Kamps Micro Turbine questions

[misterg]

Hi folks

I have the 3rd edition of Thomas Kamps' 'Model Jet Engines' and am looking at building an engine to the plans in the book. I'm not a flyer; I would just like to be able to build a working engine, making as much as possible myself.

The KJ 66 is a (very) similar design, but I'm drawn to the Kamps' design because it should work (to some extent, at least) with a home made turbine wheel.

I know there have been previous discussions about the older design which used a hollow shaft. The design in the 3rd edition uses a solid shaft:




Is anyone aware of any problems with this design? - I don't see to many build threads for it anywhere.

I've also joined the GTBA and am asking there too - apologies if anyone sees it twice!

Thanks in advance,

Andy

[racket]

Hi Andy

There shouldn't be any problems :-)

But making that turb wheel is a big job , there are plenty of NGV and wheel castings on Alibaba www.alibaba.com/product-detail/inconel-base-turbine-wheel-NGV-kj66_60747555406.html?spm=a2700.galleryofferlist.0.0.2ffe1b76Gd3yy8

Yep , saw your enquiries on the GTBA Site , the Kamps and KJ66 are pretty similar , and as you aren't wanting a "perfect" aircraft engine the instructions in Kamps book should see you out .

Cheers
John

[misterg]

Thanks, John.

I may well baulk at making a turbine wheel when it comes to it, but at the moment it’s one of those things I just need to have a go at to get it out of my system!



I would still have the option of moving to a cast turbine if I got fed up of filing, or felt the need for moar powah!

Andy

[misterg]

I may be able to get hold of some Inconel 617 for my turbine wheel (I know that Inconel 718 used to be the material of choice). Inconel 617 seems to have good hot strength, excellent creep resistance & is scale resistant but I'm not sure how workable it is in comparison to 718.

Anyone used 617 or know if it will stand being twisted into vanes?

Thanks,

Andy

[smithy1]

Inconel 617 and 718 are both very "tough"...however, they don't like to be twisted too far or stress fractures will appear at the blade roots...been there done that.

As you mention, 718 is the better of the two, but 617 is fine for our DIY stuff.

Smithy

[misterg]

Apr 14, 2021 0:43:29 GMT -5 smithy1 said:

however, they don't like to be twisted too far or stress fractures will appear at the blade roots...been there done that.

Thanks, I was going by the instructions in the Kamps book which says to twist the blades by 30-35° before grinding to shape - is that likely to cause a problem? (I can believe it will take a bit of force!).

[jetjeff]

Hi Andy,

You have to heat the blade to orange to bend it to 30-35 degrees. I've never made a turbine wheel from Inconel, just stainless. Even with heating, it takes a moderate amount of force to bend it with the claw tool.

If it's not going to power an R/C airplane, have you considered slightly enlarging the Kamps design? Reason being, the rotor becomes slightly more efficient the larger it's made.

Regards

Jeff

[misterg]

Apr 15, 2021 2:17:52 GMT -5 jetjeff said:

have you considered slightly enlarging the Kamps design? Reason being, the rotor becomes slightly more efficient the larger it's made.

Hi Jeff,

Thanks for the reply - I've already got the compressor wheel, so am intending to build to the book (however I'm just contemplating whether I could reduce the diffuser diameter by 1mm so that it will fit inside a camping gas canister!)

I have read that there are problems hot-forming inconel unless you get it to over (something like) 1000°C as moderate heat can make it more prone to cracking. I think this was a 600 series alloy (maybe linked to an incident with a thrown blade). I appreciate that all the alloys have different properties, and there are a bewildering number of them. Not sure that I can get that temperature without oxy gear (I reckon 1100°C is about the limit for an air blowlamp in ideal conditions - probably won't be able to get >900°C without insulation).

I have a 316 disc that I could use - I don't know whether just to go ahead with that, or (given the work involved) it would be worth starting with something better.

[misterg]

OK, I have another question:

One of the things that I've picked up is the KJ66 should have the bearings preloaded towards the front of the engine. I assume that this will also apply to the very similar Kamps design?

The design in the book is for a spring preloading the rear bearing towards the rear. This shows the relationship of the rear end parts as dimensioned in the book (with the turbine wheel as a blank disc):



A spring force of 15N is recommended and Kamps states: "this value should not be exceeded by a significant amount" - I think that this is somewhat less than what is usually recommended for the KJ66.


I have seen various suggestions to reverse the preload direction of simply replicating the preload arrangements around the front bearing. I am uneasy about this from an engineering point of view, as one then loses the accurate location of the compressor wheel.

As an alternative, would it be possible to preload the rear bearing from the rear using a wave spring washer and a narrow threaded ring to set / adjust the preload?

This would mean lengthening the shaft tunnel by ~3 to 4mm if the bearing position remains unchanged, but there seems to be room to do this without fouling any of the other parts:



I haven't come across this arrangement before, and wonder if it is likely to introduce problems that I'm not aware of.

Any comments appreciated.

Thanks.

[jetjeff]

Hello,

Wave washers have been used successfully before. Either spring or wave washer, you just want the bearing to move axially with the thermal expansion of the shaft.

Regards

Jeff

[misterg]

Thanks - I've seen wave washers used in the same place as the spring, but it was the idea of extending the tunnel slightly to fit a wave washer / threaded ring to the *rear* of the rear bearing that I haven't seen before. Can you see any reason why not to do it that way?

[jetjeff]

I can't see any reason you couldn't do that.

Regards

Jeff

[ripp]

Hi Andy,

I think it's well explained!

www.youtube.com/watch?v=6RWbzGIaUG0

and a promotional video

www.youtube.com/watch?v=d4_Jc4kRP54


Cheers
Ralph


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[misterg]

May 28, 2021 13:10:03 GMT -5 jetjeff said:

I can't see any reason you couldn't do that.

Regards

Jeff

Thank you

May 28, 2021 15:02:57 GMT -5 ripp said:

Hi Andy,

I think it's well explained!

www.youtube.com/watch?v=6RWbzGIaUG0

and a promotional video

www.youtube.com/watch?v=d4_Jc4kRP54

Thank you! - The arrangement in that first video is almost exactly what I'm considering doing (except I'm thinking of a threaded ring to retain the bearing rather than the circlip / snap-ring) - very helpful, as I hadn't seen that arrangement used before.

The arrangement in the second video is similar to what I've seen suggested as a modification for the Kamps. The disadvantage is that any expansion difference between the shaft and tunnel affects the compressor wheel clearance. Probably not a problem for an engineered production engine where these things can be taken into account (and if the shaft tunnel and shaft are both steel?), but I don't think is ideal for a one-off build in my shed with steel shaft and aluminium tunnel.

Thank you very much for the information.

[ripp]

Hi Andy,

I can now hardly imagine how the assembly should work, normally you screw the shaft to the turbine wheel as shown in the picture, then you put the machine in place.



www.youtube.com/watch?v=LjGCKyDaHYs


If you now imagine the rotor is inserted into the shaft tunnel and screwed to the front ball bearing and spring preload and compressor wheel, there is no longer any great movement in the axial direction, so the compressor wheel cannot rub against the diffuser.

In this picture you can see the inserted spring not pretensioned.



www.youtube.com/watch?v=d4_Jc4kRP54

Cheers
Ralph


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