Re: Belt tensioners on toothed belts

Tom,

Ron's reply reminds me of something I've seen in older torch drives...

The position feedback is de-coupled from the drive. IOW, applied to
your description: The timing belts are NOT being used for drive, but
only to determine position.

I've seen quite a few that had a "hidden" (means non-obvious; at
first glance it looks like th emotor is driving the rack)) friction
drive on the long beam, and usually rack gear and resolver/encoder
for feedback. Could your belts be feedback only? (IMO that makes your
prior description make more sense, as then the tensioning spring is
only there to keep the belt tight enough to the feedback
resolver/encoder, and flex wouldn't matter...)

Just some thoughts, I'm curious now<g>

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@...>
wrote:

>
> A belt flexing up and down, depending on it's length, would

indicate a belt that was under-tensioned. SD-PI/Sterling has a PDF
that shows the tension recommendations for different belts and the
specs are quite high. Although, it's a trade-off in application. A
belt run more tightly is more precise, but the resistance in
transmission of torque is higher. Also the higher side loads shorten
bearing life. So, one needs to find a happy medium between, tight
enough to be accurate and loose enough, not to rob power.

>
> Introducing an idler into the transmission surely increases

frictional losses. Pre-loading this idler with any sort of spring is
unneccesary. A bi-directional belt relies on perfect timing in both
rotations. In one direction under load where an spring loaded idler
is present, the idler/spring will relax under load, allowing mis-
timing between driver and driven. Simply tightening an adjustable
device into a held position should be ample. Wether it be an idler or
simply the motor itself. The latter generally being the most
efficient apart from direct coupling.

>
> As a toolmaker, I learned early on and was told by my

predecessors.."Avoid springs" they should always be a last resort and
not a first option. It's amazing what can be accomplished with a
simple counterweight and ever present gravity.

>
> Ron
>
>
>
> 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.
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>