Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

doi:10.4236/jpee.2013.17003

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Journal of Power and Energy Engineering, 2013, 1, 14-19

http://dx.doi.org/10.4236/jpee.2013.17003 Published Online December 2013 (http://www.scirp.org/journal/jpee)

Test and Evaluation of Stiffness of a Pin Turning Device

for Large Marine Engine Crankshafts

Y. H. Choi1, G. B. Ha2, D. H. Kim2, H. S. An3

1Department of Mechanical Engineering, Changwon National University, Changwon, Korea; 2Department of Mechanical Design

Engineering, Graduate School of Changwon National University, Changwon, Korea; 3Research Institute, HNK Machine Tool Co.,

Ltd., Haman, Korea.

Email: [email protected]

Received October 2013

ABSTRACT

In order to prevent unwanted excited vibrations and to secure better machining precision in large size heavy duty ma-

chine tools dynamic stiffness is one of the most desirable and critical properties. In the past decades, many researches

on machine tool stiffness test and evaluation methodology have been made. However any methodology for a Pin Turn-

ing Device (PTD), which is a special kind of turning lathe for machining big size crankshaft pins, is rarely found among

them. This study proposes a test and evaluation process of stiffness of a PTD by measuring frequency response function

at the tool center point (TCP). For conformance proving for the proposed methodology, stiffness of a PTD obtained by

the proposed method with impact hammer test (IHT) has been compared with that determined by FEM.

Keywords: Pin Turning Device (PTD); Machine Tool Stiffness; Compliance Response Function; Impact Hammer Test

1. Introduction

Recently there is an increasing demand for large scale

machine tools for large and precision parts for several

high growth industry fields like; conventional and renew-

able energy power plants, airplane structures, offshore

platforms, ships and marine engines, etc. [1,9,10]. For

large size machine tool, reduced structural stiffness com-

pared to smaller machine tool is one of problems to be

resolved because deflection of a machine structure rises

as an exponential function of its dimension while the al-

lowable deflection in creases linearly [9]. In the past de c-

ades, many researchers have studied machine tool stiff-

ness evaluation methodologies and design optimization

for high structural rigidity and lightweight [2-8]. Ma-

chine tool stiffness evaluation method [3,4] by determin-

ing the compliance frequency response function at TCP

is a more economical and analytical way than that by a

direct cutting test. Thus, this method has been broadly

applied to obtaining machine tool stiffness. Typical stiff-

ness evaluation methodologies for many different types

of machine tools have been studied by M. Weck and K.

Teipel [4] except that for a special purpose lathe like a

PTD for machining crankshaft pins as shown Figure 1.

The PTD has volum e t ric dimension of

4 14.5××

(

For turning operation of the PTD, a revolving ring

built in tool post rotates around a fixed workpiece dif-

fered from a general lathe, in which a workpiece is rotat-

ing and tool is moved (indexed) by a tool post. The tool

post of the PTD can be indexed in the radial direction

only. Thus cutting forces correspondingly occur in the

radial and tangential directions. Moreover there is a strong

possibility that the PTD does not show uniform stiffness

in the radial and tangential directions along the circum-

ference because the PTD has an asymmetric structure

and open and close type revolving ring as illustrated in

Figure 1. Therefore stiffness in the radial and tangential

directions is a critical inf luential parameter upon dynam-

ic behavior and machining accuracy of the PTD. Thus

this study proposes a proper test and evaluation metho-

dology of stiffness of a PTD.

2. Test and Evaluation of a PTD Stiffness

2.1. Test Methodology a nd P rocess

According to M. Weck and K. Teipel [4], stiffness of a

machine tool can be determined from reciprocal of the

compliance measured at TCP as shown in Figure 2.

From the view point of theoretical basis, the PTD stiff-

ness evaluation methodology in this study is almost the

same as theirs [4]. However there is not specified any

method or process for a PTD in [4], so we propose a test

and evaluation methodology of a PTD stiffness as illu-

strated in Figure 3. The static stif fness

and dynamic

stiffness

are defined by Equation (1) and Equation

Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

(a) Open & close type PTD (b) Prototype PTD

Figure 1. Solid model and prototype of a PTD.

Figure 2. A typical sample of measured compliance at TCP.

Figure 3. An explanatory flow of the proposed PTD stiffness test methodology.

Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

(2), respectively. For a PTD, static and stiffness devia-

tions

k∆

and

k∆

defined by Equations (3) and (4),

of course, will be reasonable and useful criteria consi-

dering that the PTD has asymmetric structure and its tool

post with the revolving ring turns around a work piece in

the vertical plane.

≈



=



(1 )

max



=



(2)

) )

max min

ss s

kk k∆= −

(3)

) )

max min

dd d

kk k∆= −

(4)

Where F and X are excitation force and displacement

at TCP of the machine tool, respectively.

The coordinate systems and parameters for measure-

ment position are defined as shown in Figure 4(a). Car-

tesian coordinate system is used for defining global coor-

dinates of the PTD structure and a local polar coordinate

system is used for defining coordinates of the circular

parts and revolving motions. The parameters M,

represent the measurement position, the radius of

revolving ring, the radial distance of the measurement

position (the distance between the position M and the

revolving ring center), respectively. A dummy tool shown

in Figure 4( b) is installed into the tool post of the PTD

and a tri-axial accelerometer is attached on it. The radial

distance of measurement point

is apparently de-

cided as following equation.

( )

MR TPDT

RRr h=−+

(3)

Where

and

are the feed of tool post in the

radial axis and the dummy tool height, respectively.

Compliance due to applied impulse force or random

excitation has been measured at 12 evenly indexed TCPs

as shown in Figures 4(c) and ( d). Consequently stiffness

has been obtained from the measured compliance. For

measuring compliance of the PTD, some measuring de-

vices were used as; FFT analyzer (ZonicBook 618E),

impact hammer (PCB Model 086D50), tri-axial accele-

rometer (Kistler T ype 8795A50).

2.2. FEM Analysis of PTD Stiffness

In order to predict the compliance of the PTD analytical-

ly and to compare with test results, in this study, har-

monic response analysis also has been carried out. FEM

model of the PTD for the harmonic response analysis is

shown in Figure 5. The modeling data and applied force

are given in Table 1. Compliance frequency responses

were analyzed in each case of harmonic excitation ap-

plied to the evenly indexed 12 different TCPs as shown

in Figure 4(c). As the result, th e static and dynamic com-

pliances at each of the 12 different TCPs were obtained

and consequently the corresponding static and dynamic

stiffness and their deviations were computed by using

Equations (1)-(4).

(a) Coordinate system (b) A dummy tool design

Figure 4. Coordinate system, measuring positions, and equipment for the PTD stiffness test methodology.

Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

Table 1. Modeling data for FEM analysis of the PTD.

FEM modeling Element type No. of nodes No. of elements

SHEL 63 10,439 11,031

Material property

Material Elasticity (GPa) Poisson’s ratio Density

GCD 500 172 0.275 7200

SCM 440 205 0.29 7850

Applied force at spindles Direction Radial dir. Tangential dir.

Force (N) 3623 13,523

Boundary condit ions 4 supporting nodes a t the bottom are fixed

Figure 5. FEM model of the PTD.

3. Results and Discussion

For verifying conformance of the proposed methodology,

both measured and computed stiffness at 12 evenly in-

dexed TCPs have been compared with each other and

they look alike as observed in Figures 6 and 7 and Tables

2 and 3.

As seen in Table 2, measured static stiffness (mini-

mum stiffness value) of the PTD was 1.666 kN/μm in the

radial direction and 5.000 kN/μm in the tangential dir ec-

tion, respectively. Computed static stiffness was 1.370 kN/

μm in the radial direction and 3.367 kN/μm in the tan-

gential direction, respectively.

Similarly as seen in Table 3, measured dynamic stiff-

ness of the PTD was 54.1 N /μm in the radial direction

and 111.1 N/μm in the tangential direction, respectively.

And the computed dynamic stiffness was 39.1 N/μm in

the radial direction and 100 N/μm in the tangential direc-

tion, respectively

Judged by the test results, static stiff nes s d eviation was

3.334 kN/μm in the radial direction and 28.333 kN/μm in

the tangential direction, respectively. And also dynamic

stiffness deviation was 173.2 N/μm in the radial direction

and 297.1 N/μm in the tangential direction, respectively.

So, the proposed PTD stiffness test methodology has

been proven to be valid for determining real stiffness of a

PTD.

4. Concluding Remarks

In this study, a test and evaluation methodology of stiff-

ness of a PTD, which is special purpose turning lathe for

(a) In the radial direction

(b) In the radial direction

Figure 6. Comparison of measured and computed static

stiffness of the PTD at 12 evenly indexed TCPs.

large marine engine crankshafts, has been proposed. The

proposed methodology can obtain machine tool stiffness

Test and Evaluation of Stiffness of a Pin Turning Device for Large Marine Engine Crankshafts

(a) In the radial direction

(b) In the tangential direction

Figure 7. Comparison of measured and computed dynamic

stiffness of the PTD at 12 evenly indexed TCPs.

Table 2. Comparison of static stiffness of the PTD.

Position (˚)

Static stiffness (kN/μm)

Radial direction Tangential direction

Measured Computed Measu red Computed

0 2.500 1.522 6.666 4.545

30 5.000 4.784 5.000 3.367

60 3.333 3.413 5.000 3.546

90 2.272 1.574 5.000 5.154

120 1.666 1.370 10.000 12.500

150 1.666 1.613 33.333 50.000

180 2.500 2.381 10.000 12.500

210 3.703 5.000 10.000 10.000

240 4.000 6.666 10.000 8.547

270 2.500 3.125 10.000 9.091

300 1.666 1.490 11.111 19.607

330 1.666 1.197 11.111 12.500

Table 3. Comparison of dynamic stiffness of the PTD.

Position (˚)

Dynamic stiffness (N/μm)

Radial direction Tangential direction

Measured Computed Measured Computed

0 69.4 40.7 192.3 185.2

30 93.5 68.0 158.7 113.6

60 133.3 109.9 119.0 100.0

90 78.1 63.7 111.1 116.3

120 61.0 40.3 161.3 169.5

150 64.1 39.5 289.9 476.2

180 120.5 52.6 370.4 476.2

210 163.9 101.0 344.8 270.3

240 227.3 256.4 208.3 243.9

270 144.2 125.0 200.0 256.4

300 54.1 54.3 309.3 434.8

330 57.5 39.1 408.2 588.2

from measured compliance response by impact hammer

test or random excitation test. Following the proposed

method and process, PTD compliance was measured at

12 evenly indexed TCPs and then the corresponding

stiffness was determined. For verifying conformance of

the proposed methodology, measured stiffness has been

compared with FEM analysis results. Thus the proposed

methodology is proven to be appropriate for evaluating

real stiffness of a PTD.

5. Acknowledgements

This work was supported by the Industrial Strategic Tech-

nology Development Program (Grant 10033509) spon-

sored by the Min istry of Tr ade, Industr y and Energy, and

HNK machine tool Co., Ltd.

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