Jump to Content

Examination of PT6A-67R Number-1 Bearing and Components

Summary

Technical Analysis Report " Short Bros. SD360-300, VH-SUM

EXECUTIVE SUMMARY

On 13 August 2000, a Shorts SD-360 aircraft (VH-SUM) sustained an in-flight shutdown of the right engine, which was later attributed to the seizure of the gas generator and power sections of the turbine.

Further examination of the engine by the ATSB and Pratt & Whitney Canada showed the seizure to have stemmed from the distress and severe overheating of the engine number one bearing in the compressor section.

Despite the degree of damage to the bearing races, evidence of electrical arcing damage was found on the inner race (non-thrust side) and was subsequently traced back through the accessory gearbox components to the starter-generator input shaft. Intermittent pitting on the starter-generator drive gear and continuous pitting on the driven gear indicated the primary source of electrical potential was within the starter-generator assembly.

Recommendations for safety action stemming from this examination include the further study and testing of the PT6A-67R starter-generator and engine assembly to determine the conditions under which excessive current leakage could result. It was also recommended that the assembly, as installed within VH-SUM be examined to identify the potential influence of other electrical accessory items fitted.

Overview. General construction of the PT6A engine and the relationship between the starter-generator and the number-one bearing.

FACTUAL INFORMATION

Introduction

On 13 August 2000, a Shorts SD360-300 aircraft VH-SUM sustained an in-flight uncommanded feathering of the right propeller. The engine was shut down immediately. Examination of the engine showed the gas generator and power sections of the engine to have seized.

Pratt & Whitney Canada (Australasia) subsequently disassembled the engine. The findings of the teardown examination are presented in Pratt & Whitney report number 25885.

To support the investigation of the occurrence, the ATSB carried out an analysis of the following components from the failed engine.

  • Number-1 bearing and debris. The outer race of the bearing remained within the inlet case (figs. 1 & 2).
  • Gearbox input shaft and roller bearing assemblies (fig. 3).
  • Coupling splined hollow shaft (fig. 3).
  • Starter gear and roller bearing assemblies (fig. 3)

In addition, an unused bearing identical with the engine number-1 bearing was supplied, as well as copies of reports prepared by Pratt & Whitney addressing previous engine failures experienced by the operator.

Component history

The history of the engine was reported as follows (Pratt & Whitney Canada report #25885).

Serial Number: 106223
Time Since New: 6410.9 hrs
Time Since Overhaul: 1500.1 hrs
Cycles Since New: 9704
Cycles Since Overhaul: 2463
Previous Workshop Visit: 5 June 1999 for routine overhaul

Figure 1. Component as received. Figure 2. Component as received. Figure 3. Component as received.

Figures 1, 2 & 3. Components as received.

ANALYSIS

Visual examination

Number-1 Bearing:

Figure 4 shows the general condition of the bearing as recovered from the engine. All surfaces of the bearing as well as the nearby casing showed heavy discolouration and blackening that is typical of extreme overheating. All the balls and the inner race (fig. 5) showed heavy localised wear and metal flow, signalling the sliding contact of the balls against the inner race.

Figure 4. Condition of bearing balls and races.

Figures 4 & 5. Condition of bearing balls and races.

Figure 5. Condition of bearing balls and races.

The effects of thrust loads within the operating engine had produced a bias in the wear on the inner race toward the rear inlet casing. Resulting from this loading and the amount of wear experienced, the inlet case of the turbine had come into contact with the adjacent compressor rotor disc. Closer study under the stereomicroscope showed fine pitting towards the inner edge of the non-thrust inner race (fig. 6), although scoring marks obscured much of this. Damage to the outer race was equally severe, with heavy indentation and bruising of the contact surfaces from the ingress of debris (fig. 7).

Figure 6. Indentation and pitting damage to the inner and outer races.

Figure 7. Indentation and pitting damage to the inner and outer races.pg

Figures 6 & 7. Indentation and pitting damage to the inner and outer races.

The bearing cage had experienced multiple radial cracks and cracks around the circumference, with most associated with galling, scoring and other evidence of metal to metal surface contact (fig. 8).

Figure 8. Contact damage and scoring on the external surface of the bearing cage.

Figure 8. Contact damage and scoring on the external surface of the bearing cage.

Some internal corners of the cage showed shallow fatigue cracks - these had started at areas of heavy surface scoring and metal flow (fig's. 9 & 10).

Figure 9. Bearing cage fracture surfaces showing evidence of fatigue cracking.

Figure 10. Bearing cage fracture surfaces showing evidence of fatigue cracking.

Figures 9 & 10. Bearing cage fracture surfaces showing evidence of fatigue cracking.

When compared with the new item, the extent of the damage to the bearing parts was clear (figs. 11, 12 & 13).

Figure 11. Bearing components compared with new items.

Figure 12. Bearing components compared with new items.

Figure 13. Bearing components compared with new items.

Figures 11, 12 & 13. Bearing components compared with new items.

Associated components:

The coupling gear, starter gear and coupling shaft each showed varying levels of gear surface and spline pitting damage. Mostly, the pitting was restricted to localised spots at the gear teeth pitch-line (figs. 14, 15 & 16), although the starter gear showed uniform damage over the whole contact surface (fig. 17).

Figure 14. Pitting on coupling gear teeth faces.

Figure 15. Pitting on coupling gear teeth faces.

Figures 14 & 15. Pitting on coupling gear teeth faces.

Figure 16. Fine pitting found on starter gear teeth.

Figure 17. Fine pitting found on starter gear teeth.

Figures 16 & 17. Fine pitting found on starter gear teeth.

Studying the overall distribution of pitting on each item revealed the large diameter starter gear showed the most significant damage over two or three teeth in four equally spaced regions around the circumference (fig. 18). The smaller coupling gear which mates with this item showed no suggestion of this intermittent pitting, with damage found uniformly around the entire circumference.

The male and female splined connections showed pitting over both the teeth faces (figs. 19 & 20) and end surfaces (figs. 21 & 22), with the damage appearing similar in all cases.

Figure 18. Starter gear, showing four localised areas of pitting around the diameter.

Figure 18. Starter gear, showing four localised
areas of pitting around the diameter.

Figure 19. Pitting found on coupling gear and shaft spline teeth.

Figure 20. Pitting found on coupling gear and shaft spline teeth.

Figures 19 & 20. Pitting found on coupling gear and shaft spline teeth.

Figure 21. Pitting on coupling gear and shaft end contact faces.

Figure 22. Pitting on coupling gear and shaft end contact faces.

Figures 21 & 22. Pitting on coupling gear and shaft end contact faces.

Electron microscopy

Electron microscope imaging of the pitting identified on the internal bearing race and gear teeth showed many of the pits to have a semicircular form, with a shallow concave profile. Observation at higher magnifications found evidence of localised melting, which presented as small balls of metal within the pit confines. Many of the pits showed a distinctive flattened appearance - typical of the effects, produced by normal rolling surface contact. Figures 23 through to 28 show the typical appearance of the surface pits at various magnifications.

Figure 23. Pitting on coupling gear faces.

Figure 24. Pitting on coupling gear faces.

Figures 23 & 24. Pitting on coupling gear faces.

Figure 25. Globules of remelted metal within coupling gear surface pits - typical of electrical arcing damage.

Figure 25. Globules of remelted metal within coupling gear
surface pits - typical of electrical arcing damage.

Figure 26. Pitting damage as found on non-thrust side of the inner race.

Figure 27. Pitting damage as found on non-thrust side of the inner race.

Figures 26 & 27. Pitting damage as found on non-thrust side of the inner race.

Figure 28. Closer view of bearing race pitting, also showing remelted globules.

Figure 28. Closer view of bearing race pitting,
also showing remelted globules.

Metallographic examination

Sections for microscopy were taken through a pitted area on the less damaged, non-thrust section of the inner race and prepared using conventional methods. Etching with a 2% Nital solution revealed several regions along the contact surfaces that displayed locally transformed, untempered martensite associated with the pitting and extending to around 60mm in depth (figures 29 & 30). The general microstructure of the bearing race was fine, lightly tempered martensite with areas of globular massive carbide randomly throughout (figure 31).

Figure 29. Transformed regions surrounding pitting on the inner race (X80).

Figure 29. Transformed regions surrounding pitting on the inner race (X80).

Figure 30. Closer view of a transformed region (X200).

Figure 30. Closer view of a transformed region (X200).

Figure 31. General tempered martensite and carbide bearing race microstructure.

Figure 31. General tempered martensite and carbide bearing race microstructure.

Hardness tests

The parent metal hardness, as measured away from the pitted surface was around 804 - 810 HV30.

CONCLUSION

Material and manufacture

The number-1 bearing inner race (non-thrust section) showed a microstructure and hardness level that was acceptable for the item and contained no obvious deficiencies that could have affected the bearing's service performance.

Engine failure

The breakdown of the PT6A-67R engine (S/N 106223) as installed within the aircraft VH-SUM was attributed to the failure of the number-1 turbine bearing. The bearing had experienced severe frictional damage and overheating, stemming from the seizure of the balls and the sliding contact against the inner race surfaces. Because the bearing carried a degree of rearward compressor axial thrust loading, the front inner race experienced most of the severe wear and scoring, leaving the rear race comparatively undamaged and able to be closely studied.

Development of the breakdown

Electron microscopy of the non-thrust inner race showed evidence of pitting that resembled damage produced by electrical arcing. Subsequent metallographic examination verified the damage; revealing localised thermally affected regions associated with the surface pits. In the presence of such physical and metallurgical discontinuities, rolling contact fatigue cracking developed and larger spalled areas formed over the contact surfaces. Increases in friction stemming from the spalling and the generation of metallic debris would have raised the vibration and temperature levels, eventually resulting in the thermal runaway of the bearing and the seizure as experienced. Such findings concur with the results of previous examinations conducted on PT6A-67R engines by Pratt & Whitney Canada.

Source of the electrical current

Several other components in the accessory gear train showed electrical arcing effects. The damage was found chiefly around the contact points between the gear teeth. The large diameter starter gear showed four evenly spaced regions around the circumference where the pitting was most severe. In contrast, the smaller mating coupling gear showed uniform pitting around the diameter. This observation led to three direct conclusions -

  • The electric current flowed from starter gear, through to the coupling gear and into the bearing via the turbine shaft and inner races.
  • The electric current was steady or continual and not a transient surge.
  • The current was of an alternating or pulsed nature at a frequency equal to four times the rotational speed of the starter gear.

From the general design of the engine, it appeared that the most likely source of the current was from the starter-generator unit, as the unit coupled directly to the large diameter starter gear. Armature leakage current from the unit would have conducted via the frictional coupling; through to the starter gear and into the bearing via the coupling gear, shaft and turbine shaft.

Publication date: 13 August 2000
 
Share this page Comment
Last update 23 November 2010