Simplest possible self sustaining gas turbine

[finiteparts]

Jun 21, 2023 5:50:22 GMT -5 philip111 said:

Hi guys bit of an odd question, if my turbine rotor has a weight of 2.6 kg (yes I know it's a lot), what size bearing should I choose, will 608 (8mm bore) bearings be enough or should I use 6001 bearings (12 mm bore). I'm concerned about friction.

What is your rotor speed?   
How well will it be balanced?   
How concentric can you make it?   

There are a multitude of variables that go into the selection of a proper bearing and with your very heavy rotor, I would be very reluctant to speculate on your needs.  

Rotor weight primarily plays into the static load capability of the bearing, the dynamic load capability of the bearing refers to it's ability to carry the imbalance loads, the thrust loads, shaft bending, etc.... without having an understanding on the loads that the bearing will see, makes it impossible to give you even a estimated answer.

Larger bearings are limited in rotational speed capability, due to the centrifugal forces pulling the balls radially outward.   At sufficient speeds, the load of the ball pressing into the outer race causes the compressive stress in the race to exceed it's capabilities and it will begin to produce subsurface cracks that grow rapidly to cause spall, which is a failure mode were chunks of the race surface to flake off and then likely produce secondary damage as the chuck gets chewed up by other parts of the bearing.  This is why you need to understand the rotor speed when you are spec'ing out a bearing.

As a quick example, here are the calc's to estimate the imbalance force due to a small rotor concentricity error, here it is 0.0005 inch.



Good luck,
Chris

[praendy2203]

[praendy2203]

Hi,

thats my rotor.

I am using 7003 hybrid bearings (steel cage and ceramic balls)

35mm OD
17mm Id

able for 100 000 rpm.

1 each costs 78€

Andy

[ripp]

Jun 21, 2023 14:07:46 GMT -5 praendy2203 said:

Hi,

thats my rotor.

I am using 7003 hybrid bearings (steel cage and ceramic balls)

35mm OD
17mm Id

able for 100 000 rpm.

1 each costs 78€

Andy

Hi Andy

little correction: cage is Phenolharz

www.kugellager-express.de/keramik-hybrid-spindellager-hcb7003-c-t-p4s-ul-17x35x10-mm?gclid=CjwKCAjwv8qkBhAnEiwAkY-ahvvrFlxsE2EFmH6E453Zpcmnrucl4VxLJoIrg3kziFolEbGvVSRAuxoCK3AQAvD_BwE

Ralph

[ripp]

www.pregalim.sk/files/grw-katalog-d.pdf

page 41

this is the weak point for our turbines, temp. up to a maximum of 260°C, if you forget to cool it down, the cage loses its shape

Ralph

[philip111]

Thanks for those calculations Finiteparts, and thanks for the bearing recommendation Praendy, you rotor seems to weigh as much as mine does(I enjoy your videos on Youtube btw). A small eccentricity can definitely cause big load imbalances although I plan to run my rotor at about 10k - 15k rpm, just enough to self sustain itself. the diameter is 150 mm ( giving a tip speed of about 78m/s - 118 m/s ) and I will static balance the rotor after lubing the bearings with gasoline. I may also put the bearings in those rubber seating rings to accommodate for possible misalignment, eccentricity.