Author Topic: Trevithick  (Read 8877 times)

Offline Brian Rupnow

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Re: Trevithick
« Reply #195 on: October 17, 2020, 01:18:06 AM »
Propforward--I knew that--but it fits the story I was telling.

Online propforward

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Re: Trevithick
« Reply #196 on: October 17, 2020, 02:07:37 AM »
I enjoyed it almost as much as following your engine thread. All good fun.
Stuart

Offline Robert Hornby

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Re: Trevithick
« Reply #197 on: October 17, 2020, 03:54:20 AM »
Okay Charles---Big changes with the heavier flywheel. The flywheel is rocking back and forth like crazy between top dead center and bottom dead center under air pressure.  This is very promising. Based on what it's doing now, it should just require some adjusting of the valve actuators on the slide bar. I have just blown an internal hose and have to tear things down to reconnect it. Is there any kind of liquid glue that I can permanently glue the neoprene hoses to the brass or steel tubes on the 4 way valve ? I do have some small spring wire clips from the hobby shop, but at higher air pressure the hoses keep blowing off. Crazy glue sets up so quickly that I wouldn't have a chance to put some glue on the steel tubes and then slip the neoprene hoses over them. Is there something like a "delayed action" super-glue?
I have found that keeping SUPER GLUE in the fridge provides two benefits. Firstly it seems to last quite a long time before "going off"  in the tube. I have had it last 2 months. Secondly when using it I have a bit more time to position/adjust the components before it sets, say around 15 seconds rather than instant.
Robert
Age and treachery will always overcome youth and skill

Offline Brian Rupnow

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Re: Trevithick
« Reply #198 on: October 17, 2020, 03:05:33 PM »
After a full day of fixing disconnected tubing and taking things apart and then putting the parts all back together again, it was time for some analysis as to what was happening. Right now, as things set, the valve is set so that it is fully opened at the end of the piston stroke in either direction. Now, in a perfect world, where the flywheel really was doing it's job, the inertia of the flywheel should carry the crankshaft "over the top" before the piston begins to travel in the opposite direction. Since the flywheel is bouncing back and forth between the top and bottom dead center, the conclusion is that the valve is being actuated just a tiny bit too soon. If the opening was delayed for another millisecond, the crankshaft would have that space of time to get "over the hump" and  make complete revolutions rather than bouncing back and forth thru partial rotations.  So yes Charles, I do agree with what you are saying and what you show in your diagram.  With everything "as designed", I can adjust the sliders so that the valves begin to open a bit later in the cycle, which in theory mean they would close a bit later in the cycle. That would be a "best case" scenario. If that doesn't work, then as Charles suggested, shortening the radius arm on the valve would allow the valves to operate closer to the end of the piston stroke and hopefully allow the flywheel to get "over the hump" before travelling back in the opposite direction. I have the capacity with my current design to shorten up the radius arm with no other changes.

Offline Brian Rupnow

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Re: Trevithick
« Reply #199 on: October 17, 2020, 06:42:00 PM »
Bah!! Humbug!!--The easy fix (repositioning the valve actuators)  didn't work. I have never had an engine so close to running that didn't actually take off and run. I can see a number of things which I could do to fix this, but they are all progressively more and more difficult. The next easiest fix is to shorten the radius arm on the valve. Of course this will make for an extended  cantilever on the sliding brass actuator. I have a small (3mm) ball bearing that I think I can work into the equation to take a bit more friction out of the valve action. Oh well, thanks to Covid there is damn all else to do anyways.

Offline Charles Lamont

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Re: Trevithick
« Reply #200 on: October 17, 2020, 08:53:56 PM »
So far we have only ever had the Catch Me Who Can replica at Bridgnorth running jacked up, as a stationary engine. (I am hoping to get back to work on the brakes in the next couple of weeks.) In that state it will run down to about 20 rpm, and we have had it do so on as little as 5 psi. The limiting factor is not so much the valve advance as lifting that hefty crosshead over the top. In this it is helped by our use of 'modern' wheel and tyre standards rather than the much lighter ones Trevithick would have had. Each of our driving wheels weighs in at over half a ton, so they act as pretty good flywheels. Even so starting takes a bit of practice to avoid the backwards and forwards rocking motion that Brian has at the moment. On the road it will be quite different. The all-up weight is going to be not far short of 9 tons, and that amount of inertia should smooth out the low-speed motion a lot.

Brian, connecting the wheels up and putting the engine on the floor might help a bit, especially if you could fill the 'boiler' with lead. 

Offline Brian Rupnow

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Re: Trevithick
« Reply #201 on: October 17, 2020, 09:08:38 PM »
Charles--I'm just trying to get the engine to run without having any of the gears in place.(except for the one on the end of the crankshaft.) In the next five minutes I'm going to start making a shorter radius arm on top of the valve.

Offline Brian Rupnow

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Re: Trevithick
« Reply #202 on: October 17, 2020, 11:55:58 PM »
Charles---A little math for you.  The radius on the arm which sets above the 4 way valve was 0.900" long. I shortened it to a radius of 0.775" long. If you assume that the included angle is 90 degrees then the chord length of the smaller radius is 13 % less than the chord length of the larger radius. Will it make a difference in how the engine responds?---I don't know. I have made the new pieces but haven't installed them on the engine yet. That will be tomorrows job. ---Brian

Offline Charles Lamont

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Re: Trevithick
« Reply #203 on: October 18, 2020, 07:09:05 AM »
Charles---A little math for you.  The radius on the arm which sets above the 4 way valve was 0.900" long. I shortened it to a radius of 0.775" long. If you assume that the included angle is 90 degrees then the chord length of the smaller radius is 13 % less than the chord length of the larger radius. Will it make a difference in how the engine responds?---I don't know. I have made the new pieces but haven't installed them on the engine yet. That will be tomorrows job. ---Brian
Qualitatively, yes, it should make some difference. To do timing diagrams one would also need to know the piston stroke and details of the valve and ports. But timing diagrams won't tell you if it will run anyway. Today I am off to Bridgnorth and will see our replica for the first time since the start of viral lockdown. 

Offline Brian Rupnow

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Re: Trevithick
« Reply #204 on: October 18, 2020, 04:08:22 PM »
This is my Trevithick engine doing something I haven't seen before. It has a unique 4 way valve mounted on top of the boiler, and the valve swings back and forth under the influence of the sliders which contact the valve arm. I don't have things set up quite right yet, so the engine is "stuttering"---The valve is being reversed before the piston gets far enough in it's linear travel to get the crankshaft over dead center and complete a full revolution. This is not what I am ultimately aiming for, but I hope that with some adjustment to the sliders I can coax it into full revolutions.---Brian

Offline crueby

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Re: Trevithick
« Reply #205 on: October 18, 2020, 05:29:25 PM »
As is, it would make a good sawing motion, but hopefully there is a sweet spot on the slider rings.

Offline Charles Lamont

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Re: Trevithick
« Reply #206 on: October 18, 2020, 06:10:05 PM »
COOL!

Offline Charles Lamont

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Re: Trevithick
« Reply #207 on: October 18, 2020, 06:21:32 PM »
That seems well short of going over dead centre, with about another 30 of crank rotation to go at each end. I would like to see the valve and port dimensions, but for now I suggest gradually spreading the tappets further apart. This will delay the valve motion, but of course if you go too far the ports won't open enough. It looks as though the valve is not going through a full 90 anyway. If not, strangely, spreading the tappets a bit might give more valve movement.
« Last Edit: October 18, 2020, 06:28:55 PM by Charles Lamont »

Offline Jasonb

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Re: Trevithick
« Reply #208 on: October 18, 2020, 06:42:51 PM »
Brian, looks like you are well on your way to your next project of a steam saw ;)


Offline stevehuckss396

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Re: Trevithick
« Reply #209 on: October 18, 2020, 07:30:45 PM »
I have no doubt that you will get it squared away and running slicker than spit on a door knob.
Do not be like the cat who wanted a fish but was afraid to get his paws wet.