Re: anti-backlash rack and pinion mechanism

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "pcfw" <pcfw@y...> wrote:

> Hi,
>
> Has anyone ever fabricated a home-brewed anti-backlash rack and
> pinion mechanism, perhaps something like that on this site:
>
> http://www.cntmotion.com/pdf/cnt2001catalog.pdf
>
> somewhere around 2/3 way down the page.
>
> How does an anti-backlash R&P work anyway?

It was discussed on the list several months ago. The backlash from a
rack and pinion comes from the imperfect mesh of the gear/rack. The
driving gear is only in contact with one side of the rack teeth. If
the gear does not fit all the way down into the rack teeth at any
point along the way then there can be slop. Carefully measuring and
shimming the rack can help that. There has to be some clearance
built into the design so there is some inherent backlast in any two
meshing gears inless they are perfectly aligned.

THe anti backlast rack and pinion is usually provided by two basic
methods: A split gear (imagine two separate gears on the same shaft)
that has spring tension is used. One gear is fixed to the shaft
while the other is allowed to rotate a few degrees. The spring
forces the free rotating gear teeth back against the opposite side of
the rack tooth so that one gear is in contact with each side of the
engaged rack teeth. Up to the limit of the spring tension there is no
backlash.

Another concept brought up was to use two fixed gears on one shaft
and to separate racks in parallel but offset a few thousands so that
different sides of the gear teeth were in mesh on each rack.

I would think that if you can keep the gear fully meshed with the
rack by providing some sort of "floating" suspension of the driving
gear then backlash would probably be minimal.

All that being said a lot depends on the type of work you are doing.
Rack and pinion is not meant for precision mills where you have to
move lots of weight slowly and accurately. On a plasma table you are
dealing with a cutting tool that can vary .030 in diameter just based
on speed and direction of cut. There are other considerations of
rack and pinion that have to do with torque and speed that have to be
considered in the design.