Coast down during oil pressure loss in journal bearings

[ujonsson]

I recall seeing a chart somewhere for journal bearings with load*speed on the Y and coast down time on X. The chart illustrated how large load and speed that could be managed during coast down at power loss or similar events where the oil supply is interrupted to a journal bearing.

The bearing is used in an electric motor the bearing is 3.5 in dia, 2 in wide spinning at 3600rpm. The load is about 500lb during coast down at 0 torque.

It has to survive repeated 30second coast downs without oil pressure. The speed drops quite quick but it does not stop until about 30 seconds. We will do some testing but I would like to know what experience is out there.

Ulf

 

[therealkilkenny]

Hi Ulf,

I am working on measuring oil films in a Journal Bearing. We are using an ultrasonic technique, and have sucessfully measured oil films in journal bearings in the lab and on an industrial test rig. The technique is very accurate (we have measured 50nm in a ball bearing), is non invasive, and is very robust.

If this may be a solution, then get in touch through

http://www.tribologyatsheffield.com.

Regards

 

[ujonsson]

I understant the value of such technology. The issue here as I see it is teperature rise during loss of oil and possible smearing of the babbit. I wiew it as a boundary lubrication issue more than a fluid film but I will read up on your method. I have been using optical interferometry and capacitive methods in ball bearings here and at Lulea University in Sweden and your tecnique have some clear advantage in some cases. How large are the transducers and what is the requirements for mounting them?.

 

[therealkilkenny]

If you get to the stage of smearing of the babbit, then I suspect you can forget about your bearing lasting very long.
We had a visitor from Lulea the other week, Donald McCarthy, you may know him. He popped in to see our technique running on a journal bearing. Also Sergi Glavatski came to see us last september.
With regard to the transducers, they can be from 5mm Dia to 25mm dia and from 10mm high to 30mm high. These are the sizes that we currently use, but it is possible to get transducers only 1 or 2mm in size.
They are mounted on the back of the bearing and we look through the casing to the oil film. The only real constraint is temperature as the transducers do not like to work above 70 deg C.
The content of

http://www.tribologyatsheffield.com

is rather dated (we are trying to update it at the moment) and the results are from tests done a while ago. I have much more up to date material that I can send you if you like.
You can email me at p.harper@sheffield.ac.uk,
Regards,
Phil Harper

 

[electricpete]

We have 2500hp 1800rpm electric motors with 4" sleeve bearings that have coast-down times of 10-15 minutes when uncoupled. Rotor is somewhere in the range of 5,000 pounds distributed over two bearings. Seems like a lot more severe duty than yours.


When coupled to the pump the coastdown time is quite a bit shorter... I haven't measured it.

 

[ujonsson]

>electricpete (Electrical) wrote on Jun 4, 2003
>We have 2500hp 1800rpm electric motors with 4" sleeve >bearings that have coast-down times of 10-15 minutes when >uncoupled. Rotor is somewhere in the range of 5,000 >pounds distributed over two bearings. Seems like a lot >more severe duty than yours.

Thanks Electripete
Intresting point. Do you know if these bearings are lubricated with a pump and if they have a ring lubrication backup?
My problem is for power outages where I loose the pump and have to live on residual oil in the bearing. The large electrical machines I seen has had a oil ring providing emergency lubrication at power outs.

 

[ujonsson]

therealkilkenny (Mechanical) Jun 4, 2003
If you get to the stage of smearing of the babbit, then I suspect you can forget about your bearing lasting very long.
We had a visitor from Lulea the other week


I know Sergei very well. Donald have started after I left in the spring of 2000.

The issue is if or not we will get this smearing.

Ulf

 

[electricpete]

Yes, it is an oil ring lubricated pump. We have forced oil retroffitted which also feeds the top of the bearing. By the way VG32 oil. I was thinking about maintaining the film characteristics which you clarified above are not your concern. Sorry.

 

[ujonsson]

Electripete,

Thanks for your help anyway. I was actually involved in this at Lulea a little bit where we were looking at low speed operation of these type of bearings for cool downs etc. They are extremly rugged. The scope was to se how dirt and filteration affected wear at low speed operation.

Anyway, thanks for you comments.

Ulf

 

[therealkilkenny]

Hi Ulf,

If you want to know whether smearing will take place, measuring the oil film is the critical factor (provided you are measuring in the right place). If you know your oil films, you can redesign/alter the bearing or oil supply to adjust what will happen in the event of an outage.

Phil