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Mechanism Connections

JWayman
12-Amethyst

Mechanism Connections

Good morning.

I have an assembly I would like to convert into a mechanism, but I can't
work out what connections to use to make it work.


Picture a bucket going down a well. Imagine there are two vertical rails,
u-shaped in section, diametrically opposite each other, on the walls of the
well. Also imagine there are two lumps sticking out of opposite sides of the
bucket, that engage with the u-shaped rails. Thus, the bucket can go up and
down unimpeded, but can only rotate freely until one of the lumps touches a
part of one of the rails.

Just to complicate things, the rails get closer together at the bottom, so
the bucket lumps are wedged between them. I think that is just an extension
of the basic problem, however, so let's not worry about that yet.


I immediately thought of cams, but they don't work well with a plane running
against a plane, it seems. I tried making an imaginary round thing sticking
out of the lumps, so I had a round thing and a plane, but then I was asked
about depth references for the planar cam elements. I can't work out what
the front, back and depth of a cam surface could mean, so I can't answer the
question correctly.

Sorry. I can't send a picture or model of this problem.

Anyway, how would you approach it?

WF2, M220

Regards,


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1 REPLY 1

Dear John,

How about using point on line for one of the side 'lumps' and 'rail', then use a 6-dof for the other side, with a zero displacement driver in the normal direction and another one preventing rotation about the normal axis. This leaves 2 DOF, one is the vertical motion and the other is rotation about the axis of the two lugs. What stops the bucket from spinning about this axis? If it's the shape of the lug, then just add another rotational zero driver on this axis of the 6 DOF joint as well.

This method is useful when the built-in joints don't have the correct combination of DOF you want, I call it a 'roll your own' joint. A word of caution, this only works for analysis runs, not for dragging, so don't try it, otherwise the mechanism could go horribly wrong. General tip: always take a snapshot before starting to drag.


Regards,

Rod

Rod Giles
Senior Design Engineer
Polaris Britain Ltd.


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