Suter drive system

[Eddy Matthews]

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!

[gbritnell]

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

[Eddy Matthews]

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!

[towboatjoe]

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.

[Khephre]

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

[gbritnell]

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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[Eddy Matthews]

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!

[Khephre]

That's were the problem comes - I have FOUR arms

You know Eddy in some cultures you'd be considered a god...

[towboatjoe]

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:

[Eddy Matthews]

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! :-)

[Eddy Matthews]

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 :-(

[derekwarner_decoy]

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

[mjt60a]

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.

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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