Post by dieselguy86 on Oct 2, 2021 18:56:31 GMT -5
Hi again guys,
I was looking at the ball bearing cartridge out of a Garrett gt55 turbo. They did something neat with it, where the cartridge floats on a film of oil for dampening.
While I was working on designing the shaft tunnel for my engine I was wondering if I could do something similar. Where my bearings are in their respective "shaft tunnel" or cartridge, and that is inside an outer "tunnel". An anti rotation pin will have to be utilized, but im curious if this could work? I know dampening is a big issue, and there's way smarter people than me in here. I'm trying to design this engine for as long of a service life as possible. The regular 25-50hr maintenance interval of the rc turbine will not cut it.
I've seen some use o rings, but I'm worried about how long those would survive on the turbine end.
Post by finiteparts on Oct 3, 2021 10:31:13 GMT -5
I agree with John, if you have a similar sized rotor, copying the OEM design would be a good idea. Take advantage of all the engineering work that a large, well-funded manufacturer has already done for you.
I am not a big fan of o-rings as damping devices because they offer some non-linear damping, but it changes over time due to the elastomer taking a "set". At some point, the o-ring set can be so dramatic that you will have lost all effective damping. The oil film damping method is definitely more preferable in my opinion.
Post by dieselguy86 on Oct 3, 2021 14:21:45 GMT -5
My rotor will probably be ~30% of the gt55's weight, so direct copying isn't possible.
My thought was to basically float the shaft tunnel/ bearing cartridge in another shaft tunnel, with only about .002"-.0025" diametric clearance. Basically treat it like a semi-float journal bearing. The gt55 cartridge has some deep grooves in it that I assume act as an "accumulator?" of sorts and probably handle the actual dampening forces. My GMN 61900 bearings are custom made with direct lube, where small holes were edm'd through the outer races to the non thrust side of the balls. I could position a groove directly over the bearing and use it as a reservoir of sorts to supply oil to the bearings.
This brings up another question, is the heat the bearings receive mostly from operation? Or do they get a large portion from the radiant heat from the cc, or soakback from the turbine, or both? Having the bearings isolated from a radiant heat source would have to help them live.
Post by finiteparts on Oct 3, 2021 18:07:19 GMT -5
The damping is done by "pumping" the thin film around the annulus and the grooves are usually to distribute the oil inflow to several oil supply holes. The groove is commonly termed the "oil inflow circumferential groove" and does not really contribute to the damping of the outer squeeze film. The damping of the outer film is controlled by the thickness of the oil film and when the film gets to thick, the pumping of the oil gets too easy and no real damping is accomplished. I would suggest measuring up the clearance on the original unit and match that as best as possible. Squeeze film dampers are non-linear in their response to forces and it makes designing them quite challenging.
The heat input into the bearings is definitely a combination of external heat loads and operational heat generation. As for which are the primary sources, that is hard to say. Back in the 80's, the coaking problems were driven by heat condition from the turbine to the shaft and this heat would coak the oil in the bearing regions. If the bearing housing is wrapped by a glowing combustion chamber, there could be a significant heat flux into the oil system due to the radiant heat load. I agree that reducing the radiant heat load and the turbine heat soak-back are both good directions to work towards to get a longer bearing life, but giving any guidance would require quite a bit of design work to give any real insight. I have started the design work on the oil system to meet the required heat rejection (sets the oil flow), but this requires a ton of heat transfer design and bearing loads to fully understand the bearing steady state temperatures.