21160 Governor drive shaft report 14-05

MATERIALS LABORATORY REPORT 14-05

Page No. 1 of 14

Lab No. 21160

Date Nov 14, 2014

Project or customer info: US Coast Guard ship USS Lenthall Prepared

By

Aaron Schroeder

Materials Lab

Co-op

Subject: Failure investigation of fractured governor drive shaft

FME Materials lab Tel: 608.364.8321

701 White Ave, Beloit, WI 53511

www.fairbanksmorse.com Email: [email protected]

Introduction The USS Lenthall experienced an overspeed event shortly after having the governor drive assembly replaced. After the event the engine was restarted and oversped on start up. After the second overspeed event the governor and upper governor drive assemblies were removed for inspection. Upon disassembly it was discovered the governor drive shaft had been broken. The governor had been run for fewer than 6 hours at the time of the failure.

Purpose Of Test It was requested by the customer that the root cause of the failed governor drive shaft be determined. FME lab will perform a full failure analysis to investigate the possible root causes of the failure and determine the most likely root cause/s.

Conclusion The purpose of the failure analysis was to determine the most likely root cause of the governor drive shaft failure. The Lab posits the most likely root cause of the governor drive shaft’s failure to be a combination of factors, including: possible improper shimming, use of an undersized spacer (36D73033), and missing another spacer (36D72045). This combination of factors is anticipated to have allowed for vertical misalignment which caused improper gear mating that lead to the uniformly distributed grinding damage found all around both gear’s teeth. The improper mating between the gear teeth exerted a vertical force on the drive shaft and caused vertical misalignment of the mating gears. The misalignment led to excessive gear wear and caused a bending moment to be applied to the shaft. The rotation of the shaft and the bending load caused the shaft to fail through the mechanism of bending fatigue and ductile overload. The results of the chemical test on the shaft were not within the boundaries specified by AISI 1045 (referred to in drawing #36C73035*00), but the hardness test shows the mechanical properties were still conforming. It is not believed that the lowered Carbon content would decrease the tensile strength of the material enough to be the root cause of this failure.

Component(s) Part Number

Component Drawing Number

Governor Drive Assembly 36A71069

Governor Drive Shaft 36C73035

“Small” Spacer (appears undersized) 36D73033

“Large” Spacer (missing) 36D72045

Bevel Gear 35C72029

http://www.fairbanksmorse.com/


Page No. 2 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Pinion Gear 35C72030

Relevant Specifications 81340 81342 81346 86830

Lab Procedures 1. Upon receipt all available information on governor drive assembly was gathered including blueprints, part background information, assembly, and usage. 2. All parts received by FME Materials Lab were documented by taking photographs. 3. Visual examination of fracture surfaces and other components deemed important to the failure. 4. Assembled parts were disassembled by a colleague in the sub assembly department. 5. Fracture surfaces were taken to Element Materials Technology in New Berlin, WI for scanning electron (SEM) analysis. 6. Abrasive saw was used to section a part of the shaft for chemical and hardness testing. 7. Chemical analysis was performed using a Bruker Q4 Tasman Optical Emissions Spectrometer. 8. Rockwell Hardness tests were taken at the core and along the outer diameter of the shaft using a Wilson Rockwell tester. 9. One of the fracture surfaces was sectioned perpendicularly to the surface using the abrasive saw. The sections were mounted in Bakelite and polished to 1 µm finish. 10. The mounted surface was examined metallographically both unetched and after a 2% Nital etch.

Results

Figure 1, below, shows the governor drive assembly upon receipt at FME Materials Lab.



Page No. 3 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Figure 1: Full assembly upon receipt at FME Materials Lab

Figure 2, below, shows both fracture surfaces of the failed governor drive shaft, one of the damaged gears, and the undersized spacer.



Page No. 4 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





(a) (b)

(c) (d)

Figure 2: Important components of the shaft failure.

a) Fracture surface 1

b) Fracture surface 2 (was later sectioned to be mounted)

c) Damaged gear

d) Undersized spacer



Page No. 5 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





In figure 3, the diameter near the fracture surface was measured using a micrometer and compared to areas with non-effected diameter. The diameter near the fracture surface found to be equal to 0.643”, and away from the fracture to be 0.651”. A reduction of about 0.008” was observed. The values obtained may suggest that some necking have occurred at the fracture surface. Typically, large amount of necking would occur in ductile overload that have experienced large amount of tensile stresses.

Figure 3: Measurements of shaft's diameter at failure showing some deformation.

The fracture surfaces seem to have large amount of mechanical damage which occurred after the failure. Different fracture zones were observed on the fracture surfaces a smooth zone, rough zone, and an elevated zone. See figure 6. The undersized spacer is specified to be .563” wide per the drawing, but was measured to be .469”. The spacer also appears to have significant wear on one of the mating surfaces.

Figure 4: Undersized spacer showing the damaged face.



Page No. 6 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





The gear teeth on both the pinion gear and the bevel gear appear to have significant wear and grinding signs that seem to be consistent on all of the teeth. This wear pattern seems to indicate improper mating that might have resulted in one gear trying to ride over the other.

(a) (b)

(c) (d)

Figure 5: Damaged gears. Both gears are severely worn both at the tips of teeth and on the sloped side surfaces.

a) Overall image of the bevel gear attached directly to the failed shaft

b) Closer image of the bevel gear from (a) showing the excessive wear on teeth and an impacted spot.

c) Overall image of the pinion gear, surrounded by the blue nylon gear.

d) Closer image of the gear in (c) showing excessive wear on the tips of teeth.



Page No. 7 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





After disassembly the fractured samples were taken to Element Material Technology in New Berlin, WI for SEM analysis. Before SEM analysis both fracture surfaces were ultrasonically cleaned in 10% Alkanox for five minutes. Images were taken at 9x on both faces of the fracture to give a full image, shown in figure 6, and at 500x and 5000x at selected locations. Most of the fractured surfaces showed severe mechanical damage, making it difficult to observe all features on the fractured surface. Figure 7 shows an example of the mechanical damage that was present through most of the fracture surface. Figure 8 shows location B area where evidence of dimple rupture, fatigue striations, and micro-voids were observed. Figure 9 shows more evidence of micro-voids and some secondary cracking at location D. Location C is not shown because of mechanical damage. The second fracture surface showed similar results.

Figure 6: The failed surface of the governor drive shaft with locations that were examined at higher magnification

labelled. Location C is not shown because of mechanical damage.



Page No. 8 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Figure 7: Location A at 500x showing a sample of the mechanical damage that was present in many locations

throughout the samples.

Figure 8: Location B at 5000x. Important evidence of failure is labelled. Dimple rupture and micro-voids are usually

indicative of ductile failure, and striations are often seen in fatigue failures.



Page No. 9 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Figure 9: Location D at 1000x. This sample shows secondary cracking and more micro-voids.

Chemical analysis was performed using the Bruker Q4 Tasman Optical Emissions Spectrometer to

determine whether the shaft material was conforming to the specification AISI 1045. Part was sectioned as

shown in Figure 10, below, to provide a clean, flat surface to ensure valid results. The results of the

chemical analysis of the drive shaft are shown in Table 1, below, where the chemistry is compared to AISI

1045 & 1020. The shaft material was determined to be AISI 1020.

Figure 10: Governor drive shaft after sectioning with parts labelled for how they would be tested.



Page No. 10 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Part Name Carbon wt% Manganese wt% Phosphorous wt% Sulfur wt%

Failed Shaft .217 .575 .015 .027

AISI 1045 .43-.50 .60-.90 .04 (max) .05 (max)

AISI 1020 .17-.24 .25-.60 .04 (max) .05 (max) Table 1: Chemical analysis results for failed shaft compared to specifications for AISI 1045 (specified) and 1020 (closest

match to actual) steels. All results are in percent weight.

The hardness of the shaft was tested on the outer diameter and in the core. The resulting hardness was

HRB 72-73, which corresponds to Brinnell Hardness number 114-116. These results were conforming to

the specification for normalized 1045 steel which specifies a maximum value of HB 215.

Figure 11: 2 locations along the fracture surface viewed at 50x before etching. These two surfaces are representative of

the unetched micrographs throughout.

The microstructure of the shaft was examined by sectioning through the fracture surface, mounting, and

polishing the samples for examination using a light microscope. The samples were then etched with 2%

Nital and viewed under 200x magnifications. Images were also taken at 50x before etching, shown in

figure 11, and no abnormal microstructure such as inclusions or defects were observed throughout the

sample. The microstructure is typical of normalized low Carbon steel consisting of mostly ferrite and

some pearlite with mostly equiaxed grains. The microstructure near and at the fractured surface showed

signs of grain flow. The grains near the surface showed severe compression or bending strains, especially

near the outer diameters. There are also signs of micro-voids present at and near the fractured surface and

closer to the OD. The microstructure of the fractured surface near the middle of the shaft did not show

much of deformation, due to the expectation that no bending moment would be occurring at the center of

a shaft. Figure 12, below, shows the core of the shaft and equiaxed grains.



Page No. 11 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Figure 12: Shaft core showing equiaxed grains

Fracture surface images showed compression and some flowing of grains. Figure 13 is an example of

compressed and flowing grains due to the bending. Figure 14 was taken near the center of the cross

section, where bending forces are weakest, and shows minimal deformation. Figure 15 shows signs of

opened micro-voids near the outer diameter opposite of the side showing significant signs of bending and

compression.

Figure 13: Sample surface micrographed at 200x after a 2% Nital etch. The grains show significant deformation,

evidence of bending and compression stresses.



Page No. 12 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Figure 14: Sample surface at 200x after 2% Nital etch near the middle of the sample. The grain structure does not show

significant deformation, because bending moments do not exert force at the center of a shaft.

Figure 15: Sample at 200x after 2% Nital etch. Significant micro-voids are evident.

Discussion The type of fracture surface of the governor drive shaft is determined to be caused by bending fatigue and ductile overload. The SEM images have showed signs of dimple fracture and micro-voids which are indicative of ductile fracture. SEM images have also showed some signs of striation which indicate fatigue failure. The microstructure examination of the fracture surface has also provided evidence of ductile failure exhibited by the flow of grains near and at the fracture surface mainly towards the OD. Microstructure examination also showed signs of micro-voids near the surface which indicate ductile failure. The evidences found indicate that the missing spacer, the undersized spacers, and the possible improper shimming may have allowed for the mating gears to become vertically misaligned



Page No. 13 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





which resulted in improper mating between the gears during service. Misalignment of the gears consequently caused excessive wear on the gear teeth as well as excessive forces between the gear teeth which translated into bending, tensile, and compression stresses at different locations on the cross section of the failed drive shaft. The rotation of the shaft caused cyclic bending, which led to a combination of bending fatigue and ductile overload fracture. The chemical analysis showed the material was not conforming to specification. It is proposed that the steel used was AISI 1020, not AISI 1045. However, the hardness test showed the shaft to have an acceptable hardness (HRB 72-73 ~ Brinnell 114-116), and the decrease in tensile strength has been deemed irrelevant. The SEM analysis revealed that the fracture surface was mechanically damaged at most of the fracture surface. Different areas of the fracture surface showed signs of ductile fracture, micro-voids and dimple rupture, these are indicative of fatigue and ductile overload. The ferrite-pearlite microstructure of the shaft was typical for low carbon steel. Figure 13 shows significant deformation from compressive stresses near the outer diameter. Figure 14 shows the center of the shaft that revealed no visible deformation in the grains which is expected for a bending load. Figure 15 shows the opposite outer diameter of Figure 13, where there are signs of micro-voids and some grain flow which might be indicative of a combination of compressive and tensile stresses.

References Jon Tichenor (Quality supervisor) Jack Blood (Assembly technician) ASM Handbooks Total Materials Key to Metals Database

Statement I, Aaron Schroeder, certify that the information in this report is correct to my knowledge and all testing were performed in conformance with FME lab procedures and requirements.

Certified by:

Aaron Schroeder (Materials lab co-op)



Page No. 14 of 14

Lab No. 21160

Date Nov 14, 2014


By

Aaron Schroeder

Materials Lab

Co-op





Reviewed by:

Mohamed Zreiba (Material science engineer)


Documents

21160 Governor drive shaft report 14-05