Hi Guys,
I've been putting some time into this project and it is nice to see it moving forward again. Feels like the good ol days.
I've obtained not just one, but TWO RR Allison 250 4th stage turbine wheels. Why buy one when you can get two for twice the price?
I scored these babies for an awesome low of $150 each and came with documentation. Here's the pics:
The first picture almost makes it look like the blade edges are dinged up but that's a photographic effect caused by the reflection of the bubble wrap behind it shining back from the flash. These units are in really good shape. The wheel on the left has 2345 hours and 208 cycles remaining and the other has 1018 hours and 190 cycles remaining.
I have also contacted several foundries with requests to have the nozzle guide vane section cast in 316SS - still waiting to hear back from them at the moment but I've only tried domestic companies. I dont know what kind of experience you guys have had with domestic companies as of the past few years but it seems like America has lost its drive - I can't seem to find anyone who actually wants to work to make a buck. I've even been feeling it in my business and have resorted to seeking manufacturing from offshore corporations. Seems like everyone else around the world will jump at an opportunity to manufacture products for us while I have to continually hound domestic companies just to get a response. Sorry to go all political/economics here, its just frustrating.... I digress..
So if I dont have any success finding one of my fellow americans to take my money for work I'm sure I can find someone abroad to do it. Out of a dozen companies I have emailed my request to last week I've only heard back from one and that was a short, simple, "We can't help you with what you need."
I've also been sorting out the bearings to use in the final setup. I've located an offshore company that builds full ceramic bearings and they are working on putting together a quote for a pair of duplex bearings, angular contact, full complement arrangement for use at the front end of the group (just behind the comp wheel) as well as a single full complement angular contact bearing for the rear. I've also inquired about a full ceramic roller bearing to use instead of a ball bearing for the turbine end of the group.
With a pair of duplex bearings for the front, the axial preload on the bearings is provided by the precision machining that is done to the inner and outer races of these two bearings such that when they are locked together it will properly preload the balls into the races. I've requested a quote for a pair of DB (back-to-back) bearings with a ball load configuration of "O" as seen below:
DUPLEX bearings consist of an assembly of two PAIRED bearings which provide the following advantages:
● radial displacement reduced to a minimum
● increase of the radial and axial load capacity
● accurate axial positioning with axial loads
● extremely accurate functioning
● 0 arrangement called DB (back-to-back)
● X arrangement called DF (face-to-face)
These assembly configurations are used for applications where no bearing clearance can be tolerated and where a high degree of stiffness of the whole assembly is required.
Pairing is achieved with bearings of identical tolerance classes and by applying an axial preload to the inner rings for the O arrangement and on the outer rings for the X arrangement. Afterwards the inner or outer ring is ground in order to get both side faces in the same plane with minimum clearance.
Roller bearings tend to have better radial tightness as well as the ability to carry greater radial loads and do not require axial loading like ball bearing units which is why I'm entertaining the idea of using one of these at the turbine end of the assembly - it wont require any springs or spring washers in the assembly at all.
With the success John (racket) has had with his 9/94 turbine and incorporating the labyrinth seal behind the comp wheel to pressure balance the wheel and reduce axial loading on the bearings I'm pretty confident that this bearing assembly approach I'm taking is an overbuild, which I'm more than happy to do. With the $600 some-odd dollars invested into the compressor wheel and radial turbine in the gas producer I'm wanting to do everything I can to prevent a catastrophic failure. With the T04 engine I could get those parts for under $200 total which although not exactly cheap, was still within an immediate budget.
So we'll see how things come together for the NGV section - my focus right now is primarily on the hot section of this engine as I've pretty much finished with the cold side (minus getting a plate of rulon and machining the comp labyrinth seal).
