Case Study of Crankshaft Bearing Failure of a Marine Engine
History of Failure
The shift Engineer had observed Heavy smoke from the Breather line followed by obnoxious noise from the Engine. He immediately stopped the Engine and opened up the Crankcase doors for inspection. Upon opening up the doors following damages have been observed.
1) Con rod bearing # 6 was found rotated in the housing.
2) Flow of White metal was seen from the Conrod big-end collars.
Unit # 6 was pulled out from the position and further inspection revealed the following.
a) Both the bearing shell halves were found rotated in the housing.
b) Bearing metal up to the steel backing was found rolled out and thinned up.
c) White metal has got stitched with the crank pin.
d) Conrod big end serrations were found damaged.
e) Symptoms of Big end bore overheating have been observed.
f) Small end bush has also suffered metal flaking in 1” area.
g) Both the Conrod bolts have been found overheated and suspected to be elongated.
h) Cylinder Liner was pulled out and vertical seizure marks have been observed
NDT test report
To investigate the damaged/affected area on Crankshaft NDT for superficial crakes, internal cracks and Hardness affected part was subjected to NDT test and the following results were recorded.
a) DP test- NRD
b) MPT- NRD
Opinion & Adv:-
Jobs to be carried out
a) White metal to be removed from the crankpin and to be polished properly.
b) Crankpin is to be subjected to MPT, UST & Hardness test
c) Crank deflection is to be checked.
d) Neighbourhood unit to be inspected for any damage.
e) Crankpin is to be checked for taper and ovality
Complete Report
I was contacted by the client to attend to this Engine that had suffered bearing damage from overheating on its MAN/B&WL23/30 H engine.
Inspection revealed a Conrod enlarged bore/ cam wear of this crankshaft caused an overheating of a crankpin bearing which resulted in deep scratches and heat damaged areas on the crankpin surfaces, distortion of CR big-end bore, cracks in the journal surface as well as damaged fillet radii. Our technicians performed local grinding to determine the depth of the cracks and hardness testing to determine whether or not the hardness levels were acceptable.
The first task was to determine the extent of the damage to the crankpins and to the main journals. We calibrated and tested the following:
1) Maximum and minimum diameters taper and wear of damaged crankpin.
2) Connecting rod bores for dimensions and wear.
3) The run out of adjacent main journals to check for heat-induced bents in the crankshaft.
4) Extent of cam wears in each of the crankpins.
5) Complete nondestructive testing using magnetic particles and ultrasonic for heat cracks both on the surface and the sub-surface of the damaged crankpin.
Measured Hardness readings are produced below.
Recommended Hardness Value= 225 to 265 BHN.
Comprehensive testing and profiling of hard spots and bands on the damaged crankpin.
7) Conditions of the existing bearings.
The crankpin was finish machined to a diameter of (- 1.0 mm) well within maker limits.
After our tests, we decided
) Crankpin # 6 must be ground 1mm undersize to remove hard spots and heat cracks.
c) All main journals must be inspected & polished to remove surface scratches.
d) The Crankshaft was bent in the way of main journals # 6 and 7 and had to be straightened.
These findings were beneficial as they provided the basis for determining the re-usability of significant components like the Crankshaft and the period required for the entire repair.
Knowing this, the service team could and did schedule other maintenance that Hass was carried out at the same time.
In addition, this information allowed us to identify the spares, well ahead of time correctly, that would be required for reassembly and re-commissioning of the engine.
The Crankshaft had suffered a heat-induced bent in the way of main journals # 6 and #7. The solution lies in stress relief, and this was achieved by using a peening hammer on the heat-affected crankpin # 6. After this stage of the repair, the run-out of the Crankshaft was restored to 0.025mm, which was precisely at Builder’s Specifications.
Crankpins #6 was ground to the specification sizes. Work was carried out on 16 hours basis using one grinding turbine to speed up the repairs.
While grinding was in progress, removed the cylinder liners, Servicing of Cylinder heads was performed.
In house quality control system requires that Monitored these grinding repairs daily with the following checks:
1) All micrometers were calibrated using “Standard” bars.
2) Magnetic Particle Inspection technique was used to check on the presence and conditions of heat cracks.
3) Hardness testing using handheld portable electronic testers to determine the degree of existing hardness and the sizes of affected areas.
Grounded The fillets adjacent to the bearing landing surfaces achieve a smooth transition to the webs and provide a good oil flow for the cooling lubrication oil.
Finally, the crankpin was polished with an oscillating superfinishing machine to obtain a finish of 6 micro-inches.
Inspected all the main journals, and scratches were polished.
After careful cleaning, the engine was rebuilt using a new bearing with 1mm downsize. Tested the engine and opened the bearing d to determine the contact pattern.
TOLERANCE ACHIEVED
1. The crankshaft was straightened to 0.03mm.
2. The ovality and taper of the reground crankpins was a maximum of 0.03mm
3. The parallelism of the reground crankpins was held to 0.05mm
4. Restored the hardness of the reground crankpin to 250 Hardness Brinell except at one spot o 500 Hardness Brinell.
5. Removed all heat cracks.
CRANKSHAFT ANNEALING AND MACHINING REPAIRS
· Machining of new fillet radii
· Magnaflux crack testing and Hardness testing
· Local grinding to determine the crack depth
· Rough machining to remove cracks before heat treatment
· Annealing of crankpin journal to remove hardness
· Finish machining to (– 1.00 mm undersize)
TIMING
I, along with my Service Manager, arrived at the site within 2 hours of the accident.
Inspection and complete testing and calibration took one day.
Repair work continued on a 24 hours basis.
Grinding of #6 crankpin, including fillet regrind and superfinishing, took three days. Superfinishing /Polishing of each main journal & crankpins took us 4 hours each.
SPARE PARTS CONSUMED
All the pistons were dismounted and dismantled. After cleaning and checking, we have observed that the following small end Bushes were damaged.
Cylinder unit no.1 Con Rod, IMO — 0802, 00.07.13,
Cylinder unit no.2 Con Rod, IMO — 0802, 00.07.13,
Con rod was controlled measured and readings recorded are as under.
One no. connecting rod replaced in the Cylinder Unit no 6 with a new one.
Clearance in the small end & piston pin
RESULTS
We salvaged the crankshaft within maker limits. Without the combined grinding and annealing process, this crankshaft would have been condemned and required costly replacement.
Author Hopes that the case study will be useful to the readers especially to marine Engineers