CENTER FOR AUTOMOBILE RESEARCH AND ENGINEERING (CARE)

Case Studies

Project

A DOE based approach for fundamental understanding of Large Gear Deformation with Creep.

Assigning Organization

Triveni Gears Mysore

Project Lead

Dr. Suresh Nagesh

Team Members

Mr. Punykoti (RA and a M.Tech Student at PES), Mr. SureshKumar (general manager at Triveni who provided the support from the customer end)

Project Background

Heat-treating processes have traditionally been used to greatly enhance the mechanical properties of steel parts such as bearings, gears, shafts, etc. However, heat treatment processes such as carburizing, quenching and tempering often cause excessive and uncontrolled distortion. Such distortion is predominant in the components of large dimensions. Hence, this type of distortion is a major concern in the production of high quality gears and such components. Many research groups have examined the causes of distortion and found that the thermal stresses as well as phase transformations that occur during the heat treatment play an important role in the resulting distortions. A typical heat treatment cycle of such gears involves, carburizing, re-heating and quenching. The objective is to, create locally hardened gear tooth and eliminate or reduce resulting residual stresses. This study considers the analysis of distortion of large gears including creep during the heat treatment cycle of large gears.

The purpose of this study is to understand therefore the root causes of distortion and then develop a robust numerical model for predicting such distortions in large gears undergoing heat treatment during their manufacture. In particular, in the present study also aims at a design of experiment based approach to develop a transfer function between distortion and the geometric parameters such as OD (Outside Diameter), ID (Inside Diameter), FW (Face Width). The predictions from the transfer function are used to validate the measurements from measured data.

Technical Challenges

Some of the major technical challenges involve understanding the large gear design and manufacturing processes all the way from the behavior of alloy steels used for such manufacturing under different temperature conditions to the final product, the gear and its applications. Challenges involved teaching the students and our research assistants, material mechanics of high strength forged alloy steels, heat treatment as a process, creep simulations at high temperature, finite element modeling and non-linear finite element analysis including material non-linearity, geometric non-linearity, creep and contact interfaces. Challenge also was to make the students and research assistants understand design of experiments, types of designs, transfer functions and their development. Challenge also included developing semi-emperical transfer functions to relate the distortion in three dimensions as a function of the geometric and manufacturing parameters of the gears. Also since in most cases temperature dependent material properties are not generally available, the team had to obtain such properties using semi-emperical approaches of a combination of some available test data and the measurements done during the gear manufacturing process.

Technical Solution

The following are the discrete steps followed as a part of the technical solution to this problem

Phase I

Creation of a 3D CAD model of the full gear.
Creation of Numerical (Finite element) model of the gear.
Analytical calculation of the heat transfer coefficients and fitting the rate curve against the experimental data provided by the customer, to come up with semi-empirical rate curves and the corresponding heat transfer coefficients.
Validation of the finite element model for a simple problem of a blank with a hole, for which standard closed-form, axi-symmetric elastic solutions are available. This is done using the standard numerical code (ABAQUS) for both thermal and stress analysis.
Validation of cooling curve given for the Quenchant, using lumped mass approach.
Derivation of Heat Transfer coefficient for the actual gear; Estimate and compare the thermal distribution and residual stresses for axi-symmetric model, sector model and full 3D model of gear.
Application of thermal loads to the sector model of gear; Estimate the surface and core temperatures and check the residual stresses in the component at various stages of heat treatment.
Estimation of the distortion of the gear and comparison with measured data.

Phase II

Non-linear simulation with creep (thermal problem first, followed by the structural analysis) is then performed case by case identified in the DOE matrix
Various parameters such as resulting distortion at two points, OD, ID, Helix angle and tooth width are measured after each analysis for each chosen case.
Outer Diameter is measured at four different points on two planes and the average is considered for developing transfer function.
For face width, the deviation in angle of two faces of a tooth is calculated and the difference is compared with the experimental value. This is then used for developing the transfer function.
All finite element modeling is done using Hypermesh V12. Analyses are done using ABAQUS.
MINITAB special purpose statistical analysis software is then used to develop the transfer function between the inputs and the outputs.

Results

Finite element model for large gear thermo-structural distortion analysis during its heat treatment
Analytical calculation of the heat transfer coefficients and fitting the rate curve against the experimental data provided by the customer, to come up with semi-empirical rate curves and the corresponding heat transfer coefficients.
Validation of cooling curve given for the Quenchant, using lumped mass approach.
Derivation of Heat Transfer coefficient for the actual gear; Estimate and compare the thermal distribution and residual stresses for axi-symmetric model, sector model and full 3D model of gear.
Non-linear finite element simulation including creep, starting with the thermal simulation of the heat treatment process, followed by the structural simulation.
Comparison of measured data on actual gears against the data from numerical simulation
Choosing a DOE along with the customer
Performing the DOE
Development of a TF to relate the distortion as function of geometric and manufacturing process parameters
Develop a visual basic GUI in MS/Xl for use by the design and engineers.