Member postings for Mike Tilby

Hi Byron

I just received your message and have just re-sent my earlier message. I double-checked the e.mail address I used was correct so if this one does not make it through to you it I don't know what to do.

Best wishes

Mike

Hi

I sent you a couple of messages to your e.mail address a few weeks ago. I'm just wondering if you received them since I've not recieved any reply.

Mike

Many thanks Clive and the others who responded.

I just went out and did a quick test (before the household guests all woke up). I had previously heated to bright red/yellow and let air cool without success. This time I did the same heating and quenched from that temperature in a bucket of water. The tubing definitely became malleable again, whereas prior to this treatment it had behaved like a piece of spring steel.

Mike

I am making a flash steam boiler out of kunifer brake tubing. (i.e. copper alloy with 10% nickel and about 1.8% iron). As expected, this is harder to bend than copper tubing and became more difficult to bend as it work hardened. However, I was dissappointed that, unlike pure copper tubing, heating to red heat did not make much difference to its ductility.

Does anyone know how to anneal work hardened kunifer so as to regain the original slightly softer condition?

Regards

Mike

Hello Werner

My design is based on my (possibly erroneous) understanding of how turbines should ideally be designed, but I cannot be sure to what extent the following is correct for a miniature turbine. Hopefully I’ll find out one day when I eventually get a working machine.

My nozzles need to fit in the narrow diaphragm that separates the stages of what I hope will one day be a multi-stage turbine. Also the outlets of the nozzles need to be at a small angle (18 degrees) to the plane of the rotor. Drilling holes at such a small angle would be very difficult and would not fit within the circumference of the diaphragm. As in full-size turbines of this type, the nozzles have to be curved so that steam entering from the previous stage has its direction of flow changed to give the correct exit angle. Another aspect is that, for best efficiency, the text books say that a convergent nozzle should taper down by a smoothly curved shape over a short distance to the exit width. This minimises losses due to friction and eddy formation. Frictional losses are greatest at the highest steam velocity and that is attained at the outlet. Also, frictional loss is highest at the narrowest part of a nozzle which is also at the exit of a convergent nozzle. Therefore the final and narrowest part should be kept just short enough to sufficiently control the direction of the steam. Further back in the nozzle the steam has not yet expanded /accelerated so much and nozzle width is greater. These factors all mean that steam velocities at those earlier parts of a nozzle are much lower and so friction is reduced. With my design I have almost full control of the nozzle profile. Another reason for favouring my nozzle design is that the steam path is rectangular and so matches the gaps between the rotor blades. This means that the steam should fill the gaps whereas steam jets from circular nozzles would not fill the corners. I doubt this aspect has much impact on a miniature turbine but I think it was considered important for full-size turbines.

Mike

My own experience of making jets for a model butane burner supports your conclusion about the importance of keeping the throat length very short. The bore in a series of jets that I made was 0.02” diam. The length of the bore (i.e. the thickness of the nozzle end, as shown in the drawing below) was varied from 0.109”, 0.05” and 0.024”. (Measurement of actual thickness was by inserting inside the jet body a small rod of known length and measuring the overall length of rod and jet). There was a very impressive improvement in performance of the burner at each stage of shortening.

As you know, in many turbines the nozzles have to be curved and text books that I’ve read advise that a convergent nozzle should narrow down over a short distance and have a short straight parallel section at the exit to help steam leave at the desired angle. The length of this section should be about the same as its width. For a straight nozzle like yours I imagine that length could be even shorter. So I bet the reducing the bore length to even less than 0.067” would be advantageous, if that could be done.

My own nozzles have a construction that allows the curved shape. The outlet is 0.027” wide and they have a straight parallel exit section 0.027” long, as shown in the drawing below. (The only trouble is I’ve not yet reached the stage of using them to turn a rotor).

Both those latest files work fine Duncan - hooray!

Thanks Lee. Duncan has already try saving in .dxf format but that gave the same duff result as .dwg in my version of SE.

Thanks Andy. Useful to remember about the .SHX files, although for the present task loss of a bit of text would not matter. There seems to be something else different about Duncan's latest .dwg file.

Thanks Adam for the tip about SVC. My SE does not list such a file type in its file opening options. However, it does list the IGS / IGES file type for opening and saving. I gather that is another CAD format. Can you save in that format Duncan?

I'm Duncan's collaborator who can no longer open Duncan's .dwg files

Following Martin's post I installed the Autodesk Freeview and tried opening Duncans file. That generated a warning message: "This DWG file was saved by an application that was not developed or licensed by Autodesk". It asked if I wished to cancel and when clicked proceed with opening it then said that "one or more SHX files are missing" (whatever they are). It asked if I wanted to specify the missing files or just continue. I clicked the latter and the drawing opened and, as far as I can tell, it looked O.K. There is no save option with this app so next I tried converting Duncan's file to a 2004 version .dwg file.

It seems to offer no alternative but to overwrite the original file. It went throuh the motions of doing this but there was no message to say if it was successful. Anyway, the file still gave the same warnings when opening in Autodesk Freeview and it still gave a blank drawing when opened in my Solid Edge so I suspect it did nothing because it was not a valid file.

I then tried opening a much older .dwg file that Duncan had sent. This one does open fine in S.E. and when opened in the Freeview there was no warning, just a message to say it was a trusted DWG file last saved by Autodesk or a licensed application.

I then tried an on-line converter (https://cad.online-convert.com/convert-to-dwg) with exactly the same -ve result as with AD-Freeview.

I feel I've learnt a bit about dwg files etc and added a useful app to my PC but we seem to be no further forward with solving the problem.

I'm not keen on changing my SE version in case it causes problems with my many files of projects and block libraries etc.

Mike

I’ve just noticed there have recently been posts on this old thread.

Andre: I too am curious about what you mean by “novel designs of 'semi-turbine' that straddle the definition of 'pure' turbines and displacement type of engine”. Could you please explain? Presumably you don’t mean a turbine on the exhaust of a reciprocator?

Regarding turbogenerators on locos and road vehicles, in the past, a few designs for working miniature turbogenerators for various models have been published in M.E. I can post details if anyone wants them. (They were summarised in the article that started on p.299 of M.E. 4632, 14 Feb 2020).

Last thing is just to recommend Turbine Guy’s comprehensive thread which shows how turbine efficiency can be measured and improved by careful attention to details.

Mike

Hi Greensands

Glad to hear someone is interested in the articles. I can send you a copy of the article by Bond if you get in touch via a message.

Regards

Mike

Byron, did I send you a copy of the article by Jim Bamford describing his syrup-tin dynamometer? If not I’ll do so. His device, made in the 1950’s, was a very simple water-brake dynamometer. It had a simple paddle that rotated in water inside the syrup-tin. The tin was pivoted and had an arm attached on which weights could be attached and adjusted in distance from the axis until the arm was balanced horizontal when the turbine was stationary. The shaft from his turbine entered the tin through the centre of one of the pivots and as the paddle turned it tended to rotate the tin. The load on the turbine was adjusted by bending the blades of the paddle (or the water level). Prof. Dennis Chaddock, who had used spinning bars equivalent to your propellers, later wrote that Jim Bamford’s syrup-tin dyno was a brilliant innovation.

Too many years ago I made one of these syrup-tin dynos and played around with it. Now I’m older and greyer I thought I’d make a proper one with discs as invented by Froude. That is part built. My initial intention was to use one of the cheap electronic scales one can buy to measure force on the arm instead of weights, but it would be much better to record torque continuously rather than writing down the reading from time to time while also controlling a boiler etc. Recently, a friend who is an electronics expert, built a dyno with a motor/generator to provide an adjustable load. In his device the motor is only the load and torque on the motor is measured directly with a load cell. I understand that analysing the signal from a load cell requires a very high gain accurate amplifier and,based on past experience, I foresee problems with interference etc. So combining a torsion bar with a water brake has some attraction for me. However, at current rate of progress the time when I’ll need the dyno is a long way off.

Regards, Mike

Thanks for that - I see I was making things much too complicated. I guess you used trigonometry to calculate the the reduced torque as the arm with the weight dipped down below horizontal - in addition to calculating the actual angle.

I think I've identified the issue that your article appeared in - by looking at images of mag. covers. The issue for June 2017 (No 255) has a list of 5 projects for the workshop, one of which was "Arduino dynamometer". Does that sound like the one? If so I'll buy a copy.

Mike

Thanks Dave.

On thinking about the torsion type dyno I am not sure how I would easily measure the angular twist accurately when the shaft is stationary - while keeping the loaded lever dead horizontal. Maybe with a dividing head to hold and rotate one end of the rod while the weighted lever is attached to the other end?? A finely divided rotary encoder disc might do the job but would add complexity. These thoughts together with what you say about the length of the shaft make me think that the load-cell approach might not be so bad after all. Nevertheless I'd still be very interested to read your article. I looked for an index to MEW but the only one I can find does not cover anything more recent than 2016.

I'll ask Duncan about the dynamometer car.

I'd be interested to hear if you find that book. Parsons designed his dynamometer very soon after the first trials of Turbinia had been carried out in 1894.

Regards

Mike

Hi Dave

I'd like to look at the article you mentioned in which you describe your shaft torsion dynamometer. Can you please tell me the issue/year of MEW in which it appeared? I am wondering if it would be a more suitable alternative to the water-brake dyno that I have partly built. I am thinking it would enable me to have automated data collection without having to tackle the electronics needed for analysing a load cell. I feel happier about using optical sensors and microcontrollers since I've used them before.

Are there any problems with that type compared to the water brake type? e.g. calibrating the torsion shaft which I guess is done using static weights hanging off of a lever?

I'm not sure, but I think the first torsion dynamometer was the one invented by Charles Parsons in order to investigate lack of power in the Turbinia. His dynamometer is on display here in the Discovery Museum in Newcastle and led to the Denny Brothers of Dumbarton developing the shaft torsion dynamometer for use in the ships they built.

Regards

Mike

Hi Turbine Guy

This is the company: www.smbbearings.com

They are in Oxfordshire, UK

They have been very helpful and had no minimum order size. This is the advice they sent me (it was a few years ago, so the price info will be out of date):

"My manager has recommended that you consider a dental bearing that we have available, with a high speed cage & ceramic balls – the SCBR144TWZW09 SMB - which can operate at 250000rpm without the need for lubrication (dimensions are 0.125x0.25x0.1094&rdquo. We currently have 14pcs available from stock at £9.48 each plus VAT."

Below is the data sheet for those bearings.

Regards

Mike

Hi Turbine Guy

63,00 rpm does sound good. My comment was triggered by the max speed of 38,700rpm shown on the data sheet you posted for the CER155SC bearings. Since the dental bearings I've bought (but not used yet) can run dry or lubricated with water, they should avoid problems of oil and its emulsification. But I am still thinking of trying graphite initially, especially in view of the information you provided about its durability. Unfortunately I am still a long way from getting to the stage of experimenting with bearings.

Mike

Hi Turbine Guy, very interesting discussion on bearings. One thought crossed my mind - previously I asked the company SMB Bearings for advice on use of their stainless steel bearings which are rated at 50,000rpm. I told them that I planned to run them at 50,000 rpm and they advised strongly against them because they said at that rated speed they would not last very long since the rated speed was the absolute maximum. I guess different manufacturers may mean different things when they specify a speed rating. So, if you've not asked them already and before paying a lot of money, I was just wondering if it is worth checking with Lily how their bearings would peform at the speed you plan to run at since it seems only slightly below the max. rating. SMB advised me to use bearings rated at 200,000 rpm for a 50,000 rpm turbine. The 200,00rpm bearings are hybrid - ceramic balls and s/s rings. [Apparently the key factor that wears out bearings at high speed is centrifugal force of balls against the outer ring. ceramic balls ar less dense than s/s balls and so the centrifugal forces are less].

Regards

Mike

Here is another company.

I've used them for ground round stainless steel:

https://technifast.co.uk/ground-bar

They supply it in A2 or A4 grades, finished to H9 or M6 tolerances.

Mike

Hello Turbine Guy,

I was very interested to read about your experience with turbine bearings. For the tubine I am building I originally planned to use plain phosphor bronze bearings with forced lubrication. Then I was pursuaded that ball bearings would be more efficient, although I worry about power being lost in forcing water etc out of the way of the balls. I buy my bearings from a helpful UK company called SMB bearings. They sell stainless steel bearings rated at 60,000 rpm but they told me that is their absolute limit and if my design speed were 50,000 rpm they would not last very long. They suggested small bearings used for dental drills which are rated at 200,000 rpm. They are stainless with ceramic balls, are fairly cheap and can be run either dry or in water. I bought these but have not used them yet. More recently I have been thinking of using plain graphite bearings as such bearings were used in a turbine built by another model engineer in the past. I have obtained some rods of graphite of a grade made for making seals etc. I gather that some grades can be too abrasive for such an application. Did you use any particular type of graphite for the turbine that you mentioned?

Have you seen the articles in Model Engineer written by Prof. Chaddock in the 1950s? His turbine ran at over 80,000 rpm in plain bearings with forced lubrication. Also he determined the power output using a fan air brake. This consisted of plain rectangular wood bars of specified width and thickness mounted on a shaft mid-way along their length. That shaft was geared down 10-fold from the turbine shaft. For bars of various lengths he gave graphs which directly show the hp required to spin them at various rpm. Apparently the theory for predicting the power requirements for these bars is well established and the bars need no further calibration. Please let me know if you don't have his articles.

Regards, Mike