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Power Harrows

A rather important PTO-driven machine for secondary tillage espe­cially for heavy soils is the rotary harrow (Fig. 16.). There are two types of power harrow. Those with rotary tines are preferred by most farmers, but a few use a power harrow with reciprocat­ing tines.

It consists of a number of rotating disks, which usually are equipped with two or three tines that are angled towards the soil. Thus, they mainly beat and crush clods on the soil surface and press them into the soil. During the operation of the rotary harrow, a wall of earth builds up before the machine but due to the horizontal movement of the tools, almost no mixing occurs. This earth wall creates a level plane surface after the passage of the machine. Compared to the rotovator, the energy consumption is higher. This is partly due to the necessary drawbar pull and partly to the less effective tool operation. The machine is not suitable for plant residues as the great number of rotating axles may wind up plant residues, thus causing blockage of the machine. If the machine should be used in unloosened soil, it is necessary to angle the tines away from the direction of travel in order to achieve a rake angle that breaks up soil towards the surface. Rotary harrows need a roller to bear the weight of the machine, for depth control and for recompaction.

Rotary power harrows have vertical rotating tines bolted to a series of rotor heads across the full width of the machine. The tines are driven by the power take-off through a gearbox and system of gears attached to the tine rotors. Adjacent tine rotors turn in opposite directions and each rotor is carefully timed with its neigh­bour to prevent the tines hitting each other and to achieve the best possible tilth. Although it is more common for power harrows to be used on the rear hydraulic linkage, some models can also be attached to a front linkage and driven by a front-end power shaft. This leaves the rear linkage free for a drill, fertiliser distributor or a sprayer.

Plate 16. A 4 m power harrow with a packer roller.

Plate 17. Hydraulic rams fold the two halves of this power harrow to a vertical position for transport.


Plate 18. The underside of a power harrow with the clod bar and coil packer behind the tines. The gears on the tine rotors are assembled so that each pair of tines are at 90 degrees to the next pair.

Figure 19. Rotary power harrow.


Rotor speed can be varied from approxi­mately 125-500 rpm depending on model. This is done by using different pairs of gears in the gearbox or with a lever change gearbox. For most conditions, a rotor speed of 250-350 rpm is recommended. Except for some narrow models, most power harrows are driven from the 1,000 rpm power take-off shaft. Power requirement is high: a 3 m harrow requires at least 56 kW (75 hp) at the power take-off shaft and a heavy duty 6 m model needs up to 150 kW (200 hp). A slip clutch or a shear pin mechanism protects the drive from the power shaft to the main gearbox.

Working widths vary from 1.5-6 m; the smaller models are lightweight in design and are mainly used by horticulturalists. Power harrows over 4 m wide have a more robust frame and gear drive than narrower machines. Very wide power harrows are either towed endways or hydraulically folded for transport. A 6 m power harrow with a pair of transport wheels at one side and a drawbar attached at the opposite side is less expensive than the folding model. This is really two 3 m harrows hinged on a central frame with separate drive to each half.

Adjustable soil deflector plates are fitted to both sides of power harrows to give a level surface between each pass and a floating clod comb or a backboard follows the tines. Both improve clod breaking performance, but a back­board holds the soil for longer in order for the tines to have more effect.

The soil is consolidated to varying degrees by a rear crumbier roll, open coil roller or packer roller, which also regulates working depth. A crumbier roll works best when soil conditions are good. The open coil roller has a greater con­solidating effect and the packer roller compacts the soil to the same degree as a Cambridge roll. A drill or a planter can be attached to a three-point linkage frame and power shaft extension on many models of power harrow so that two operations can be completed in one pass. The drill and harrow are both lifted when turning on the headland. A bridge link is a method used by some farmers to couple a trailed drill to a power harrow. The trac­tor driver lifts and lowers the harrow on the three-point linkage and operates the drill with auxiliary rams from the tractor hydraulic system.

Plate 20. Power harrow tine and gear.

Plate 21. Types of rear roller used with power harrows. Top: crumbier roll. Centre: open coil roller with star scrapers. Bottom: packer roller with scrapers.

Plate 22. A floating clod comb is fitted in front of the open coil roller.

The star cleaners rotate to clean the coils.

Plate 23. Power harrow tine depth is adjusted by using different holes in the quadrant (left) and the height of the levelling bar is varied by moving the limiting pegs up or down in the linkage.

Adjustments.The fineness of tilth produced with a rotary power harrow depends upon the speed of the tine rotors and the forward speed of the tractor. A fine tilth is achieved with a low gear and high rotor speed. The height of the clod comb or back board also affects the tilth. It should be high enough to let broken soil through but keep the clods in. Clods will be buried just below the surface if the board is set too deep.

Working depth, up to a maximum of 250 mm, is adjusted by raising or lowering the rear roller.

Power harrow-drill combinations.Although the implement is more suited to working on ploughed and pressed land, many acres of cereal crops are sown in a single pass with a drill mounted on a power harrow. How­ever, it is slower than a tined combination outfit and is limited in its working width.

Plate 24. A combination power/harrow drill with pneumatic grain feed mechanism.

Power harrow maintenance.Frequent lubrication is important. Gearbox oil levels need checking. Make sure all nuts and bolts are tight, paying special attention to the tine retaining bolts. The tines need to be replaced when worn. Some power harrows have quick-release clip-on tines. If the tines are allowed to get too short the tine holders will also wear away and the tine rotor oil seals may be damaged.


Rollers.Rollers serve mainly for recompacting, leveling and crumbling. At the same time, their task is to create a better contact between seeds and soil. Furthermore, they can be used as a means of spreading the tractor weight evenly across the whole width of the vehicle.

Figure 25. Various types of rollers: (1) plain roller, (2) grooved roller, (3) Cambridge roller,

(4) cross kill roller, (5) cage roller, (6) spiral roller, (7) toothed packer, (8) disk roller,

(9) tyre furrow press, (10) conventional furrow press with cross kill roller.


Usually, rollers should be used under dry conditions since wet soil will clog them eas­ily unless scrapers are used to clean them permanently. According to their form, rollers can only compact the surface or act at a certain depth if they are shaped to penetrate through the topsoil. Many different forms are available (Fig. 25). Among them, the furrow press should be noted, which is used directly after plowing in order to give back to over-loosened soils optimum porosity and to make them traffickable again.

The two main types of roll are the flat roll and the Cambridge or rib roll. A furrow press is similar to a Cambridge roll, but the rings are spaced further apart. Cambridge rolls have a number of cast iron rings of 500-600 mm diameter on a heavy axle which leave a corrugated surface. Flat rolls are less common and have a number of cast iron rings with a flat outer rim on an axle. Others have one or more lightweight steel cylinders on an axle or a much heavier cylinder which can be filled with water to make a very heavy roll for grassland.

Rolls are normally trailed and were tradition­ally used in gangs of three - one wide and two narrow with a working width of up to 8 m. They are hitched one behind the other for transport. They can only be pulled at low speeds on public roads and have been replaced on most arable farms by hydraulic folding models up to 12 m or more wide with pneumatic tyred transport wheels. The outer sections are swung inwards on some models (Fig. 26), while others are folded in a gull wing formation.

Plate 26. Hydraulic rams fold this roll for transport.

Plate 27. A roll hydraulically folded for transport.

Plate 28. This flat roll has a heavy steel cylinder which can be filled with water to add weight when rolling grassland.

Plate 29. Furrow press with rear crumbier roll. One of the walking stick shaped drawbars is left in the correct position by a linkage on the plough so that the press is automatically re-connected to the plough when it starts the next furrow.


Cambridge rolls:

• Breaking clods.

• Firming the soil.

• Pushing stones into the soil to reduce the risk of them damaging harvest machinery.

• Leaving a finely ridged surface for broad­casting grass seed.

Flat rolls:

• Breaking clods.

• Providing a smooth, firm surface for drilling.

• Pushing in stones on pastures (especially with the very heavy or water-filled type).

Maintenance.The axle bearings need regular lubrication when in use. The roll sections should be kept tight on the axle to make the rings turn in a straight line.

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