why scrape sleeve bearings? 3

[electricpete]

As has been discussed before on this forum, typical practice for some folks after replacing a bearing is as follows:

Do a blue check or a dry roll check to estimate the contact pattern between the shaft and the babbit.

Scrape to get the desired contact pattern (> 80%).

What is the purpose of this procedure?

To my way of thinking, the cylindrical geometry of the sleeve should be a natural result of manufacture. The diameter of bearing should be a little larger than shaft to develop a wedge. The clearance can be checked separately by plastigage. Also important to note, the position where the shaft rests during blue check (bottom dead center) is much different than where it will sit while running.

So... I don't get what it is that we are trying to gain through this blue-check and scraping procedure.


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[unclesyd]

I never seen a precision or OEM babbitt bearing hand scraped. In fact I think that any scrapping is prohibited for such bearings at our site. As you are interested in large motors we buy all bearings as replacement parts.

Where we poured a split bearing it is fit by the millwrights or the company that poured the bearing by bluing and scraping. Bluing is used as plastigage can't give the whole picture.

 

[JJPellin]

I rarely work with large motors. But for other rotating machinery (pumps, compressors, turbines, gearboxes), scraping is not usually needed or desirable. But, proper contact must be achieved somehow. The shaft is going to rise up on a film of oil (wedge). This oil wedge must be able to support the weight of the rotor plus any dynamic forces during operation. The ability of an oil wedge to support a load is specified in PSI. It is dependent on area. The load that an oil wedge can support is dependant on the oil properties, shaft speed and the thickness of the wedge. For the most part, the loads are fixed, the oil properties are fixed and the shaft speed is fixed. I need to control the amount of surface area where the oil wedge is the optimal thickness to hold that load. If the contact was very poor and only contacting along one edge of the bearing bore, the same force (pounds)would be distributed over a very small area(square inches), producing a very high pressure loading (pounds per square inch). If the PSI is too great, the oil wedge will break down and the shaft will contact the bearing.

With that said, it is usually preferable to buy a precisely machined bearing and mount it in a precisely machined housing. If the contact is not good enough, we will usually reposition the bearing housing to get better contact. With a motor, this may not be possible since you also have to maintain the rotor to stator concentricity (air gap). And my limited experience with motors is that they tend to be less precisely machined and less robust by construction. Scraping may be the only alternative that achieves the necessary contact area.


Johnny Pellin

 

[Newmanite]

scraping bearings arent usually done anymore in this throw away and technologically advanced world.It was done to remove high spots that had the potential to score a journal,and promote excess wear.Pretty much any irregularity from the manufacturer when it came to plain bearing scraping was reduced when scraping (or fishscaling)was done.This is an old art which has died a natural death as manufacturers can achieve much higher quality than what was before.(Funnily enough I still have my tools)

 

[unclesyd]

Anecdotal:

I still have all my grandfather's and father's tools for bearing work. There are all manner of "spoons" for scraping and what I call ladles, also called spoons, for pouring bearings. There are also numerous burnishing tools, radius tools, and oil groove cutting tools.

I've gotten to use the smaller ones numerous times. These tools got a workout during WWII in the US Steel machine shop fitting bearings to shafts. My father ran the horizontal press that put bronze bearings on ship shafts and if there were no shafts in the shop they worked on bearings. The majority of the Liberty ship shafts came through this shop.

These tools are good conversation pieces.

 

[Tmoose]

I'm working for an outfit involved in many phases of power generation, especially coal burning.
Turning many tons of coal per hour into a precise, dry fine dust requires a pulverizer like this.

http://www.babcockpower.com/images/stories/firing_mps1.gif

Down at the bottom is a gear box with a thrust bearing that must handle TONS of load from the crushing process.
Tilting pad thrust bearings are a popular design.

Despite careful manufacturing a blue check is often used at assembly to confirm each pad is sharing the load correctly. The penalty can be thrust bearing life measured in minutes.

 

[stgrme]

Scraping of tilting pad journal bearings is probably still done. The pads are generally fitted to a mandrel, which is made to a diameter equal to the shaft diameter plus twice the radial clearance. Hand scraping is done at the job site since fixturing for securing the pads for machining is probably only availalbe at a bearing shop (manufacturer).