360 Degree Thrust Bearing

[Birnie]

I hear the above term used every now and then referring to turbos but I have NO idea what the hell it means.

Please explain!!!

Thanks...

[discopotato03]

Bush bearing turbochargers have to have a thrust bearing to stop the turbine/compressor assembly moving back and forth within the bearing housing - end float . So at the compressor end of the turbine shaft is a collar or cylindrical spacer with a square section groove machined into it . The front of the turbos bearing housing has a plate fixed to it and the shoulders of the thrust collar bear on it .

Most turbos have this plate with a gap or slot in one side so that this thrust plate can be slid in between the shoulders of the collar at assembly time , once the bush bearings and their circlips are installed the turbine/shaft goes in then the thrust collar and plate .

One of the down sides of bush bearing turbos in high speed high boost applications is that they get a fair bit of end float loading and the thrust bearing assembly doesn't have enough bearing area to be long term reliable . So usually the thrust plate suffers rapid wear allowing excessive end float and the wheels generally have a big "rub" on something and destroy themselves .

These standard style thrust plates are said to be 270 degree and the full circle ones 360 degree . As a bit of a band aid upgrade the full circle ones have a bit more bearing area in the plate itself but its not really enough in the scheme of things . Their use means you have to have a two piece thrust collar to be able to assemble it . I'm not sure whats available now but in my time with bush turbos it meant having to use a carbon or dynamic compressor end seal which is not prefferable in a blow through EFI application because it adds drag for no very good reason . Speaking of drag you will get a little more oil shear drag with 360 degree thrust plates because thats the nature of the beast .

Ball bearing turbos use annular contact ball races which means they support the turbine shaft against radial and end float loads . The fact that they do so with a fraction of the drag and much closer tolerances than bush turbos is a huge advantage . Bush turbos have a very short life in high shaft speed use because of the limitations of their bearing system . Todays modern wheels are designed to run faster than the ones they replaced which is why from an OEM perspective they can be compact and make quite reasonable torque/power .

Cheers A .

[dontfeelcold]

do you mean angular contact ball races?

Whats an acceptable amount of side to side play and end float in a ball bearing turbo when it is cold (I would assume there is some allowance for expansion)?

[Dale FZ1]

The term is annular - as spelt. It is the name of a bearing design.

Take it as read that both radial and axial float in a BB unit is significantly reduced over a plain bearing unit especially in a bench comparison. The plain bearing unit of course relies on the film of oil to carry the rotating assembly while the engine is operating. So clearances tighten up and float reduces by a big margin when operating - much the same as the plain bearings within the engine itself.

Getting acceptable wear from thrust bearings in turbos used on early petrol engine applications was a challenge for turbocharger engineers when the throttle was installed between compressor and inlet valves ie. blow through as Adrian indicated. No such problems with diesels which have no throttle.

This wear problem was partly attributable to pressure wave reversions on closed throttle, which induced opposite-direction axial thrust in the rotating assembly. The old school draw through carburettor installations did not suffer nearly as much so enjoyed a longer life.

On-task, the answer is a small amount. Detectable to feel, but would take finely calibrated instruments to measure fractions of a millimetre.

Edited April 2, 2007 by Dale FZ1