# Building a better crankshaft (crank and slider)

In summary, the dual input crankshaft uses the piston instead of a flywheel to turn the engine, which eliminates the inherent tendency to jam. This is an easy solution that is more efficient than the current crankshafts.
some bloke said:
Here's a mechanism which I think would work. You could have the central oval cam (which doesn't rotate) rock back & forth to change direction.

View attachment 285697

I am imagining the piston to the right of the wheel, pushing the pin to the left. The pin interferes with the fixed cam, so cannot move past it. The pin is in a slider. The image is the 4 stages of half a stroke, and it behaves the same on the way back.

The idea is to have a mechanical movement where the first parts of the pistons stroke are converted into non-linear motion - in this case, the pin is deflected sideways by the central cam, and starts the wheel rotating anti-clockwise. At the start of each rotation, the pin slides along the cam and starts the wheel going in the correct direction.
I am still not understanding your diagram lol.

These are my questions on the first diagram also:
some bloke said:
If you're interested, this was inspired by the mechanism inside of a clicky pen, which converts repeated reciprocating motion to rotary in a similar way to what you are requesting - have a look at a transparent clicky pen and you'll see the mechanism spins as you click it!
This implies that the concept is mathematically possible.

hmmm27 said:
So, since it isn't actually a flywheel, beyond having a rotational component, like many other bits and pieces of the system like the crankshaft, crankarm, piston-rod, etc...Without rotational momentum, a basic system (piston->piston-rod->crank-arm->crankshaft) is never getting past tdc/bdc, regardless of how friction-free the bearings are.
There is another concept that proves it is mathematically possible without adding a flywheel type extra weight. While this contraption is not exactly ideal, with collisions on the walls at 90 degree angles, and not exactly 100% the same as a typical crank, but does imply it is mathematical possible.

What does Charlie Brown's shirt have to do with anything ?

hmmm27 said:
What does Charlie Brown's shirt have to do with anything ?
Lol it is a 2d overlay but that is the shape you put on a 3d cylinder to give 1 way rotations (you put a disk into the slot from the piston.)

Lol it is a 2d overlay but that is the shape you put on a 3d cylinder to give 1 way rotations (you put a disk into the slot from the piston.)
Ah, so now you're powering the piston from the crankshaft ? Or, a rotary motor connected to the zig-zag on a cylinder (or perhaps The cylinder), powering the piston through the zig-zag, thence to the crank, etc.

Were you traumatized by a flywheel as a child ?

berkeman
hmmm27 said:
Ah, so now you're powering the piston from the crankshaft ? Or, a rotary motor connected to the zig-zag on a cylinder (or perhaps The cylinder), powering the piston through the zig-zag, thence to the crank, etc.
No, the piston is supposed to power the zig zag cylinder, but it could work the other way around.

hmmm27 said:
Were you traumatized by a flywheel as a child ?
-_- No and let's keep the convo on physics... let's steer the convo away from talking about traumas...

No and let's keep the convo on physics... let's steer the convo away from talking about traumas...
Great! When does it start?

Averagesupernova
This thread was discussed in the Mentor forum, and the decision was made to allow it to continue.

There are many mechanisms for converting linear to rotational motion, some of which do not require flywheels. I'm not aware of any mechanism that can convert a linear motion with sinusoidal velocity to smooth rotational motion without rotational inertia (flywheel).

An earlier post linked to a mechanism consisting of a slot with two racks and a pair of half gears. That mechanism could convert linear to rotation motion without a flywheel, but has two fundamental shortcomings:

1) There is a point at each end of travel where neither gear is in contact with its rack.
2) Getting smooth rotational motion requires the rack to have infinite acceleration at each end of travel.

If a requirement is smooth (constant angular velocity) rotation output with a smooth (minimum peak acceleration) linear input, then a flywheel is necessary. If herky jerky input and/or output are acceptable, then there should be mechanisms that will work.

There is an old book available online and free that consists entirely of various mechanisms. Search 507 Mechanical Movements to find a copy. It's a good read for anybody who finds this thread interesting.

hmmm27, paradisePhysicist, pbuk and 1 other person
jrmichler said:
This thread was discussed in the Mentor forum, and the decision was made to allow it to continue.
Thanks

jrmichler said:
There are many mechanisms for converting linear to rotational motion, some of which do not require flywheels. I'm not aware of any mechanism that can convert a linear motion with sinusoidal velocity to smooth rotational motion without rotational inertia (flywheel).

An earlier post linked to a mechanism consisting of a slot with two racks and a pair of half gears. That mechanism could convert linear to rotation motion without a flywheel, but has two fundamental shortcomings:

1) There is a point at each end of travel where neither gear is in contact with its rack.
2) Getting smooth rotational motion requires the rack to have infinite acceleration at each end of travel.

If a requirement is smooth (constant angular velocity) rotation output with a smooth (minimum peak acceleration) linear input, then a flywheel is necessary. If herky jerky input and/or output are acceptable, then there should be mechanisms that will work.

There is an old book available online and free that consists entirely of various mechanisms. Search 507 Mechanical Movements to find a copy. It's a good read for anybody who finds this thread interesting.
Yesterday I found a vid that overcomes 1) , haven't had time to test it in the sim yet but I think it can produce smooth or almost smooth rotational movement. The problem is that in order for it to be smooth it needs sawwave type input instead of sine wave.

I also tried to build another concept someone had, I put it in the simulation and my current implementation of it somewhat sucks, but it at least shows something like this maybe can be done.

I am still not understanding your diagram lol.
View attachment 285700

These are my questions on the first diagram also:
View attachment 285704

I'll try to describe them better!

In the first one (the sequential images):

• Yes, it's a side view, not 3d
• That thing is a slot in which the connector can slide inwards and outwards along the radius of the crank. The circle inside is both where the con-rod would attach, and what interferes with the fixed oval cam inside.
• The arrows were just what was available in powerpoint, don't pay any heed to their 3dness!
• The piston does attach to the little circle, it is being pushed left & right.
• The big circle is the crank, so it rotate about its center.
• There's no piston pictured as it's simply pushing the little circle left and right, I didn't think it was needed to draw it! My mistake!
The second one (crank in a crank):

• 3d arrow is not relevant, the design is drawn in 2d!
• "Fixed Gear Around Crankshaft" means it is fixed to the engine casing and doesn't turn, and the crankshaft passes through it to reach the first crank (larger circle)
• The little circle is a gear which is fixed to the crank, and orbits around the fixed gear as the crank turns. It turns the secondary crank in the opposite direction, with a ratio of 1:1 (so it turns once backwards whilst the crank turns once forwards).
• "is it just to show where not to put the conrod..." that circle is the central axis of the second crank, which is attached to the first crank, so the secondary crank orbits the crankshaft whilst also cranking on its own.
• <Piston here> Yes, that's right. The piston connects to the small circle and pushes it around.
• "what is this rod for" - it was just visual to mark that the final conrod position is fixed to the secondary crank and not to the first.
One day I will animate this, and it will make more sense!

some bloke said:
I'll try to describe them better!

In the first one (the sequential images):

• Yes, it's a side view, not 3d
• That thing is a slot in which the connector can slide inwards and outwards along the radius of the crank. The circle inside is both where the con-rod would attach, and what interferes with the fixed oval cam inside.
• The arrows were just what was available in powerpoint, don't pay any heed to their 3dness!
• The piston does attach to the little circle, it is being pushed left & right.
• The big circle is the crank, so it rotate about its center.
• There's no piston pictured as it's simply pushing the little circle left and right, I didn't think it was needed to draw it! My mistake!
I guess I am confused about the elongated ellipsoid in the middle, what is its function? Is that the central oval cam you refer to that rocks back and forth? I do not see it rocking back and forth in the image, and I'm wondering how it is connected to the pin. In the first one also the center circle seems offset from the others, I am wondering if there is a significance for that.
some bloke said:
The second one (crank in a crank):

• 3d arrow is not relevant, the design is drawn in 2d!
• "Fixed Gear Around Crankshaft" means it is fixed to the engine casing and doesn't turn, and the crankshaft passes through it to reach the first crank (larger circle)
• The little circle is a gear which is fixed to the crank, and orbits around the fixed gear as the crank turns. It turns the secondary crank in the opposite direction, with a ratio of 1:1 (so it turns once backwards whilst the crank turns once forwards).
• "is it just to show where not to put the conrod..." that circle is the central axis of the second crank, which is attached to the first crank, so the secondary crank orbits the crankshaft whilst also cranking on its own.
• <Piston here> Yes, that's right. The piston connects to the small circle and pushes it around.
• "what is this rod for" - it was just visual to mark that the final conrod position is fixed to the secondary crank and not to the first.
One day I will animate this, and it will make more sense!
Ok I think I have a clear blueprints of how to build this one now, one of these days if I get some energy if I will try to build an animation of it.

hmmm27 said:
Well, there you go.

After 4 pages, and coming in with that specific problem, by now you should be able to tell whether charlie-browns-t-shirt, sliding-siding, or gearpunk-porno-123 meet your requirements or not, those being - if you'll pardon the assumption -

Create a mechanism to translate (reciprocal) linear motion to continuous rotational, without using rotational momentum in any manner.

Some_bloke's solution is the most mathematically sound method.

If I may be allowed a slightly different instantiation of the concept . . .

For reference draw vertical and horizontal axes, and a circle centered on their origin.

For the mechanism of the "second input", picture
- an ellipse, similarly centered, touching the reference-circle from the inside ;
- a tiny circle, resting on top of the ellipse, centered on the vertical axis : and
- a 6,371km radius circle underneath the big circle, not touching, centered along the vertical axis (or you may want to simply assume it's there)

Movement of the ellipse is rotation around its centerpoint ; movement of the small circle - forced against the top side of the ellipse by gravity - is to be restricted to up and down, only.

(tldr; same as some_bloke's except inside-out : the cam moves, the follower is fixed)

Sorry, M\$-Paint plus a Trackpoint isn't a terribly good combination.
6,371km radius? Is that a typo, or what?

I guess I am confused about the elongated ellipsoid in the middle, what is its function? Is that the central oval cam you refer to that rocks back and forth? I do not see it rocking back and forth in the image, and I'm wondering how it is connected to the pin. In the first one also the center circle seems offset from the others, I am wondering if there is a significance for that.

Firstly, the central circle being out of position in the first one is not relevant, that's a mistake!

So, the oval piece in the middle doesn't move - it's fixed to the engine casing, no rocking or anything, it's just there to get in the way of the pin.

The pin is where the piston's con-rod attaches. When the piston pushes it to the left, the pin slides inwards on the slot (towards the middle of the crank) and it knocks into the oval cam. The oval cam being angled means this causes the pin to be pushed upwards, and this gets the crank past its initial few degrees.

From there the crank operates as normal, pushing it around to the 180° mark. As it reaches the far point, the pin slides back outwards on the slot, allowing it to clear the oval cam.

Then the process repeats in the other direction (not pictured). The pin is pulled back to the right, it collides with the cam and moves downwards, rotates the crank past the ambiguous point where it could turn either way, and starts it rotating in the same direction.

some bloke said:
Firstly, the central circle being out of position in the first one is not relevant, that's a mistake!

View attachment 286355
So, the oval piece in the middle doesn't move - it's fixed to the engine casing, no rocking or anything, it's just there to get in the way of the pin.

The pin is where the piston's con-rod attaches. When the piston pushes it to the left, the pin slides inwards on the slot (towards the middle of the crank) and it knocks into the oval cam. The oval cam being angled means this causes the pin to be pushed upwards, and this gets the crank past its initial few degrees.

From there the crank operates as normal, pushing it around to the 180° mark. As it reaches the far point, the pin slides back outwards on the slot, allowing it to clear the oval cam.

Then the process repeats in the other direction (not pictured). The pin is pulled back to the right, it collides with the cam and moves downwards, rotates the crank past the ambiguous point where it could turn either way, and starts it rotating in the same direction.
this idea is bloody brilliant.
https://www.myinstants.com/search/?name=brilliant2

if i ever get rich from this contraption (or at least make a decent profit) i shall try to give u a reasonable reward. not trying to "general electric" people out of their fair share.

had this been invented 200 years ago, this may have revolutionized society, the steam industry, cars, everything.

Last edited:
pbuk
A single cylinder engine cannot be dynamically balanced without a flywheel, so it will shake itself, whatever it is attached to, and you, to pieces. That is why we use multi-cylinder engines, they can be balanced. Happily, multi-cylinder also resolves the stationary problem at BDC and TDC.

Single cylinder steam engines were inefficient, so were rapidly replaced by compound engines having both a high and a low pressure cylinder. The most efficient steam-driven piston engines are tripple expansion engines.
https://en.wikipedia.org/wiki/Compound_steam_engine#Multiple-expansion_engines

had this been invented 200 years ago, this may have revolutionized society, the steam industry, cars, everything.
Don't kid yourself. You are so far behind, that you think you are first.
I repeat.
Baluncore said:
Before setting out to discover a better solution you need to understand the existing solutions. Replace the excitement of "uninformed independent re-discovery" with the excitement of "historical research" into the technology.
More than half of science and technology is the review of literature.

pbuk, Averagesupernova and gmax137
That's a very nice animation in Post #82, but it does not work. Keeping in mind that the goal is to convert linear motion to rotary motion without using rotational inertia (a flywheel), let's take a close look at the process.

Start with the system in this position:

The pin is bottomed in the slot. The piston decelerates to a near stop because the moment arm is very small. Small movement of the piston with large rotation means that the output torque decreases to near zero.

Next, look at the pin sliding outward in the slot:

Again, the piston slows almost to a stop. The output torque decreases to near zero. And what is moving the pin outward in the slot?

Next, at bottom dead center (BDC):

There is an instant where the pin is at the extreme tip of the elliptical cam. At that instant, the output torque is zero. Since the assumption is zero rotational inertia, the mechanism stops at this point. There is a similar, but not shown, effect at TDC.

Next, the pin is sliding outward as the piston moves away from the crankshaft:

The animation shows the piston at a dead stop while the pin slides outward, and does not show what is making the pin slide outward. With the piston at a dead stop, the output torque is zero, and the mechanism stops.

These types of mechanism can be analyzed by starting at a point, typically TDC, rotating the output shaft in small steps, and calculating the position of the piston at each step. Piston velocity and acceleration is calculated from the piston positions. Piston position, velocity, and acceleration is then plotted against output shaft position. Output torque is calculated from the ratio of piston velocity to crankshaft angular velocity.

Since you are comparing this to a conventional crankshaft with flywheel, you need to do the same analysis, using the same methodology, to a conventional piston with crankshaft and flywheel. Look closely at the peak piston acceleration (about 2000 G's), and the sizes of the wrist pin, connecting rod, and crankshaft. then consider the peak loads in your mechanism and the strength of the parts needed.

While this thread has been entertaining, it's time for the OP to do some actual analysis and less wishful thinking. The OP will do an analysis as indicated above, or this thread will be locked.

Averagesupernova and berkeman
Baluncore said:
Don't kid yourself. You are so far behind, that you think you are first.
I repeat.
Not me, but I think the user known as "some bloke" is the first to invent this. You may be right and maybe this was invented already, I'm not 100% sure.

Single cylinder steam engines were inefficient, so were rapidly replaced by compound engines having both a high and a low pressure cylinder. The most efficient steam-driven piston engines are triple expansion engines.
Early steam locomotives had a problem of getting stuck at BDC, then the locomotive would start to go in reverse, this invention would have revolutionized that. My comment about this potentially revolutionizing cars may have been hubris though.

More than half of science and technology is the review of literature
pbuk suggested looking into the Neilson steam locomotive, unfortunately the book about it seems to be missing. Maybe society needs to do a better job at preserving the literature?

https://openlibrary.org/books/OL14474327M/Neilson's_single_cylinder_locomotives.
jrmichler said:
That's a very nice animation in Post #82,

thankyou

jrmichler said:
but it does not work. Keeping in mind that the goal is to convert linear motion to rotary motion without using rotational inertia (a flywheel), let's take a close look at the process.

Start with the system in this position:
View attachment 286432
The pin is bottomed in the slot. The piston decelerates to a near stop because the moment arm is very small. Small movement of the piston with large rotation means that the output torque decreases to near zero.
Yes but this is a universal problem with all crankshafts not just this one, when near TDC or DBC its harder to create torque.
jrmichler said:
Next, look at the pin sliding outward in the slot:
View attachment 286433
Again, the piston slows almost to a stop. The output torque decreases to near zero. And what is moving the pin outward in the slot?
I stopped applying input to the piston, then it is being carried by inertia. Admittedly, not the best example to prove that this device works, could have uploaded a different test run with more friction and showing the action at BDC.
jrmichler said:
Next, at bottom dead center (BDC):
View attachment 286434
There is an instant where the pin is at the extreme tip of the elliptical cam. At that instant, the output torque is zero. Since the assumption is zero rotational inertia, the mechanism stops at this point. There is a similar, but not shown, effect at TDC.
That picture does not show the pin at the tip of the elliptical cam, but underneath the elliptical cam (ellipsoid). In this pic it is almost at TDC, when it is at TDC it will be underneath the ellipsoid.
jrmichler said:
Next, the pin is sliding outward as the piston moves away from the crankshaft:
View attachment 286435
The animation shows the piston at a dead stop while the pin slides outward, and does not show what is making the pin slide outward. With the piston at a dead stop, the output torque is zero, and the mechanism stops.

These types of mechanism can be analyzed by starting at a point, typically TDC, rotating the output shaft in small steps, and calculating the position of the piston at each step.
I performed that analysis as requested, the device works (depending on the friction inserted and the amount of power to the piston.)

jrmichler said:
Piston velocity and acceleration is calculated from the piston positions. Piston position, velocity, and acceleration is then plotted against output shaft position. Output torque is calculated from the ratio of piston velocity to crankshaft angular velocity.
Torque can only be calculated by force, momentum, mass of the piston, not velocity alone. A crankshaft made of styrofoam would have fast angular velocity but not much torque.
jrmichler said:
Since you are comparing this to a conventional crankshaft with flywheel, you need to do the same analysis, using the same methodology, to a conventional piston with crankshaft and flywheel. Look closely at the peak piston acceleration (about 2000 G's), and the sizes of the wrist pin, connecting rod, and crankshaft. then consider the peak loads in your mechanism and the strength of the parts needed.
I am not sure what the peak piston acceleration of a car engine is, but I am willing to consider my original comment that this could have revolutionized cars, may have been hubris. That being said, the device should in theory work, but maybe not as a car engine. A website on marine engines said 221 m/s/s which is 20x the acceleration of gravity.
http://marinediesels.info/Theory/piston_acceleration.htm

jrmichler said:
While this thread has been entertaining, it's time for the OP to do some actual analysis and less wishful thinking. The OP will do an analysis as indicated above, or this thread will be locked.
When I was analyzing this with a fine tooth comb, there was a brief moment I could see why you think it doesn't work. The reason why it works is because when at BDC when pushing the piston forwards it cannot decide to do a clockwise or counterclockwise rotation, but leading up to BDC it always chooses to do a clockwise rotation. Then when the piston retracts, it cannot decide either but the ellipsoid makes the decision for it.

that being said, when I increased the friction of the crank to 40nm it got stuck. But IRL it should work in theory if the parts are solid, well oiled and don't bend too much. How much it rotates is entirely dependent on friction and how much force is being applied to the piston. If there is too much friction it will get stuck and not rotate past the apex of the ellipsoid. This can be fixed by adjusting the ellipsoid angle more, reducing friction, or increasing power of the piston. Further optimizations could be done by experimenting with ellipsoid angles and slider lengths.

Early steam locomotives had a problem of getting stuck at BDC, then the locomotive would start to go in reverse, this invention would have revolutionized that.
That was not a problem. It is easy to rotate the flywheel through about 5 degrees over BDC or TDC since there is very little piston movement at that point in the cycle, and the wear in the bearings gives some freedom. Only in a new engine with tight bearings might it have been hard work to advance the phase by hand.

In single-cylinder steam engines there is little difference between BDC and TDC because steam pressure is applied alternately to both faces of the piston, in effect eliminating the dead point at BDC. I remember when starting and driving a steam traction engine in the late 1960s. It did not matter if the flywheel started and moved initially in the wrong direction, that just put it in a better position to start in the required direction. The direction was decided by the valve gear through a Stephenson linkage. If it stalled and did not start forwards, then flipping the valve drive linkage to reverse and then forwards again, always resolved the problem within a couple of seconds, and no one noticed the quickly-learned natural process.

Any single cylinder engine needs a flywheel, or a weighted crankshaft, simply to balance and smooth the power flow to the drive. Arguing that a flywheel is not needed, or can be eliminated, is pointless since we no longer use inefficient single cylinder steam engines, and the inertia of the driven load was sufficient to maintain rotation once things were moving.

Yes but this is a universal problem with all crankshafts not just this one, when near TDC or DBC its harder to create torque.
Which is the exact problem you are trying to solve. Any practical mechanism for converting linear to rotary motion without a flywheel must meet several requirements:

1) It must work with a significant friction load on the crankshaft.
2) It must work with a roughly constant input force. That's why I asked you to calculate the ratio of input force to output torque for a complete revolution, and compare to a conventional slider crank with flywheel.

You have not done so. Thread closed. If you want the thread reopened, PM me with the appropriate calculations.

hutchphd and Averagesupernova

• Mechanical Engineering
Replies
8
Views
3K
• Mechanical Engineering
Replies
3
Views
4K
• Mechanical Engineering
Replies
6
Views
5K
• Mechanical Engineering
Replies
1
Views
2K
• Mechanical Engineering
Replies
11
Views
1K
• Mechanical Engineering
Replies
7
Views
1K
• Mechanics
Replies
7
Views
4K
Replies
10
Views
2K
• Mechanical Engineering
Replies
2
Views
2K
• Mechanical Engineering
Replies
6
Views
2K