I would like to get some input on were a floating bearing should be installed in a overhung fan assembly. I have heard that the floating bearing should be on the fan side and fixed bearing on the drive/pulley side. I have also heard the opposite. Is there a standard? Or does each application differ? This would be for a belt driven fan.
ThanksThis message has been edited. Last edited by: Xracer,
Fixed end to pulley side so that pulleys stay in alignment.
We use SKF Carb bearings on overhung fans with some success.
floating bearing on fan side...
i.e fan nde bearing must be floating to allow axial movement and also thermal expansion....
fan de bearing remains fixed...This message has been edited. Last edited by: rotary,
Thank you all for your replies
Why would a manufacture recommend that the floater bearing be installed on the pulley side and fixed bearing on the fan side?
Let's assume a direct drive. The NDE bearing ( fan side) is loaded radially more then that of DE ( since impellor weight is closer ). It is better for a bearing to have axial load developed during operation applied on the bearing which has high radial.In order to satisfy that the NDE one has to be fixed.
Just my opinion...
Maybe a very, very close clearance between the fan rotor and housing?
I would like to refer you to this presentation made at a SIRFrt Meeting in Australia back in 2004.
Spherical Roller Bearings And Overhung Fans
It is important to get it right as I have seen fan bearings catch on fire where the wrong bearing was fixed.
Nice presentation, but it did not say which bearing should be fixed for axial or thrust load. What do you say David?
I believe the presenter was emphasising that fans, overhung in particular, can have low radial loads and relatively high axial loads. The right load ratio is essential to ensure correct operation of the bearing, Fa/Fr < 0.3 (approx). - I am not sure of the presenter's source for this as I have seen SKF documentation indicating
For Spherical Roller Bearings Fa/Fr < 1 and,
For Self Aligning Ball Bearings Fa/Fr < 0.25.
This can best be achieved by ensuring that the axial load is taken by the bearing carrying the higher radial load and therefore this is the bearing to be locked. If this ratio cannot be achieved with either bearing then a redesign is recommended.
Gentlemen, I am the author of the presentation referred to earlier. The slides from the presentation are not really a stand alone document, so for a more detailed description of the issues and solutions please refer to the 2 documents that I have attached. These are "Maintenance Skills and Practices Flyers" that we have produced on this topic. They should clarify things for you about which bearing to lock, but if you have any questions after reading them, please ask.
A word or two on Carb bearings. These are often promoted as the ultimate solution for overhung fan bearing problems. The reality is that if you have spherical roller bearings and an overhung fan that has catastrophic bearing failures, then Carb bearings are probably not the answer. As I explain in the Flyers, the problem is often with the FIXED bearing not having enough radial load. The floating bearing is not usually the problem. But Carb bearings cannot be used as a fixed bearing, so they don't help in this situation. And using them as the floating bearing doesn't achieve anything - apart from complicating setup and parts compatibility issues. There is, however, one situation I'm aware of where a Carb bearing does help. I know of a manufacturer who had a fan that rapidly went out of balance, and they were unable to rebalance the fan often enough. This resulted in the plummer blocks being pounded to the point where the bearings began to turn in the housing, and this ultimately led to bearing failure. In this situation the Carb bearing can help because the bearing outer race is actually clamped in the plummer block (standard plummer block/bearing arrangement has clearance between bearing and housing)and any axial float is accomodated within the Carb bearing.
My final piece of advice is that if you are having failures, then do a root cause analysis on your bearing failures. Only then can you determine what the problem is that needs to be fixed! At that point you may find that these Flyers offer a solution.
_43_OH_Fan_Brgs_Part_1.pdf (81 Kb, 421 downloads) Overhung Fan Bearings - Part 1
2nd Flyer attached.
_44_OH_Fan_Brgs_Part_2.pdf (93 Kb, 327 downloads) Overhung Fan Bearings - Part 2.
Does anyone have a good guide for typical bearing types / arrangements for other common machine types? Something similar to these two PDF's but covering a much wider range of machines?
Thanks for the attachments. I was advocating this concept all along for a good chunk of time ( see my brief post above ) and glad to have an ally.
There is another related issue here though. The DE bearing in a direct drive is not meeting most of the time minimum load requirements. This leads to overheating. Not changing the outlined approach it appears to me that the DE bearing has to be of a smaller size then NDE.
This is an excerpt from an SKF paper in Plant Services on this topic:
One cause of fan failure is confusing the fixed and floating bearing positions. A typical fan has two bearings supporting the shaft. In most cases, the bearing exposed to the highest radial load should be fixed, or axially held, within the housing. Radial bearings generally perform better when the axial-to-radial load ratio is low.
Also, the axial force needed to float a bearing is lower when the radial load is lower (axial force to move bearing = coefficient of sliding friction x radial load) (or Fa = µ x Fr). The fan’s other bearing should be allowed to float internally or within the housing bore, thus accommodating shaft expansion and contraction.
In most fan applications, the bearing closest to the motor or belt drive has the highest radial load and should be the fixed bearing.
I have recently learned that one particular belt driven over hung fan manufacture requests the fix bearing be at the fan wheel side and the floating bearing at the pulley side. The answer that I received was that the load on the bearings were too low, thus why they specify the fixed at wheel and floater at pulley. We just replaced bearings on this brand fan, and have installed them per the drawing/mfg specs. Time will tell if they last longer than the previous bearings.
I wonder as to what was the failure mode of these bearings? Overheating?
Being just a bit of the devil's advocate, assuming the amount of axial float within the "floating" bearing is reasonably limited, and that the fan wheel-to-housing clearance is adequate, what happens if you allow both bearings to float? For most of the bearings I see on overhung fans, fixed vs. floating is simply a matter of inserting or removing the "rings" - otherwise the bearings are usually identical.
IMO it will be not healthy for the machine. After all for proper machine operation one wants shaft's position fixed but the bearing can potentially move inside the pillow block housing within a 0.25" range. Obviously one can't make both bearings fixed due to thermal expansion of the shaft. Therefore just one bearing is fixed with maximum (undesirable) shaft movement in axial direction limited to thermal growth.This message has been edited. Last edited by: David_G,
|Powered by Social Strata||Page 1 2|