Too poor Antifriction Bearings (AFB) lives in general & delay plant restarts production loss from Impossible to remove Antifriction bearings.
| Article Type: | Root Cause Analysis (RCA) |
| Category: | Mechanical |
| Equipment Type: | Miscellaneous Rotating Machine Problems |
| Author: | S. Raghava Chari |
Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.
Antifriction bearings (AFB) Problems
The author gathered from several daily reports several Antifriction Bearings (AFB) causing long plant outages, excessive maintenance efforts problems plaguing the plant for almost a decade as under:
- Most AFBs’ Shock Pulse Meter readings reach the danger red zone within few months service, though designers select AFBs for 7 to 25 years 24x7x365 runs
- AFBs & coupling halves were so stubborn that crew failed removing after many shifts of permitted and
- unpermitted violent efforts; consequent, components final flame cutting, its ill effects of injured shafts, extensive shaft repairs and even making new shafts etc. extended plant downtimes, curtailed production and boosted maintenance expenses to unacceptable levels,
Author Root Cause Analysis
The author’s long time not used rusty AFB knowledge refreshing studies tell that:
Designers select AFBs to serve 50, 000 (≈6.5 years} to 200,000 hours (≈27 years} called the L10 lives depending on machine criticality.
≈50% of the selected bearings serve even five times longer. In short most AFBs outlast the equipment.
Several German made 10‑year serving gearboxes had not been dissembled as the top cover opening showed the gears in good condition, noise and vibration levels near same as at start and the oil showed no significant iron content increase.
AFB makers prescribe the below given shaft and coupling bore dia machining tolerances are must to realize item 1 lives:
- shaft dia tolerance where the AFB fits: k7
- Coupling bore to shaft dia tolerances H7/m6
Keeping the information in mind, the author summarizes the plant faced AFB problems, their reasons and AFB design lives realizing remedies as under:
Removal difficulties and final flame cutting ill effects
The reasons are:
- Machinists not educated on AFB fit up class arbitrarily machined the shaft dia +50 microns (0.002”) the measured AFB bore dia
- This is way too high from the catalogs specified +1/8 microns / mm shaft dia for AFBs and +1/7 for coupling halves (CH). For a 50 mm bore AFB shaft dia shall be 50+7 microns
- Too tightly fitted bearings inner race expand and reduce the designed in clearances. The bearings run hot and fail prematurely
- Too loose AFBs on shaft slip on the shaft, injure the shaft, over heat and fail prematurely
- The above are the general reasons for stubborn to remove AFBs & CHs and too poor AFB lives.
How could the crew install AFBs on oversized shafts?
Easy; the crews flame over heated AFBs without measuring the metal temp. They might have heated it even 250 to 300o C.
Such overheated bearings easily slip on oversized room temp shafts. On the other hand, while heating the AFBs for removal, the shaft also heats up.
Hence, removals were mostly impossible, and flame cutting was the only way.
Why even correctly fitted easily removable bearings last 2-3 months only?
One reason could be the spare bearings purchase requisition raising engineers not including AFBs suffix and prefix numbers either due to ignorance or overlooking.
Suffix and Prefix are as important as basic numbers. Unfortunates due to space non-availability most AFBs come with basic No. only punched.
Hence, never buy bearings from numbers given on them; consult the equipment catalog and order by furnishing basic and suffixes in the order. Next problem is an apt example.
He found that all including the persons recognized, as ‘AFB experts’ were unaware of this basic information.
The author got the shaft tolerances table painted on the workshop wall, impressed the above table information on all AFB handlers in small batch classes, and instructed them to follow it without fail.
Author’s efforts Benefits
AFBs lasted better than catalog lives, AFB consumption reduced 90%, and plant availabilities increased 10%.
Author: S. Raghava Chari
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