How to start Walschaerts Design as a beginner?

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Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Sun Dec 21, 2014 3:09 pm

To the audience who had used Docstader's software including Jos, how would you go about
checking to see if your valve gear parts dimensions are OK?

Here's what I'm thinking:

Top Level Design:

1. My design is freelance loco, 16mm/ft (1/19th scale) of British 2ft gauge 0-4-2T

2. Outside frames of thick steel plates for weight and strength (not cheapy bendable 1/16" thick)
so that horn guides are not necessary, re-gauge-able (32mm to 45mm), and that the wheels
can be taken out easily when the loco is upside down

3. Max. of 45 minute run on one full butane and boiler water

4. Sprung and equalized chassis to over come track imperfections and twigs/leaves on
rails

5. Use All-Square valve gear layout for simplicity

Lower Design:

1. Ease of maintenance (easy to take it apart). Chassis and valve gears are integral,
apart from the structure and boiler. Doesn't need to remove the latter two to adjust
the valves and to remove the wheels

2. Manual controls. No R/C

3. Get the cylinder stroke as long as possible to take advantage of steam expansion

4. True scaled wheel tire contour and dimensions (4.5" wide tire, 1"w x 1"L flange with
fillet between the flange and tire, no sharp edges) - no ugly Lionel deep flanged
pizza cutter wheels

5. Dimensions of valve gears are constricted by determined wheel axle positions and cylinder
mounting locations (per all-square layout, the piston axis is in-line with the driver axle centers)

Dockstader Software Usage:

I have determined all of the valve gear dimensions so all I need is to plug them into the Docstader's
parameters. I've already played with a drawing of 2x the size to see if the Expansion swing
is of recommended amount and symmetrical. Unfortunately, the total swing was over 60 degrees.
The combination lever was somewhat OK but I hope that the software would help me to remove these
kinks. I haven't experimented on paper drawing for correct valve motion.

1. But problem is, in his program, I can't change the angle of the Return Crank. I can change this somewhat
by changing the length of the crank and/or the length of the Eccentric Rod and also the Expansion
Link's tail pin location. Valve stem length can also be adjusted

2. I can blow up the Docstader's valve graphic portion by the 'scan' bar to verify the valve's port covering
motion by slowing the motion to a crawl

3. I can also adjust the die block travel for above valve motion

4. Adjust the back set of the Expansion Link

5. Then I can check and optimize the steam utilization* At this time, I won't want to learn
about the curve graph interpretation

6. Repeat above for Reverse direction

*Don't think that affects much in my small scale

Dorian

JJG Koopmans
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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Mon Dec 22, 2014 5:01 am

Hi,
I do not know what you mean by "Return crank". If this is the eccentric crank, changing its
location is deadly, since it is derived from the layout of the gear, including the height difference between cylinder and valve chest.
Kind regards
Jos Koopmans

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Mon Dec 22, 2014 8:06 pm

Hi Jos,

Yes, I agree about being careful about changing the Eccentric Crank's location.
It can ruin the valve gear layout design made prior.

Return Crank is a British term for Eccentric Crank. When I said to change the angle of Return
Crank, I meant to rotate it a bit along the Crank Pin, and perhaps to change it's length
(Pitch Circle diameter). Not the crank pin position away from the axle center. That would
change the piston stroke. This Crank Pin rotation around the crank pin would tilt the Expansion
Link forward or backward away from the vertical position* when the crank pin is set at front
or rear dead center.

Maybe this is what I must do with the Docstader's graphic. But of course, again, the software
won't allow the Eccentric Crank's rotation slightly or more. I need to change just the components
I've listed on the previous chat. I don't think the software would calculate the lengths of other rods
just because I've changed the length of one component to compensate so the valve would move
correctly. The software just saves you time not cutting metal over and over again? Yes, it does have
some alarms e.g., showing us the die block swung way pass the Expansion Link slot. That's nice too.

*This vertical position of the Expansion Link would keep the valve stationary on that side of the
locomotive as the Die Block is moved up and down the Expansion Link's slot. But the rotation
of the Eccentric Crank would off set the valve motion. You can push or pull the valve stem to
compensate so the Expansion Link is vertical again. I've done this adjustment to different
already made model (designed and built by somebody else) to achieve fair symmetrical valve
movements to open the ports for both loco's right and left side. I need to be able to do the
same for this 16mm/ft free lance model.

Thanks for your response,

Dorian

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Fri Dec 26, 2014 4:27 am

Merry Xmas!

I'll try to log this 'how to use Docstader's program' step by step so others would
benefit. His instruction won't help this far. I would request the audience to
help this process out when I get stuck as Jos had done so far. If it's already written
on-line somewhere, I would like to use it.

Plugged in my valve gear parameter values into Dockstader's software*. Then ran it.
The valve gear picture didn't look good with the tiny rods crunched up together and
the radius rod detached away from the expansion link. I didn't care except I had to
turn off the annoying beeps. I need to fix the piston motion 1st. The piston was
shooting out of the cylinder front cover! I reduced the piston rod length bit by bit,
while keeping the main rod crank pin location with respect to the driver's axle
center unchanged (stroke value kept constant). I thereby achieved piston motion
contained inside the cylinder covers and symmetrical. I.e., almost equal amount of
clearance volume at both front dead center and rear dead center were obtained.

___
*The plug-in values are close to what I've listed and shown on my rough drawing
at my 'Walschaerts - All Square Layout Design' blog.

To verify the piston motion, I had to learn how to blow up the piston/cylinder portion
of the graphic and to counter the picture wanting to shift out of the screen. That
was a bit annoying. This doesn't happen on a typical CAD software. There is a
software bug that at times, if you inquire to display what the parameter
stands for, an error message pops out and can't go back to the animation screen.
I just simply terminate the session. It's a good thing to frequently save the parameter
values in your hard drive. If you forget what those parameters stands for, it's better
to run another separate session which uses the working default values and inquire
it there while you're doing the trials session.

I then went back to tackle the valve motion to make the valve cover and uncover
the live steam ports symmetrically too. I first had to adjust the infamous ReverseArm
twin parameters of prior blog so that the die block won't shoot outside the Link slots.
I just set these parameter values to .200 and .400 as a start (since full die block travel
was computed to be ~.400) but reduced it further to .160 and .170. I still don't know
exactly what those ReversArm to CenNorm and ReversArm to Max mean however. I just
want to get the valve gear to give symmetrical valve motion for the steam ports.
I figured once the valve gear dimensions are OK, I can always get back to the
Reverse pivoting linkages and figure their component lengths by mechanical experiment.
To do this, I'll start out by setting the Main Crank pin to rear dead center, adjust the valve gear
components till the Expansion Link is vertical. This would dictate the valve motion (or
valve stem) be stationary while I switch the Reverse gear up and down the Expansion
Link slot (Rev/Cen/Fwd). This method must be repeated with same result for the pin
at front dead center. Hope this dead center method would be easy and work! Geeze,
won't it be wonderful if someone would update the software so it would automatically
correct the error generating parameter values?

Oh oh, it's almost 2AM. Better goto bed now.

Dorian

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baggo
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Re: How to start Walschaerts Design as a beginner?

Post by baggo » Fri Dec 26, 2014 6:23 am

Maybe a bit late in the day now but you can download a spreadsheet from Don Ashton's website for designing both Walschaerts and Stephensons gear from scratch. Just enter your fixed dimensions and the spreadsheet does the rest.

http://www.donashton.co.uk/html/downloads.html

Simulators are great for seeing the results from a certain valve gear but not ideal for designing one unless you really know what to alter to get things right.

John
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Fri Dec 26, 2014 2:37 pm

Thanks John.

I've tried Don's software along with buying his book and read it. Both didn't work for me.
It's hard to decipher his variables as to what they mean. Unlike a science test book, he
doesn't explain them very well. I don't think the variable names, e.g., x doesn't mean
the same throughout his pages of equations. So though spread sheet can be EZ to plug
numbers, without knowing them exactly what they are, it's impossible to use either his
book or the software. His examples at the back of the book didn't help either.

But after saying all that, was it successful for you to use his software?

Dorian

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Fri Dec 26, 2014 2:46 pm

You can see my layout in other article of mine based on Walschaerts' Valve Gear, a small
book by Henry Greenly, Fig. 1. There, the combo lever is vertical and the Exp. Link is slightly angled forward.
This angle is measured to be 21.5 deg. Mine isn't angled. Both are vertical. And I think that made the
measurements I've taken from my drawing for the Doc's plug-in got off with crazy result (alarms). Parts were
shooting out of their confines (piston, die-block, etc.). I assume he had drawn the layout in Fig 1. correctly so
I'll go back to my drawing board.

Oops, Gota go!


Dorian
Last edited by Dor_Crank on Sat Dec 27, 2014 9:04 pm, edited 2 times in total.

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baggo
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Re: How to start Walschaerts Design as a beginner?

Post by baggo » Fri Dec 26, 2014 2:54 pm

Hi Dorian,

Yes, I agree that Don can be a bit hard to follow at times. I've exchanged emails with him over the years and sometimes I don't really understand all he says!

I use the Stephenson spreadsheet quite often but it took some time before I fully understood it. It's not the easiest of things to follow. I've not used the Walschaerts one recently so can't remember much about it but I have used it in the past with success.

Most of the time though I'm just looking at and improving existing valve gear designs so I don't need to design entirely from scratch very often.

John
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info

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baggo
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Re: How to start Walschaerts Design as a beginner?

Post by baggo » Fri Dec 26, 2014 3:40 pm

Yes, I think you've got the position of the expansion link relative to the combination lever wrong in your drawing?

With the reverser set to mid gear, the combination lever should be vertical when the piston is in the middle of the stroke (as you have it). At this point the expansion link will be at it furthest point forwards or backwards depending on whether the piston is on its forward or backward stroke. The valve should also be about central over the ports at this point.

The expansion link will be vertical when the piston is at front and back dead centres.

These screen shots of the results from the simulator I tend to use (by Allan Wallace) may help to illustrate this. I've set the first example up similar to your diagram hopefully.
Attachments
Walschaerts example.jpg
Walschaerts Example 2.jpg
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Sat Dec 27, 2014 12:22 pm

Thank you John for your inputs. Yes, I would use your diagrams above to re-draw my scaled valve gear
layout, re-measure the gear parts, then plug those numbers back into Dockstader's software.

No, I have had no contact with Mr. Ashton yet. Perhaps he can really explain his software or book better
in person.

Sounds like Mr. Wallace's program is easier to use? May try that after my re-draw effort. Yet, my
no understanding of what the twin ReverseArm parameters is of concern. I faked the values for the
reversing portion of the gear for plug-ins to use Mr. Doc's program.

Dorian

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baggo
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Re: How to start Walschaerts Design as a beginner?

Post by baggo » Sat Dec 27, 2014 1:14 pm

I tend to use the Wallace simulator as I prefer the type of indicator diagram that it gives. It makes it very easy to see the differences in the valve events between the front and back halves of the cylinder. That's not so easy with the oval diagrams that Charlie's and other simulators give. There's also a lot of dimensions asked for in Charlie's software that aren't relevent to the actual valve events and are unnecessary unless you want to check for bits fouling others etc.

If you do try Allan's software and get stuck with it give a shout. It's pretty straightforward to use.

I've never really understood the two different reverser travels given in Charlie's simulator but, thinking about it, I wonder if you set the 'reverse arm to centre max' to give the full available die travel in the expansion link and the 'reverse arm to centre norm' to give the maximum cut off that you actually want e.g. 75% ? That's the only explanation I can think of.

John
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

http://www.modeng.johnbaguley.info

Dor_Crank
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Joined: Tue Sep 10, 2013 1:08 pm
Location: LA, CA

Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Tue Dec 30, 2014 1:03 am

Hi John,

Now that I scan all of Dock's parameters (EditDimen --> FrameDimensions and CylinderDimensions),
there are no explicit the Link Die travel parameters. That is disturbing. Jos had suggested above
that the 2 ReverseArm parameters are related to the Expansion Link slot dimensions by plugging in
extreme values to them and the Die Block had shot out of the slot. But we don't know exactly
what values to plug in.

I can try the two ReverseArm parameter interpretation as you suggested. For the ReverseArm to Center Max,
I can try plugging in full die travel length and also try 1/2 the travel which 'Center Max' may suggests.

I never thought the ReverseArm to Center Norm meant to be cutoff value. Yes, for my loco, the valve length, ports,
lap (no lead necessary for my small scale model), and valve travel dimensions were made for 75% cutoff, same as you
suggested. I can plug .75* into Dock's Center Norm parameter. What's strange about Dock's parameters under his
'Cylinder Dimensions' is that filling all of his valve port and valve length (lap size is inherent in this valve length)
parameters, the cutoff value is determined automatically by Reuleaux diagram, e.g., Henry Greenly's
Walschaerts' Valve Gear book, Fig. 8, page 11. That means stating the cutoff value in his plug-in explicitly can
contradict the valve and valve port dimensions. Despite this contradiction, I'll try your suggestion. But 1st, I need to
re-draw my layout.

Thanks again for making me think and for your kind help over this busy holidays.
________
*I did try higher than 100% (1.000 as a decimal representation), viz., 1.300 (130%) for the Center Norm
with and the Max set to 3.000. Other parameters remained as default values. But nothing happened
(no alarm) when I moved the die block from 'cen' to 'rev' or 'fwd' position. It doesn't looks like this is of
cutoff percentage parameter. The software doesn't like the max value to be bigger than the Center Norm
value. It automatically sets the max = center Norm.

I'm more inclined to think of abandoning Dockstader's program. We're wasting time on these two
ReverseArm parameter interpretation.


Dorian

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