Paddleducks
Paddler Modelling => Paddlewheels/Drive Systems => Topic started by: Eddy Matthews on June 26, 2005, 03:25:57 AM
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Here's the basics of the drive system for the Suter sternwheeler - Still a lot of work to do to finish it, but it's coming along and it actually looks like it will work! :-)
The motor drives a shaft via a toothed belt, the shaft turns the crank arms which will be linked via wooden arms to the slider mechanism at the back - This in effect turns rotary motion into linear motion - The slider is then linked via two more wooden arms to the paddlewheel itself. I hope that makes sense?
The motor in the photograph is a dummy just used to show how things will go together. I've just ordered a 50:1 ratio motor and gearbox from MFA/Como which gives 158 RPM on 6 volts, so it should be fairly close to what I need, and I can always vary the motors top speed by increasing or decreasing the voltage slightly.
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Eddy,
I'm assuming that the crank at the end of the countershaft has the same stroke length as the crank on the paddlewheel. My next question is: What is the length of the arms that go from the countershaft crank to the slider mechanism? Are they going to be the same length as the main pitman arms that go from the slider mechanism to the paddlwheel crank? The reason I'm asking is because you will get into a mathematical problem with the angle of the crank when it's between 0 degrees (top dead center) and 90 degrees. I will draw this up in autocad and try to post it for you.
George
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Eddy, where did you obtain your toothed belt and pulleys? And what specs did you buy to, i.e. is the ratio 1:1, number of teeth, size of belt, etc.
I've been tossing up drivetrain options for the split-sternwheeler Jean build I've been thinking about, and am torn between the toothed belt option running pitman arms and a chain-and-sprockets alternative connecting directly to the wheels.
Gary Nelson did a corker version of Jean using chains direct to the wheels but from the few photos I have I believe that the prototype was driven by pitman arms on each wheel, so I'm more enthusiastic about that option and a couple of sets of toothed belts, pulleys and a couple of MFA Como geared motors, etc wld fit the bill nicely.
One other question - aren't you overspeed at 158 rpm on 6V for a sternwheeler? I have it in my mind (somewhere in the dark corners, just behind the cobwebs) that a sternwheeler should be geared to 30-40 rpm.
Many tks for your clarifications
Tony
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Eddy,
I'm assuming that the crank at the end of the countershaft has the same stroke length as the crank on the paddlewheel. My next question is: What is the length of the arms that go from the countershaft crank to the slider mechanism? Are they going to be the same length as the main pitman arms that go from the slider mechanism to the paddlwheel crank? The reason I'm asking is because you will get into a mathematical problem with the angle of the crank when it's between 0 degrees (top dead center) and 90 degrees. I will draw this up in autocad and try to post it for you.
George
Yes the cranks are identical George, I made two left hand and two right hand, all exactly the same length.
To be honest I didn't consider different length arms to be a problem - But since the motor/countershaft unit and the slider mechanism are seperate items I can easily adjust the length to make that distance the same as the distance to the sternwheel.
I'd be interested in more information on this if you could do something in AutoCad as you suggested George.
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Eddy, where did you obtain your toothed belt and pulleys? And what specs did you buy to, i.e. is the ratio 1:1, number of teeth, size of belt, etc.
The toothed belts and pulleys I got off a friend at my local club, but they originally came from http://www.hpcgears.com in the UK. The pulleys are 1:1 ratio.
I'm not sure if I can get all this stuff to work properly, as the slightest difference in lengths or angles will cause the whole thing to lock up solid. But I'll give it a try - If the worst happens, I can always resort to using a belt or chain drive, but like you I'd love to have a system of working pitman arms that looked like the original.
One other question - aren't you overspeed at 158 rpm on 6V for a sternwheeler? I have it in my mind (somewhere in the dark corners, just behind the cobwebs) that a sternwheeler should be geared to 30-40 rpm.
I'm basing my speed on the Dumas Creole Queen I used to have - It used about 110 RPM. So allowing for the drag of the drive system and the drag of the water on the paddlewheel, I hope that I'll be somewhere close to what I need. I like to have a reserve of power to get out of trouble quickly and to have the power needed to go against a headwind when required.
I can always vary the top speed by reducing or increasing the voltage - An easy thing to do as I use Nicads, or I can change the pulley sizes. If I'm way off the mark you'll hear about it here when I get the model onto the water for testing.
I hope that helps?
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Tony, Let me speak from experience. If you don't have a minimum of 100 rpms on your wheel(s), you'll kiss your boat goodbye to the cross winds and current. Remember that most paddle boats float like a styrofoam cup on the water and the wind loves to play with that. Even the real boats have to fight cross winds.
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Eddy,
Do you or someone you know have the capabilities of opening an AutoCad file? If not I'll print it out and take a picture of it and then post the picture. I have used this method before and it works fairly well.
George
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Eddy,
Do you or someone you know have the capabilities of opening an AutoCad file? If not I'll print it out and take a picture of it and then post the picture. I have used this method before and it works fairly well.
George
I can view AutoCad files George, but perhaps a normal image posted on here would be better, then everyone can benefit from the information?
I'm still confused by your statement about the length of the arms - NOT that I doubt you - just my poor little brain cannot grasp why it should be a problem! :-)
Work on the drive system has stopped until I hear from you again - That's not a problem, I have plenty of other bits to do! So don't worry if it takes a few days to sort out...
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Tks Joe and Eddy, without your advice I'd be dead in the water. Tks also re the contact for the belts and pulleys.
Tony
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Eddy,
What happens is this. With one end of the arm sliding along in a flat plane and the other end going around the crank pin you have a situation which is referred to as the angularity of the rod. Lets say you have a crankpin that is 1inch away from the center of the paddlewheel therefore giving a 2inch stroke. Now you have a pitman arm which is 10 inches long. When the pitman arm starts from top dead center and travels halfway down it's guideway (1inch), the crankshaft has not turned 1/4 of a turn. Depending on the length of the pitman arm it could vary by as much as 3 degrees. Therefore if you use a pitman arm up front that has a different length than the pitman arm going out to the paddlewheel you will have a mathematical difference in angular stroke at 90 degrees. I will post a picture as soon as I get a chance.
George
Paddleducks <drivesystems@paddleducks.co.uk> wrote:
The following message was posted on Paddleducks, by Eddy Matthews
gbritnell wrote: Eddy,
Do you or someone you know have the capabilities of opening an AutoCad file? If not I'll print it out and take a picture of it and then post the picture. I have used this method before and it works fairly well.
George
I can view AutoCad files George, but perhaps a normal image posted on here would be better, then everyone can benefit from the information?
I'm still confused by your statement about the length of the arms - NOT that I doubt you - just my poor little brain cannot grasp why it should be a problem! (http://www.paddleducks.co.uk/forum/images/smiles/icon_smile.gif)
Work on the drive system has stopped until I hear from you again - That's not a problem, I have plenty of other bits to do! So don't worry if it takes a few days to sort out...
Regards
Eddy
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Thanks George, I'm sure your right, but I still don't understand why.... Hopefully when you get round to a drawing of the situation it will become clearer?
My few brain cells take a while to grasp things like this! :-)
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Crikey, you said a mouthful there George.
Can I clarify - by using an output rod or pitman arm that is exactly the same length as the con rod from the motor crank, you're mirroring the angularity of the con rod with the pitman arm, and by doing that you're eliminating any tendency to bind at difference angles of the crank?
And - in effect, Eddy's slider mechanism is equivalent to the cross-head guides in a slide valve steam engine?
Yours in bewilderment :thinking
Tony
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Tony,
That's it exactly. When I looked at the picture Eddy posted it looked like he was going to have a short link up front which would have caused the problem. Thanks for the simplification.
George
Paddleducks <drivesystems@paddleducks.co.uk> wrote:
The following message was posted on Paddleducks, by Khephre
Crikey, you said a mouthful there George.
Can I clarify - by using an output rod or pitman arm that is exactly the same length as the con rod from the motor crank, you're mirroring the angularity of the con rod on the pitman as well, and by doing that you're eliminating any tendency to bind at difference angles of the crank?
And - in effect, Eddy's slider mechanism is equivalent to the cross-head guides in a slide valve steam engine?
Yours in bewilderment (http://www.paddleducks.co.uk/forum/images/smiles/thinking.gif)
Tony
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Crikey George, you said another mouthful there!
Did you actually use the word "simplification"? My limited math is stretched beyond its outer limits mate so you give me too much credit. I'm simply trying to understand what you've already spotted. :thinking
Having said that, this has been an invaluable series of postings so thanks a heap, team! It's helped me start to grasp the mechanics behind Joe's pitman arm drive system that he has on his website.
http://www.dragg.net/jbrown/construc2.htm
Joe, do you have any photos of your pitman arm drive mechanism other than the ones on your website?
And another question or two for you - I'm OK with soft soldering but not up to scratch on silver soldering or brazing. What are the loads like on the mechanism? Will soft soldering be strong enough or wld you recommend a more substantial approach? Any other learnings or recommendations that you'd make for someone wanting to follow your approach?
Praise Be for knowledgable people!
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Hi PD's - Tony you question the strength of soft solder which is naturally considerably less than silver solder, but if you apply some simple principals all will be OK for say most model work & typically model wheel linkages or the like
If you had two sections of 1/4" brass angle iron normal thickness from a hobby shop [typical 0.013" flange thickness] - good cleanleness [steel wool the joint surfaces & good clean liquid flux etc] the resultant 1/4 of a square inch of wetted soldered joint will be extremely strong and I suggest the brass angle iron would distort under load before the soldered joint failed
Another trick for highly loaded soft soldered joints [brass paddle wheel frames or ribs] of thicker 0.050" brass material is to use a few 10BA brass screws & nuts and combine them into the soldered joint
Having said this I do understand soft soldered joints are not acceptable for a model steam engine construction & this is where silver solder comes into its own as a correctly dimensioned & silver soldered joint has similar yield strengths to the parent materials [grey cast iron or steel] being joined
Segmented belts - you will find a limited range of 6 mm wide segmented belts & pinions [or differential housings] at model shops that cater for fast electric or nitro R/C cars - Derek
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After doing a search on Google about this, it appears that the length of the connecting rods affect the dwell at the end of the stroke, and that's where the problem comes in - In effect the end of the rods will be moving at different velocities when they approach the end of the stroke....
I've tried in vain to find a decent definition that I can post here, but I can't find one. But at least I think I'm eventually starting to get my head round this! :-)
Fortunately I can easily make the four rods indentical in length, and there's plenty of room in the model to cope with that - I can now see why Towboat Joe did his drive mechanism the way he did, it virtually eliminates all these problems!
I'll persevere with my system to see if I can get it to work though - I hate giving up on anything unless I'm forced to!
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Eddy,
Here's a picture taken from my AutoCad drawing. It shows what I have been trying to explain about the different length arms and the effect they have when converting from rotary to linear motion. The picture shows 2 different length rods with the same stroke and the effect it has when the crank has turned 90 degrees. The numbers at the left are taken from the initial position when the rod was flat and the crank was at 0 degrees or the 3 o'clock position. When I originally built my model and was planning on powering it I layed out several designs of motion transfer to do what you are trying to do but with no luck I must say. If I can be of any more help without adding to the confusion please let me know.
George
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Thanks for that George. I have to admit that I still don't fully understand why the different length arms have that effect, but I certainly accept that they do...
I thought I'd figured out all the problems with the pitman arms until you threw that spanner in the works! :-) However, I'm glad you did, or I would have been scratching my head for ever trying to figure out why it didn't work!
This is even more confusing than when I studied all the maths involved in making a skirt for a hovercraft - Using Sines, Cosines and Tangents etc etc, something I hadn't done since I was at school! Like most pupils I sat there and thought what possible use could I have for this rubbish? Only to find out 30 odd years later that I did have a use for it!
It just goes to show that you never know everything, there's always something new to learn!
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It amazes me how you guys can get so technical and mathmatical over simple pre-industrial age mechanics. Relax guys, as long as you have four cranks with mounting holes exactly the same length and two pitman arms with mounting holes the same length, everything will work fine provided you fixed them at a 90 degree angle to one another.
I only have one other photo of the mechanism I built years ago. I retired the boat to a museum, but I still have the drive unit. Using regular rosin core solder with a 60 watt iron has worked well for me. It has stood up over 10 years now. Just be sure where you solder the brass that it's clean and shiny. Use flux on the metal for better adhesion.
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oh get away with you Joe - what's the point of being obsessive-compulsive if you can't obsess over something!
Many thanks for the additional photo - easily worth a thousand words on dwelling at the end of a stroke (I always thought that was a medical issue!)
cheers
Tony
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Paddleducks <drivesystems@paddleducks.co.uk> wrote:
Gentlemen,
I didn't mean to cause such a fuss over this it's just that I hated to see Eddy spend alot of time building this mechanism and then have trouble making it work. Sometimes it seems like something should work but we can't figure out why it won't and it comes down to something simple. I have been building engines and models for years and with them you have to be accurate or they won't even turn over. I believe I had posted this link before but if not you're welcome to visit and see my engines etc.
George
http://photobucket.com/albums/v43/gbritnell/
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Joe, the problem we're discussing is because of my drive mechanism design.... I have a motor driving a shaft with the crank arms fitted, this then has two pitman arms that go to a slider mechanism, another two arms then go from the slider to the paddlewheel..... That's were the problem comes - I have FOUR arms, not two as you do with your setup.
So the length of the arms becomes critical as George has pointed out.
Your drive system eliminates this problem, either by accident or design, but mine is prone to all sorts of problems if I don't get everything absolutely exactly right.
At the start of all this I thought my system was simpler than yours, but it appears that it isn't! Since I HATE working with metal, I went with my setup as it was easier to construct, but now I'm wondering if I should have bitten the bullet and used your design?
Anyway, time will tell, so watch this space - Hopefully next week will see me trying to get this to work!
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That's were the problem comes - I have FOUR arms
You know Eddy in some cultures you'd be considered a god...
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I can see your idea in the photo. I don't see why it wouldn't work. you just have to make two pitmans identical from the drive unit to the slide and make two more identical from the slide to the wheel cranks.
That's not the hard part. The hard part is having the four identical cranks lined up where two are verticle on one side and two are horizonal on the other side to be 90 degree to one another. If they are a hair off your wheel will lurch link a person trying to run with a bum knee.
I've seen a "fake out" as I call it where you have a drive unit sitting higher than the wheel with matching cranks and the pitman is connected from crank to crank. The drive unit has to sit higher to keep the pitmans from bottoming out on the deck. It works as the same principal as the connecting rods on steam engine wheels (or whatever connects the drive wheels together).
When I built mine, I knew I wanted the pitmans to push back and fourth, and I just sat and thought about the enginerooms I've been in and how things worked. The first thing I knew was I needed a crank the same size as on the wheel so the rotation would be the same.
Then I thought how to make the crank push something back and forth. I came up with putting a wheel on the side of the crank and letting it slide inside a "U" shape shaft long enough for it to rotate full height up and down. All I had to do the was to solder it to a horizonal piece of rectangle tubing and it would push it upon another one.
The next thing was to figure out supports to keep everything in place and let it move freely. I greased all areas where things rub together with black axle grease once a year.
When all was said and done, I GOT LUCKY!!! :roll:
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That's were the problem comes - I have FOUR arms
You know Eddy in some cultures you'd be considered a god...
Hehehehe thanks Tony, that made my day! :-)
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I've seen a "fake out" as I call it where you have a drive unit sitting higher than the wheel with matching cranks and the pitman is connected from crank to crank. The drive unit has to sit higher to keep the pitmans from bottoming out on the deck. It works as the same principal as the connecting rods on steam engine wheels (or whatever connects the drive wheels together).
If I get stuck totally, the "fake out" will probably be the method I'll use Joe. It's not ideal as the action won't be correct, but it's the second best solution. After that I'm left with gear, chain, or belt drive to the wheel :-(
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Hi PD's - I don't suggest for one moment that George has gone over the top here, but quite the reverse & whilst I am not asking George to justify his comments - I am asking a renound model engineer his thoughts on the following
1) if we drill a 1/8" diameter hole with a new conventional HSS twist drill, does not the hole accept a precision ground pin of 0.125" diameter?
2) if the above is true, then the 1/8" or 0.125" diameter hole is marginally oversize
3) if the pitman arms & link pins were not jig drilled on centers could they not vary in center distance by a greater amount than x 4 times the marginal oversize hole?
4) so if the above is true it would not really matter if the center distance of all the components was out by 0.0??" as long as they are all the same :?: - Derek
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Many years ago I had a try at making such a mechanism out of 'bits of maccano' and though it worked (well, kind of...) it did have a jerky action as described, I suspect that was due to the 'push-pull' component not being in line between the paddle shaft and driving shaft, though I could be wrong about that, I didn't go on to build a model using this system.
(http://www.btinternet.com/~mjt60a/ubb_link_images/testhull02c.jpg)
If I were to build a sternwheeler I'd probably use a 'prop shaft' with a crownwheel & pinion - type arrangement on one side (cunningly disguised as valve operating gear?) and have the pitman arms slide freely in a box of some type on each side..
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