Re: [CAD_CAM_EDM_DRO] Re: Belts & Pulleys

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.

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.

One example,
Taking .012" as the backlash amount:
PD=2" pulley circumference is 6.283"
.012"/6.283" * 360 degrees = .687 degrees of backlash.
.687/360 * .2" = 0.0004" (for a 5 turn per inch ballscrew)

I'm planning on using a 2:1 reduction so this will further halve
backlash at the leadscrew.

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

Cheers, Peter



gittt2000 wrote:

> --- In CAD_CAM_EDM_DRO@y..., Peter <prenolds@s...> wrote:
>
>
>>backlash for two 20 groove pulleys with 6.6lb belt tension is about
>>.012" for a 3mm GT belt and .029" for a 3mm HTD belt. Sounds like a
>>lot!
>>
>
> That's interesting - I wonder why it is stated as a linear distance
> rather than a degree of rotation?. If it stays constant then it
> suggests a big diameter pulley will suffer much less backlash than a
> smaller one, and given that the pulley is driving a leadscrew it's
> the rotational backlash we are interested in.
>
>