Design and simulation analysis of cowpea dehulling Machine

The dehulling of cowpea seeds will have a great influence in increasing the overall production, consumption, processing into more dietary, hygienic products and the advancement of cowpea processing industry. The technology of cowpea processing industry is still at its lowest ebb. Therefore, designing a machine to dehull cowpea beans would be of greater benefit to the common man protein intake and the afore-mentioned development of the cowpea industry. The machine is designed to be mechanically operated by an electric motor as the prime mover with dehulling capacity of 6.67x10-4m3/s or 37.2kg/min. The beans will be fed by rotating force received from the feed wheel in the hopper into the plate's gap interface that will be pre-set that the total volume of feeder is approximately the volume of the space between plate surfaces. The rotating action of moving against stationary plate effect the rasping of soaked cowpea off their cotyledons. The design consideration applied appropriate technology, ease of dehulling soaked cowpea and the attrition action of plate surfaces and readily available engineering materials with possible equipment in the country. The dehulled efficiency was 79% at 81.25% machine efficiency shown from the simulation analysis, this greatly show a high level improvement when compared with other related designs.


Introduction
Cowpea (Vigna unguiculata) is one of the common food source in rural and urban homes in Nigeria, as it is cheap and it has significant constituent of protein, minerals and vitamins [1].This crop is mainly cultivated in west and Central Africa with an annual production of 3 millions tons [2].In Nigeria, cowpea are eaten in various forms; as porridge along with fried or boiled yam or plantain, Dehulled cowpea is necessary in the preparation of some Africa food process (fried beans,akara, boiled moi moi).In spite of the widespread use of this staple food among Nigerians [3],dehulling of cowpea has been identified as the major problem culminating against the utilization and processing of cowpea into a more versatile food product that eliminates flatulence and repulsive odour effects. The existing method such as the traditional method of soaking in water and hand rasping to remove the seed coats is time consuming, tedious and discourages large scale processing even though it gives better efficiency. Also, the machine method employing either the wet or dry milling, peeling or polishing machines has a lot of setbacks in terms of tedious labour during pre-treatment, time wastage, complexity of machines, cost of operation and acquisition and their inefficiency. The developments of agriculture in developing countries of the world depend on the part played by modern agricultural machinery and equipment. Many local cowpea dehulling machine have been developed but reliable information and performance data on them are limited.
There is need therefore to design, fabricate and evaluate the performance of a local cowpea dehuller that provide lower cost and yet achieving good quality products [4]. In the case of burred plates grinding machine, the dry seeds are poured into the hopper of the machine with the plate surface fairly adjusted to crack the seeds [5], thereafter pour water to float the husks. It is still less efficient as rice polisher. The application of these existing machines still makes cowpea dehulling inefficient and an uphill task in terms of labour, time consumption, product result, cost of machines and operation [6].
The objectives of this study is to explores the needs arising in applying appropriate technology in designing a simple cowpea dehulling machine which are:  Cost effective, qualitative, easily affordable  Will aid higher production capacity and reduce time wastage to manual effort.  Can be used in urban and rural areas with important contribution to the development of small-scale agricultural food processing industry 2. Methodology

Machine Description
The cowpea dehulling machine simply will consist of various components that are designed to perform specific tasks. This includes two circular surface plates, one attached to a rotating shaft supported by two roller bearings and the other to a spring-loaded adjustable screw stationary shaft. The Material on plate surfaces will be made up of Butyl rubber, and the entire assembly is to be enclosed in a cylindrical casing mounted on a rigid frame that also accommodate the prime mover (electric motor).The power from the electric motor is transmitted to the rotating shaft of the dehulling machine by means of V-belt and pulleys. The cowpea would be poured through vertically mounted hopper on top of the casing which is fed at an angle through off centered hole on the adjustable stationary (non-rotating) plate. To satisfy efficient feeding of cowpea to the action zone, an automatic feeding device (feed wheel would be incorporated in the hopper in order to compensate for negligible gravitational effect and supplied constant feed rate. Basically, the seeds are fed by rotating force received from feed wheel in the hopper into the plate's gap interface that is pre-set to accommodate average size of cowpeas across width. Then the rotating action of moving plate with the stationary plate effect the rasping of coats of soaked cowpea off their cotyledons before they finally dropped with aid of centrifugal force or reduction in sizes into the delivery end.

Research Methodology
The lean availability of data from existing literatures necessitated the carrying out of series of experiments in order to obtain data for the mathematical basis of the design. Cowpea samples used was purchased from Yelwa Tudu market, Bauchi State as dry seeds with about 15% moisture content. First Point considered was the determination sizes of cowpea; this was done by random sampling measurements, both for dry and soaked seeds. Ten samples were measured, each across the length and width using venier caliper. Then a mass of 100 grams dry seeds was weighed in the laboratory and the volume noted, it was soaked in excess tap water for 30minutes with observation within 10 minutes interval, after which water was drained off and the weight and volume of wet seeds noted.
The force suitable to rasp the soaked beans was experimentally determined by application of weights and sliding plates mechanism with rubber sheets bonded to their surface as shown in fig 1.

Figure 1: Sliding Plate Mechanism
The maximum crushing force under static and dynamic load was also determined. In order to ascertain or compare the value of coefficient of friction of soaked cowpea with butyl rubber and that obtained from sliding plates mechanism, a calibrated incline plane was used as illustrated in fig:2.

Force sufficient to dehusk soaked cowpea (82 seeds per experiment)
Taking the weight of sliding plate into consideration (determined tobe 1.75 N), the optimal result was obtained from average of the reading as = 12.0 + 12.5 + 13.0 3 = 12.5 With corresponding load taking weight of hanger as 0.25N, optimal result from average readings gotten was  Comparing with that obtained between cowpea with rubber surfaces (Taking average readings for computation when using incline plane). This shows proximity in value to that obtained from sliding plate mechanism.
The calculation from data shows a reduction in bulk density of soaked cowpea due to higher rate of increase in volume than in weight due to water absorbed. Experimental observation portrayed that 15.8N force tends to crush the soaked cowpea instead of smooth Dehulling under dynamic loading on the other, the static loading effect would show no effect of breaking or crushing the soaked cowpea except under impact. Thus more than 50N forces were required to crush seeds statically when applied on single seed.
Random sampling measurement of cowpea sizes illustrates a varying size of 6-11mm across length, 3-7mm across width for dry seeds, while approximately 9-11mm across length and 5-7mm across width for soaked seeds. In comparison with that obtained from literature (3-7mm across width and 9-12mm across length for soaked beans) [7]. Also the coefficient of friction was seen to be higher for soaked cowpea than dry because the husk tends to stick to the rubber surface due to the seed being laden hence required higher angle of inclination.

The Mathematical Basis for the Design
Engineering design is a mathematical model which shows exactly the behaviour of the engineering system in form of equations that correlates different parameters to each other in a system. Thus, design is the concept of impression to bring into being a system, determining the elements and their pertaining parameters, shape, choice of materials and condition of operation [8].
It embraces also cost analysis and manufacturing method for construction and fabrication of the system   It is also assumed that the distributions of cowpea on sliding plate on machine are equal, so number of seed on circular plate will be; No. of seeds on sliding plate -area of sliding plate The permissible force on sliding plate that dehulled cowpea efficiently was 12.50N Therefore, the permissible force on circular plates in machine will be:

Figure 4 schematic view of feed wheel
The wheel has 5 sections with length 47mm and diameter 50mm  The value is approximately the volume of the space between plate surfaces to accommodate cowpea; Therefore, each feeder section will contain: The values determined for both power and number of motor revolutions will be the basis for choosing electric motor as a prime mover.

Determination of feed rate
Volume seed rate(Q) per minute will be = × The hopper has the shape of a frustum of a cone that is inverted, the volume will be: Therefore, time (t) taken for its content to finish if the stimulated time to finish dehulling after soaking of cowpea is 20 minutes for greater efficiently, which is suitable for optimal operating condition. The FEA simulation report shows the contact pressure experienced by the cowpea due to the effect of the rotating plate and he fixed plate. Using the calculated torque to drive the plate (T = 5.36Nm) and the coefficient of friction between the cowpea and plates as 0.5. All material remain properties remain as specified in the material selections.    Hence, the obtained force from the Contact pressure simulation is acceptable for safe operation of the Dehulling Machine.

Conclusion and recommendation
The result showed that the machine have dehulling efficiency of 79% with a corresponding machine efficiency 81.25%. This tends to show that if the machine is be commercialized, cowpea utilization will be increased.
The cowpea dehulling machine will be effective in rural and urban area, especially in the small-scale cowpea processing industry due to its low, simple, profitable and efficient design.
The machine was designed considering the conditions and Size of soaked cowpea as design parameters thus a high level of performance will be obtained, if the machine is use to dehull cowpea according to the design specifications. The machine is restricted to dehulling of only wet seeds.