Vibration problems with an electrical generator 1

[AlbertFM]

Dear all,

I am dealing with a 35 kW and 90.000 rpm electrical generator to work coupled to a micro gas turbine.

I face some problems when I try to ramp-up the generator. When it pass through 550 seems to stay in a critical speed and later on at 750 Hz an other one. We can pass the first one but the second one destroys the system.

Trying to find out what is the source of the problem I have simulated some parts and I've used the hammer test to find the natural frequences of the system and all the components. What annoys me is that all the natural frequencies are at least 3 times bigger and I am not able to find the source of the problem.

Does anybody have any usefull comment that can helps me to solve the problem?

I will be really pleased with your help.

Thanks in advance

 

[AlbertFM]

Tmoose,

The brand is GRW. Hybrid bearings 12-28-8. Angular contact (15º). Specially designed to be used in high speed and high temperature systems.

We are working in some different protos and the arrangement varies a little bit from proto to proto. In general you can imagine a shaft with a bearing at each end. In most protos the inner part of the bearing is what rotates. In a future design it will be the other way round. The shaft enters into the inner ring due to the adjust tolerance of the shaft (is slip fit). Then is fixed in the axial direction by a case that is thighten by the nut. If the rotor wants to move in the axial direction (all the rotor not only the shaft) we need a preload at the outer race to avoid loosing the angular contact of the bearings. This preload is given by a wave spring that presses an outer case. See attached file.

As far as I know, the bearing manufacturer has approved what we are using the bearing for (speed, temperature, dimension and masses of the shaft...). I am not certain if they know how the complete arrangement is working. I will let them know.

You can find attached a simplified scheme to understand the arrangement.

Thanks