geneva cam assembly - intermittent relation

Josh_20

Hello all,
Is there a way to make an intermittent relation to allow a Geneva Cam mechanism to work ? 

Public Geneva Cam Document 

philip_thomas

@Josh_20 - No, Onshape does not currently support intermittent contacts.
That said, there are solution partner apps that might be what you're looking for.
Check out Simwise4D in the simulation section of the app store.

philip_thomas

@Josh_20 - No, Onshape does not currently support intermittent contacts.
That said, there are solution partner apps that might be what you're looking for.
Check out Simwise4D in the simulation section of the app store.

mahir

It's a longshot, but you might be able to create a path surface that can be used with a tangent mate. The plan view would look something like the green line below, but you'd have to change it's elevation to make sure it doesn't intersect itself. I didn't draw it very well, but the large radii correspond to the distance from the blue pin to the dark gray pin, so no motion would occur at those locations... in theory. Like I said, it's a longshot but worth a try.

philip_thomas

Despite @mahir awesome freehand sketch the set of faces that the pin would have to be tangent to, also have to be continuous (meaning that the track cannot intersect itself) - i don't see how that could be achieved. Additionally - there should be only one solution - meaning the pin cannot simultaneously touch both sides of the slot.
Finally - the guy that wrote the code (Bo) has a Phd in '3D kinematics' and is rolling his eyes at me!

konstantin_shiriazdanov

@mahir thats cool idea but i'm afraid that tangent constraint in assembly doesnt work with surfaces with spline cross section. though if we had "on path" constrain it would be a lot easier to make complex assembly motion

mahir

Oh yee of little faith. I haven't tested it since I don't have access to the Geneva parts, but this is the kind of surface path I was talking about. It is constructed purely of tangent arcs and straight lines, not splines.So it should be compatible with a tangent mate. It also does not intersect itself. Nor does it touch the 5mm pin at multiple points. Might not work, but like I said, it's worth a try.

mahir

Here's a link to the part studio in case someone wants to try throwing it into an assembly.

Btw, OS's "add" functionality with surface part patterns needs some work. Surfaces that should merge just fine don't merge. The only way I could make the above surface one piece was by patterning the path and creating a single sweep. I should have been able to sweep 1/12 of the whole and create the rest with 2 circular patterns. But doing that results in 12 separate surfaces.

https://cad.onshape.com/documents/57acdfaae4b005c413ed9b6f/w/3fd585a46d3af1b3ba413c53/e/fc18786802f2ffec50ebb4a3

philip_thomas

@mahir - WOW!

philip_thomas

That said - there are 6 points on your 'track' where there are two solutions - i am therefore dubious this would work (of course, i have been wrong before)

mahir

I haven't futzed much with tangent mates. If OS doesn't internally extend surfaces, then it should work ok. But if OS treats the surface as one big self intersecting extrusion, then yeah, that won't work.

mahir

Nope, doesn't work, but not because of the intersections. Tangent just doesn't want to accept any part of that surface. Even if I try using the non-patterned, non-intersecting version, tangent acts like it's an unsupported surface, even though it's really just some extruded lines and tangent arcs. Odd.

Josh_20

@mahir That's amazing stuff! Even if it did work would that require the pin and attached drive wheel to be travelling free to rotate around the driven cam? In the Geneva Mechanism both the axles are fixed. 

mahir

@Josh_20, it's moot since the surface doesn't work with the tangent mate, but the intention was to simulate the intermittent rotation of the Geneva drive. If the tangent had worked as planned, a hidden dummy pin could have been used that was concentric to the drive pin, but free to move axially. Dare to dream

jon_sorrells

Here's one I made: https://cad.onshape.com/documents/c6d9b482ab29c1ec8e1cf3b6/w/83b692918163273cacba0ae5/e/bc5a5ff3e2a8d1276c472b1b

I made the slots larger than the pin, because the tangent mate was having trouble when the slot was the exact size to fit the pin.  There's still more than 1 solution to the tangent mate, so it sometimes won't give the result you want.

mahir

Duh. I was going for exact, but this probably gets the job done. Just attach to an invisible surface mated to the real gear, and that'll do the trick. Nice.