Unusual Bearings

[StevePParticipant @stevepye68246]

 

I’m not sure if this will be of any interest to any one but here goes.

In 2011 a good friend of mine asked if I’d like to work for a startup company, I became employee number 11.

The company were developing a generator driven by a micro gas turbine, the turbine performed but the project was shelved as the sponsor of the development pull the plug ,the company was able to continue though with the help of new investors.

From what was learned a much small capacity generator and turbine were designed for a different market, the generator and turbine would be coolant and oil lubricant free! The new design would utilise hydrodynamic bearing unlike the previous unit which use hydrostatic bearings. An American professor created the bearing designs, radial and thrust.

It took  nearly two years before we could manufacture the bearings to the required quality to even get an engine to run consistently. At that time the bearings were performing faultlessly on our test rig.

The radial bearing consists of a stainless steel sleeve inside which , sits a bump foil which resembles corrugated cardboard and follows the bores profile, sitting between the shaft and bump foil sits a top foil, evenly spaced pairs of foils are attached to the inside of the sleeve.

The thrust bearing consists of a stainless steel washer with several triangular shaped bump foils evenly attached around the bore, the bore is clearance for the turbine shaft to pass through,  above each individual bump foil  sits a top foil with one end angled down and attached to the washer.

All the foils are manufactured from Inconel 718 and are only 0.076 mm (0.003 inches) thick, the material is supplied in its annealed state and the foils have to be age hardened before use.

These bearings work in a similar way to hydrodynamic oil bearings,  the oils viscosity is a magnitude higher than airs viscosity so the air film thickness is minute, air likes to adhere to surfaces, so when the shaft rotates it drags air in between its self and the foils, the assembled foils create a wedge shape  entrance aiding entry of the air, the clearance is then closed down generating a pressurised air film. The thrust bearing works in the same way,  air is drag up the angle top foil by the rotating thrust disc creating the air film.

All these bearings require a small static clearance between the foil and shaft or thrust disc, to allow the air  film to form.

Hydrodynamic bearing only start to work ,when the rotating shaft reaches a certain surface speed, lift off speed, 8 to 10,000 RPM in our case at which point an air film is created between the top foil and shaft, up until that point the shaft and foils are in contact,  wear is minimal because the alternator accelerates the shaft upto lift off speed within several revolutions.

 

Running the engines for longer and longer periods resulted in catastrophic failures, obliterating the bearings and causing irreparable damaging to the whole power unit , it  was quite disheartening as I was making all the bearings at this time. The cause of all the failures couldn’t be determined at that time because the parts were so badly damaged,  we then had a stroke of luck,  on stripping down an engine that had been running for a number of hours we noticed something which could be the cause of all our failures. On measuring the clearance of the radial bearing I noticed a slight increase in clearance,  I then heard, new to me phrases ,transient loads and bending modes, at that time I didn’t even know that  a plain round and balanced shaft, at a certain rpm would start to bend, like a tuning fork. These bending modes can be predicted by software programs,  bearing position, stiffness and the profile of the shaft alter the frequency of the bending modes.

Predicted frequency Hz of the bending mode x 60 = rpm approximately that the bending mode appears, these modes can be extremely destructive.

These loads on the bearings were compressing the bump foil spring units beyond their yeald point causing the bearing clearance to open up, eventually causing the failures. At this point at the request of the director a brilliant American engineer visited the factory for few days to mentor a few of us on a different approach to foil bearing architecture. With a renewed enthusiasm, I was given a free reign to design and develop some new bearing. New sleeves and foils were cut using wire Electrical discharge machining, the foils weren’t suitable because of the recast layer on the edges and slight distortion, at that point I had to design and produce small press tools to produce foils of the correct form and profiles. Inconel is not an easy material to work with. After  many design iterations, taking many months, the new bearings  even when flexed to their limits didn’t  yeald, performance was simular to the previous bearings when tested on the rig.

It was a great relief when the first engine ran with the new bearings fitted and it kept on running.

 

A little more about these new Bearings.

The new bearings have no static clearance when the air film is generated it pushes the foil away from the shaft.

Lift-off rpm is similar to the previous bearings.

The bearing do generator a small amount of heat because of viscous shear in the air film.

The bearings us the air that leaks around the rear of the compressor wheel to purge the used air being exhausted from the foils, this purging air is already at a temperature of 200 centigrade.

The radial bearing nearest the turbine runs at a temperature of between 400 and 500 centigrade, this bearing has to cope with the turbine shaft growing in size because of the temperature difference between the bearing sleeve and turbine shaft. It also has to cope with a tapered shaft because of the temperature gradient along the turbine shaft.

The turbine alternator produces maximum power at 135,000 rpm, over 2000 revolutions a second.

Just before I retired some units had accumulated 20,000 hours of running time.

20,000 hours x 60 minutes x (being Conservative) 115000rpm  = 138,000,000,000 revolutions.

 

I still find it incredible that these delicate bearings can survive in such a harsh environment.

 

Feel free to correct anything or disagree.

I’m only a retired tool maker that loves engineering,  design and solving problems, I’m not an academic.

Steve

 

 

 

 

 

 

 

 

[SillyOldDufferModerator @sillyoldduffer]

Woof woof, I love reading this sort of post!

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Great stuff, Steve … that must have been very satisfying.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Before computers and FE analysis they used to work out whirling speeds with paper and pencil and a device called a planimeter which measured areas on the cross section of the wheel/shaft assembly. Completely forgotten how!

I’ve done it with Rayleigh’s method, but that only works for simple shapes. Again I’d have to look it up to repeat.

[Chris MateParticipant @chrismate31303]

Very interesting. Would like to know when setups were tested if the turbine was moved around abruptly for centrifical force disturbances to see how everything holds up, or if it was bolted stationary in place-?

[StevePParticipant @stevepye68246]

Thank you for your comments.

Dave , I’ve had some really interesting jobs working for some good companies and working along side some great (in my eyes) engineers. I may bore members with a few more posts.

Michael it is good when you can contribute something to a project, it make you want to go to work.

The American engineer demonstrated a crude way to predict the first bending mode  by suspending the entire rota assembly by a fine wire and striking it with a tuning hammer,  with an app on his phone he measure the frequency of the note multiplied by 60 which gave an estimate of the rpm which would excite the bending mode.

The generator is a static unit for replacing a diesel generator set, complete units were tested on shaker test equipment  in a similar way  vehicle manufacturers test their vehicles. These tests were carried out to check that units could be transported around the world without suffering damage in transit.

 

Steve.

[Steve P]

On Steve P Said:

… complete units were tested on shaker test equipment  in a similar way  vehicle manufacturers test their vehicles. These tests were carried out to check that units could be transported around the world without suffering damage in transit.

I spent a fascinating eleven years of my life [from mid 1970s] doing that sort of work, Steve … there was rarely a dull moment.

MichaelG.

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Edit: — a few minutes about ‘plate modes’ for anyone not familiar:

https://youtu.be/YedgubRZva8?feature=shared

https://demos.smu.ca/index.php/demos/waves/104-chladni-patterns

 

[Mike HurleyParticipant @mikehurley60381]

A quite fascinating read! Thanks for sharing it Steve

 

[John HaineParticipant @johnhaine32865]

Interestingly much of the early work on whirling modes in shafts was done by Prof. David Robertson of Bristol University in the 1930s, also famous for the Robertson Clock which has been mentioned here in the past.

[StevePParticipant @stevepye68246]

Your welcome  Mike

Steve

[mark costello 1Participant @markcostello1]

Very interesting.

[StevePParticipant @stevepye68246]

Thanks, glad you found it interesting Mark.

Steve

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