Showing motion where there are inter-related revolute joints

[R4SMEs]

I am developing a model of an expanding table of the so-called butterfly type,
in which there are two wooden panels joined by a hinge (i.e., a revolute joint) and one of the panels (the upper of the two of the expansion part) has another revolute joint to the structure, i.e., to the framework of the table.

I have successful implemented the components and their revolute joints.
However, I should appreciate advice on how to drag the joints/ animate the overall expanding table such that it behaves/ performs in a reasonably realistic way.

My main objective is to test the motion and check how parts inter-relate to each other.

The overall concept for the table can be seen by looking at the very start of the following YouTube video which shows the expanding leaf being opened and closed in a similar type of table.
https://www.youtube.com/watch?v=rI6Uz4gt_-M

As can be seen from the demonstration of such a table, it is not only the revolute joints but also the weight of a panel that comes into play.

As one part is lifted, the other part tends to either stay where it is (when sitting on the bottom support in the low position) or to move down by itself, as a result of its weight.

Unfortunately, this effect of gravity is something that is not represented in the Fusion model!

ElipticalButterflyExpandingV30_3Dview.png provides a 3D view of the Fusion model.

ElipticalButterflyExpandingV30_EndView.png
This shows the view from one end, including the drawing that was used to extrude the two wooden panels joined by their hinge and the position of the revolute joint between the upper of the two panels and the structure of the table.

I tried to do a generalised drawing.
For this reason, there is a gap at the left between the moving panels to take into account that there may, in general, be a small gap between the panels where they close at the hinge.
[This gap is currently exaggerated for testing purposes, i.e., in the final implementation the gap will hopefully be close to zero.]

Equally, I have the moving panels thicker than the main top of the table (which is seen in cyan).
In practice, the thicknesses of the moving panels will be reduced to be similar to that of the table top itself.

It was an interesting challenge to define where the pivot point between the top of the expansion panels and the table itself should be positioned.
This is seen as the position of the revolute joint in the centre of the image, above the two panels (i.e., the blue and brick red panels).
It is the blue panel that is joined by this revolute joint to the table structure.
[It is seen apparently floating in mid-air but in practice there would be a block of wood attached to the top of that blue panel with an axle going through it, as in the YouTube video.]

To define the position of that upper pivot point, I drew the two construction circles and constrained their centres to be concentric (thereby defining the position of the pivot point).

By constraining the two corners of the upper (blue) panel to be on these circles, it ensures that the top of the blue panel will be flush with the top of the table (cyan) when the blue part is lifted/ rotated into its raised position.

Incidentally, I should mention that there is more than one solution to the geometrical requirements based solely on the two construction circles.
The lower resting position (left to right position/ height/ angle) can vary.

I used "Fix" for two lines in the sketch (note the two green lines at the bottom and left edges of the lower rectangle of the sketch) to fix the location of the sketch corresponding to a particular solution.

In practice, there are other constraints that need to be considered including ensuring that panels in their lowered position fit within the available space.
[They rest at the bottom on a horizontal piece of wood, which is currently drawn in an artificially low position (this is seen at the bottom right of the end view).

At the top, in the stored position, they need to be lower than the bottom of the T-shaped runners which move into this area as the ends of the table are pushed together.]
Also, as the moving parts are lifted up to the raised position, we need to ensure that they clear the apron at the side of the table.

For these and other reasons, it is important to be able to play with/ test the motions ...

Being hinged at its left (to the upper blue panel), the left edge of lower (brick red) panel has to follow the edge of the upper panel.

In practice, its weight would keep it more or less horizontal but, unfortunately, the Fusion model doesn't appreciate this fact!
Consequently, when dragging on either panel, they move in unrealistic ways.

Furthermore, it is difficult to even move things into particular settings (such as "expanding leaves fully up").
I suppose that could be done by playing with joint limits and joint rest positions - but this doesn't solve the issue of how to view the motions/ animate things in a reasonable way.

I can provide the model if anybody is interested and wants to play with it/ hopefully assisting this endeavour.

[TrippyLighting]

The joint to be used for this sort of motion (if I understand the video properly)  is a pin-slot joint.

I'll see if I can rig up a model to demonstrate how that works. The Pin-Slot joint is a bit tricky.

[R4SMEs]

I'm not sure that a pin-slot joint is involved anywhere.
Physically, the system certainly involves only two revolute joints:
a) The hinge between the two panels of the expanding part of the top (they are hinged at the left).
b) The axle about which the upper panel rotates.
As I already mentioned, the upper panel is attached to the axle via a block - you can see that on the video whilst in my model, that upper pivot point is implemented in a virtual way at the moment since I haven't drawn the block and define the pivot point simply by a point.

However, maybe a pin-slot joint can in some way emulate the functionality of the two revolute joints?

I attach the model which will facilitate understanding.

Note that in the model the history has been (and should be!) rolled back to before the "Butterfly assembly OLD" component.
I previously drew (and extruded) the expansion panels from above, i.e., in their raised position.
That approach was unhelpful in defining the geometry associated with the pivot points and their movements.

So I subsequently changed to do the drawing from the end view (and moved the old stuff to the end of the history, from where I have rolled back to suppress the old approach.
I expect to delete that "Butterfly assembly OLD" component in due course.

[davebYYPCU]

I think you are after the Assemble > Motion Study, not done many of them.

Slide the table top open,

then rotate the folded pair,

then open the folded pair.

Select the study, and then Edit, there are play buttons on the bottom left....

Might help....

[R4SMEs]

Many thanks.

This approach looks promising - I shall have to study the subject of Motion Studies.

I don't understand why the brick red (lower) panel jumps suddenly from being together with the blue panel to going to its final raised position at the other side of the table.

There is a big hysteresis in the behaviour - after that sudden jump, going back in single steps doesn't reverse the motion before a lot of steps.
Such an effect seems inconsistent with the linear slopes of the MS graphs.

The associated revolute joint between the panels jumps through the full approx. 180 degrees for just one step of the Motion Study, despite the graph having a linear slope.
The (linear changing) blue curve of the MS doesn't seem to be followed.

The panels should start to gradually rotate apart at their hinge from the start of the upper panel being lifted since gravity will result in the lower panel staying where it is.

I believe that, in usage (as seen in the YouTube video), only the upper panel is lifted and the lower panel gradually moves itself into position.

At present, the MS simulation is like lifting the two panels held together then throwing them apart!

[davebYYPCU]

I saw that, tweaks to the ramping, and timing may help, as said, not done much with these.

[TrippyLighting]

@R4SMEs Before you attempt to make any joints or motion studies you should really create all the parts/geometry to make the joints. 

[TrippyLighting]

While part of the movement can certainly be approximated with a pin-slot joint the entirety of this cannot really be simulated with the existing joints only. In the video the "free" leaf when full expanded contacts the apron.

Once the rotating leaf starts to move the free leaf will start contacting a lower rest piece and loose contact with the apron. This is necessary so the free leaf can be fully lowered below the sliding table top.

In order to simulate this properly, during the simulation one joint would've to be suppressed and another one be activated. Ultimately this is an area where contact sets should be able to do the job. However, none of my experiments with contact sets have been very successful. In the transition from the apron to the lower rest piece the contact sets in whatever combination I've used just ignores that lower rest piece.

A word of caution: Contact sets are resource hogs and should only be used early on in simple designs to study motion. Once the motion has ben studied and the design verified, disable the contact set so it does not interfere with the normal joins in the design.

@jeff_strater should this work or am I expecting too much here ?

[R4SMEs]

Thanks for your continued efforts to solve this challenge!

In an actual table, there are more than one way to lift the pair of expanding leaves from their lower (stored) to their raised (table expanded) position.

One could lift the upper of the stored leaves, with the lower one following by itself (whilst being subject to gravity, which would tend to open the hinge between the panels).

Alternatively, both panels could be lifted together from the bottom, and only later in the lifting process (by using two hands) separate them at their hinge.
In this sense, there are an infinite number of ways in which the leaves can be lifted into position from their stored to raised position.

If approximated by a pin-slot joint (if actually possible?), in effect, a different design of table mechanism would be designed in which the motion is guided along one particular route.
That could be an interesting design and quite possibly even a better design - but it would not be the same table which is using the two revolute joints.

I have refined the overall model and moved some parts closer to their expected final positions.
I attach the current model.

I have included an axle to better illustrate where the pivot point is (previously it was only shown by a point).
The axle is apparently floating in air since I have yet to draw the supports for the axle and bearing components in which the axle will rotate in the table structure.
However, the functionality of the revolute join is unaffected by such details.

I worked to constrain (fully define) the pivot point in the relevant sketch (which is located within the "Butterfly assembly" component), i.e., the sketch which I have named "YZ plane at centre".

Previously, I had used "Fix" on the bottom line of the lower panel in the sketch to change it (i.e., lines of the rectangle of the lower panel) from blue to green.
Now I have defined a parameter for the angle of that lower panel in its stored position relative to the horizontal.
By adding that, the sketch changed from blue to black, i.e., became fully constrained (as it had been previously by using the "Fix").

This change has, however, resulted in it now being only possible to rotate the upper revolute, whereas previously I could also exercise (by dragging components) the revolute joint at the hinge position.
I am perplexed as to why it is no longer to do that.
Any ideas?!

The final design will likely have a horizontal lower resting position.
I added the angle parameter to facilitate exploring the effects/ possible benefits of having an angle - maybe clearances can be improved, etc.
The angle can, of course, be changed to 0.001 deg to achieve the horizontal position.

[TrippyLighting]

This isn't perfect, but it should point you in the right direction. I believe the curved slot can be a straight slot as well.

[chrisplyler]

I approximated it with a linkage component that I can just hide. I only extruded the linkage for demonstration purposes here in my video. You could, of course, just put the joints on the sketch points without creating any physical geometry.

[TrippyLighting]

That is in essence the same solution s mine 😉

[R4SMEs]

[In reply to TrippyLighting (message 10)]
Many thanks for your effort!

It is an interesting idea and a nice implementation of a table.
However, you have effectively designed a different table which includes the extra pin-slot joint.

The table as seen in the YouTube video and a similar elliptical one that I am trying to model do not have the pin-slot joint.

As I wrote in my previous post, with the two revolute joints (which the tables I am talking about only have), there are an infinite number of ways (i.e., trajectories) in which the folded leaves can be lifted from their lower stored to the upper (in-use) position (and vice versa as they are lowered).

By adding the extra pin-slot joint, you in effect constrain the possible trajectories of the motion to only one.
As such, the approach could be useful as a tool for evaluating the movement for that particular trajectory.
[A "virtual" pin-slot joint can be used for that purpose - "virtual" in the sense that no slot would be physically provided for it.]
Different slot curves could determine different trajectories.

Nevertheless, this leaves open the issue of how one can move the two folding panels about (either by manually dragging them with the mouse) or by a motion study?

davebYYPCU showed the potential of a motion study.

However, as I replied, there are some strange aspects:
a) despite the linear definition of joint movements in the MS, a "snap" effect occurs whereby a sudden rather than the gradual movement that would be expected occurs, and
b) there is a hysteresis effect, whereby single stepping backwards through the curves of the motion study produce a completely different effect compared to that of single stepping forward.

Are these bugs of the Fusion motion study or is there some mistake in the way it is being implemented?

[R4SMEs]

chrisplyler is also reiterating what I just wrote (our posts crossed).

Such approaches (whether an extra pin-slot joint or a bar constraining the motion) serve to restrict the possible trajectories to just one.

In the real table, a person with two hands can rotate the leaves into position in an infinite combination of rotations of the two revolute joints.

I attach a patent from the year 1924 - it looks like Mr. Fox beat chrisplyler by over 100 years!
So no chance of patenting that table design 😞

[chrisplyler]

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@TrippyLighting wrote:

That is in essence the same solution s mine 😉

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Then I'm almost as geniusy as you are!

@R4SMEs  You wanted to be able to animate the leafs. We've shown you a way to do it. You're right, of course, that locking them into any certain movement for an animation prevents you manually moving them around in their full range of real-life motion. You can't have it both ways simultaneously. An animation requires defined movement. If you want to go back to free movement, you'll have to suppress those joints that restrict the movement for the animation, leaving only the main pivot and the hinge pivot joints active.

[R4SMEs]

I am not so interested in the animation aspect.

Animation would be an extra unexpected bonus but was not what I was asking about (refer my first post).

I am more interested in just being able to manually drag the joints around to explore the range of possible motions that a user may subject the components to.

In that way, whether or not parts can catch on each other could be identified (before making the table!).

As I have explained, unfortunately, I am currently unable to do this (whether from my lack of expertise or whether from there being bugs in Fusion).

Furthermore, referring to the @davebYYPCU motion study, using such an approach it would seem possible to define the relative time sequences of joint motions
(without the need to introduce extra "virtual" joints to constraint the motion in artificial ways).

However, as can be seen, the MS does not behalf in ways that would be expected - I mention above the sudden jump and strong hysteresis effects.

[chrisplyler]

Well, a Motion Study acts to define/limit movement also. So if you want to just freely drag the leafs around, create only the main pivot joint and the hinge joint. Define appropriate joint limits. Maybe establish some contact sets if you like. Drag them around until you're blue in the face.

[TrippyLighting]

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@R4SMEs wrote:

Nevertheless, this leaves open the issue of how one can move the two folding panels about (either by manually dragging them with the mouse) or by a motion study?

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I am totally lost as to what you are trying to do and that is with 30 years professionally in CAD and 3D modeling mechanical devices (among other things) that are vastly more complex than this simple table.

If you set proper joint limits you shouldn't be able to drag the components beyond their mechanical joint boundaries.

Where that becomes a problem is when there is no joint but physical interference exists. That is what I was hoping the contact set would allow for, but it only does to a degree. 

I doubt that can be solved with a motion study as a motion study also only allows parts to follow certain pre-determined constraints.

While I obviously have no problems to understand this very simple mechanism I am having a problem understanding what you are trying to simulate and what knowledge you are trying to gain that isn't already obvious based on the previous posts.

[R4SMEs]

The Motion Study was another unexpected bonus that can obviously have its uses - but not what I was initially asking about.

Yes, I accept that a Motion Study is based on a particular sequence of inter-related pre-defined joint movements (as should be defined by the graphs).
[But the sudden jump and strong hysteresis effect is an issue in using this - maybe if I study this MS subject and play about with it, I can work out a solution myself,
but I was hoping for a simple explanation of the puzzling issues seen in the @davebYYPCU motion study.]

The challenge/ problem seems to be that it is not possible to manually drag just one of the two revolute joints.

I have now removed all joint limits and the rest position boxes are unchecked.
There are no contact sets to complicate matters at the moment.

Is there a way to manually drag just one of the joints whilst the other doesn't change?

Ideally, one would click on a flag and drag a particular joint around - and it alone - but that doesn't seem to be possible.

[TrippyLighting · Accepted solution]

Aha! If you double click on the little flag in the joint symbol in the viewport you can rotate just that joint.