Hot and Cold, or How Omaha Steaks Fixed My Drill Press

Things fall apart, the center cannot hold....

Yeats, The Second Coming

My last  job was managing a planing mill and one of my duties was running dry kilns and maintaining a wood fired boiler.   So as you might imagine there are a lot of bearings that run 24 hrs a day, seven days a week.  Over time they would break down and need replacing.  To diagnose bad bearings is sometimes easy; they will rumble or create a vibration when they run.  Other times it's hard to tell, usually they will run hotter than other bearings.  A rule of thumb is if you can hold your hand on the bearing for  5 seconds, it's ok.  If it's too hot to do that, change it.  Because it's easier (and cheaper) to change a bearing than a whole shaft, believe me on that.  So what does this have to do with a drill press? 


You can see the bearing on the right the different parts.  This is a pretty
standard roller bearing with an outer race (the band of steel on the outside)
ball bearings ( usually covered or sealed) and an inner race.  It's the inner
race that I gonna talk about today.  This part is what contacts the shaft, so
this has to be heated so the
bearing can be changed.  Sometimes, especially with smaller bearing, you can simply pull them right off with a puller.  Other times, you
have to heat the inner race, which makes the race expand, thus allowing the bearing to be removed.

A rough analogy is a canning
jar.  If the lid band is heated up and screwed on when hot, it cools and tightens up on jar.  If you try to take it off, it comes off with difficulty.  However, run it under hot water and it'll pop right off.  Same principle applies to bearing.  The trick is to heat just the inner race and not the shaft, because if you heat the race and the shaft, then everything expands, which is pointless.  It's the differential that is the key.    If you simply heat everthing up, you really are not accomplishing anything.

So back to my drill press.  The chuck kept falling off, not all the time but enough to be annoying.  I knew how to fix it, but had no practical way to cool the shaft.  But when I got some frozen steaks packed in dry ice, I used the dry ice to cool the shaft.  Wearing gloves, I packed the dry ice around the shaft as best as I could and wrapped it in a towel.  I then took the chuck and put it in the oven at 225º for about an hour.

Below you can see the frost on the towel.

Then, when the shaft has cooled and wearing gloves, quickly put the hot quill on to the cold shaft, couple taps with a dead blow hammer and viola! fixed.  I just hope I never have to take it back off.

 

Back at my old job, we used to change the bearings on the big planer ( about the size of a pick-up truck) every year or so.  The bearing are 6" O.D. and the shaft diameter ( the size of the inner hole) is 2 7/16".  These are fitted bearings, meaning the hole and the shaft are exactly the same size, thus the only way to remove and replace them is by using the heat differential.  Usually we'd do this in the winter.  By placing the shaft outside overnight when it's -10 or -20, you can skip the dry ice.  Then the next morning we'd heat the bearings up and they would slip right on.  At $800 per bearing, and four per shaft, you really did want to get it right.  Beating them on with a big hammer is not the answer, rather it's simple thermodynamics.  Now I have to change the bearings on my lathe, again.  That's another post.