Sealed Bearings: What are the options?

May 22, 2020 Uwe Sund

Too much dirt in the bearing?

It’s common knowledge that most plain bearings are much less sensitive to dirt than greased ball bearings but no matter the bearing type, too much dirt is to much!

Especially within applications in the field of agricultural and construction machinery, we can assume that the “right grain size” will find its way into the bearing. The results of this are predictable: damage to the shaft and bearings and reductions in system service life.

What are the possible solutions to seal off the dirt?

Of course, the classic remedy is lubrication – according to the principle “a lot helps a lot”. The grease gushes out at the sides, virtually forming a grease ring seal. This can be quite effective. However, this method is only effective as long as it is also heavily re-lubricated. Therefore, the solution is maintenance-intensive making it not very lean, let alone clean. Often the end user has many machines to service and regular maintenance may not happen resulting in the “too much dirt in the bearing” scenario previously mentioned.

Otherwise, of course, all the usual suspects are considered.

O-rings:
Advantage: Cost-effective, also very effective depending on the design of the bearing point.
Disadvantage: The tolerances must be relatively tight.

Lip seals:
Advantage: Cost-effective – depending on the version.
Disadvantage: Rather sensitive, should be slightly greased in order to work, quite tight tolerances between bearing and shaft required. Lip seals are designed to only be installed in the inner diameter of the bearing. So, they are effective axially but struggle radially.

Possible alternative? What about felt?

Felt isn’t just for funny looking hats, shoes, pool tables, and tennis balls anymore! Actually, there are many companies that deal exclusively with technical felt. The main uses are as insulating material (e.g., automotive) and in filtration equipment and there are a variety of qualities. There is also a history of felt being used as a seal in sealed bearings, even in crankshaft bearing in Volvo and Borgward engines. If you Google “felt seal” and look for pictures, you will get many results from the “Spare parts for vintage cars” area.

All over the world I regularly find felt as sealing material in agricultural machinery. These are generally considered antiquated designs but the material also repeatedly appears as a problem solver in present-day machines.

Therefore, I decided to carry out an experiment in our factory test facility. For this test, we allowed a chassis part to run in the sand. The test was designed so that the sand could only enter the bearing radially.

In order to introduce the felt ring (thickness 2 mm) in the application, we made a 1 mm deep recess in the bearing. To ensure that the ring stayed in place during assembly, some superglue was used (felt with adhesive backing is also available).

 

 

 

 

 

Test rig: Sealed bearing runs completely in the sand

“New” sealed bearing before the test
 

Sealed bearing (without felt ring) and shaft after 65,000 cycles
 

Sealed bearing (wit felt ring) and shaft after 90,000 cycles

The pictures speak for themselves, the bearing without the felt ring is extremely worn out in the inner diameter and on the flange.

Which specifications of felt make this possible?

Felt is highly compressible without much resistance. Large tolerances and misalignments can be compensated, embedded dirt is pressed against the shaft or the holding fixture only with very little force. What takes place then is rather a polishing effect instead of the counter partner getting “scratched”.

Felt acts like a filter. The dirt is filtered until the filter is congested. If the filter is tight dirt will not get in any further.

With an initial lubrication, felt keeps the grease in the bearing. Thus, for example, a maintenance-free bearing can be used instead of a bearing point that requires regular maintenance. And, the dirt stays outside. Felt also has a low coefficient of friction. About 0.22 (dry) against steel and felt can be used over a very broad temperature range.

To speak with an expert about which plain bearings might be best for your application, please do not hesitate to contact Nicole Lang, iglide® Product Manager. 

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