Building a better crankshaft (crank and slider)

[Dr.D]

The animation in post #1 appears to be about a 4 cylinder IC engine.

I do think you were inadvertently correct when you said,

Idk what you are going on about honestly.

You are wasting the time of everyone on this board.

 

[paradisePhysicist]

The animation in post #1 appears to be about a 4 cylinder IC engine.

I do think you were inadvertently correct when you said,

You are wasting the time of everyone on this board.

Noone's making you post in this thread, if you don't want to then leave. If you aren't going to fix the specified problem then you are wasting my time as well and I just generally don't like dealing with toxicity/naysayers.

The post wasn't about 4 cylinder IC engines, sorry if the gif wasn't clear enough.

 

[gmax137]

I just watched the video linked in the original post #1. At best, it makes up its own terminology ("input," "cam"...). It is misleading as well. The stroke of the piston is determined by the throw of the crank, calling this the "semi major axis of the cam" only obfuscates what is going on. It isn't entirely incorrect, and might have some use if the device under consideration was say a cam opening a valve, but that is normally done with a follower and pushrod, or the cam acting directly on the valve stem, neither of which is a slider-crank mechanism. Finally the notion that the piston ("slider") cannot drive the crank's rotary motion can be put to bed by considering the millions of piston engines happily running since the dawn of the industrial revolution.

 

[paradisePhysicist]

I just watched the video linked in the original post #1. At best, it makes up its own terminology ("input," "cam"...). It is misleading as well. The stroke of the piston is determined by the throw of the crank, calling this the "semi major axis of the cam" only obfuscates what is going on. It isn't entirely incorrect, and might have some use if the device under consideration was say a cam opening a valve, but that is normally done with a follower and pushrod, or the cam acting directly on the valve stem, neither of which is a slider-crank mechanism. Finally the notion that the piston ("slider") cannot drive the crank's rotary motion can be put to bed by considering the millions of piston engines happily running since the dawn of the industrial revolution.

Do you have a better video to suggest? I did not intend for this video to be the "holy grail" of crankshafts just as a quick example to illustrate a problem. I also said several times (actually one of the first things I said) that of course with a flywheel the problem can be solved, and motion can be achieved, but that I was not interested in using a flywheel.

 

[Averagesupernova]

I've read through this thread as well as watched the video and I also don't 'get' it. The crank mechanism such as the one in the video works as it does because top and bottom dead center are undefined with respect to push or pull on the slider. The flywheel is a remedy to allow us to do what needs to be done while still leaving these two areas undefined. There is no way around it without adding complexity. It can be a flywheel, more sliders with timing that determines when to push, etc. But you cannot get around it by simplifying what we already have.

 

[paradisePhysicist]

I've read through this thread as well as watched the video and I also don't 'get' it. The crank mechanism such as the one in the video works as it does because top and bottom dead center are undefined with respect to push or pull on the slider. The flywheel is a remedy to allow us to do what needs to be done while still leaving these two areas undefined. There is no way around it without adding complexity. It can be a flywheel, more sliders with timing that determines when to push, etc. But you cannot get around it by simplifying what we already have.

Looking back at my original post, its not 100% perfect, in retrospect I'd have rephrased one of the lines above the gif, I didn't anticipate the minor mistake was going to result in stress and drama.

Anyway, I am willing to add complexity as long as the complexity is robust and doesn't require a flywheel, I am looking to be able to just push the slider and get full rotations without having to add a bunch of mass and decrease the acceleration.

 

[Averagesupernova]

You do realize you will be adding a new set of problems right? I guess if you want to find that out, go ahead.
-
Make your slider engage two gears with a ratchet pawl mechanism. They will each turn the opposite direction but only half the time. Stop and reverse the direction of the slider at will. Connect the two opposite turning gears together with more gears to get the rotation constant and in the same direction. No flywheel. New set of problems include more reciprocating mass as compared to previously, a ratchet mechanism that will wear quickly, more parts in general. Looks like you haven't built a better mousetrap, but by golly that pesky flywheel is gone.

 

[paradisePhysicist]

You do realize you will be adding a new set of problems right? I guess if you want to find that out, go ahead.
-
Make your slider engage two gears with a ratchet pawl mechanism. They will each turn the opposite direction but only half the time. Stop and reverse the direction of the slider at will. Connect the two opposite turning gears together with more gears to get the rotation constant and in the same direction. No flywheel. New set of problems include more reciprocating mass as compared to previously, a ratchet mechanism that will wear quickly, more parts in general. Looks like you haven't built a better mousetrap, but by golly that pesky flywheel is gone.

I am generally leery of rachet type mechanisms due to the reasons you mentioned. Currently the best solution on the table so far is the half-gear mechanism, however that has some problems as well. I will link to the video of the half-gear mechanism here, it is similar but somewhat different than the gif posted by Lnewqban. Here it is:
https://www.ytprivate.com/watch?v=DfDPyWdlfz0

So far this one is my favorite but I am on the lookout for other options.

 

[hmmm27]

magnets.

 

[paradisePhysicist]

magnets.

Hmm I thought of this yesterday, but haven't yet worked out the specifics. The idea was going to be something involving the crank turning another gear, and that both gears had a magnet attached that boosted the rotation, in such a way that it also didn't interfere with rotation on the stall angles. Haven't really explored the idea yet because of all the sweltering heat, AC is on yet still been sweating all day. Though I don't have Lego magnets, I am willing to cross that line and use magnets, breaking legos is simply not my policy.

 

[some bloke]

Just so that I am on the right page here...

When you say a dual input piston, are you maning one of the pneumatic pistons the Lego makes, and that it is going to both push and pull the crankshaft, and that the problem is getting from the end of the push to the start of the pull? Is that about the summary of it?

I've made a piston engine out of lego technic before using the pneumatics, but it was 4 pistons and a flywheel to make it run, all pistons pushing and pulling. They were 90° offset to allow 2 of them to be pushing/pulling whilst the other 2 were switching.

I am mulling this over at the moment. One thing I am considering is if you were t owork out a mechanism where the start and end of the stroke is translated by 90° to push the crank up or down, passing the problematic point of the 179-181° changeover. Alternatively, a crank-in-a-crank could do something similar...

I shall ponder. Maybe even crack out the lego later!

 

[paradisePhysicist]

Just so that I am on the right page here...

When you say a dual input piston, are you maning one of the pneumatic pistons the Lego makes, and that it is going to both push and pull the crankshaft, and that the problem is getting from the end of the push to the start of the pull? Is that about the summary of it?

I've made a piston engine out of lego technic before using the pneumatics, but it was 4 pistons and a flywheel to make it run, all pistons pushing and pulling. They were 90° offset to allow 2 of them to be pushing/pulling whilst the other 2 were switching.

I am mulling this over at the moment. One thing I am considering is if you were t owork out a mechanism where the start and end of the stroke is translated by 90° to push the crank up or down, passing the problematic point of the 179-181° changeover. Alternatively, a crank-in-a-crank could do something similar...

I shall ponder. Maybe even crack out the lego later!

Interesting, what is a crank in a crank? Are you referring to adding a gear underneath the crank?

 

[some bloke]

Interesting, what is a crank in a crank? Are you referring to adding a gear underneath the crank?

I am pondering exactly that right now.

The thing I am considering is whether you could have the crankshaft such that where the conrod would connect, there would instead be a second crank which is turned relative to the original crank. This would be such that (in a horizontally laid out engine, crank on the left and piston on the right) the main crank would be at 3:00 (180°, no power from the piston) whilst the secondary crank it at 12:00. The piston would push on the second, smaller crank, turn it, which in turn pushes the main crank beyond the 180° mark and then when the piston pulls back, it would be pulling the secondary crank at 3:00 (so have no control over which way it turns) but the main crank at beyond 3:00, meaning the net forces will make the secondary crank turn in the correct direction.

It's all in my head right now, I'll see if I can put it into some doodles later!

 

[some bloke]

The doodle is complete but I fear that without animating it in some way it is going to appear as utter nonsense...

I will try and make a gif for it later if I get the time!

I will also say that this will likely make a very poor engine as I think it will result in the speed juddering all over the place... but it could negate the need for a flywheel, anyway!

 

[paradisePhysicist]

Interesting and this gives me some ideas, think I'm going to try them in the simulation.

 

[some bloke]

The basic thinking behind it is that if you have two cranks, both of which are turned by the same piston, then when one is at it's maximum/minimum then the other is not and the net result is that one will turn and the other will not resist, so the whole assembly will turn. No idea if it'll work!

 

[paradisePhysicist]

The basic thinking behind it is that if you have two cranks, both of which are turned by the same piston, then when one is at it's maximum/minimum then the other is not and the net result is that one will turn and the other will not resist, so the whole assembly will turn. No idea if it'll work!

Hmm, based on this I tried to put two gears in the same spot in the simulation. Then a rod at then minimum location to gear 1, then a rod at 90° location to gear 2, both attached to the same piston. It will not work and how do I explain this... I used springs to give it some leeway incase the rotations and positions were not exactly precise, still didn't work, I think the reason is because when gear 2 reaches the maximum position, it wants the piston to go in reverse, but at the same time, gear 1 wants the piston to still go forward, therefore it can't rotate. I will draw a picture to make it clearer.

Actually I think I could have just used 1 gear for this actually, but in the simulation I used 2 gears stacked on top of each other.

I haven't tried your doodle yet but I will try that next.

 

[hmmm27]

Sorry if I missed the post explaining this, but is it a powered upstroke ? And, the only problem is that you can't seem to go through 0 or 180 without rotational momentum... ?

 

[paradisePhysicist]

Sorry if I missed the post explaining this, but is it a powered upstroke ? And, the only problem is that you can't seem to go through 0 or 180 without rotational momentum... ?

Yes.

 

[hmmm27]

okay... so, you've got

- a big circle, featuring
- a little circle near the edge.

Make the little circle a slot : same placement, same width but say a length of 3x, the diameter of the original. Angled, as measured at BDC(or TDC), at 45deg.

Works fine in 2d ; probably too messy in 3d, but might give you something to work off of.

 

[paradisePhysicist]

Make the little circle a slot : same placement, same width but say a length of 3x, the diameter of the original. Angled, as measured at BDC(or TDC), at 45deg.

Works fine in 2d ; probably too messy in 3d, but might give you something to work off of.

Hmm, can you draw me a rough sketch, I'm not quite sure what the visualization of this is. Are you saying to cut a slot into the gear, put the conrod into the slot, or something else?

 

[hmmm27]

Something like...

The first one is normal, it's the one you have issue with. The second one uses a slot, not a circular hole, to hold the crankpin.

 

[Averagesupernova]

Something like...

View attachment 285613The first one is normal, it's the one you have issue with. The second one uses a slot, not a circular hole, to hold the crankpin.

What happens at the top of the stroke?

 

[hmmm27]

What happens at the top of the stroke?

Same as what happens at the bottom... except upside-down. 45deg before tdc/bdc, the pin switches from the inside end of the slot to the outside.

 

[paradisePhysicist]

Something like...

View attachment 285613The first one is normal, it's the one you have issue with. The second one uses a slot, not a circular hole, to hold the crankpin.

I will try running this in the simulation.

I tried this idea (post #21) into the sim, and it cannot work for linear input, no matter how many teeth are in the circular gear (the simulation is very forgiving).
https://grabcad.com/library/reciprocating-mechanism-1

I attempted to put post #44 into the sim but the instructions seem unclear, is the small gear anchored to the world space or supposed to slide around the big circle, I am not sure, I have a lot of questions about that drawing.

 

[hmmm27]

I will try running this in the simulation.

#55[edit: #52] * idea does rely on inertia ; if the piston-arm is comparatively too heavy, you'll still be stuck.

* so, how does one link to a specific post in a thread ?

 

[paradisePhysicist]

#55 * idea does rely on inertia ; if the piston-arm is comparatively too heavy, you'll still be stuck.

I tried post #52 it in the sim, it works if the friction is low, but if there is a decent amount of friction it won't work, gets stuck when the diagonal is in the outer region. Post #55 contains different concepts so idk which one you are referring to.

* so, how does one link to a specific post in a thread ?

You can use the quote button.

Also, I tried the idea I said was my favorite (post #38) and it sucks for linear input, back to the drawing board ig.

 

[hmmm27]

I tried post #52 it in the sim, it works if the friction is low, but if there is a decent amount of friction it won't work, gets stuck when the diagonal is in the outer region. Post #55 contains different concepts so idk which one you are referring to.

Sry, meant #52, not #55. post amended. Given length of slot was for an example, not a calculation.

 

[some bloke]

Hmm, based on this I tried to put two gears in the same spot in the simulation. Then a rod at then minimum location to gear 1, then a rod at 90° location to gear 2, both attached to the same piston. It will not work and how do I explain this... I used springs to give it some leeway incase the rotations and positions were not exactly precise, still didn't work, I think the reason is because when gear 2 reaches the maximum position, it wants the piston to go in reverse, but at the same time, gear 1 wants the piston to still go forward, therefore it can't rotate. I will draw a picture to make it clearer.
View attachment 285607
Actually I think I could have just used 1 gear for this actually, but in the simulation I used 2 gears stacked on top of each other.

I haven't tried your doodle yet but I will try that next.

Yes, this was similar to my first thought before I dismissed it for the same reasons!

The trick is finding a way to have 2 cranks which can be turned by a single con-rod. That's where my design came in, though I am struggling to find a way to make it on any online gear generators - none of them assume a gear is fixed and the rest rotate around it!

 

[paradisePhysicist]

Sry, meant #52, not #55. post amended. Given length of slot was for an example, not a calculation.

I will post gif of experiment, I don't think it will work at any length or configuration without assistance from rotational inertia (ie., a heavier gear or flywheel needs to be added.)

I am looking into the one cylinder locomotive, the only website I can find is this website, which says the diagram is wrong, so I haven't figured out how they built it yet. http://www.douglas-self.com/MUSEUM/LOCOLOCO/onecylinder/onecylinder.htm

Yes, this was similar to my first thought before I dismissed it for the same reasons! The trick is finding a way to have 2 cranks which can be turned by a single con-rod. That's where my design came in, though I am struggling to find a way to make it on any online gear generators - none of them assume a gear is fixed and the rest rotate around it!

Hmm can you edit your diagram because I'm a bit confused about it.