Re: [CAD_CAM_EDM_DRO] Re: Hexapod question

I think the best (only) usable joint is that each strut end must be
hemispherical (a half-ball) that mates with the hemisphere of the mating
strut, constrained in a ball-socket.

RayHex

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> From: Kevin Martin <kpmartin@...>
>
> I may not be able to explain exactly *why*, but I have to disagree with

you,

> ballendo...
> If the two cross-axes of the U-joint are in the same plane the joint can
> flex in any direction without any change in "length", which is the same
> motion as you would get with a ball joint whose centre was where the

U-joint

> cross-axes met.
>
> What a U-joint does *not* give you is smooth transmission of *rotary*

motion

> as one shaft of the joint rotates the other.
>
> And of course, a U-joint it really two joints in series, so has twice the
> play of a ball joint built to the same clearances.
>
> It is difficult to tell from the photos whether the U-joints satisfy the
> above geometry; at first glance they appear to.
> I am actually not convinced that having extended u-joints (where the
> cross-axes are not coplanar) produces more degrees of freedom either,
> although it does complicate the positioning math, and may increase the
> effects of play and frame flex. I have certainly seen photos of

commercial

> hexapods which do not use ball joints (not that I would be able to find

one

> now).
> -Kevin Martin