Location of bearings in MV motor 5

[edison123]

Recently, I got a 3.3 KV, 2000 HP, 990 RPM motor for a rotor rewind. It has a ball bearing (6234) on the Drive End (DE) and a roller bearing (NU 234) on the slipring end (ODE).

First time, I am seeing a roller bearing on the ODE and a ball bearing on the DE.

When asked, the OEM says the arrangement is correct.

Any views on this ? Anyone else has seen such a bearing set-up ?

 

[sreid]

Not my area of expertise but a ball bearing at one end can be a locating bearing and a roller bearing at the other end allows for axial thermal growth.

 

[edison123]

Thx sreid.

I understand the functions of the bearings. But I have only seen that in a Roller & Ball bearing combination, the roller bearing is always at the Drive End due to its higher radial load rating.

In this motor, the roller bearing is at the non-drive end (ODE), where there is much less radial load as compared to the drive end.

 

[Skogsgurra]

I take it they have the same outside/inside diameters?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Yes Gunnar. Both the bearings have the same ID/OD/Width (170/310/52 mm).

As per SKF manual, the dynamic load rating of the roller bearing 616 kN is while that of ball bearing is only 212 kN. So, it makes sense to have the roller bearing on the Drive End, the torque transmission point. But, OEM is insisting that their set up right without giving any reasons.

I hope the forum members can shed some light.

 

[Skogsgurra]

Sounds unusual to mee, too. Is there a pulley or is it coupled to load directly? I think that it is critical only when you have a pulley and a belt that puts radial load on the bearing.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[powerjunx]

Hi, edison.

Your observation is keen enough to distinguish the difference! Such bearing location is unfamiliar to me! I been dismantling machines ranging from general purpose to integral horsepower or even DC machines but yet i can't relate such arrangement. Weird!

Though, ID/OD of bearing are the same - obviously it 6234/NU234, i wonder why OEM configured that design?

I been through with rolling mills apllication (DC or Wound Induction Machine) but this configuration is nowhere to be found.

Ed, your post is interesting. Maybe we could clear this bit by bit..
What is the load your machine?
How's the coupling configuration?
COuld you give us actual scenario in which the machine is in actual operation?










regards,

bil

 

[edison123]

This is a banbury mill motor (rubber mixer) used in a tyre company. The motor is directly coupled to a gear box system (where the speed is whittled down to about 10 RPM with mutiple stages of gearing). The NDE has just an external cooling fan for the top mounted air-to-air cooler.

I intend to swap the bearings.

I am still hoping some one might have come across this type of bearing set up and have some explanation for it.

 

[ozmosis]

edison123
Is it vertically mounted? I know Siemens will sometimes fit ball bearings in the DE if axial loads permit, and rollers on the nde.

 

[edison123]

Sorry, I should have mentioned it is a horizontally mounted motor.

 

[powerjunx]

Ed, could you describe further the coupling assembly? Is it flexible coupling with rubber or spring support, with spacers?

 

[edison123]

It is a rigid coupling connected directly to the high-speed gear coupling. The motor and coupling beds are also rigid.

 

[powerjunx]

It's rigid, whew! This really uncommon. I thought, it's flexible. Interesting.

Do you mind to give me the Motor manufacturer and Specification? I would like to discuss this configuration to our senior consultant.

 

[edison123]

It is a (or an) ABB, Finland make.

 

[aolalde]

Edison.

I have seen that array on other European motors. The only reason is to fix (lock) the driving end side of the shaft and allow for thermal expansion of the shaft over the roller bearing.

The motor application described does not have any extreme radial load on the driving end since it is rigid coupled to the gear box, then a ball bearing on this location is the best solution. If you swap those bearings, a thrust force (due to thermal expansion) will be exerted against the gear bearing.

Only problem with roller bearings light radial load is skidding, generating heat and noise. I hope the unbalanced magnetic pull plus the rotor weight make the rollers "roll" instead of "skid".

 

[edison123]

Thx aolalde. All the machines I have seen have had roller bearings on the DE and most of them were rigidly coupled. So, if the axial expansion via the roller bearing (on DE) causes thrust on the driven equipment, why many motors are configured that way ?

Also, isn't the radial load higher on the DE due to torque transmission even in directly coupled machines ?

 

[aolalde]

Any shaft will expand due to the temperature gradient, If you allow that length increase on the drive end and the shaft has no movement allowance, a force proportional to the shaft cross section area, Young modulus and length increase will react against both shaft anchored ends.
A perfect aligned motor-load shafts will have no radial force due to the torque transmited to the load. A torque is the result of an identical pair of oposed forces.

If your transmision is via belts or chain, the tension perpendicular to the shaft will impose a moment to the shaft extension that result in a radial load reaction on the bearing.

 

[raisinbran]

Edison 123,
I would be tempted to leave the bearings as is. In a solidly coupled configuration, you want the axial shaft growth to be opposite of the driven machine, in your case - out the back end of the motor. This way, the motor shaft can do what it has to do and not interfere with any other machine.
We have played games with bearing location many times in the mining industry, and that has been out biggest concern - axial growth vs. axial clearance.

Raisinbran

 

[edison123]

Thx aolalde and raisinbran. I will bear that in mind.

 

[stardelta]

Seen it a few times with larger Schorch machines designed for custom applications. It does go against convention and first time we saw it we decided to verify with Schorch to ensure the layout was correct and not a mistake made during a previous repair. They were happy to confirm the layout as being correct but could offer no real explaination other than provision for thermal growth and the machine having a large and heavy cooling fan on the NDE that imposes its own radial forces.