Re: Hexapod question

--- 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...
> 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.
>

... now).

> -Kevin Martin
>

The U-joint can act as a true ball joint IF the end mounted in the
plate is free to rotate... this necessitates a good rotary bearing
mounting in the plate. Doing this at both ends leaves the strut free
of any cyclic binding, but the strut itself must be telescopic and
not dependent on being rotationally anchored to change length. The
drive then, must be rotationally isolated within the strut (i.e. no
torque transmission through the U-joints), the ballscrew nut and the
motor keyed linearly to the same element. A little hard to do it
right, especially in a small and cheap package... not so hard for a
large expensive machine.