interference between pump DE AND NDE bearing and bearing top cover

[pradeep4u]

Dear All
I want to ask question regarding to boiler feed pump journal bearing and bearing top cover interference. We use HDB 125/9 KSB make pump. it has 9 stages. In journal bearing at DE AND NDE side we keep interference of 0.06mm between bearing and bearing top cover. I just want to ask why inteference is kept. Please give your valuable suggestions
Regards
Pradeep

 

[TugboatEng]

Journal bearing? The interference keeps the bearing from rotating in the housing.

 

[1503-44]

Clearance for lubrication.

https://oaktrust.library.tamu.edu/b.../163654/T13179-190.pdf?sequence=1&isAllowed=y

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[pradeep4u]

Dear 1503-04
I am not talking about clearance between shaft and bearing. I am talking about interference between top bearing part and top cover. For antirotation of bearing normally pin is provided so that bearing not to be rotated with shaft.
Regards
Pradeep

 

[TugboatEng]

You said journal bearing in your original post and now you have said anti-rotation bearing in your current post. Can you clarify, is this an anti-rotation bearing or are you trying to prevent the bearing from rotating? I don't know of any anti-rotation journal bearings but I do know rotation of journal bearings can be a problem. I see pins used to prevent journal bearings from rotating but this is a poor choice as the pins often shear. An interference fit is the best option for preventing rotation of journal bearings but, except in the case of precision insert, requires a line bore of the bearings after installation.

 

[Artisi]

Have you thought of addressing your question to KSB?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[pradeep4u]

DEAR TugboatEng
I did not said of anti rotational bearings. I said that journal bearings locked by pin to prevent rotation. I don't have contact details of persons of KSB but i thought you all will be able to solve my questions
Regards
Pradeep

 

[JJPellin]

Rotation of the bearing within the housing is not the only risk. The pin can prevent that, as you noted. But, any movement of the bearing will result in fretting corrosion which will damage the bearing and the housing. The crush (interference fit) between the bearing and the housing is needed to prevent movement and fretting damage.

Johnny Pellin

 

[electricpete]

I think the question is already well answered.

I’ll just throw in some thoughts in the discussion because I don’t completely understand what drives the selection of bearing/housing innterface as interference vs clearance and the level of interference

The purpose of interference as mentioned is to prevent relative movement of any kind. Yes anti-rotation pins do almost the same thing but they don't prevent fretting as JP mentioned. And they may not be as reliable even for gross movement as Tugboat mentioned... Like Tugboat, we have also had the bearing/housiong anti-rotation pins snap. The ones that are problematic for us are the electric motors that have one or more insulated bearing, so the anti-rotation pin is made of some kind of non-metallic composite which is not as strong as steel and one one family of machines that pin has broken twice for us with some ugly consequences (pin broke and bearing rotated during startup when friction is highest, so the oil ring groove was no longer at the top, so the oil ring could no longer effectively lubricate the bearing, which wrecked the bearings and damaged the journals... unfortunately we had no temperature monitoring on those machines).

Selection of interference fit (as opposed to clearance) also tends to provide tighter positional control of the bearing.

The bearing-housing interference may affect the as-assembled shaft-bearing clearance, especially for thin compliant bearing shells like bronze (plastigage check after torquing housing fasteners takes this into account)

For journal bearing housing-to-shaft fit, I have seen that high-speed pumps (> 3600 rpm) with small diameter shafts 1-2.5” tend towards high interference, while low speed motors (<=3600rpm) with larger shafts tend toward lower interference and sometimes clearance (up to 0.002” clearance is allowed by most motor repair specifications). Those are just the particular machines I have been exposed to... I don’t know if the speed is the main factor (slow/fast) or the application is the main factor (pump/motor) or the shaft size is the main factor. I can speculate about reasons. My best guess is that precise positional control of the pump within tight impeller clearances may be more critical than precise positional control of motor within its airgap. (This one makes the most sense to me... multistage high speed pump rotors are often somewhat flexible and some are subject to galling if the impeller touches). Also perhaps smaller shafts with their smaller bearing clearances may be more sensitive to misalignment which dictates tighter positional control. Or maybe the high speed makes fretting more likely. Or some combination, who knows.


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(2B)+(2B)' ?

 

[pradeep4u]

Dear All
Thanks for your replies. I am overwhelmed. Does interference fit applies for all journal bearings. e.g turbines bearings
Regards
Pradeep

 

[JJPellin]

There may be some exceptions. If the machine is large and slow and the bearing is consistently heavily loaded in only one direction, an interference fit may not be needed. But, there is still no reason, in most cases, to have the fit any looser than size on size. As far as I know, all of our turbine bearings (general purpose and special purpose) call for an interference fit for the radial journal bearings.

Johnny Pellin

 

[TugboatEng]

Just to throw a wrench in the gears, turbochargers have clearance on both sides of the bearing and are intended to rotate in the housing.

 

[electricpete]

Ha, yeah that would be a special case.

Some more info for motors:
EPRI NMAC Repair and Reconditioning Specification Guidance for AC Squirrel-Cage and Salient Pole Synchronous Motors with Voltage Ratings of 2.3 to 13.2 kV Report 1016679 (also Revision 1 of 1000897). You can download for free (or the cost of giving up your email address) here:

The fits between the bearing OD and housing shall be checked to ensure they are within the following limits. For thick shell bearings, unless otherwise specified by the motor manufacturer, such fits should be size-for-size to 0.001 - 0.002 ins. tight. For thin shell bearings such fits should be size-for-size to 0.002 in loose

... Thinking about why they (EPRI) might treat two types of bearings differently, I can speculate that maybe they don’t want the assembled shaft-bearing clearance to be sensitive to the housing fit, so for thin shell bearings (that might be significantly deformed by sturdy housing), they make it size-for-size or slightly loose. (but they still recommend plastigage checks with housing tightened which means you have to assemble the housing onto the bearing twice, once for plastigage check and again for final assembly). Or else there is some concern about stresses in the thin bearing clamped by interference, maybe with thermal expansion / differential expansion effects. Who knows.

But we do at least one motor where the OEM tells us to have a clearance 0-0.002”, even though the bearings are is clearly thick shell imo.

And the Electric Apparatus Services Association (respected trade organization for motor repair) says

Most electric motor sleeve bearings perform best with housing clearances of 0.001” to 0.003”

It seems like a bit of a hodge-podge on the electric motor side. So from my side, all I can say is punt: look to your OEM, or applicable standards, or experts in your particular equipment and bearing.


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(2B)+(2B)' ?

 

[JJPellin]

There is one other possible exception that didn't occur to me at first. If the bearing is bronze backed in an iron or steel housing, the differential thermal growth will increase the interference slightly when the machine is running. In this case, it might be acceptable to have a slight clearance when measured cold in order to avoid an excessive crush when running hot. An excessive interference could cause another problem in some applications. In many of our gearboxes, the split-line of the housing contains a groove with pressurized oil that feeds the oil to the bearings. If the bearing interference was high, it would make it difficult to seal that pressurized split-line which could lead to internal or external oil leaks. So, as electricpete notes, it is best to use the fit recommended by the machinery manufacturer.

Johnny Pellin

 

[pradeep4u]

DEAR ALL
THANKS FOR YOUR VALUABLE KNOWLEDGE SHARING TO ME. I WILL BE GRATEFUL TO YOU ALL
REGARDS
PRADEEP