Member postings for John Lluch

No need for apologise, you have been very helpful. I just hope Davis Plastics will eventually reply with a positive answer. I eventually found a local supplier that has the "bearing grade" PEEK that I would like to try, but they ask me to buy a full length rod, which has a prohibitive price. I will let you all know what's the reply from Davis Plastics.

Hi Fowlers, this is the text of the email that I sent to them:

"

Dear Sirs,

I have been advised that you may be able to supply PEEK based materials in small quantities. My application is a small sliding valve for superheated steam. The part will slide on a flat surface with steam ports to allow steam in and out depending on the valve position. The matting flat surface will be stainless steel 303 or 304 grade. Dimensional stability is important but not critical because it's a flat sliding valve after all that can be designed with enough tolerances.

I am particularly looking for a reinforced bearing grade PEEK with low friction and small wear of both the plastic and the matting surface, such as KETRON PEEK-HPV, or VICTREX PEEK 450FC30. These grades are reinforced with carbon fibre and contain graphite and PTFE lubricants.

I have looked at your online store site (shopforplastic) and your ebay store, but I did not find the specific PEEK grade that I require available.

However, I found that the PEEK-HPV material is listed on your davis-plastics site. So I wonder if I can place an order by email.

Ideally, I would purchase a 20 mm diameter rod with 200 or 300 mm length, or a minimal quantity of sheet with at least 8mm thickness.

Are you able to supply this?

Thank you very much.

Kind Regards,

"

Edited By John Lluch on 28/11/2017 08:45:33

Hi Fowlers, Following your advice I just sent an email to Davis Industrial Plastics. I asked for a bearing grade PEEK. Let's see what they reply.

Duncan, It may take some time before I am able to post any result. Unfortunately, I'm partially disabled due to a chronic illness, and everything takes a lot of time for me. I also need a lot of time to rest. Maybe you want to try the material too, if you have some opportunity. Said that, I wonder if you have already some previous experience with carbon loaded PTFE. I would like to know too how the reinforced PTFE works.

Edited By John Lluch on 27/11/2017 19:46:24

Thanks for your replies. They are all very helpful.

So it looks to me that the best material for this must be PEEK with Carbon Fibre + PTFE and graphite. Eventually, I found this material on the RS-Online site (**LINK**).

Its not clear what's the original manufacturer, but based on the RS specs it seems to me that it could be Victrex PEEK 450FC30 (**LINK**) I am also comfortable with a statement on the RS site and elsewhere that PEEK has excellent resistance to superheated steam.

Of course, there's still the "iglidur X" material offer that I keep from IGUS, which is cheaper than the material found on the RS-Site. It may turn that "Iglidur X" is just a rebranded version of the "Victrex PEEK 450FC30", because the specs are suspiciously similar. But I think I will finally discard any products from IGUS despite having found them cheaper, as they do not give me enough confidence or information.

Maybe someone can suggest where to buy the "Victrex PEEK 450FC30" or equivalent product at a better price than RS-Online, so I stop looking further?

As per what I have found, many if not all manufacturers state the base polymer for their compositions, and most of them also the type of fillers embedded. Of course, they will not provide the exact formulation but at least you have an idea of what's in there so you can compare specs with other manufacturers of similar products.

IGUS seem to be the exception. They are extremely secretive about the actual polymers being used, which in my opinion makes difficult to know what alternatives you may have from other manufacturers. They are also more expensive, and they tend to say that their products are the solution for everything, so you end trusting them without really knowing what you are buying, which makes the purchase a bit frustrating and uncertain. I honestly dislike this way of selling things, which is not saying that it may be bad for them, because I may end buying their Iglidur X product anyway.

Edited By John Lluch on 25/11/2017 21:16:05

Hi Jeff and Fowlers.

Thanks for your replies.

The matting surface may be Stainless Steel 303 grade (yes I know this is very uncommon). So in theory suitable for a sliding valve made in plastic.

I would REALLY like to know what's the actual base polymer of the "Iglidur X" material from IGUS. I hate that they provide all kind of mechanical and property data at low to moderate temperatures but fail to give the same data at their upper range of working temperatures. According to their spec: "With a permitted long-term application temperature of 250°C, the iglidur® X tolerates even 315°C on a short term. Like in all thermoplastics, the compressive strength of iglidur® X decreases with rising temperature".

Well, that's nice to know, but to what extend? It looks as they are marketing a material for high temperatures only to fail to give actual data at these temperatures (?)

**LINK**

Their specs make me think on this: "Ok, the material isn't going to melt that easily, but you better don't use it at all if you are serious about high temperature"

That's why I ultimately asked for your practical advice which I appreciated.

Edited By John Lluch on 25/11/2017 18:10:09

Hi Michael,

Thanks for your reply.

I have an offer from the local IGUS representative for 20 mm diameter round bar, 200 mm length, of the "Iglidur X" material. Price is 23.32 EUR (plus transport). Not totally cheap, but fairly affordable and enough quantity to make several parts. The rep said the material "should" stand superheated steam, but I guess that's an uncommon application.

Has anyone tried PEEK as the material for sliding valves?. I found an old discussion that partially covers this question in the forums **LINK**  which discourages the material for use in piston valves, but I think that the described problems should not happen in a sliding valve. Any experiences?

I also have a secondary question: I have been pointed to "Iglidur X" from IGUS for the same purpose (by an IGUS representative). The specs seem very similar to PEEK, but I am unable to find the actual base polymer of said IGUS branded material. Can it be just PEEK filled with some particular or proprietary fillers?

Edited By John Lluch on 25/11/2017 12:05:01

Edited By John Lluch on 25/11/2017 12:05:58

Indeed, the thread is about making the hobby more accessible and appealing for the next generations. He also was concerned that the LBSC designs would eventually fall into oblivion.

Dean da Silva proposal was creating updated 3D plans of existing LBSC designs, and possibly adapt them to what's available or more common today.

One of the questions he raised was whether he should keep the plans in imperial units, or switch to metric. He said he would rather choose metric. I feel some responsibility for your discomfort because I first ignited the debate about the practical difficulties of moving existing plans from imperial to metric. My view was that I would rather stick to imperial for the existing LBSC designs, but only move to metric for any new developments. Others seemed to agree on that, or otherwise they highlighted the feasibility of moving to metric.

However, instead of reviving the LBSC designs, my preferred proposal would be to work on a set of totally new locomotive designs, with several levels of difficulty, created from scratch in 3D CAD, and starting with a very simple entry level locomotive. The entry level locomotive should be easy to make for the average person, and require only simple tools to be completed. For example, it could be designed in a way that it would use a majority of laser cut parts, and avoid or minimise the need for castings and complicate machining. Essentially, a revival of the old LBSC spirit, but not by using his old locomotive designs, but by creating totally new ones.

Edited By John Lluch on 05/09/2017 14:53:21

Edited By John Lluch on 05/09/2017 14:57:49

Edited By John Lluch on 05/09/2017 15:01:26

I miss a 'like' button in this forums.

Marcus. I would expect that as younger generations enter the hobby, the development of open source communities supporting and updating model engineering plans in CAD format will eventually become a reality. You are making very valid points on this subject.

IanT. I also want to add that Integer numbers up to a certain figure are EXACTLY represented in floating point formats. Also floating point arithmetic of integer numbers is EXACT. I have no doubt that CAD software uses floating point numbers to store internal dimensions, but I sometimes suspect that the actual stored values are scaled up to a particular factor to prevent accumulation errors. I do not know about other CAD packages, but I never, and I mean NEVER, have seen or suffered an accumulation error on SolidEdge. It behaves for example as if 1 was internally stored and computed as 10000, or 0.1 as 1000 and so on. So unless you get to specify a really small number or a dimension with a ridiculous amount of decimal places, the chances of accumulation errors are nil. This, or the software uses a really clever approach at rounding floating point arithmetic errors.

Edited By John Lluch on 04/09/2017 10:17:25

Hi Neil, You are an interesting man lol. Actually, while I was posting my 0.1 decimal to floating point binary conversion example, I wondered if you (or somebody) would refute that by showing fractions that are powers of two. I honestly didn't think this would be the case on a model engineering forums. But I'm absolutely delighted about your reply!. Having written a BASIC interpreter puts you in a strong position for that. My background is computer software development, and I too have written a couple of interpreters of common (old) computer languages, and a compiler for a specialised computer language aimed at the knitting industry. I am also the main developer of the HMI Draw and HMI View apps in the App Store, which also contain a real time pseudo-compiler to RPN for fast execution of Ruby expressions.

So yes, you are right. Indeed, fractions commonly used in imperial dimensions have a better representation in computers.

Hi Neil,

I am not totally sure of that because I'm not too used to it, but I recall having seen fractions in LBSC plans that were divisible by 3. This creates a periodic number with an infinite number of decimals.

As it has been mentioned, this is further complicated because computers store numbers in binary format. Indeed, numbers in computers are represented in a convened binary notation (IEEE-754) that uses a normalised floating exponent of two (also in binary). This representation is commonly known as floating point number. The reality is that an apparently simple decimal number may require an infinite series of binary digits to be an exact representation of the same number. For example the number 0.1 in decimal notation is a periodic number in floating point binary representation, in fact, when converted back from single precision floating point representation to decimal notation, the result gives errors from the 9th decimal place (0.1 converted to IEEE-754 and back to decimal turns to be 0.100000001490116). But I think this is beyond the topic of this forums and of little interest for the discussion that is going on.

Just to clarify my points on my earlier posts, my suggestion all the time was to stick to inches and imperial units for the existing LBSC designs when redrawn in CAD. I am NOT advocating any translation to metric, but just the opposite. However, I manifested my preference for the metric system in general and for any new designs, which should not become the main topic of this thread.

That's indeed an important point and one which is very difficult to place a proper suggestion for. Both jpg and pdf are nowadays pretty standard file formats and ones that will hardly be broken or made incompatible with other formats in the future. However, they both are meant to hold 2D information (such as pictures or printable content) and they are not editable, so in case an error is latter found the original CAD file will always be required. Between the two I would chose pdf, provided the purpose is only to distribute static content to be printed by the end user

About CAD, I am not aware of any format that would open (in an editable form) in all software platforms. However, for the purposes of dynamic display only, there are several file types that will open in most CAD systems. This does not solve of course the problem of maintaining an editable version of the 3D plans, which will be required to keep future compatibility. I am used to SolidEdge from Siemens because that's what I learned, and that's what I would chose, but I suppose SolidWorks can be considered a better choice because I understand it is more widespread. Other CAD systems have options to import SolidWorks files, though generally in a non fully editable way.

Finally, if the possible legal issues about ownership can be solved, a good thing to do would be to open source the 3D plans files to let everybody propose improvements or correct errors by pushing updates based on experience while building the actual locomotives.

Edited By John Lluch on 03/09/2017 21:18:52

Edited By John Lluch on 03/09/2017 21:20:13

Edited By John Lluch on 03/09/2017 21:20:50

Hi Neil, I appreciate your comment and to be honest I sort of expected something like that. I can't fully agree with you though.

I meant the SI is a superior system in the sense that physical properties have single measurement units as opposed to the imperial system, which is more inconsistent in that respect. . For example power unit is 'watt' in the SI regardless of thermal, mechanical, electric... as opposed to 'HP' and 'btu/h'. There is no clear unit for work in the imperial system, the equivalent to 'joule' in the SI. There is a difficult distinction between force (or weight) and mass in the imperial system, which is not the case on the SI where force is 'newton' and mass is 'gram'. The SI also completely avoids conversion factors in formulas of practical use, as opposed to the imperial system that often requires them. This is among other advantages of the SI.

Fractions are of course exact while they remain fractions, but my point is that they can not be reliably represented in decimal numbers. Specially when converted to mm. Decimal numbers is what CAD/CAM and CNC will use, so when converting from inches to mm it all becomes a mess of dimensions with infinite (to be exact) decimal places. That causes trouble due to accumulation of errors and rounded display of dimensions. If you keep what is already in inches as it is, then there is no problem. CAD systems will handle fractional inches gracefully, and CNC machines will have anyway some working tolerances. This obviously works and have been for long time, and this is also my point.

I suppose I do not fully understand the problem you explain with laying out a slide valve and its eccentric, but I can imagine what you mean. I concede that if you are grown up using fractions it may have sense to apply them in some cases. But the industry has evolved to avoid them for some of the practical reasons mentioned above, and not using them is not a design constraint at all. Virtually everything is drawn in metric units nowadays. I have drawn a freelance locomotive from scratch (in mm of course) and I never felt the need to use any fractional dimension. In fact, from my background point of view, fractions are rather an inconvenience. In plans drawn in inches with fractional dimensions I have seen that sometimes decimal numbers must be specified to account for accumulation of fractional dimensions (specially in assemblies) that don't really add to a nice number. This never happens if everything was specified as decimal numbers already.

Edited By John Lluch on 03/09/2017 20:03:17

Dean, I would also like to see LBSC designs drawn in 3D CAD and converted to metric. I am a metric system (the SI system to be more precise) guy, and I only learnt about the existence of something else (imperial) in recent years. I mean, I knew that an inch was 25.4 mm, but I never thought that this was something that was used in practice, specially on engineering plans. When I first ordered locomotive plans from Reeves, I became highly disappointed at learning that fact. So that's to make clear that I REALLY would like to have model engineering plans in mm units rather than something else.

Now, said that, I want to make the point that converting existing plans in inch units to mm is not as easy or straightforward as it might seem. Yes, it is of course just a matter of multiplying everything by 25.4, but the problem is that the imperial system is fractionary, not lineal. Dimensions in inches are often expressed with fractions, which means they are not actual exact measures, but just approximations.

The 3D CAD itself will essentially work because computer numerical precision goes up to at least 10 decimal places and therefore approximations may be good enough, but they will still be approximations, and accumulation errors may appear when adding dimensions, possibly causing trouble in assemblies.

The fractional nature of the inch system is problematic because you do not enter a fraction to a CNC lathe or CNC mill. Also a CAD-CAM program will not generate fractional output but numbers with a limited resolution (several decimal places).

Furthermore, plans that were originally drawn in inches will become really odd when translated to mm. To begin with, you will have to carefully round part dimensions to exact numbers in order for the CAD system to avoid accumulation errors. This is difficult, because it is easy to make a lot of assembly mistakes. If you chose to keep the parts dimensions as they are with all their decimal places, then you will either have very long numbers over the dimension lines, or they will appear rounded anyway, not reflexing the actual measures, thus causing further trouble.

Whatever approach you take, the converted dimensions will be bizarre, as opposed to simple and clean of newly drawn plans, making it evident that the original plans were in inches rather than millimetres.

The International System of Units (formerly metric) is exclusively used elsewhere except the UK and some USA. It's a superior system, which has greatly facilitated the latest developments in physics, and it is definitely the way to go.

Steam locomotives in continental Europe where all built in metric units, and model engineers there only use metric units for their models. But if you have some old British plans in an alternative system, then keep them as they are because attempting to do otherwise calls for a lot of trouble.

Edited By John Lluch on 03/09/2017 19:07:33

Thanks for this fully steamed discussion . So basically to answer my original question: since a model locomotive will rarely have any loading gauge constraints, provided everything is well designed I can just go for 2 sideways, double acting cylinders of the appropriate size.

John

Thanks Tim, I am not for a 'true' model, so what you posted is just what I wanted to hear. Of course, I will take care of the correct phasing between left and right pistons, though I suppose it is not the first time someone arranges them to fire in parallel, LOL!. If you think on it, a 2 cylinders loco is like a 8 cylinder internal combustion engine as far as pulses per cycle are concerned !. Should be more than enough for smooth running I guess !

Variable valve timing is definitely used on high performance internal combustion engines, such as sports street cars and racing cars.

On street sports cars (say BMW M3-M4) valves are still mechanically driven from the crankshaft but an electro-hidraylic system is continuously adjusting the "cam-phasing" on both the intake and outake valves to achieve the best possible performance and low emissions at any time.

On track racing cars (say F1 cars) a more complex system is used that involves not only cam-phasing adjustments but also duration. In such case I believe (not sure though) that the actual valves are directly driven by the system by means of electric solenoids, i.e not from the crankshaft through mechanical means. This of course requires the system to be fast enough to cope with the rotation speed of the engine.

My guess is that although all this is possible and definitely much easier on steam engines -simply because they turn very slowly, as you point out-, I wonder if this would bring any benefit at all. Maybe from the point of view of efficiency we could squeeze one or two drops of water, but I believe that hardly more power could be achieved from the engine.

Efficiency on a rolling steam engine is very poor for a multitude of reasons starting by the fact that we need to heat water before we can use it and we need to keep it hot and build pressure at all times regardless of the actual load conditions, so a lot is lost through the relief valve alone!. A fully controllable cut-off should achieve some efficiency improvements on the cylinders though.

From the point of view of raw power I think that no improvement could be achieved. Max power and torque is obtained at high steam pressure and long cut-offs (at the expense of efficiency). In this scenario I believe that variable timing can not deliver any extra power to the system, essentially because existing mechanical systems (valve gears) are already doing what it is supposed to do.

However, this does not preclude us to implementing if for the sake of it, of course . In such case I think that even a small sized, off-the-shelf, PLC (Programmable Logic Controller) has enough processing power to *fully* control a steam engine.

These devices are readily available and I have plenty of experience with them, so that part should not be a major issue. However, I am not aware of any electrically driven, fast switching, small valves that could be used reliably on hot steam. Could you point the community to some kind of solenoid valve that could be used to this purpose?

John.

Hi,

A loco with internal cylinders is harder to build due to more parts involved and so on. But does having internal cylinders really pay the extra building effort?

Considering a locomotive with 2 sideway external cylinders, and another one with 3 cylinders, can both be considered to perform the same provided that the total volume displacement is the same (i.e cylinders on the first loco are 50% bigger to compensate for the lack of the internal cylinder)?.

OR, what is the ultimate reason why locos would have more than 2 cylinders. Is it a mater of compactness (constraints on loco total width) or is there a more subtle reasons, such as smother running? Is this really significant on a model locomotive?

Thanks.