Bearings -- Should they spin or not??

[bitsnpieces1]

  I was reading the topic about disassembling an ST head and wanted to make a few comments about bearings.  In my 25+ years of wastewater work, I've seen lots of bearings replaced.  We had professionals of all types teaching us at seminars about how to work with them.  SO, I am familiar with how to handle them. 
  Now just a few things said in a general look for way. 
1) Whichever part of your piece of equipment truns should have the bearing race a heavy press fit.  Heat then slip on.
2) Whichever part of your equipment sits still while it's running should have the bearing race be a slip fit. 
  The reason is: Bearings are designed to slowly rotate when in use.  Think of it this way.  The weight of things sit on a small spot at the bottom of the bearing, if the races never turn all the weight will cause enormous wear on that spot and nowhere else.  The material of the races is designed to 'squish' just a litle bit under pressure.  So, in the case of an ST head, if the end bell side race is a slip fit, the 'squish' will cause that race to VERY SLOWLY rotate, thereby exposing a new surface at the bottom as it runs.  This will extend the life of the bearing tremendously. 
  The race sitting on the rotating shaft needs to be a very tight fit so that it won't rotate.  Here the situation is that when the rotor starts to spin, and when it changes the rate of spin, the race tends to lag behind.  This tends to scrape  or gall the surface of the shaft itself. 
  SO, The rotating piece gets a tight fit, the stationary piece gets a slip fit (NOT a loose fit), and the bearing and equipment lasts much longer. 
  3) Bearings are made of pretty hard metal.  Never strike them with a hard metal object.  This will cause them to crack.  SOFT metal like brass can be used with much better luck.  You may not see the crack that you create, but, it will show up later as a failed bearing.  NOW, this isn't to say that some highly skilled, highly motivated people can't use a get-a-round and have it be succesful, just not the run of the mill people like most of us.  My Father used to remove old bearings from 6" & 8"  shafts by using an acetylene torch and ice water.  Heat a spot on the outer race to red hot with a torch, chill with ice (or cold) water, repeat the operation at 180 degrees. Then strike each spot with a heavy hammer,  The race will shatter at each spot and the race will fall off.  Use a pair of pliers (or other item) to remove the balls and cage,  then repeate the heating-chilling-striking procedure on the inner race.   This will get off the old bearing without harming the shaft at all.  Some people (including my Father occasionally) use a cutting torch to cut the races in half, BUT, you probably will cut into the shaft material itself while doing it.  This means hundreds of dollars to machine down the shaft and fit a new specially machined sleeve if you are VERY carefull and lucky, OR,  several thousands of dollars for a new shaft and its' installation if you're not VERY careful and lucky.  City governments frown on this kind of expensive wastage. 
  Since it's our own personal possesion that we're working on here, it would be nice to use the best technique you can manage.  Any other comments are welcome here. 
  If your bearings are loose where they should be tight, it can be fixed.  If it's tight where it should be loose that's even easier to fix. 

[okiezeke]

Bits N Pieces,
All good advice.  When I was much younger I tried to remove a bearing race with a hammer and chisel.  I still have a piece of the race imbedded in my hand.  Someday I'll try your method with the torch,,Carefully.
Zeke

[mobile_bob]

Bits :

sound advice on the fit of races, the stationary race needs to creep and the rotating race can be press fit.

as far as driving on races it is amazing the difference in cultures and how they look at this, the japanese recommend hardened steel punches to drive races in, while we use soft steel or brass drifts.

heating needs to be controlled however, i would not recommend over 250 degrees or so, because of seal damage and melting out the grease.
i have never had to heat a brg or race in 32 years to install

the trick to cutting off the race is to cut it perpendicular to the shaft and not inline with the shaft as would intuition might lead one.

if you cut perpendicular to the shaft, the shaft will stay cool enough to have the flame curve around it and not scar the shaft, if you cut in line with the shaft
however you don't have that protection.

have cut hundreds of brgs off of shafts over the years, although i would not on a generator shaft, don't want a flame to lick the windings.

overall good advice though

one other point a stationary race in an aluminum housing is a tight fit, because as the aluminum warms it expands more than the brg and allows it to creep
as designed.

bob g

[Doug]

There are tables that a machine shop uses for a guide when repairing shafts and housings. I can't find my tables to post but EASA provides memeber shops with all this sort of technical data ....

I don't know to what amount an outer race is suposed to rotate, I've never heard it explained that way or been told that but it makes sence. I believe we are talking about cleanrences no more than .001 limmit and as I recall generaly less on a rebuild.

Doug

[bitsnpieces1]

  The rotation is slow.  Our equipment ran 24/7/365 and rotations of once per week or once per month would have been OK.  The key point is that the rotation is continuous and at a steady rate to equalize wear over the entire surface.  And Bob is right about a tighter fit for aluminum since it expands more when heated.  Also, we had some large 1200cfm centrifugal blowers that ran cool at the intake end and say around 300-400 degrees F at the exhaust end, so the exhaust end housing needed to be much tighter. 
  The basic point was to let people know if the bearing fits are right on their ST head, tight where they should be tight (on the rotating shaft) and a light press fit (make sure housing is clean, a tiny bit of lube and some force from your hands) in the end bells. 

[ZackaryMac]

Just to add a little fun to the equation, Rotax have their crankshaft bearings (ball bearings) press-fit to the crank, and pinned in the aluminum crankcase so they wouldn't turn at all. Ever. These are engines used in snowmobiles and watercraft. Though not high-speed, they still can turn to 8500 rpm.

On the other hand, with over 20 years experience with Rotax under my belt, I've seen the engines usually fail for one reason or another, 99% of the time due to owner neglect or ignorance. So the pinned bearings didn't matter.   

[mobile_bob]

some pieces of equipment have their races pinned for reasons of over rotation caused by various means, torques and speed being a couple of them.

when a brg is pinned it is usually done because the common method of allowing the brgs to creep happens at such an accellerated rate that the case mount is prematurely worn out.

cases cost more than brgs, so the brg is thought of as a consumable and usually these types of equipment have shorter brg lives because of the pinning.

it is just like everything else in life, that is everything is a comprimise.

the engineers decide up front or from experience which way works for them for the intended use.

i would expect though that the st heads should be a slip fit in the case so as to provide for some creep of the races, with cast iron as the case material that expansion rate would be very close to that of the races unlike that of aluminum cased genheads.

i am with the others, the inner race should be tight, press fit and the outer should be a lubricated slip fit.
my bet is this is close enough for a good long life

bob g