F

LI

a

ARRA

KCFFE

1

mfwet

psssanpaeamp

0d

Journal of Materials Processing Technology 211 (2011) 467–474

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

lexible forming tool concept for producing crankshafts

.M. Alves, P.A.F. Martins ∗

DMEC, Instituto Superior Técnico, Univ. Tecn. Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

r t i c l e i n f o

rticle history:eceived 12 August 2010eceived in revised form 6 October 2010ccepted 27 October 2010

eywords:rankshaftslexible forming toolinite element methodxperimentation

a b s t r a c t

Forging, casting and machining compete on quality and price for the production of crankshafts. Forgingand casting are commonly utilized for mass production because the capital investment in equipment andtooling are very high. Machining is employed only in case of small production batches of high qualitycrankshafts made from materials that are normally difficult to forge or cast because it is time and energyintensive, generates a lot of waste and is generally more costly than forging and casting.

As a result of this, conventional manufacturing routes for crankshafts are not suitable for flexible smallto medium-batch production and, therefore, are not appropriate for the growing agile manufacturingtrends requiring very short life-cycles and very short development and production lead times.

This paper is concerned with these issues and is focused on the development of an innovative formingtool concept for producing small to medium-batches of cost competitive crankshafts. The proposed tool

concept combines knowledge on buckling of solid rods under compression with flexible constructionsolutions based on modular dies to allow crankshaft production to change output rapidly. Single cylinderto multi cylinder crankshafts including multiple main bearing journals, crankpins and crank webs can beeasily produced by fastening or removing appropriate die modules in the overall tool set.

The presentation is illustrated with test cases obtained from finite element modelling and experi-ory pin col

mentation with a laboratexhibiting high ductility

. Introduction

Crankshafts are employed to convert circular into reciprocatingotion or reciprocating into circular motion. Its application draws

rom ancient water powered saws, which combined crankshaftsith connecting rods for cutting rectangular blocks of stone to mod-

rn internal combustion engines where crankshafts are necessaryo translate the reciprocating motion of pistons into rotation.

Forging, casting and machining are competitive manufacturingrocesses in crankshaft production industry. Forged crankshafts arehaped in a sequence of stages. Starting with a solid rod, the cross-ectional area of the rod is first altered in shape by roll forging,ubsequently formed into the final shape by close die forging oper-tions and then trimmed. The intermediate stages in forging areecessary for distributing the material and filling the die cavitiesroperly (Thomas, 1986) but trimming can be eliminated by thepplication of precision forging technology. The work of Behrens

t al. (2007) presents a comprehensive investigation on the subjectnd shows the potential of precision forging technology to reduceaterial waste and energy consumption and to improve the overall

hysical and mechanical properties of crankshafts.

∗ Corresponding author. Tel.: +351 21 8417561; fax: +351 21 8419058.E-mail address: pmartins@ist.utl.pt (P.A.F. Martins).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.10.024

rototype tool conceived to operate exclusively with lightweight materialsd forming.

© 2010 Elsevier B.V. All rights reserved.

Casted crankshafts are less expensive than forged because theycan be made close to the required shape and size in a single oper-ation. They are favoured for low cost production of engines thatoperate under moderate loads whereas forged crankshafts are cho-sen in case of engines working under heavy load conditions. This isbecause forged crankshafts generally offer higher toughness, resis-tance to impact and fatigue and better strength to weight ratio thancasted crankshafts (Montazersadgh, 2007).

Machining starts with a solid piece of material, usually in theshape of a cylinder, from which the desired crankshaft is achievedby removing away unwanted material. The process is mainlyused for small batch production of high quality and high pricedcrankshafts made from materials that are normally difficult to forgeor cast.

A comprehensive comparison of the above mentioned manu-facturing processes with respect to fabrication aspects, mechanicalproperties and final costs is available in Zoroufi and Fatemi (2005).The study allows concluding that conventional manufacturingroutes are not suitable for flexible small to medium-batch pro-duction (that is, low to medium volume of custom and specific

products) and, therefore, are not appropriate for the growing agilemanufacturing trends requiring very short life-cycles and veryshort development and production lead times. In fact, the presentneed for flexible manufacturing processes requires the develop-ment of innovative technological solutions that are capable of

468 L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474

F a) Strec on inl

rli

isddfpcc

rctra

disbsn

s

ig. 1. Mechanical and tribological characterization of the aluminium AA6061-O. (urves showing the changes of the internal diameter as a function of the reductiubricated conditions.

educing the fixed and capital costs of crankshaft production to aevel where small to medium-batch production becomes econom-cally feasible.

Although the above conclusion may, at first, appear obvioust represents a serious technological challenge for the currenttate-of-the art of crankshaft production due to the necessity ofeveloping new manufacturing processes that are capable of pro-ucing components of high quality with low tooling costs. Theulfilment of this challenge will open new trends in crankshaftroduction and will be of great interest to the automotive, motor-ycle and other industries that incorporate large or small internalombustion engines in their products.

The work of Matsumoto et al. (2008) provides a first attempt toeduce manufacturing costs and increase flexibility by assemblingrankshafts from several pieces. The solution is based in a new plas-ic joining method for fixing bars to a plate in which bars kept atoom temperature are pierced into a hot plate without lubricationnd fixed to the plate by thermal shrinkage of the plate after cooling.

This paper presents a cost competitive alternative for the pro-uction of single piece crankshafts. The approach is based on an

nnovative tool concept that combines knowledge on buckling ofolid rods under compression with flexible construction methods

ased on modular dies to allow production to change rapidly fromingle to multi cylinder crankshafts with various main bearing jour-als, crankpins and crank webs.

As with all new forming processes there is a need to under-tand the deformation mechanics in terms of its major parameters

ss–strain curve obtained from uniaxial compression tests. (b) Ring-test calibrationheight for several friction factors and the experimental data obtained under dry

with the objective of designing and fabricating a flexible tool sys-tem to successfully shape solid rods into crankshafts. Under thesecircumstances, the aim of the present paper is threefold: (i) tointroduce an innovative flexible tool concept that allows low cost,single-stage, forming of solid rods into crankshafts, (ii) to investi-gate the deformation mechanics of the proposed forming processand (iii) to demonstrate its overall feasibility by experimentationwith a laboratory prototype tool system conceived to operate exclu-sively with lightweight materials exhibiting high ductility in coldforming.

The overall methodology was based on independently deter-mined mechanical properties of the material, experimentation andprocess modelling using an in-house finite element computer pro-gram. The presentation is illustrated with selected test cases andis expected to contribute to transferable of original technologicalknowledge and to stimulate the extension of the proposed toolconcept to hot forming of steel rods that are commonly utilizedin crankshafts.

2. Material and tribological conditions

The experiments were performed with solid rods of AA6061aluminium alloy with 20 mm of diameter that were annealed byheating at 415 ◦C during one hour and subsequently cooling in air.The stress–strain curve of AA6061-O was determined by means ofcompression tests carried out at room temperature and is defined

L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474 469

F hemat mpon

a

�

Tasb

artTpse

Fs

ig. 2. Flexible forming tool concept for shaping a solid rod into a crankshaft. (a) Scool set utilized for producing a crankshaft with a single crankpin. (c) Active tool co

s a power function of the strain (Fig. 1a).

¯ (MPa) = 235ε̄0.16 (1)

he compression specimens were produced from the supplied rodnd the tests were performed in accordance with the ASTM E9-09tandards. The effects of strain rate and anisotropy on material flowehaviour were neglected.

The role of friction at the contact interface between the rod andctive tool parts (e.g. platens and dies) was determined by means ofing compression tests and made use of samples with 6:3:2 propor-

ions in outside diameter, inside diameter and height, respectively.he tests were performed in dry lubricated conditions and sam-les were compressed between platens of the same material andurface roughness as the active tool parts that were utilized in thexperiments with crankshafts.

ig. 3. Single-stage forming of a round bar into a crankshaft. (a) Tool equipped with twoolid rod. (c) Tool equipped with four slide die modules.

tic representation of a tool set equipped with a slide die module. (b) Picture of theents together with a picture of a crankshaft.

The procedure for evaluating the friction factor made use ofcalibration curves, relating the changes in inner diameter withreduction in height during deformation. These curves were pre-pared in advance using the in-house finite element computerprogram I-form and the stress–strain relationship of AA6061-Odefined in Eq. (1). The experimental results plotted in Fig. 1b allowthe authors to estimate a friction factor m = 0.35.

3. Flexible forming tool

As with all new forming processes there is a need to performexperiments with prototype tools for the purpose of formationof knowledge. Although the prototype tool utilized in the inves-tigation was designed and fabricated to operate exclusively withlightweight materials in order to reduce development costs, this

slide die modules. (b) Picture of a crankshaft with two crankpins made from a PVC

470 L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474

F iscretit

dc

pcw

tap

F5

ig. 4. Finite element model utilized in the numerical simulation of the process. Do one-half of the geometry.

oes not excludes the potential of applying the proposed formingoncept to other materials such as steel.

Fig. 2 illustrates both before and after stages of the laboratoryrototype tool set build upon the innovative flexible forming tooloncept that was utilized for shaping a solid rod into a crankshaft

ith a single crankpin (that is, for a single cylinder engine).

The tool consists of an upper driver plate, a lower fixed plate,wo die holders connected by horizontal sliders, a slide die modulend an adjustable die stop. When the rod (preform) is com-ressed axially via a loading ram attached directly to the upper

ig. 5. Photographs showing samples of successful and unsuccessful crankshafts made f.5 and 6.0. Details of the slide die module at the final stage of deformation are included

zation of the preform by means of eight-node hexahedral elements was restricted

plate, the die module moves downward but also horizontally bysliding along the horizontal guides. The horizontal movement ofthe die module is not driven by the press and allows plasticinstability of the solid rod to develop outwardly in a controlledmanner. The sense of displacement is towards the die holder

located in the opposite side of the die stop and the amount ofdisplacement, which depends on the initial unsupported portionof rod between the slide die and the plates, defines the distancefrom the centre of the crankpin to the centre of rotation of theshaft.

rom solid rod preforms with values of the slenderness ratio lu/d0 equal to 2.0, 4.0,for the left and rightmost experimental test cases.

L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474 471

F tage fa

cr(F

cbmm(sajcflFw

adpurlofcmw

sar

4

ctlm

ig. 6. Evolution of the load–displacement curve for heading, buckling and single-snd two crank webs.

The slide die module is split in two halves for allowing therankshaft to be removed from the tooling after forming. Theesulting crankshaft, which consists of two main bearing journalscrankshaft support), two crank webs and a crankpin, is shown inig. 2c.

Even though each slide die module serve only to shape arankshaft with a single crankpin similar to that in Fig. 2c, the num-er of crankpins can be increased by summing-up additional dieodules on top of each other. Fig. 3a shows a solution with two dieodules that was utilized for cold forming the crankshaft of PVC

polyvinyl chloride) that is pictured in Fig. 3b. The crankshaft con-ists essentially of two crankpins placed at equal angular intervalsround the axis of the shaft that are separated by a main bearingournal. Each crankpin has two adjacent crank webs and the appli-ation in PVC is a good example of the suitability of the proposedexible forming process to shape other materials than metals.ig. 3c shows a complete tool system for producing crankshaftsith four crankpins.

The active parts of the flexible forming tool are dedicated tospecific outside radius of the preform. In principle, the initial

istance between the slide die modules and the upper and lowerlatens should cope with the minimum threshold ratio of thensupported length to the diameter of the solid rod (slendernessatio) that is necessary to ensure buckling. The adjustable die stopocated in one side of the sliders avoids the formation of concentricr near-concentric parts and stimulates material to flow outwardlyrom its axis of compression in order to form the crankpins and therank webs. This is critical because if the movement of the slide dieodule is prevented due to absence of buckling, the applied loadill bend the horizontal sliders and may damage the tool.

The laboratory prototype of the flexible forming tool concepthown in Figs. 2 and 3 was installed in a mechanical testing machinend the experiments were performed with a constant displacementate of 1.67 mm/s (100 mm/min).

. Finite element modelling conditions

Because the experiments were performed under a quasi-staticonstant displacement rate of the upper-table of the mechanicalesting machine, no inertial effects on forming mechanisms areikely to occur and therefore no dynamic effects in deformation

echanics are needed to be taken into account. These operating

orming of a solid rod into a crankshaft with two main bearing journals, a crankpin

conditions allowed numerical modelling of the process to be per-formed with the finite element flow formulation and enabled theauthors to utilize the in-house computer program I-form that hasbeen extensively validated against experimental measurements ofmetal forming processes since the end of the 80’s (Alves et al.,2003).The finite element flow formulation giving support to I-formis built upon the following variational statement:

˘ =∫

V

�̄ ˙̄ε dV + 12

K

∫V

ε̇2V dV −

∫ST

Tiui dS +∫

Sf

(∫ |ur |

0

�f dur

)dS

(2)

where K is a large positive constant enforcing the incompressibilityconstraint and V is the control volume limited by the surfaces SU

and ST, where velocity and traction are prescribed, respectively.Friction at the contact interface Sf between workpiece and toolingis assumed to be a traction boundary condition and the additionalpower consumption term is modelled through the utilization of thelaw of constant friction �f = mk. The friction factor m was set to 0.35(refer to Section 2).

The numerical evaluation of the volume integrals included inEq. (2) was performed by means of a standard discretization proce-dure based on the utilization of eight-node hexahedral elements.In order to ensure the incompressibility requirements of the plas-tic deformation of metals, both complete and reduced Gauss pointintegration schemes were utilized.

The finite element computer models were set-up in order toreproduce material flow in a tool system equipped with a singleslide die module (Fig. 2) and on account of symmetry the discretiza-tion was simplified and restricted to one half of the initial preform(Fig. 4).

The numerical integration of the friction boundary integral in(2) is performed by means of a five Gauss point quadrature andthe active tool components (plates and dies) were discretized bymeans of contact-friction spatial linear triangular elements (Fig. 4).Further details on the finite element procedures for discretizationof Eq. (2) can be found elsewhere (Alves et al., 2003).

The numerical simulations were accomplished through a

succession of displacement increments each of one modellingapproximately 0.1% of the initial preform length. The convergenceof the numerical simulations was stable and the overall CPU time fora typical analysis containing roughly 5800 elements and 7000 nodalpoints was below 4 h on a standard laptop computer. No intermedi-

472 L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474

F h a sinc e initi

aob

5

fseam

5

mltapba

ig. 7. Flexible forming tool concept applied to the production of a crankshaft witomputed evolution of the finite element mesh for 8%, 16% and 25% reduction of th

te remeshing operations were utilized and, therefore, no influencef field variable recovery techniques on the final results needed toe taken into consideration.

. Results and discussion

The work on the proposed flexible forming concept was per-ormed on a prototype laboratory tool set-up and consisted onhaping solid rods into crankshafts with a single crankpin. How-ver, since the basis of the experimental and numerical analysisre the same for multi-crankpins, the procedures utilized could beodified and implemented for other types of crankshafts.

.1. Modes of deformation

The experiments confirmed the existence of three differentodes of deformation. For small values of the slenderness ratio

u/d0 (which is the ratio of the unsupported length lu to the diame-

er d0 of the solid rod preform) material flows uniformly around itsxis of compression in order to form a concentric double-headedart instead of a crankpin while for large values of lu/d0 materialuckles, shifts sideways and restrictions are set by the maximumllowed horizontal displacement of the slide die module (Fig. 5).

gle crankpin. (a) Detail of the tool set-up at the end of the process. (b) Initial andal unsupported height of the preform and comparison with an experimental part.

This difference in behaviour is due to the fact that for small val-ues of lu/d0 material flow prevents the horizontal movement of theslide die module whereas for large values of lu/d0 the crank websare extensively formed into a ‘V-shape’ before being compressedbetween the tool plates and the slide die modules.

In-between the abovementioned modes of deformation it is pos-sible to shape solid rods into crankshafts with appropriate crankwebs. The initial unsupported length of the rod preform placedbetween the upper and lower die plates lu defines the volume andshape of the crank webs and determines the distance from the cen-tre of the crankpin to the centre of rotation of the shaft (refer to theintermediate crankshafts in Fig. 5).

5.2. Forming load and material flow

Fig. 6 shows the evolution of the load–displacement curve forthe fundamental modes of deformation of solid rods under uniformaxial compression (Lange, 1985): (i) heading in case of a short rod

with a slenderness ratio lu/d0 = 2.0 and (ii) buckling with the upperend free in case of a long rod with a slenderness ratio lu/d0 = 4.0.Both loads grow monotonically at the beginning of deformationbut, while heading gives rise to a rapid increase of the compressiveload as deformation progresses, buckling experiences a low rate of

L.M. Alves, P.A.F. Martins / Journal of Materials Processing Technology 211 (2011) 467–474 473

Fv

ifec

fboltptpac

tccetg

eoieovlt

5

t

ig. 8. Computed distribution of the effective strain at the end of the process. Theiew of the sliced crankshaft show details of the distribution inside the component.

ncrease up to a maximum value and is followed by diminishingor large amounts of deformation. This is because bent forms ofquilibrium, resulting from buckling, need small values of the axialompressive load as the degree of bending increases.

As seen in Fig. 6, the evolution of the load–displacement curveor a crankshaft with a slenderness ratio lu/d0 = 4.0 is located in-etween heading and buckling with the upper end free. A closebservation of the experimental and finite element predicted evo-ution of the load–displacement curve allow the identification ofhree different phases: (i) the transient beginning of axial com-ression, (ii) the low rate of growing typical of buckling and (iii)he rising at a moderate rate of growing due to material beingrogressively deformed over the surfaces of the die side modulend platens. At the end of the process the crankshaft is totallyonstrained between these active parts of the tool set-up (Fig. 7a).

From what was mentioned before, the deformation mode ofhe proposed flexible forming process to shape solid rods intorankshafts may be classified as ‘controlled buckling’ under axialompression via slide die modules. The predicted finite elementvolution of the geometry of the crankshaft during forming is plot-ed in Fig. 7b and the correlation with the experimental part is veryood.

Fig. 8 contains the finite element predicted distribution of theffective strain at the end of the forming process. The mechanicsf the process is a combination of buckling and shearing along twonclined planes placed inside the crank webs. The highest values offfective strain are located in these planes (refer to the sliced viewf the crankshaft) and are due to the accumulation of high localizedalues of strain-rate during the forming process. If necessary, theevel of damage in this region can be eliminated by means of heat-reatment.

.3. Flexibility and potential

One of the key features of the proposed forming process ishe influence of the initial unsupported length lu of the solid rod

Fig. 9. Crankshafts with one and four crankpins. In case of the crankshaft with fourcrankpins the rightmost crankpin was shaped with a smaller amount of materialthan the other three.

preform placed between the upper and lower plates of a toolmodule on the geometry of the crank webs and on the distancefrom the centre of the crankpin to the centre of rotation of theshaft.

In order to highlight the flexibility and potential provided by thisfeature of the proposed forming process authors decided to forma non-symmetric crankshaft that presents differences between theleft and rightmost crankpins (Fig. 9).

The non-symmetric crankshaft pictured in Fig. 9 was obtainedsimply by shaping the rightmost crankpin with a smaller amountof material than the other three crankpins and was included inthe presentation to demonstrate the flexibility and potential of theproposed forming process.

6. Conclusions

This paper is about looking to the fundamental modes of defor-mation of solid rods under uniform axial compression in a differentperspective. The slenderness ratio related to the unsupportedlength of the preforms that defines the threshold for produc-ing sound concentric parts was deliberately exceeded in orderto develop an innovative flexible forming tool concept that takesadvantage of buckling and non-uniform material flow around theaxis of compression to successfully shape the crankpins and crankwebs of crankshafts.

The feasibility of the proposed concept was investigated bymeans of finite element modelling and experimentation usinga laboratory prototype tool conceived to operate exclusivelywith lightweight materials exhibiting high ductility in coldforming. Tryouts were also performed with polymers, namelyPVC.

The flexibility inherent to the proposed tool concept allows dieset modules to be easily attached or removed in order to quicklyrespond to a need for producing customized crankshafts in smallto medium lots, in very short time frames and with a cost effectivemode of operation.

Acknowledgement

The work of Fabio Silva during the investigation is greatlyacknowledged.

References

Alves, M.L., Rodrigues, J.M.C., Martins, P.A.F., 2003. Simulation of three-dimensionalbulk forming processes by the finite element flow formulation. Modelling and

Simulation in Materials Science and Engineering – Institute of Physics 11,803–821.

Behrens, B.-A., Doege, E., Reinsch, S., Telkamp, K., Daehndel, H., Specker, A., 2007.Precision forging processes for high-duty automotive components. Journal ofMaterials Processing Technology 185, 139–146.

Lange, K., 1985. Handbook of Metal Forming. McGraw-Hill, New York.

4 rials P

M

M

74 L.M. Alves, P.A.F. Martins / Journal of Mate

atsumoto, R., Hanami, S., Ogura, A., Yoshimura, H., Osakada, K., 2008. New plasticjoining method using indentation of cold bar to hot forged part. CIRP Annals –Manufacturing Technology 57, 279–282.

ontazersadgh, F., 2007. Stress analysis and optimization of crankshafts subject todynamic loading. MSc Thesis. University of Toledo, USA.

rocessing Technology 211 (2011) 467–474

Thomas, A., 1986. Forging Methods. Materials Forming Technology, Sheffield, UK.Zoroufi, M., Fatemi, A., 2005. A literature review on durability evaluation of

crankshafts including comparisons of competing manufacturing processesand cost analysis. In: 26th Forging Industry Technical Conference, Chicago,USA.