Bearing Problem

[ewee]

A piece of equipment that we sell continues to fail bearings faster than would be considered normal (1 month or less running 24/7). The unit has a 20" propeller that spins at 3600 rpm. The propeller is used to fracture water introduced by a spray manifold at 100 psi and a maximum flow rate of 100 gpm. The propeller is mounted to a drive shaft with two 1-15/16" deep groove ball bearings mounted in a 4-bolt flanged housing. The bearings are spaced at 21.5" apart. The drive shaft is then coupled to a 25HP motor using a standard Omega coupling. The entire assembly is mounted in a clam shell housing to protect the motor and bearings from the "wash-down" type conditions and to prevent contamination. The bearings have Perma automatic lubricators mounted to them to provide proper lubrication. The lubricators are set to provide 60cc of lubricant over 6 months.

Unfortunately, I did not design this product and was not with the company when the engineering work was done. So I am trying to start from the beginning and run calculations to determine whether the current bearings are suitable for the appplication, etc. I have no experience in bearing design and a little confused on where to start. So I will start with the following questions:

1) Are deep groove ball bearings the correct bearing for the application?

2) How do I calculate thrust and radial loads for this application? The propeller weighs 37lb and the shaft weighs 30lb.

3) How do I calculate estimated bearing life? In other words how do I determine the dynamic capacity (C) and the equivalent radial load (P) when I don't have the radial factor and the thrust factor?

Any help that can be provided would be appreciated. Thanks for your cooperation and assitance in advance.

Jeff

 

[diamondjim]

The bearing manufacturer should have
and application department that should
be glad to take your input and give
the information that you seek. If
not, try their competitors. That is
a dandy rpm and at what temperature
do these run? Who specified the
lube interval? Is what you stated
for each bearing?
3600 x 60 x 24 x 30 equals the revolutions
per month = 155,520,000 revolutions.
That seems like a bunch for deep groove
ball bearing.

 

[rnd2]

ewee
With all that rpm and water going for grabs 24/7 have you considered using water lubricated sleeve type bearings?

 

[dwedel]

The manufacture would be an excellent place to get exact information. Calculating the life is something that almost needs a text book explaination. Here are some online catologs.

http://www.timken.com/products/bearings/catalogs/

http://www.timken.com/industries/torrington/catalog/

Continue talking to the bearing supplier, they should have someone on staff that can help you. If not, find another supplier. I have talked through problems with suppliers before, and they have a real intrest in providing you with the correct aplication.

Some installation problems would be:

Not enough allowance for thermal growth, or shrinkage.
If the water would turn to ice and jam the bearing that is supposed to slide with thermal growth.
Improper installation that fixes the bearings in the housings and the shaft not allowing for some movement. See the bearing manufacturers for more info on installation configurations.

Improper alignment of the Omega coupler. Perhaps the loads are high enough on bearing to cause problems but doesn't affect the coupler. You might be able to see this with vibration. Update International has great information and classes on coupling alignment.

Improper calculation of forces. You talk about weight, but what about reactionary thrust loads, loads from different orientation, coupling alignment loads (you will always have some), fan housing distortion.

This sounds like a snow making machine? Shred the Gnar!!

dwedel
Hotrod Big Engines!
For site policies and guidelines
see faq731-376

 

[ewee]

Response to diamondjim

The equipment runs in temperatures from -20 to 140F. The lubrication interval was arrive at by working with the automatic lubricator manufacturer and the bearing supplier.

Response to rnd2

The water used in this application can be sometimes be acidic or basic so I would'nt think that sleeve bearings would be a good fit (correct me if I am wrong).

Response to dwedel

I have talked with the bearing suppliers about thermal expansion and they said there was not enough growth to worry about floating one of the bearings. However, another similar unit that we manufacture does have a floating bearing. I am planning to do a side by side test with two units, one without the bearing floating and one with the bearing floating. However, this may be a lengthy experiment before we have any results.

I don't believe that ice forms and jams the bearing because they are fully enclosed. However, ice or residue can form on the propeller blades causing an imbalance and excessive vibration. We are considering the installation of a vibration sensor to turn the machine off should excessive vibration occur and to prevent damage to the unit.

Bearing, shaft and coupling alignment are issues we have considered and at times struggled with. We currently dial indicate the motor shaft to the drive shaft to achieve alignment (is this a good practice?). The 4-bolt flange bearing housing we use is self aligning and will allow for up to 5 degrees of misalignment.

Can you elaborate on what you mean by reactionary thrust loads and loads from different orientation. The equipment is typical run in the vertical position and some times 10 to 20 degrees from vertical. Fan housing distortion is something we have considered. The enclosure the motor and shaft assembly are housed in, is carefully fabricated using jigs to hold the bearing mouting pads square to the motor mounting holes, etc. However, the enclosure is not stress relieved and post machined, so there could be distortion caused from stresses retained after fabrication or during assembly.

Thanks

Jeff

 

[rnd2]

ewee
Fluctuation from mildly acid to midly caustic water will not be a problem. The speed and load requirements indicated do not look at all onerous for water lubricated hydrodynamic bearings. You maight want to check out

http://www.maritexbearings.com

 

[sreid]

I think the answer to the problem will be found by looking at the bearings prior to total failure. If you monitor bearing vibration/noise there is usually a marked increase in noise or vibration prior to total faiure. Disassembly and inspecting the bearings at this point will almost always pinpoint the source of the problem. Bearing manufacturers are usually glad to inspect the bearings and tell you the likely problem, they have seem many more failed bearings than either you or I will.

 

[willdo]

The limiting speed for your bearings is 4700 RPM. Your speed being above 75% of that limiting speed often makes the bearing manufacturer nervous and is worth consulting directly with them. I don't see where it said how close the propeller is actually mounted to your bearings but a 20" propeller @ 37 lbs could be quite an overhung load even before any imbalance is considered. Your supplier should get the failed bearings analyzed for you; this can tell you many things about your loads and lubrication. Based on the failure analysis the manufacturer should be able to offer good replacement recommendations. I would think a spherical unit would be best suited for your loads but your speed may be too high. Good luck to you and I hope you will let us know how you proceeded!

 

[ewee]

Response to willdo

The propeller is mounted approximately 9-3/4" from the bearing. I will try and report back once the bearing suppliers report back to me. However, the suppliers are not moving as fast as I would like. Can anyone comment on the bearings suppliers that have given the best technical support and service?

Thanks

Jeff

 

[dwedel]

I was just trying to state some ideas to start discussion. It sounds like you have some good information to start on.

Most of my ideas or thoughts were ways that you might get unforseen loads on your bearings.

Good Luck,

dwedel
Hotrod Big Engines!
For site policies and guidelines
see faq731-376

 

[enfield]

Just my 2 cents worth here 37 kg spinning at 3800 rpm would impose a wee bit of bearing load if it was unbalanced..
I would be looking at what others have mentioned and those bearing engineers or at least the ones I have spoken to really are helpfull ( it save me a wee bit of time )
Also it may be a 50 mm shaft but ( I can only speak for crankshafts , but they flex all over the show ....crankshafts that is different loads , but I am constantly surprised just what they get up to

Stephen ( be interested in what u find out!!)

 

[ewee]

Response to enfield

The propeller weighs 37lb not 37kg. Do you comments still apply?

Thanks,

jeff

 

[enfield]

oops sorry about that 37lb is still 16 kg!! and yes They would (the comments that is) look at even a slow rotating childs bike wheel even 5 grams out of balance has the thing hopping around ...
37 kg at 3800 on a 2 inch shaft !! ( I would watch through the window!! that would be fun!! sorry about that !!)
Anyway balance is not hard to check, I made a v block using large dia bearings then some stick on wheel weights placed at the radius of gyration ...U can get good results if you are carefull ( the one I did was an old compressor flywheel which was 12 kg at the end of a crankshaft)
Once again sorry for that
Good luck in finding the solution keep us posted

Stephen

 

[williamoh]

EWEE,
What kind of motor drive do you use for the application? If the drive is VFD, you may be dealing with the electrical bearing damage. Have you cut the bearing and check the bearing races?

 

[ewee]

Reply to willaimoh

The motor runs at full speed at 60Hz. No VFD is used. Ten Failed bearings were sent to the manufacturer for analysis with the following observations made by their engineers:

Ineffective lubrication
Insufficient lubrication
Unsuitable lubrication
Excessive thrust load
Soft-particle lubrication

Thanks,

Jeff

 

[willdo]

I originally missed that the equipment is run vertically. A ball bearing 4-bolt flange with high thrust loads at that speed is not a good choice.
Rexnord, who makes your Omega coupling, also makes very good spherical bearing units which can better handle your loads. The speed is another factor. Is your Omega coupling even rated for 3600 RPM? Some Omega couplings have to have high speed rings or special matched assemblies above certain RPMs.
Could your equipment work at a slower RPM?
I would call Rexnord and ask for an engineering specialist and go from there. Rexnord's phone number is 414-643-3000. Your bearing suppliers sound like maybe they'd do best if you could call them with the exact part numbers you need.

 

[diamondjim]

If you submitted all of your application
data and drawings to the oem, they should
have given you some realistic expected
life. If 3 out of the 5 failed because
of inadequate lube, obviously the lube
system is not delivering enough lubrication.
It would be nice to see an application
drawing to have a better idea of what the
system looks like. The excessive thrust
load analysis suggests that there seems to
be some unknowns in the application and the
wrong bearing may be in use. Did they
define what the soft particle is?

 

[diskullman]

Have you done a failure analysis to determine the exact cause of the failure. My guess would be excessive vibration. You may want to look into some of the Air Handler ball bearing units offered by many manufacturers. These feature a loose fit in the housing allowing the bearing to self align easier. The biggest problem we have with fan & prop applications are they are generally lose their balance very quickly, subjecting the other components to excessive vibration. The Omega coupling should absorb some of the vibration.
The Air handler bearings are very loose in the housing, much like a worn out housing. Maybe try a bearing with an anti-rotation pin in the outer race, fitting into the housing so that you can self-align with your fingers.

Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant

http://www.uniquetechnologies.com

 

[DDRSS10]

The 100 gpm of water combined with the volume of air that a fan, that sounds to be quite large, would create a very large thrust load. This sounds to be a snow making machine, I have seen the conditions the run in the lack of lube could easily be caused be cold flow problems. Even if the bearings see 140deg the oil is being cooled by ambient temp and the condensing if the water. Is there a measurement of the actual oil supplied to the beaaring before it fails. Just a thought.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[diskullman]

Your speed is off the charts for a deep groove bearing. You may try a spherical roller to attain that speeed range, something like a Brownin SPB1000x1-15/16", or Dodge S2000x1-15/16

Russell Giuliano