Calling all MechEng's: Advice Requested...

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Calling all MechEng's: Advice Requested...

Qbert
Please see:  http://www.10bitworks.com/dokuwiki/doku.php?id=3d_printed_robotic_arm

for a design of the first DOF of a robotic arm, based on this plastic stepper motor design:  


I have been able to get the stepper motor to “almost” work, so by adding the magnetic cores to the bobbins it should work fine under no load and low load conditions.  Hopefully the gears will raise the torque by the gear ratio of four.  Please inform me of any thoughts you may have on this design!  Thanks in advance.  

Les



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Re: Calling all MechEng's: Advice Requested...

Kevin Crowley
Bearings go where weight or force is causing parasitic friction.


On Mon, Aug 18, 2014 at 3:01 AM, Les Hall <[hidden email]> wrote:
Please see:  http://www.10bitworks.com/dokuwiki/doku.php?id=3d_printed_robotic_arm

for a design of the first DOF of a robotic arm, based on this plastic stepper motor design:  


I have been able to get the stepper motor to “almost” work, so by adding the magnetic cores to the bobbins it should work fine under no load and low load conditions.  Hopefully the gears will raise the torque by the gear ratio of four.  Please inform me of any thoughts you may have on this design!  Thanks in advance.  

Les



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Re: Calling all MechEng's: Advice Requested...

Qbert
I knew it, the bearings should be aligned to each other and to the gears ideally, right? 

Les


On Aug 18, 2014, at 3:25 AM, Kevin Crowley <[hidden email]> wrote:

Bearings go where weight or force is causing parasitic friction.


On Mon, Aug 18, 2014 at 3:01 AM, Les Hall <[hidden email]> wrote:
Please see:  http://www.10bitworks.com/dokuwiki/doku.php?id=3d_printed_robotic_arm

for a design of the first DOF of a robotic arm, based on this plastic stepper motor design:  


I have been able to get the stepper motor to “almost” work, so by adding the magnetic cores to the bobbins it should work fine under no load and low load conditions.  Hopefully the gears will raise the torque by the gear ratio of four.  Please inform me of any thoughts you may have on this design!  Thanks in advance.  

Les



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Re: Calling all MechEng's: Advice Requested...

PolyVinalDistillate
In reply to this post by Qbert
Looking at your design and the following mention of bearings gave me an idea..you could use a planetary gearbox design if you incorporate a sun gear at either end of the rotor. The rotor would then be held central by the planet gears, and this would act like bearings if your gears print well enough (in my experience a 0.2 mm nozzle with 0.1 mm build layers creates very smooth running planetary gear sets if you set the backlash on the gears to 0.4, and the backlash on the annulus to -0.4). If the annulus(es?) are part of the fixed structure, the planet carrier could be used as the output at one end.

As I'm on holiday, I'm on a really crap little netbook, and can't offer examples of what I mean in picture form! Hopefully my explanation makes some sense. Essentially, something like http://www.thingiverse.com/thing:53451 at either end of your rotor, and then use the planet carrier for output torque at one end (something along the lines of: http://www.thingiverse.com/thing:8460 ). You should easily get a 1:4 reduction with sensible gear sizes (have a look at the wikipedia entry on epicyclic gearing for the equations).
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Re: Calling all MechEng's: Advice Requested...

Qbert
PVD, great idea and what fun to work on a planetary gear system.  I’ll be printing up a storm of these experiments when I get my printer back and will give that one a try for sure!  It could be my answer to the gearing for the entire robot arm, methinks!  

Les


On Aug 18, 2014, at 8:19 AM, PolyVinalDistillate <[hidden email]> wrote:

> Looking at your design and the following mention of bearings gave me an
> idea..you could use a planetary gearbox design if you incorporate a sun gear
> at either end of the rotor. The rotor would then be held central by the
> planet gears, and this would act like bearings if your gears print well
> enough (in my experience a 0.2 mm nozzle with 0.1 mm build layers creates
> very smooth running planetary gear sets if you set the backlash on the gears
> to 0.4, and the backlash on the annulus to -0.4). If the annulus(es?) are
> part of the fixed structure, the planet carrier could be used as the output
> at one end.
>
> As I'm on holiday, I'm on a really crap little netbook, and can't offer
> examples of what I mean in picture form! Hopefully my explanation makes some
> sense. Essentially, something like http://www.thingiverse.com/thing:53451 at
> either end of your rotor, and then use the planet carrier for output torque
> at one end (something along the lines of:
> http://www.thingiverse.com/thing:8460 ). You should easily get a 1:4
> reduction with sensible gear sizes (have a look at the wikipedia entry on
> epicyclic gearing for the equations).
>
>
>
> --
> View this message in context: http://forum.openscad.org/Calling-all-MechEng-s-Advice-Requested-tp9354p9358.html
> Sent from the OpenSCAD mailing list archive at Nabble.com.
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Re: Calling all MechEng's: Advice Requested...

PolyVinalDistillate
They're definitely good fun :D I built my first one for a windmill testbed for 1st year engineering students to use to measure the power they were getting with their own designs of wind turbine blades - waay better than the lego windmill I made previously lol It's at http://www.thingiverse.com/thing:272351 It's 2 stages, ratio 1:30.

It only has 2 planets per gear stage and you'll need at least 3 of course, but I can vouch for the design style I've used - it's worked beautifully and provides a torque of about 2 kg.cm with the paper turbine blades the students make. I'm fairly confident you could extract a few times that if you can generate enough input torque, and planetary gearsets have (when running smoothly) about 95% efficiency if memory serves. With PLA you may have to run the thing for a bit with an electric drill to smooth the gears a little after construction.
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Re: Calling all MechEng's: Advice Requested...

Qbert
I finally figured out how to use the involute_gears.scad library so I’ll model one in that and share it i guess.  

Les


On Aug 18, 2014, at 12:40 PM, PolyVinalDistillate <[hidden email]> wrote:

> They're definitely good fun :D I built my first one for a windmill testbed
> for 1st year engineering students to use to measure the power they were
> getting with their own designs of wind turbine blades - waay better than the
> lego windmill I made previously lol It's at
> http://www.thingiverse.com/thing:272351 It's 2 stages, ratio 1:30.
>
> It only has 2 planets per gear stage and you'll need at least 3 of course,
> but I can vouch for the design style I've used - it's worked beautifully and
> provides a torque of about 2 kg.cm with the paper turbine blades the
> students make. I'm fairly confident you could extract a few times that if
> you can generate enough input torque, and planetary gearsets have (when
> running smoothly) about 95% efficiency if memory serves. With PLA you may
> have to run the thing for a bit with an electric drill to smooth the gears a
> little after construction.
>
>
>
> --
> View this message in context: http://forum.openscad.org/Calling-all-MechEng-s-Advice-Requested-tp9354p9360.html
> Sent from the OpenSCAD mailing list archive at Nabble.com.
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Re: Calling all MechEng's: Advice Requested...

Qbert
oops cannot make planetary gears using the MCAD/involute_gears.scad library because the gear ring is not supported.  

Les


On Aug 18, 2014, at 2:18 PM, Les Hall <[hidden email]> wrote:

> I finally figured out how to use the involute_gears.scad library so I’ll model one in that and share it i guess.  
>
> Les
>
>
> On Aug 18, 2014, at 12:40 PM, PolyVinalDistillate <[hidden email]> wrote:
>
>> They're definitely good fun :D I built my first one for a windmill testbed
>> for 1st year engineering students to use to measure the power they were
>> getting with their own designs of wind turbine blades - waay better than the
>> lego windmill I made previously lol It's at
>> http://www.thingiverse.com/thing:272351 It's 2 stages, ratio 1:30.
>>
>> It only has 2 planets per gear stage and you'll need at least 3 of course,
>> but I can vouch for the design style I've used - it's worked beautifully and
>> provides a torque of about 2 kg.cm with the paper turbine blades the
>> students make. I'm fairly confident you could extract a few times that if
>> you can generate enough input torque, and planetary gearsets have (when
>> running smoothly) about 95% efficiency if memory serves. With PLA you may
>> have to run the thing for a bit with an electric drill to smooth the gears a
>> little after construction.
>>
>>
>>
>> --
>> View this message in context: http://forum.openscad.org/Calling-all-MechEng-s-Advice-Requested-tp9354p9360.html
>> Sent from the OpenSCAD mailing list archive at Nabble.com.
>> _______________________________________________
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>
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Re: Calling all MechEng's: Advice Requested...

PolyVinalDistillate
In reply to this post by Qbert
Les Hall wrote
I finally figured out how to use the involute_gears.scad library so I’ll model one in that and share it i guess.  

Les
I used http://www.thingiverse.com/thing:3575/ before I discovered OpenSCAD has an involute gears lib! I imagine they're similar..
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Re: Calling all MechEng's: Advice Requested...

kintel
Administrator
In reply to this post by Qbert
On Aug 18, 2014, at 15:29 PM, Les Hall <[hidden email]> wrote:

> oops cannot make planetary gears using the MCAD/involute_gears.scad library because the gear ring is not supported.  
>
Isn’t that just a cylinder minus a herringbone gear?

See e.g. http://www.thingiverse.com/thing:53451

 -Marius

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Re: Calling all MechEng's: Advice Requested...

PolyVinalDistillate
kintel wrote
Isn’t that just a cylinder minus a herringbone gear?
See e.g. http://www.thingiverse.com/thing:53451
 -Marius
I hunted online to look at examples of planetary gears, and came to the ultimate conclusion that it is just a cylinder minus a gear (I was using spur gears). Just make sure you use a positive backlash on the gears themselves (I found 0.4 worked for the scale I was at) and a negative backlash on your subtractor gear for the annulus.
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Re: Calling all MechEng's: Advice Requested...

Qbert
Oh that is such good news!  I will be happy to use this technique, only thing is I don’t understand the backlash.  How is that indicated in the below module for a double helix gear?  

Les




module double_helix_gear_1 (
teeth=40,
circles=8)
{
//double helical gear
{
twist=200;
height=5;
pressure_angle=30;

gear (number_of_teeth=teeth,
circular_pitch=350,
pressure_angle=pressure_angle,
clearance = 0.2,
gear_thickness = height/2*0.5,
rim_thickness = height/2,
rim_width = 5,
hub_thickness = height/2*1.2,
hub_diameter=bearingSize[0]*1.25,
bore_diameter=bearingSize[0],
circles=circles,
twist=twist/teeth);
mirror([0,0,1])
gear (number_of_teeth=teeth,
circular_pitch=350,
pressure_angle=pressure_angle,
clearance = 0.2,
gear_thickness = height/2,
rim_thickness = height/2,
rim_width = 5,
hub_thickness = height/2,
hub_diameter=bearingSize[0]*1.25,
bore_diameter=bearingSize[0],
circles=circles,
twist=twist/teeth);
}
}





On Aug 18, 2014, at 2:42 PM, PolyVinalDistillate <[hidden email]> wrote:

> kintel wrote
>> Isn’t that just a cylinder minus a herringbone gear?
>> See e.g. http://www.thingiverse.com/thing:53451
>> -Marius
>
> I hunted online to look at examples of planetary gears, and came to the
> ultimate conclusion that it is just a cylinder minus a gear (I was using
> spur gears). Just make sure you use a positive backlash on the gears
> themselves (I found 0.4 worked for the scale I was at) and a negative
> backlash on your subtractor gear for the annulus.
>
>
>
> --
> View this message in context: http://forum.openscad.org/Calling-all-MechEng-s-Advice-Requested-tp9354p9365.html
> Sent from the OpenSCAD mailing list archive at Nabble.com.
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://rocklinux.net/mailman/listinfo/openscad
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Re: Calling all MechEng's: Advice Requested...

PolyVinalDistillate
I'm not sure.. as I recall, for the library I used, it was specified in gear () as a parameter:
gear(..., backlash = 0.4, ...)

But that was with the library I was using - Greg Frost's V5 involute gears I think... *checks again*

yup - http://www.thingiverse.com/thing:3575/#files (parametric involute gears V5 by GregFrost).

The builtin OpenSCAD lib should specify a similar parameter I think.

I used the backlash to allow more clearance between the planetary gears - I *think* this is the right way to do it. If you've made gears before and printed them without changing the backlash, you might get away without it. I find that my printer added about 0.1 to 0.15 mm excess material to the outside of objects and the backlash parameter allowed me to deal with that.

the SCAD code in http://www.thingiverse.com/thing:247369 might help...
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Re: Calling all MechEng's: Advice Requested...

Qbert
I continue to work on this, taking a break now.  All advice incorporated into solution, love it when that happens!  gonna be a fine planetary gear stage when done, attached to a 3d printed stepper motor as part of a robotic arm - nice…

Les


On Aug 18, 2014, at 3:17 PM, PolyVinalDistillate <[hidden email]> wrote:

> I'm not sure.. as I recall, for the library I used, it was specified in gear
> () as a parameter:
> gear(..., backlash = 0.4, ...)
>
> But that was with the library I was using - Greg Frost's V5 involute gears I
> think... *checks again*
>
> yup - http://www.thingiverse.com/thing:3575/#files (parametric involute
> gears V5 by GregFrost).
>
> The builtin OpenSCAD lib should specify a similar parameter I think.
>
> I used the backlash to allow more clearance between the planetary gears - I
> *think* this is the right way to do it. If you've made gears before and
> printed them without changing the backlash, you might get away without it. I
> find that my printer added about 0.1 to 0.15 mm excess material to the
> outside of objects and the backlash parameter allowed me to deal with that.
>
> the SCAD code in http://www.thingiverse.com/thing:247369 might help...
>
>
>
> --
> View this message in context: http://forum.openscad.org/Calling-all-MechEng-s-Advice-Requested-tp9354p9367.html
> Sent from the OpenSCAD mailing list archive at Nabble.com.
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://rocklinux.net/mailman/listinfo/openscad
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Re: Calling all MechEng's: Advice Requested...

Chow Loong Jin
In reply to this post by PolyVinalDistillate
On Mon, Aug 18, 2014 at 06:19:08AM -0700, PolyVinalDistillate wrote:
> Looking at your design and the following mention of bearings gave me an
> idea..you could use a planetary gearbox design if you incorporate a sun gear
> at either end of the rotor. The rotor would then be held central by the
> planet gears, and this would act like bearings if your gears print well
> enough (in my experience a 0.2 mm nozzle with 0.1 mm build layers creates
> very smooth running planetary gear sets if you set the backlash on the gears
> to 0.4, and the backlash on the annulus to -0.4). If the annulus(es?) are
> part of the fixed structure, the planet carrier could be used as the output
> at one end.

FWIW, I made some changes to involute_gears.scad in
https://github.com/hyperair/MCAD that added an "internal" option which handles
reversing the backlash and clearance, as well as a "helical_angle" option that
makes helical and heringbone gears easier to generate (the twist option is cool,
but requires manual compensation as the pitch diameter and rim_thickness
changes).

</shameless-plug>

--
Kind regards,
Loong Jin

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Re: Calling all MechEng's: Advice Requested...

Chow Loong Jin
In reply to this post by PolyVinalDistillate
On Mon, Aug 18, 2014 at 01:17:11PM -0700, PolyVinalDistillate wrote:

> I'm not sure.. as I recall, for the library I used, it was specified in gear
> () as a parameter:
> gear(..., backlash = 0.4, ...)
>
> But that was with the library I was using - Greg Frost's V5 involute gears I
> think... *checks again*
>
> yup - http://www.thingiverse.com/thing:3575/#files (parametric involute
> gears V5 by GregFrost).
>
> The builtin OpenSCAD lib should specify a similar parameter I think.
>
> I used the backlash to allow more clearance between the planetary gears - I
> *think* this is the right way to do it. If you've made gears before and
> printed them without changing the backlash, you might get away without it. I
> find that my printer added about 0.1 to 0.15 mm excess material to the
> outside of objects and the backlash parameter allowed me to deal with that.
There are two options: backlash and clearance.

Backlash makes the gear teeth thinner, i.e. it allows one gears to rotate back
and forth a little bit without causing a corresponding amount of rotation in the
meshing gear.

Clearance makes the grooves deeper, to avoid the teeth tips of one gear
impacting the bottom of the groove of the other gear.

When making an annulus (internal) gear, you negate backlash to make the teeth
fatter, but clearance can't simply be negated for internal gears, because you
want to make the teeth tips slightly longer in order to make space for the gear
meshing with it.

An example of a planetary gear set, with corresponding calculations and
animation is at https://github.com/hyperair/planetary-gears.

--
Kind regards,
Loong Jin

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Re: Calling all MechEng's: Advice Requested...

Qbert
Thank  you for your explanation, Chow Loong Jin.  This helps me understand! 

Les


On Aug 19, 2014, at 1:48 AM, Chow Loong Jin <[hidden email]> wrote:

On Mon, Aug 18, 2014 at 01:17:11PM -0700, PolyVinalDistillate wrote:
I'm not sure.. as I recall, for the library I used, it was specified in gear
() as a parameter: 
gear(..., backlash = 0.4, ...)

But that was with the library I was using - Greg Frost's V5 involute gears I
think... *checks again* 

yup - http://www.thingiverse.com/thing:3575/#files (parametric involute
gears V5 by GregFrost). 

The builtin OpenSCAD lib should specify a similar parameter I think.

I used the backlash to allow more clearance between the planetary gears - I
*think* this is the right way to do it. If you've made gears before and
printed them without changing the backlash, you might get away without it. I
find that my printer added about 0.1 to 0.15 mm excess material to the
outside of objects and the backlash parameter allowed me to deal with that.

There are two options: backlash and clearance.

Backlash makes the gear teeth thinner, i.e. it allows one gears to rotate back
and forth a little bit without causing a corresponding amount of rotation in the
meshing gear.

Clearance makes the grooves deeper, to avoid the teeth tips of one gear
impacting the bottom of the groove of the other gear.

When making an annulus (internal) gear, you negate backlash to make the teeth
fatter, but clearance can't simply be negated for internal gears, because you
want to make the teeth tips slightly longer in order to make space for the gear
meshing with it.

An example of a planetary gear set, with corresponding calculations and
animation is at https://github.com/hyperair/planetary-gears.

-- 
Kind regards,
Loong Jin
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Re: Calling all MechEng's: Advice Requested...

Qbert
One other question:  how do i determine proper spacing between the gears?

Les


On Aug 19, 2014, at 4:02 PM, Les Hall <[hidden email]> wrote:

Thank  you for your explanation, Chow Loong Jin.  This helps me understand! 

Les


On Aug 19, 2014, at 1:48 AM, Chow Loong Jin <[hidden email]> wrote:

On Mon, Aug 18, 2014 at 01:17:11PM -0700, PolyVinalDistillate wrote:
I'm not sure.. as I recall, for the library I used, it was specified in gear
() as a parameter: 
gear(..., backlash = 0.4, ...)

But that was with the library I was using - Greg Frost's V5 involute gears I
think... *checks again* 

yup - http://www.thingiverse.com/thing:3575/#files (parametric involute
gears V5 by GregFrost). 

The builtin OpenSCAD lib should specify a similar parameter I think.

I used the backlash to allow more clearance between the planetary gears - I
*think* this is the right way to do it. If you've made gears before and
printed them without changing the backlash, you might get away without it. I
find that my printer added about 0.1 to 0.15 mm excess material to the
outside of objects and the backlash parameter allowed me to deal with that.

There are two options: backlash and clearance.

Backlash makes the gear teeth thinner, i.e. it allows one gears to rotate back
and forth a little bit without causing a corresponding amount of rotation in the
meshing gear.

Clearance makes the grooves deeper, to avoid the teeth tips of one gear
impacting the bottom of the groove of the other gear.

When making an annulus (internal) gear, you negate backlash to make the teeth
fatter, but clearance can't simply be negated for internal gears, because you
want to make the teeth tips slightly longer in order to make space for the gear
meshing with it.

An example of a planetary gear set, with corresponding calculations and
animation is at https://github.com/hyperair/planetary-gears.

-- 
Kind regards,
Loong Jin
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Re: Calling all MechEng's: Advice Requested...

Chow Loong Jin
On Tue, Aug 19, 2014 at 10:01:25PM -0500, Les Hall wrote:
> One other question:  how do i determine proper spacing between the gears?

Add up the pitch radii of the two gears. The pitch circles of both gears need to
intersect each other at exactly one point.

Circular Pitch = distance[1] between the same point of the teeth along the
                 circumference of the pitch circle.

Circumference of the pitch circle = Pitch * number of teeth

Radius of the pitch circle = Circumference / 2π or Circumference / τ


[1] In MCAD/involute_gears.scad or MCAD/gears/involute_gears.scad, for
    hysterical raisins, I mean backward compatibility, the circular pitch is
    expressed in units of 180 / PI, so just multiply your desired circular pitch
    by 180 / PI when passing it in, or use the convertcp() function at
    https://github.com/hyperair/MCAD/blob/master/gears/involute_gears.scad#L592

--
Kind regards,
Loong Jin

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