A field thread from a cooling-fan retrofit in a foundry ventilation room, forty-five minutes between a clean bearing press-fit and a hot cover plate. Edited for names and part numbers.
From: T. Nguyen, Production — To: R. Almeida, Design; cc maintenance
Subject: fan shaft hub running hot after bearing change — 6208, both ends
Ricardo, the roof fan unit FU-04 was down this morning for the planned bearing swap, we fitted new deep-groove bearings at both the drive end and the free end, fresh grease, dialed the fits per the old drawing, and the drive-end housing is already 62 C at the cover bolt after forty minutes, hand too hot to hold comfortably, and there is a low whine on the free end that clears when you speed it up. Motor at 1450 rpm, fan at 960 through the belt, so the drive end sees the full belt pull, maybe 900 N radial, and the free end sees nothing but the dead weight of the impeller, about 150 N. The old bearings that came out were 6208s with the C3 printed on the shield, our stockroom handed me one C3 and one CN from the same box without noticing, and they both went in with the same shaft interference because the drawing does not call a clearance class. Question before I pull both housings again: is one CN bearing on a 0.018 mm shaft interference enough to explain the hot cover, or should I look at the grease plugging the drain first?
From: R. Almeida, Design — To: S. Kaur, Bearing Supplier Applications; cc T. Nguyen
Subject: RE: 6208 running clearance — check my arithmetic before we change parts
Simrit, the production foreman fitted a mixed pair, one C3 and one CN, on a shaft seating with 0.018 mm measured interference at the drive end, housing bore a slip fit at 0 to +0.008. Belt pull about 900 N radial at 1450 rpm input. The CN bearing is the hot one, 62 C at forty minutes, and the C3 one on the free end is fine, maybe 38 C, which is what this unit always ran at. I always thought the interference fit takes a big bite of the internal radial clearance, I want the actual number so I can tell the shop why we do not mix classes, and whether the CN bearing at that fit was doomed on paper, before we order spares that repeat the mistake. I do not want a lecture, I want the clearance budget in micrometers: manufacturer radial clearance for CN and C3 in this size, the reduction for 0.018 mm shaft interference at a 40 mm bore, and how much is left for the belt load and the temperature growth of the inner ring before lunch.
From: S. Kaur, Bearing Supplier Applications — To: R. Almeida; cc T. Nguyen
Subject: RE: 6208 running clearance — the budget, in numbers
Ricardo, the arithmetic, all in micrometers at the 6208 bore of 40 mm. Manufacturer radial internal clearance, unmounted: CN takes 5 to 20, C3 takes 20 to 38. Your 0.018 mm shaft interference, 18 micrometers, reduces the mounted clearance on the inner ring by roughly 80% of the interference, so about 14 of the 18, because the ring stretches elastically, and the remainder goes into the raceway groove elastically bending. So the CN mounted clearance lands at 5 minus 14, through 20 minus 14, which is minus 9 to plus 6, in other words the CN bearing is at best near-zero and quite possibly preloaded negative on the balls before you spin it once. The C3 lands at 20 minus 14 through 38 minus 14, call it plus 6 to plus 24, and that is enough to run. Then the temperature growth: inner ring on a running shaft warms faster than the outer ring through the housing, differential 15 to 20 C is typical, and at 11.5 micrometers per C per 100 mm of diameter, the 40 mm bore adds roughly half a micrometer per degree, 8 to 10 micrometers of growth by the time you are warm, which shuts the last of the C3 window and pushes the CN firmly negative, preloaded balls, squeezed grease film, rising torque until the cover plate heats to your 62 C. The belt load is the smallest player here, 900 N radial moves the rolling elements a micrometer or two in their clearance window, but a negative window does not move, it binds.
Two more lines so nobody reads the table wrong. First, the housing fit matters almost as much as the shaft fit: a transition housing at plus 0.008 does nothing to the clearance, but if the housing were an interference, the outer ring would squeeze inward as well and the budget would be worse, so keep the housing a slip fit for this arrangement. Second, never mix classes in one machine, and never let the stockroom decide the class, put CN or C3 on the drawing line, not in the parts box. I will send the same note to your stores team. You can run CN in this position, but only with a light shaft fit, 0 to plus 0.008, and then the CN window becomes about the same as what you just got with a C3 and a heavy fit, so you made the correct call on the drawing change, swap the drawing to C3 for the 0.018 fit.
From: R. Almeida, Design — To: T. Nguyen; cc maintenance, S. Kaur
Subject: RE: 6208 running clearance — revised fit sheet, please fit today
Revised fit sheet attached for FU-04, both ends now specify 6208 C3, shaft seat 0.012 to 0.018 interference, housing slip 0 to plus 0.008, and the drawing line for clearance class is filled in so the store does not gift us a CN again. Please pull the mixed pair this shift, fit two C3s off the same batch, and do not reuse the grease from the hot housing, it has broken down, clean the cavity, new grease, one third of the free volume. Log the cover temperature at start, thirty minutes, and sixty minutes for my sheet, and I want the free end whine on video if it comes back, because that one I still do not own.
From: T. Nguyen, Production — To: R. Almeida; cc maintenance
Subject: RE: 6208 running clearance — refitted, numbers in
Both housings fitted with 6208 C3 off the same batch label, shaft seats stoned to the 0.012 to 0.018 band, housing bores measured slip, my vernier gave plus 0.004 on the drive end. Cover temperature log: start 24, thirty minutes 41, sixty minutes 44 C on the drive end, free end 39 steady, and the unit draws the usual current on the nameplate. So the hot cover is gone, and I owe you the video anyway, the free-end whine came back at 40 minutes this time, low pitch, and it does clear above 1000 rpm. It is the C3 end, so it is not the classic preload sound, it is steady and it does not care about belt tension, video attached, phone under the end cap.
From: S. Kaur, Bearing Supplier — To: R. Almeida, T. Nguyen; cc maintenance
Subject: RE: free-end whine — listen to the phase, not the pitch
Saw the video. That is not a bearing ring tone, if it were element passage it would broadband, and if it were raceway damage you would hear it constantly, but the video shows it appears roughly at 40 minutes and clears above about 1000 rpm, and it is exactly that signature of a grease channel forming, when it is warm enough to slump and one side of the pack thins, at low speed the balls push a fluid wave back and forth and the housing wall rings like a small bell. Bearing is fine, and your temperature log agrees, 44 C with a fresh C3 and a 0.018 fit is a healthy running joint. What I would change before you call this closed: a grease-recirculation groove on the free-end end cap, or just a shield-side breather, because the free end has no through-flow and warm grease packs to one side. That is a housing drawing change, not a bearing change, so it belongs to your side, and you can ship the machine now and do the cap at the next PM if you want the schedule.
From: R. Almeida, Design — To: T. Nguyen, S. Kaur; cc maintenance
Subject: RE: 6208 whine — closing, and the drawing change
Confirmed, ship the unit, the whine is grease imaging, not distress, we log it at 44 C and 39 C through the shift and the current is nominal. Drawing change DRG-FU04-2211: end cap at the free end gains a 0.5 mm radial vent groove, 4 mm deep, and the PM instructions get the note about greasing one-third free volume and not topping off a warm bearing, which is the original mistake that let the pack slump in the first place. Thank you, Simrit, for doing the clearance arithmetic instead of quoting a catalog page.
Reference data — 6208 bore 40 mm; CN unmounted clearance 5-20 µm, C3 20-38 µm; shaft interference 0.018 mm eats ~14 µm of clearance; thermal growth +8-10 µm at 15-20 C differential; belt pull 900 N radial at 1450 rpm; final fit C3, shaft 0.012-0.018, housing slip; temps 44 C drive end, 39 C free end.
Glossary — internal radial clearance: the total radial gap between the balls and the two raceways, measured unmounted; interference compression: the stretching of an inner ring that reduces the effective clearance; thermal growth: ring expansion with temperature that further reduces running clearance; preloaded negative: a ball squeezed with no running gap at all, heating and binding; grease recirculation groove: a machined channel that lets excess grease return instead of packing one flank.
Field note — the clearance that vanished before lunch
An unmounted clearance class is a promise that the fit must keep. Put 18 µm of shaft interference on a 6208 CN and the 5-20 window becomes minus 9 to plus 6 before the shaft spins; warm it 20 C and the window shuts entirely, and now a 900 N belt is pushing against a squeezed film that cannot deform, which is why the cover plate was at 62 C by the time the shop went to break. The class belongs on the drawing, the fit belongs to the shop, and the temperature log belongs in the PM file; when a bearing runs hot or whines, ask which of the three changed, not which bearing is on sale.
From: R. Almeida, Design — To: stores, maintenance
Subject: 6208 clearance class quick reference, print and pin on the rack
One sheet for the stockroom so we stop this discussion on a Saturday. For a 6208 carried on a 40 mm shaft with a normal turned seat, use the class shown for your fit band. If the shaft is ground and the seat is light, CN is fine; if the seat is interference over about 0.010, move to C3; if the housing is an interference fit or the machine runs over 80 C oil splash, C4 becomes the honest choice, and then call me before you order, because a C4 for no reason is a noisy bearing with a shorter grease life.
| Condition | Class | Running clear. (µm, approx) | Result |
|---|---|---|---|
| Light shaft fit 0 to +0.008 | CN | 5-20 | correct |
| Interference 0.012-0.018 | C3 | 6-24 after fit | the FU-04 choice |
| Same, hot or high-vibration duty | C4 | 38-55 basic | rare, justify |
| Mixed pair, interference fit | CN + C3 | CN goes negative | the original fault |
Stores: the clearance class goes on the pick ticket in handwriting, not in memory, and a C3 in the CN slot is a downtime event, not a convenience. Maintenance: after any bearing job, the first stick-on pass is the temperature at thirty minutes; if it climbs past 60 C on a fresh joint, suspect the fit, not the bearing. That closes FU-04 and the reference sheet goes into the drawing file as DRG-FU04-2211.
