don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Nov 13, 2018 18:16:37 GMT
Hi Don, The Main Crankpin throw is 1.25" and the Length between centres of Return Crank is 1.460" but I don't know the Diameter of the circle described by the Return Crank. Where is this specified on the drawings? Brian Sorry I don’t know but there are two Black 5’s in our group so I can ask their builders if they can help. They are both Don Young locos.....is yours? Edit:- Well I should have realised that yes it is....seeing as you have your other thread about it 🤪 Cheers Don
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Post by runner42 on Nov 14, 2018 0:35:04 GMT
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Post by Jim on Nov 14, 2018 5:13:03 GMT
It would be worth your while getting Don Ashton's book Camden Books has it. Don describes the drawing you have in detail along with much motre valuable detail.
Jim
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Post by silverfox on Nov 18, 2018 11:15:15 GMT
Having read this thread several times I still cannot figure out what diameter my return crank should be. It is not shown anywhere on the drawings and TBH I am getting a headache on the information that has been written Lisa says adjust the length of the retun crank to get the 90 degree angle ( if that is correct why does the designer specify a length?) Stevep says an all square set up ,the tail of the exp link sits on the line between the two axles Now the B17 set up has the cl of the piston road passing along the lines of the centres of the axles,likewise the little end of the con rod is also on this line.BUT the B17 exp link tail crosses this line twice My Excel cannot open Don Ashtons excel chart, but IIRC from some years back when I first tried to sort this out, in advance of any building, there were measurements required on it that I did not have any idea osf! So Here is the layout as taken from the drawings with the following given measurements. And a few notes clarifying some dims Although it looks like the cylinders are angled they are not Drawing is in mm converted from the imp given if it is shown as decimal below, that is what is on the plan Radius of Crank Throw 1 5/32 Length between centres of return crank 1.417in Radius of exp link through trunnion centre 6 13/32 Anchor link between centres 1 ¼ Off set of tail on expansion link on a radius of 1 ¾ from trunnion centre it is 23/64 to the rear Length between centres of radius rod 6 11/32 Eccentric rod between centres 6 27/64 ( check from job) Combination lever distances between centres 3in and 0-444 in Drop link length from crosshead (piston rod centre) 23/32 Although on another plan it is 0.700 It will be noted that some of the centres on the drawing do not connect, as I was unsure what would happen to the diagram if I started to do this On the crank circle the segment on th LH side is the radius of the return crank The other arc from top to bottom is the radius rod arc. The marks on the piston rod line indicate the position on the crosshead pin at the 4 quarters So armed with all this What diameter should the return crank be set at? Ron
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Nov 18, 2018 14:36:32 GMT
I don't claim to be an expert on valve gears, but some of your dimensions don't seem right.
I would expect that the radius of the expansion link and the length between centres of the radius rod were the same. With the motion in centre stroke (as you have drawn), you should be able to move the reverser from full forward gear to full reverse, with no movement of the valve.
The movement of the valve with the reverser in mid-gear should be 2 x (lap + lead). The proportions of the combination lever, and the stroke of the piston give you that.
If your expansion link tail pin crosses the centre line through the axles/cylinder twice, as it swings back and forth, then I think you have an 'all square' setup. Therefore, I would expect the expansion link tail pin to be directly beneath the trunnion pin. (The backset is to equalise the swing when the eccentric rod is angled up. As yours oscillates about a horizontal position, I think the backset should be just to bring the pin in vertical alignment).
This also means that the return crank should be set at 90 degrees to the crankpin. As I said in my previous post, once of the first decisions a valve gear designer makes, is the angular swing of the expansion link. We don't know that on your deign, but we do have the dimensions of the return crank, and the throw of the crankpin. Using Pythagoras, you can work out the radius of the circle that the return crank pin describes. (And incidentally, by using the 1 3/4" dimension of the expansion link tail, the angular swing of the expansion link).
Hope that helps.
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Post by silverfox on Nov 18, 2018 15:23:02 GMT
Steve
The drawing is not 100% correct I just put the exp link in an 'upright 'position and the crank positions at the 'four quarters'so i have the full travel of the centre of the con rod. I daresay that given the time, knowledge etc i could have done better. But as i said the measurements are correct,and all i have seen three other B17 built to these plans hopefully are correct. However the builders cannot remember what this magic circle diameter should be
The measurements as listed are taken direct from the plans so if by some alchemy it can be ascertained that would be great.
Edit
Steve
JUst found out that 3/16 and 5/32 does not add up to 13/32 Yes the rqdius aof the exp link and the radius rod are the same at 6 11/32
That should make a difference (lol)
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Nov 18, 2018 16:50:27 GMT
Ron,
Using Pythagoras, you get Sqrt(1.417*2 - 1.15625*2) = 0.819"
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Nov 18, 2018 19:24:06 GMT
Hi Ron, I have been looking at photos of B17s and it seems fairly certain to me that they are indeed “all square” in the valvegear department.
I agree with what stevep has posted and that your Return Crank pin should be set at 90 degrees to the Main Crankpin. At first I thought that the radius of 0.819 (ie diameter 1.638) of the Return Crank circle was a bit large, but photos clearly show that these engines had the expansion link tailpin quite a way from the trunnions, so I’m sure these figures will be correct.
Cheers Don
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Post by silverfox on Nov 18, 2018 20:46:42 GMT
Steve, Is it really that simple? if so put me down as a complete failure! and me with 1/3 of an OU degree in maths! Not wishing to play devils advocate, i have been looking at Martin Evans B1 series ( some of which helps me with the B17) anyway The B1 crankpin throw is 1.125, return crank between centres at 1.375 and shows the radius of the return crank as -75 The set up is I would say all square as the cylinders are horizontal No the pedantic bit! When i drew the crank throw, it is apparent on Autocad that there are two possible points 1) where the 1.375r crosses the horizontal line this is about 40 thous (3/64) outside the .75r 2) where the 1.375r crosses the circle of .75r it forms an angle of 92 degrees Would either of these differences have an adverse effect of the geometry of the set up? The drawing isn't as clean as on Autocad due to the cropping and enlarging of it
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Post by RGR 60130 on Nov 18, 2018 22:12:59 GMT
Ron,
Is your B17 to the M L Breeze design? I think he did one.
Reg
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Post by silverfox on Nov 18, 2018 22:27:03 GMT
Reg Certainly is I have seen two others running to the same design
One is Grahame Ainge in the NLSME and another belongs to a chap near Harwich ! I think a third one is on the go in NE London. Last time i spoke to Michael i think he said he has sold about a dozen plans. not that that means anything
One other question
Does the Expansion link trunnion centre have to be on the Valve spindle centre line?
I have learnt more this evening on valve gears than the last 20 odd years!!!
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Nov 18, 2018 22:49:36 GMT
Ron, the way I understand this is that the Return Crank is set at 90 degrees to the main crankpin when the tailpin of the expansion link lies on a line between the little end pin and the driving axle centre when the piston is at either front or back dead centres. Whether the cylinders are inclined or horizontal doesn’t matter. This is so that the expansion link is in exactly the same place with the piston at either end of the cylinder.
The B1’s tailpin is definitely not on this line, it is above it, hence its Return Crank is a bit more than 90 degrees (like Black 5’s & 9Fs for example). The B17’s tailpin does however seem to be on this line, hence it is set at 90 degrees.
Hope I’m not confusing things !
Cheers Don
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Post by silverfox on Nov 18, 2018 22:58:43 GMT
Don
When i was drawing the motion, the arc of the tailpin passes through the CL twice and not at one point
Now stever has given me the password i shall have another go at redrawing it
Update... the tail centre does not touch the CL of piston but is about 0.063 above it
Ron
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Nov 19, 2018 0:06:22 GMT
It is an interesting subject but it’s not easy to put it into words to discuss with someone else AND keep it short!
I’ve just been re-reading Don Ashton’s description of Walshaerts valvegear, in particular about Return Cranks & Tailpins etc. There’s no doubt it’s a complicated motion to fully understand properly but the beauty of it is, it will still probably work ok for our small engines even if not perfectly built.
I still think the B17 should be set at 90 degrees but I can’t be 100% sure without seeing the drawing of its valvegear.
I’ll close for now with a sentence taken directly from Don’s description:-
“The correct angular setting for the Return Crank has the pins lying at 90 degrees to a line from the Tailpin to the axle centre”.
This is what I was referring to earlier and would be at one of the dead centres, when the expansion link is in such a position that movement of the reverser imparts no motion to the valves. (my interpretation).
Cheers for now Don
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Post by runner42 on Nov 19, 2018 2:43:46 GMT
Some words from Allan Wallace.
The eccentric pin radius comes from a Reuleaux diagram which relates port opening to valve travel, lap and full gear admission. My simulator doesn’t do it that way but achieves the same result when you manually use trial and error (much easier in software than by cutting metal!). One aims to get about 70% admission in full gear.
Brian
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Post by silverfox on Nov 19, 2018 8:53:53 GMT
Runner I agree, and it is a lot quicker! Burnt a bit of oil last night and drew the arcs front and centre of the travel of the eccentric rod and the movement of the expansion link tailpin look like it is ok, now to go and find out what lead,if any, is needed and at 1/16 full size means 5 thou
Ron
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Nov 19, 2018 10:00:52 GMT
Reg CLIP Does the Expansion link trunnion centre have to be on the Valve spindle centre line? No - the expansion link trunnion will be above or below the valve spindle centre line (depending on whether you are using piston or slide valves) by the distance between the top two holes of the combination lever (assuming you are using straight connections). If the connection to the combination lever is angled, then the position would have to be adjusted to suit. Here's a picture of the setup of the Stanier 2-6-4T I built. You can see that the valve crosshead has an offset connection to the top of combination lever, so in this case, the expansion link trunnion is in line with the valve spindle.
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Post by silverfox on Nov 19, 2018 12:06:42 GMT
Steve A bit tied up today with hospital visits for the good lady and a meeting tonight. I will have a go at doing a more detailed drawing just for the ctlinder valve spindle and expansion link trunnion
( there more i use Autocad, the better i get!!)
Ron
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Post by runner42 on Nov 21, 2018 1:10:21 GMT
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