Converting bearings from grease to oil

[Markeco]

I have been experiencing periodic bearing failure with a pressure blower that is used to move 350F gas. The blower is equipped with two Sealmaster NP-23 bearings and the shaft diameter is 1-7/16”. The wheel rotates at 3600rpm and the shaft and bearing temperatures are 250F.The ambient temperature near the blower is 140F. In order to minimize gas migration from the nearby housing shaft seal I have installed a slinger ring between the housing seal & bearing. This has reduced bearing seal failure slightly. The use of “high temp” greases and the purchase of factory modified (high temp seals) Sealmaster NP-23 bearings had little affect.
I have read an article by Russ Hink (SKF, USA) that discusses converting grease lubricated bearings to oil. I am specifically interested in circulating oil because I can install a cooler in the circulation loop. The only concern that I can see is a failure with the recirculation pump. Does anyone have any experience with this and can you offer any pros or cons?

Mark

 

[satchmo]

We have equiped some process lines with oil cooling on bearings. They are 2 7/16 pillow blocks. I wasn't involved in the design, but I'm pretty sure that a constant supply of air is supplied (which helps keep the bearing cool) and oil injected periodically. The pillow block housing had to be drilled out to allow the oil to drain. Bearing failures on this line are almost nil, whereas we will fail bearings occasionally on similar lines with grease.

The main con is that you can get oil on the floor, so housekeeping can become an issue.

 

[unclesyd]

I would go with a good oil mist system. The type oil would be determined by your particular application and the particular equipment used.

Here is one of several good systems on the market.

http://www.lsc.com/flash/index1.html

 

[electricpete]

I have never encountered a rolling-bearing machine with an oil cooler. Considering you have a hot ambient and hot machine, you'd have to consider the effects of differential expansion from temperature gradients carefully. Perhaps you can anticipate this an adjust the internal clearance and fits accordingly. But seems very tricky to me.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[diskullman]

You could retrofit to the old Fafnir syle housings, still available from Ketchie-Houston. These have nice large oil reservoirs built-in, almost a drop-in replacement.

These high speed, high temp applications usually spell out "good customer!" at bearing sales warehouse. These type of bearings don't like either. I would suggest a solid lube if the speed weren't so high. Otherwise I would suggest the type of bearing,lube/cooling used on automotive turbochargers.

Russell Giuliano

 

[Tmoose]

How hot is the shaft and bearing housing?

What have been the bearing "failures?"
If confined exclusively to the bearing close to the hot stuff then the temp would be all the more suspicious. If both fail, then I'd suspect more is going on.

I saw some big spherical roller bearings on a fan in a gypsum plant with oil circulation added. The Gypsum dust snuck past the lab seals pretty easy. The heat flinger/slinger tended to move air in a way that added to the bearing contamination, and the breather on the oil sump let more in. Gypsum is pretty soft, but still not a good bearing lube.

If heat is really the root cause then cooling the housing may reduce bearing internal clearance and bring on big trouble. Cooling the shaft directly can offer some relief.