Merchant Navy in 7.25" project

[pondok2]

Hi all,

You wait years for Bulleid pacific projects, then two come along at once!😉

I've been reading Martyn's West Country project thread with great interest, and at the risk of flooding the forum with Bullied pacifics, this is Merchant Navy "Canadian Pacific" in 7.25" gauge.

As with Martyn's build, this is also a 3D CAD design and yes, practical considerations in assembly and maintenance, as well as cost effectiveness in component manufacturing result in what we think are judicial simplifications.

To add to the risk of confusion, the project owner, who is commissioning the build, is also called Martyn! 😆

The design work in CAD started in the new year, and the main push has been to model enough components to run the 3D model on the screen, as always to confirm the valve gear centres for complete valve event accuracy. Being able to run the model on the reverser is an important objective.

Various other design considerations come into play from what we want to achieve, but I'll try to post pics of what we have so far for interest.

The goal is to have all 3 cylinders running in 3D animation by the end of August, so I can see some drawing being done during the holidays!

[pondok2]

The BFB driving wheels CNC machined:

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http://instagram.com/p/CrQ6dd5NXoU

[brucevoelkerding]

a couple of Questions -
o which CAD Package are you using ?
o how easy is it to input an Angle for the Driver to then measure the Position of a Valve ?

(I used to have access to SolidWorks at Work and designed many Cam-operated Production Machines which could be rotated thru their Cycle via the Mouse or animated. I never got to the Point of being able to feed a Model Parameters from an Excel File. I did model a Locomotive during Lunch Breaks. I would attach a stationary "Degree Wheel" to the Reference Frame and just rotate the Driver to a given Degree Reading and then measure the Valve Position. If instead I tried to input the Angle as a Constraint on the Driver, the Model would sometimes lock up.)

[pondok2]

thanks for the interest Bruce,

I'm using fusion360, it's not the best in any respect, but it does everything I need it to.

Ok, I assume you mean, how do you place the valve gear into different positions to observe the valve events for various cut off?

Well fusion360 sadly doesn't offer a joint feature that is able to simulate a die block following the curved slot of an expansion link - as far as I have been able to discover!

but the workaround is to create a false expansion link with a line of holes at the various representative cut offs. Then it is "just" a question of setting up regular rotating joints between the radius rod die block pivot and each cut off respectively.

Fusion then allows you to activate each joint in turn and the radius rod jumps to that position for testing.

I'm sure every CAD package allows for objects to be visually sectioned, and this allows you to observe the valve events very closely.

Accuracy is really as close as you like, although there is no function to factor in bearing and bush play, but that doesn't matter much once the correct centres are found.

There is also the very handy option of being able to modify rid and link lengths without havibg to rebuild all the joints.

At the moment I have "only" the outside slide bar bracket and motion bracket to model before I can start drawing the valve gear and rods with a view to simulations.

Probably a little controversial on this project, inside admission is to be used on the outside cylinders, so the main visual change is to the suspension link outer clevis.

regards,
Andy

[pondok2]

(forgot to mention, yes I have sometimes come across limitations and the animation locking up)

[masahiraoka]

Good luck Andy on your journey.
I look forward to seeing the animations
we’ve found that the design / development time took at least twice as long as we estimated at the outset
ciao martyn 

[pondok2]

Thanks Martyn, it will be quite a journey and I am not at all surprised about the development time. Once the chassis is running on screen, I would be happy to get the laser cut parts in and build up the completed main frames, stretchers and motion brackets as the first stage, so I hope to have that complete by Christmas. The stretcher sub-assemblies will mostly be TIG welded, so that part of the process should be fairly quick.
How far off a running chassis is your light pacific?

[masahiraoka]

Andy
it’s hard to say at this stage but I guess we’ll have the chassis running on air in 6-9 months. Nigel is very busy working on full size steam locos and has to fit in working on the West Country around this. We have a production meeting this afternoon so may have a better idea after that.

[pondok2]

I try keep things going in a strict sequence, which is usually main frames -> rolling chassis -> running chassis -> boiler & cab -> painting and fittings, with the tender in there somewhere in parallel, but for me the trickiest looking shapes are the boiler throatplate and manifold cover, so I had to jump the gun and start getting the shapes together as solid models at first. These are only really for making formers out of, so more a guide than complete precision. Must admit I can't quite see how to get the stepless transition between the Belpaire shoulder and the manifold take-off cover. The drawings show vague outlines and suggest that the cover doesn't protrude further out than the Belpaire, but photos suggest to me that it does protrude... more drawing there needed I think.

My attempt to post using an instagram link really didn't work, so I'll mess about with my old flickr account:

[https://live.staticflickr.com/65535/53066303446_e1948a47cb_b.jpg]

[https://live.staticflickr.com/65535/53065714682_04e5b36b86_b.jpg]

[https://live.staticflickr.com/65535/53066693655_844abcb131_b.jpg]

[https://live.staticflickr.com/65535/53066799903_ab04a9a421_b.jpg]

[pondok2]

Well I obviously have no idea how to embed the image url into a post, so I'll try the image button here....

edit , ok great, that obviously worked.  Although the actual copper boiler inside is quite simplified, without the complex throatplate curves, I left enough space to make sure that these shapes can be faithfully reproduced with the cladding, as it is such a feature of the loco. 

[pondok2]

Still quite a few brackets and bigger things like the ashpan before the main frame is fully drawn, but the priority is to simulate the valve gear to ensure perfect valve timing to be able to drive on the reverser, and we have now reached the stage where that is possible, so the other brackets will wait.

Also to look closely at the when the valves open to exhaust to ensure evenly spaced 6 chuffs per revolution.

Why is this not automatic using the full size drawings? Because the outside valve geometry needs to be changed for inside admission cylinders - a change we think is worth it for a few reasons.

Personally I'm looking forward to seeing that massive inside crank axle with its crank webs spinning ☺️

[pondok2]

[pondok2]

Here we have the critical stage of running chassis, having modified the outside valve gear for inside admission, except that as yet I'm finding some asymmetry in the exhaust events. Admission in all 3 cylinders is very accurate, so I'm a little stumped by the exhaust, but it's not the first time and usually if I watch the simulation for a bit, and/or sleep on it, something comes to me. In fact, I have an idea.....






if that works, I'll put the simulation video up here shortly.

[pondok2]

3D model running with all 3 cylinders, around 80 CAD joints required to get this far with the animation.  A valve events chart attached out of interest.

The proof of concept gets the green light with the modification to inside admission in the outside cylinders, and fires the starter gun to get metal in.  I can start on the stretchers and brackets, but the main frames still need the smokebox saddle, reverser leadscrew and running board brackets, ashpan and foundation ring support drawn.

[pondok2]

Main frames permanently bolted together now. Next up all the outside brackets.

[pondok2]

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[pondok2]