NuclearJ, I like to scrape plain bearings. It's not a case of not being round, it's a case of providing a surface which supports the shaft evenly and provide many small cavities which contain lubricant. I use blue on the shaft and rotate the shaft to transfer the blue to the bearing surface. As far as I know there isn't a machine in existence which can machine a surface with small indents. This is one of the benefits of scraping. I have never used lubricant and have never damaged a bearing or shaft by rotating.(yet). There is no great danger of removing much of the bearing surface, plus it's only the parts of the bearing which would be taking an unequally large part of the load.
Scraping is a skill. Don't try it if you're unsure. I find that young engineers don't understand the reason for scraping and are very nervous of the procedure. I haven't yet scraped a bearing which is shaped (lemon bore etc.), only bearings which are reputedly round.
Best regards,
Joe McCormack

Joe,

Thanks for the info. I do have experience scraping bearings for the last 12 years. It is an old skill and I am a firm believer it is still required in some applications (not all). If my equipment is 35 years old or more installing new bearings isn't the issue, its the banging and moving the housing and end bell supports have been doing over the years that doesn't allow things to line up the way they did when they were new. And let's face it, line boring and machining the end bells and housings and grinding the shaft isn't always the most financially sound decision. You are correct about the new engineers but I have found the same knowledge and attitude with old maintenance supervisors who have been in the same job for 20+ years and haven't seen the process performed correctly.

I am also having a hard time believing that scotchbriting the bearing surface actually does damage that would create a temperature rise. I would be surprised if a thermal camera would pull much of a difference when comparing one with scotchbrite and one without. When it is all said and done the shaft still theoretically runs on oil and not the Babbitt.

quote:

find that young engineers don't understand the reason for scraping and are very nervous of the procedure.

... and old engineers know better than to do this.

For a non-critical application, one can get away with a lot, including this. Only having scraped 'round' bearings suggests that the application may not be too tough.

I've heard stories dating from around 50 years ago about certain experienced mechanics being valued for their touch with bearings and getting a machine to run. Today, precision bearings (not the off-the-shelf type) take care of this. This does not in any way suggest that good craftsman are not required too. Quite the contrary, everything still has to be put right by qualified people.

quote:

Originally posted by William_C._Foiles:

quote:

find that young engineers don't understand the reason for scraping and are very nervous of the procedure.

... and old engineers know better than to do this.



Today, precision bearings (not the off-the-shelf type) take care of this.

Bill,

I have seen very seasoned mechanics install a completely repoured/remachined (precision)bearing into a motor and destroy it in 15 seconds because they didn't know and take the time to scrape and form fit to the shaft.

People think theses are just like rolling element bearings where the outer race and inner race issues are with-in spec and uniform with each other and that isn't the case.

Have you any idea of the geometric parameters involved in a precision bearing? The radius of curvature is often not the same as the installed radius or radii for the bearing surface(s); an elliptical bearing is a simple example of this.

I am not sure what you mean by form fit to the shaft, but bearing surfaces should not have the same radius of curvature as the shaft.

Most bearings are bored with a small amount of shim between the top and bottom halves to form an elliptical shape as well as have a .0015 per shaft diameter inch larger bore. When all said and done for operation there should be a very thin line straight down the middle of the lower half where the shaft contact is equal (80% inline) to allow for equal oil film and temperature distribution.

What normally occurs is a new bearing is installed and the contact pattern is at a low percentage on the bearing surface which leads to localized heating and increased bearing temperature.