Author Topic: Interpreting a drawing  (Read 1827 times)

Offline mike21alpha

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Interpreting a drawing
« on: February 06, 2023, 01:54:10 PM »
I think I am either being thick or missing something. See the portion of a Molly drawing attached. In my head, the two sets of eccentrics are backwards ( the fwd ones seem to be rearward and vice versa. Am I reading this incorrectly, or is the fwd/rearward referring to something else?

Offline Brian Rupnow

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Re: Interpreting a drawing
« Reply #1 on: February 06, 2023, 02:24:46 PM »
Be aware of where the drawing came from. Drawings made in Europe have their "views" projected opposite hand to drawings made in North America. Look up "right projection" and "left projection".---Brian

Offline crueby

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Re: Interpreting a drawing
« Reply #2 on: February 06, 2023, 02:44:58 PM »
It also depends on what the valves are, inside admission or outside admission, that reverses things. Same with the linkages...

Offline mike21alpha

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Re: Interpreting a drawing
« Reply #3 on: February 06, 2023, 04:13:15 PM »
Be aware of where the drawing came from. Drawings made in Europe have their "views" projected opposite hand to drawings made in North America. Look up "right projection" and "left projection".---Brian

Thank you for responding. It is, most definitely European, Brian. It is by Mr Lawrence. I was looking at the partially sectioned elevation. It shows the upright crank, the RH one, on the the LHS. So, we are to the front of the loco, looking rearwards (in my head).  This put the eccentrics in the opposite sense than that labelled.

Ian

Offline mike21alpha

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Re: Interpreting a drawing
« Reply #4 on: February 06, 2023, 04:16:27 PM »
It also depends on what the valves are, inside admission or outside admission, that reverses things. Same with the linkages...

Ah!  So this may be a terminology thing?  This is easily the most complicated thing I have turned and I will machine it as I see it. What could possibly go wrong🤪?

Ian

Offline crueby

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Re: Interpreting a drawing
« Reply #5 on: February 06, 2023, 04:38:13 PM »
It also depends on what the valves are, inside admission or outside admission, that reverses things. Same with the linkages...

Ah!  So this may be a terminology thing?  This is easily the most complicated thing I have turned and I will machine it as I see it. What could possibly go wrong🤪?

Ian
What type of valves are they, D valves or spool/piston valves?  Also, is it a Stephenson style reverse linkage or something else? Some of the linkages reverse things from direct connections.

Offline mike21alpha

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Re: Interpreting a drawing
« Reply #6 on: February 06, 2023, 04:49:32 PM »
They are slide valves with the steam chest underneath the cylinders… - ah! A penny just dropped, I think. Are they reversed because they are inverted?

Offline crueby

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Re: Interpreting a drawing
« Reply #7 on: February 06, 2023, 04:54:43 PM »
They are slide valves with the steam chest underneath the cylinders… - ah! A penny just dropped, I think. Are they reversed because they are inverted?
Inverted?  Could you post a picture of what the steam engine/linkages/reverse gear as a whole looks like? I am not familiar with a Molly engine.

Offline mike21alpha

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Re: Interpreting a drawing
« Reply #8 on: February 06, 2023, 06:10:43 PM »
It is LBSC’s 3 1/2” gauge model of a Jinty. It is a two cylinder (inside), slide valve engine with the steam chest underneath the cylinders. I could take a picture of the linkage portion of the drawings, if you please, but I suspect that you will have the image in your mind, by now.

Offline crueby

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Re: Interpreting a drawing
« Reply #9 on: February 06, 2023, 06:18:53 PM »
If its a Stephenson style linkage, then yes, I think the drawing is backwards. For that setup with d valves, the eccentric centers ought to be a bit over 90 from the crank pins.

Offline AVTUR

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Re: Interpreting a drawing
« Reply #10 on: February 06, 2023, 06:37:57 PM »
Initially I thought this was an obvious projection problem, either first or third angle. First angle is popular in the UK with those who design model engines (most of industry use third angle).

However I see that it is an LBSC design. Having started on an LBSC Speedy I think you are on your own. Looking at his drawings I have the impression he did not have a clue about projections (once very common). He does seem to have used third.

Any good drawing will state whether it is first or third projection (or use the modern symbol).

AVTUR
« Last Edit: February 06, 2023, 06:42:19 PM by AVTUR »
There is no such thing as a stupid question.

Offline crueby

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Re: Interpreting a drawing
« Reply #11 on: February 06, 2023, 06:55:57 PM »
Initially I thought this was an obvious projection problem, either first or third angle. First angle is popular in the UK with those who design model engines (most of industry use third angle).

However I see that it is an LBSC design. Having started on an LBSC Speedy I think you are on your own. Looking at his drawings I have the impression he did not have a clue about projections (once very common). He does seem to have used third.

Any good drawing will state whether it is first or third projection (or use the modern symbol).

AVTUR
Regardless of the projection, look at the position of the eccentrics in relation to the crank pins. They are towards the crankpin, less than 90 degrees away from them. That either means they goofed, or it is inside admission, or it is not a stephenson linkage but something that inverts the motion.

Offline Zephyrin

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Re: Interpreting a drawing
« Reply #12 on: February 06, 2023, 08:17:59 PM »
on this image of the underside of the loco found on the Internet
it seems that a rocking lever inverts the valve movement, hence the confusion.

https://www.stationroadsteam.com/thumb2/phpThumb.php?src=%2Fimages%2Fproduct%2F4001-k.jpg&ar=x&w=800&h=&hash=9d346fe16d4ad382cbc2b17dd22af5e9

Offline crueby

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Re: Interpreting a drawing
« Reply #13 on: February 06, 2023, 08:38:04 PM »
Aha! That's the missing piece. So the drawing is correct as is. Great find!

Offline mike21alpha

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Re: Interpreting a drawing
« Reply #14 on: February 09, 2023, 06:13:51 PM »
Ah! Thanks for that!  That seems to explain it for me.

 

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