structural optimization of a powered industrial lift truck frame

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),

ISSN 0976 – 6359(Online), Volume 5, Issue 10, October (2014), pp. 45-56 © IAEME

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STRUCTURAL OPTIMIZATION OF A POWERED

INDUSTRIAL LIFT TRUCK FRAME

Harshal D. Shirodkar1,

Dr. S.B.Rane2

1Post Graduate Student, Sardar Patel College of Engineering, Mumbai-400058, India 2Associate Professor, Sardar Patel College of Engineering, Mumbai -400058, India

ABSTRACT

Purpose: The purpose of this paper is to re-design a lift truck frame (Chassis) with the optimum

mass while maintaining stress constraints. The paper also demonstrate how Optimization techniques

can be applied mainly when product is already launched in market and optimized design is to be

implemented without altering any existing assembly fitment parameters/functional requirement.

Methodology: This paper describes the use of topology and size optimization technique using CAE

software OptiStruct of Altair Engineering to redesign Frame of a lift Truck and comparison of result

using alternate solver Radioss.

Findings: Up to 15% weight reduction in frame is achieved without altering any existing assembly

fitment parameters or compromising any functional requirements. Also considerable cost saving of

Rs.10000 per vehicle is achieved through this process.

Practical Implications: It is completely feasible to implement the optimized design in actual

practice/production & other organization can also benefit by implementation of similar process.

Limitation: This paper has limitation in terms of further optimization using all the features since the

design is already ready and the frame production is continued. Therefore the optimization cannot be

achieved through major change in the shape which may affect the existing production activities.

Originality/Value: This paper can help derive common methodology for optimization techniques

specific to Industrial Equipment and other Off-highway equipment like Earthmoving and

Construction equipments.

Type of Paper: Applied Research.

Keywords: Hyper Works, Lift Truck, Structural Optimization, Size, Topology.

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 5, Issue 10, October (2014), pp. 45-56

© IAEME: www.iaeme.com/IJMET.asp

Journal Impact Factor (2014): 7.5377 (Calculated by GISI)

www.jifactor.com

IJMET

© I A E M E



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I. INTRODUCTION

Optimization [3] is a mathematical technique that deals with computing minima or maxima

of functions subjected to design variables or constrains. There are essentially two stages of the

design process in which structural optimization [2] can be applied. In the early stage of concept

generation, topology optimization [1] should be used to develop an efficient structure from the

beginning. At this level, an automatize variation of optimization parameters is proven useful to find

the best feasible design possible. In the later stage, shape and size optimization [6] should be used to

fine-tune the structure realized from the topology optimization. Using optimization in this manner

gives great possibilities to save time; mass as well as it may produce innovative designs. As first

design step topology optimization is used for finding out proper material distribution followed by

size optimization for computing optimal thickness of structural members of Frame.

The whole challenging task, starting with pre -processing, solving and post processing is

completed using Altair’s HyperMesh, OptiStruct [4] and Hyper View FE package. Results are

compared using alternate solver of Altair Radioss [4] which are discussed in the concluding part.

II. PROCESS METHODOLOGY

Here is a simplified overview of the traditional design process and where the different types

of structural optimizations can be applied. There are basically two different areas where structural

optimization should be performed; in the early design phase where topology optimization is used to

generate a good concept and in the detailed design phase where size- and shape-optimization [7] is

used to further improve the structure.

1. Concept Generation

A flow chart for the concept generation is presented followed by a description of each step.

Choice of parameters and other details are discussed in the respective sections. The process of

concept generation should be seen as an iterative process where the problem formulation and design

domain is incrementally improved until the best possible solution is found. Refer Fig 1.

Figure 1: Flowchart – Concept Generation [4]



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2. Detailed optimization Process In this stage a CAD-model of the structure is available that is similar and have the same

topology as the end result. The objective in this stage is to fine-tune, or refine, the structure to make

it as good as possible. There are basically two different types of detailed optimization: size

optimization and shape optimization. Refer Fig.2

Figure 2: Flowchart – Detailed Optimization [4]

III. PRE-PROCESSING

In the pre-processing part, required surface model is created using Hyper Mesh. The FE

Model with design space for initial boundary condition is shown in Fig.3. Shells (QUAD4, TRIA3),

Solids (Hex 8) and rigid elements are used accordingly to define geometry and constrain conditions.

Out of different load cases, critical laden Static load case having maximum Stress value is

considered for initial structural optimization.

Figure 3: Meshing, Load & Boundary Conditions [5]



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3. Material Details

Hot Rolled Structural Steel Specification as per IS 2062:2011 [10].

Grade: E250 A (Old Designation: Fe 410 WA)

Tensile Strength: 410 Mpa

Yield Strength: 240 Mpa

Young’s Modulus: 210 Gpa

Poisson’s Ratio: 0.3

Factor of Safety: 3 [Considered for Dynamic load Conditions]

Allowable Bending Stress: Y.S / F.O.S = 80 Mpa.

Allowable Deflection for Simply Supported Beam according to Deflection Span

Ratio [8] = Span L/360 = 2200/360 = 6 mm.

Additional safety factor of 1.1 is considered for each of above weight which considers the

overloading factor. For example the Counterweight is Cast Iron Component hence casting during

manufacturing can come above the desired weight or operator while lifting load may not exactly lift

rated load hence safety factor takes into consideration all these uncertainty.

4. Load and Boundary Conditions

Summary of loads on Frame are shown in the Fig.4 & Table: 1

1) Reaction force due to counter weight resting on structure

Total Force on each edge = 21500 / 2 = 10750 N

2) Force due to engine, transmission etc.

at their mounting:

Load on each Engine Bracket = 1408 N and

Load on each Transmission Bracket = 1343 N

3) Reaction due to steering (rear) axle to pivot

Load on each pivot = 40000/2 = 20000 N.

4) Upright load on the front bushing brackets.

Load on each bracket = 65000 / 2 = 32500 N

5) Force due to tilt cylinder operation.

Force on each cylinder = 35500 N.



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Table 1 Component Weight in Kgs

Rated Load 5000

Lifting Mechanism

( Mast + Carriage + Forks)

1500

Front Drive Axle 300

Transmission 300

Engine 550

Rear Axle 145

Counter Weight 2150

Figure 4: Summary of Loads on Frame

IV. STRUCTURAL OPTIMIZATION

The structural optimization is carried out in three phases. In the first phase, Frame is subjected to

topology optimization. Depending on the density plots, material distribution with cut outs has been

finalized to reduce the weight. In the second phase, the Frame designed after topology run is used for

size optimization for computing optimal thickness of all the structural members. In third phase, the

output model is run for remaining load cases.

5. Topology Optimization

This present work adopts Density approach for optimal material distribution. Available

design space is defined with proper loading and boundary conditions with objective to minimize the

global compliance of the structure, subjected to mass constraint.

The density plot by topology run is shown in Fig.5.

The below mentioned criteria's are used for topology optimization.

Design variable : Density of each element within design space.

Design Constraint: Stress with specified limit.

Design Objective: Minimize the mass.



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Figure 5: Topology optimization plot for critical load case [5]

6. Geometry Interpretation

Although the topology results appear reasonable, the design is definitely not ready to hand

over to the machine shop for fabrication. The results of the topology studies are merely rough

geometric proposals, and some interpretation is required to create the final design

Material removal was considered such that the material removed should be scraped but

utilized to fabricate other small brackets required on the Frame. The Geometry has given appropriate

result acting as cross member between two main Frame plate and transferring weight of

counterweight through the weldment ultimately to ground.

Refer Fig.6.

Figure 6: OptiStruct Proposed Topology Design & Final Geometry Extraction



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7. Size Optimization

OptiStruct has the capability of performing size optimization. Size optimization can be

performed simultaneously with the other types of optimization. In size optimization, the properties of

structural elements such as shell thickness, beam cross-sectional properties, spring stiffness, and

mass are modified to solve the optimization problem.

The output design by topology optimization with introducing cutouts is set for size

optimization to get the optimized thickness of all structural members. The following criteria are

defined for size optimization.

The output thickness result are seen in Fig.7

Design Variable : Thickness of the components

Design Objective: Minimize mass

Design Constraint: Stress with specified limit

Design by size optimization is considered for remaining load cases followed by adjustment of

cut outs for maintaining C. G. location of Frame

Figure 7: Thickness values by size optimization [5]

7.1 Interpretation of results

The output *.out file contains a summary of the size optimization process. From the

information in the *.out file, one can see how the objectives, constraints, and design variables are

changing from one iteration to the next. Below Table 2 & 3 show excerpt from *.out file showing

Lower and Upper Bound Design variables converged to desired thickness. OptiStruct used six design

iterations to reach optimum shell thickness for given conditions.



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Table 2: Result of Size Optimization

DESIGN

VARIABLE

I.D

DESIGN

VARIABLE

LABEL

LOWER

BOUND

DESIGN

VARIABLE

UPPER

BOUND

06 Inside P 8.000E+00 1.159E+01 2.000E+01

07 Inside P 8.000E+00 9.189E+00 2.000E+01

08 Outer si 4.000E+00 4.000E+00 1.200E+01

09 fender 6.000E+00 1.351E+01 1.600E+01

10 counter 1.200E+01 2.025E+01 2.400E+01

Table 3: Result of Size Optimization

DVPREL1/2 USER ID PROP-TYPE PROP-ID ITEM-CODE PROP-VALUE

DVPREL1 10 PSHELL 14 T 4.000E+00





V. FINAL RESULT AND CONCLUSION

8. Result Obtained After Topology And Size Optimization In OptiStruct.

After the size optimization, the stress value should be reviewed to make sure that the stress

constraints are not violated. Displacement and stresses results of optimized design are shown in Fig:

8 & 9 respectively.

Figure 8: Displacement plot with OptiStruct Solver [5]



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Figure 9: Stress plot with OptiStruct Solver [5]

The work has shown how topology and size optimization tools can be used for the design of

Frame. Through optimization techniques weight of Frame is reduced by around 15% with no

significant increase in stress and deflection value. Comparative results for existing and optimized

design of Frame are tabulated in Table 4.

Table 4: Summary of Results

Weight(kg) Deflection(mm) Stress (Mpa)

Existing design 910 0.6 70.8

Optimized design 780 0.65 79.7

9. Result Compared Using Alternate Solver – Radioss

The analysis is carried in Radioss Solver & comparative results for stress and displacement

are shown in Table 5.

The Stress has increased marginally with no significant rise in deflection.

Figure 10: Displacement plot with Radioss Solver



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Figure 11: Stress plot with Radioss Solver

Table 5: Summary of Results

Solver Deflection(mm) Stress (Mpa)

OptiStruct 0.65 80

Radioss 0.68 86

9. Justification of Optimized Design

The stability of Lift truck is very important criteria which limits the value for reducing the

weight of the Frame. The Lift truck has to pass the various stability tests as per the guidelines in

Indian Standard IS 4357: 2004. [9].The location of C.G of entire Lift Truck is calculated with

reference to front drive axle as pivot point for calculating moment. Next, front & Rear laden as well

as un laden reactions are found.

Then Stability Ratio of the Lift Truck is calculated as below as per Industrial Norms of Lift

Truck Manufacturer:

Stability Ratio = ��

��

Stability Ratio Range between 25 to 28% is considered as Average Range.

Stability Ratio Range between 28 to 30% is considered as Desired or Optimum Range.

Mentioned range may vary depending upon the Tonnage of Lift Truck.

As calculated the values are as below;

Stability Ratio before optimization = 30.1%

Stability Ratio after optimization = 28.9%



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The significance of Stability ratio can be explained as below:

When the Lift Truck is un laden the front & rear reactions are approximately same since

weight is equally distributed. But when the rated capacity is lifted in front the entire C.G is shifted &

the rear reaction reduces.

Stability ratio of 28% means even after lifting rated load of 5000 kgs the reactions on the rear

wheels are not allowed to reduce to zero instead a buffer is maintained to compensate the dynamic

change in Laden C.G which take place when the load is lifted at various height and also tilted in

front by max 6° for stacking the load at maximum height of 3.5m.

10. Benefits Summary

1. With this approach, the design cycle time is reduced by 25%; the optimum configuration is

achieved directly through the use of OptiStruct optimization tool.

2. Cost reduction of Rs.10000/- per vehicle is achieved as a befit of this research activity.

3. The research has demonstrated a methodology of how to use optimization techniques when the

product is already in market & challenge is to implement optimized new design with far higher

strength to weight ratio to that of original design prepared without altering any existing

assembly fitment parameters/functional requirement.

11. Future Scope

1. Future scope would be towards implementation of Shape Optimization along with Topology

optimization to limit the design domain and shape in early phase of development.

2. Also Dynamic load conditions would be simulated so as to have optimum design which would

ensure excessive safety factor is not considered.

12. Acknowledgement

We would like to express our gratitude to Mr.Pinaki Ghosh, GM - Engineering R&D for

giving us an opportunity to work on this project in Voltas Material Handling Pvt Ltd, Pune and

providing the necessary approval for publishing this paper. Special thanks to Mr.Khusal Kesrod,

Design tech for his support and encouragement throughout this project. And last but not the least

senior Professors and colleagues from Sardar Patel College of Engineering, Mumbai for their

valuable guidance and constant encouragement towards completion of this research.

VI. REFERENCES

Thesis:

1. Martin Fagerstrom and Magnus Jansson, 2002, “Topology optimization in the design

process”, Master's thesis, Chalmers University of Technology.

Books:

2. Martin Philip Bendsoe, 1995, “Optimization of Structural Topology, Shape and Material”,

Springer-Verlag Berlin Heidelberg.

3. Singiresu S. Rao, 1996, “Engineering Optimization | Theory and Practice”, John Wiley &

Sons, third edition.

4. Altair Engineering Inc, 2009, “OptiStruct 10.0 help files”.



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Proceedings Papers:

5. Dr S.B.Rane, Harshal Shirodkar, P.Sridhar Reddy, 2013, “Finite Element Analysis and

Optimization of Forklift Chassis”, India Altair Technology Conference.

6. Gerald Kress and David Keller, 2007, “Structural Optimization”, Swiss Federal Institute of

Technology Zurich.

7. Marco Cavazzuti and Luca Splendi, “Structural optimization of automotive Chassis: Theory,

set up, design”, Mille Chili Lab, Dipartimento di Ingegneria Meccanica Civile, Modena, Italy.

Journal Papers:

8. Hirak Patel, Khushbu C. Panchal, Chetan S. Jadav, April 2013, “Structural Analysis of Truck

Chassis Frame and Design Optimization for Weight Reduction”, International Journal of

Engineering and Advanced Technology (IJEAT), ISSN: 2249 – 8958, Volume-2, Issue-4.

Standards:

9. IS 4357:2004,Methods for Stability Testing of Forklift Truck, The Bureau of Indian

Standards (BIS)

10. IS 2062:2011, Hot Rolled Medium and High Tensile Structural Steel, The Bureau of Indian

Standards (BIS).