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Post by steamer5 on May 23, 2020 6:09:57 GMT
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Post by Roger on May 23, 2020 7:53:54 GMT
I don't really understand the question. Unless you're designing the valve gear, you need all of the dimensions to be defined one way or the other. You can't infer a missing critical dimension. If you don't have enough to draw it in CAD then you're back to the design phase. Then you'd have to make a reasonable guess at the missing dimension and put it into a simulator and see whether it produced acceptable valve events. You're asking the wrong person really, I'm just an Engineer. |
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Post by Roger on May 23, 2020 7:59:42 GMT
I have no idea without seeing a drawing. With a drawing, I'd put it into a CAD program to get any required dimensions that I needed. I'm not sure that CAD can help as without the PCD or the angle you are missing to much info to draw it. As Stevep pointed out the return crank is not at 90 degrees for most, can't say for 'all' as I don't know enough about other designs. Certainly for Doncaster the angle ia less than 90. I think I must have missed part of the discussion, I thought Reg had all of the dimensions else how did he know what to make? You can only draw what you have dimensions for. You can extract other dimensions not explicitly given from those which you are, but no more. If you don't have the whole design then you're into designing it yourself, presumably experimenting with different dimensions for the missing one. If you're missing more than one dimension, then you've really got a problem. |
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Post by Deleted on May 23, 2020 8:20:42 GMT
I'm not sure that CAD can help as without the PCD or the angle you are missing to much info to draw it. As Stevep pointed out the return crank is not at 90 degrees for most, can't say for 'all' as I don't know enough about other designs. Certainly for Doncaster the angle ia less than 90. I think I must have missed part of the discussion, I thought Reg had all of the dimensions else how did he know what to make? You can only draw what you have dimensions for. You can extract other dimensions not explicitly given from those which you are, but no more. If you don't have the whole design then you're into designing it yourself, presumably experimenting with different dimensions for the missing one. If you're missing more than one dimension, then you've really got a problem. Hi Roger This wasn't in relation to Reg's drawing, it was in answer of Ron's about how do you work out the PCD if no dimension is given. I don't know the answer, perhaps it was a matter of drawing all the parts and seeing what worked....I'm sure there must be a simpler formular, although you do read of how designers play around with various templates...the photo of the LNER drawing office with a large board for scaled motion parts to be experimented with would perhaps suggest the same approach? Pete |
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Post by RGR 60130 on May 23, 2020 9:21:16 GMT
My little 'How To' has certainly taken off! Hopefully someone will find it of help. I think I need to clarify that this thread was only to show how to accurately set the return crank to a previously derived dimension and has nothing to do with the valve gear design process its self. I had the pitch circle dimension before I started. It's perhaps unfortunate that my drawing appears to show an all square arrangement which seems to be confusing some people a little. My usual starting point when looking at valve gears is Don Ashton's site here www.donashton.co.uk/html/downloads.html (Ron, I've emailed you a copy of a spreadsheet I think will help you) As alluded to by others, setting the return crank accurately to a given measurement will not produce great results if the underpinning valve gear design is flawed or if you haven't built your engine accurately to the design data. Reg |
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stevep
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Post by stevep on May 23, 2020 9:23:03 GMT
Roger If you have not been given the PCD, how do you calculate it and what dimensions do you need? Ron You are venturing into complicated territory here. The eccentricity of the return crank pin is the amount that the expansion link will swing back and forth. (They are connected by the eccentric rod). The designer of the valve gear will design the gear using a number of assumptions - one of which is how much angular swing they want impart to the expansion link. They don't always tell you what this is! What you are normally told is how long the return crank is from crank pin to return crank pin centres. Then the normal way of setting is to achieve the effect that Julian described, and others have alluded to. First, construct an adjustable eccentric rod, and fit the return crank in approximately the right position. Then set the crank on FDC or BDC (exactly) and adjust the eccentric rod until moving the reverser from full forward to full reverse doesn't move the valve rod. Then move from FDC to BDC (or vice versa) and see if the valve rod still doesn't move. If it does, lengthen or shorten the adjustable eccentric rod, and re-position the return crank to try and achieve no movement. Keep doing this until success is achieved, and then fix the return crank permanently, and make the final eccentric rod using the centres from your adjustable one. Then do it all over again on the other side of the engine! |
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Post by Roger on May 23, 2020 9:37:06 GMT
I think I must have missed part of the discussion, I thought Reg had all of the dimensions else how did he know what to make? You can only draw what you have dimensions for. You can extract other dimensions not explicitly given from those which you are, but no more. If you don't have the whole design then you're into designing it yourself, presumably experimenting with different dimensions for the missing one. If you're missing more than one dimension, then you've really got a problem. Hi Roger This wasn't in relation to Reg's drawing, it was in answer of Ron's about how do you work out the PCD if no dimension is given. I don't know the answer, perhaps it was a matter of drawing all the parts and seeing what worked....I'm sure there must be a simpler formular, although you do read of how designers play around with various templates...the photo of the LNER drawing office with a large board for scaled motion parts to be experimented with would perhaps suggest the same approach? Pete Hi Pete, Without a copy of that drawing, I don't think there's much that anyone can say about it really. We're all guessing what's on the drawing, and what isn't |
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oldnorton
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Post by oldnorton on May 23, 2020 10:12:53 GMT
Roger If you have not been given the PCD, how do you calculate it and what dimensions do you need? Ron Don Ashton's spreadsheet. But you will need all the basic dimensions for the position of cylinder, valves, axle, weigh shaft, etc. These should be on all construction drawings. Then you have to assume the engine has been built to exactly the drawing dimensions. |
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JonL
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Post by JonL on May 23, 2020 10:23:03 GMT
Thats a superb post, thank you. I confess the way I set mine was to rigidly mount a set of dividers next to the locomotive (set to the diameter required) and moved the wheels 180 degrees on the rollers. Nowhere near as precise.
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Post by builder01 on May 23, 2020 13:50:55 GMT
Roger If you have not been given the PCD, how do you calculate it and what dimensions do you need? Ron You are venturing into complicated territory here. The eccentricity of the return crank pin is the amount that the expansion link will swing back and forth. (They are connected by the eccentric rod). The designer of the valve gear will design the gear using a number of assumptions - one of which is how much angular swing they want impart to the expansion link. They don't always tell you what this is! What you are normally told is how long the return crank is from crank pin to return crank pin centres. Then the normal way of setting is to achieve the effect that Julian described, and others have alluded to. First, construct an adjustable eccentric rod, and fit the return crank in approximately the right position. Then set the crank on FDC or BDC (exactly) and adjust the eccentric rod until moving the reverser from full forward to full reverse doesn't move the valve rod. Then move from FDC to BDC (or vice versa) and see if the valve rod still doesn't move. If it does, lengthen or shorten the adjustable eccentric rod, and re-position the return crank to try and achieve no movement. Keep doing this until success is achieved, and then fix the return crank permanently, and make the final eccentric rod using the centres from your adjustable one. Then do it all over again on the other side of the engine! This is exacly how I did it for my Super Simplex. Evans gave no PCD for the return crank. No CAD or valve simulator, just adjust the rod length and return crank until it is correct. |
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Post by Roger on May 23, 2020 18:24:25 GMT
Roger If you have not been given the PCD, how do you calculate it and what dimensions do you need? Ron You are venturing into complicated territory here. The eccentricity of the return crank pin is the amount that the expansion link will swing back and forth. (They are connected by the eccentric rod). The designer of the valve gear will design the gear using a number of assumptions - one of which is how much angular swing they want impart to the expansion link. They don't always tell you what this is! What you are normally told is how long the return crank is from crank pin to return crank pin centres. Then the normal way of setting is to achieve the effect that Julian described, and others have alluded to. First, construct an adjustable eccentric rod, and fit the return crank in approximately the right position. Then set the crank on FDC or BDC (exactly) and adjust the eccentric rod until moving the reverser from full forward to full reverse doesn't move the valve rod. Then move from FDC to BDC (or vice versa) and see if the valve rod still doesn't move. If it does, lengthen or shorten the adjustable eccentric rod, and re-position the return crank to try and achieve no movement. Keep doing this until success is achieved, and then fix the return crank permanently, and make the final eccentric rod using the centres from your adjustable one. Then do it all over again on the other side of the engine! Hi Steve, Thank goodness Don Ashton supplied me with a complete stick diagram of the valve gear with all of the relevent dimensions. I'm staggered that there are published designs where this basic information is missing. I just made everything to the drawing. The only adjustment on mine is the position of the bobbin. |
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Post by builder01 on May 23, 2020 19:49:56 GMT
Hi Steve, Thank goodness Don Ashton supplied me with a complete stick diagram of the valve gear with all of the relevent dimensions. I'm staggered that there are published designs where this basic information is missing. I just made everything to the drawing. The only adjustment on mine is the position of the bobbin. By making everything to the drawing, I assume you mean to Don's specifation, not the original drawing. What is particularly interesting, is that despite all of the shortcomings of the drawings, and lack of CAD and valve simulators, people have managed to create working miniature steam locomotives from poorly made drawings. Perhaps they were not built in an optimal way, but, they worked well enough to provide the builders and riders with much entertainment. There are, of course, no "perfectly built" miniature locomotives, but, neither were the full size. The drawings for my loco were done in 1989, I was sure there were going to be problems, but, as many had already been sucessfully built, I was not worried that there was not anything I could not overcome. |
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jma1009
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Post by jma1009 on May 23, 2020 20:10:51 GMT
I don't think that Reg's method is of general application, and is only going to be applicable where the most detailed design details have been considered and worked out, and then the loco chassis, cylinders, and valve gear have been made to 'toolroom' standards that mirror exactly the detailed drawings of these 'superior' designs.
There are very few of these 'superior' designs, and very few who can then replicate the making of the bits of these to the standard of accuracy required to rely on Reg's method.
The length of the return crank centres is of far greater significance than it's PCD, and I would not wish a 'tyro' to be beguiled or misled by Reg's method in applying it to designs that are deficient, or even where not adhering to 'toolroom' standards in a 'superior' design will negate it's application.
I thought I would check Don's book before work today, and on page 7 he pretty much states what Stevep and I said yesterday evening.
I am in no way criticising Reg. I am wishing to send a warning to others building say the designs of Don Young, LBSC, Martin Evans et al for Walschaerts that the PCD method of setting the return crank is going to result in failure unless you have also done the work via CAD or Don's spreadsheet or one of the computer valve gear simulators and can make everything to the required 'toolroom' standard, and know that your centres of the return crank and it's PCD is going to do the necessary.
Cheers,
Julian
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Post by Oily Rag on May 23, 2020 22:37:22 GMT
Then do it all over again on the other side of the engine![/quote]Hi Steve,
Thank goodness Don Ashton supplied me with a complete stick diagram of the valve gear with all of the relevent dimensions. I'm staggered that there are published designs where this basic information is missing. I just made everything to the drawing. The only adjustment on mine is the position of the bobbin.[/quote]
YES it is staggering.
To my mind it is pathetic that designers often hide the fact they don't have a decent grasp of the subject and or could not be bothered to get a good grasp, and or could be bothered to expend their time to do a proper job with checking and checking again all the aspects. So they just leave it all in sitting in a situation of drawing/geo haze for the builder's to sort out. Fudge and leave the the builder contemplating their process their own perceived inadequacies, a sort of shifting of responsibilities off the designer's shoulders. Many of the designers cannot be ignorant of this, they deliberately move on.
Considering most of them they took gold coin for their published designs, no matter how meague the amount of coin, it is not a "Professional Proper Job"
OK, drawing mistakes are made, that is human, expected but the obvious and deliberate haze of fudging drawings, missing dimensions to expedite their own process is infuriating to me.
I have seen and heard over the decades the constant grief of ME motion geometry drawings and it is why even on my little garden steamer I had to expend a lot of time sorting it on CAD and books and the simulator to ensure I avoided the such grief, because the geo with hazy and also incorrect dimensions that I paid coin for when I checked was never going to work any where near properly. A lot of compromise to get it to even run and then it would not run in reverse. (must have been the builder's work is the get out of jail cry) So using Don Ashton's book and CAD and the Dockstraeder simulator and hours of time.
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mbrown
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Post by mbrown on May 24, 2020 10:37:57 GMT
I agree that some published designs are deficient in various respects, but I think we should avoid being too harsh on the designers. Don Ashton was an absolute master of valve gears, but in the absence of his published findings, how many people understood the finer points at all? LBSC, Martin Evans, Don Young and co. had to be reasonably familiar with all aspects of design - not just valve gears - and also know how to put it across as journalists (probably against the clock - that's how magazines work). I think they did their best within the limitations they had to work with.
Full size was not always that brilliant either I am re-reading Eric Langridge's memoirs ("Under 10 CMEs") and although he had a valve gear model to play with, a degree of "rule of thumb" was also involved. He says that the inside valve gear for the Schools class as taken direct from the Drummond D15 design, and he comments elsewhere that "Walschaerts valve gear more or less designs itself". Then there is the case of the Joy valve gear on the LNWR 0-8-0s - noted for highly uneven valve events. Late in the day, someone managed to shift the position of the Joy slideshaft and got perfectly even beats - but "management" decided it was a major operation and it was more cost effective to let them plod on going "dot and carry one" for the rest of their lives.
My suspicion is that, if Don Ashton had published his work half a century earlier, a lot of full sized locos would have had better valve gears. The model designers, for the most part, were amateurs like us, sharing the knowledge they had - and where would the hobby be without them?
Malcolm
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Post by steamer5 on May 24, 2020 11:01:42 GMT
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Post by d304 on May 24, 2020 12:06:03 GMT
Finding the PCD
To find the PCD you need to know the dimension of the return crank. Drawing in CAD is probably the best way to get the proper dimension then compare it in one of the valve gear programmes.
In CAD draw the main drive wheel with the centre of the return crank. On the line generated by front dead centre and back dead centre draw a line that is 90 degrees through the centre of the axle. Rotate the return crank dimension until it bisects the 90 degrees. This is you PCD!
Depending if you have inside or outside admission ( and where the top pin for the of the combination is) the crank will either lead or follow.
For LNER people you will notice this also generates the position of the corner of the “ square” drive of the return crank.
Have look at the National Railway Museum bog no 42 on the Sir Nigel Gresley A4 rebuild and they have images of them setting the crank position without the use of the eccentric rod. I also believe the person in one of the images is building an A4 in 7 1/4 from works drawings.
I have been trying to understand and fine tune valve gear for my A4 using Don Youngs,s Doncaster as a basis. Apologies to 44767, I am almost there!
regards
David
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Post by ettingtonliam on May 24, 2020 12:23:36 GMT
My little 'How To' has certainly taken off! Hopefully someone will find it of help. Yes, Reg, its helped me! On Locomotion, the crankpins on an axle are not quartered, they are in line, and the quartering for the coupling rods is done by having a return crank mechanism at one end. I'd been pondering how to set these return cranks, and you've just shown me how! Thanks very much. Richard |
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Post by RGR 60130 on May 24, 2020 12:52:08 GMT
I'm pleased it's been of help to you Richard.
Malcolm's mention of inside valve gear got me thinking about the inside Walschaerts valve gear on the likes of Peppercorn Pacifics using an eccentric. I'll have a look and see if I have enough photos to do a little 'How To' on those.
Reg
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jma1009
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Post by jma1009 on May 24, 2020 13:24:06 GMT
To find the PCD you need to know the dimension of the return crank. Drawing in CAD is probably the best way to get the proper dimension then compare it in one of the valve gear programmes. In CAD draw the main drive wheel with the centre of the return crank. On the line generated by front dead centre and back dead centre draw a line that is 90 degrees through the centre of the axle. Rotate the return crank dimension until it bisects the 90 degrees. This is you PCD! Unfortunately, David, this is not correct unless the expansion link tail pin is on the centre line of the motion, which it very rarely is. |
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