A novel manufacturing method of propeller for autonomous underwater vehicle (auv) using cold forging process

Transcription

1 Indian Journal of Geo Marine Sciences Vol. 41(3), June 2012, pp A novel manufacturing method of propeller for autonomous underwater vehicle (auv) using cold forging process Samad Z a, *A. B. Abdullah a, H. M. T. Khaleed a, M. H. Abu-Bakar a & M. R. Arshad b a School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus Nibong Tebal, Penang, MALAYSIA. b USM Robotics Research Group, School of Electrical and Electronic Engineering Universiti Sains Malaysia, Engineering Campus Nibong Tebal, Penang, MALAYSIA *[ mebaha@eng.usm.my] Received 25 March 2011; revised 20 July 2011 Present article consists the design process of modular AUV propeller using cold forging process. Modular approach emphasis on enhancing parts manufacturability and assemblability by simplifying their configuration. In addition, details steps involve in manufacturing process are also described. Optimization using FEA takes place before product is manufactured. In this paper, error assessment was performed to evaluate the accuracy of the forged part. Based on the experiment conducted, the performance of the propeller is still within the specified design requirements even though the error is quite large. [Keywords: Autonomous Underwater Vehicle, FEA, Modular approach, Cold forging] Introduction The recent demand of underwater vehicle requires smaller and energy efficient technology. Three major areas that intensively being highlighted are selection of the suitable material, design of the propeller and sources of energy 1. Marine propellers are made from metal based material such as Manganese Aluminum Bronze (MAB) and Nickel Aluminum Bronze (NAB) 2. Recently, there is a tendency in choosing composite plastic propellers due to their flexibility 3. The materials in many ways determine their suitability when matched to engines of varying horsepower. For example, composites based propellers are used on engines of less than 50 hp, while aluminum propellers are preferred to be used on engines of up to 150 hp. In other case, bronze is also used but for lower power demand. Composite propellers cannot be repaired and difficult to produce small propeller. Even though aluminum propellers are more expensive than composite, but most of the aluminum alloys are better in terms of corrosion resistance 4 and fortunately meet the required part strength 5 for this application. In addition, aluminum blades can also be repaired. The most versatile material is stainless steel, but it is quite expensive. In addition, propellers made from stainless steel are stronger and allow for any modification and repair 6. Conventionally, the propellers can be manufactured by casting 6,7 or machining 8. The disadvantages of casted propeller are on part strength and surface finish and usually it requires machining to get the desired surface quality. The strength of the propellers which are manufactured by casting is 17% lesser compared to long glass fibre reinforced polyamide thermoplastic 9. In the cast products, the flaws and thermal problems have to be solved and grain structure cannot be controlled 10. Compared to cold forging, the production time of casting process is significantly higher. The collision between tool and work-piece, and material wastage and tool vibration is more 11 compared to forging process. Similar to the casted propeller, the grain structure also cannot be controlled through machining. Materials and Methods There are three types of marine propellers, i.e. controllable pitch propeller, Skewback propeller and modular propeller. A modular propeller provides more control over the boats performance. In order to increase the speed, there is no need to change an entire propeller, as the pitch can be adjusted in the case of repairing the damaged blades. Normally the propeller is completely assembled by sliding/attaching the base of the blades into the corresponding slots of

2 SAMAD et al.: A NOVEL MANUFACTURING METHOD OF PROPELLER 243 the center hub, and then placing the rear hub before fastening. Since the parts are easily can be detached, it can be carried in less space. To date, there are two modular propellers available in the market. The characteristics are as listed in the Table 1. In general, both have complex design and consist of many components, which may affect the assembly process. Furthermore, the sizes of both propellers are only suitable for a boat. Basically there are two most main methods of propeller design i.e. vortex line method and 3-D panel method 12 and few other methods were then developed for example streamline-adapted method 13 to make the design process easier. Optimizations of the design variables such as twist angle and chord length get a great attention by many researchers recently. The purposes of this paper are to present the design and development of the AUV propeller using modular approach. The paper is arranged as follow, begins with introduction and next the design requirement is outlined and then the details steps involve in manufacturing process is discussed. After that the quality assessment is presented and the paper ends with conclusion. Design Variables There are many methods for propeller design. Here, propeller vortex lifting line (PVL) code was employed. First of all, the specifications of the AUV were set. Aspect of propeller design including the thrust, speed, no. of propeller required, propeller location and depth of the vehicle were taken into consideration. In addition, the designs also consider the effect of blade number, rate of propeller rotation, chord length distribution, propeller diameter and hub size. As a result, the profile of the propeller was obtained. Kerwin 16 simplified all these parameters in terms of advance ratio, thrust coefficient, and power coefficient. The efficiency depends on thrust, consumed power and velocity. All these parameters were summarized in the Table 2. Where v = Velocity, m/s D = Diameter, mm N = Rotations per second, 1/s ρ = Density of air, kg/m³ P= Power, W T = Thrust, N Characteristics Table 1 The modular propeller designs available in the market Name Piranha Propellers 14 ProPulse Propellers 15 Advantages 1. Replaceable blades 2. In-expensive to manufacture 3. Stronger than prior art units 4. Better performance 5. Half cost of metal propellers Material Aluminum core with fiber-reinforced composite polymer resin Diameter cm cm Method of blade Plug-in Snap-fit attachment Number of Blade 3 or 4 blades 4 blades Blade orientation Fixed angle Varied angle Motor power hp hp 1. Easy to dismantle 2. Replaceable blade 3. Adjustable pitch 4. 40% lighter than aluminum 5. Corrosion free Metal hub with composite blade

3 244 INDIAN J. MAR. SCI., VOL. 41, NO. 3, JUNE 2012 Table 2 List of design variables in quantifying the propeller performance 16 ρ= Design Parameter Nomenclatures Formula Thrust C T CT T n 2 D 4 Power C P P C P = ρ n 3 D 5 Advance Ratio ν / nd ν ν/ nd = n D Efficiency η η = ν n D C C T P Figure 2 Orientation of the blade and the twist angle definition Where R = Total radius r = Local radius α = Angle of attack = 1.54 o (Ideal), refer to Figure 2 k = Constant (k>0) After that, the hydrodynamic design of propeller blade has been optimized to achieve the required thrust before proceed to the manufacturing stage. Figure 1 Illustration of the propeller configuration in determining the twist angle Figure 1 defines some terms that are used to describe the propeller s performance. The total radius, R of the propeller is the distance from the center to the tip. The chord length, c is the straight-line width of the propeller at a given distance along the radius. Depending on the design of the propeller, the chord length may be constant along the entire radius, or it may vary along the radius of the propeller. Another variable is the twist angle, β of the propeller, which may also vary along the radius of the propeller as shown in Figure 2 and the value of the twist angle at local radius, r can be determined by 17 ; Rα β = t α t k 1 r r R Manufacturing Stage Manufacturing process is one of the most challenging stage and it become more challenging to get the optimum end product without defect at minimal wastage. This can be done by simulating the process involve in the manufacturing and do all the changes and modifications before manufacturing take place. Therefore in this research CAD/CAE/CAM tools were employed. The process of the design and fabrication of the propeller will be further discussed in details as the following sub-sections; CAD Model The profile of the propeller obtained from programming code namely PVL 18 has been converted to CAD model. It can be achieved by setting the required design parameters such as trust and power, the efficiency of initial profile of the propeller as discussed in the previous section. In addition, the model also can be utilized to determine the performance analytically in the CFD 19. The CAD model constructed in the SolidWork is as shown in Figure 3. FE Modeling The numerical simulation performed was more focus on the blade, since the shape is more complex

4 SAMAD et al.: A NOVEL MANUFACTURING METHOD OF PROPELLER 245 and the performance of the propeller is much depends on blade rather than the hub. Initially the simulation is performed at 2D view. Material properties of the workpiece and the die are as listed in Table 3. The simulations are running simultaneously and from the result, the tooling will be designed. Khaleed et al., has successfully simulate the 3D view of forging process Figure 3 CAD model of the propeller using DEFORM 18. The velocity of the punch is set at 250 mm/s, the friction coefficient is assumed to be 0.15, and the initial temperature of the workpiece, punch and die is at 25 o C. Optimal Process Steps and Sequence At this stage, numerical simulation is performed to get the optimal process. Here several parameters are taken into consideration including the maximum capacity of the press machine i.e. maximum load, complexity of the tooling set and machine-ability of the punch and insert. From the simulation, the optimal process sequence involve five stages, i.e. punching to get the preform, shaping, trimming, pin heading and finally twisting as shown in Figure 4 (a) to (e) respectively. For the hub, the process involves two steps, i.e. preparation of the billet and end by forming process. Since it is quite direct, therefore it is not highlighted in the discussion. Fabrication Finally, the propeller is fabricated using 100 tone mechanical press machine as shown in the Figure 5 and the final assembly of the propeller is as shown in the Figure 6. Table 3 Material properties of workpiece die, punch, and others Parameter Workpiece Die Material type AISI 6061 AISI D2 Modulus Young (GPa) Yield Strength (MPa) Poisson ratio Hardness HRC-24 HRC-62 Figure 4 The optimal forging steps involve in manufacturing of the blade

5 246 INDIAN J. MAR. SCI., VOL. 41, NO. 3, JUNE 2012 Figure 5 C-type mechanical press machine used in the forging of the propeller Results and Discussion Here two aspects will be discussed, the advantages gain from the modularization and error assessment conducted on the forged propeller blade. Figure 6 Final assembly of the propeller Table 4 Comparison between new design and current propeller design Current Design New Design Factors Piranha Propulse Number of parts Size (radius) 40-45cm 26-37cm 4.85cm Optimization In the optimization stage, two aims i.e. to minimize the flash and at the same to avoid underfill 20. The flash-less forging leads to reduction in load required to forge, wastage of material, time and number of operations for the complex component like AUV propeller. Similarly for the hub as discussed in 21. Advantages of Modular Approach Advantages of the modular propeller compared to current available modular propeller in the market can be highlighted in two factors i.e. number of the components and size of the propeller. Compared to current design of propeller, the size of the new propeller is much smaller i.e. approximately 4.85 cm radius compared to 42.5 cm to 31.5 cm average radius for current design. In terms of number of components as well, the new modular propeller has only 3 components compared than 6 and 5 respectively for the available modular propeller. The result can be summarized in the Table 4. Form Error Evaluation Since the profile of the blade is complex and geometries are varies in terms of part thickness and twist angle. Therefore the blade is divided into five sections. Here for the measurement, the twist angle is defined as the mid-line of the cross-section of the blade perpendicular to the normal. Similar analysis has been performed to determine the thickness and twist angle error measured on the forged blade. The measurements are made based on the CAD model and the result will be compared to measurement

6 SAMAD et al.: A NOVEL MANUFACTURING METHOD OF PROPELLER 247 Figure 7 Two methods of profile construction, (a) CMM and (b) Alicona 3D-imaging system Table 5 Blade thickness profile deviation Section Measured, mm CAD Model, mm % of Error Table 6 Twist angle error Section CMM, degree CAD Model, degree % of Error made using CMM and Alicona imaging system as shown in Figure 7. The results are as demonstrated in the Table 5. Similar pattern found for the twist angle as summarized in Table 6. The data obtained from CMM measurement will be mapped and the twist angle will be determined. The deviation between the twist angle obtained from CMM and the CAD model is then determined by referring the definition given by 22. For further exploration, the AUV propeller was successfully modeled using CFD 23. From the result the different of the thrust between simulation and experiment was approximately 5.41%. Based on these results as well, the forged propeller is still performed within the required specification including the thrust. Conclusion The objective of the paper is to present the design and manufacturing process of AUV propeller. In the design stage, a computer code namely PVL has been employed and based on the specific requirements set within the code, a new design of propeller had been proposed. Since the profile of the propeller is very complex, a modular approach had been introduced to the propeller design in order to simplify the product configuration tend to make the manufacturing process easier. The propeller has been successfully fabricated using cold forging process. Modularity can simplified the product configuration as the number of component reduced. In terms of size, the new design is much smaller than the competitors. For further evaluation, error has been quantified using non-contact 3D measurement equipment namely Alicona. Even though the error is large, but the performance of the AUV seems to be within the specified thrust. At this stage, it is clearly shown that, the new design is better than the current design while the performance of the propeller in the water is still ongoing. Acknowledgement Authors would like to acknowledge the Ministry of Higher Education and Ministry of Science, Technology and Innovation for their sponsorship in this project through Science fund grant scheme. References 1 Chyba, M., Haberkorn, T., Singh, S.B., Smith, R.N. and Choi, S.K. Increasing underwater vehicle autonomy by reducing energy consumption, Ocean Engineering, 36(1) (2009) Motley, M.R., Liu, Z. and Young, Y.L. Utilizing fluid structure interactions to improve energy efficiency of composite marine propellers in spatially varying wake, Composite Structures, 90(3) (2009)

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