bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Feb 28, 2017 17:43:21 GMT -5
hi John, i was wondering if i could use normal steel bearings and apply kj66 bearing cooling style?
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Post by racket on Feb 28, 2017 17:55:26 GMT -5
We can use ordinary steel bearings but we need to keep rpm down and use C3 type "rattly" bearings and preload.
I used steel 20 mm ID bearings up to ~30,000 rpm ( ~50% N1) in my FM-1 engine , then made hybrid ones using ceramic SiN balls in the steel raceways with minimal lube and bleed air cooling like the KJ66 but they still failed, eventually went time expired Allison 250 turbine bearings with full pressure jet lubrication .
Because I was using a radial inflow turb wheel which doesn't "pressure balance" the comp like an axial turb wheel disc does, I also fitted a labyrinth seal against the back of the comp wheel to "unload" it of some of the static pressure trying to force it forward.
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Post by jetjeff on Feb 28, 2017 18:07:22 GMT -5
Hi bournaVee,
GRW makes high speed bearings of the angular contact, full compliment type (no cage), ceramic balls that can withstand high temps and very high rotational speeds. They require spring preloading via a spring or wave washer to work correctly.
They are not cheap BTW.
They will not tolerate axial force if the engine is disassembled, you'll find the races will separate and the balls become loose. They can be reassembled if you heat the outer bearing race slightly with a propane torch.
Regards
Jeff
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Post by jetjeff on Feb 28, 2017 18:22:47 GMT -5
Question to all, in regards to the rotor. Which force wins, in the effects on the compressor pulling the rotor forward, or combustion pushing the rotor rearward?
Jeff
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Post by racket on Feb 28, 2017 19:25:07 GMT -5
Hi Jeff
Generally with an axial turbine wheel coupled to a centrif comp wheel the static pressure on the back of the comp roughly balances out the static combustion products pressure on the turbine disc , but theres no certainty in being able to accurately determine which way any residual thrust goes as it probably changes with rpm/P2 pressures as well as a myriad of other factors , best to have some preload going in both directions :-)
I know some of the homebuilt micro engine have their preloads favouring one end or the other , but thats probably been determined by trial and error.
At the small diameters of most homebuilt engines the forces aren't that great due to low PRs , and the actual bearings are probably stronger relatively than once into larger sized engines.
Cheers John
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:11:29 GMT -5
thanks a lot John and Jeff
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:13:33 GMT -5
hi John, hope you are doing well. most of the engine designs ive seen have axial preload of the front part
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:15:04 GMT -5
and Jeff, those ceramic bearings are really expensive
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:19:02 GMT -5
im taking part in a science and tech exhibition this month. i cant take my kj66 engine because i havent been able to run it to full power, im using normal steel bearings on it.
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:26:17 GMT -5
so, im trying to complete this bigger one.
Hi john, you mentioned that with normal steel bearings, the engine can go 30,000 rpm, at that speed, the engine should be able to deliver useful power(shaft power) right?
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Post by racket on Mar 3, 2017 19:42:38 GMT -5
Hi
That 30,000 rpm was with 20 mm bore bearings , smaller bores will be able to go faster , 10 mm ones possibly 50,000 rpm
Cheers John
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 19:51:10 GMT -5
wow.. thanks John.. what style of lubrication did you use?
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Post by racket on Mar 3, 2017 19:58:18 GMT -5
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 20:39:58 GMT -5
hi John, thanks for the link i will be using reverse annular combustion chamber design.. i just sketched this up
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bournaVee
Junior Member
Joined: January 2017
Posts: 66
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Post by bournaVee on Mar 3, 2017 20:48:54 GMT -5
brown indicates spring preload, black is aluminum bearing housing, ash is compressor
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