Re: [CAD_CAM_EDM_DRO] Re: Belts & Pulleys

Peter wrote:

> Jon,
>
> Never say "NO" backlash... Murphey's law (or some other guy) always
> applies! You're right, quality parts, properly tensioned will minimise
> but not eliminate backlash.
>
> I think Les is taking the numbers for belt tooth tolerance (another
> variable...) rather than backlash but as this is also a non cumulative
> error - assuming one doesn't jump a tooth it too, is of pretty small
> magnitude.

Well, I worried over this error, so I put the encoders directly on the
leadscrew, and had the motor drive the screw by belt. This works very
well.

> A larger diameter pully will reduce the amount of rotational backlash.
> The numbers quoted in the SDI manual are for two equally sized pulleys
> with a pitch diameter (PD) of only .75". Given more realistic PD's of
> say 2" - 4" the rotational error will be quite small (but not
> insignificant) in angular terms.

I can really tell you, there is no 'backlash'. There is probably a small amount
of belt stretch as the motor applies force one way or the other, but this would
amount to tens of microinches of table motion. When the belts are worn,
then backlash ocurrs - you can hear the belts make creaking noises when the
direction is reversed. My original belts, which were definitely 20+ years old
when I put the machine together creaked like that. I replaced them as soon
as the machine was working.

>
> None of these magnitudes of backlash or belt tolerance should be cause
> for panic for our requirements...

Right.

Jon