Re: Belt tensioners on toothed belts

I just did the calculations for my machine.

The error introduced by the belt flexing by 10 mm is about 0.01mm
This is about the same as a single step.

For what I am doing it is negligible and the error is not cumulative.

Chris

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@...> wrote:
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Ken Campbell" <deltainc@>
> wrote:
> >
> >
> > ----- Original Message -----
> > From: "caudlet" <thom@>
> > >
> >
> > ----- Original Message -----
> > From: "Chris Horne" <chris@>
> > Subject: [CAD_CAM_EDM_DRO] Re: Belt tensioners on toothed belts
> >
> >
> > > The error introduced is directly related to the slackness of the
belt,
> > > the diameter of the pullies and the screw pitch.> Chris
> > *******
> > Hi ... we are in the r/c car business , many use toothed belts (
> usually HTD
> > type.. strong for their weight ) .. anyway, tensioners are common,
and a
> > couple of flanged ball bearings also common and do the job fine ;
For
> > these bigger apps, consider a spring loaded arm, and stop by your
local
> > hobby shop and check out the small shock absorbers ( coil overs
> available )
> > used on dune buggies ... that would stop harmonics from building up,
> > besides being " real cool" to show your buddies.
> >
> > (g) ken campbell, deltawerkes.
> >
>
> The difference here is that:
>
> 1. Postional integerty is important. It's probably not a problem on
> an RC car if the position of the drive wheel is off by .005 in
rotation.
>
> 2. The CNC usage is under conditions where the direction of drive
> changes hundreds of times in a minute.
>
> I was dismayed watching the belt bow up and down under the direction
> changes by as much as 1/2". That change in drive length has to go
> somewhere. I guess that it might tend to cancel out over a lot of
> direction changes but could be cumulative over a long run if not
> exactly equal. I would think the repeatability of the system would
> suffer if belt flexing is allowed to happen.
>
> Since this is the first time I have seen this design in a CNC machine
> (spring loaded idler arm) I didn't know if the designer had discovered
> a new approch or just wanted to avoid having to design a movable motor
> mount. I have seen rigid rollers on belt drives to force a fuller
> wrap around the drive pinion but the tensioning mechanism is at the
> ends of the belt and can be tightened enough to prevent any flex.
>