dumb question, does turbo spin freely

[bb]

This is probably a dumb question. I was watching a show where they had a car with a turbo and the guy was messing with it an spun the impeller while he was talking.
Here's the question... should my impeller spin freely (where if i spin it, it will continue to spin after I stop). I ask this because mine has a small amount of drag to it where it will not spin totally free.

Thanks
Oscar

[Bill Verburg]

It should spin w/o any noticable friction at all

[DeutschMark]

Additionally, there should not be any play in the impeller bearings.

[sammyg2]

Um, Here my take on this:
There are two common designs of turbos, one had piston ring-like oil seals, the other has a mechanical seal with contacting faces. This style has more friction, at least until it gets up to speed.

A turbo should spin freely by hand, but if you spin it with your fingers I would expect it to stop in a second or so. I've never seen a turbo in good condition spin so freely that it would take more than a second or two to coast down to a stop. The oil in contact with the tight clearances will create a slight drag, plus it just doesn't have very much mass to overcome that slight friction.
I have seen worn out turbos that would freewheel and take longer to coast down.

[beepbeep]

It should spin w/o any noticable friction at all

Additionally, there should not be any play in the impeller bearings.

Wrong and wrong.

A turbo with plain bearings (as most of turbos are) will not spin without noticeable friction. It's plain bearings are suspended by thin layer of oil and when oil-pressure is present, it's shaft will spin on oil-film, not touching the bearings. When oil pressure is not there, there will be a drag and if you try to spin the impeller by your fingers it would stop in the matter of seconds, often a fraction of second.

Also, every plain bearing turbo (even brand new one) will have some play in the bearings, to allow oil-film to form between shaft and bearing itself. Otherwise it would be impossible for oil to escape and bearing would seize.

My brand new never-started Garrett GT40 has some play (and no, it doesn't have BB cartridge).

So don't worry, your turbo is probably OK even if it doesn't spin freely and has some play (as long as play is under factory margins).

BB-cartridge turbos will spin much easier when dry though. It was probably one of those you saw. It's actually a bad thing to spin a turbo without lubrication but BB's are sturdier when it comes to this (even if they last shorter than plain bearing ones when used due to constant mechanical contact).

[Jim2]

Plain bearing (bushing) turbos have a noticable amount of play designed in. This tolerance provides an oil cushion which absorbs minor balance issues when the turbine is running at high RPMs.

Neither the impeller or turbine should rub against the housings when turning the turbine shaft by hand. If you do notice this, it indicates the bearings are worn out.

[DeutschMark]

Thanks beep, I stand corrected. I guess what I really meant was that if bearing play becomes excessive, the impeller contacts the housing, causing expensive damage.

[Cory M]

The shaft should spin smoothly (assuming there's some oil in there) and there shouldn't be any axial play, there may a bit (we're talking thousandths here) of radial play because the shaft is meant to ride on an oil film inside the journal bearings. The compressor and turbine wheels shouldn't rub the housings at all.

[thabaer]

There are specs for radial and axial play, I just checked the SC's K27 a few months ago and (from memory here) I think they are on the order of .004" and .009". DON'T USE my numbers, check a book! Just posting these to confirm for the curious that there is measureable play in both planes on journal bearing turbos that are in proper working condition.

You just use a dial gauge clamped to the housing, pretty straightforward check.

I can't, however, say the same thing for other designs like the Garret bearing cartridge parts, don't have clue there, never looked at using one... yet.

[donstevens]

I agrre with Beep Beep. It wil spin much more freely with oil pressure supplying and oil film.