Bearing Selection 4

[Lewy07]

Hello

We are in the cement manufacturing industry and we have a primary air fan that operates at 2970rpm and over the years we were using self-aligning ball bearing 2316 C3 as specified and provided by the equipment manufacturer. Until recently we have been changing the bearings at a frequency of once every two weeks. On mounting we check the fan for balancing and it is within specifications. On initial start up and after a week of running both the fan and motor bearing vibrations are within specifications but the bearing condition of the fan is out of spec. Thus after the fan runs for two weeks the fan starts vibrating and the bearings eventually seize. The lubrication that we use is grease (Shell Alvania R3) as specified by the equipment manufacturer. My question is what could be the cause of this? Is it also possible to substitute the bearing on the impeller side with a cylindrical roller bearing of the NU or N type, whilst keeping the self aligning ball bearing on the motor side of the fan because I think we are easily exceeding the load limit of the self aligning ball bearings. The bearings are mounted in SNR316 plummer block housing.

 

[GregLocock]

3000 rpm*24*7*2= 1 million cycles. That is way less than any high speed life I have seen for a bearing. Is it underspecified?

Replacing an SA bearing by a roller bearing may work, unless you are seeing significant misalignment or shaft bending.

bearing life calcs are fairly easy, why not get your bearing supplier to look at the design - you must be one of their best customers!



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dimjim]

I would suspect that cement dust is fouling the
grease in these bearings. Any evidence of this?

 

[boo1]

would the two week cycle be
3000 rpm*60*24*7*2= 60 million cycles

consider contamination, over heating, improper lube, fatigue

post some pictured of the failed bearings and races

 

[diskullman]

That speed is too fast for grease lubrication. NTN shows a limiting speed of 2500 RPM for grease, and 3300 RPM for oil lubrication. You may be able to rig the bearing housing to a recirculating oil system. You could also try a 22316, although the limiting speeds are even lower, it may stand up to the vibration better, maybe even try a shaker screen bearing.

Russell Giuliano

 

[automatic2]

you do have a floating bearing to compensate for shaft expansion?

 

[BobM3]

With all the assembly and disassembly, is the shaft seat and bearing bore in the housing still in good shape? Out-of-round or incorrect press fits would shorten the life of the bearings.

 

[volpegrigia]

The life of the product is about 60,000,000 cycles which would give a C/P allowed of 3.91 (3.91^3=60). Thus the equivalent load on the bearing should be around 34.5 kN (7750 Lb). If the load on the bearing is in line with the above, you have an application that is designed poorly (the bearing appears to be much smaller than required). If your bearing loading is not close to the load above, then you must look at other possible sources of failure.

 

[GregLocock]

Oops sorry about the 60. dang rpm

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[boo1]

just a number

 

[ischgl99]

I would look to see if anything has changed since the failures started, is the fan running faster or hotter? Is the shaft aligned properly? Are the bearings installed in the pillow blocks properly? Are the housings being overfilled with grease preventing the bearings from expelling the excess?

I would not replace with a cylindrical bearing without checking with a bearing applications engineer. The self aligning bearing might have been installed for misalignment purposes and the cylindrical bearings are less able to tolerate this. A spherical roller bearing might be a better choice, your bearing supplier is best able to help you with this.

 

[gary50]

If it is axial flow fan, the axial load may cause overload for the bearing in that direction.

Kari

 

[Lewy07]

We machined a new shaft for the fan when we noticed that the shaft had bent due to vibration. The bearing will fail such that the clearances will be greater than the recommended. The grease application is to recommendations and there are no possible sources of contamination due to cement as the bearings are covered. The fan is a centrifugal type. Recently when we tested for vibrations the bearing vibrations where in spec, but the bearing conditions where out of specification, what possible solution can we have?

 

[GregLocock]

How does a shaft bend due to vibration?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[diskullman]

I would call the engineers at SKF and inquire about their CARB bearing for these dimensions. Their limiting speed shows 3400 rpm for grease lube, and 4500 for oil lube. They also have a design for high vibration applications, part # C2316C4VG114. I'm sure their engineering department will find a solution.

Russell Giuliano

 

[Tmoose]

I'd get the bearing application or fan manufacturer on site to change bearings with me. I'd write a PO to get them there if necessary. In the mean time I'd order the largest diameter standard sheaves/pulleys I could get to maintain the ratio I need, with the fewest belts allowed for the power required. I'd order a belt tension gage at the same time, and call the belt manufacturer's engineering department to provide a calculated tension specification. Until the belts arrived I'd loosen the existing belts until they just stop squealing at speed. At 9 am each morning I'd s-l-o-w-l-y apply 1 stroke of Mobil SHC 100 grease to each bearing from a dedicated standard grease gun after wiping off the grease fitting.

When the new large 1 or 2 groove sheaves and narrow section belts arrived I'd get the belt supplier to install the belts with me. If the belts did not squeal on start up I'd loosen them a little, unitl they just stop squealing at speed.

 

[mayt4u]

Hi lewy07

Far as I can figure you might have, an alignment problem. The so called "self aligning bearings" are NOT designed for misalignment, but minor (minor<or= 0.005") imperfections of the whole system. (slight deflections of the bearing area) The bearing housing might be tilted, or be twisted (<90degrees) from parallel. The larger sheave might not be true. (<0.001/1"radius TIR). The sheave to sheave might be out more than 0.010". These are big numbers that I'm using, so something closer could help, but they work for a base line. I like to look to <0.001" all way round for measurment and < 0.3 ips on fans. (That should get somebody going).

All the Best.
Robert
Could be worse, could be me.

 

[Tmoose]

Because the fan speed was not 1800 or 3600 (minus slip) I assumed it is belt drive.
Is it direct drive (coupled) running on 50 Hz or on a VFD?

 

[Lewy07]

It is direct drive coupled, running on 50Hz