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Hi-Res Spectrum

Chronology and Vibration levels (ips). Note: vibration was rapidly and steadily increasing 0.19 ips to 0.45 ips in 2 days. Hi-Res Spectrum. of. Zoom in around 1x. TWF. Overview of observations on the two bearings (more later).

mckinneyb
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Hi-Res Spectrum

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  1. Chronology and Vibration levels (ips).Note: vibration was rapidly and steadily increasing 0.19 ips to 0.45 ips in 2 days

  2. Hi-Res Spectrum of

  3. Zoom in around 1x

  4. TWF

  5. Overview of observations on the two bearings (more later)

  6. Bearing seating surface measurements (average of 3 per measurement)Outboard bearing shaft is 8 mils small and bearing bore is 3 mils big

  7. Left photo - Outboard bearing remained in endbell when removed, since bearing was loose on shaft (Later bearing was easily removed from endbell by hand). (This is single shielded bearing with shield toward winding)Right photo – View of grease cavity after bearing removed from housing – cavity is FULL. (grease is otherwise in good condition, no discoloration) Cavity full Of grease Shield

  8. Photo showing general as-found condition of grease in bearing on winding side = non-shield side.Fairly good condition. IB OB

  9. Seating surface of bearing on shaft (outboard). Ridges on both sides of bearing that can be felt with fingernail (red arrows) No evidence of axial movement… distance from shoulder appears correct.

  10. Outboard bearing inner ring bore:1 – OB Darker color than IB. Indicating perhaps higher temperature at this location when OB was spinning.2 - OB Pitted appearance across axial center (see next slide)3 - OB has a smoother, more mirror-like finish than inboard. (see slide after next) OB IB

  11. Closer view of bearing OB brg ID shows pitting.

  12. Outboard bearing has smoother more mirror-like finish than inboard. Presumably polished by relative motion OB IB

  13. 1 - After cleaning, Inboard bearing has more permanent staining (presumably from baking on lubricant during high temps). Higher inboard temp makes sense since inboard is cooled better in TEFC…. Presumably spinning outboard bearing did not yet create overheating (certainly would have after more time)2 - Inboard shows more thrust loading. It is not typical to see sign of thrust loading, however more likely on inboard which is fixed than outboard. OB IB

  14. Inboard bearing has distinct circumferential tracks on OD after cleaned, outboard does not. I am used to seeing some signs of movement/fretting, but inboard looks strange. OB IB

  15. Photo of bearing cavity after grease removed. Estimate cavity volume next slide

  16. Grease cavity volume estimation ~ 13 cubic inch. 1.30” 1.75” 0.375” Brg A1 A2 Grease Cavity 5.51” 2.1” 2.38” 0.25” 1.65” 1.25” 0.5” 1.28” 2.56” Shaft A1=0.5*Pi*(2.38^2-1.28^2) = 6.3 in^3 A2 ~1.25*Pi*(2.1^2 – 1.65^2) = 6.6”in^3 Total cavity volume ~ 13 in^3. (6.5 free after pack half full). Lube schedule: 1.2 ounce = 2.1 cubic inches every 18 months. (in-line with EPRI, not as often as recommended by SKF etc) After 4.5 – 6 years will be full IF no shrinkage (*) This had been installed 8 years -> perhaps we should not be surprised that cavity is full? (*) I have heard that grease can shrink substantially, but can we count on that?

  17. General Greasing discussion • We never have any luck with grease coming out of the drain or relief, even though we let it run for 2 hours after greasing with plug removed. • Why do OEM’s even bother to put a grease plug in? I think grease may expel if bearing is lubricated very frequently (like monthly or continuous… such that it never hardens). But not when following EPRI schedule like once per 18 months

  18. How close to failure was this motor? • Pretty close. • Rapidly increasing vibration levels suggested rapidly degrading condition (see slide 1) • Fits are specified to be clearance at outer-ring/housing interface and interference at inner-ring/shaft interface. • Clearance at inner-ring/shaft interface tends to degrade much faster than looseness at outer-ring/housing interface due to the fact that the machine weight load always acts down… the inner-ring/shaft interface is moving with respect to this load and therefore the load tends to force relative motion at the interface. • This action is illustrated below (worst case scenario assuming weight load completely overcomes friction to maintain contact at 6:00 loaded position. Rotate 90 CW Rotate 90 CW Rotate 90 CW Rotate 90 CW Inner Ring w w w w w shaft Slip distance in 1 rev We assumed that the weight load forced the contact to be maintained at 6:00 position. Therefore surface velocity of inner ring is same as shaft. => During one revolution of the shaft, the dot on the shaft and inner ring both traveled the same circumferential distance = *D_shaft. However the circumference of the inner ring is *D_innerRing, which is greater than *D_shaft, so the inner ring did not complete a full revolution. Slip distance in 1 rev = *(D_inner_ring – D_shaft) For our measured 10 mil diametrical clearance, slip distance in one rev is *0.010 = 0.0314” per revolution. In one minute at 3600rpm, slip distance could be up to 3600*0.0314” = 113”… almost 10 feet. In one hour, slip distance could be 565 feet. In one day, slip distance could be 2.5 miles. This relative motion tends to enlarge the clearance, which can lead to accelerating failure. The scenario is much more concern on inner ring than outer ring due to the fact that inner-ring/shaft mating surface rotates with respeect to the load as shown above (outer ring/housing does not)

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