This is a 1200 RPM centerhung ID fan with a 4" dia shaft and sleeve bearings. About 8 years ago someone has changed the viscocity of the oil used on these bearings from ISO 68cSt/40C (recommended by the OEM)to ISO 100cSt/40C.

This fan is still running all these years but had 4 occurances of IB bearing failure. Currently, the temperature is 130F on IB (equipped with thrust collars) and 100F on OB. The babbit insert is fabricated in a local shop according to OEM dimensional specs but of unknown material composition. This fan has an automatic damper on inlet operating sometimes at 20% openning which potentially may produce unstable operation in particular in axial direcion but vibration pattern/amplitudes is not affected. No misalignemnt, excessive amplitudes are present. Installation is OK (blueing showed close to 100% contact on the callar faces). The OEM is no longer in business to ask for an advise.

Questions:
-Is there a prudent accurate method (other then trial and error) to determine the correct oil viscocity for this application? Of course, one can always follow general recommendations, but they are just general...
- How to detect and measure excessive thrust (possibly static) as one of the accelerated wear causes?

I'll appreciate your view.

David

This message has been edited. Last edited by: David_G,

According to all my trainging, you should use the highest viscosity oil you can get away with without causing abnormally high temperatures. The thinner the oil, the lower the tempeature on the bearing as long as fluid film wedge is adequate, once the oil is too thin to provide a proper wedge, then damage occurs. When vendors specify a viscosity, it is normally specified along with a ambient temperature range the equipment will see. Some plants in far north have to change from one viscosity to another for winter vs. summer temperatures. I would say the 100 cSt oil you are using is fine if the temperatures you gave are accurate. Did the previous bearing failures happen after a startup... perhaps on a very cold day? The thicker oil could be a problem in that situation.

I give my thoughts here with a very important preface: I am an electrical guy and I never took a lub course of any kind. I would not change lub viscosity on my equipment without consulting OEM or some other expert.

My understanding is the same as Ed's.

Lower viscosity lowers temperature but also moves you closer to viscosity breakdown under load.

Higher viscosity gives higher temperatures but moves you further away from viscosity breakdown under load.

Also of course viscosity decreases with temperature but I think you will usually be further from viscosity breakdown under load with the higher viscosity oil even though it runs at a higher temperature. The reason for temperature limit is generally the lubricant. Normal oils will start aging more rapidly somewhere in the neighborhood 200F - 220F. Synthetic oils can sometimes tolerate higher temperatures and sometimes are a solution where temperature is high.

Did the failures occur during periods of high loading? If so maybe you are suspecting still too light oil and I would think further increases in viscosity are possible based on your relatively low temperatures.... as long as you remain below 180F or so there should be no temperature-related problem with the bearing. Maybe the caution would be perhaps the oil distribution associated with rings etc may be affected locally in some parts of the bearing. So of course monitor the machine closely with vibration, oil samples, inspection after changing.

Viscosity index also plays in there somehow. I heard an OEM field technician tell me that bearing with 90C operating temperature was reduced to 80C just by changing from VG68 normal to VG68 synthetic with higher viscosity index. This seeems a little backwards to me. Both oils have same viscosity at 40C. Both reduce viscosity as temperature increases but the high-viscosity synthetic reduces less. So at operating temperature the synthetic by my thinking should have higher viscosity and higher operating temperature... but that's not the way it works. Maybe someone can explain to my why I'm confused on this part.

Isn't it all a matter of geometry, load and speed? Thicker oil leads to a thicker wedge but doesn't it also increase friction (the high temps Ed was talking about) and increase the chance of instability? If the operating temps have increased maybe that's why they increased the viscosity.

There are plenty of equations and charts to get film thickness based on viscosity--Mark's Handbook has a whole section on fluid film bearings. Or, what about asking another OEM?

Pete, as far as I know, higher viscosity index just means it's more stable across a wider temperature range. So a synthetic 68 is 68 cSt at 0º or 200º, or close to it. And I've seen the same kinds of decrease in temps by switching to synthetic. Something to do with the longer chains and maybe some van der Walls forces or...ok, I'm making that part up. But the rest is true.

I think we're saying the same thing.

Let's say we have synth oil and normal oil both VG68 and assume the synth has higher viscosity index.

Both will have 68cSt at 40C.

What is viscosity at operating temp ~ 85C?

Le'ts say the normal oil will have viscosity in the range 12 cSt at 90C (based on chart for 95 viscosity index)

The synth oil may have viscosity in the range 20 cSt AT 90C. (maybe higher, I'm not sure).

The synth oil with higher viscosity should force the bearing to run slightly hotter to my thinking.

But as explained above someone told me synth oil runs cooler in this exact scenario.

It's magic.

Good Day All,
Lots of interesting opinions and thoughts in this thread.
In answer to your first question yes there are formulas out there to get you started. For example at 1200 rpm and a 4" shaft and an in operation of 130F you could run an ISO
46 oil. Now that's where you start and where you will have to modify that particular viscosity is with the particular parameters of your application i.e. duty cycle, number of stops and starts, ambient startup temperatures, recirculation oil system etc.

You mentioned that there have been a few failures over the years. It's likely that there has been some reduction in the shaft diameter, and excess clearance in a babbited bearing if you have periods of instability (because of damper - temperature conditions) that might be causing you some problems. There are some of the lead based babbits that will start to creap (Johnsons apparent elastic limit) in the temperature range you mentioned 180F. You might want to consult with the local shop and possibly consider one of the ASTM B23 tin based babbits.

Re the issue of the VI generally with a typical refined oil Group 1 or 2 base stock you will get a VI of around 95. The group 3 base stocks (Castrol's "synthetics some Petro Canada products) the VI improves up to 130 or so and with the traditional Group 4 synthetics the VI is around 155 or so. What was said previously was correct the viscosity is +/- 10% of the ISO reference point so 100 cSt @ 40 C give or take 10% could be anywhere between 90 and 110 cSt, however when you get to the 100C the higher the VI the thicker the oil. THIS IS A GOOD THING. There are 2 reasons why a synthetic (group 4) or a group 3 might give you a lower operating temperature, for instance the group 4 has a higher lubricity -it slides against itself easier therby reducing internal fluid friction, internal fluid friction is a major source of heat build-up. At elevated temperatures it has a higher viscosity than a lower VI product and therfor is able to mantain seperation between the metal to metal components. Those are a couple of the reasons why a synthetic can be a wise choice, however if your don't need them they are a waste of money.
David, I don't know how to directly measure (other than with strain gauges) the loads you refeered to however you could monitor the effects of the loads through oil analysis. I'd suggest you consider looking at a typical set of tests initially to get a base line and monitor the lubricant's health and then look as some debris analysis (patch initially, and analytical ferrography if required later) this should help you to determine when the failure starts and give you some thoughts on how to solve the problem. Your local lab should be able to help you if you describe to them what you are trying to accomplish.

hope this helps...

regards.......

quote:

the higher the VI the thicker the oil [at high temperature]. THIS IS A GOOD THING

A good thing with respect to avoiding metal/metal contact, but not with respect to minimizing operating temperature, correct?

quote:

There are 2 reasons why a synthetic (group 4) or a group 3 might give you a lower operating temperature, for instance the group 4 has a higher lubricity -it slides against itself easier therby reducing internal fluid friction, internal fluid friction is a major source of heat build-up. At elevated temperatures it has a higher viscosity than a lower VI product and therfor is able to mantain seperation between the metal to metal components

Lubricity is somewhat a new concept for me. I googled it and at first impression it sounds to me like it is associated with coatings formed on solid parts which reduces friction. I was under the general impression from textbooks that vast vast majority of heat came from fluid friction a direct function of fluid shear and viscosity (not lubricity). After all most temperature calcs are based on viscosity (along with geometry speed etc) and NOT lubricity. Am I misunderstanding it?

I agree with your comment on metal/metal contact. Certainly if below minimum viscosity at operating temp the normal oil would run hotter... along with other problems. Maybe that was the case with the example I was told. One would think a machine generating that much heat from metal/contact due to viscosity breakdown would be in pretty sad shape with other symtpoms.

Thx.

Alan, Great write up.
Viscosity is the resistance to fluid flow. It relates directly to adhesion and cohesion of the fluid, and relates directly to shearing stress, but does not address how much heat is generated during shearing.
Lubricity is a term that has been around for a long time, but is starting to be used more often. Lubricity is the property of how slippery the oil is, and is directly related to film strength. Synthetic oils typically have better lubricity than an equivalent viscosity mineral oil.
For instance, a typical bearing that runs 120 deg. F with a high grade mineral oil will run 10 to 20 degrees cooler with the same viscosity of Royal Purple or Mobil SHC synthetic oil. The viscosity at these temperatures is the same, but the lubricity of the synthetic means it generates much less heat while shearing.
As Alan said, the VI or Viscosity Index determins how flat the curve is across a temperature range. Notice in the attached graph how the SHC 264, which is a 32 cSt oil, has a higher viscosity at 100C than a mineral 46 oil.

Pict0002.JPG (167 Kb, 14 downloads) Viscosity Curves

I was under the impression viscosity IS directly related to heat generated by fluid shear in sleeve bearing hydrodynamic lubrication, and this lubicity is irrelevant.

Look at equation 9 on the first page of
http://www.engr.mun.ca/~katna/index1/hydrodynamic_bearing.doc

F = A * mu * [U/h]
F = friction force
A = area
mu = absolute viscosity
U = velocity as function of depth
h = film thickness
[U/h] = shear rate

Multiply both sides by velocity and you will have on the left side F*u = power = friction heat (watts)

This equatio describes the mechanism for friction from fluid shear in hydrodynamic film lubrication mode (to the right on the Stribeck curve) such as sleeve bearings. It seems here this friction is dependent on viscosity... and lubricity is nowhere to be found. Furthermore Juvenal* has all the charts mentioned before. Viscosity is a component of all those dimensionless parameters, but where is lubricity? (not there). I would think that would suggest lubricity viscosity is relevant and lubricity is not FOR HYDRODYNAMIC FILM LUBRICATION such as normal operating range for sleeve bearings. To me hydrogynamic film lubrication is derived from the fluid shear relation above and not dependent on lubricity.

Lubricity once again is something new to me and it seems like all the applications I saw in searching google had something to do with engines etc, not sleeve bearings. My thinking is possibly it may be important in auxiliary components associated with a sleeve bearing (pumps? oil rings?) but if it is related to sleeve bearing itself than Juvenal must have not known about it. So please explain, have there been advances since Juvenal to discover lubricity or is it not relevant to hydrodynamic film lubrication?

* Chapter 13 of "Fundamentals of Machine Component Design" by Robert C Juvenall, Wiley 1983

This message has been edited. Last edited by: electricpete,

quote:

For instance, a typical bearing that runs 120 deg. F with a high grade mineral oil will run 10 to 20 degrees cooler with the same viscosity of Royal Purple or Mobil SHC synthetic oil

THAT is exactly the part that doesn't make sense to me. Please clarify are you talking sleeve or rolling element bearing. If sleeve bearing it makes no sense to me.

quote:

Notice in the attached graph how the SHC 264, which is a 32 cSt oil, has a higher viscosity at 100C than a mineral 46 oil.

I don't understand what is the relevance of comparing two different grades of oil. My question related to one viscosity grade, examining the effect of change in VI upon viscosity at operating temperatre far above 40C..... see T/F question below.

True or False question: Two lubricants, both VG68, one synthetic high VI and one normal lower VI, both heated to 100C. The high VI has higher viscosity at 100C - True or False?

My answer is true and that is the basis for my suspicion the high VI oil would in general run hotter in a sleeve bearing machine with operating temperature far above 40C, all other things being equal.

This message has been edited. Last edited by: electricpete,

Ed and Alan and others - I appreciate your taking time to respond to my questions. If something doesn't sound right to me I want to explore it so I can learn from you guys if I'm wrong.
Thanks

Pete,
Good job of questioning! I have been looking through my references as well, and can't easily answer you. I am familiar with the calculations you detailed above as the classic ones taught in machine design. By the definition of lubricity, it should not really have any impact on heat generation if fluid film is thick enough to prevent any contact. With that assumption, viscosity is all that matters, and viscosity is the ratio of shear rate to shear stress. All I can say at this point is that I have swapped many pieces of equipment over to synthetic oil of the same viscosity and recorded temperature drops of 20 deg. F or more. By all reasoning, the superior VI of the synthetic should make it have a higher viscosity at higher temperatures than the mineral oil, thus it is "thicker" at any temperature above 40C than the same VG mineral oil, yet the bearing temperature runs cooler.... I've seen this primarily in rolling element bearing machines wear you might be able to make a case for the lubricity having a big impact, but I've also seen it occur on sleeve bearings and kingsbury thrust bearings. Maybe the answer lies in the fact the traditional equations while accounting for the velocity as a function of depth, with depth being film thickness, don't take into account that the shear rate of individual layers of the lubricant is not linear across the film thickness? I think the equations are originally derived from at least 2nd order differentials, with the assumption that the velocity change with depth is a linear function. Does a oil with higher lubricity flow "faster/easier" at the point of surface contact than a mineral oil, thus lowering the shear stress in the middle of the oil film, thereby creating less frictional heating?
I'm not a lubrication specialist, just trying to figure things out as well... This might be a good question to ask the folks at NORIA, I sure would like for someone to set me straight if I'm off in left field!
To answer your direct question: Yes, the viscosity of a VG68 High VI oil like a synthetic at 100C would be higher than viscosity of the normal VI oil.

Thanks Ed. It is good to have some direct report of results of varying VI and also some good though-provoking comments.

quote:

Originally posted by Ed Hudson:
All I can say at this point is that I have swapped many pieces of equipment over to synthetic oil of the same viscosity and recorded temperature drops of 20 deg. F or more. By all reasoning, the superior VI of the synthetic should make it have a higher viscosity at higher temperatures than the mineral oil, thus it is "thicker" at any temperature above 40C than the same VG mineral oil, yet the bearing temperature runs cooler.... I've seen this primarily in rolling element bearing machines wear you might be able to make a case for the lubricity having a big impact, but I've also seen it occur on sleeve bearings and kingsbury thrust bearings.

Sleeve bearings I have enough literature which seems to provide a good model for heat generation so I can pretend to begin to understand it. Rolling bearings I don't have much at all. I have a lot of questions about rolling bearings that I think would form a good new thread.... will post within a few days.

quote:

Originally posted by Ed Hudson:
Maybe the answer lies in the fact the [#1] traditional equations while accounting for the velocity as a function of depth, with depth being film thickness, don't take into account that the shear rate of individual layers of the lubricant is not linear across the film thickness? I think the equations are originally derived from at least 2nd order differentials, with the assumption that the velocity change with depth is a linear function. [#2]Does a oil with higher lubricity flow "faster/easier" at the point of surface contact than a mineral oil, thus lowering the shear stress in the middle of the oil film, thereby creating less frictional heating?

Regarding #1 - The derivation I see in the link starts with a differential equation and solves for the velocity profile... there is no inherent assumption of the form of the velocity profile before the problem is solved. For the simplest case infinitely long bearing the solution to the differentail equation does end up being a linear term plus a quadratic term but that form is not as assumption but a solution to the diff eqn. Other forms where we consider axial flow the problem is solved by numerical methods but no assumption on velocity distriubtion. Many other assumptions may be questioned and I agree like all models it is by no means perfect.

[#2] I had never given much though to the fact that we always assume zero slip between the lubricant and the bearing solid parts. Now that I have had time to reflect on your comment I think that strong adhesion between the fluid and the solid is an essential part of a good lubricant. Think about all the oil you have ever handled.... it adheres well to everything it touches and very difficult to remove. If there were slip between the fluid and the solid rotating shaft in a bearing, then there would be less shear between the layers of fluid and the result is equivalent to operating at a lower speed of the solid rotating parts.... we have smaller film thickness and closer to metal-to-metal contact. One more thought experiment... think of bread dough that you knead. If you add some flour you can get it so it doesn't stick to your fingers at all. I'm sure it has high viscosity. But if I put it in a bearing I'm sure the shaft would burrow down through the dough and make metal contact. Not due to the lack of viscosity (cohesion) but due to lack of sticking to the shaft (adhesion). In summary based on the logic above I believe that good adhesion between the oil and the shaft/bearing is essential for good lubrication. But then again I may be way off-base. Open to any comments.

This message has been edited. Last edited by: electricpete,

I had a rare opportunity (rare for me) to participate in testing the effect of oil type on temperature on a sliding bearing. (The reason for the testing - troubleshooting an intermittent hight temperature problem.) The bearing is combined radial/thrust tilting pad bearing (similar to Kingsbury) on top of a large vertical motor. Temperatures monitored at Thrust pad (TH) and Upper Guide (UG)

The motor was run unloaded with the following results after temperature stabilization:
UG(C) TH(C)
VG32n 56 53
VG46n 62 59
VG68n 63 60
VG68S 63 60
VG100n 70 66

The "n" suffix means normal oil (Mobil DTE series) and the S suffix means synthetic.

As expected temp increases with viscosity.

But at the VG68 we tried both the Mobil DTE Heavy Medium and a synthetic oil. We got identical temperature results for the normal and synthetic oil (63C/60C both times).

This seems to contradict my prediction that high VI oil above ref temperature would run hotter.

Maybe this lubricity effect or something similar is helping to reduce temperature of the synthetic below my prediction. Could be. It still seems that lubricity should not apply to hydrodynamic lubrication where there is no contact. Would anyone like to comment?