Re: [CAD_CAM_EDM_DRO] sourcing miniature epicylics/planetarys

Hi Doug,

If you are going to use something like this you should perhaps think
carefully about the backlash and its effect on the accuracy of the readout.
Remember, whether straightforward or epicyclic, each gear in a train has
some 'freedom' which must be takem up when the gear train reverses
direction - the more gears, the more backlash and the more inaccurate the
final reading. Under 'very' light loads as in driving an encoder, some of
this may be masked by using a heavy grease in the gearbox but, as soon as
there is any drag on the shafts, all the backlash will come into play as the
'freedoms' are taken up and this will give an unpredicatable inaccuracy.
Have you considered the possibility of using belts and pulleys? They would
take up a little more space but by having a couple of parallel shafts and
putting several sets of small to large pulleys free running on these you can
get an increase in ratio without backlash. Flat belts are also better than
toothed.
___ _____
| | | |
| | | | free running
in | | ___| |
___| | | |_ etc.
|___ | | _
| | |___ |
| | _||_ | | ___
| | | | | | | |
|__| | | |___| | |
|| | | || | |
__||_| |__||__| |
__| || |__
|__ || __
|____ ||____ |
| | | | out
| | | |
free | | | |
running |___| |___|

Best wishes,

Ian.

----
Ian W. Wright
Sheffield UK

----- Original Message -----
From: Doug Fortune <pentam@...>
To: <CAD_CAM_EDM_DRO@egroups.com>
Sent: 17 August 2000 01:47
Subject: [CAD_CAM_EDM_DRO] sourcing miniature epicylics/planetarys


>
> re: minature epicyclics for rack-and-pinion to rotary encoder
>
> Accuracy of gear-on-rotary encoders running along a rack
> can be improved perhaps by mounting a miniature epicyclic
> between the encoder and gear (to increase the encoder shaft
> rotation relative to the gear rotation).
>
> Anyone seen such a unit? (part numbers/web addresses please,
> not just vague Berg/SDP/mcmastercarr/Boston Gear/Dayco/
> Grainger/etc suggestions).
>
> Anyone tried this? For instance a 32 pitch 0.5" PD spur gear
> travels 1.570795"/revolution. Using a US-Digital S2 Optical
> Shaft Encoder (2048 CPR) gives 0.000 766 inch per count.
>
> Using a 7.7 : 1 ratio planetary would give approx 0.000 100
> per encoder step.
>
> Thanks
> Doug
>
> PS: The next post will tell me there will be backlash. Instead of
> telling me why it won't work, try to be positive! I thought of
> sandwiching a gear between two racks (one fixed, one attached
> to the table). The rate of rotation of the gear is half as much,
> but there is likely to be less backlash and way less radial force
> on the encoder shaft. This arrangement will allow a much
> higher planetary gear ratio as well (say 10 or more).
>
>
>
>
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