uuu
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Post by uuu on Dec 1, 2020 13:31:36 GMT
...What i don't know yet is what the typical running temerature is. How hot is the superheated steam of a miniature loco? How hot do the cylinders get when in contact with the steam. Does anyone have some empirical values? The late Jim Ewins did some tests on a 5" gauge loco with thermocouples in various places. He was recording the superheater itself at about 850C steam at about 325C in the cylinder inlet and cylinder temps of 135C, exhaust 160C This table nicked from a John Baguley post on another site: linkWilf
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Post by Roger on Dec 1, 2020 14:43:29 GMT
One article I read explained that one method of getting the right size is to put the cylinder in the oven and get it to the maximum running temperature and then adjusting the fit to be a light sliding fit. What i don't know yet is what the typical running temerature is. How hot is the superheated steam of a miniature loco? How hot do the cylinders get when in contact with the steam. Does anyone have some empirical values? Good question. Saturated Steam at 90PSI is 166C so you know it's going to be more than that. How much Superheat you add depends on whether you've got Radiant types and how many of them, as well as many other factors. Without doing tests on your particular setup you probably won't be able to find out with any degree of confidence. The melting point of PTFE is 327C, so your upper limit is going to be sub 300C. I'm afraid I don't have any practical figures for any given design, perhaps someone has done measurements. I suspect that most designs don't create a great deal of superheat. I'm pushing my luck with my design, where the calculated temperature peaks at 309.5C with four Radiant Superheaters. I may end up having to replace my PTFE rings with something metallic. We'll see. My guess is that the temperature will actually be less than this due to losses in the pipes and the cooling effect of the bulk of the cylinders. Only time will tell.
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Post by mikessme on Dec 1, 2020 19:59:22 GMT
As you have the set screws I think they will be the same as mine so should drift out. The rear half can be removed first as it has a closed end, I tried to attach pictures but the thread limit is exceeded so maybe I can email them to you. The steam ports are opened up along their complete length, but be careful not to extend the ellipse length where they enter the valve bore as this is where the O rings seal, again I have a picture of the ports before and after modification. I removed some of the metal between the ports on the mill but finishing I did with needle files
Mike.
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Post by mikessme on Dec 1, 2020 20:22:38 GMT
As you have the set screws I think they will be the same as mine so should drift out. The rear half can be removed first as it has a closed end, I tried to attach pictures but the thread limit is exceeded so maybe I can email them to you. The steam ports are opened up along their complete length, but be careful not to extend the ellipse length where they enter the valve bore as this is where the O rings seal, again I have a picture of the ports before and after modification. I removed some of the metal between the ports on the mill but finishing I did with needle files Mike. Another thought looking at the work you have done on the valve linkages, the pins in the valve cross heads are loctited in place, I wasn't happy with this and sure enough in the early days of my ownership one came loose and as the slots in the cross head guides are bigger than the pin it caught and locked up. Fortunately little damage was done but I remade the guide blocks without slots and allowed the pin to float.
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Dec 2, 2020 9:48:59 GMT
SNIP I tried to attach pictures but the thread limit is exceeded so maybe I can email them to you. SNIP Mike. Mike, You cannot attach pictures to your post. You need to store them in a sharing site and use a link in your post to display them. There is a 'sticky' post at the top which tells you how to do it.
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Post by michaelfive on Dec 5, 2020 9:52:39 GMT
Thanks Roger and Wilf for the information about steam temperature. I haven't had the time to machine the ptfe tyres for the valve bobbins yet. But i think i will try the method with preheating and adjusting. Furthermore i'm thinking of giving the tyres some clearance between the bobbins and the tyres and add a silikon O-Ring there.
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Post by michaelfive on Dec 5, 2020 9:57:03 GMT
As you have the set screws I think they will be the same as mine so should drift out. The rear half can be removed first as it has a closed end, I tried to attach pictures but the thread limit is exceeded so maybe I can email them to you. The steam ports are opened up along their complete length, but be careful not to extend the ellipse length where they enter the valve bore as this is where the O rings seal, again I have a picture of the ports before and after modification. I removed some of the metal between the ports on the mill but finishing I did with needle files Mike. Sounds like a lot of filing! The bores of my engine are only 3.5mm in diameter. I dont think that i can reach to anywere near half of the bores depth on the mill. Maybe a thin file on the Dremel could be an option...
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Post by michaelfive on Dec 5, 2020 10:05:16 GMT
As you have the set screws I think they will be the same as mine so should drift out. The rear half can be removed first as it has a closed end, I tried to attach pictures but the thread limit is exceeded so maybe I can email them to you. The steam ports are opened up along their complete length, but be careful not to extend the ellipse length where they enter the valve bore as this is where the O rings seal, again I have a picture of the ports before and after modification. I removed some of the metal between the ports on the mill but finishing I did with needle files Mike. Another thought looking at the work you have done on the valve linkages, the pins in the valve cross heads are loctited in place, I wasn't happy with this and sure enough in the early days of my ownership one came loose and as the slots in the cross head guides are bigger than the pin it caught and locked up. Fortunately little damage was done but I remade the guide blocks without slots and allowed the pin to float. Thanks for offering pictures. I will send you a PM with my E-Mail address. Regarding the pin of the valve crosshead: I faced the same problem that the pin got locked up in the slot of the guide. I overcome this issue by making new pins with a thin shoulder. The shoulder sits between the brass sliding block and the steel crosshead. For the other side i made a washer with the same thickness. The crosshead is now centred between the blocks with free of lateral play and the pins cannot slide out...
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Post by mikessme on Dec 5, 2020 18:05:38 GMT
I will be interested to see how the ptfe rings work out, the comment from Roger re my rings being shaved off in the ports I think explains why they failed so quickly.
The footplate looks a lot tidier, but it might be worth checking the firebox doors, I found a big gap between the doors and the firebox ring, I have reduced this with a brass ring which the door frame clamps in position.
Mike
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Post by michaelfive on Dec 5, 2020 20:53:03 GMT
I will be interested to see how the ptfe rings work out, the comment from Roger re my rings being shaved off in the ports I think explains why they failed so quickly. The footplate looks a lot tidier, but it might be worth checking the firebox doors, I found a big gap between the doors and the firebox ring, I have reduced this with a brass ring which the door frame clamps in position. Mike I noticed the gap when repainting the doors, but didn't pay it much attention. Thanks for pointing it out. Could you share a picture of the ring as well?
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Post by michaelfive on Dec 6, 2020 19:05:32 GMT
I finally found the time to test if there is a leak path between the valve sleeves and the cylinder block. I printed a valve piston that could be fitted with o-rings and tested in three positions: - Centered with all ports closed - Admission to front ports - Admission to back ports
Luckily the seal of the sleeves seems ok. In central position i could not hear any air noises at all. If the piston is set to admission i hear a very slight blow im the exhaust pipe. This could be the sleeves but i rather think that its the main pistons with the silikon o-rings that leak a bit. I don't think that this is a problem for the momment. I will check back when i made new main cylinder pistons.
Nevertheless i might need to remove the valve sleeves to enlarge the steam path from the valve to the cylinders as Mike did.
But first i will attack the valve bobbins. This should then allow me to see the engine running on air for the first time.
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Post by mikessme on Dec 6, 2020 23:55:53 GMT
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Post by mikessme on Dec 7, 2020 23:04:40 GMT
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Post by michaelfive on Dec 8, 2020 17:08:06 GMT
Thanks for the pictures Mike! I put the ring for the firebox doors on my ToDo list. As for the slots i keep looking for a machining option that prevents me from filing 😉
I have another question Mike: Does your engine have the factory modification with offset expansion links?
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Post by mikessme on Dec 9, 2020 22:53:42 GMT
It does indeed and required a bit of work to get the radius rods to operate freely without binding. I spent a lot of time around the expansion links to provide sufficient clearance in the motion and eliminate the kickback from the weigh shaft which was initially present, and to get the reverser to operate freely over its full range.
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Post by michaelfive on Dec 10, 2020 12:45:15 GMT
It does indeed and required a bit of work to get the radius rods to operate freely without binding. I spent a lot of time around the expansion links to provide sufficient clearance in the motion and eliminate the kickback from the weigh shaft which was initially present, and to get the reverser to operate freely over its full range. The engine i got doesn't have the offset expansion links. I was wondering why MW did this mod and if it was only for aesthetical reasons since full size has that offset. But i'm begining to think that they extended the lenght of the links and had to offset it to prevent fouling with the motion?! Now that i have the valve gear sorted out, i measure a valve stroke in full gear that allows the ports to open only around 70 percent. Which i guess they corrected with extending the links. Can you confirm that? Do the valves fully open on your engine? Do you know what the lenght of your expansion links is? Thanks for your help!
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Post by mikessme on Dec 10, 2020 21:06:53 GMT
I don't have the measurements for the expansion links or details of what the differences were, but from the build manual the mod kit fitted was extensive and included: Valve bobbins, valve ring spacers, valve rings, valve bobbin ends, expansion link girders, expansion links, rod link pins, return cranks, lifting arms, weigh shaft, and radius rods. I have retained dimensions of the valve bobbins, and can measure the return crank centres if this is of help. I don't have the valve opening measurements but the ports were not fully open, I think from memory they were about 80% in full gear, but didn't at the time think this significant due to the number and area of the ports, but if you wish I can measure the valve travel, and as I still have the valve liner dimensions this will give a definitive figure. At the time I was more interested in getting what I had to work correctly rather than making any changes, but speaking to one of the UK steam workshops last year, I was advised they had rebuilt several of these kits and had in all of them identified that the valve bobbins and stroke were too short, so it may be worth comparing dimensions of these between our two locos.
Mike
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Post by michaelfive on Dec 12, 2020 8:11:58 GMT
Thanks again Mike for the useful information. If you could measure valve travel in mid gear and full gear would be very helpful.
I measured the port width (the ports are round so its more a diameter than a width) and the position of the ports. Since i tried to measure it without removing the sleaves which was tricky its prone to errors. Maybe you could check with what you measured with sleaves removed.
I made a sketch as basis for the new bobbins in three positions:
a) centered with both ports closes b) At front dead center with ports open with the amount of lead c) At maximum travel in full foreward gear
In mid gear i measure a valve travel of 8mm. With a port width of 5mm and a distance between the ports of 42mm (center to center), i defined a lap of 3.8mm and 0.2mm lead. Thus 2x (lap + lead) = 8mm. In fulll foreward i measure a valve travel of 14.5mm. With the valve bobbin set to equal lead the vlave travels another 3.2mm from FDC and 3.3mm from BDC. Which is A not symetric and B opens the ports only about 3.4mm (front) and 3.5mm (back) I did not yet measure in full reverse, since i already have doubts that foreward is correct. To fully open the ports a valve travel of 2 x (port with + lap) would be required thus 2 x (5+3.8) = 17.4mm Maybe the ports must not fully open but i guess more than just 70 percent?! And i don't think that the timing can be correct like that. Or do i miss something?
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Post by John Baguley on Dec 12, 2020 10:28:14 GMT
Hello Michael,
The width of the valve head will be determined by the width of the ring, not the total width of the head (8.8mm) as it's very unlikely that the outside edges of the valve bobbin and the retaining plate will seal. These are usually made a smaller diameter than the bore to allow the edges of the ring to determine the valve events. You may have to make some new bobbins so that you can fit wider rings. The existing rings are probably too narrow.
Don't worry about the port openings in full gear not being equal. It would be unusual if they were. It's the time that the ports are open for that matters, not by how much they open. One port may well open less but it will be open for a longer period to compensate.
It's difficult to say how far the ports should open in full gear without knowing the geometry of the complete valve gear but it isn't necessary for the ports to open fully in order to get a suitable cut off in full gear e.g. 75%. It depends how the valve gear has been designed.
John
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Post by michaelfive on Dec 12, 2020 15:23:56 GMT
Hello Michael,
The width of the valve head will be determined by the width of the ring, not the total width of the head (8.8mm) as it's very unlikely that the outside edges of the valve bobbin and the retaining plate will seal. These are usually made a smaller diameter than the bore to allow the edges of the ring to determine the valve events. You may have to make some new bobbins so that you can fit wider rings. The existing rings are probably too narrow.
Don't worry about the port openings in full gear not being equal. It would be unusual if they were. It's the time that the ports are open for that matters, not by how much they open. One port may well open less but it will be open for a longer period to compensate.
It's difficult to say how far the ports should open in full gear without knowing the geometry of the complete valve gear but it isn't necessary for the ports to open fully in order to get a suitable cut off in full gear e.g. 75%. It depends how the valve gear has been designed.
John
Dear John, thanks for your inputs. I was probably not clear about the size of the heads: This is not the size the engine came with (actually the bobbins were not even the same size on both cylinders!) but the size i taught i need to make them. Since i'm planning to make ptfe heads i intended to make the ptfe tyres the whole lenght, covering the retaining caps. The idea was that the outher edges of the tyres would seal the ports and give sharp events. But maybe the way i tried to determine the size of the heads was wrong?! What i did, is measure the valve travel in mid gear. In mid gear when no motion is imparted from the links, i expect the maximas of the valve travel at both death center. I found the travel to be 8mm. If i dont have exhaust overlap, then lap plus lead equals half of the valve travel. Thus i have 4mm that i can devide out between lap and lead. I literature a read that typical lap is around 0.75 times port with. Thus i decided to go for 3.8mm lap and 0.2mm lead. That leads to a head size of port width plus lap, which equals 8.8mmm. But maybe i'm already wrong with that?! I can now understand, that the time of admission is more important than how much the valves open. But i guess to determine if the events are ok and the admission is correct i will have to measure all the parts of the gear and put the numbers in one of the simulators? Or is there any other way to determine if the design of the gear is well done? Thank you all for your time and help!!
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