My weird geneva mechanism doesn't work. Looking for suggestions

AI Thread Summary
The discussion revolves around the challenges faced in designing a non-traditional Geneva mechanism for a toy project. The creator lacks a physics background but aims to minimize parts while achieving a functional design. Key issues include jamming and dragging of gears, attributed to the absence of a locking mechanism, which is essential for proper operation. Suggestions emphasize the importance of incorporating a lock to prevent freewheeling and ensure correct tooth engagement. The creator acknowledges the need to address these issues and refine the design based on feedback received.
DefinitelyAnEnjinear
Messages
10
Reaction score
6
TL;DR Summary
I designed a weird geneva mechanism where the driving pin is mounted on the geneva wheel and pushes against gear fixed to the central axis to rotate the geneva wheel.
Two things that I think I should say before I describe what I did: First, I do not have any physics-related background (but I have a degree in CS, and I'd like to think I have a good enough grasp on math to be able to understand equations describing motion of objects). So please forgive me if I'm missing things that may seem obvious to you.
Second, there's a (good, in my opinion) reason why I went for this approach rather than just using a normal geneva mechanism, which will be explained in part 2.

Part 1: What I'm going for​

The parts I designed, rendered:

images.png


And in action:
here it is working!


and here it is, without cheating, not working as intended.


The idea is that what would've been the driving pin in a normal geneva mechanism (here, a gear with only 1 of its teeth, located at the bottom) pushes against a gear that doesn't rotate, mounted on the central axis, and causes the thing it's mounted on to rotate.

when I drive the thing with my hand, it works fine. but when I place the gear that's supposed to drive the whole thing on the center axis, it either jams when the single tooth at the bottom engages with the gear at the bottom, or it simply gets dragged along with the big gear (as @Baluncore pointed out, I did not include a locking mechanism - I never understood why it's needed in normal geneva mechanisms, so I've learned to ignore it, but clearly it's needed here) (the jamming is harder for me to understand, the dragging along isn't much of a mystery to me).

Part 2: Why not use a normal geneva mehanism?​

I think this can be boiled down to my set of priorities.
This is all for a toy - it's not going to be subjected to any serious stress, there is no cost if it breaks, no maintenance... It's just something designed for fun by someone who barely knows what he's doing.

What is important to me is using a minimal number of parts however, and this approach makes it easier because of what I'm going to use it for:

Untitled.png


I have these 8 knobs (each attached to a stick; not depicted). I need a mechanism to engage with each one, push it away, then pull it back in, and then disengage with the knob before moving on to the next one.

My simpler idea was to use a very simple cam to guide each of those
rough sketch.png

Imagine the blue circle is centered, the black lines inside the square are the cam, the red arrows depict the direction the cam is supposed to push the knobs as the cam rotates around the blue circle. I hope you have a vivid imagination because I'm not great at drawing.

However that didn't work out because the cam applied too much force on the knobs to the side rather than forward/backward, and I would have to make the whole thing a lot bigger to make it work, as far as I can tell.

So instead I had the idea of making this weird geneva mechanism, figuring the smaller gear (the one not missing teeth) could double as a cam for a mechanism (to be added later), and I could use its rotation (minus the part where the bottom, lone tooth engages with the bottom gear) to move another cam-and-follower mechanism that will push and then pull the knob.

in addition to having a fewer number of parts, I figured that (assuming I have no choice but to have a cam-and-follower mechanism orbiting around to engage with each knob) I would have no choice but to have a rotating gear at the center of the circle (only place to put a gear to ensure it's always in contact with another gear orbiting the center), so rather than driving the geneva mechanism and the orbiting cam-and-follower with 2 gears, I could combine it and make the whole arrangement of what I'm designing easier.

After a few failed attempts, I am here asking for your help (and forgiveness for any sins I may have committed against engineering)
 
Last edited:
Engineering news on Phys.org
Welcome to PF.
The geneva mechanism requires two things.
1. A pin or tooth that advances the mechanism by one step.
2. A lock that holds the driven gear when it is not being advanced.
I can not see a lock in your design.
 
  • Like
Likes DefinitelyAnEnjinear and Bystander
Baluncore said:
Welcome to PF.
The geneva mechanism requires two things.
1. A pin or tooth that advances the mechanism by one step.
2. A lock that holds the driven gear when it is not being advanced.
I can not see a lock in your design.
Ah, the part I never paid attention to. of course.

But it wouldn't solve my jamming problem

Edit: I'm suddenly having memories of considering adding the lock but not knowing how to calculate when exactly the teeth would start meshing, though given that I can extract the outer radius of both gears and I know the distance between them it should actually be trivial, so at least I can fix the dragging part it seems.
 
DefinitelyAnEnjinear said:
Ah, the part I never paid attention to. of course.
Of course.
The lock part of the mechanism serves two distinct but coupled purposes;
1; It prevents the system freewheeling while it is not being advanced, and;
2; It sets the required tooth engagement position for the next state change.
Designing a new mechanism, (or state machine), requires you make a complete list of requirements that must be satisfied by each component. Prototypes that fail must revise that critical specification.

The transition between states is the critical part of any design.

If a later request for an engineering change is made, you must consult the list and verify satisfaction, before permitting the change order.
 
Posted June 2024 - 15 years after starting this class. I have learned a whole lot. To get to the short course on making your stock car, late model, hobby stock E-mod handle, look at the index below. Read all posts on Roll Center, Jacking effect and Why does car drive straight to the wall when I gas it? Also read You really have two race cars. This will cover 90% of problems you have. Simply put, the car pushes going in and is loose coming out. You do not have enuff downforce on the right...
Carburetor CFM A Holley Carb rated at 500 cfm 2 barrel carb has venturi diameter of 1.3/8". There are 2 barrel carbs with 600 cfm and have 1.45 diameter venturi. Looking at the area the 1.378 bore has 5.9 sq. Inch area. The 1.45 dia. has 6.6 sq. inch. 5.9 - 6.6 = 0.70 sq. inch difference. Keeping the 500 cfm carb in place, if I can introduce 0.7 sq inch more area in the intake manifold, will I have the same potential horsepower as a 600 cfm carb provide? Assume I can change jetting to...
I'm trying to decide what size and type of galvanized steel I need for 2 cantilever extensions. The cantilever is 5 ft. The space between the two cantilever arms is a 17 ft Gap the center 7 ft of the 17 ft Gap we'll need to Bear approximately 17,000 lb spread evenly from the front of the cantilever to the back of the cantilever over 5 ft. I will put support beams across these cantilever arms to support the load evenly
Back
Top