poniedziałek, 9 marca 2009

reaction turbine overhaul - part five

This part is the last one, dedicated to reaction turbine overhaul, I supervised. In last Friday I got info from our departament resposible for on site activity, that the trial run of this has been finished and no major defects were found.
Today, I would like to spend a couple of minutes, presenting bearings overhaul.
As it was not a point of outage scope and rotor's journals were not machined, the bearings could be used again.
In general, the most important features of the bearings are: geometry with right clearancess and the condition of the white metal. The clearancess depend on journals diameter. Sometimes, at small diameters, rotating machines can be operated with equal clearancess (bearing is a circle) but very often clearancess are not same on the sides and up, bottom. The bearing has lemon shape.
The another problem that can cause supply of new bearings is the lack of right adhesion between white metal and bearing casing. It is checked by US examination. Certain, % of surfaces can have a lack of adhesion.
In my case project managers wanted to have new, round bearings. NDT examination done on site showed the lack of adhesion and it was necessary to manufacture new bearings.
We used support of external supplier and it prepared bearings for us. However, I refused to accept first delivery, as geometry was out of tolerance. I required that the diameter is 0.00/+0.02 mm and the shape defect (cone) is not more than 0.02 mm - the bearings are very precision parts of the rotating machines and such requirements are nothing strange. Unfortunatelly, our supplier sent us bearings wrong machined and ... thought we did not discover it or whatever.
I asked our Quality Control to check dimensions against documentation I prepared prior the bearings were sent to supplier. It turned out that we have bigger diameter than can be and additionally they have ~ 0.20 mm of shape defect (cone) it is completly not acceptable and bearings were sent back to new white metal cast and machining.
The supplier did the mistake for the secound time, what is horibble but we were very sensitive and found it. The third time was the best. I think that after these mistakes we have right opinion.
What, I wrote above did not affect the overhaul deadline as the bearings were prepared parallel to other works. Anyway it can happen that new bearing delivery date is the critical one and "supplier's issue" can make us very nervous.

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