CT12a Information (shaft play)

[Lambolica]

In my never ending quest to learn how shit works I have pulled apart my old CT12a's to try and work out "Shaft play"

Now alot of you more knowledged ones may know all this already but I have never seen what makes turbos tick. So I do what I normally do when I want to find out... I pull it apart and no put back together....

I am hoping that some constructive information here from the more experienced users will make this a helpful thread for prople like myself who are new to Turbos.

Firstly Shaft play... I was under the impression that Shaft play was the movement of the shaft in and out. ( I used the theory that as a turbo creates pressure in the piping the compressor wheel would be pushed back against the rest of the turbo and over time as the wearing surfaces wear down this would create in-out shaft play).
After pulling apart the turbo I found that this really cannot occur (at least before the shaft bearings go) as the compressor wheel is clamped to a bearing on the housing and held in check with the rest of the bearings on the shaft.
On both turbos (the rear being worse than the front turbo) there appear to be 2 bearings on the shaft that have worn down and worn the shaft as well. I will measure the movement of the worn bearings on the shaft but I would say that it is around 0.25mm bearing to shaft and a little less bearing to housing. giving a total sideways movement of the shaft of a little less than 0.5mm at each bearing (more on the rear turbo) this gets exaggerated as you move down the shaft to the compressor wheel where I can push the wheel hard against the housing. ie greater than 2mm play.

This is all well and good I have shaft play. what I can't work out is how does shaft play relate to increased oil consumption? I can see minor oil loss issues but can't explain the oil losses I experienced before I pulled them off. I had a great deal of oil in the turbo piping and oil was blowing out of the BOV as well on hard changes and particularly on the rear turbo there is evidence oil comming out of the exhaust wheel.

The following is a picture of the turbine shaft with all the bearings on it.....



From left to right:-
1. Exhaust wheel
2. shaft ring.
3. shaft bearing 1
4. spacer
5. shaft bearing 2
6. oil baffle thingy (appears to do nothing more than act as a spacer and locate a oil gallery plate.
7. end plate Has a small bearing that the compressor wheel sits on and is held in place with the nut at the end. this bearing spins inside another housing.
8. Compressor wheel
9. ( not there but...) nut to hold compressor in place (note left handed thread)

Would I be correct in saying the shaft ring (No. 2) and the end plate (No. 7) be where the oil is leaking out? As I can't see (on the few brief measurements I took late last night) that there is a huge amount of wear on these parts.

Incidently, after pulling the shaft down and photographing it I put it back in the centre housing (with out compressor and exhaust housing) filled the oil gallery with new oil, connected a dremel tool to the shaft and spun the turbo in my and at around 20,000rpm with out any axial vibrations for several miniutes...In my hand That was kind of cool until I realise what may have happend if it decided to get too close to my hands. didn't see any loss of oil. My guess here is the housing heating up plays a distict part in distorting components???

[wilbo666]

Yep No 2 is the rear oil seal, leakage there can occur there I believe?

No 7 is the thrust bearing, as you said when the turbo creates pressure it wants to move back, this absorbs the load I believe. On the turbo I disassembled (T03 similar) the thrust bearing was a bronze bush I believe, and there was what looked like a carbon? seal behind the bush/bearing.

I would suggest the oil leakage would be the result of the pressures created, very high vacuum and also boost. As I see it the compressor side could allow oil to be burnt under vacuum, and the exhaust side I'm not really sure to be honest (someone enlighten me?!), I would have thought that the exhaust would always have positive pressure, I maybe wrong on that thought?


Cheers
Wilbo

[Draven]

exhaust side always positive pressure yes, but not necessarily higher positive pressure than your oil pressure

[Lambolica]

I disassembled the rear turbo and cleaned it up on the weekend. once again I put it back together (minus comp and exhaust housings) and filler the gallery with oil and let it sit on the ground. I came out the next morning and the complete contents of the gallery were spilled all over the floor.

Both the compressor and exhaust ends were leaking and lost the full content of the gallery in about 10 hours. I'd hate to see what it is losing when spooling.

Regardless I've nearly got the other turbos on the car and should be on the road this weekend blowing a little less smoke

The No.7 has a thrust bearing in it but it appears to be an alloy, or at least not brass. and there was a brass backing plate on it.

On closer inspection of the rear seal (No.2) the ring didn't appear worn and located in the housing nice and tight but it appears that it has significantly worn through the shaft material itself (particularly on the rear turbo) I'm assuming oil is going between the shaft and the ring to leak out. I was looking for ring to housing leaks.

I still think it wouldn't be that expensive to rebuild as the prices I've been quoted but meh. job is almost done.