Bearing Problem

please post it in the mechanical engineering section. you will get more professional answer.

From BC bearing engineers:

The amount of clearance is standardized in the bearing
industry and most Radial Bearings are classified into groups
designated by the C clearance groups.
Clearance Group Suffix Bearing Clearance
C1 Smaller than C2
C2 Smaller than normal
CO or CN Normal
C3 Larger than normal
C4 Larger than C3
C5 Larger than C4
The amount of clearance depends on the bearing type
and size.
The suffix for the clearance group is always added to the
bearing code number except for the CO or CN for normal
clearance.

So so replaced the bearing with one that has tighter clearance. Depending on the machine this could well result in poor running and early bearing failure, as the bearing will now have more preload than it was designed for.

Good article...

http://www.ntnamerica.com/pdf/2400/brgclear.pdf

By the way this was all easily found with a quick Google search....

I am assuming that the OP is aware of the C designations (he is most likely)

Theory

The bearing clearances are selected on the operating conditions of the bearing to have the optimal clearance (in fact pre-load) on it.

have too much additional clearance and the bearing will run with this and as a result the like of it goes down, the unstressed rollers will tend to slip rather than roll.

Have too less clearance and the oil wedge (EHD Film) will not form, forcing metal to metal contact.

The life of the bearings drops sharply with reduced clearance and a bit less sharp on increased clearance.

Since you have reduced the clearance (C4 to C3), the life is likely to drop sharply. However whether the rubbing was due to it or not will depend on few informations

a) Did the rubbing existed when the motor was cranked?

b) When the rubbing sound was observed? as soon as motor started (cold rubbing) or developed after some time (after the bearings and shaft heated up, the reduction in end plays created it)

c) What is the interference between the shaft and the bearing? Was it re-checked before mounting ?

d) Do you have any FFT analysers around? you may check the vibration pattern and immediately get the answer.

PS: you have reduced the cold clearance (unmounted) from 125/165μ to 85/125μ

Dear Mr. Lalit Nawada,

First step is WE SHOULD NOT GO BLINDLY WHAT BEARING THE MANUFACTURER HAS SELECTED and FITTED. We should check up whether the Bearing selected and used is TECHNICALLY CORRECT and IF NOT CORRECT We should take up the matter with the supplier.

First please analyse this and come to decision. It appears to me the solution adopted is adhoc.

DHAYANANDHAN.S,

INDIA.

For a maintenance man (I assume Mr Nawada to be one), it will be quite difficult to do this.

As a practice, we go, in that role blindly by the OEM selection. Simply because the stresses on a complex machine are not too easy to calculate. That is my view as one dual role.

As maintenance incharge - I have approximately 60 machines to look after. It ranges from VMC, VTL, cenetr lathe, Cyl grinding, slotting drilling..., all with different capacity, some CNC, some NC, some conventional.

Suppose I go ahead and find a spindle bearing (or even a lead screw bearing need replacement). Of course i don't have any calculations with me (and I don't think Dorries or OM or Skoda will give that to me ?). So we go to the part-list/manual provided by the OEM and indent the component (bearing) may be from other company, but the same spec.

The second role is simpler, as Production Incharge, I review the performances of the bearings as well as different suppliers and can take the action since there I (or my designer) has the calculations, and there we are dealing with hundreds (in some cases thousands) of similar bearings that make up the products.

My advice will be

- Check the life of the bearing (after how many hours the OEM bearing failed)

- Check whether it is - premature and whether it was the equipment fault or due to our own error (This we know that the lubrication, sealing, etc are one in a while skipped for long and that causes the failure rather than the design). For this a lot of detective activity may be needed. After all nobody accepts that they have skipped an activity.

- have a look at the failed bearing.

- consult the motor OEM or bearing men (but for that this must be alarming failure - often, costly (it is by the size), not attributable to maintenance failure.

Take there suggestion and try out.

Also since it is a vibrating screen, there will be a lot of vibrating/shock loads. had the motor been corerctly selected for the application ? Is there a vibration damping device/coupling in between? The bearing has interference on both ID and OD?

Contact SKF,

I would use vibrating screen spherical rollers from them on both ends. Icreasing your interface is of no advantage if you selected the wrong bearings. I think I would lean toward double row spherical roller bearings for this application.