Author Topic: Reverse Link  (Read 9342 times)

Offline Bernd

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Reverse Link
« on: May 01, 2011, 03:12:22 PM »
As I mentioned in the Water Cooler section I have a question about expansion/reverse links. I know how it operates. The problem is the drawing on the engine I want to build. This would be Bertha the Steam Engine in the June-July 2000 Modeltech Magazine. There are dimensions missing. Below you'll find two drawings. The first is from the Bertha engine and the second is From Rudy's Steam Launch engine. Note that there are are some crucial dimensions missing, specificly the two holes where the valve rods will be connected.

On Rudy's drawing he gives the distance from the center of the slot to the two holes that the rods will be connected to.


Bertha drawing



Rudy's drawing



Rudy shows the center of the slot to center of the holes to be .281" and from center of slot down .312". The Bertha Steam Engine is missing those dimensions.

Can anybody tell me how you get these. I take it you just don't pull them out of thin air. The Bertha Engine has a stroke if 1" of that will help.

Bernd
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Offline Bogstandard

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Re: Reverse Link
« Reply #1 on: May 01, 2011, 05:13:13 PM »
Bernd,

No need to go to all that trouble, for the Bertha, the figure you require is 2.375" rad from the same rad centre position of the 3" to the centre of the slot.

You work out downwards from the centre of the slot using the dimensions given, to the bottom of the radius around the holes, then come up 3/16" to the hole centres, which means the centre of the holes are 5/8" from the centre line of the slot.

The Rudy's one is calculated out in a totally different way from the Bertha one.


John
« Last Edit: May 01, 2011, 05:17:41 PM by Bogstandard »
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Offline Bernd

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Re: Reverse Link
« Reply #2 on: May 01, 2011, 07:44:58 PM »
Thanks John for that explination. I'll study what you said tomrrow as the brain is a bit foggy tonight.

Bernd
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Offline Bogstandard

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Re: Reverse Link
« Reply #3 on: May 01, 2011, 11:06:49 PM »
Kenneth,

I had totally forgot about that dimension.

Bernd needs to look maybe elsewhere on the plans for that angle of separation, then that should be a fairly easy calculation, as using what is shown, it cannot be determined.

I have also noticed that the third hole has no size or outer rad shown, if those two are missing as well, then difficulties will arise working out where that one should be.


Bogs
« Last Edit: May 01, 2011, 11:08:29 PM by Bogstandard »
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Offline Bernd

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Re: Reverse Link
« Reply #4 on: May 02, 2011, 09:07:49 AM »
John,

Went back and looked at how you described finding the dimension from center of slot down to the two 3/16" holes. Why I didn't see that is beyond me.  :palm:

I went back and looked at the full plans and I can't find anything more, even in the write up there is nothing giving a clue on where to place those holes horizontally . In your opinion do you think one can take what is there and "guesstimate" the horizontal dimensions.? It seems to me all your doing with that link is place the whole works vertically under the valve for a full opening and closing of the valve in the steam chest.

I think I might have a way of getting approximate dimensions. I did this before to find a dimension of an HO scale drawing to be able to determine the size and shape of a part I wanted to create in 1 1/2" scale. What I did was copy the drawing into PDF formate. Then I scaled to a known dimension on the screen and was then able to find the size of the part that was not dimensioned. I'll post my results a soon as I get "round-tuit".

Again thanks for the help John.

Bernd
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PatJ

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Re: Reverse Link
« Reply #5 on: May 02, 2011, 11:45:47 AM »
The link appears to be drawn non-symmetrical.
If you draw it symmetrical in CAD, starting with a vertical centerline, and then using the dimensions above, you get 30.61 deg from the vertical centerline to the rod hole, and 20.84 deg from the vertical centerline to the edge of the slot.
To the link lift hole, measured from the vertical centerline is 30.21 deg.

It seems like the slot and rod holes would have to be laid out symmetrical to a vertical centerline, but maybe I am missing something.

Pat J

Edit:  That 1/4" (typ.) dimension can't be correct, since you can see in the drawing that the sides are not equal.
If you hold the 3.25" dimension, then the angle from the vertical centerline to the lift link hole is 33.2 deg.
« Last Edit: May 02, 2011, 12:04:25 PM by PatJ »

Offline John Stevenson

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Re: Reverse Link
« Reply #6 on: May 02, 2011, 01:52:39 PM »

I have also noticed that the third hole has no size or outer rad shown, if those two are missing as well, then difficulties will arise working out where that one should be.


Bogs

?? Drawing says three  3/16" holes ?

Missing dimension between the holes is 1 1/4"
« Last Edit: May 02, 2011, 02:05:08 PM by John Stevenson »
John Stevenson

Offline Bernd

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Re: Reverse Link
« Reply #7 on: May 02, 2011, 02:11:50 PM »
I just got done with doing a bit of research on this valve gear. For those members that are new to steam engines, the valve gears shown above are called “Stevenson Valve”. They were basically invented to reverse a steam engine. That is what has been written up in two books that I have from Lindsay. One is called “Steam Propelled Vehicles” by James E. Homans and the other is “Valve Gears” , International Correspondence Schools.
 
Apparently the distance between the two bottom holes, the holes that are marked .281” , on Rudy’s engine, are not critical, as is their horizontal spacing. As long as the rod that is connected to the eccentric imparts the travel necessary to open and close the valve opening inside the steam chest.

That’s about the best I can explain it. So based on that I can sort of “guesstimate” the dimensions on the Bertha engine.
If you are further interested in how a Stevenson valve works may I suggest a an internet search on steam valves.

Bernd

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lordedmond

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Re: Reverse Link
« Reply #8 on: May 03, 2011, 05:47:41 AM »
Be careful with stevensons valve gear there are there types

launch, loco and marine

you have a launch link the loco link has the eccentric strap holes on the ends the marine link has all the holes on the centre line

with holes as in your the link amplifies the eccentric strap motion the lot link reduces it

this link may give some info to sort out the hole positions its not the stroke that matters

[url] http://www.donashton.co.uk/html/stephenson_s_gear.html/

Offline Bernd

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Re: Reverse Link
« Reply #9 on: May 03, 2011, 09:39:52 AM »
launch, loco and marine

Thanks for the link.

Won't the launch and marine be the same? I know the one on the loco's is different than on a launch.

Bernd

Note: The link you gave doesn't work. Hopefuly this one will.  http://www.donashton.co.uk/html/stephenson_s_gear.html
Saved that page for further study. Very interesting information there.
« Last Edit: May 03, 2011, 09:47:48 AM by Bernd »
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lordedmond

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Re: Reverse Link
« Reply #10 on: May 03, 2011, 11:06:12 AM »
sorry got a extra url in there  :doh:

but you got the gist limbo on duty today here dyslectic head and fingers good job numbers are Ok else I would be in trouble in the WS

Stuart