Geneva Size Limitations

[John T]

Hi Art,
On the little set I've made (fit on a 4 by 5" mount) I've had a problem of the pin just bumping into the Geneva and not sliding down into the pocket.  I've "carved" it out until I get the motion (slides into pocket)  but obviously that's not what was intended I'm sure.

The lantern in 1.25" on the DP and the Geneva is the same.

My question is - Is there a lower limit to which the Geneva's should be cut or is it just the material I'm using (MDF)?

John
PS Still having fun.

[ArtF]

John:

  Its likely an issue of tolerance. In a perfect world the "pins" are actually bearings. This is because the pins do not roll
over the respective curves in lanterns, but rather rub as they move. In inverted geneva's, its common to have looseness
at the changeover point, these ones have only a very small amount, so I suspect sanding them to an appropriate contact
angle ( the vector of the point at where the incoming tooth hits the pin when its slightly out of position ), is normal, though I can look into narrowing that angle so it hits at a more deflective angle. ( this is much like the point where a
deadbeat hits the pallet arms, that angle is usually optomised for a power vector less than 90 degrees.
  Ill look into if I can modify it to something liek 45 degrees.. In the meantime, the material probably dictate how much one woudl have to sand that point..

Thx for the report,
Art

[John T]

Thanks Art,
Other Geneva's that I've made don't have the problem so it's no big deal for me.

John

[ArtF]

Hi John:

  Checking the numbers, it seems that when the tooth just engages the
released wheel, there is a small degree of rotational slop ( which is normal
in this type of geneva ), and this means the entry pressure angle will be 45 degree's
or less depending on the position the wheel takes at engagement. The large the wheel
the more rotational pressure is exerted on the pin to make it turn into its slot. This
means I can see where a very small gear coudl get excessive pressure and bind if
there is much friction. Explains the result you found at any rate.

  Let me kow how things work out for you. I take it your finding the larger the geneva the better
the pin meshes during rotation?

Thx, preciate the feedback
Art

[John T]

Hi Art,
I'm finding that on 2 inch geneva's its important that the outer edge of the geneva is as close as possible to the pins (lightly rubbing) and then it goes fine.

John

[ArtF]

Thx John:

  I noticed that the pinion is set for a particular radisu to match a particular wheel. In other words you have to ensure that when you dropa pinion, no more pins are added to the wheel before its dropped. This ensures the pinion al;ways rubs the pins. I guess the trick if one wshes to use various pinwheels on one driver, is to make the pinion with the smallest pinwheel first, place them both, then increase the wheel sizes. You can probably get away with that.

Thx for the testing though, its nice to know these things.. :)

ARt

[John T]

Hi Art,
The Geneva's I've built so far have all been "sliding" pins - Is there a way to provide for a "roller" pin in the Geneva design.  I can easily make rollers by using a piece of brass tubing over a steel pin.  However to do that I need one size for the pin and another for the roller - when I change the pin size it changes the whole Geneva design and I can't use gCode to drill the pin holes.

Hope that makes sense.

John

PS I can easily live with sliding.

[danmauch]

Why can't you simply make two drawings? One that shows the pin holes and one for the roller diameters. Then merge the two drawing and delete the parts you don't want.
Dan

[ArtF]

John:

  Dans suggestion is a good one. The pinwheel doesnt really change when you change pin radius, only the pinion tooth changes. Id make a small pin version and a large bearing version, then use the pinion from the large bearing version
and only use the smaller pin version for drilling purposes..

Thx
Art

[John T]

Thanks Guys that should work well.

I'm also going to try making the pin holes 1/8" and using the 1/8"OD Brass tubing as an "insert" up to the surface of the Lantern wheel, and then inserting the 3/32" rod inside of that and finally the roller will be the 1/8" tubing installed over the 3/32" that the "tooth" will engage.  I'll let you know how it progresses.

John

[John T]

Hi Art,
I've made a Geneva with rollers and while it does turn a lot easier, it still seems that the roller strikes the geneva tooth at a very steep angle - doesn't look like the 45? you said you thought it would. Am I doing something wrong?

[ArtF]

John:

  Looks liek your hitting just above 45 degree's..not your problem, more mine.
You ve proven I think that the angle is less than optimal. I may have to change it to a
much lower angle to work smoothly. The problem is that its a tradeoff. Look at this
more typical example of a geneva inverted.
http://newgottland.com/2013/02/06/inver ... ecahanism/

  As you can see my design tries to eliminate as much of the "looseness" in the
changeover from tooth to lock phase. Whats your opinion, you have more experience
at this point that I do in terms of how the inverted design runs? Woudl you like more
"looseness" factored in to get a more hi power changeover hit angle?

Art

[John T]

Hi Art,
I like how positive the action is "lack of looseness"  its just the actual entry of the pin on the tooth.  To get the action and feel that appeals to me I've "ground" in an approach that is more gently.  Since I don't have the vocabulary I'll post a photo instead.  The pencil marks were put in to mark the initial contact and then ground down toward the gullet.  I made no change in the actual gullet nor the tip just the "approach slope.  A very slight rounding on the exit slope stopped in binding on the way out.

Forgive the colouring  -  I was bored.

John

[John T]

Hi Art,
I thought I'd try something - I made a Geneva wheel with 4 teeth spaced as close as possible.  The entry into the teeth has the same bumping problem but once the teeth are engaged its smooth as silk.  I don't know if this helps you but I thought it might.

[ArtF]

Yep, thats the tradeoff, between a positive locking action or looseness. I like your solutioon though, that looks like it lowers the pressure
angle enough for a more posative engagement. Ill see if I can roughly duplicate that within a couple releases. I appreciate that kind
of feedback, saves me gobs of time..

Thx
Art