Chapter 5: Design of sowing and planting machine 5.1. Design of seed-cum fertilizer drills
The functions of a well designed seed drill are as follows: •
Meter seeds of different sizes and shapes
•
Place the seed in the acceptable pattern of distribution in the field
•
Place the seed accurately and uniformly at the desired depth in the soil.
•
Cover the seed and compact the soil around it to enhance germination and ermergence.
The seed cum fertilizer drill machine consists of •
seed box
•
fertilizer box
•
seed metering mechanism
•
fertilizer metering mechanism
•
seed tubes
•
furrow openers
•
seed rate adjusting lever and
•
transport cum power transmitting wheel
The seed metering mechanism is discussed in detail.
Seed metering devices
Metering mechanism is the heart of sowing machine and its function is to distribute seeds uniformly at the desired application rates. In planters it also controls seed spacing in a row. A seed drill of planter may be required to drop the seeds at rates varying across wide range. Common type of metering devices used on seed drills and planter are:
1. Adjustable orifice with agitator: in this type or metering device figure 5.1, seed flow is regulated by changing the size of the opening provided at hopper bottom. An agitator fixed above seed opening helps in continuous flow of seeds. This does not give precise control over the seed rate and uniformity of distribution in rows. Many designs of conventional animal and tractor operated machines have adopted this mechanism on account of its simplicity and low cost. The design of stationary orifice with agitator type seed metering mechanism for drills below.
The physiological of seed size and shape of orifice, shape and dimensions of seed box affect the flow rate of seed as given by Bernacki (1972). For free flow seeds through hopper bottom openings, the sizes openings are given as under: (i)
Circular openings (d) d =
(ii) b=
2P
1
ρ s sin 2α
Rectangular openings (b) P
2
ρ s sin 2α
Where, P = Unconfined yield strength of material, kg ρ s
=
Specific weight of seed, kg/cm
3
α = 45 − ϕ / 2 , Angle of inclination of pouring places φ = Internal friction of seed, degrees
d = Diameter of opening, m d = Diameter of opening, m
Figure 5.1 Orifice agitator type seed metering device
The flow rate of seed through circular orifice/ opening is given by
P Q = F ∗ ρ s ∗ Ao ∗ 2 g ∗ ρ s
0.5
3
Where Q = flow rate of seeds, g/s F = Flow rate index depends on geometry of seed box and opening
ρ s
=
Bulk density of seeds pouring out of orifice, kg/cm
Ao
=
Area of orifice, cm
3
2 2
g
=
Acceleration due to gravity, m/s
P
=
Static pressure of seed mass, g/cm
2
So, based on type of seed, its size, height of seed hopper, the opening of hopper bottom (d) is calculated by using equation 1 and 3 given above for free flow of seeds through orifice. A rubber roller is used for agitation of seeds in the seed box. Similarly, for metering fertilizer an adjustable orifice with star type agitator is used for metering the fertilizer.
2. Fluted roller:
Fluted roller metering mechanism figure 2 is a more positive metering device. Axial or helical flutes are mechanical or cast on an aluminium, cast iron or plastic roller. Rotation of fluted roller in housing, filled with seeds, causes the seeds to flow out from housing in a continuous stream.
Figure 2 Fluted roller seed metering mechanism
Seed rate is controlled by changing exposed length of fluted roller in contact with seeds and fairly accurate seed rate can be achieved for a variety of medium size seeds like wheat, soybean, sunflower, safflower etc. However, metering of small seeds like mustard and sesamum at 2 to 5
kg/ha seed rate is not accurate with normal size flutes which are designed for seed rates of 20 to 120kg/ha. Therefore, fluted rollers with smaller size flutes are used. Flow of seeds in seed tube
After release from the seed plate, the seed travel in seed tube and the time of seed residence in seed tube (t 1) is calculated by the following equation: t 1
=
(2h1 / g )0.5
4
Where: t 1
=
time, seconds
h1
=
Length of seed tube,
g
=
Gravitational acceleration, m/s
2
Time for seed to strike ground (t 2) is calculate by using 0.5 gt 22
h2
=
v 2 t 2
Where, h2
=
Distance of seed tube end and ground surface, cm
v2
=
v, Final velocity of seed at end of seed tube, m/s
+
5
Horizontal distance moved by seed on the ground (S): S =
V ∗ t 2
3.6
6
Where, S = horizontal distance moved by seed after release, m V = Forward speed of power source, km/h t 2
=
Time travel of seed from seed tube to ground, s
Total time of vertical travel (t) t = t 1
t
+ 2
7
Figure 3 flow of seed in seed tube
