Optimum Material and Process Modification to Reduce Lead Time of Pedestal Manufacturing Used in Gearbox Assembly

Optimum Material and Process Modification to Reduce Lead Time of Pedestal Manufacturing Used in Gearbox Assembly

Dr. Prashanth Thankachan [1] Mr. Prabhakar Purushothaman[2] [1] Chief Technology officer – R & D [2] Design Engineer- FEA UCAM PVT LTD, Bangalore

Abstract – In recent years computer aided engineering techniques has developed to great extent, the choosing right materials and processes are great challenge for the engineers. This paper discusses about material and process selection based on computer aided technique. The output from all CAE tools is based on the accuracy of the provided input, therefore the requirement for using this tool is to understand the behaviour of the product and to specify the appropriate input. The CAE Techniques provides better understanding of the processes and material selection and helps to modify the products as it is essential to compitate in todays global market. The one such software is CES, (Cambridge Engineering Selector) which is used to modify the material selection and manufacturing processes used for the pedestal of the gearbox without affecting the functional requirements. This paper illustrates various consideration made for modification and the material and process of pedestal. The performance of the existing material and modified material is evaluated using finite element analysis and comparison of result is shown.

Keywords: Material selection Parameters, Material selection for Gearbox, Material selection using CES software & Finite Element Analysis.

most often existing pedestal designs are manufactured with six parts welded together, which involves cutting to required shape and machining to the dimensions, welding, stress reliving, inspection etc shown in Figure 1. The lead time for processing of pedestal is high, therefore in order to reduce the lead time, alternate material identification and process optimization is discussed in this paper.

1. INTRODUCTION

   Material science and manufacturing technology has witnessed large scale development in recent years, providing the design engineer with large choice of novel materials and processes to engineer from. This also calls for informed decisions in the choice of materials for engineering application design failing which can lead to flaws in modulus, strength, toughness, cost etc. In architectural sciences, Pedestals have been used since the time of the Romans for architectural work in temples, followed by Italians and Chinese to support the statues and position the statue at particular heights.[1] In engineering sciences Pedestals have been designed to form the supporting elements of gearboxes, motor and as a means to hold measuring tools within a framework of machine design. In later stage the word is commonly used for all supporting element, as in gear box assembly, the gearbox is supported and raised by this element therefore this is named as pedestal.[2] The pedestals are also used in rolling machines for fixing the machine. [3] This often calls for the structure of the pedestal to be rigid, however,

   Figure 1 Pedestal with gearbox assembly

   The various considerations have to be made while selecting the raw material and processes shown in (Figure 2). To select a material or process the following constrains also needs to be satisfied, only the major criteria are criteria is listed apart from this based on application of the product some other criteria may be must for material and process selection. This can be arrived only by systematic study to understand the behaviour of the product. The very precise input is required to apply constrains in CES software in order to select very precise materials

   proportional to strain (x) the constant proportionality (E) is called Youngs modulus.

   Figure 2 Material and processSelection Considerations

2. ANALYSIS OF ALTERNATIVE MATERIALS AND PROCESSES FOR PEDESTAL MANUFACTURING [4]

   Materials selection charts are a graphical way of presenting material property data in a systematic arrangement. Most mechanical characteristics extend over several orders of magnitude, so logarithmic scales are used to array the materials as per the properties from low to high.

   In order to select the suitable material the performance index is derived for the pedestal component. The performance index is the function of following parameters, functional requirements, geometry parameters and material properties [4]

   P= f {F, G, M}

   Were, P = Performances (mass, volume, cost, etc.) F = Functional requirements

   G = Geometric parameters M = Material properties

   The objective arrived for pedestal is

   m = AL

   Were, m= mass, A= Area, l = Length and = Density. The defined objective are mass depends on volume and density, which has to be less and the constrains focused are,

   F/A < y.

   Were, F= force, A= Area and y = yield strength of material. This defines that the stress levels should be always below the yield strength of the material, this constrain ensures the material is in elastic limits and only elastic deformation takes place for the applied load. The equation is re-arranged to eliminate the free elements as

   m > (F) (L) (/ y)

   The weight can be minimised or increased using the variables density and yield strength. In order to arrive at the materials with light weight yet with high stiffness the performance indices are arrived to materials as

   Were, E = Youngs modus

   = Density of material

   In log space: log E = 2 (log + log M) this is a set of lines with slope=2

   Youngs modulus is the measure of stiffness in a material. As per Hooks law within the elastic limit stress (x) is directly

   x = E x

   Michael Ashby of Cambridge University has developed the material plot graph, displaying two or more properties of material together that enables the user to select the appropriate material. The CES software functions using the Ashbeys plot. The CES [5] software has the material database in which level-

   2 with eco and durable property database is selected for analysis having 98 materials under different families.

   In order to select the material with qualities of low weight and high stiffness using CES software the material of all the families are plotted in graph stage for density to Youngs modulus and the slope will be 2 has to be generated as shown in Figure 3. The material falls below the line of the slope are suitable materials and can be taken forward for further refinement.

   Figure 3 Youngs modulus Vs Density graph

   The various family of materials carried forward for refinement are Foams, Natural materials, composites, polymers, metals and alloys. The suitable constrains are applied by considering the functional requirements. By adding constrains in the graph stage such that the material should have minimum Youngs modulus value of 100GPa. The materials are refined and were end up with 3 family of materials with 17 material candidates as shown in the Figure 4.

   Figure 4 Youngs modulus vs density graph with 100GPa constrain.

   For the further refining, constrain given in CES software in limit stage. The limit stage helps to set the upper and lower limit in order set constrain. In this stage for Youngs modulus the limit of 180GPa to 230GPa is set in order to further refinement.

   Application of this constrain to the list of materials results in filtering of materials such as ductile cast iron, low alloy steel, low carbon steel, medium carbon steel, nickel, nickel based super alloy, nickel chromium alloy, stainless steel and zirconium. For the further refnement of material the graph plotted for the castability and the yield strength as shown below in the Figure 5. The result shows the ductile cast iron has higher castability and the range of the yield strength is also similar but slightly less than the low carbon steel.

   Figure 5 Castability Vs Youngs modulus graph

   As the pedestal manufacturing process involves various machining processes such as drilling for clamping holes and jig boring to fit bearing for output shaft, the machinability is the important property required for the component and therefore the graph of machinability and compression strength is plotted and shown in Figure 6.

   Figure 6 Machinability Vs Compressive strength graph

   For the further refinement as the pedestal has to hold the total weight of the gearbox it is subjected to tensile loads and therefore the graph of tensile strength in MPa is placed the result shows the ductile cast iron has higher tensile strength than the low carbon steel as shown in Figure 7.

   Figure 7 Tensile strength graph

   As the structure of pedestal should be stiffer and stronger, the preferred material is ductile (nodular) cast iron as the material is suitable for bending and torsion loads were gray cast iron is not preferred. [6]

   The comparison shown for the properties of ductile (nodular) cast iron and low carbon steel in order to get an idea of the various parameters required for the pedestal is met to modify the material taken from the CES 2009 software database shown in Figure 8[5].

   to the Dandong Foundry, China [7] in gray cast the presence of graphite will be in the form of flakes due to this it has lower strength and it is used to produce the components such as used machine bases, housings etc. were the component is subject to less load. And in ductile cast iron the graphite is present in the form of spherical shape due to this it can withstand high tensile and compressive loads and used to produce components such as brackets, crankshafts, connecting rods etc.

   For eliminating the gray cast iron from the material candidate the further comparison is made between the gray cast iron and ductile cast iron material properties as shown in Figure 9. According to the Sumitomo drive technologies [8][9] in Cast iron Vs ductile iron housing materials topic has quoted Ductile iron is typically twice as strong as many grey cast irons, and nearly as strong as steel and shown the table comparing the gray cast iron and ductile iron shown below:

   Figure 9 Cast iron and Ductile iron comparison

   In order to ensure the functionality of pedestal, finite element analysis is conducted for pedestal with same load and boundary conditions, by applying the properties of low carbon steel and ductile iron. The deformation and stress results are compared.

3. FINITE ELEMENT ANALYSIS [10]:

   Figure 8 Ductile cast iron and low carbon steel comparison

   The comparison of the low carbon steel material and ductile cast iron material shown in Figure 8. It can be observed that the Youngs modulus value is lesser for the ductile cast iron 15% lesser than low carbon steel but the rest of the properties like elastic limit, tensile and compression strength of the material is much better than the low carbon steel. According

   Finite Element Analysis is an engineering analysis technique which is widely used in various field of engineering, implemented to identify behaviour of complex structures for which no exact solutions exist. The basic concept of finite element analysis is to convert the complex problem into a simple form by descretised into many small parts called elements, each elements has nodes which has degree of freedom and it enables to solve the complex problem easily, by finding the solution to all small parts and the sum of behaviours of all parts are assembled into one solution for the overall problem.

   1. Mesh Generation and Elements:

      The meshed model is generated using Hyper Mesh 2009 software, in order provide appropriate loads and boundary conditions the whole mass of the gearbox assembly is idealized as a mass node at the center of mass position as per Saint Venants principle. [11] The elements used in meshing are 3D elements (Solid92-3D 10-Node Tetrahedral) the 1D element (Mass 21) and 2D elements (CERIG) rigid elements connecting mass node with solid elements are used in Finite element modelling as shown in Figure 10

      Figure 10 Meshing and Idealization

   2. Displacement Result Comparison

      The finite element analysis involves three stages; pre processing involves creation of finite element model, Processing involves matrix generation, solving and evaluating the result. The post processing involves viewing of deformation, stress results. The deformation in the pedestal with plain carbon steel material is 0. 12 mm and deformation in ductile iron is 0.014 the Youngs modulus considered for plain carbon steel is 210 GPa and for ductile iron is 180GPa as properties obtained from CES shown in Figure 6. This shows the percentage of difference in Youngs modulus for plain carbon steel to ductile iron is 15%. The percentage of difference in deformation for plain carbon steel pedestal to ductile iron pedestal is 15%. This is due to the material is considered to homogeneous isotropic and therefore it obeys Hooks law. From the comparison of result it was concluded that by changing plain carbon steel material to ductile iron the deformation will be increased by 15%, if that is acceptable than the material and process can be modified.

      Figure 11 Deformation result of pedestal with Plain Carbon Steel

      Figure 12 Deformation result of pedestal with Ductile Iron

   3. von-Mises Result Comparison

   The von- Mises stress criteria is also termed as distortion energy criteria for predicting the failures in theories of failures, widely used criteria as it predicts the failure accurately for metals and alloys which are ductile in nature. As per this criteria when the stress is equal to or greater than the distortion energy the material fails. [12]

   Where, v = von-Mises stress criteria, a, b and v = Principle stress and y = yield strength of material. As per von-Mises stress criteria the maximum stress in plain carbon steel is 139 MPa and 137 MPa therefore in the both the cases the pedestal is in elastic limit and only temporary deformation will take place.

   Figure 13 von-Mises Stress result of pedestal with Plain Carbon Steel Material

   Figure 14 von-Mises Stress result of pedestal with Ductile Iron Material

4. CONCLUSION AND RECOMMENDATIONS:

By the analysis conducted on pedestal the suitable material recommended is ductile iron and the suitable process recommended is sand casting processes.

The benefits of recommendations are:

• Pre machining of low carbon steel plates can be totally eliminated.

• Welding operation is eliminated

• Stress reliving operation is eliminated.

• Assembling and inspection operation is eliminated.

• Reduced machining time

• The deformation can be further reduced by adding stiffeners as the casting provides shape flexibility.

• By conducting topology optimization analysis optimal shape can be arrived with less weight in pedestal as shape freedom is there in casting process.

REFERENCES:

1. Unknown, Pedestal available online http://en.wikipedia.org/wiki/Pedestal retrieved on 02/01/2014.

2. Web page, JTOutfitters, Pedestal used in gear box available online http://forum.ih8mud.com/fj55- classifieds-corner/791038-wtb-mini-truck-ps-gearbox- pedestal.html Retrieved on 05/01/2014

3. Howard A Gries, Die Rolling Machine, US Patent Patent number US4322961 A available online, http://www.google.com/patents/US4322961 retrieved on 12/01/2014

4. Jeremy Gregory, Material Selection For Mechanical Design 1, Massachusetts Institute of Technology Cambridge, Masschusetts available online http://ocw.mit.edu/courses/materials…materials- selection…/lec_ms1.pdf retrieved on 09/12/2013.

5. CES 2009 – Cambridge engineering selector software database, carried out in MSRSAS, Bangalore

6. Unknown, Failure in brittle material under static loading, Machine Design An integrated approach available online

   http://www.unm.edu/~bgreen/ME360/Brittle%20Materi als.pdf retrieved on 13/02/2014.

7. Dandong ruiding founding co Ltd, Ductile iron vs. gray

   iron http://www.ironfoundry.com/ductile-iron-vs-gray- iron.html retrieved on 14/12/2013.

8. Dandong ruiding founding co Ltd, Ductile iron vs. gray iron http://www.ironfoundry.com/ductile-iron-vs-gray- iron.html retrieved on 14/12/2013.

9. Sumitomo drive technologies Cast iron Vs ductile iron housing materials

   http://www.smcyclo.com/uploads/product/files/file- 1283 .pdf retrieved on 14/12/2013.

10. Kurowski, Paul M. (2004) Finite Element Analysis for Design Engineers, Society of Automotive Engineers, USA.

11. Dr. Whelan, Saint-Venants Principle and Stress Concentrations, strength of materials- session-3 available online

    http://coefs.uncc.edu/mwhelan3/files/2010/10/ICD_Sain t_Venant1.pdf retrieved on 03/01/2013.

12. Saeed moaveni.(2011) finite element analysis, 3rd edision dorling Kindersley India pvt.ltd

13. Rolf Sandström, Stockholm Criteria in Material http://core.materials.ac.uk/repository/eaa/talat/1502.pdfr etrieved on 03/01/2013.

14. QJ Harmer PM Weaver KM Wallace Desgn led component selection Computer-Aided Design, Vol. 30. No. 5, 1998, p 391-405.