by the rear crumbier roller which can be replaced with a packer roller where greater consolidation is required.

Figure 11. A ground driven rotary cultivator

Another design, first seen many years ago, has been re-introduced to meet a demand for a rotary cultivator with a low power requirement (see Plate 10.18). It has three pairs of rotor shafts running at an angle with cross-shaped blades which drive themselves by ground contact. It is made in various widths between 2.75 and 4.8 m and can be used for seedbed work and stubble breaking.

4. Harrows

Harrows. Harrows are the most important machines for seedbed preparation. They are still used in low input agriculture to bury the seedlings spread by hand or with disk broadcasters. In a similar way, they serve to incorporate fertilizer. They have to perform crumbling, loosening, mixing, leveling and to a certain extent stratifying the seedbed. At the same time, they are used to control weeds after the emergence of the actual crop. A further task of harrows is stubble cleaning; that is, cutting, loosening and mixing of the root zone in order to induce germination of fallen out grains and prepare the field for primary tillage.

The conventional harrow consists of tines grouped together in a frame and several of these frames grouped together for the complete harrow; various tines of different intensity (rigid or spring) and additional rollers for better crumbling and good leveling are available (Fig. 12). These elements are grouped in carrier frames for seedbed preparation. The tillage intensity is comparatively high. A harrow with a smaller working intensity is the chain harrow (Fig. 12), which adapts well to any soil profile or ridges, and which is used especially for weed control and for leveling dams and ridges.

Figure 12. Harrow, harrow elements with rigid and spring tines, roller with cutter bars and chain harrow (right below) with tines.

Spike Tooth Harrows. (Fig. 13) These consist of simple frames with tines bolted to them where the frame members cross. There is a variety of harrow sizes and weights from light harrows for final seedbed preparation to heavy harrows used in some areas for breaking down ploughed land. Spike tooth harrows will be found in many farmyards but their use has declined in recent years. Traditionally these harrows were used in sets of four or more, attached to a bar or pole with a wheel at each side, often towed behind a cultivator or roll.

The modern method is to suspend a number of harrows with chains on a three-point linkage mounted frame which folds for transport. Pressure harrows up to 8 m wide or more on a mounted frame with a closed hydraulic system to distribute the weight of the toolbar equally over the sections are a more recent develop­ment. By equalising the pressure, each harrow follows the ground contours more accurately and the individual harrow frames have a levelling and scrubbing effect. Small diameter crumbier rollers or open coil rollers can be attached to the rear of the implement to con­solidate the surface.

It is important to keep harrow tines tight. Failure to do this will wear the holes in the frame.

Plate 13. Spike link chain harrow.

Chain Harrows. The links may be plain or spiked. Plain linked harrows are used by some farmers to spread droppings and molehills on pastures or to cover seed. Grassland can be aerated with a spiked link chain harrow.

Figure 14. Disk harrow (half in transport position and half in work position) (left) and rotary spade harrow (right).

Disc harrows. A machine with a very great importance is the disk harrow. It usually consists of several sets of disks on a horizontal axis, which are angled towards the direction of travel with an adjustable angle (Fig. 14.). This allows to control the intensity of the soil engagement of this machine. In order to avoid lateral forces and to avoid soil transport to one side, there usually are two (symmetrically) angled sets of disks grouped one after the other. In order to achieve central force application, disks of one row are angled in two opposite direction. Due to its scraping effect, the disk harrow is well-suited for stubble cleaning. The comparatively strong inverting effect allows tillage with disk harrows alone as its effect comes close to that of a disk plow. At the same time, it can also be used for secondary tillage to break clods, create a seedbed and level the soil after plowing.

A problem of this device can be that it may create a smear zone; furthermore, it is not pulled into the soil by the form of its implements, which means that it must be fairly heavy in order to sufficiently penetrate into the soil.

Disc harrows cut and consolidate the soil. Two or more sets of saucer-shaped discs are fixed to a frame which may be mounted or semi-mounted. It is more common for the larger models to be trailed with large diameter trans­port wheels raised and lowered with a hydrau­lic ram. Disc diameter varies from 300-750 mm or thereabouts and each set of discs is carried on ball or taper roller bearings. Some machines have plain discs while others have scallop edged discs, usually on the front gangs, and plain discs at the back. Plain discs have a better cutting and consolidating effect and scallop edged discs give better penetration in hard ground.

Working widths vary from 2.75 to 7m (9-23 ft) or more. Mounted disc harrows will be transported on the lift linkage and trailed models have a pair of pneumatic tyred wheels lowered by a ram for transport purposes. Wider disc harrows are folded hydraulically for road transport. Some models have three gangs wide with folding wing sections. The double tandem disc harrow (Fig. 14) has all four gangs rotated inwards with the angling rams to give a transport width of about 2.5 m. Another design (Fig. 15) has both gangs raised to the vertical position for road haulage.

Plate 15. Hydraulically folded disc harrows in the transport position.

Discs are used by some farmers in combina­tion with other equipment, including tined implements or a roll to make a seedbed in a single pass.

Adjustments

Disc angle Hydraulic rams are used to vary the cutting angle of the discs. When set at the widest angle to the direction of travel, soil movement will be at a maximum. When run­ning almost straight the discs will move the soil less and consolidation is increased. Some models have a ratchet operated turnbuckle arrangement and elderly disc harrows usually have hand levers to alter disc angle.

Working depth is controlled with the tractor hydraulics or ram operated depth wheels. Some models have a depth sensing on the hydraulic ram which returns the discs to a pre-set working depth. The disc angle will also affect work­ing depth and extra weight may be added to increase penetration in hard ground.

Disc scrapers are adjustable and should be set close enough to keep the discs clean.

Uses

• Seedbed preparation, especially after plough­ing in grass where a tined implement is likely to pull turf to the surface.

• Cutting up crop residues such as kale stalks.

• Stubble breaking.

• Some farmers use heavy disc harrows, often with scallop edged discs on the front gangs, as an alternative to ploughing.

Maintenance. Regular greasing, and checking the tightness of nuts and bolts are the main servicing needs for disc harrows.