Modelling chains

Does anyone have a slick way of modelling roller chains as an assembly of
links.
It's only a short length of chain (maybe 20 links) and does not form a
continuous loop.
A rough representation is:-
O
O
O
O
O
O
O O O

I need it to wrap 90 deg around a sprocket and be able to vary the position
of one end and have the other end (and all the inbetween links) move along a
corresponding distance, so that a mating component gets dragged along with
the chain. Better still would be the mating component drags the chain
along.

I'd hoped it might be possible with a "cam" mate, but I've not yet been able
to make this work.
I can bodge it by having the 2 straight sections as linear patterns, and the
sprocket section as a circular pattern. As you add one link to one straight
section, you have to remove one from the other straight section to keep the
total number constant - this can be done with a design table. The added
complication is that if you move by the chain by less than one pitch, you
also have to vary the angular position of the circular pattern so that the
sections mate up properly.

Can anyone think of a better way than this?

It's not important for the sprocket to rotate in sync - that would just be a
visual nicety.

Regards,
John H

 

SW2007 has a chain/timing belt sketch tool, why does everyone always
want to model every link in an assembly? A layout sketch would seem to
be one 'slick' way.
If you search the group, you will find lots of discussion about this
over and over.

 

It's a mechanism driven by a chain linkage and I need to ensure that there
is sufficient travel without the end connectors fouling on anything - our
customer says they foul on the existing design which wasn't modelled up
fully, and no-one is quite sure where the root problem lies.

Also I need to produce a drawing showing the correct number of links, with
the correct end pieces, and I also would like the BOM to call up the correct
quantities. There is also a requirement for a manual, and an exploded view
of an assembly sketch doesn't quite convey the same information.

It's the same argument as whether or not to add fasteners to an assembly - I
personally think it's important to do so.

Regards,
John H

 

Would a curve driven pattern of the link work for all but the end links?
Just a thought.

 

What is the name of the latest Inventor instructional book?

Sorry, I couldn't resist the Jeopardy answer.

Muggs

 

Theory:
Start a sketch (wherever makes sense - at asm level, skeleton part,
in envelope, whatever methodology works for you and your stye) Draw the
arc along the sprocket (always constant length, right?) tangent
vertical line then tangent horizontal line (and maybe not tangent -
note item about pitch adjustments around sprocket later in post). Add
length dims to all three (to measure arc length, pick the ends of the
arc and the arc itself and then plant the dim).
Create an equation - horizontal leg = total distance - arc length -
vertical leg . This keeps all three in sync after a rebuild, assuming
that the vertical leg length drives it. If I have it backward, just
change the equation accordingly so the horizontal length drives it.
You can even use in context between component and vertical leg and turn
that dim into a driven and the equation will correctly evaluate for the
horizontal leg (ignore the warning about using driven dims - its just
a heads up, not a limitation)

If I have the mechanism all wrong in what I described above, just
ignore what follows... but let's pretend for now that I didn't screw
up yet

You could then use equations referencing the segment dims to
automatically drive your three patterns (instances and distance) but
what fun would that be? OK, it's a little fun, but still - as you
mentioned, its not going to account for when the pitch doesn't
exactly line up, which might be important for interference detection,
etc. I hate imprecision.

Lets go for the fence.

So now, edit the sketch again and add a single fitspline to the three
segments. You have just turned three segments into one segment driven
by the three (can do it in a second sketch if you like - again,
personal style rules).

Now you *might* be able to use this single spline-segment for a curve
driven pattern, but I'm not set up to try it right now and I kinda
doubt it will work especially for the partial pitch situations. And,
I'm pretty sure that curve driven patterns only work in parts so you
have to pattern chain link bodies in a part. Or pattern features, then
mate a single link component to the seed feature and use a feature
driven pattern to make the assembly pattern (and, even so, I haven't
tested if feature driven patterns are supported when the pattern is a
curve-driven pattern - always keep an eye out for little pitfalls
like this)

So how could we get this thing to update correctly and even incorporate
the pitch? The following is a little Vegas (a gamble), but if you make
small changes when you make changes I bet it will work. it might even
work for big changes depending on how the sketcher feels in this
release.
In a new sketch, draw 20 line segments end to end (make sure no
automatic relations are applied, like collinear, vert, or horiz) .
Window select them and make them equal, then dim one to your chain
pitch. Now you have a sketch that acts like a length of chain. Flop
segments around to test it. That's kinda fun.

I *think* if you then make all the 21 pitch endpoints coincident to the
fitspline, and one end coincident to the driven end of the fitspline,
you will have a sketch mimicking a chain that will follow your layout
sketch exactly even as the ends are altered. Note - don't constrain
the other end of the chain, because that is NOT accurate - a faceted
radius (pitch around a sprocket) is not the same as the arc length of
that sprocket - there is an adjustment that needs to be accounted for
with the way the sketch is set up with the tangencies and all.

Then you can mate chain segments to those 20 sketch lines. If SWx
sketcher were really robust (and who knows in this case?) it would work
and update wonderfully. I advocate that you test it first with just
the sketch before burning time making mates. That would be fun to try
out. Those pitch segments might follow the spline perfectly like so
many ants on a scent trail.
And if it doesn't work the failure will be interesting and educational.
Sorry I don't have the time right now to give it a shot myself.

Ed



..

 

Thanks for the suggestions.

It seems that neither the sketcher nor assembly mates allow a point to be
coincident with a compound curve - it lets you create it, but then the point
only follows the first portion you picked.
i.e. if you have a straight-radius-straight curve and you make a point
coincident with one of the straights, the point will track the straight
section as if it were infinitely long.
I've a feeling this worked in I-DEAS sketcher - might fire it up again to
take a look.

If I want to do it as an assembly, then it seems I'm left with the linear
and circular pattern option.

If I do it as a part, I think the curve driven pattern might work. I was
hoping a curvedriven pattern could be used to drive a component pattern - it
lets you pick it, but then gives an error something like "can't find start
location".

I may take another look at "cam" advanced mates - it seems like this should
work, but didn't when I tried it the other day.

Bummer.


John H

 

hi john

I model chains as a part. Sketch the centre line, model a link then use
a curve pattern to attach to centre line, you have to make the length
of the centre line divisable by the pitch obviously. I have examples if
you like.

steve

 

i'll be thaksfull if you send it on mail..
i need somethin similar on my assy..
thanks

 

I have done something very similar to what you are trying to do, only
in my case I was modeling a length of cable carrier. With a large
number of links the mates will sometimes become unstable (the links
will fold over each other instead of following in line), but for short
sections it works well.
First I created a part to define the path that the links will follow.
The path must be a spline, for the links to be able to wrap around the
arc. Usually sketching the path with lines and arcs and then applying
a fit spline works well. I then create an extruded surface using that
spline.
Next I add a sketch to my links that consists of a single point
concentric with one end of the link.
Finally add your path and the links to an assembly. Mate all of the
links in a chain (ends concentric). Mate one of the end links to the
end of the path and then mate the sketch point on each link to the path
surface.
By changing the lengths of the "legs" on the spline you can force the
links to follow.
Alternatively, you can make the "legs" of the spline longer that the
chain. Then if you don't mate one of your end links to the end of the
path, you can drag the chain back and forth along the path.

 

Thanks jdc.

The key step in your procedure was using a spline to extrude a surface that
the chain runs against - I had been trying the same method as you but using
lines/arcs extruded which doesn't create a single surface.

Excellent!

John H

 

Please read the fifth paragraph of my post above:
Applying a 'fit-spline' to a series of lines and arcs (compound cuyrve)
creates a single curve that a coincident relationship will follow.
That single curve (the spline) is driven by all of the segments of the
compound curve

Ed

 

I often wonder that myself. I remember back in the days of the drafting
board where I had to keep a pencil and paper next to me and actually do the
math myself. I'm surprised that technique has been left out of all these
new-fangled drafting tools.
From: RaceBikesOrWork<>
To:comp.cad.solidworks
Date: 9/19/2006 3:56 AM
Subject: Re: Modelling chains
SW2007 has a chain/timing belt sketch tool, why does everyone always
want to model every link in an assembly? A layout sketch would seem to
be one 'slick' way.
If you search the group, you will find lots of discussion about this
over and over.

 

Lofting a surface between profiles consisting of tangent arcs/lines also
produces a single continuous edge. This also allows you to follow edges in
3d space.

 

I haven't been following this discussion closely, so I'm nost sure what
others have already suggested, but the Path function which is new in
2007 allows you to set up chain/belt motion with associated sprockets.
I don't know how it would handle an open loop. You would probably have
to still make a closed loop and only use a portion of it. Still, I
think putting the chain together is going to just be a series of mates.
If you set up the half link parts with mate refernces, it should snap
together fairly quickly if you don't have a hundred links to do. Using
a full link (or even several links) as an assembly and then mating the
assemblies together and either making the subassemblies flexible
(sounds a bit scary) or by dissolving the subassies into the upper
level assy,

good luck