Help making model of Walschearts valve gear.

[Simon Robinson 4Participant @simonrobinson4]

What dimensions are the various rods?

[Simon Robinson 4Participant @simonrobinson4]

Im trying to better understand how Walschearts valve gear works. So I have decided to make a working model of it, that I can hang on my wall. Does anyone know where I can find the dimensions of the different rods and parts eg the Radius rod, Combination lever, expansion link etc. (With the dimensions I can calculate them to the scale of my model)

I have looked at engineering diagrams on google images and the proportions of rod length vary. Some for example have the radius rod the same length as the combination lever.

Does the rod length vary in different locomotives and are there variations on the design of Walschearts valve gear?

Thanks in advance.

[RedsetterParticipant @redsetter]

Excellent project! My father made me a model of Walschearts gear about 50 years ago, using mainly cardboard and drawing pins, and we both learnt a lot from it.

A set of drawings for a locomotive such as Martin Evans Simplex, Don Young's Black 5 or even LBSC's Tich will give you good dimensions for the components. You will need to model the valves and ports as well in order to see what is happening.

Greenly's "Walschearts Valve Gear" and Martin Evans "Model Locomotive Valve Gears" got most of it right and are well worth reading, though some of the finer points of design are glossed over.

There is a lot of variation in the design, and generally the gear has to be designed to suit a particular locomotive. Once you get started you will see why.

You will find it fascinating, but dangerously addictive.

[Simon Robinson 4Participant @simonrobinson4]

Posted by Redsetter on 07/04/2018 17:22:30:

Excellent project! My father made me a model of Walschearts gear about 50 years ago, using mainly cardboard and drawing pins, and we both learnt a lot from it.

A set of drawings for a locomotive such as Martin Evans Simplex, Don Young's Black 5 or even LBSC's Tich will give you good dimensions for the components. You will need to model the valves and ports as well in order to see what is happening.

Greenly's "Walschearts Valve Gear" and Martin Evans "Model Locomotive Valve Gears" got most of it right and are well worth reading, though some of the finer points of design are glossed over.

There is a lot of variation in the design, and generally the gear has to be designed to suit a particular locomotive. Once you get started you will see why.

You will find it fascinating, but dangerously addictive.

Thanks for your reply. I will try and find those drawings. A cardboard model of Walschearts seems a good place to start.

[RedsetterParticipant @redsetter]

You need to make the model reasonably large so that you can see what is happening, as very small dimensional changes can have large effects on the valve events. The only significant metal part in my father's model was the connecting rod, which was a 12 inch hacksaw blade. The design was scaled up to suit this from one of the full page drawings in Martin Evan's book, so the model fitted on a board 18 inches long, which was quite a practical size. I wish I still had it!

[duncan webster 1Participant @duncanwebster1]

There are plenty of computer models for Walschearts gear, making physical models is sooooo old hat. Doing it on computer means you can change lengths of rods in minutes rather then hours.Send me a PM if you're interested and I'll send you some links

[RedsetterParticipant @redsetter]

Posted by duncan webster on 07/04/2018 21:01:59:

There are plenty of computer models for Walschearts gear, making physical models is sooooo old hat. Doing it on computer means you can change lengths of rods in minutes rather then hours.Send me a PM if you're interested and I'll send you some links

Model engineering is in fact about getting your hands dirty and making physical models.

You will learn far more about the practicalities of valve gear by making a model than you will by playing on the computer. Certainly once you understand the basics the computer is a valuable design tool, but the OP has exactly the right idea.

I strongly object to being called "old hat". You just want to impress us all with your computer skills, don't you? Well, jolly good, I'm impressed.

[SillyOldDufferModerator @sillyoldduffer]

Dare I suggest Redsetter has got the wrong end of the stick!

To get a computer to simulate a valve gear, it's necessary to express the geometry in mathematical form. Hard work. However, once you've done that it's easy to repeat calculations with different dimensions and angles. Results can be animated graphically and analysed in numeric form.

Likely for any given gear and locomotive there is an optimum layout, and, as has been pointed out, small changes can make a big difference to a valve gear's performance. Building a succession of physical models to find the sweet spot can be a monster time waster. Like a real model, a computer model can be used to show how a gear works. But the computer's real advantage is it's ability to quickly explore many different configurations. Millions if necessary. This is why almost all modern design uses computer tools.

Not knocking physical models. They're interesting and valuable for complementary reasons. Dozens of good justifications for making them; there is no real conflict with computers which are just another tool. ( Damned powerful ones!)

Dave

Edited By SillyOldDuffer on 07/04/2018 23:47:13

[julian atkinsParticipant @julianatkins58923]

Hi 'Redsetter',

That last post of your's was out of order. Duncan is a highly respected miniature locomotive builder with considerable practical experience, as well as understanding the theory of what we make.

I have never made cardboard and pin templates for my valve gears, and I doubt they will give you any understanding of 'backset' for the expansion link connection to the expansion link, or any understanding of the 90 degree components via the (1) eccentric movement, and (2) the combination lever out of phase with the above by 90 degrees to provide the lap and lead component.

The computer simulators allow us to do this in a matter of minutes as Duncan states. The work of Don Ashton is invaluable in understanding 'whats what' and what makes a difference to good Walschaerts valve events.

Simon,

If you want a simplistic explanation just read LBSC's TICH construction book on the valve gear.

Cheers,

Julian

[JasonBModerator @jasonb]

Why not use the best of both worlds. Download Don Ashton's spreadsheet for Walschaerts which has all the sizes for Britannia and use those to make your cardboard/meccano/lego physical model.

Probably a lot easier than trying to find a set of Loco drawings that may well have errors in them and the sketches on Don's chart give the position of all the parts and their relation to each other.

 

J

Edited By JasonB on 08/04/2018 08:13:18

[Anonymous]

As and when I get around to redesigning the valve gear for my traction engines I will be using a mix of mathematics, graphical charts and computer simulation. No need to actually build a model.

Of course the valve gear is of secondary concern. The first issue is to specify the size and spacing of the steam and exhaust ports, and to decide on basic valve events such as cutoff range and lead and lap.

Andrew

[FMESParticipant @fmes]

**LINK**

or

**LINK**

May be useful

[HopperParticipant @hopper]

Actually a working model that you can hang on the wall would be a pretty cool project in itself. You could make a matching Stephenson's linkage to go with it too.

[Ian S CParticipant @iansc]

This is not quite the subject; when I was at High School in the early 1960s there were models in the mechanics lab of just about all the different valve linkages, these were large, and made of wood(4'/5' long x 2'/3' high. The length of rods etc and leavers all adjustable. The school was used by the NZ Railways as part of their apprentice training scheme, and in those days steam was still in use in NZ..

Ian S C

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