Pitman revistited

[eddieprice]

Came across PD while researching sternwheel paddle steamers and was interested in an earlier thread on Pitman drives back in April. I rather liked Version 6 and thought I would try to create an animation to prove that it worked.

The Pitman drive would be ideal for an electric drive, but I would quite like to make a steam driven unit. I came across a photograph of the starboard engine for a sternwheeler, I think it was in SteamboatMuseum.com, It looks as though the piston drives the rear paddle directly via a crank, but I can't see how the slide valve is driven. Normally one would expect an eccentric, which would have to be attached to the axle of the paddle. Would this be right? In photographs of sternwheelers there does not appear to be an eccentric visible.

Thanks for any help that you can provide, PD looks an interesting forum.

Eddie Price

[bogstandard]

Hi Eddie,
I'm just looking into the various ways a slide valve can be driven and eccentric is just one of many.
I am hoping to do another engine build on here and will be looking at non eccentric drives for the control of the valves, just to be a bit different and maybe a little easier to make. One method is a small crank on the very end of the crankshaft and have it driving a layshaft that in turn drives the valve gear, the advantage of this is that one small crank can drive any number of valves just by extending the layshaft and taking a drive from it.

John

[eddieprice]

Thanks guys. You have given me something to think about

Eddie

[Bill Hudson]

Eddie,

Check Bill Hudson's Photo Gallery. There  are several views of the stern wheel, crank and eccentrics of the Portland sternwheeler.  The first picture is of the drive mechanism off the pitman rather than eccentrics out on the wheel. Not a real good picture but you may get the idea.  The rectangular bar in the lower right hand of the picture goes to a swivel slide mounted on the pitman.  As the pitman goes through its elliptical movement the bar slides in the swivel and rotates in a circle and thusly the motion is converted to a circular motion which turns the eccentrics just to the left.  A another picture shows the two eccentric frames and another one shows the other end where the reversing gear is.  
Hope this helps,
Bill

[Bierjunge]

Regarding electric drive:  

Came across PD while researching sternwheel paddle steamers and was interested in an earlier thread on Pitman drives back in April. I rather liked Version 6 and thought I would try to create an animation to prove that it worked.

 
Eddie, I am absolutely delighted seeing your animation of my variant six! (The original posting can be found at http://www.paddleducks.co.uk/forum/viewtopic.php?t=2838 ). Excellent animation!  
 

The Pitman drive would be ideal for an electric drive, but I would quite like to make a steam driven unit.

 
 
Well, i fact it's just the other way round: The pitman drive was ideal and minimalistically simple for the direct drive of a low rev reciprocating steam engine, but recreating that motion in a realistically manner by an electric model drives can cause some headache, see the posting mentioned above.  
 

I came across a photograph of the starboard engine for a sternwheeler, I think it was in SteamboatMuseum.com, It looks as though the piston drives the rear paddle directly via a crank, but I can't see how the slide valve is driven. Normally one would expect an eccentric, which would have to be attached to the axle of the paddle. Would this be right? In photographs of sternwheelers there does not appear to be an eccentric visible.

 
I had the same problem as you when I did some research on sternwheel drives, since I didn't find many pictures with eccentrics visible either.  
So my first idea was the same as David's, a potential striking of the valve rod by stops on the piston rods, as in some pumps.  
The only problem with that idea: I did not find any proof in 1:1 scale at all!  
Such a striker gear would only allow a full pressure engine (hardly any expansion of the steam) with a very bad efficiency, and modifying it to enable expansion would be a major complication, making it worse than other valve gears.  
For models, however, it could be a very simple solution with the only drawback that reversing isn't easy at all, once the engine is running.
 
The simple solution of the riddle: In fact, there were many eccentrics directly on the wheels shaft indeed. The only reason that they are not prominent in pictures (unlike steam locomotives) is that the stroke of the eccentrics (and therefore the eccentrics themselves) are very small compared to the stroke of the piston and the crank (piston stroke up to 10 ft, in locomotives typically 2 ft), and mostly hidden on the inner side of the longitudinal beams carrying the wheel. So you often simply won't notice them!  
An example of eccentrics can be seen in the attached picture of the wheel of the sternwheeler Petaluma (see my 6 year old son to get an idea of the size).  
 
Another solution, if you can't see eccentrics at all, is described by Bill: Sometimes, the eccentrics were driven by the pitmans, so the mechanism was hidden and protected) inside the superstructure.  
The only drawback of this solution is, as Bill mentions, that a point in the middle of the pitman describes an elliptical movement. For the valve gear, the movement component with a 90 deg phase difference to the piston is quite important. If you look at the ellipse however, this 90 deg (in this case vertical) component is much smaller that the horizontal component (only responsible for cutoff lead). So designing a valve gear derived from an elliptical motion is much more difficult, and in a model more prone to tolerances, as the circular motion of a regular eccentric (I hope I made myself understood).
This disadvantage of the elliptical movement can be partially compensated however, if you use a cam instead of an eccentric, as described in http://www.paddleducks.co.uk/forum/dlman.php?folder_id=153 , showing some sophisticated trochoid shaped cam.  
 
Regards, Moritz

[eddieprice]

Thanks Bill, nice photographs. We don't have sternwheelers in the UK, so it is nice to see some actual technical photographs.

and thanks Mauritz. I was struggling a bit with the detail of your descriptions, but I think I get the idea. I didn't realize the wheels were so big.

I am thinking of getting some plans of the Thomas Edison from John Fryant, if he is still in business. I have e-mailed him for some more information.

Eddie

[Bierjunge]

I am thinking of getting some plans of the Thomas Edison from John Fryant, if he is still in business.
Eddie

 
If you're interested in The Edison, I think it is a must to get the reprint of the 1902 catalogue "The Marine Iron Works of Chicago", for example at http://www.steamlaunch.com/books/marine.html as well.  
This company used to build and sell sternwheeler engines (being depicted in this booklet) as well as complete kits of small sternwheelers.  
And there is a photograph of the edison in it in her original form, before the hog post system was modified.
 
By the way, the engines of the Edison were salvaged after she burned and re-used by Edison's friend Henry Ford in the steamer Suwanee in his open air museum village Greenfield Village (next to the Henry Ford Museum) at Dearborn on the outskirts of Detroit.  
 
I have attached two pictures of her engine I took there just last week. And coming back to your original question: Yes, you can see that there are eccentrics on the wheel shaft, but somehow hidden behind the wheel carrying beam.  
The link (just the ordinary stephenson valve gear) acts on some kind of rocker shaft (being well visible above the crosshead), which again acts on the valve rod. Get the idea?
 
Moritz

[Bierjunge]

...and the second one

[towboatjoe]

Here's how I made my realistic operating pitman drive with an electric motor.

[Bierjunge]

Please allow one more comment on the engines of the Edison:

By the way, the engines of the Edison were salvaged after she burned and re-used by Edison's friend Henry Ford in the steamer Suwanee in his open air museum village Greenfield Village (next to the Henry Ford Museum) at Dearborn on the outskirts of Detroit.

I had remembered reading this story somewhere (in fact at http://www.steamboats.com/museum/models3.html ), but I get more and more doubts, and maybe it isn't true after all.

The Edison had engines with a 36" stroke. But if you compare the Suwanee's engines to the height of the banister on the pictures, they look smaller to me.
And finally, http://ul.bgsu.edu/cgi-bin/xvsl2.cgi (usually a totally credible source) tells a total different story if you search for vessel name "Suwanee".

Anyway, the design of the engines (and their type of valve gear) is absolutely similar, judging from the Marine Iron Works catalogue.

Moritz

[Bierjunge]

Here's how I made my realistic operating pitman drive with an electric motor.

 
Beautiful work, Towboatjoe!  
That's a perfect example of the Scotch Yoke type of sternwheel drives, classified in http://www.paddleducks.co.uk/forum/viewtopic.php?p=12963  
 
This weekend, I've seen another beautiful example at the Great lakes Model Regatta at Auburn, IN, of which I've attached two pictures.  
The white plastic part is the sliding part with the groove for the crank pin, guided in brass channels. From there, the motion is transmitted to fake cylinders even including a moving western river type lever valve gear! What a pity you almost don't see it if the cabins are on!  
 
By rethinking thiss type of sternwheel drive, I wondered why nobody builds it with a slightly curved groove (explained in the topic linked above) in order to minimize jerking and jamming?
 
Regards, Moritz

[AlistairD]

This is what I was able to find out about the history of SUWANEE for my book "Directory of the World's Steamships


1927: Built. The engine was built using parts of an earlier Suwanee which had been long since sunk in Florida’s Suwanee River<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
1930: New boiler built by Henry Ford
1935: Superstructure added
1954: New boiler fitted
1968: Original wooden hull replaced
1988: Rebuilt with a steel hull

Alistair

----- Original Message -----
From: gretzschel (paddlewheels@paddleducks.co.uk)
To: paddlewheels@paddleducks.co.uk (paddlewheels@paddleducks.co.uk)
Sent: Friday, August 10, 2007 3:38 AM
Subject: Re: Pitman revistited


Please allow one more comment on the engines of the Edison:



gretzschel wrote:


By the way, the engines of the Edison were salvaged after she burned and re-used by Edison's friend Henry Ford in the steamer Suwanee in his open air museum village Greenfield Village (next to the Henry Ford Museum) at Dearborn on the outskirts of Detroit.




I had remembered reading this story somewhere (in fact at http://www.steamboats.com/museum/models3.html ), but I get more and more doubts, and maybe it isn't true after all.

The Edison had engines with a 36" stroke. But if you compare the Suwanee's engines to the height of the banister on the pictures, they look smaller to me.
And finally, http://ul.bgsu.edu/cgi-bin/xvsl2.cgi (usually a totally credible source) tells a total different story if you search for vessel name "Suwanee".

Anyway, the design of the engines (and their type of valve gear) is absolutely similar, judging from the Marine Iron Works catalogue.

Moritz






No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.476 / Virus Database: 269.11.10/943 - Release Date: 08/08/2007 17:38

 Post generated using Mail2Forum