108

CHAPTER 6

FAILURE ANALYSIS AND OPTIMIZATION OF

SHEAR PIN DIMENSION

6.1 INTRODUCTION

The shear pin is a mechanical sacrificial component like an electric

fuse designed to break itself as and when the mechanical overload arises to

prevent the severe damage of the expensive components in the gearbox

assembly and an electric generator in the wind turbine. The nacelle comprises

of the gearbox assembly, coupling assembly, brake disc assembly, generator

and cover. The coupling connecting the gearbox assembly and an electric

generator of the WTG are fitted with shear pins numbering seven. The rubber

buckles fitted in the coupling provide flexibility during operation. The driver

and driven ends of the coupling are shown in Figure 6.1(a) and 6.1(b)

respectively.

The sudden switch over of the generator from high speed

(1007 rpm - 6 poles) to slow speed (750 rpm - 8 poles) whenever there is a

sudden change in wind velocity is termed as down coupling. It was observed

that the frequent pre-mature failure of the shear pins occurred during down

coupling while operation of the WTG even it did not attain the rated power.

The gearbox in the WTG is subjected to shock loads due to fluctuating wind

force, sudden grid drop, non -synchronization of pitching and sudden braking.

109

Figure 6.1(a) Driver end of the coupling

Figure 6.1(b) Driven end of the coupling

110

In this research work, a specific case of shear pin failure that occurred in a 350 kW capacity WTG commissioned on 31/12/ 2001 is undertaken for investigation. The WTG ceased to function on 06/06/2007 due to the failure of the shear pins causing disconnection of transmission of drive from the gearbox (drive end) to the generator (driven end). It was observed during the inspection that all the seven shear pins were broken at the neck diameter (Figure 6.2). As informed earlier in this section the shear pins are generally designed in such a way that it should fail as and when the mechanical overloads come up so as to protect the highly expensive units, namely, gearbox and generator.

Figure 6.2 Shear pins with damages at the neck diameter

The sudden and frequent change in wind velocity, misalignment between the gearbox shaft and the generator shaft and bearing (rolling contact type) failure are the major sources for mechanical overload. Figure 6.3 shows the assembly of drive train components in WTG and Figure 6.4 shows the arrangement of shear pins in the coupling respectively. The researchers in the field of wind energy have been working on the development of advanced shear pin materials, new heat treatment methods and designing large size coupling in order to tackle the problem of failure of shear pins due to

111

mechanical overload. Many research papers have been published on failure analysis of Kaplan turbine, steam generators, elevator drive shaft and air compressor; but an attempt has not been made to analyze the reasons for the failure of shear pins employed in the wind turbine generator, to the best knowledge of the investigator. Therefore, it is imperative to investigate the problem of failure of shear pin in WTG.

Figure 6.3 Drive trains of the WTG

112

Figure 6.4 Shear pins arrangement in the coupling

6.2 SHEAR PIN FAILURE HISTORY AND CONCERNS

A study was carried out to investigate the frequency of failure of

shear pin used in the coupling. Table 6.1 gives the history of shear pin failure

in WTG under consideration. It is obvious from Table 6.1 that the shear pin

fails within 6 years of usage against the recommended life of 10 years. In the

event of such pre-mature failure, the shear pins has to be replaced

immediately for getting continuous power supply from the wind turbine. The

frequent replacement of shear pin results in increase in bore diameter of

locating bush leading to weakening of the coupling assembly by way of an

increased clearance between the bush and the shear pin. The replacement of

the defective coupling necessitates the dismantling of generator and to be

brought down from the wind turbine nacelle located at the top of the tower.

The de-erection of generator to swap the coupling in the turbine nacelle needs

a crane of higher capacity of the order of 200 Tonne to 400 Tonne at the wind

turbine site.

113

Table 6.1 History of shear pin failure

Commissioning date of the wind turbine

Date of shear pin failures

Time taken in hours for shear pin replacement

31-12-2001

06/01/2007 7

16/06/2007 4

20/06/2008 5

02/01/2009 12

15/02/2009 5

06/03/2009 4

17/03/2009 4

04/04/2009 4

07/04/2009 5

10/04/2009 7

16/06/2009 5

A team of operation and maintenance engineers and the product

development engineers has been trying to explore the possibility of designing

stronger pins and employing new manufacturing process for trouble free

operation of the wind turbine. This research study is intended to predict how

and why the shear pins fail in the wind turbine and the remedy to minimize

the occurrence of failures either by design changes or by design modification

of neck diameter or by introducing surface treatment at the neck portion of the

shear pin. More than 150 gearboxes were examined for radial and axial play

(Figure 6.5) to check whether the failure of shear pins had been due to radial

play and axial play between the bearing outer race and the gearbox housing.

The examination revealed that a radial play of 0.05 mm and above is found to

increase the failure rate.

114

Figure 6.5 Radial play measurements in the gearbox

6.3 ANALYSING THE ROOT CAUSE FOR THE SHEAR PIN

FAILURE

The gearbox chosen for this research work is designed for rated

power 350kW and output speed 1007 rpm to achieve the final step up speed

ratio of 1: 31.5. The high speed shaft of the gearbox is coupled with the

driver end of the coupling and the driven end is connected with the generator

by means of seven shear pins. The driven end of the coupling is connected

with the generator shaft by a key with shrink fit. The technical team inspected

the damaged shear pins (Figure 6.6) and presumed that the failures may be

due to over load by wind force or misalignment between the gearbox and the

generator assembly.

115

Figure 6.6 Damaged shear pins

6.3.1 Material and Compositional Analysis

As a preliminary step in the failure analysis the material of the

shear pin was inspected for any imperfection in its composition and material

defects. The shear pins are made of ETG 88 unique (drawn) special steel that

has very good mechanical properties combined with extraordinary good

machinability.

The material properties of ETG 88 are given below:

Young’s modulus (E) : 2.1×105 MPa

Tensile strength : 800 ~ 950 N/ mm2

Hardness : 29 HRC

Yield strength (Rp) : > 685 N/ mm2 (max 800 N/ mm2)

116

The hardness of the shear pin and the locating bush were measured

in Rockwell Hardness Tester in ‘C’ scale. The hardness of the shear pin found

to be 25 ~ 27 HRC and that of locating bush found to be 35 HRC hence there

is no chance for the shear pins to fail due to material properties. The chemical

analysis of the shear pin was carried out by spectrophotometer and the results

are presented in Table 6.2. It is observed from Table 6.2 that the composition

of failed shear pin confirms to the specifications of ETG 88 material.

Therefore the shear pin failure cannot be due to the compositional effect.

Table 6.2 Chemical analysis of the shear pin material

Element Specifications of ETG 88 (wt %)

Composition of failed Shear Pin

Carbon (C) 0.42 ~ 0.48 0.47

Silicon (Si) 0.10 ~ 0.30 0.108

Manganese (Mn) 1.35 ~ 1.65 1.54

Sulphur (S) 0.24 ~ 0.33 0.284

Phosphorus (P) 0.04 (max) 0.0160

Lead (Pb) 0.0 0.00600

Chromium (Cr) --- 0.0780

Nickel (Ni) --- 0.0810

Molybdenum (Mo) --- <0.002

Iron (Fe) Remaining Remaining

6.3.2 Visual Inspection

The visual inspection revealed that the fracture occurred at the neck

region of the shear pin (Figures 6.2 to 6.6). When the shear pin cuts into

pieces, the two halves of fractured surface rub against each other leading to

surface damage. Machining marks were observed at the neck portion of the

117

failed shear pins due to poor manufacturing process followed during

production. While analyzing the fractured surface (Figure 6.6), typical

torsional cyclic fatigue fracture was noticed. Fatigue crack initiated at the

corner of the neck region where the machining marks were present and

propagated along the whole surface as seen in Figure 6.7. The crack nucleates

and propagates due to the cyclic loading until a critical crack length was

attained resulting in pre-mature failure of the shear pin.

Figure 6.7 Crack path at X100 magnification factor

6.3.3 Metallography

The specimen was prepared by rough, followed by intermediate and

fine polishing and then etched with 2% Nital solution for examination.

Microstructures of the damaged portions of two different shear pins were

118

analyzed using optical microscope as well as SEM. The photographic view in

Figure 6.8 shows micro crack in the shear pin. Figures 6.9 and 6.10 shows the

experimental observation from microscope examination. Micro cracks clearly

seen on the shear pin surface (Figure 6.8) has initiated from the outer surface

of the machined place, and then reached the centre portion of the shear pin.

During micro structure examination, ferrite plus over tempered martensite

structure (Figure 6.9) is clearly seen on some pins and retained austenite plus

over tempered martensite structure (Figure 6.10) is seen on some other pins.

The specification of the material (Metals hand book 1990) includes tempering

treatment; the same was clearly evident from the microstructure examination.

Further, the shear pins were hardened and tempered after the notch (neck

diameter) was made.

Figure 6.8 Microstructure of shear pin showing crack at X 200 magnification factor

119

Figure 6.9 Microstructure of shear pin material showing ferrite and over tempered martensite structure at X 200 magnification factor

Figure 6.10 Microstructure of shear pin material showing retained austenite and over tempered martensite structure at X 200 magnification factor

120

6.3.4 Fractography Investigation

The fractured shear pins were cleaned ultrasonically in acetone and

examined at different magnification scale using SEM and the outcome of the

investigation is presented through Figures 6.11 to 6.14. As recommended in

ASM Handbook (1987), no surface preparation and coating were done on the

fractured surface of the shear pin during the examination. Furthermore, an

accelerating voltage of 25 kV was applied during the SEM analysis. Micro

cracks and oxide particles (Figures 6.11 and 6.12) were seen on the surface of

the fractured shear pin due to overload. The mode of failure of the shear pin is

either due to brittle fracture (Figures 6.13 and 6.14) or fatigue fracture

(Figure 6.15) because of low cyclic fatigue phenomenon.

Figure 6.11 SEM examinations at the neck portion caught micro-crack and oxides at X200 magnification factor

121

Figure 6.12 SEM examination at the neck portion caught micro-crack at X30 magnification factor

Figure 6.13 SEM fractograph at the neck portion showing brittle fracture at X500 magnification factor

122

Figure 6.14 SEM fractograph at the neck portion showing brittle fracture at X30 magnification factor

Figure 6.15 SEM fractograph at the neck region showing fatigue fracture at X30 magnification factor

123

6.4 FINITE ELEMENT ANALYSIS FOR EXISTING DESIGN

6.4.1 Force and Stress Calculation

For the given data the tangential force and the nominal shear stress

for various neck diameter are calculated using the standard Equations (6.1) to

(6.4)

Power (P) = 350 kW

Speed (n) = 1007 rpm

Coupling pitch circle diameter (d) = 314 mm

Diameter of the shear pin (d1) = 10 mm

Neck diameter of the shear pin (d2) = 5 mm

Total number of the shear pins (n1) = 7

Torque (T) =P×60×10³2× n

= 3319 Nm (6.1)

Force (Ft) = 2T

d= 2×3319

0.314 = 21141 Newton (6.2)

Nominal shear stress for 5 mm neck diameter ( )

= 4T×2×10³×d2

2×n ×d

= 4×3319×2×10³3.14×(5)²×7×314

= 154 N/mm² (6.3)

Nominal shear stress for 10 mm diameter shear pin ( )

= 4T×2×10³×d ²×n ×d

= 4×3319×2×10³3.14×(10)²×7×314

= 38.47 N/mm2 (6.4)

124

The assumption made during the calculation:

All shear pins take up the total load equally and hence the load on

each shear pin is

= = 3020.14 Newton

6.4.2 FEA results

Figure 6.16 shows the dimensions of the shear pin. The 20 node

SOLID 192 element type chosen for meshing has three degrees of freedom

per node. Figure 6.17 shows the finite element meshed model of the shear

pin. Table 6.3 gives the mechanical properties of the shear pin and mesh

details used for the FEA. The shear pin is assumed as a cantilever and the

loads (the tangential force and the bending shear force) are applied at the free

end. The total number of elements used in 5 mm neck diameter model is

41616, 41960 elements for 5.5 mm diameter model, 50534 elements for 6 mm

diameter model, 54944 elements for 6.5mm diameter model and 57134

elements for 7 mm diameter model.

Figure 6.16 Dimensions of the shear pin

125

Figure 6.17 Finite element meshed model of the shear pin

Table 6.3 Mechanical properties and mesh details

Material Properties : Young’s modulus (E) 210000 MPa Poisson’s ratio 0.3Mesh details : Element type 20 node SOLID 192 Mesh type and size Very fine mesh, mesh size 1 No.of nodes for 5 mm neck diameter shear pin 59850

No of elements for 5 mm neck diameter shear pin 41616

126

6.4.3 Loads on the Shear Pin

Two cases of loading namely i) twisting force and ii) twisting force

combined with bending force are considered for investigation. In case i), the

shear pins are subjected to pure shear loading (Tangential force) and in case

ii) the shear pins are subjected to shear load combined with bending load due

to misalignment between the gearbox and the generator. The analysis is

carried out to determine the shear stress and von Mises stress. Figure 6.18

shows the distribution of shear stress and Figure 6.19 shows the distribution

of von Mises stress for case i) where shear pin with 5 mm neck diameter is

considered.

Figure 6.18 Shear stress distribution in shear pin with 5mm neck diameter

127

Figure 6.19 von Mises stress distribution in shear pin with 5mm neck diameter

The combination of shear and bending load was applied in different

percentage levels. The maximum shear strength is derived from the maximum

shear stress theory which states that the material shear strength is 0.5 times

that of the material yield strength (Shigley 2008). The maximum shear

strength is 400 N/mm2 as the maximum yield strength of the shear pin

material is 800 N/mm2 (Section 6.3.1). It is observed from Table 6.4 shear

stress and von Mises stress against various misalignment) that the von Mises

stress exceeds beyond 400 N/mm2 when the misalignment between the

gearbox housing and the high speed shaft bearing outer race is 0.05 mm and

above, whereas the shear stress is crossing the nominal value 154 N/mm2

(Section 6.4) when the misalignment is around 0.20 mm. Figure 6.20 shows

the distribution of shear stress and Figure 6.21 shows the distribution of von

Mises stress respectively for shear loading combined with 5% bending load.

128

Table 6.4 Shear stress and von Mises stress against various misalignments

Misalignment in mm

Shear stress (N/mm2)

von Mises Stress (N/mm2)

0 74.495 287.947

0.05 78.099 668.534

0.1 87.773 688.086

0.15 101.842 989.238

0.20 171.223 1276

Figure 6.20 Shear stress distributions in shear pin with 5mm neck diameter and 0.05 mm misalignment

129

Figure 6.21 von Mises stress distribution in shear pin with 5mm neck diameter and 0.05mm misalignment

6.5 FINITE ELEMENT ANALYSIS OF MODIFIED DESIGN

A brainstorming session was held to investigate the reason for pre-mature failure of shear pin. It was decided to undertake field study to investigate the radial play between the gearbox and the generator. The pre-mature failure of the shear pin may be due to the cyclic fatigue load because of the radial play (Section 6.2). Hence, a modified design which will facilitate the shear pin to withstand cyclic fatigue loading is made and validated using FEM. The chemical analysis of the failed shear pin has proven that the composition of the shear pin is in accordance with the specifications of the ETG 88 steel and hence, there is no materials defect. Therefore, design alternatives are tried by geometrical modification since the neck region of the shear pin is the critical zone. The design modification was done based on the neck diameter. Figure 6.22 shows the dimensions of the proposed design. The shear pin in accordance with the new design is modeled and analyzed using

130

ANSYS software. Figure 6.23 shows the shear stress distribution for 6.5mm neck diameter and Figure 6.24 depicts von Mises stress distribution for the modified shear pin. The FEA envisages that the shear pin with 6.5 mm neck diameter would not fail since the induced stress is well within the specified stress limit.

Figure 6.22 Proposed shear pin with 6.5mm neck diameter

Figure 6.23 Shear stress distributions in shear pin with 6.5mm neck diameter

131

Figure 6.24 von Mises stress distribution in shear pin with 6.5mm neck diameter

Table 6.5 gives the shear stress and the corresponding von Mises

stress obtained using ANSYS corresponding to various neck diameters

considered for analysis. It is evident from Table 6.5 that the von Mises stress

(139.17 N/mm2) for 6.5mm neck diameter is less than that of the nominal

shear stress (154.0 N/mm2) of 5.0 mm neck diameter (Section 6.4), also the

induced stress is within the safe limit of the material shear strength

(400 N/mm2) i.e. 0.5 times of the material yield strength as specified in

section 6.4.3.

132

Table 6.5 Shear stress and von Mises stress for different neck diameter

Neck Diameter in mm

Shear Stress ( N/mm2)

von Mises Stress (N/mm2)

5 74.495 287.947

5.5 58.214 222.828

6.0 46.46 172.87

6.5 37.678 139.17

7.0 29.896 111.787

6.6 RESULTS AND DISCUSSION

The visual observation and the micro-fractographic examination

confirmed that the fracture was nucleated at the machining marks near the

neck region of the shear pin. The results of the chemical analysis (Table 6.2)

showed that the shear pin material is in accordance with ETG 88

specifications. Failure analysis suggests that the crack propagation is due to

the cyclic fatigue load until a critical crack length was attained. The finite

element analysis considering pure shear loading effect and the various levels

of combined shear and bending loads shows that the maximum stress is found

along the neck region of the shear pin. The shear stress for the pure shear

loading was 74.495 N/mm2 for 5mm neck diameter shear pin with negligible

misalignment (Table 6.4) and it is found to be 171.223 N/mm2 for 0.20 mm

misalignment which exceeds the nominal shear stress (154 N/mm2) for 5mm

neck diameter shear pin. Further, according to maximum shear stress theory

the material shear stress is 400 N/mm2, but the von Mises stress is

668.534 N/mm2 for 0.05mm misalignment which is more than the material

shear stress. It is well understood that the frequent failure of the shear pin

happened due to the combined bending and shear loading which caused

reversed fatigue loading in the shear pin.

133

6.7 CONCLUSION

The major conclusions of the present doctoral work regarding the

failure analysis and optimization of the shear pin dimension are summarized

below:

i. Based on the evidence collected at the site and the

investigation carried out at the laboratory, the failure of the

shear pin is attributed to the growth of the pre-existing fatigue

crack to the critical size scale which triggered the frequent

tripping of the wind turbine during operation with

misalignment.

ii. The metallographic and chemical analyses reveal that the

failure is not related to any defect in the material or with any

abnormal operating conditions like temperature.

iii. Visual observation and the micrographic analysis indicated

that the fracture was nucleated on machining marks along the

surface of the shear pin. The pre-mature and frequent failure

of the shear pin was because of fatigue due to misalignment

(radial play) between the gearbox and the generator assembly

in the nacelle.

iv. Finite element analysis reveals that for pure shear loading

condition, the von Mises stress exceeds the tensile yield

strength of the ETG 88 material. It is because of the reduced

neck diameter and misalignment between the driving and

driven shafts. Due to misalignment and incorrect neck

diameter, fatigue loading was produced with combined

bending and shear load, which causes the pre-mature failure of

the shear pin.