Originally Posted by
NC Cams
In case you've never looked, ALL bearings comply to the same ABEC standards. Whether it be for ABEC 1, 3 5, 7 or 9. Thus they are all interchangeable. THings like raceway smoothness, raceway coaxiality & runout and other "secret" internal features ultimately determine one from the other. Even Hardinge spindle techs, at the time, couldn't figure out why the NSK bearings ran quieter and had less axial runout on their spindles than their former bearing supplier. Theoretically same bearing, noticeably different performance.
Aside from this, whether bearings came from CHina or NSK, they all react the same to kluge job versus "NSK" standards of assembly.
WHy NOT to use press with a/c's?
SImple. THe raceways are off of verticle by 15, 25, 30 deg. even deg 60 when it comes to ball screw bearings. Thus, they become more and more sensitvie to axial distortion by pressure applied to the SIDES (you know, where you put the pipe spacers againgst to press them on). Besides, depending on the press fit, metal to metal rubbing can induce scuffing or microwelding from metal to metal rub. Gall the bore or the OD while presing, you WILL distort the raceway.
COnsidering that bearing internal clearance is as low as 0.000005mm depending on size and literally 0.000000 in a preloaded AC, it don't take much press induced distortion to fubar the raceway no matter WHO makes the bearing. IT is simply too easy to "drive it home" or "get er done" and wreck the geometry of the raceway with a press.
Ok, so use some lube. As things heat up, they expand unevenly. ANY lube under the bearing can affect uneven heat transfer- why lobe the bore when youre' relying on press fit and FRICTION to keep it from spinning? Much of bearing cooling takes place by conductivne transfer via the bore and OD. Uneven heat transfer will distort the raceway. No big deal right? My engineers found that writnig on the back of the bearing with magic marker affects the heat rise and noise output of bearings. When disassembled, they found literally uneven raceway contact that coincided with the marker.
Pressing off bearings is not an issue. Reason: it is presumed tht the bearing is being replaced so who cares? Re use of the pressed off bearing becomes the question.
The scuffing of the pressed ring becomes the first concert. Second is the fact that most bearing forks or clamps (press off tools) can easily apply load to outer and inner ring concurrenlty - thus you press thru the balls which we alrady agree is a no no.
FInally, distortion. Depending on how well you can grip the ring, you often end up gripping just the very top of the inner ring or at two points rather than evenly. This is the WEAKEST part of the ring and easiest to distort. Distort the ring, you distort the raceway. Guys reuse bullets after being in and out of the chamber. The only time I reuse a bearing is if it slip fits the shaft/housing. I avoid repressing at assembly simply by thermal fitting. Period Paragraph.
In case you didn't know, thermal fit is quite common in the top spindle shops. THis is why they have induction heaters. Bridgeport/Hardinge uses only that method. THe shops I saw all used them to heat outer rings for preload set/check - no damage potential is the reason. Not a press to be found in the assembly rooms I was in - and many did not use NSK bearings..
Finally the issue is that of time. THe more bulky the bearing, the harder it is to press squarely and do so expediently. IT would take a good press and a lot of GOOD tooling (no hacksawed pipe) to press in a 8" od outer ring. ONce the guy had the housing hot at the railroad shop (lsshrxoeh above), he literally 'Shook" the ring into the housing as if he was shoveing pizza into a box.
One of my guys took the same speculative view toward press vs heat with me, until he saw me replace a 6310 (50mm bore) bearing on a lathe motor. Wasn't about to take out the armature to press that puppy. It would take a crane that I didn't have. So, I Iced down the sahft, heated the inner ring with propane torch, Literally dropped it on. Spent more time cleaning up the melted ice than screwing with the bearing.
I probably won't/can't convince you that thermal fitting is a superior way to installing bearings over anything that involves a press or god forbid a hammer. I don't care and am not about to try. As far as Im' concerned, you're more than welcome to "press baby press" I avoid it like the plague.
One can pound screws with a hatchet. IT works but the use of the screwdriver is much less primitive. Likewise is that of the thermal fitting of bearings. THis diatribe is being written for the benefit of the unconvinced lurkers.