GearHeads Forum

Hi Justin:

 
When it comes to the world of "strange" gears, like geneva's, or inverted geneva's ( which is what mine are), there
is no real reference. Its all part of the gearing world where unique or strange forms enter the domain. In this type of gearing
here is no spec or right or wrong way, its always just a question of "Does this work?". If you search hard on the web youll find
any manner of strange mechanisms designed specifically to have features that match a particular need. Clocks are that way on steroids.
Many unique forms are out there, usually designed by an individual who needed an answer to a particular problem.

  The greatest problem when discussing these is language, humans dont speak in a language that easily adapts to mechanisms, hence everyone tends to speak in abstracts, like looseness or deterministic engagement. But I digress...

  The inverted geneva does have looseness in the capture point, mine less than most as I took the liberty of adding more cycloidal
tooth to mine. A true geneva doesnt, but neither is a true geneva a gear by tight definition,  its more of a mechanism. Look at a true
geneva and youll see, as in your photo, that it has several parts, not just two gears, it has a separate locking layer which really is two more gears,
and they function as a 4 gear set really which we term a geneva mechanism, more so than a geneva "gear" set.


  Im experimenting to see if I can do more to allow a better subset of such things.

Art

I'm trying to find references to this type of gearing -- are there alternate terms for it? This isn't the "Geneva Drive" I'm familiar with.

I see that it is a form of lantern/cage gear -- with an intermittent action. It doesn't have a the deterministic transmission usually seen in a gear set. There is a position of the cycloidal gear at which the lantern gear can bump back and forth.

Do you guys have links to any on line references for this gearing?


---I found this, which is sort of similar, but it is driven by the pinned gear, not the cycloidal toothed gear:

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

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.

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:

  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

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?

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:

  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

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