[Arm-netbook] physics analysis needed of belt-driven pulley system

[Hrvoje Lasic]

below are answers

On 19 August 2017 at 12:52, Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
>
> On Sat, Aug 19, 2017 at 11:38 AM, Hrvoje Lasic <lasich at gmail.com> wrote:
> > Ok, so correct me if I am wrong but you are asking some mechanical
> > questions.
>
>  mechanical / physics, yes.
>
> > so, I speak in general now and if you have question ask deeper.
> >
> > Force needed to drive whatever you want to drive will be exactly the same
> > at the end part of system.
>
>  on the print-head yes.  but because of the doubled speed, the DRIVE
> end *NO*, the force is NOT the same.
>

so, if on the print head-side force is the same then it is on the drive
side same as well (work can not be missing somewhere). However, if you are
doubling the speeds, then overall work required will be some higher value
so, correct would be to say that because of double speed you are having
more force on print head needed and as result drive will also have to work
more, losses will be bigger etc.

anyway, that is all theoretical. Real value would be to find out force you
need to have on print head then do some sort of calculation back to the
drive. I guess you don't know that but maybe you can see what has been done
on system that have other drive and then try to extrapolate what you will
need. We can also assume here that someone has already put some reserve on
what is now there, so possibly you can leave same motor or if you have
problem increase force at disposal assuming you can do that on same motor
size (nema 17 or so)

I think backlash here will be your biggest problem here. Backlash is in
fact inertia of the system. So, for example when you run you want to stop
you cant do it right away, you need some time/distance. Or comparing to
electrical engineering you can compare it to inductance. Inductance is
resistance plus reactance.

You said that you expect lesser backlash on pulley system. If that is the
case (I am not sure why or why not this statement would be truth) you will
be able to increase speed up to certain point but you will have to test
where is that point. Basically you will have to do a lot of testing.


>
> > But efficiency of belt system could be slightly
> > different then efficiency of `non` pulley system (whatever it is). But
> > again, efficiency could be neglected for sake of easy of calculation or
> you
> > can put around some value that (guess) could be 80-90%.
>
>  i'm assuming that there are no significant losses around pulley bearings.
>
> > What you will be looking is gear ratio on pulley that could alter
> > force/speed ratio and same thing on other system.
>
>  the gear ratio on a single pulley system is 2:1.
>
> > also, you will be looking on backlash.
>
>  *thinks*... backlash should be reduced (halved) due to the better
> effectiveness of the pulleys - assuming that there is no "play" in the
> bearings (we can assume decent bearings / idlers).
>
> l.
>
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-- 
Hrvoje Lasić
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tel +385 1 6152 706 <+385%201%206152%20706>
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