Farm Power & Machinery

Types and components of Secondary tillage implements

Secondary tillage implements, Types and components

Harrow

• Harrow is a secondary tillage implement that cuts the soil to a shallow depth for smoothening and pulverizing the soil as well as to cut the weeds and to mix the materials with the soil.
  
• There are several types of harrow used in India such as disc harrow, spike tooth harrow, spring tooth harrow, acme harrow, patela, triangular harrow, blade harrow,
Disc harrow

• It is a harrow, which performs the harrowing operation by means of a set, or a number of sets of rotating slat discs, each set being mounted on a common shaft.
  
• Disc harrow is found very suitable for hard ground, full of stalks and grass fields. It cuts the lumps of soil, clods and roots.
  
• Discs are mounted on one, two or more axles which may be set at a variable angle to the line of motion.
  
• As the harrow is pulled ahead, the discs rotate on the ground. The disc harrow in field operation is shown in Plate 14.1.


(Courtesy: http://www.woodsequipment.com)
Plate : 14 Disc harrow in operation


Classification of disc harrow

• The disc harrows are classified based on disc arrangement as Single action, Double action disc harrows, tandom disc harrows and Off-set disc harrows.
  
Based on power source, they are classified as Tractor drawn and bullock drawn disc harrows.

a) Single action disc harrow

• It is a harrow with two gangs placed end to end, which throw the soil in opposite directions.
• The discs are arranged in such a way that right side gang throws the soil towards right, and left side gang throws the soil towards left as shown in Fig. 14.1.


Fig. 14.1 line diagram of disc arrangement in single action disc harrow

b) Double action disc harrow

• A disc harrow consisting of two or more gangs, in which a set of one or two gangs follow behind the set of the other one or two, arranged in such a way that the front and rear gangs throw the soil in opposite directions (Fig. 14.2).
  
• Thus the entire field is worked twice in each trip. It may be of two types i) Tandem and ii) Off-set type.


Fig. 14.2 line diagram of disc arrangement in double action disc harrow

c) Tandom disc harrow
It is a disc harrow comprising of four gangs in which each gang can be angled in opposite direction as shown in Fig.14.3.


Fig. 14.3 line diagram of disc arrangement in tandom disc harrow

d) Off-set disc harrow

• It is a disc harrow with two gangs in tandem, capable of being off-set to either side of the centre line of pull.
• Two gangs are fitted one behind the other. The soil is thrown in both directions because discs of both gangs face in opposite directions.
• It is very useful for orchards and gardens. It travels left or right of the tractor.
• The line of pull is not in the middle, that's why it is called off-set disc harrow.
• Off-set disc harrow is based on the basic principle that side thrust against the front gang is opposed by the side thrust of the rear gang.
• Hence the gangs are arranged at suitable angles so that both thrusts are counter balanced with each other.

Fig. 14.4 line diagram of disc arrangement in offset disc harrow

Components of a Disc harrow

• A disc harrow mainly consists of disc, gang, gang bolt, gang central lever, spool or spacer arbor bolt, bearings, transport wheels, scraper and weight box.
• Disc: It is a circular concave revolving steel plate used for cutting a inverting the soil (Plate : 14.2).
  
• Disc is made of high glass heat-treated hardened steel. Tractor drawn disc harrows have concave discs of size varying from 35-70 cm diameter.
  
• Concavity of the disc affects penetration and pulverization of soil. Usually two types of disc are used in disc harrows, plain disc and cut away disc.
  
• Plain discs have plain edges and they are used for all normal works. Most of the harrows are fitted with plain discs only.
  
• Cut away discs have serrated edges and they cut stalks, grass and other vegetation.
  
• They are not effective for pulverization of soil but it is very useful for puddling the field especially for paddy cultivation.


(Courtesy: http://www.sumoindia.com)
Plate : 14.2 Different types of disc used in disc harrow

Gang: Each set of discs that are mounted on a common shaft is called the gang.
Gang bolt or arbor bolt: It is a long heavy of square headed bolt from the other end, a set of discs are mounted on the gang bolt. The spacing between the discs on the gang bolt ranges from 15 to 25 cm for light duty and 25 to 30 cm for heavy duty harrows. The angle between the axis of the gang bolt and the direction of travel is called the gang angle.
Gang control lever: A lever, which operates the gang mechanisms of the disc harrow, is called the gang control lever.
Spool or spacer: The flanked tube, mounted on the gang bolt between every two discs to prevent the lateral movement of the disc on the shaft is called the `spool’ or `spacer’. It is just a device for keeping the discs at equal spacing on the gang bolt. It is usually cast in special shapes and sizes and is generally made of iron.
Bearing: Bearing is essential to counter act the end thrust of the gang due to soil thrust. The harrow bearings are subjected to heavy radial and thrust loads Chilled cast iron bearings are used for heavy radial and thrust loads and they are also used due to their durability.
Transport wheel: In trailing type disc harrow, the transport wheels are provided for transport work on roads and for preventing the edges of the discs from damage. Mounted type disc harrows do not require wheels for transport work.
Scraper: It prevents disc from clogging. It removes the soil that may stick to the concave side of the disc.
Weight box: A box like frame is provided on the main frame of the harrow for putting additional weight on the implement. Additional weight helps in increasing the penetration of the disc in the soil.
There are several factors which affect the penetration of disc harrow in the field. If the disc gangs are set perpendicular to the line of draft, the penetration is not adequate. Penetration can be increased by adding some additional weight on the frame of the harrow. For obtaining maximum penetration, the gangs should be set with the forward edges of the discs parallel to the direction of motion. If the hitch point is lowered, better penetration is achieved.
A sharp edged disc has more effective penetration compared to blunt edged disc. It is observed that penetration is better in low speed than in high speed.

The following are a few adjustments for obtaining higher penetration

i. By increasing the disc angle
ii. By adding additional weight in harrow
iii. By lowering the hitch point
iv. By using the sharp edged discs of small diameter and lesser concavity
v. By regulating the speed.