rotor to journal bearing clearance

It is difficult to salvage in case of the severe damage on the journal diameter unless you re-machine back.

The damaged surface of the journal is going to immediately damage the new bearing by cavitation.

The frame 5 Bearings (as per my experience of about 10 years back) are offset machined and that too with the vertical and horizontal axes not equal (a shim is put in the parting line while boring/ Exact amount of offset i don't remember but i think it is about 0.15mm or so.

This is to ensure the rotor centering when floated by the hydrodynamic film.

The clearance between the rotor and the journals were (again memory 0.40 mm or so.

You can take this value since I remember checking the centering by shims and lead wire check.

Excess clearance of the journal (as well as the scoring on the journals) will unstabilise the hydrodynamic film (based on the visosity, rpm, etc the optimum journal clearance is maintained for the rotor centering) so that the EHD film on all sides put equal pressure and centers the journal. The errors are bound to result in excessive machine vibration and the less clearance/ journal run-out are going to kill the white metal.

The rotors are machined and balanced at overspeed (in our case in vacuum tunnel) to ensure the behaviour of the GT in operation.

I advise you to send back the rotor to OEM for rectification. Local polishing may not bring the required parameters. the OEM (may be us or others, I don't know and currently I am in other product line) will re-machine the journal, balance and manufacture a bearing suitable to the new journal diameter.

Normally my advise is not to carry out major maintenance for high speed machines (5200 RPM if I remember ?) unless you have all the facilities and expertise.

However if the damage is superficial, contact the service personnel of the OEM and take their advise.

Just one question why the GT didn't trip on vibration before the journal got damaged ? The equipment protection logics are their in the system.

PS your journal diameter is 8" with a tolerance of 0.0016" approx ?

The tolerance on the clearance is upto a max of about 0.02" or so.

Thank you for your detailed reply.

Can you please clarify what you mean by rectification? We dont have a facility for in site machining of the journal.

Regarding GT trip on high vibration, the unit didnt show any sign of abnormal vibration before the damage took place ?

Your evaluation of severe depends on your experience at evaluating this type of damage, the key thing you are looking for is; will this shaft and bearing condition allow for the generation of a proper hydrodynamic film.

The question about bearing clearance leads me to believe that you had a wiped bearing that was severe enough to score the shaft. OEM's usually give design clearances on the drawing or as a multiple of shaft diameter. The design clearance is usually in the realm of 1 to 1.5 mils (thousandths) per inch of shaft diameter, double design clearance is often used as the point to re-babbitt a bearing.

The type of bearing involved will enter into the equation in that some bearings are designed to dampen vibration by creating pressure areas in the proper places.

Have you looked at on site machining, better safe than sorry.

Why don't you contact the OEM ? he may assist you in gettin it machined in anywhere nearby you have a big workshop with a centerlathe big enough to handle the rotor assembly.

The journal may have to be re=machined and then polished to a high finish (we normally spacify an ra of 0.4μ or better for the journals.

But always you have a high risk in the balancing portion and at least none of our customers have a nearby facility to do this. Check whether you have any other high speed machine manufacturer nearby, even steam turbines may do since those rotors are also balanced.

But you need a dynamic balancing and not a static balance.

I advice strongly to contact OEM/ other OEM (even an OEm with non GE models may do). My experience as mentioned is a bit outdated and may not be exactly concurrent and also I am and was form OEM area where we do not repair (unless sent back from site for this purpose) and in that case as mentioned we re-machine the journal and change bearings to the dimensions.

GE has their service personnel in all the continents you may log onto their web site and ask for help.

Exact extent of defect and rectification normally can not be ascertained unless seen physically (Some times now metal spraying is resorted to , but even after that, the journals are re-machined and re-balanced- also we have observed that the hardness of the sprayed area is significantly lower than the parent)

Check whether there was a contamination in oil (solid or water) since these normally may cause damage to bearings. Any sort of failure of MOP and EOP didn't start in time ?

Just for curiosity how did you know the damage if no vibrations ? How many days it has run before failure ?

In a failure analysis pl concentrate on what we say as "What parameter has changed" these questions if you find may help you preventing in future failures.

In an recent case, in one of our equipments (not GT) , same feature has happened during shop testing- it was traced to a small defect in shape of lobe (four lobed bearing). For you it may be a bit difficult to trace to root (better to stick to OEM or OEM recommended sources for the critical components)

PS: As mentioned do not expect much detail from an Ex OEM, however will like to help as much as possible.

We have already carried out local polishing then oil stone to get the desired surface finish.

The bearing clearance was measured using lead wire and found 0.022 (22 thou) which is maximum according to OEM specification.

The question is what could go wrong if we run the GT in this condition with max clearance reached?

As the the clearance measured at site is maximum as the OEM.you should change the beaing. By the way what is the surface condition of the bearings?

I think this has been resulted due to poor quality of lubricant purification system and bad filters.in this process no high bearing metal temperature or shaft vibration is noticed by the protective devices.Please review.

Insitu machining of the rotor is possible but it will also take considerable time. dynamic balancing will be required to make it safe for running.

Increase bearing clearance may lead to hydrodynamic film failure or unstability of the rotor and severe damage may occur.

Were the surface defects deep or superficial ? since you are mentioning the yo could polish with oil stone that means it was not too deep. Also in most of the cases, serrations being peripheral (ie aligned to direction of rotation) will not give much problem- unless it is deep since the clearance - the EHD film will be around 10 thou, the aberrations may not affect the lift unless they are signifinat with respect to this value.

Unbalance caused (as i understand - polishable) may not be significant either.

Ensure good finish (if possible why not polish with some 400 or 600 grade emeries ? available in met labs scrathes in journals will have additional effect in the fatigue life - it is the weakest portion of the rotor)

replace the bearing and check the vibration pattern and proceed , take a chance (i know as an OEM how much time we will take to repair your rotor (don't tell my marketing man ) Check with hawks eye the vibration pattern as you speed up with starting mechanism - throgh AGB - till you put the system on load only after ensuring healthy pattern.

As you have mentioned the problem with machining is the time it take to repair the rotor and manufacturing of undersized bearing.

Thank you for your technical advice

By EHD do you mean elasto-hydrodynamic lubrication?

If it is so such a behaviour does not appear in journals since pressure is too low, it will appear in ball/roller bearings where pressure locally is very high and modifies in an important way the lubricant viscosity. In usual bearings it is only a hydrodynamic edge which supports the shaft.

A company called Nicol and Andrew, web address onsitemachining.com can usually repair this damage on site, or they will tell you if it is possible.

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