DECISION MAKING OF HAND TRACTOR GEAR BOX DESIGNS

Decision making on selection of hand tractor Gear Transmission Box (GTB) “Improvement Designs” is carried out according to many criteria. Based on the focus group and brainstorm performed by product-development teams, seven criteria are finally identified as follows; Material corrosion resistance, Manufacturability, The ability of the design to withstand the maximum load operation, influence on the other components process, manufacturing cost, mass of GTB design, and processing time. Those criteria are categorized into positive and negative characteristics. Positive criteria indicate that score which is the greater the better, by contrast, negative is the less the better. There are 3 alternatives namely Initial Design, Design 1 and Design 2. The selection is performed based on Analytic Hierarchy Process (AHP) Method. The evaluation is analyzed separately according to each category. The best alternative is the one which has the highest ratio between positive and negative criteria.


INTRODUCTION
In a product/component development project, the various designs can be produced.Since the difficulty of evaluation criteria determination and the complexity of product coverage exist, the assessment and consideration of decision making tend to be bias and subjective.These reasons trigger decision maker assess and consider the choice intuitively (Luo et al., 2008).On the other hand, failure on fabrication frequently occurs because of the deficient in deciding the best of them (Atuahene- Gima and Li, 2004).
Design selection should be performed carefully, comprehensively and inwrought in order to minimize risk of development failure.
Many decision making tools/ models have been applied for the selection of a suitable design.
Based on the literature review established by Seram (2013), decision making tools which are commonly used can be mentioned as follows: Technique For Order Performance by Similarity to Ideal Solution (TOPSIS), AHP and TOPSIS, Decision Matrix (DM) and Robust design (RD), value analysis/ engineering (VA/VE), design for X (DFX), axiomatic design (AD) by Coelho and Mourao (2007) which shows how AD allows for perceiving the relationships between each product and the related manufacturing processes, Cariaga et al. (2007) Integrating QFD, Value analysis (VA) and data envelopment analysis (DEA) used on a high-tech research facility construction project, and Barajas and Agard (2008) applied Fuzzy Decision-Making for product selection.
In mechanical components design and development, Technical aspects and capabilitiesprocess become important points for the selection consideration (Rosen et al., 2012;Boothroyd and Dewhurst, 2002).There are technical aspects which are able to be quantified with engineering consideration, but there are other aspects can be appeared which are qualitative and intangible such how to determine the importance of the selection criteria.Both aspects must be evaluated.
In view of this purpose, The Analytic Hierarchy Process (AHP) does seem useful (Battistoni et al., 2013).AHP is a theory of decision making which uses pairwise comparison matrix and relies on the judgments of experts in order to derive priority scales.It is these scales that measure intangibles in relative terms.The comparison matrices are constructed by using a scale of absolute judgment that represents how big one element dominates other with respect to a given attribute (Saaty, 2008).This paper deals with the implementation of AHP in order to find an appropriate alternative related to design development of mechanical component; a pair of Gear Transmission Box (GTB).This research provides evidence that multi criteria analysis is very useful in making a decision for Product Development Engineer in the preeminent design selection.

Analytic Hierarchy Process (AHP)
AHP was introduced by Thomas L. Saaty and designed to solve complex problem entangling many criteria.The Analytic Hierarchy Process (AHP) is a structured technique for dealing with complex decisions.Based on mathematics and human psychology, The AHP provides a comprehensive and rational framework for structuring a problem, for representing and quantifying its elements, for relating those elements to overall goals, and for evaluating alternative solutions.Qualitative attributes are changed into quantitative in a pair-wise comparison set as presented in Table 1 (Saaty, 2008).
Since using human perception, AHP model can incorporate data both qualitative and quantitative.So complexity of problems can be tackled well by using AHP model.In addition, Ratio is greater than 10%, we need to revise the subjective judgment (Saaty, 1990).CR is calculated by dividing Consistency Index (CI) with Consistency Random Index (RI) (see Table 2).Consistency Index (CI) is calculated based on the maximum Eigen value.All the equation as shown below; Fed T  Source: Saaty, 1990 comparison of element l to element i, p i is priority or weight of element i  According to Norton (2014) And Forces (T j ) caused by couple Fe, given b Where e is horizontal distance to rivet systems center of gravity in meter, F is an eccentric force that causes load in rivets systems in Newton and d k is variable that show distance each rivet to center of gravity of rivet systems in meter.

Loads in Bearing because rotation of gear and shaft
Load in bearing that caused by power transmitted consist of two forces.Tangential force (F t ) and radial force (F r ) (Norton, 2014).
Tangential direction is the axis heading to the center of bearing whereas radial force direction is opposite with radial movement.Both are given by F r = F t tan Ф (7) And Pitch velocity (V p ) is given by  Where Ф is pressure angle of gear in degree, hp is power transmitted by gear in horse power, torsion (T) in inches-pound, angular velocity n in rpm, gear diameter d in inches, V p in ft/min, F t and F r in lbf.

Processing Time (machining time)
Design is fabricated through two machining processes which are fraise and drilling.Singal et al. (2008) explained that cutting time (t c , min) for fraise depends on length of cutting track (l t , mm) and cutting speed (v f , mm/min) that are given by, Where l v is starting track in mm and l n is finishing track in mm.

Cutting Metal Plate and Forming Force
The piercing process is cutting process where resulted part in the inside of cutting line.
Very important factor this process is shearing resistance (Ks) got from maximum amputation style (Fs).Shearing Resistance (Ks) is factor influenced by factor clearance between punch and dies, tool worn-down, material characteristic, thickness of sheet and form of the blank.Ks must be bigger than maximum tension shift so that the material can be cut.Maximum tension shift of material is equal to 0,8σ u (ultimate tensile    By using equation ( 5), ( 6), ( 7), ( 8), ( 9) and ( 10), maximum load can be calculated.It is 57.06 kN in GTB RH.

Initial Design
As shown in Figure 4, Initial Design consists of two parts; GTB RH and GTB LH.Initially,   3).
Overhead rate is estimated as 5% of material cost and 10% of machining, labour and assembly cost.
Total manufacturing cost for initial design is Rp 429,671.

Design 1
Design 1 uses ST 37 material.Main body consists of a part.Thickness of main body is 3.2 mm.there is supporting plate 8 mm thickness in every side of GTB.This design consists of 11 parts in a side.(Shown in Figure 5).GTB are produced by combination of metal forming, welding and machining all in its own shop.There are 5 metal forming operations, 16 machining and 2 assembly operations in GTB LH.
In other sides there are 4 metal forming operations,     7.

Decision Analysis
The best design selection is conducted by using In this paper, the criteria are distinguished into two types; positive criterion and negative criterion.
Positive criterion means a criterion which is the greater the better.The criteria involved in this type are criteria 1,2,3,4, and 5 which are shown in figure 7.

Weight of criteria:
The following table shows pair-wise comparison matrix among five criteria.By equation ( 1), ( 2) and (3), CR/Consistency Ratio can be calculated.
By equation 1, eigen value can be calculated;  Because there are five attributes, so n value is equal to 5 and Random consistency index (RI) = 1.12 (Table 2).By using equation 2 and 3, consistency index and consistency ratio can be calculated.CI = 0.0542 CR = 0.0540 = 0.0482 1.12Because CR < 10%, the inconsistency is acceptable

Score of Alternatives under criterion 1
Hand tractor product is operated normally in the mud wallow.This condition makes team chooses material which has good corrosive resistance in water or mood.If it continually incurs water, corrosion in graphitic stage of FC 25 will occur.Then ST 37,compound of Fe-C-S-P-Al, is a good corrosive resistance if rate of S and P is small (Bodude et al., 2012).So, assessment of each alternative can be performed as follows.

Score of Alternatives under sub criterion 2a
In the Following

Score of Alternatives under sub criterion 2b
In the Following Table, pairwise comparison matrix for weight of alternatives with respect to sub-criterion 2b (Manufacturability-Machiningability) are shown.CR = 1.3% (since CR is smaller than 10%, so inconsistency is acceptable)

Score of Alternatives in sub criterion 2c
In the Following

Score of Alternatives under criterion 3
In the Following

Score of Alternatives under criterion 4
In the Following

Weight of Criteria
The following table show pair-wise comparison matrix among three criteria.CR = 0,6% (since CR is smaller than 10%, so inconsistency is acceptable)

Score of Alternatives under Manufacturing Cost Criterion
In the Following

Score of Alternatives under Mass of GTB Criterion
In the Following Table, pairwise comparison matrix for weight of alternatives with respect to mass of GTB criterion is shown.As well as the manufacturing cost criterion, the alternatives' score are obtained by calculating the each percentage of the total.

Score of Alternatives under Processing Time criterion
In the Following

Score of Alternativesunder sub-criterion 5a
In the Following

Score of Alternatives under sub-criterion5b
In the Following  The greatest ratio is Design 1's.

Figure 2 .
Figure 1.Manufacturing cost elements Source: Ulrich and Eppinger, 2000 the element of each category can be seen in Figure 1.Component costs include standard component cost and custom component cost.Standard components are the items which have been standardized in terms of shape, measure or materials such as shovel, bolt, and pipe.Whereas custom components are the items which are made based on the design regardless of standardization.Assembly costs include workers' wage cost and equipment cost.Overhead cost is a category which represents another cost besides Component and Assembly Cost.It involves two elements: support and indirect allocation.(Ulrich and Eppinger, 2000) Eccentrical Loaded bolt/rivet GTB needs many mechanical analyses for the best performance of the design.Load analysis is used to find-out how many and how big loadforce must be shored up by GTB.It is useful in measuring design strength.
fraise l n = d/2; for vertical fraise Similar with fraise process, cutting time of drilling and boring process are given by feeding speed (f z , mm/rotation) for each tool f z = v f / (n.z) ; z = 2 (13) Cutting time (t c , min): t c = l t / v f (14) l t = l v + l w + l n (15) Total machining time can be derived by summing nonproductive time (t a ) , tool changing time (t d ) and cutting time (t c ).t o = t a + t d + t c (16)

Figure 3 .
Figure 3. Seven main components of hand tractor

Figure 8 .
Figure 8. Priority Hierarchy of negative criteria Based on the seven criteria and five sub-criteria (Material corrosion resistance, Manufacturability [Availability of Facility, Machining-ability, Assembling-ability], The ability of the design to withstand the maximum load operation, Material availability in market, Influences to other component process [Others hand tractor component, Others product component], Manufacturing cost, Mass of GTB, and Processing time), Design 1 is admitted as the best design among three (Initial Design, Design 1 and Design 2).

Table 1 .
(Saaty, 2008) Definition pairwise comparison set(Saaty, 2008) Fudhla: Decision Making of Hamd Tractor Gear Box Designs103AHP has capability to solve multi-objective and multi criteria problems based on the preference comparison of each element in the hierarchy.In AHP, Consistency test and sensitivity analysis are emerged to perceive the reliable judgment because as we know that the judgment relies on the expert's opinion which is possibly inconsistent at any time.Consistency test is addressed for priority hierarchy that was built.Generally, consistency test is performed by using Consistency Ratio (CR).If the value of Consistency Ratio is smaller or equal to 10%, the inconsistency is acceptable.If the Consistency

Table 3 .
Manufacturing Cost Initial Design circumference of cutting line in mm, So is thickness of material in mm, and Ks is shearing resistance equal to 0,8 σ u in kg /mm 2 A Simple equation is used to determine forming force of bending;

Table 4 .
Processing step of Design 1

Table 6 .
Processing step of Design 2

Table 5 .
Design 1 manufacturing cost recap By using equation 4, the manufacturing cost is calculated based on component costs, assembly costs and overhead costs.Component cost consists of material cost, machining cost and machining labour cost.(Shown in Table

Table 8 .
Weight of five decision criteria

Table 9 .
Weight of sub-criteria 2

Table 12 .
Alternatives Score under sub-criteria 2a

Table 15 .
Alternatives Design Score under criterion 3

Table 16 .
Alternatives Design Score under criterion 4

Table 17 .
Alternatives Design Score in sub-criterion 5a

Table 19 .
Total score recapitulation of positive criteria

Table 20 .
Weight of negative criteria Table, pairwise comparison matrix for weight of alternatives with respect to in Processing Time Criteria is shown.The scores are percentage of each processing time.

Table 22 .
Table, pairwise comparison matrix for weight of alternatives with respect to Alternatives score in Mass of GTB

Table 23 .
Score of Alternatives in Processing Time criterion

Table 21 .
Score of alternatives in Manufacturing Cost Criterion Fudhla: Decision Making of Hamd Tractor Gear Box Designs115

Table 24 .
Total score recapitulation of Negative criteria Because of the characteristics of both are different, then the selection is conducted by calculating ratio.The best alternative is the one who has the highest ratio between positive and negative criteria.