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Member postings for Bob Worsley

Here is a list of all the postings Bob Worsley has made in our forums. Click on a thread name to jump to the thread.

Thread: What RCD NVR for a workshop?
03/07/2021 11:02:34

Slight complication if you use inverters to run three phase motors from single phase, you must use the Type B ( as far as I can remember) type of RCD.

The RCD works by measuring the imbalance in the current in the live and neutral wires, the problem comes when there is a DC current also in the wires, this saturates the inductor and it will never trip. Hence no safety. Some other devices also put a DC load on the mains, but it is the inverter that is the big problem with model engineers. There is discussion about the old practice of using half wave rectification in TV sets, but they didn't have RCDs then, just the ELCB, earth leakage circuit breaker, no where near as good.

Never ever run a high current load through an extension lead just for the hell of it! If you forget to FULLY unwind it then you will get one hell of a surprise as it explodes. Always fully unwind extension leads for loads over just a 100W or so. The reason? The wound extension lead is a transformer or choke winding, and forcing current through it causes it to heat due to the magnetisation and hysteresis losses as in any inductor. Added to this you are adding in some extra resistance, but more importantly lots of synchronous impedance which will effectively stop the welder working as it should. The welder uses a variable choke to introduce synchronous impedance into the output winding to limit the current.

Thread: What V belt cross section for use on a standard sized Picador pulley
18/06/2021 12:16:32

I did a search for stepped Vee pulleys, and was surprised at how many people make them. It was just in the application information section of their web sites.

There seem to be fairly strict limits on minimum pulley diameters, maximum ratios and shaft centre distances.

I needed a four step pulley to get the speed range on a mill, and to get a nice geometric ratio of speeds needed 3, 4, 5 and 6 inches at least, but 4" would be better to get the thing to actually drive when in the step up direction.

Use poly-vee belts? Expensive though.

16/06/2021 18:35:49

Doing Vee belts as well, and the minimum diameter A section pulley is 3" from my findings.

Thread: DHSS access to your bank account
22/05/2021 17:46:58

If you read the paypal and ebay T&C then if you sign up you give them permission to deduct any amount and any time from your credit or debit card.

THAT is criminal!

Thread: Hydrogen home heating
19/05/2021 09:54:50

Somewhere I was reading about sealing at high pressures, hydraulics probably, but as a side comment it was stated that sealing hydrogen is near impossible. The problem seems to be that the molecule size of hydrogen is so small that no type of non-metallic sealing ring will keep it in, things like O rings. You have to use a metal to metal seal. The article also said that over about 250 bar metal to metal seals, like BSP, start to leak, you have to use O rings to seal.

Is this correct? It was long a go and just remembered the conclusions, perhaps incorrectly.

So how was the old coal gas piped if 50% hydrogen? It must have been in a mix with something to make the molecule larger? I can remember as a child the bubbles coming to the surface after rain all along the gas pipelines where we lived, all mud in those days, no hard surface. Of course the gas pressure was very low, not like what is needed in a car application.

Thread: Shipping to the EU - beware!
06/05/2021 09:42:01

Possibly the important word here is TO.

I have bought a couple of books from the EU recently and arrived no problems, no real delay.

I wonder if the EU really are being awkward, could try holding the import of French and German cars to see what response that results in?

Thread: Crampton Loco Drawings
24/04/2021 19:25:19

Wow, made more progress than me. I fancied making the Blackgates Crampton but after three attempts to contact them to ask about drawings and copies of articles etc, had no reply.

They are in the EIM from a long time ago, I have a few of the articles but since so little support is available I am just sitting on my hands at the moment. I would be able to say what year the articles were, but EIM didn't bother to put the date or issue number on the pages, amazing. Mind you, some MEs didn't have the date or issue number on the contents page, amazing squared.

Certainly an interesting looking loco, make a good gate guard.

Thread: Clayton wagon valve gear
08/03/2021 09:42:40

Reading a book about the Crampton Loco and it seems one of those also used the Dodd's valve gear. Poor copy of a drawing but to my eye undoubtably the same.

Thread: Making a M20 x 8tpi 'rope thread' tap
06/03/2021 16:38:49

My picture of this thread is not very deep but nice curved form to it.

Sounds just like what is needed to roll boiler tubes to make them not smooth, to increase the heat transfer from gas to water. These are made by Babcock & Wilcox, also over 100 years ago.

Thread: Design of boilers
02/03/2021 16:38:22

I am not interested in diesel engines.

Re-read my original post, asked lots of questions, where are the replies.

Anyone do experimental work?

Found a heat book that used photography to measure the nucleate boiling bubbles, 1 to 6mm diameter.

LBSC must be spinning in his grave, I would like to communicate with him.

Conclusion, forget trying anything different.

Thread: Where to find a *good* optically flat mirror?
25/02/2021 11:21:07

H&W mirror is about 6" by 5" with corners cropped, about 1/2" thick, mounted in an aluminium casting.

For other metrology bits interested in a Watts Circular Division Tester? For measuring angles with circular graduation scale and microscope to read it. Calibration certificate says it goes to the few odd arc seconds. In a large and heavy wood box!

Thread: Design of boilers
24/02/2021 17:49:26


"As to Mr Worsley's inflammatory and unproven statements, I'd just say "don't feed internet forum trolls.""

Please provide a list of these, and as to why they are both inflammatory and unproven.

I used the exaple of tunnel diodes as a comparison, different bits of engineering might be useful to other bits.

Why not give us some calculated examples of thermal expansion and stresses rather than just stating that it happens? If you read Babcock & Wilcox's book on steam you will see that they use a spiral shaped tube to provide thermal flexibility. This is also in the Clark book from over 100 years ago. Clark also mentions joints in the fire tube that also gives thermal flexibility. The question for experimental engineers is why hasn't this been tried?

Clark mentions notching which is caused by thermal changes, often caused by feed water introduced into the very hot boiler. Even opening the fire door introduces a gale of cold air. There is thermal expansion, but what seems to be the killer is uneven thermal expansion, hence feed water and fire door air.

Stefan-Boltzmann is THERMAL transfer of energy, and as such does not cover all wavelengths of electromagnetic radiation for the simple reason that only around the infra red is heat transmitted. Perhaps you need to read a few books and then apologise.

Thread: Where to find a *good* optically flat mirror?
24/02/2021 17:27:49

Found a front silvered mirror off of a H&W shadowgraph? Also thought I had one of the mirrors you pictured but so far can't find it.

Thread: A milling anomaly
24/02/2021 17:25:39

A 1/4" cutter is pretty small, as well as being long series, and could easily bend. Try a 1/2" cutter? 1000rpm also sounds rather slow.

Thread: Design of boilers
20/02/2021 10:49:39

The maths is over most people's heads, but it is easy to use what others propose, such as Reynold numbers, Stefan-Boltzmann laws etc. This is where the Schaum books are quite good, worked examples which can be adapted.

I too have been reading, on to Clark's The Steam Engine now. What is apparent from all the tests done years ago is just how little of the heat actually goes through the fire tubes. Is this why modern boilers are water tube? Read any book on heat transfer and in my experience you won't get far, they are not very well written. But all of them discuss Stefan-Boltzmann and the fourth power between TWO environments, the boiler and the world.

My initial comments were about water spaces, that bubbles are not scaled. Again, any book on heat transfer will describe the stages of boiling with the difference in temperature between fire and firebox. It is intriguing that there is a period of negative resistance in the heat transfer, tunnel diodes if you know about them. The firebox has about 15% of the heating surface and contributes about 60% of the steam raising, read the tests done. But, my point, to repeat, is that the bubble formation on the firebox is limited by the flow of water to replace the steam, pressure is irrelevant. With a 1/4" water space then this does, or doesn't, happen? Even in full size there are comments about the too close spacing of tubes, so putting them 1/16" apart in a model boiler is? sensible? not when the bubbles are larger than that.

Go into the kitchen, put some water in the kettle, and boil it. Look at the bubble formation, they are not 1/16" in diameter.

Clamp a sheet of glass to a steel sheet with a gap between, fill with water, get the torch out and boil it.

It is worth pointing out that the word experimental means that you have an open mind.

Thread: Advice on benchtop milling machine
20/02/2021 10:12:22

It is a real machine and 70 years old, might be time to clean the slides? With real metal and fitting the slides should move without any real force needed. Also check that things that need lubricating, screws, slides etc are lubricated.

The knee is a different problem, they are always heavy to raise and lower, get a couple of gas struts to counterbalance the weight. Also, after 70 years the nut might be on its last legs, when they strip they cause some terrible accidents, well worth a look.

An old solid machine is worth its price in gold, you will really notice the lack of rigidity. You have it, with all the bits, don't even think of replacing it until checking the slides etc.

Thread: Design of boilers
05/02/2021 10:35:41

All my books say that the Stefan-Boltzmann law is the radiated loss between two temperatures, the boiler and the environment.

What is Martin Johnson's article? Why not put in a link so someone without 120 years of ME can read it?

All this flow in tubes is down in the noise, most heating is done through the firebox and with a 1/4" of water space my point was that that was not all it could be. Just look at the size of the bubbles when a kettle boils. Another gotcha is the design of crown stays, how any water is supposed to flow around them is not at all clear. Look at the ones specified for Minnie as a poor example. No cross holes, only 5/8" apart.

It seems that the large firebox stays used in the Allchin are a good thing, in that they conduct heat to the water. So making the stays even larger could be an even better thing.

The point is that with an incandescent fire, temperature about 700C or so, then there is a lot of heat there. And extracting the maximum possible from radiated heat could be more effective than using tiny flue tubes. Gases have almost no specific heat capacity, why they don't heat up tubes. The suggestion about using solid copper heat sinks from the front of the firebox into the boiler might be more effective.

Thread: Clayton wagon valve gear
31/01/2021 09:59:09

Found another reference to the Dodds valve gear in the Colburn Locomotive engineering book.

Thread: Design of boilers
24/01/2021 10:26:59

Found something that might be of interest to a wider audience.

Spon's Dictionary of Engineering, mine is dated 1874.

There is a whole long section on boilers, but at page 441 it starts talking about heating ability of tubes and fireboxes. One test done was to divide a boiler, 5' long, into six sections. First was the firebox wall and 1" of tubes, the second 11" of tubes, the remaining four each 12" of tubes. After three hours the evaporated amount for the sections were 2ib 12oz, 2ib 13oz, 1ib 14oz, 1ib 6oz, 1ib 2oz and 1ib 1oz.

Many other thoughtful comments and tests.

Also a diagram showing heat distribution across a flue when divided into a core and two circular rings. Temperature in 700deg, centre out 700deg, middle ring 700deg, outer ring 200deg. Linear flow is very poor at passing heat to the tube, as you would expect. But why not put some of that stainless steel shavings sold as pan scourers in the tube? Even increase the diameter of the tube?

22/01/2021 10:00:49

The dismal response was aimed pretty well at everyone, the near complete lack of any analysis and thoughtful comment on what took me several weeks to work out.

One person says that the smoke box gets red hot, another says that the boiler is 70% efficient, can't both be correct. Reading old books and they get efficiency readings of about 5%, which I would believe. This is why I introduced the Stefan-Boltzmann law, the boiler radiation losses are probably more than 50% of the heat that goes into the boiler.

I have also just spent several long evening trying to calculate form first principles the theory of staying, and it is now beyond my maths ability, see if I can find another book. I have used the Schaum's outline books but now find them incomprehensible. What is the difference between shear and bending moment stresses? I can understand the shear applied to a rivet, but in a beam?

I see that copper prices have risen over the last 6 months or so by $1000 tonne to $5,500, so probable can't afford to make them now.

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