sternwheeler crank timing problem

[grumpysumpy]

hi guys
restoring an old sterndrive that has parallel coupling rods fitted. rebushed the links and rods to take the slop out of the system and now having trouble with jambing. i can trim the cranks and pivot bushes to get a smooth operation in one direction but jambs in the opposite direction. have tried all angles from 70 degrees to 110 degrees on the slave crank - no help. put a slot in one of the motor drive rods - no help. backing off by half a turn the screws that locate the pivot bushes reduces the current draw and gives a smoother operation in both directions. the links are identical and so are the rods.
is there a correct procedure for setting up this type of drive system
before photo shown
gordon AKA grumpysumpy

[kiwimodeller]

Hi Gordon, this subject has been of interest to me for a while now as I start on the build of a sternwheeler. I have recently looked at a Mantua kit of a 50" Mississipi sternwheeler and the cranks on that were set at 90 degrees as per the plans. There was very slight slop in the ends of the links and I could detect just a very slight tightness in certain positions as the wheel was spun by hand but I am sure the motor could overcome it relativally easily. The model has never beem finished but the motor and drive system has been run. All the other reading I have done suggests 90 degrees too so I was not contemplating anything else. It will be interesting to see if anyone else has done any experimenting. Cheers, Ian.

[grumpysumpy]

Ian thks for your input. I thought I had it beat when I was rotating the paddles, nice and free both directions but then I tried it from the motor end and all hell broke loose. Jambed in both directions. Back to fiddling with cranks. Incidentally in forward rotation the followup crank lags by 90 degree and the followup crank at the motor end should have an elongation in the rod. but........in reverse the cranks change roles and now the elongated slot is the primary rod. What is the slot for? To allow for any mismatch in dimensions.....maybe

regards

Gordon

[R.G.Y.]

I have no experience of stern wheelers, but your set up is similar to a rail locomotive's. I do know they need the ALL cranks to be exactly the same throw. All square and the same distance apart. Be fore you tightened things up there probably was enough play to compensate for any inaccuracies. 
Hope this is some help. R.G.Y.

[capnbob]

Hi Gordon,

From the pictures it looks like you have two cranks on either end of a shaft in the boat and two cranks on either end of the paddle wheel shaft.  In a setup like this all four crank arms need to be exactly the same length Also the two pittman arms need to be exactly the same length.  If everything is perfect the system will run smoothly.  To compensate for the lack of perfection there needs to be some built in slop or adjustments.  The slop can be an oversized hole in the pittman.  The slots in the pittman is either for slop or if there is a screw there, to adjust the lengths to match.  Good luck with the setup.

Bob

[grumpysumpy]

Bob
been doing more work on dimensions. obviously .005" has been too much out of tolerence. thks for your help

[grumpysumpy]

finally got my act together. got the timing right and she is on the water

[kiwimodeller]

Hi Gordon, congratulations on getting it back on the water. What did you do to solve the problem of the links binding up? Thanks, Ian

[grumpysumpy]

Ian
basically made the cranks identical (down to .002") and drilled the holes in thr crankrods identical (down to .002") assembled leaving the trailing motor crank assembly bearing loose. ran the motor at minimum voltage and watched the loose bearing slightly move back and fwd with each half rotation. removed that pin and filed a slot radially in the crank a few strokes at a time until the bearing stopped moving then locked it down. no more binding.

will get better as high points wear down so i am told

gordon

[kiwimodeller]

Great, thanks for that, it is good to know that the simple system can be made to work. I dont have the skills or machinery to make the Scotch Link system discussed in an earlier thread and thought I would have to use to either chains or toothed pulleys and belts but the links such as you are using will be easier to fit I hope and will look at least part way authentic. Appreciate your help. regards, Ian.