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One thing to remember about double shielded sealed for life bearings is just what is life? Most machines are designed with a service life of about 11 years. We have found that doubled shielded bearings had a life of around 2 years. Kinda kicks reliability in the butt!! Oil is the best lubricant, but poses problems with leaks and contamination. Grease will run hotter and often trap contaminants. One must look at the machine and consider which will best accomplish what you needs so as the machine will run for a good long time.

Gary B
Thank you Gary and Stephen for your experience.

There is a rule wich requires calculate the D*N value of the bearing, where:
D=bearing bore in mm
N=shaft speed in rpm

For D*N below 80000(until 108000)are preferred sealed, lifetime lubricated bearings

For D*N between 80000 to 300000 are preferred regreasable bearings

For D*N greater than 300000 is recommended oil lubrication for bearings.

For example :
1.-For D*N=108000 (life time lubricated bearing):
When N(rpm) is 3600 D(mm)is 30
When N(rpm) is 1800 D(mm)is 60

2.-For D*N=300000 (regreasable bearing)
When N(rpm)is 3600 D(mm) is 83
When N(rpm) is 1800 D(mm) is 167

Have you seen some application out of this rule?
I have many applications outisde of those rules.

Lots of DN below 80k which have double-shielded bearings. That's different than sealed.

Lots of DN in the 80k - 300,000 are double-shielded. Do you consider them relubricatable?

A few DN above 300k that are still grease lubricated with all three configurations open, single-shielded, and double-shielded.

I had an article which spelled out similar guidelines as yours with a few different twists. I think the author was Heinz Bloch. I've been looking around for it but can't find it.
I think you have to evaluate the application.

The factors that push you towards shielded or sealed bearings:
1 - Small or slow speed bearings (low d*n as we have talked about)
2 - infrequent operation.
3 - bad environment with lots of potential for contamination.
4 - difficult to access for maintenance.

Maybe all these were talked about before except #2. Nevertheless #2 also makes a big difference because infrequent operation means the grease lasts a lot longer.

We have a lot of crane bearings. Very infrequent operation and for only a few minutes at a time. Difficult to get to. Outdoor environment. Ideal for shielded/sealed bearings IMO.

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