Stephenson’s Rocket

Bob Wild · Nov 6, 2023

Basil said:
Thanks Bob, Still trying to get my head around this. Great 3D scan of the original. Is the upper valve shaft not 2 piece? Just wondering why one upper valve levers would need to be disengaged for normal running. Now I'm afraid I am going to start showing my complete lack of knowledge when it comes to steam engines. Got me intrigued though.

Hi Basil. You are indeed correct, insofar as the shaft is made from two pieces. It appears to me however that they are keyed together. There has to be some means to only allow one gab rod at a time to drive the left and right crank levers. This why I think that one of the gabs has to be disengaged with the keep (catch). I might try to make a trip to York to see the sectioned model and Sheldon to see the replica, if 'Er Indoors will permit. Any help from the experts would be most appreciated.

Bob

Bob

Bob Wild · Nov 11, 2023

Still trying to work out how the levers work. On all the videos I have seen both of the valve levers are cycling up and down. So there must be a way to drive the main shaft from only one of them. But I’m darned if I can figure out how?!?

Bob

Bob Wild · Nov 28, 2023

Finally sussed out how it all works. Like all good engineering the beauty is the elegance and simplicity (once you get your head round the drawings!). Here's my current understanding:

The eccentrics are mounted 90 degrees apart on the sliding sleeve. The left eccentric drives the left cylinder and the right eccentric drives the right cylinder thus the two cylinders drive the two driving wheels separated by 90 degrees. The right eccentric drives the inner countershaft via the eccentric rod, and then the gab rod drives the valve lever and inner drive shaft (not shown in my original drawing and hence the right cylinder. Similarly the left eccentric drives the left cylinder with the outer shafts a sliding fit on the inner shafts. Thus the gab rods are permanently engaged. The key to reversing is the relative angle of the driving dogs which are keyed to the axle. They are offset so that the valve opens from one dog at just the right moment to drive in a clockwise direction, and the other dog (when engaged) opens the valve at the right moment to drive in the opposite direction.

Goodness only knows how I will get all the angles right in my model. I can't key them in place due to the small shaft sizes, so I'll figure out hoe to make then adjustable. We wil see. I'll soon have something to post as the eccentrics are almost finished.

Bob

Bob Wild · Jan 10, 2024

Having finally got my head round the drawings, started on building the bits. Here are the eccentrics complete with reversing mechanism. Also the shafts and linkages for the lower half of the assembly. I have tried to follow the design as near as possible. However the drawings show some very intricate parts at the scale I am using. For instance using keyways in a three mm shaft is way beyond my skill and ability. Additionally there are wrought iron parts with many sculptured outlines - much too hard for me. But I’ve ended up with a working mechanism that is dimensionally correct. Next up the valve mechanism.

Bob

Samet ÇALIŞKAN · Mar 14, 2024

Hello, I was very happy when I saw this title. I've been wanting to build a steam-powered model of Stephenson's Rocket for a while now. However, all the plans I found either do not work or work with air, not steam. Will the model you made work with steam? If so, I'm very happy now.

Bob Wild · Mar 16, 2024

Hello Samet. That's a very interesting question. I am using the plans from Julius de Waal which are not fully complete, and he says it is designed to be built in wood! He has omitted the valve gear, so I obtained further drawings from the National Rail Museum, here in York. This is a very accurate model built many years ago. There are eleven sheets and I think it was designed for steam. Unfortunately the cost for the whole set is £243, which was a bit expensive for me. So I just bought a couple which gave details of the valve gear. I do hope that mine will run, but it will only be on air. Here's the link to the NRM drawings:

https://collection.sciencemuseumgro...he-rocket-locomotive-drawings-set-of-drawings

Hope this helps,

Bob

Bob Wild · Mar 16, 2024

Another major milestone reached today. Got the whole valve mechanism built. Very fiddly and I spent ages getting all the links working correctly. Had to remake a few parts because they were a sloppy fit. When you look at the number of moving parts, you can see that all the clearances stack up and lose quite a bit of movement. Anyway they do all fit together, and the valve openings look OK for now.

And just to prove it works here are some videos:

Next up are the pistons and crossheads. Big issue is how on earth to make the slots in the crosshead that the guides fit into. Not only are the guides a square section, but the are also at 45 degrees to the normal. Any suggestions would be most appreciated.

Bob

Samet ÇALIŞKAN · Mar 16, 2024

I was very happy to see your work. You built it with great workmanship. You gave me courage too. I follow your work with great excitement.

Richard Carlstedt · Mar 16, 2024

Bob, Any chance you can post a drawing of the part here ?
I have been able to come up with some crazy solutions at work on intricate parts and would like the challenge.
Rich
That video and your work is awesome !

Bob Wild · Mar 18, 2024

Hi Rich. Here are the drawings. I know what it is supposed to achieve, but it's very hard to interpret how it looks. And what on earth is the pump plunger ?!?

Bob

Richard Carlstedt · Mar 18, 2024

Bob, I assume you are referring to the crosshead as the problem part as I see no "pump plunger"part ?

Wait, I see it now--need my glasses
Rich

Bob Wild · Mar 18, 2024

Yes, it’s the crosshead that is puzzling me. If you look closely there is a hole with a label for a pump plunger.
I am thinking of tilting the head of my mill at 45deg to cut the grooves and then machining the guide rails to fit. I was wondering about screwing the upper and lower parts to a block so I could machine both together in an attempt to keep them in line.

Bob

Richard Carlstedt · Mar 18, 2024

Bob
I saw that pump plunger and have to assume they had a feed water pump on one side of the engine coming off the crosshead.
For the "V" grooves I would suggest using a 45 degree fixture for the milling instead of rotating the mill head
The rear of the crosshead looks pretty straight forward, but the front side is a nightmare of sorts.
First the 45 degree angle. I want you to draw a square and then a single diagonal line between opposite corners and label the line 1 "
Now the horizontal and vertical sides are .707 as that is a 45 degree angle.. in other words, a box with a 1" diagnal always has .707 " sides
I remember the number easily because the first commercial jet plane in the USA was a Boeing 707...
On the print, the "V"s have a 9/16" Centerline or .5265...so that means your diagonal line is .5265" and that makes the two legs( .5265 x .707)= .3976"
SO when you mill say the top grove on the fixture, stop with the finish pass, and then move over (Y?) .3976" and then drop the spindle the same .3976"
and make your lower pass. the two V's will be exactly .5265" C/L
More later
Rich

Richard Carlstedt · Mar 19, 2024

Well Bob, I can see why the crosshead parts are "Unique" in construction.
I drew up the rear crosshead ( Inside half ) and it is complex
Here is a sketch--Tough part if all solid.. Silver solder solution I am afraid
Rich

Bob Wild · Mar 21, 2024

Thanks Rich. I was also thinking about making it in two parts. Machine the grooves and profile first an the solder on a boss. I don’t really see what that big vertical hole is for. There should be a slot for the Conrod and a hole for the piston rod in line with the grooves. Anyway, I am on holiday for a week or so, which gives me time to ponder it all.
Bob

Paul2206 · Mar 22, 2024

Bob Wild said:
Hello Samet. That's a very interesting question. I am using the plans from Julius de Waal which are not fully complete, and he says it is designed to be built in wood! He has omitted the valve gear, so I obtained further drawings from the National Rail Museum, here in York. This is a very accurate model built many years ago. There are eleven sheets and I think it was designed for steam. Unfortunately the cost for the whole set is £243, which was a bit expensive for me. So I just bought a couple which gave details of the valve gear. I do hope that mine will run, but it will only be on air. Here's the link to the NRM drawings:

https://collection.sciencemuseumgro...he-rocket-locomotive-drawings-set-of-drawings

Hope this helps,

Bob

the plans are downloadable for free.

Steamchick · Mar 22, 2024

Thanks!
K2

Bob Wild · Mar 23, 2024

Ah, but … the free plans are very low resolution and if you look, they are barely legible; certainly no use for modelling from. So, I’m afraid there is no choice but to pay for them. See the snippets I have shown in earlier posts. I have been talking with a very helpful young man (he/him) at the NRM who tells me they are in the process of publishing the high resolution drawings on their website, but it won’t be for some time yet.
Bob

Bob Wild · Apr 9, 2024

Hi Rich. Cracked it. Done a bit of digging and found out that there were several arrangements of the crosshead assembly, as Stephenson made various modifications and improvements. So, here is the version that Glithero photographed on the NRM original. From that I have produced a version that I think I can make, albeit with difficulty and very carefully! As you suggest, I think it will have to be fabricated, with the bosses for the conrod and piston shaft soldered in place. Quite a challenge (for me).

Bob

Bob Wild · Jun 16, 2024

Got the crossheads made now. I tilted the milling head at 45 degrees to make the guide slots. The two halfs were machined from a single piece of brass to make sure that the grooves would line up truly. I then cut the material in two.
When the piston, crossheads and the guides were assembled I then had a go at manually moving the valve stem backwards and forwards. To my amazement and astonishment the piston did indeed move as intended. I had been very nervous about whether it would work since the ports and valve mechanism was to my own design!

Bob

Stephenson’s Rocket

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