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Question From a Millwright

05/28/2009 6:02 PM

Please end a dispute as to wether or not having air blow across a hot bearing housing will help to cool down the bearing while it is running or will it only cool the housing making things worse?

I look forward to you reply,

Thanks

Greg Davis

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#1

Re: Millwright

05/28/2009 6:42 PM

Depends on whether you are blowing hot air or cold air.

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#2

Re: Millwright

05/28/2009 6:43 PM

First let me welcome you to the insanity.

It is my feeling that anything that removes heat from the bearing assembly, is going to help. If you only remove 5 degrees from the bearing housing, you will still be removing some heat from the bearing itself.

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#3

Re: Millwright

05/28/2009 6:53 PM

I can only answer with a question. Does the air cooled cylinder of a motor cycle keep the piston cool?

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#4

Re: Millwright

05/28/2009 7:29 PM

About three projects back, we worked on large motors with cooled bearings. Anything helps, but air won't do a lot.

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#5

Re: Millwright

05/28/2009 8:02 PM

A small stream of water will do far more to cool things off than a lot of much more expensive air in most cases.

As to whether it will help, I supose that could be answered by considering all of the millions of TEFC (Totally Enclosed Fan Cooled) electric motors built and used. There will be some cooling from the expansion of the air and some heat carried away by convection.

So, I suppose unless you are using very hot air, yes, it will help cool things off some. If you are blowing a bunch of abrasive trash into an open bearing with your air stream, then you may be doing more harm than good.

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#6

Re: Millwright

05/28/2009 9:41 PM

It all depends on...

As the previous posts have suggested the example, the not so hot air or water will cool things down.

However for bearings, we need to take the heat off from point of generation (is the rolling surface) where the lubricant is to be.

You cool the housing down - then it gets cooled obviously.

That cools the outer race

Inner race and rollers are still hot.

End play (ir the clearance) goes down

bearing may seize in extremes (however that extreme is not likely for practical circumstances)

Is your bearing fits designed for air-blast cooling? And what is the size of bearing, what is the hot -ness of the bearing ? all these matter.

As far as the temperature of bearings do not overshoot the allowed why do you bother?

If it is antifriction - roller bearing- then we normally allow 90-100 deg C, though the bearing manufacturer says 120 deg C is safe.

But that is the bearing temperature. The housing temp will be at leasts some 10 deg or so less than it (even may be more than 10 deg) depending upon the configuration.

Always remember, reduction in the endplay has a much much higher damaging potential than the increase in endplay.

Only your winning point is (assuming you are the proponent of the air cooling) the air cooling (by flow) may just take up a few degrees of heat off and that may not have so much effect on the endplay (or even temperature of the hsgs either)

So the controversy is controversial. More data is required before you proceed with serious cooling, unless you cool the bearing itself - through lubriation line.

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#9
In reply to #6

Re: Millwright

05/28/2009 11:02 PM

GA. I go for cooling the lubricant. Add cooling fins or install a tube sheet system to the lube sump. sb is right about heat reducing the tolerances, creating further heating, until that moment when molten babbitt occurs, verrry quickly. As little as 3 seconds to total seizure.

If it's not a pump lubed system, why the cover?

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#7

Re: Millwright

05/28/2009 9:53 PM

Using air to cool down a hot bearing housing will definitely help you to cool down the bearing temperature but you got to calculate the cost of your energy wasted by your air compressor. Depend upon the application and location of your bearing usage, you can also use water to cool down your bearing housing.

Don't worry about it as it will not cause any problem to your bearing. We did that all the time. We use blower fan to cool down some of the motor bearing when we detected the bearing temperature is high.

But at the end of the day, you still need to solve your bearing temperature problem.

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#8

Re: Millwright

05/28/2009 9:55 PM

Grdavis I had a 6th grade science teacher that insisted that if you were to blow across a steaming bowl of soup you would raise the temperature due to the friction of the moving air. I could not see a person blowing a strong enough stream of air to raise the temperature of the steaming soup. Therefore I believe you could carry away an amount of heat sufficient enough to lower the temperature. In your case you have to carry away more than is being generated so the greater the difference in the two temperatures the more heat you would dissipate from the bearing. Although a small amount of lubricant might do more. Jerrell

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#10

Re: Millwright

05/29/2009 12:38 PM

If it's a lubricated bearing, the lubricant will help transfer heat from the inner bearing to the housing. That's how the housing is getting hot in the first place. Rapidly cooling the bearing housing could cause the problems mentioned above, but slowly pulling heat from the bearing housing should cool the lubricant, which will in turn cool the inner bearing. The bearing losing heat to the environment around it is what's keeping it from overheating now.

You might even get away with putting fins on the bearing housing rather than blowing air at it.

How about adding a copper or aluminum fan to the shaft next to the bearing? This would both cool the bearing housing with airflow and conduct heat away from the shaft side. If you used a copper/aluminum bushing, you could get heat to flow directly from the inner bearing to the fan, which would then act as a heat sink.

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#11

Re: Question From a Millwright

05/30/2009 1:16 AM

It will heat up the air. (I can't believe I typed that answer out loud)

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#12

Re: Question From a Millwright

05/30/2009 1:34 AM

Air on the housing will cool the bearing. The cooled housing will become a heat sink for hot lube oil and the bearing thermal conductivity to the same housing. There will not be a significant delta T to 'crush' the bearing OD since as soon as the heat sink removes heat over to the air, the generated heat migrates along the same path as one smooth constant temperature gradient, not a sharp delta T to cause additional interference.

Pumps with fan cooled bearing housings run significantly cooler than non-fan cooled.

If this is a ball bearing, just glue a RTD from your Fluke meter to the OD face of the bearing race and try it. If you glue it to the ID race you will need much more wire !

Do not glue any RTD on the back of a sleeve bearing and mount it and try it as this will raise a significant high spot to the ID.

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#21
In reply to #12

Re: Question From a Millwright

06/03/2009 5:27 AM

Best so far - GA, but would have given you one just for the laugh that this evoked "you will need much more wire !"

Extract from Pump Handbook on our very own site - "Air-cooling is becoming more popular as cooling water is in limited supply or of poor quality in many parts of the world. It is also less expensive to operate. In its simplest form, a fan is mounted on the shaft on the outboard side of the bearing housing to direct air over the bearing frame toward the pump, thus creating forced convection of heat from the bearing frame. Sometimes a heat deflector is also utilized which is..."

http://www.globalspec.com/reference/9354/121073/Section-6-1-1-Centrifugal-Pump-Bearings-Cooling

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#13

Re: Question From a Millwright

05/30/2009 1:43 AM

Just use ceramic bearings and have a nice day

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#14

Re: Question From a Millwright

05/30/2009 5:57 AM

Hi,

Why is the bearing so hot in the first place as to raise this issue. If correctly designed for the particular application bearings should not need to become hot if they are not overloaded or are being used to resolve inappropriate forces; i,e axial forces being imposed upon a bearing lacking end thrust capabilities.

Of course removing heat by air , water or any orher means will benefit the situation because it will increase the temperature gradient between the point where the friction in generating the heat and the 'cold sink'.

But I would seek to identify why the bearing needs the cooling in the first place ;solve that and forget the rest. Everyone will sleep easier then!

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#15

Re: Question From a Millwright

05/30/2009 12:56 PM

When looking at cooling bearings...............the three methods of heat transfer are used..........coduction.........convection.............and radiation............both in the luricant and the air surrounding the bearing.

In shot.......yes, blowing air on the bearing will cool...........by how much depends on several factors.

As some have answered...........Why?

What has caused this increase in temperature, that requires it to be cooled by air............something has changed.......find out what it is and fix it.

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#16

Re: Question From a Millwright

05/30/2009 8:48 PM

The op never said the bearing was too hot, only that it was hot. How hot we never got to find out. If the bearing is on a conveyer shaft in a big freezer, 50 degrees will be hot. If in an oven 400 degrees would not be hot.

The question was weather blowing air across a bearing housing will cool the bearing, or only the housing. Unless there is some very good insulation between the bearing, and the housing, some heat will be removed from the bearing.

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#17

Re: Question From a Millwright

05/31/2009 8:30 AM

In many cases in the past I have used air to cool bearings, and it works to some degree.

As I do not have enough information on the size speed and the temp increase ramp it would be hard to determine if it will work in your application.

Depending on the amount of heat that the bearing is generating will determine amount of air flow, depending on the thermal conductivity of the bearing and housing will determine how much heat can be removed.

There have been cases where we have used chilling nozzles, these are used in the machine industry to cool parts while machining, but as they use a lot of air to create cold air output it may not be economical to use.

As a Machine Design Engineer I would approach this in a different direction, "Why is the bearing getting hot" and if it is getting hot and there is no way of stopping the heat buildup then install the correct bearing that will handle the heat.

But to answer your original question. YES air can reduce the heat buildup in bearings, and it has been done for years when there is on other alternative, and there is no bearing that will handle the heat.

We have also installed heat sinks on several applications, these were on bearings with a slow rise in temp, but you must look at how fast the bearing is generating the heat as you only have the thermal conductivity of the outer bearing race to the housing. Reducing the ball heat is what is critical.

One last comment: There are hundred's of Bearing Engineers working for the big bearing Manufactures, have you used their expertise to assist you? Most will come to your location and help to determine cause and corrective action.

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#18

Re: Question From a Millwright

05/31/2009 7:04 PM

Does the bearing over heat? How hot does it get? did you measure the temperature? what is the max working temperature according to the manufacturer? All these question in my opinion must be asked before any patching is done. Cooling down an over loaded bearing says something about the basic design, and load calculations. or If it is well designed than the culprit might be the bearing itself? Cooling is aleays good and necessary when friction is present. But if a machine is well desigend either it is built in and if addtional cooling is necessary is a hint for trouble. My 5 cents...

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#19
In reply to #18

Re: Question From a Millwright

06/01/2009 4:37 PM

Quickest,is pump white lead thru it.Been there done that.

Just keep on pumping.

Joe in Texas

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#20

Re: Question From a Millwright

06/02/2009 9:37 AM

I am not sure of the application you are speaking about, but from my experience as a mechanical contractor and lubrication rep, the type of lubricant will greatly affect the ability of the bearing to reduce the generated heat of friction. If the environment is extremely hot, then we need to discuss exact parameters.

You can reach me at 216-401-1845.

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#22

Re: Question From a Millwright

06/03/2009 9:56 AM

My thanks to everyone for their input - some replies covered much more than I was looking for, but all were appreciated. The question indispute was simple wether or not applying air would cool the bearing or make things worse by only cooling the bearing housing. At any rate my thanks again to all.

Sincerely,

Greg Davis

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