Failure Analysis and Maintenance of Compressor Bearing Shells

Bearing shells are among the most critical components of compressors and located at the core of the machine. Subjected to heavy alternating loads, 

uneven force distribution and strong impact loads, bearing shells are prone to damage. Common failures include bearing seizure, alloy spalling and cracking, 

bearing shell scoring and severe abrasion. Proper lubrication maintenance, reasonable selection of lubricating oil and adjustment of bearing assembly clearances are effective measures to reduce bearing shell failures. 

Correct diagnosis and maintenance of bearing shell faults are essential to guarantee long-term stable compressor operation.

Not long ago, the main oil pump of a reciprocating coke oven gas compressor at a chemical enterprise in Hunan under our administration displayed low oil pressure. 

The oil pressure remained low even after the auxiliary oil pump started. The unit was shut down for inspection. It was found that partial spalling occurred to the babbitt alloy on the upper bearing shell of the connecting rod big end, 

and several cracks existed on other contact surfaces. Large-area spalling was found on the lower bearing shell, with complete loss of alloy material in the middle section. Nevertheless, no abnormal noise was detected by on-site operators. 

Such hidden faults are difficult to identify without adequate experience.

Fault Analysis
1. Bearing Seizure
Bearing seizure generally occurs when no lubricating oil film is formed or the oil film breaks down between the bearing shell and crankshaft journal due to insufficient oil supply or other factors. Root causes are listed below:
(1) Severe shortage of lubricating oil in the lubrication system

Insufficient lubricating oil leads to rapid temperature rise on the friction surfaces of crankshaft journal and bearing shell, resulting in seizure. Major causes of oil shortage include severe blockage of lubricating oil filter, 

oil pipeline clogging or severe leakage, oil pump damage, fractured pipe joints, or failure to replenish lubricating oil in a timely manner.

(2) Improper assembly clearance between crankshaft journal and bearing shell

The clearance governs the formation of lubricating oil film. If the clearance is too small, oil cannot easily flow into the friction interface to form an oil film. Excessive clearance reduces oil film thickness and fails to fully separate friction surfaces, 

raising seizure risks. Meanwhile, excessive clearance intensifies vibration and impact between journal and bearing shell, triggering oil film rupture.

(3) Grinding repair of crankshaft destroys wear-resistant and fatigue-resistant layers on the journal surface

Crankshaft journals normally undergo elaborate heat treatment to form high-performance wear-resistant and fatigue-resistant surface layers. If the crankshaft is reground after a seizure fault, 

these protective layers will be removed, which will lead to recurrent seizure failures in a short period.

(4) Deterioration of lubricating oil
Impure lubricating oil or oil degraded after long service hinders the formation of effective oil film and eventually causes bearing seizure.
2. Bearing Shell Alloy Spalling and Cracking

Without oil film isolation, direct contact frequently takes place between friction surfaces of crankshaft journal and bearing shell. Micro protrusions generate fatigue cracks under relative motion. 

Lubricating oil penetrates into cracks and creates hydraulic pressure, accelerating crack propagation and causing alloy particles to peel off rapidly. 

Direct surface contact further raises bearing shell temperature and lowers the fatigue strength of the alloy layer, accelerating crack initiation and alloy spalling.

Alloy spalling will increase the fitting clearance between journal and bearing shell, reduce oil pressure and generate abnormal noise.
3. Bearing Shell Scoring
Scoring arises from instantaneous oil starvation or sudden oil film rupture between the bearing shell and journal, characterized by scratch marks on both surfaces. Severe abrasion of bearing shells occurs under repeated intermittent oil shortage.
4. Recurrence of Bearing Shell Alloy Cracking and Spalling

Improper operation of air compressors causes frequent direct metal contact (without oil film separation) between bearing shells and journals. Micro protrusions develop fatigue cracks under interaction. 

Penetrating lubricating oil produces hydraulic force that expands cracks and accelerates detachment of metal particles from the bearing shell surface.

Phenomena and Troubleshooting
During compressor operation, bearing seizure, burning or spalling of the babbitt alloy layer on the connecting rod big end leads to temperature rise, high-temperature ablation and melting of babbitt alloy.
Symptoms of bearing seizure: High bearing temperature, excessive operating current and loud noise inside the compressor.
Solutions: Oil replacement, bearing scraping and fitting, bearing shell replacement.
Common Improper Maintenance Practices
1. Polishing bearing alloy with abrasive sandpaper

During bearing fitting, some maintenance personnel use fine sandpaper to grind bearing alloy when the contact clearance and contact area fail technical requirements. This practice is extremely hazardous. 

Hard abrasive grains detached from sandpaper embed into the soft bearing alloy, triggering severe abrasive wear between bearing shell and journal. This causes premature scoring or spalling, shortens service life and may induce serious accidents.

Correct method: Use turning tools or scrapers to achieve proper fitting clearance and qualified contact area.
2. Checking bearing contact marks with colored powder or chalk

Some technicians apply colored powder or chalk to inspect contact traces and areas between bearing shells and crankshafts. This method is incorrect, as hard particles contained in these materials will cause abrasive wear. In addition, 

chalk cannot be evenly coated, leading to misjudgment and improper operation, and damaging journal and bearing surfaces.

Correct method: Use oil-based pigment such as red lead powder for contact imprint inspection.
3. Adjusting fitting clearance by adding shims on the back of bearing shells

When excessive wear enlarges the clearance between crankshaft journal and bearing shell, some maintenance personnel insert tin foil or thin copper sheets between bearing shells and bearing saddles to adjust clearance. 

This repair method is wrong. It damages the roundness, cylindricity and concentricity of connecting rod bearings relative to the crankshaft and accelerates journal wear.

Correct method: Replace with qualified spare parts to prevent accidents.
4. Single-piece replacement of connecting rod big end bearing shell

In crankshaft bearing maintenance, the upper bearing shell usually suffers mild wear while the lower shell wears heavily. Some technicians only replace the worn lower shell and reuse the original upper shell. 

This increases roundness and cylindricity errors of the crankshaft bearing, destroys concentricity, raises crank rotation resistance and aggravates wear of journals and bearing shells.

Correct method: Bearing shells must be replaced in pairs instead of single upper or lower shell replacement.
5. Randomly adding shims on one side of connecting rod big end bearing cap

To adjust bearing clearance, some maintenance personnel add extra shims only on one side of the connecting rod big end bearing cap. This distorts bearing roundness, 

cylindricity and concentricity between bearing and journal, accelerates wear and may lead to accidents.

Correct method: If clearance is insufficient, add shims of equal thickness or quantity on both sides of the bearing cap simultaneously. Conversely, remove identical shims from both sides until clearance meets specifications.
6. Hand scraping of finished bearing shells for fitting

Finished bearing shells are commonly selected directly during maintenance. However, some technicians scrape the alloy layer to satisfy contact area requirements. This practice is not recommended. 

Manufacturers strictly control dimensional accuracy, geometric tolerances and surface roughness of finished bearing shells, and scraping destroys these parameters. Besides, 

the alloy layer of finished bearings is thin (normally 0.1~0.3 mm). Scraping damages the alloy lining, impairs maintenance quality and shortens service life.

Correct method: Measure and confirm the repair grade of crankshaft main journals and connecting rod journals, then select matching main and connecting rod bearing shells of corresponding repair size.
7. Over-tightening bearing bolts under the idea "tighter is better"

When tightening connecting rod bearing bolts, many technicians tend to fasten bolts excessively to avoid loosening. The tightening torque significantly affects bearing service life. 

Insufficient torque shortens bearing service life and may cause mechanical failures. Excessive torque results in overly small fitting clearance between bearing and journal and potential accidents.

Correct method: Use a torque wrench to tighten bolts progressively, alternately and symmetrically according to specified torque values. Progressive tightening means torque is increased in multiple steps until reaching the standard value, 

instead of one-step tightening.

Summary

For safe and stable compressor operation, timely and proper fault handling as well as routine equipment management are equally important. Prevention is always superior to troubleshooting. 

To ensure safe operation of compressor bearing shells, end users shall comply with the following requirements:

Select lubricating oil of appropriate grade;
Perform regular maintenance on the lubrication system;
Check oil level before compressor startup and replenish oil to standard level when insufficient;
Guarantee formation of effective oil film between bearing shells and journals;
Ensure the contact area between journal and bearing shell reaches over 70% in contact imprint inspection;
Regularly inspect fitting clearance and adjust or repair as required;
Monitor changes in oil pressure and cooling water pressure during compressor operation;
Strengthen compressor operation and maintenance management.