Micro Turbojet Engine Beginner - need help
 

So just as an overview, it is my first time building a model turbojet, and I am basing it off the model in Thomas Kamp’s Model Jet Engine Texbook. In the textbook there are some bits that are unexplained. Firstly, in the engine; there are spacer discs on each end of the shaft - what are they used for and where shoul they be placed? Secondly, the book says to use a preload spring only on the rear bearing. Shouldn’t that also apply for the front bearing?

I deleted your duplicate post, you only need one thread per question, this is the best area to get you a proper answer in any case.

No pre load on the front bearing, using a pre load assumes the bearing can travel axially on a tunnel, you can see, by the design of the engine, that is not possible on the front of the engine.

Do you have a schematic on the engine? It is necessary to understand what is being called a spacer on the text.

Designs I have seen using a spacer, they may serve different purposes, it could be to aid in the front bearing lubrication, (this behind the bearing), or to properly align the compressor (front of the bearing) not to reduce shaft play between bearings, the designs I have seen using spacer discs or washers to eliminate play, do not use pre-load.

Basically, using a pre load eliminates the need for axial play washers or discs, but you may still have a spacer, probably in front of the front bearing, behind the compressor

it all depends on the engine design

Thank you for your reply, it has cleared up everything. Yes I think you are right, in the schematic I am following it appears that the spacer is being used as a wall between the compressor & front bearing, as well as the turbine & rear bearing. Unfortunately I am unable to attach any picture (it is saying I need to have at least 10 posts before uploading URL's, which picture attachments also seem to be URL's). In one similar drawing, it is showing that they are using a spacer also between the rear bearing and the spring, which if I'm not mistaken is to fill the gap between the spring and the rear bearing, so that the spring is actually doing its job? I will be ordering disc springs for ISO 6001 bearings such as the ones found in "spring fasteners" website with dimensions of Outer diamter: 27.7mm, Inner Diameter, 17.3mm, Thickness: 0.4mm, Overall Height: 1mm (I'm not allowed to attach URL's).

In summary, the bearing should either be trapped between two walls, or preloaded and not move axially at all?

If you are serious about making an engine then you need to follow the design exactly with the same part number compressor etc. The plans are detailed enough to make a working engine. I made one that ran quite well and was flown a number of times. It is not a very good design though and you would be much better following the KJ66 plans and buying the compressor, turbine wheel and NGV. If you google KJ66 you will find the plans and some parts. Just basing a design on what is published will only work if you recalculate the flow through the engine.

John

Well, to give more of an overview.. I am a 3rd and final year Aerospace Engineering student, and I am building this engine for my final semester project, so just building an engine to the exact dimensions of a previous engine, won't get me a good mark. The smallest wheel size I could find locally is an HX-35 Turbo Compressor, and I simply do not have enough time for overseas purchases, given the crisis we all are in, which is the issue I am having with bearing preload disc springs, as I cannot find them at all locally. Also, I couldn't find the turbo that was cited in the book neither locally nor online (at least for orders).. but the book says if I plan on making the engine but with a different compressor, then I should scale up the dimensions. This wheel is the smallest wheel size I could find locally.. and we all know the crisis we are currently in if I need to purchase something overseas, which is the issue right now I am having with bearing preload disc springs, as I cannot find them at all locally. The problem is our whole country is starting to go into complete lockdown and quarantine, but my lecturer still wants the project done by the end of May.

Furthermore, the compressor wheel used in the book is 66.6mm. So, the book recommends to use a scale factor of 80.96/66.6=1.2266666666... for all dimensions. Additionally, I tried doing some of the vector triangle calculations, and they were sort of correct.. but the issue is that they lead to nowhere.. they lead to determining parameters such as peripheral speed, flow angle etc. but that seems to be only enough to calculate the angle of the guide vane of the compressor diffuser. Things such as calculating the diameter, the height of the vanes, the thickness of the disc have not much of calculations. If you have any useful equations for such parameters, then it would be very helpful if you send them to me. Anyway, in Thomas Kamp's textbook, it seems that he is using the kj66 design, however he did not make it clear if he is or if he is not.. he has also shown 2 different compressor diffuser designs, the most common one which has both radial & axial vanes, and one I have generally only seen in his book which shows vanes that start radially and then extrude in the axial direction at the maximum diameter of the diffuser housing. I apologise if the description is confusing, but I'm not allowed to upload any document or URL.

Building a small turbojet that will work is quite a long term project and needs a lot of skills. If you are in the final year and only have a few months to build it in with possible disruption due the virus I would doubt you have the time. If I was in your position I would opt for doing a software simulation of the flows though the engine. You would ideally need a commercial engine to base the calcs on and verify the results, an impressive end to the project would be how to gain more thrust and revised flow calculations.

Incidentally not sure if you have thought that there are many variants of the HX35 wheel. You will in any event need the pressure map of the wheel you have as a starting point. Manufacturers are very coy about releasing them. The HX35 incidentally is a really large wheel. There are lots of various wheels of a more suitable size available on Ebay. Remember to check on the direction of rotation as there are both directions used these days. (This will affect the shaft screw thread direction). Smalley in the USA makes a series of different size and strength wave spring washers that can be used instead of a coil spring for pre -loading. Just Google the name. You will need a turbine wheel that matches the compressor and you will be unlikely to able to make one. Similarly the NGV is not easy to make and both need to be made from the correct materials.

If you got as far as making an engine it will need balancing which is a problem all on its own.

Good Luck

John

If you mean to find the turbine wheel that is matching to the compressor I shall use.. then I have that. In fact I have the whole HX-35 turbo cartridge. But, to quote directly out of the book:"However, for quite different reasons the use of a radial turbine appears to he a poor choice for the amateur. One reason is the mass of the rotor. Even at model sizes the rotor might weigh anything up to 0.4 kg. That means a high moment of inertia and a correspondingly poor ability to accelerate."

If you think it still would be a better idea to use the radial turbine out of the turbo cartridge, then I think it's better to do so, but please do inform me if there are any readily available designs of radial turbine guide vane systems (to guide the air correctly towards the turbine entry).

Additionally, I will most likely be using Computer Aided Manufacture to machine the turbine and nozzle guide vane out of a billet, bar, block etc. which the book says is another option of fabrication. If there is still any issue with this, then inform me of that as well.
Also, I am using the 8-blade HX-35, which with the Caliper I measured a maximum housing diameter of 80.96mm. I don't know if there are other HX-35 8-blade, 80mm variants. I also have the shaft within the cartridge, along with the associated nut. The nut loosens in the clockwise direction.

give us a scheme

The part between the rear bearing and the pre load spring is not a spacer per se, it is a “tube” intended to exert the spring pressure on the outer race of the bearing evenly, you are aware that the turbine bearings use conical races, right?

Oh okay, I guess it was labelled incorrectly..

Firstly, may I ask how is the spring incapable of spreading the force evenly? I mean shouldn’t the spring have been designed in order to fully suit standard ball bearings?

Secondly, I am not aware that they use conical races, all the designs I have seen so far use standard ball bearings.. and if I were to use conical races, that would mean I need to use a different type of roller bearing, correct?

Additionally, I would like some advise regarding ignition.. so I am planning to use a spark plug or glow plug, to eliminate the possibility of the fire going out. Many designs I have seen do not account for the inclusion of the plug. Is there a specific point of the combustion chamber I should place it on? Such that the aerodynamics aren’t affected, no disruption of airflow etc.

Sorry I used the incorrect term, the bearings are not conical, the correct term is angular contact bearing

Without the “spacer” you risk uneven pressure on the bearing and possible binding on the bearing seat, this will destroy the bearing.

all microturbines use angular contact bearings, otherwise there is no component holding the shaft in position, regular bearings would fail almost immediately and cause catastrophic engine damage.

The bearings function both as radial and thrust bearings at the same time, this also means that they are directional, meaning that they go into the engine on a specific direction, the front bearing goes in in the opposite direction of the rear bearing.

Angular contact bearing
 

On the attached image you can see the difference between a regular bearing and a angular contact bearing
https://cimg3.ibsrv.net/gimg/www.rcu...ac5acaf1a.jpeg

Oh right okay. Now, for the "spacer" between the bearing and the spring, how should it be designed? Should the thickness basically match the thickness of the outer race? Or are they obtainable from a store? And as far as I'm aware, the spring should be touching thicker part of the outer race, correct? Should I use this type of bearing for both the compressor and turbine section, or just the turbine section?

Angular contact bearings should be used on both the front and the rear, if you only use one, that bearing will pull the inner race of the regular bearing destroying it,

Yes, the spacer needs to contact the thick side of the outer bearing race, it does not have to be exactly the same thickness, but if it is, I would think would be better, so you can visualize that the spacer is just a tubular metal section that can slide inside the shaft tunnel, with no play, the spring needs to also be a perfect fit to the shaft tunnel so that it contacts the spacer with no possibility of slipping into the spacer hole while under pressure

you should by now realize that the thin outer race of both bearings should be directed towards the shaft ends on both sides, This means that outer races seat against the bearing balls, the balls seat against the inner race, the inner races seat against both, the turbine and the compressor or their spacers if so included, this will keep the shaft in position, shaft cannot detach the bearing races as it would happen with regular bearings

Forgot to mention, bearings should be high speed/high temperature ceramic, most common manufacturer is GRW

How about bearings that reach a speed of 130,000 rpm (with oil) and utilise ceramic balls? Are these good enough? And I've noticed from different manufacturers that the lower the contact angle, the higher the rpm speed (with oil) so given that this is correct, I should go for the bearing with the lower contact angle?

I have no idea about the contact angle implications and effect on rpm, I suggest you avoid complex details which may not really help you achieve your goal faster and go with what’s already available and proven, I would use the most common bearing which is the:
GRW D608/602C which should fit a KJ66 shaft dimensions

see below:
https://www.bocabearings.com/product...0RC&model=KJ66

Correct me if I'm wrong, but since I will be using a larger compressor+turbine, shouldn't I be using a thicker shaft if I want it to rotate at the same speed as a normal kj66 engine? Besides - shouldn't this be the consideration when generally using components with higher mass and higher load? If that is the case, then I will use a D6001/602C bearing, which also utilises ceramic balls, and according to the specs (can't attach) it is utilised for jet turbines.

I would say it makes sense, but since you are not following a proven design, unless you recalculate all variables, it’s difficult to say, a larger compressor may not need as high rpm, but still may not work because the engine can’t self sustain, I’m no engineer, I can’t help you outside of existing working designs

Alright, I'll probably make the existing shaft and my own shaft, then compare the two as a research project. Hopefully I can make it self sustain with either design.. but if not, my lecturer is usually generous when he sees the correct amount of effort being put regardless of outcome, along with a valid and strong evaluation of the outcome.

Anyway, thank you very much for your help! It really gave me a confidence push and a clearer sight.