HPST 2020 Jet Engine

[racket]

Hi Andy

Love your choice of background music , rather appropriate .

Just some minor "adjustments" and she'll be "burning grass" again ;-)

Cheers
John

[praendy2203]

Hi,

John already know, what I will explain!

The cause for the incident was the critical shaft rpm!
Maybe also a not sufficient lubrication.

Got a nice Excel table to calculate the values.

So with my shaft diameter (24 mm) and weights also bearing distance the result was
52 507 rpm🤘

Now I ordered already a piece of 42crmo4
steel, to lathe the new shaft.
Then the 70 000 rpm are possible👍
with a shaft diameter of 29 mm.

Andy

[madpatty]

Sept 19, 2020 7:38:48 GMT -5 praendy2203 said:

Hi,

John already know, what I will explain!

The cause for the incident was the critical shaft rpm!
Maybe also a not sufficient lubrication.

Got a nice Excel table to calculate the values.

So with my shaft diameter (24 mm) and weights also bearing distance the result was
52 507 rpm🤘

Now I ordered already a piece of 42crmo4
steel, to lathe the new shaft.
Then the 70 000 rpm are possible👍
with a shaft diameter of 29 mm.

Andy

Hi.

I was encountering very similar vibrations with my engine but at a fairly low rpm of around 23000.
Your analysis makes me wonder maybe I was also having some sort of resonance vibrations.

Cheers.

[racket]

Hi Andy

46% more "meat" in a 29mm vs a 24 mm , sounds good :-)

Cheers
John

[sauerkraut]

Sept 19, 2020 7:38:48 GMT -5 praendy2203 said:

Hi,

John already know, what I will explain!

The cause for the incident was the critical shaft rpm!
Maybe also a not sufficient lubrication.

Got a nice Excel table to calculate the values.

So with my shaft diameter (24 mm) and weights also bearing distance the result was
52 507 rpm🤘

Now I ordered already a piece of 42crmo4
steel, to lathe the new shaft.
Then the 70 000 rpm are possible👍
with a shaft diameter of 29 mm.

Andy

Hi Andy,


First of all, really nice work on the engine.

If I may, how are you calculating your maximum shaft speed?

Sean

[praendy2203]

Hi,

its very simple with the Excel software!

But, all is in German language!

So if you want to calculate,I can do it and need all datas in metric system.

All apply to steel shafts!

Shaft center diameter

Distance mid bearing fwd, to mid bearing aft (centre line)

Weight of shaft

Weight of compressor wheel

Weight of turbine wheel

Point of gravity compressor to mid fwd bearing

Point of gravity turbine to mid aft bearing

Max RPM you want to run

Bearing dimensions

Inner diameter
Outer diameter
Thickness

To figure out the centre of gravity compressor, I fittet a small strip of tape and put it on the wheel, until the wheel hangs vertical.


How to upload pictures?

Cheers Andy

[wannabebuilderuk]

imgbb.com/upload

Once uploaded select "bb code thumbnail linked" and press the copy button, then paste it in the reply box

[praendy2203]

Ok,

I got it! JUHU

Andy


ibb.co/rMgWdqz
ibb.co/StdnNc1

[finiteparts]

Hi Andy,

Great work on this engine. I had a couple of thoughts to share on your latest vibration issue.

It appears that you have o-rings on the compressor side bearing and just a slide fit on the turbine side bearing. This makes me wonder how much the o-rings engage the compressor side bearing? My thought here is that if you have any significant "support" of the bearing on the o-rings, you have reduced the front bearing stiffness to trade for visco-elastic damping from the o-rings. A softer compressor side bearing would likely result in larger rotor deflections on the compressor end, which is what appears to be the case due to the compressor vs turbine damage.

A rolling element bearing is inherently very stiff, which is good for controlling rotor deflections, but as you are probably aware this is also bad for vibration transmissibility, and thus it is often good to try to add damping to help control the bearing near its critical. When we say that roller bearings are stiff, we are talking about the stiffness of metal, i.e. how much force it takes to deflect the centerline by some unit of length,for example a rolling element brg might be around 10^8 MN/m while a hydrodynamic bearing might be more like 10^7 MN/m. So when you add in an o-ring or two, if the o-ring supports any of the rotors mass, you get a much softer bearing, since the compressive strength of the o-ring will be very low compared to the bearing or the housing.

I had a simple rotor already modeled for a different turbocharger like system, so I thought that I would plug in a change in the compressor end bearing and illustrate what I am referring to.  Now, keep in mind that this is a really simple rotor model, which does not account for rotor internal damping, gyroscoping stiffening(or other gyro effects fo that matter), lumped mass distributions, isotropic bearing/structural stiffnesses (how the load gets to ground), very high level of balance (this was running with ISO balance grade of G2.5), etc....so don't get hung up on values.  I was just trying to show trends with this model. A final note on the balance, the imbalance was placed on the compressor end, with the turbine balanced to a finer level.  I did this just to better illustrate the compressor bearing stiffness effects, but we might argue that this could have been the case with your run due to the damage to the compressor side only.

So in the first image, you can see the 8 element model with the compressor assumed to be on the left side.  The bearings were moved closer to the rotors and the shaft was considered to be a uniform diameter.  If we look at the mode shapes in the bottom images, you can see how the mode shape becomes more complex and suggests higher eccentricity at the compressor bearing (node 2)
 


If we look at the Campbell diagrams, we can see that the one per rev (lower dashed line) and two per rev (upper dashed line) crossings are moved to lower rotational speeds.  Now I circled the crossings at the backward-whirl curves, which is a whole other topic, but it should illustrate the idea that reduced system stiffness moves the critical frequency crossings to lower speeds.



In this final image, we can see that the response magnitude of the rotor has substantially increased by the reduction of the compressor side bearing stiffness.  If you look at the Response Magnitude scale in the upper figures, you can see that the softer system (the right side image) has increased by several orders of magnitude, and if the rotor speed did get high enough, it would enter damaging levels of response.



I wonder, because of what your damage is telling you, if you might want to stiffen up the compressor bearing.  If you are getting any real support of the rotor on the o-rings, you might think about removing them.  Also, remember that the calculations that you are doing from Kamp's book assume rigid bearings, so if you soften the bearings in any way, your critical speed will be incorrect.

Good luck!

Chris

[praendy2203]

Hi,

During the last weeks, waiting for a new compressor wheel, I build an afterburner for the spaghetti pot.

Here the first results!

Tested only in idle with Propane gas.

youtu.be/ONHo5eq0zMM

Andy

[slittlewing]

Looks awesome!! Fantastic job. Are you using a flame holder inside your afterburner?

Cheers
Scott

[wannabebuilderuk]

Man that is a beautiful little engine

[praendy2203]

Hi Scott,

yes, there is a flameholder.

You can see it during startup, the glowing thing, shaped like a star.

Andy

[praendy2203]

Hi,

after some testing and trying different settings, I got my afterburner almost producing nice noise and flames.

Have to install new spray nozzles and check function again.

Cheers from cold Germany😉

Andy

[racket]

Hi Andy

How are the "vibrations" going , has the new shaft cured your problems ??

Just a bit more "tuning" and the A/B will be doing its thing :-)

Cheers
John