**3. Maize sheller design considerations**

The design objective is to obtain maximum shelling performance from the equipment. The performance of shellers in terms of shelling efficiency, grain

damage percentage, output capacity, cleaning efficiency, and power requirement is a function of design parameters, operating factors, physical and engineering properties of maize [21].

### **3.1 Design parameters**

Design parameters include: cylinder diameter, cylinder speed, shelling length, clearance between the spikes and the concave, diameter holes in the concave, spike shape, size, and arrangement on the shelling drum and the blower type. Uttam et al. [11] recommended 886 rpm and 12.05–13.64% for shelling speed and moisture content, respectively [1] for the best shelling results. At these conditions, the study concluded that the shelling efficiency, cleaning efficiency, grain recovery efficiency, total grain losses, and output capacity were 87.08, 95.89, 95.48, 2.96, and 623.99 kg/h respectively. Chilur and Kumar [22] developed and evaluated the performance of a modified dehusker cum sheller. In their study, they recommended a clearance of 25 mm between the spikes and the concave for good shelling results.

### **3.2 Operating factors**

Operating factors include grain moisture content, shelling speed, and the feeding rate. An evaluation of these factors depends on the knowledge and understanding of the equipment's mode of operation.

Shelling efficiency is increased by reducing the moisture content [23]. This can be attributed to less resistance to the removal of maize grains from the cobs due to low moisture. The grain damage percentage increases with a reduction in moisture content [1]. This can be attributed to less deformability of the grains which reduces the breakage at low moisture content. The sheller output capacity also increases with a reduction in moisture [24]. This can be attributed to the reduced time needed to remove maize grains from maize cobs as moisture content lowers. Likewise cleaning efficiency increases with a decrease in moisture content [25]. This can be attributed to the negligible moisture content of the chaff as the grain moisture content reduces.

The shelling efficiency is increased by an increase in shelling speed [23]. This can be attributed to the increased ease in the removal of maize grains from the cobs as a result of increased impacts and resistance created between the shelling drum and the concave as the shelling speed increases. Increased shelling speed increases the grain damage percentage [1]. This can be attributed to the more force exerted to the maize grains on the cobs as a result of higher cylinder speed and frequency of impacts at higher shelling speed. Increased shelling speed causes an increase in the output capacity. The output capacity of the sheller also increases with an increase in shelling speed [24]. This can be attributed to more removal of maize grains from the maize cobs due to increased impacts and resistance created between shelling drum and the concave with the increased shelling speed. Likewise, the cleaning efficiency increases with an increase in the shelling speed [25]. This may be attributed to an increase in the air flow rate produced by the sheller blower as the shelling speed raises.

Increasing the feeding rate decreases the shelling efficiency [26]. This can be attributed to the increase in unshelled grains that comes with the increase in the feeding weight as the feeding rate increases. The increased feeding weight causes an imperfect contact between concave and shelling drum. Also, increasing the feeding rate, decreases the broken grain percentage. This is due to increasing the weight entering the sheller through the hopper which acts as a cushion that reduces the effect of the grains with the shelling unit and this reduces the broken grain percentage.

### *Improving Maize Shelling Operation Using Motorized Mobile Shellers… DOI: http://dx.doi.org/10.5772/intechopen.101039*

To find out how different design and operating factors of maize shellers affect their performance, studies have been conducted. Aremu et al. [27] designed, constructed, and assessed the performance of the motorized maize shelling machine. The experiment used three pulleys to change the shelling speed between 623 and 886 rpm with moisture content at levels of 13, 15, and 17%. Their study noted that maize grains of lower moisture contents were easily removed from the maize cobs. This was in agreement with what [28] found out when they conducted a similar experiment under the same conditions. The study further noted that shelling speed is directly proportional to the shelling efficiency and output capacity.

In most of the earlier studies, one operation factor was studied at ago using different experiments. However, using factorial experiments, the researcher can compare all treatments that can be created by different factor levels [29]. Factorial experimentation is highly recommended because every observation gives information about all the factors in the experiment. Srison et al. [30] used a factorial experiment to study different factors affecting losses and power consumption of axial flow corn shelling unit at different levels of the main effects. The study results revealed that peg tooth clearance, concave rod clearance, and concave clearance had significant difference on the shelling losses and power consumption, but not on grain breakage. Ugwu and Omoruyi [31] conducted an experiment to find out the effect of moisture content and feeding rate on the shelling efficiency. A 2 hp electric motor was used to provide the drive through belt connections to drive the pulley on the shelling chamber. The factorial experiment was conducted using three different moisture contents and feeding rates. The feeding rates were 3.75, 4.75, and 5.75 kg/s. The moisture contents were 10, 15, and 20%. The study observed that the shelling efficiency of the maize sheller was significantly and negatively affected by moisture contents of more than 15%. The results obtained also showed that shelling efficiency of the equipment was 99.01% at a moisture content of 10%.

### **3.3 Physical factors**

The important crop physical factors include the moisture content, the biometric properties such as length, width, arithmetic and mean diameter, shape, volume and surface area of the grains [32], grain cob ratio, grain bulky density, sphericity, angle of response, terminal velocity, one thousand grain mass, and porosity [2]. One thousand grain weight, density, sphericity, and surface area of different grains are required when designing different separating, handling, storing, and drying systems. Bulky density, true density, and porosity are needed when sizing grain hoppers and storage facilities [33]. They can also affect the rate of heat and mass transfer of moisture during aeration and drying processes. Density is used to separate materials with different densities or specific gravities.

The arithmetic mean diameter (*Da*) in mm and geometric mean diameter (*Dg*) in mm of the grains can be calculated using Eqs. (1) and (2) according to [32].

$$D\_a = \frac{(L+\mathcal{W}+T)}{\mathfrak{Z}} \tag{1}$$

$$D\_{\mathfrak{g}} = (L \times \mathcal{W} \times T)^{\frac{1}{5}} \tag{2}$$

where

*L*: length of the maize grain, mm *W*: width of the maize grain, mm *T*: thickness of the maize grain, mm

The sphericity (*ϕ*) is surface area of a sphere with the same volume of the maize grain can be determined using Eq. (3) according to [34].

$$\phi = \frac{(L \times W \times T)^{\frac{1}{3}}}{L} \tag{3}$$

The surface area, *S* in mm<sup>2</sup> of agricultural products generally indicates the patterns of behavior in a flowing fluid such as air, as well as the ease of separating additional materials from the product during cleaning by pneumatic means. The surface area of the grains can be calculated using Eq. (4) according to [33].

$$S = \pi \left( D\_{\mathfrak{g}} \right)^{\mathbb{Z}} \tag{4}$$

The bulk density of the main grains can be calculated using Eq. (5) according to [34].

$$
\rho\_b = \frac{4M}{\pi D^2 h} \tag{5}
$$

where

*ρb*: bulky density, gcm�<sup>3</sup>

*M*: mass of grains that fills the height of 150 cm measuring cylinder, g *D*: internal diameter of glass sampler, cm

*h*: height of the maize in the glass jar sampler, cm

The angle of response can be calculated using Eq. (6) according to [35].

$$\theta = \tan^{-1} \left( \frac{h\_0}{r} \right) \tag{6}$$

*θ*: angle of response, degrees

*h*0: height of the maize heap, m

*r*: radius of the maize heap, m

For primary processing of maize, particularly maize shelling, it is important to determine these physical properties mostly dependent on moisture content. Atere et al. [36] carried out a study on the physical properties of the maize varieties commonly grown in Nigeria. Properties determined included tri-axial dimensions (length, width, and thickness), sphericity, bulky density, true density, porosity, one thousand seed grain weight, and co-efficient of static friction. The data obtained was subjected to analysis of variance (ANOVA) and least significance difference (LSD) tests. The moisture contents of maize in this experiment were 11.35, 11.34, and 11.25%. The ANOVA results showed that maize grain properties of length, thickness, and effective diameter, bulky density, true density, porosity, and response were significantly different (*p* < 0.05) within the three varieties.

### **3.4 Engineering factors**

Engineering properties are divided into frictional and aerodynamic properties and they are used in designing equipment for solid flow, conveying systems, and separation equipment [37]. Frictional properties include the coefficient of friction and angle of response, which can be measured using the angle of response apparatus (**Figure 5**). It consists of a plywood box of 60 mm � 60 mm � 60 mm (a) and a protractor (c) for measuring the angle in degrees and provided with a fixed and adjusted plates [32]. It also has a control (b) for raising and lowering the box during *Improving Maize Shelling Operation Using Motorized Mobile Shellers… DOI: http://dx.doi.org/10.5772/intechopen.101039*

**Figure 5.** *Angle of response apparatus [32].*

measurements. The box is filled with maize and adjustable plate inclined gradually allowing the grains to slide and assume a natural slope. The static coefficient of friction of maize grains on different surfaces can then be determined by this apparatus. Aerodynamic properties include drag coefficient and terminal velocity measured using the terminal velocity apparatus [37].

Identifying the physical and engineering characteristics of grains is important when designing, improving and optimizing the separation and cleaning equipment [34]. The engineering selection and design of grains equipment requires knowledge of these grain properties because they are of great importance in the simulation and design of these equipment. Their influence is more pronounced in problems of conceptual design where a wrong estimation of a property can lead to a design plan that is not feasible. The knowledge of maize properties also gives information about the product quality, its acceptability by different groups of consumers and its behavior in post-production, during storage, and consumption.

### **4. Designing a maize sheller**

To ensure safe food, the equipment used for shelling maize should be designed, fabricated, and tested according to the required food grade design requirements. Mild steel can be used for maize sheller fabrication because it does not contaminate dried foods like maize grains. Besides, mild steel is smooth textured, mechanically stable, easily cleaned, and readily available at a relatively low cost. Bako and Batule [38] used mild steel to construct the shelling drum, spikes, conveyor, sieve, upper casing, hopper, exit cutes, and the frame of the maize sheller. Akoy and Ahmed [39] noted that mild steel can be used to achieve the equipment objective at the lowest cost possible. Designing a maize sheller requires designing the individual parts and then assembling them. These parts include main and other shafts, hopper, power transfer systems, and other parts.

The main shaft of the maize sheller can be designed using a hollow shaft because it has less weight, it is better in absorbing torsional loads and with great strength to weight ratio. Torsion theory [40] as shown by Eq. (7) can be used to calculate the minimum and maximum shaft diameters.

$$\frac{T}{J} = \frac{\pi}{R} \tag{7}$$

where

*T*: applied external torque, Nm

*J*: polar second moment of area of the shaft cross section

*τ*: shear stress at radius *R* and is the maximum value for both solid and hollow shafts

*R*: outer radius of the hollow shaft

For hollow shafts *J* is calculated using Eq. (8) [40].

$$J = \frac{\pi \left(D^4 - d^4\right)}{32} \tag{8}$$

where

*D*: outer shaft diameter

*d*: inner shaft diameter

Calculation of the Torque generated by the available power required to shell the maize can be done using Eq. (9) [27].

$$P = T\alpha \tag{9}$$

where *ω* is angular velocity in rad/s calculated from Eq. (10) [27].

$$
\rho = \frac{2\pi N}{60} \tag{10}
$$

where *N* is shelling speed in rpm

Using a diameter ratio of *d* = 0.833*D* and a maximum allowable shear stress *τ*max of 42 MNm�<sup>2</sup> for a mild steel hollow shaft, the minimum shaft diameters can be calculated [40].

The concept of calculating the volume of the frustum of the pyramid using Eq. (11) can be used to size the hopper [1]. Volume of the frustum (hopper) is the difference between big pyramid volume and the small pyramid volume.

$$V = \frac{1}{3}bh\tag{11}$$

where

*V*: volume of the pyramid, m<sup>3</sup>

*b*: base area, m<sup>2</sup>

*h*: pyramid height, m

The maximum bending moment *Mb*max can be obtained by taking moments about any point along the shaft while considering all the forces acting on the shaft and their respective distances from the chosen point [1]. A shear force diagram and bending moment diagram are then drawn from which the maximum bending moment is read.

The torsional moment *Mt* can be determined using Eq. (12) according to [24, 27].

$$M\_t = \frac{P}{2\pi N} \tag{12}$$

where *Mt*: torsional moment, Nm *Improving Maize Shelling Operation Using Motorized Mobile Shellers… DOI: http://dx.doi.org/10.5772/intechopen.101039*

*P*: power, watts

*N*: speed, rpm

The bending, load, bending stress (tension and compression) can be calculated from Eq. (13) [24].

$$\mathcal{S}\_b = \frac{M\_b R}{I} \tag{13}$$

But for hollow sections, *<sup>I</sup>* <sup>¼</sup> *<sup>π</sup> <sup>D</sup>*4�*d*<sup>4</sup> ð Þ <sup>64</sup> [40]. where *Sb*: bending stress, MNm�<sup>2</sup> *D*: outer diameter *D*: internal diameter *I*: moment of inertia The torsional stress can be determined using Eq. (14) according to [41].

$$
\pi\_{xy} = \frac{M\_t R}{J} \tag{14}
$$

where

*τxy*: torsional stress, Nm�<sup>2</sup>

*Mt*: torsional moment

*R*: outer radius of the shaft

*J*: polar moment of inertia

*d*: inner diameter of the shaft

Torsional rigidity of the shaft can be based on permissible angle of twist. The amount of twist permissible depends upon the particular application and it can vary from 0.3 m�<sup>1</sup> for machine tools shaft to 3 m�<sup>1</sup> for line shafting [41]. Torsional rigidity can be calculated from Eq. (15) according to [41].

$$\theta = \frac{\text{TL}}{\text{GJ}} \tag{15}$$

where

*θ*: angle of twist, degrees

*L*: length of the shaft, m

*G*: torsional modulus of rigidity, Nm�<sup>2</sup>

The lateral rigidity of the shaft can be based upon the permissible lateral deflection for proper operation, accurate machine tool performance, shaft alignment, and other factors. The amount of deflection can be calculated by two successive integrals shown by Eq. (16) according to [40].

$$\frac{d^2y}{dx^2} = \frac{M\_b}{EI} \tag{16}$$

where

*Mb*: bending moment, Nm�<sup>2</sup>

*E*: modulus of elasticity, Nm�<sup>2</sup>

*I*: moment of inertia, m4

The sheller main shaft speed and the engine shaft speed can be related by power transfer equation shown by Eq. (17) according to [24].

$$N\_1 D\_1 = N\_2 D\_2 \tag{17}$$

where *N*1: speed of the driver pulley, rpm *D*1: diameter of driver pulley, m *N*2: speed of the driven pulley, rpm *D*2: diameter of the driven pulley, m

### **5. Economic feasibility of maize shelling as a business**

Most fabricators, wholesalers, and retailers of maize shellers in many countries do not have definite capacity building and after-sale services to the maize sheller users [42] and no adequate instructions on equipment maintenance. Hence the entrepreneurs mostly learn on their own the operation and maintenance of their maize shellers. As a result, the economic lives of maize shellers become shorter and cause a financial loss to entrepreneurs. Thus, determining the key indicators relating to the financial feasibility of a maize shelling business is of greater importance before getting into the maize shelling business. These indicators include benefit– cost ratio and payback period [43]. The payback period is the period within which the initial investment will paid. It can be estimated using Eq. (18) according to [24].

$$P = \frac{I}{NA} \tag{18}$$

where

*P*: payback period, years *I*: investment cost, USD *NA*: net annual returns, USD

The benefit–cost ratio can be defined as the comparison of the present worth of the costs with the present worth of the benefits [42]. The benefit–cost ratio can be calculated using Eq. (19) according to [24] and it is recommended to be greater than one for the shelling business to be financially viable.

$$BC = \frac{DB}{DC} \tag{19}$$

where *BC*: benefit–cost ratio *DB*: discounted benefits *DC*: discounted costs

$$\text{Discounted Benefits} = \sum\_{t=1}^{n} \frac{B\_t}{(1+r)^t}$$

$$\text{Discountted costs} = \sum\_{t=1}^{n} \frac{C\_t}{(1+r)^t}$$

*Bt*: returns for year *t*, USD *Ct*: cost for year *t*, USD *t*: economical life, years *r*: discounted rate
