Location of the drive

The rotor may be driven from the one end, both ends, or from the centre. Driving the rotor from the centre reduces the total number of drivesystem components. Power is transmitted from the PTOshaft, through the gearbox, to the rotor.

However, driving from the centre usually requires close fitting blades next to the gearbox. This means that a small centre strip will generally remain uncut. A small chisel may be installed in the centre to remove the unworked strip for seedbed preparation, but should be removed for primary tillage like weed control.

Driving the rotor, from one end on smaller rotary tillers eliminates the problem of an uncut centre strip, but requires more driving components. Power may be transmitted to the rotor from the gearbox or upper drive shaft by a series of gears or through heavy duty roller chains (see Figures 1 and 2).

Driving from both ends permits more uniform application of power for long rotors on large machines, and again eliminates an unworked centre strip. It requires a more complex drive train, but may allow the use of lighter components in some places because the driving power is split. The use of end drives increases overall machine width, which may not pose problems in field operations but could be a factor in orchard or vineyard work. All rotary tillers are equipped with slip clutches.

Power requirements

Specific energy required for rotary tillage, that is energy used per cubic meter of soil moved, is about three times as much as that required for mouldboard ploughing. Heavy soils, a faster operating speed, or a greater operating depth would require more kW for the same size machine. The capacity of a rotary tiller is a very important factor in mechanisation planning, or else it may delay the planting process if tillage is not completed in time.

Although a rotary tiller with a given capacity, will cost more than an individual implement required for conventional tillage (plough, disc, and harrow), the total investment is generally less, particularly if a stalk shredder is also eliminated. The power requirements of rotary tillers are generally 25 to 30 kW per metre working width (see Figures 3 and 4).

Principles of rotary tillage

Rotary tillers apply tractor engine power through the PTO directly to the soil, without excessive tractor weight. But rotary tillers cannot cure poor tillage management, and they may even make poor management look worse. However, with good management and proper operation, they can provide a valuable tillage alternative to ploughing, disking, and harrowing. The rotor normally turns in the same direction as the tractor wheels.

Normally, two or three pairs of right-hand and left-hand blades are used on each flange on the rotor shaft, depending on soil conditions and the degree of pulverisation desired. A left-handed blade is one where the cutting end points to the left, when viewed from the rear of the cultivator. A right-hand blade points to the right (see Figure 5).

Three pairs of blades per flange are usually recommended for general operation on most rotary tillers. In wet, sticky soil it may be desirable to remove one pair of blades per flange so that the soil is more easily thrown away from the rotor. The rotor speed must be increased when using only two pairs of blades per flange, because the remaining two pairs of blades on each flange must provide a proper cut. The increased speed moves the soil faster and helps to prevent plugging.

Under normal conditions, most rotary tiller blades are self-sharpening and they are arranged in a scroll, or spiral pattern on the rotor so that not more than one blade strikes the soil surface at the same time. Failure to maintain the scroll pattern will cause uneven work, excessive vibration, and thumping,  when more than one blade hits the soil at the same time (see Figure 6).

The size of the cut, or bite, of rotary tiller blades is determined by the forward speed, the number of blades per flange and the rotor speed. Slower rotor speeds require much less power, help maintain soil structure, reduce blade wear, reduce fuel consumption, and save time.

Next month we shall look at seed drills and planters, tractor preparation, and the PTO shaft. We thank the ARC Agricultural Engineering in South Africa who made the information on rotary tillers available to the readers of ProAgri Zambia.