[chbot] Cheap Rack and pinion servos?

[Yani Dubin]

Sorry, accidentally hit send.

Was trying to paste the following in:

"The way these actuate from memory is to rotating a toothed inner cylinder,
driven by the motor at one end, and gripped by a tooth (a float?) at the
other end - which is held rigid against an outer cylinder / housing to stop
it rotating with the inner cylinder, forcing it to slide along. In most
cases, this extends a retractable metal cylinder with a bolt in the end
(fixed to the load being lifted)."

On Wed, Apr 22, 2015 at 4:28 PM, Yani Dubin <yani.dubin at gmail.com> wrote:

> Hi Brahm,
>
>
> On Tue, Apr 21, 2015 at 9:16 PM, Brahm Boelee <brahmboelee84 at hotmail.co.uk
> > wrote:
>
>> That linearmotion site is super interesting
>>
>
> I have salvaged some of the hospital bed type actuators (which feature on
> the homepage of this site - not sure if these are the ones you are
> interested in).
>
> However not sure the ones I have had more than 30cm of travel. But I
> thought I'd mention a few things to be aware of with these, from my brief
> experimentation, if you are considering them.
>
> The two brands of unit I have pulled apart were based on a 24V brushed DC
> motor design (one was German, one Taiwanese from memory).
>
>
> The insulation rating in each case means you only get a 10 percent duty
> cycle (the name place specifies something like 2 mins on 18 mins off).
> Therefore they are designed for brief spurts at maximum power, and if you
> exceed this, you risk damaging the insulation and reducing the life of the
> motor.
>
> One (or both) of them had a ~40:1 reduction gearing ratio between the
> motor worm and the toothed actuator arm (orthogonal coupling), meaning it
> took quite some time to actuate (at a very rough guess, 15 seconds for 30cm
> - about what you might expect to gently raise/lower a patient). So not
> suitable for rapid movement.
>
> I evaluated the speed of the motor by cobbling together a DIY tachometer.
> The Dewert motor was achieving 2000rpm with the 15.7V my bench supply could
> provide, and sucking 200mA. So maybe 3500 rpm at rated load, and I think I
> saw a 50W mentioned somewhere.
>
>
>
> No idea if this is of interest, but if you want to know more about these
> or see one in motion, let me know and will see whether I can reassemble one
> :)
>
> Regards,
> Yani.
>
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