Engineering consulting services for the design, analysis and testing of reciprocating

internal combustion engines and engine systems.

  • office: (858) 456-3511
  • fax: (858) 456-3512

Case Studies

Substantial Collateral damage was sustained by the engine crankcase.
  • Project: Failure Analysis of a Natural Gas Engine Connecting Rod
  • Client: Hartford Steam Boiler

Approximately 480 operating hours after a routine major overhaul, a spark-ignited, natural gas fueled engine suffered a catastrophic failure.  The damage was so extensive that neither the owner nor the repair company was able to identify the probable cause or the origin of the failure.

The owner`s insurer hired Vronay Engineering Services to conduct a joint inspection with all stakeholders to determine the most probable cause of failure and to assist with determining whether it was more cost-effective to repair or to replace the engine.  Vronay was also tasked with managing and overseeing the re-commissioning the cogeneration system as the insurer desired to return the cogeneration plant to operational status as quickly as possible prior to the onset of their peak season.

Vronay coordinated a joint engine disassembly with all stakeholders, including representatives from the repair firm whom had completed the last major overhaul, their insurer, the owner and the owner’s insurer and a consulting firm hired to investigate the failure on behalf of the repair company’s insurer.

During the engine disassembly process, it became clear that the failure had initiated with the #6 Left bank connecting rod, which had separated from the crankshaft.  The collateral damage caused by the loose connecting rod included damage to the piston, cylinder head, crankshaft pin and structural damage to the cylinder block and crankcase.  Further inspection discovered that the two bolts securing the connecting rod bearing cap to the connecting rod had both failed; however, their failure modes were very different.

After reaching a consensus with the stakeholders on a course of action, the owner shipped both the non-failed connecting rod assembly and the failed rod assembly to Vronay for further analysis. 

 Subsequent analysis of the two bolts found that the outboard bolt (bolt #1) had failed in high-cycle fatigue while the inboard bolt (bolt #2) had failed as a result of ductile rupture. Based on this observation, bolt #2 appeared to have failed because of the loss of the clamping load provided by bolt #1. 

The fracture surface of bolt #1, clearly showed evidence of a failure in reversed bending. What was also of interest was the overall pristine condition of the bolt. This seemed to indicate that the bolt failed in fatigue, broke into two pieces and simply, “dropped” out of the connecting rod prior to separation of the connecting rod bearing cap and the subsequent catastrophic failure.

Calculations and analysis by Vronay estimated that if the technicians inadequately tightened the bolt, it would failure in fatigue after less than 10 million cycles. It appeared to the investigators that the root cause of failure was improper tightening of the bolt during the last major overhaul.

However, discussions with the maintenance personnel who assembled the engine found that due to the level of experience and their excellent record, such a scenario seemed improbable. Therefore, Vronay performed additional analysis to ensure the bolt pre-load was not lost sometime after assembly. In addition, it was necessary to rule out material or design defect of the bolt and connecting rod.  

 
At this juncture, the following alternative root causes of failure were contemplated:
  1. The bolt may have been improperly installed – over tightened and cracked.

  2. The bolt may have worked its way loose or there could have been an obstruction at the rod spilt line (e.g. misaligned dowel pin).

  3. The blind hole into which the bolt was installed may have been drilled to shallow or may have had an obstruction in the bottom of the hole that prevented the bolt from clamping the bearing cap to the connecting rod.

  4. The bolt may have been defective suffering from inadequate strength.

  5. There may have been one or more metallurgical defects – e.g. thread root cracks, improper thread hardness profile, or improper heat treatment of the bolt.

  6. The bolt may have been re-used from an earlier installation such that it had pre-existing damage, i.e. yielded material in the way of the thread roots. Note the engine manufacturer requires replacement of these bolts after each disassembly.
Metallurgical analysis of the failed bolt, and comparison with several other new bolts and the other ruptured bolt from the failed connecting rod, ruled out of these other possible failure modes, leaving looseness as the remaining candidate failure mode.
 
Examination of the fit of the threads of the #1 bolt and the #2 bolt, along with similar analysis of both bolts from a non-failed connecting rod from the same engine discovered that the thread fit of the #1 bolt, which failed in fatigue, was loose. On further microscopic examination and detailed measurements of the internal and external thread dimensions, Vronay discovered that the apparent loose thread fit was evidence that the mechanics failed to tighten the bolt significantly.
 
As shown in figure 5. the non-failed bolts exhibited full contact on the thread faces, a result of the threads yielding during the high installation torque. The specification effectively specifies tightening the bolt to 90% of its proof strength, yielding the threads to ensure tightness. In contrast, the thread fit of the failed bolt shown in figure 6, shows large gaps between the external threads of the bolt and the internal threads of the connecting rod, indicating the bolt was never torqued to specifications.
 

 

Figure 5:  Full Contact On Thread Faces of Non-Failed Bolts

 

Figure 6:  Lack of Thread Face Contact On Failed Bolt

 

Epilog

 Vronay Engineering Services demonstrated that not only was the most probably cause of failure an improperly torqued bolt, but through judicious direction of laboratory analysis was able to provide unambiguous proof that the bolt had in fact never been significantly tightened.