Model Engine Maker
General Category => Chatterbox => Topic started by: Captain Jerry on July 27, 2019, 03:36:25 AM
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What do you think this is?
[youtube1]https://youtu.be/4J0GyH2EjqA[/youtube1]
Alibre' has no means to show a release animation so the attached jpg shows the result of the toggle release activated by contact with the dark green ramp and the light green peg on the toggle, letting the valve drop closed.
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In my unrelenting search for a valve release mechanism that I can build, at a size that I can afford to work with, I am doing a bit of research on the TTNW (Things That Never Were) engine from the Pipe Division of Pipe Dreams Industries. It provides a steam valve early release under governor control. I have made several attempts to build a Corliss type engine at small scale and have been unable to make a valve release that works reliably.
The release is not the problem. The failure point is in having the release cog reconnect. The latch connect face is very small and it is difficult to design and build a spring with enough tension to close the latch and hold it. There is also the pivot of the latch is a problem. Because of the small size of the latch, it tends to wobble on its pivot pin which makes it disengage at will.
The TTNW design is an attempt to overcome these difficulties by replacing the latch with a toggle. The mechanism is more like the Greene engine, in that it replaces the oscillating plate with a reciprocating plate which caries the release mechanism, however, it the latch faces with a toggle mechanism. There are a pair of knock of ramps whose angle can be controlled by the governor. When the toggle knock off pin contact the ramp face, the toggle is knocked over-center and the dashpot closes the valve. on the return stroke, the toggle resets.
I have used Alibre' to model the reciprocating motion under eccentric control, but Alibre' cannot simulate the toggle release and collapse so I included a still shot of the motion immediately after the collapse. The distance between the steam valve bores on this model is 2.5 inches which I believe to be the minimum size for an engine with working release. Parts are of a size that I can manage and all pivots have a clevis design to eliminate wobble and twisting.
I have been fiddling with some iron and will decide shortly if I have enough confidence to make this a project. Your comments will help make that decision.
Jerry
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Real live proof of concept test. Please click on the tiny line below. It is a link to a YouTube video.
https://youtu.be/vkfL3Pz6b6U (https://youtu.be/vkfL3Pz6b6U)
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Neat, as well as ingenious.
Jerry (JerryNotts)
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The usual term for this type of valve gear mechanism is "trip gear".
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The usual term for this type of valve gear mechanism is "trip gear".
Thanks for the reply Mr. Key. May I call you Chuck? I have certainly heard the term "trip gear" but I thought that applied to all types of early cut-off designs. As I am sure you know there are many variations of which "Corliss" and "Greene" are commonly heard but there are many subcategories such as "crab claw" and "Reynolds type" and there are also many versions within the subcategories. Most, but not all of them, use some version of a cam to actuate the release. Greene types use a pair of ramps, but as far as I know, they have one thing in common. There are two mating faces that are held in contact against each other and to actuate the mechanism, some form of cam forces the two faces to slide against each other until their sharp corners or tooth slide past each other and let a spring or a dashpot or the weight of a bucket of maple syrup snap the valve closed. A latch, or a cog or something that lets two sharp corners disengage.
I agree that the mechanism that I have been fiddling with is a form of "trip gear" but it does not include any sharp corners or latches. It uses an "over-center" hinge that is much like the human knee. It will easily support your weight but if someone kicks you in the back of the knee, your leg will collapse and you will fall down but your knee is still connected. As far as I know, this method was never used on a steam engine, hence the name "TTNW" (Things That Never Were) but that doesn't it from being a real challenge, for me at least.
If you know of any "real" engine that works this way, I would like to know more about it.
I spent the day fiddling with it again. Mostly replacing simple screw pivots with shoulder bolts to eliminate slop and filing clearances to eliminate binding. I will probably spend a few more days with it before I decide to dig in or move on. There hasn't been much interest but .......
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I have now had time to give your post the attention it deserves. The late prof Heinz Wolff (see The Great Egg Race on YouTube) would have described it as "an Ellegant solOOtion". By arranging your ramp to be curved, and to start horizontal and tangential to the unlatching pin, you could eliminate the impact.
Incidentally, upon release, the toggle mechanism is of course unstable, i.e. has insufficient constraints, so I don't imagine any CAD system would be able to model the action.
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Captain Jerry, I think you've got it. Keep looking through those TTNW archives, please. I actually understand how it works. Thanks, JFLingg
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Many thanks to ChuckKey and JFLingg for the replies and for the time and effort to understand the design. My study archives are limited to the left over bits and pieces that I uncover as I am cleaning up my shop and organizing it for its potential closing. The bits that inspired me to revisit this concept were a pair of bent rods with eyes at each end and a pair of small blocks of steel with several holes in them. I first recognized the bent rods as part of a toggle system that released under tension and that helped to remind me that the steel blocks were part of a toggle design that released under pressure. I had spent a fair amount of time with the tension system before putting them in a little box under the bench. I never throw any thing away. I may throw it against the wall but never in the trash.
I decided to do a little fiddling with the pressure release system. I wont go into the many bad designs I considered before hitting on the sliding platform of the current design. It has some similarity to the Greene engine, an engine being built by Kvom in another thread here.
I have been refining my design a little, with an eye on aesthetics and material requirements. I am going through my limited remaining stocks of raw materials and will soon begin testing my ability to meet the challenge. The knock of ramps are still not finalized. As you have suggested, Chuck, a curved profile is a possibility. I am not nearly as concerned about impact as I am about adjustment and range of required motion. The angle of the ramp will need change only a few degrees to cause the trip to occur within the .100" movement of the toggle. And of course, the governor design and linkage are a big part of the problem. What the heck, I'll fake it if I have to.
As to the limitation of CAD systems to simulate release actions, I believe that the inability to simulate gravity is critical. There are five primary constraints and more than twenty secondary constraints on the position of the toggle and only one of those need be suppressed to release the toggle, but without gravity to cause the dashpot to drop, nothing happens.
[size=78%] [/size]
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You can see a slide type trip mechanism if you Google 'Mill Meece pumping station'.
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As I said, a few days ag, I gathering raw stock. This is an example of what I am finding.
(https://live.staticflickr.com/65535/48427991456_93ab40c7d0_z.jpg) (https://flic.kr/p/2gMq7DW)IMG_0185 (https://flic.kr/p/2gMq7DW) by captain.jerry Ginn (https://www.flickr.com/photos/140341766@N06/), on Flickr
Crappy old HF drill press vice which has now been converted to this
(https://live.staticflickr.com/65535/48427990476_fa4d872683_z.jpg) (https://flic.kr/p/2gMq7n3)IMG_0186 (https://flic.kr/p/2gMq7n3) by captain.jerry Ginn (https://www.flickr.com/photos/140341766@N06/), on Flickr
It is a pretty good source of cast iron and it has been aging in a bottom drawer for about 15 years. I guess since I have gone to the trouble of sawing up a vise, I should begin a legitimate build log, which I will do tomorrow. Now that I have resolved my difficulties with Flickr, there will be frequent pictures.
Jerry
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One more broken promise. I did not start a build project on this engine. I realized that I had no real plans, just ideas, which make it hard to hold the line and manage a build log. So that will be put off for a while.
A few things did happen last week. I am now 81 years old and am the reluctant holder of a "Disabled Parking Permit." I am not seriously disabled, just have a problem with balance and wobbly knees. I still won't park in a "blue" space if its the only one available.
I have been getting some shop time in and refining the design of the TTNW Toggle Release mechanism. It has been a series of small design changes as I slowly learn some of the basic requirements and principles. Mos of the parts were made to fit rather than built to plan. I may now refine the design to reduce the bulk now that I know where it can be done without affecting function.
This is a video of the most recent operational test:
I have left the dash pots uncushioned for this test. The sound of the weight hitting bottom as the valve is snapped closed helps keeping track of the action. I have tested with a bit of cushioning and it has no effect on the operation. I may do a some testing with different weight.
(https://live.staticflickr.com/31337/48515786747_2cd19571b6_z.jpg) (https://flic.kr/p/2gVb68R)Toggle Release #1A (https://flic.kr/p/2gVb68R) by captain.jerry Ginn (https://www.flickr.com/photos/140341766@N06/), on Flickr
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Nice action on the links :ThumbsUp: but I must admit that I can't see the variable cut-off mechanism in action .... :thinking:
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Nice action on the links :ThumbsUp: but I must admit that I can't see the variable cut-off mechanism in action .... :thinking:
Thanks for looking in and taking the time to comment. You are of course correct. Variable cutoff is not shown in this video. It is there, but it is not shown. What is shown is early and quick cutoff. When the toggles on the slider collapse, it is because they are pushed over center by contact with the cut off control which is a pair of ramps. In order for the cut of to be variable, the height of these ramps must be controlled by some load or speed sensing device. the governor has not yet been designed or built but there is provision for it.
In the center of the slide, you can see a vertical screw thread that is part of the adjustment for the slider friction. Just behind that screw thread you can see the head of another screw. At present that screw can be turned to raise or lower the cut off ramps. At some point in the future, that screw will be replaced by a link to a governor which will raise or lower the ramps by a lever, not rotation.
I can adjust it for a very quick cut off to no cut off at all with a hex nut driver and I will soon have a linkage lever to do the job but the actual governor is a long way off. You can see some wobble in the ramps and in the adjusting screw due to the poor fit of the screw thread in the guide. When the screw is replaced with a polished rod, that wobble should be greatly reduced.
Jerry
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I had deducted that it was on early cut-off (but thanks for confirming) and I also got the ramps correctly identified - what I absolutely didn't get was how they are moved / controlled - so thank you for explaining :cheers:
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It took me a day to knock up something as an example of what might be done. I say might because until there is a governor attached, I cant be sure what will work. The length of the levers on the crank are the critical element. The corresponding crank at the governor and the power developed by the governor will be part of the final design.
From this example, it seems to need either a longer lever arm or a slotted connection where the lever connects to the lift rod. The lift rod is a close fit in the guide for stability of the ramps and that constrains the travel to a straight vertical path and the curved path of the lever arm causes some binding. A slotted connection will probably be the best solution.
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Seems to be coming along nicely. One problem I see is that the tripping force will be transmitted to the governor, but, having only just read up to date, I have not thought how to avoid or minimise that yet. It probably relates to clock escapement design.
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Governor feed back is always a consideration and I have done what I can to minimize it with this design. As far as I can see, every cut off design faces the same problem. The design consideration comes down to how long the sensing mechanism is in contact with the cutoff control surface as it moves the latch faces until they separate. One factor is the depth of the latch faces or how far the moveable face must move from latched to released. The other factor is how much force is applied to the moveable face, usually a spring.
On the toggle release, the force that is applied is a fraction of the weight of the toggle itself and a fraction of the weight of the arm that links it to the valve, The depth of the contact faces is a bit harder to describe. It involves two lines. Line one is the line between the pivot at the small end of the toggle and the pivot at the valve end of the link arm. Line two is the line between the two pivots on the toggle. For the toggle to latch the second line must be below the first. when the knob on the toggle contacts the ramp, it begins to raise the second line and it remains in contact with the ramp untill the two lines become one (and a little bit more). At that point the toggle releases and the knob flies away from the ramp.
Aha! I see what you mean. With the method that I have shown, during the time that the knob is in contact, there is a reaction force applied to the ramp and transferred to the lift rod, the lift crank and ultimately to the governor. It helps to have questions like yours and it helps to talk about the problem. I have an answer! That answer also solves the problem that I found with the crank binding as it raised the lift rod.
If I get time in the shop today, I will post a revision of the lift mechanism that relies on a cam to raise the ramps. There will still be some feedback but it will be greatly reduced.
What do you think? Cam shape not yet final.
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OK, no cam, unless you consider a wedge to be a flat cam. I think it works better, with less slop than a cam and I think it completely handles the backfeed issue.
(https://live.staticflickr.com/31337/48530924402_f7dfc4a159_z.jpg) (https://flic.kr/p/2gWvF2U)IMG_0195 (https://flic.kr/p/2gWvF2U) by captain.jerry Ginn (https://www.flickr.com/photos/140341766@N06/), on Flickr
Jerry
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That works. Just an idea: instead of raising and lowering the trip ramps, how about having them separate, sliding in a track on the shelf, and moving towards each other for shorter cutoff. That way it might be easier to have a ramp each side of the toggle, which will avoid the twisting force that would induce wear in the single-sided mechanism (which I realise you are only using for trying out the idea). Rasing the ramps also precludes the use of the curved tripping surface I suggested earlier, but of course I am not saying you are obliged to like that idea.
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I see you are doing a little design thinking. Great! The idea of separate ramps, moving opposite to each other is a good one and I also considered separate ramps tha pivoted rather than lifted but the linkage involved to keep them in sync was more than I wanted to deal with. Balancing the twist with a matching ramp on the outside is also a good idea and it might be possible to do that with a one piece lifting mechanism. On a larger model where it could be done without looking so out of scale it might be possible to do that by lifting the ramps from the bottom. That is the way it is done on the Greene Engine.
Keep on thinking and thanks for posting,
Jerry
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I may be beating this to death but I'm having fun with it. I see that it is getting a few views but not much conversation.
The current condition is the result of some refining of the design. The general principal works but since the pieces have been hacked out of scrap with no real plan, just a general idea. Everything has been a "Make to fit", eyeball measured, do it over effort and there has been a lot of slop in the linkage so some of that has been improved. Clearances and unnecessary spacing and oversized components have been tightened up and still more is needed. The most important feature on this version is a greatly stabilized knock off ramp design and the first effort to locate the governor control link to the bottom of the mechanism. It works, It needs smoother action and finer control and can be reduced in size.
More to come. I hope you enjoy the video.
https://www.youtube.com/watch?v=IRvKUFFvn3Y