Re: Hexapod question (joints)

Two Cents:

Universal joints have a number of problems, besides the play, and all
that other stuff the rotational velocity is NOT constant. The output
shaft rotation velocity changes in proportion to the angle of the
joint, the greater the angle the greater the change.

That's why Detroit developed the constant velocity (CV) joint for
front wheel drive cars, otherwise the vibration would shake your
teeth out.

Although you do not rotate the shaft through a full rotation you
still get the ill effects because you will always be operating the
joints at an angle hence the velocity will not be truly linear.

Similarly, a standard ball joint will provide a difficult problem
insofar as the under stress it is probable that the ball will spin in
the socket preventing linear extension of the shaft.

I would guess a pinned-ball joint would be the best solution.

Thanks

Aaron Moss
www.IndustrialHobbies.com

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "afaIII" <afa_mma@a...> wrote:
> I don't think ball joints are required. In fact, they would not work
> well at all because then the motor and/or screw would be free to
spin
> and do nothing.
>
> The bad news is a U-joint or gimbal isn't the same as a perfect ball
> joint, either. As you move the bottom end of a strut around, it will
> rotate slightly relative to the top which will screw/unscrew the
strut
> and change the length slightly.
>
> I don't know if this will cause enough error to matter or not. For a
> first experiment, maybe not. For real precision, probably so.
>
> It ought to be possible to account for the U-joints in the control,
> but I don't believe the EMC calculations do this. Working out the
> proper math for the U-joints would be possible, I think, but not
easy.
>
> Andy A.
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Kevin Martin"
<kpmartin@t...>
> wrote:
> > I may not be able to explain exactly *why*, but I have to disagree
> with you,
> > ballendo...
>
> snip...
>
> >
> > -----Original Message-----
> > From: ballendo [mailto:ballendo@y...]
> > [...]
> > The u-joints you've used are creating the problem. You need a ball
> > joint at the end of the struts.