Remote Motor Extruder (Shaft Transmission)

Hi,

Excuse me starting a new thread, I couldn't work out how to do this before, so I've started transferring my comments over to this, but may need a moderator to help tidy up the last thread I hijacked.

See Quick "Rough and Ready" andymation on: [www.youtube.com]

Bowden Cable Alternatives.


I'm thinking of changing the location of the Extrude feeder motor to where the X-Axis Motor is.

A uniform Cross-Section shaft could be used to transmit the motion of the motor to the first stage pinion of the extruder (logic: I think this will transmit hi speed/low torque better than it will transmit the lower speed/higher torque). Either a round shaft with a flat or a "trifoil" arrangement with three BB's within the hollow shaft of the Extruder Pinion could do the job.

The Uniform Cross-Sec would allow the pinion to be moved along the shaft without the "X-Co-ordinates" of the print head affecting the amount being extruded.*

(*with the premise of large polar moment of area and/or modulus to reduce the amount of torsion in the bar at the extruder).

I've not done any calcs on this yet, but think it may be a better way of controling feedrate relaiably than allowing a Bowden cable arrangement (where the filament is under buckling/compressive strain).

I'm as yet to build my "Rep/Strap", but I just thought I'd pitch this and ask if anyone has tried this config on an extruder head yet?

(n.b. This arrangement is not compatible with the cartesian configuration of my RepStrap, but will hopefully work with more conventional Mendels).

Points and pointers are welcome.

-AS

-------------------------------------------------------
> I can't visualize this. Could you do up a napkin
> drawing?

-------------------------------------------------------

I'll throw together a few calcs on the torsion, I'll do a few ProE models (or is a different format more acceptable?) and post it somewhere in the next few days.

Should we move this topic to its own thread?

HTH,

Yes, similar, but I'd have possibly used PTFE sleeve over S/Steel pair of shafts or profiled shaft.

Two round shafts nearly touching would be able to transmit torque with no need for machining the shaft.

-Andy

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[attachment 3743 delocalised_stepper.jpg]
[attachment 3744 delocalised_stepper2.jpg]
[www.youtube.com]

Here are some very quick 3d Models I did. Nothing too fancy, no bearing configs. etc.

The Animation is on YouTube for discussion.

The models and animation were done on SolidEdge ST3.

The concept is to remove from the print head the Stepper used for driving the ABS strip through the nozzle.

This will remove the mass from the print head and give faster acceleration, smoother motion and hopefully stop overshoot.
This only works if the force to overcome the friction along this shaft is less than that needed to accelerate the motor.

This is not to scale, has had no engineering calcs performed and is meant for a discussion piece.

Three h6 S/Steel tubes could be used for the rotating shaft and go into a reprap'd pinion with PTFE sleeves in it. I've not said Aluminium, because of the temptation to use extruded Al Tube, which has a fair bit of mold jitter fromt he etruder head.

Either this lot has to be smaller than the 8mm ID of standard bearings used (608RS) or new, larger bearings have to be used.

The Three Shaft config has been used as little machining is required, whereas if a shaft with a keyway or other profile were to be used, more precise machining would be required.

I'll keep you informed as to how this concept develops.

Next step is calculation, so I'll have to look into the spec's of existing Wade/Adrian Geared Extruders.

Has anyone else used a similar configuration yet?

-Andy

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Edited 1 time(s). Last edit at 02/07/2011 03:46PM by andysuth.

I love andymations!

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

I guess I've been typing my name too much and just thought I'd leave it in there(!).

What do you think Bob? has anyone had any success with this or similar?

I've been up since 4 am so probably have another crack at the calculations tonight, but should be just a few quick mods to one of my standard Excel sheets. I hope.


-Andy.

Why not just put the gears on the motor and drive the pinch wheel directly with the shaft(s)? If the gear is on the printhead, then it will use up more room there and reduce the maximum build height.

Thanks for the great post and the information i like it.

Cefiar Wrote:
-------------------------------------------------------
> How do you intend to keep the small gear (the one
> on the shafts) coming off the big gear when you
> move the carriage back and forth?
>
> Ideas:
> Print a lip on both sides of the big gear. The
> issue is that you need a printer that can do
> overhang for one of them, or you have to print the
> gear in 2 or more parts (eg: like the Mendel
> Z-axis Pulley).
> Use a herringbone gear, which will not go
> sideways due to the herringbone teeth. The issue
> here is that they tend to need fairly fine-tuned
> printing capabilities.

Yes they both sound great. I was going to contain the shaft with two 608 bearings. Whilst they are not supposed to take side loads, they should be able to cope with the small loads from friction (the whole concept here relies on friction being minimum).

Thanks for your interest.

brnrd Wrote:
-------------------------------------------------------
> Why not just put the gears on the motor and drive
> the pinch wheel directly with the shaft(s)? If the
> gear is on the printhead, then it will use up more
> room there and reduce the maximum build height.


Brnrd,

There is a bit of optimisation to be done, but with the shaft work:
Power = Torque x Angular Velocity

So to transmit the same amount of power at low speed a higher torque is required.

But, Shaft Torsion is proportional to the Torque transmitted:

Torque = J x G x tors / L
J = Polar moment of Area
G = Mat'l Const.
tors = Angle of Torsion, in Rads
L = Length of shaft.

Since you need to step down the motor speed at some point, it is much better to have a higher shaft speed, as the higher torsion will have higher deflection leading to more inaccuracy in the extruder position and possible higher angle at the pinion, making the friction increase.

This has to be offset with the whole acceleration/deceleration of the shaft, but I'm not sure how often that happens, and can be minimised by keeping mass of shaft down.

It was the original concept, to locate the gearbox at the motor, as its a little easier, but I think there are benefits.

-Andy

....Also the gear ratio reduces the effect of torsion on the filament.

Imagine a ratio of (say) 3:1.

The pinion has to rotate three times to every once the gear wheel goes around. So if the Torsion on the shaft gives 3deg of inaccuracy, what is seen at the shaft is only 1deg inaccuracy, so reduces errors by a 3rd. This translates as directly into linear movement of the filament through the pinchwheels.

No compound that with the Torque in the GearWheel Shaft being 3x greater than the torque in the pinion shaft, so if the shafts are identitcal geometry and material and length, the Torsion will be 3 times greater.

This effectively means the Torsions effect on accuracy will be nine times worse if the long transmission shaft is between the Gear Wheel and the Pinch Rollers than between the Motor and the Pinion.

(3 = ratio of Pinion to Wheel, improving accuracy)
(3 = ratio of Torques in Shafts)
3 x 3 = 9

As the idea of this suggestion is to reduce hystorhesis problems with Bowden, this factor of 9x the Torsional Errors will be undesirable.

-Andy

ledvinap Wrote:
-------------------------------------------------------
> andysuth Wrote:
> > Yes they both sound great. I was going to
> contain
> > the shaft with two 608 bearings. Whilst they
> are
> > not supposed to take side loads, they should be
> > able to cope with the small loads from friction
> > (the whole concept here relies on friction
> being
> > minimum).
>
> I't no so bad. 608 is deep grove, so from
> [www.skf.com]
> n&maincatalogue=1&newlink=1_1_13
>
> If deep groove ball bearings are subjected to
> purely axial load, this axial load should
> generally not exceed the value of 0,5 C0. Small
> bearings (bore diameter up to approx. 12 mm) and
> light series bearings (Diameter Series 8, 9, 0,
> and 1) should not be subjected to an axial load
> greater than 0,25 C0. Excessive axial loads can
> lead to a considerable reduction in bearing
> service life.

Nice find on the datasheet there Ledvinap, though a lot of the bearings used on repraps are non-SKF bearings, eBay specials, so I'd be a little cautious as SKF is one of the higher quality bearings and may be a little more capable than the 608rs that I'm using.

Do you mind if I just relocate this reply to the other thread I've started, when I put my first post here I didn't know how to start the new thread?

Cheers,

-Andy

The SKF data sheet should be describing the generic properties of ABEC bearings. Any SKF-specific features or qualities will be noted as special. So, any Chinese (or whetever) bearing that meets an ABEC tolerance class should be just as good as an SKF bearing of the same class (except as noted by SKF). In other words, I wouldn't hesitate to load a generic class 3 bearing from anywhere the same as I would an SKF bearing.

I suspect that, once you dive into the numbers, you will find that the steppers don't have sufficient torque to damage a 608, even if the pinion binds tight to the shafts.

Dale,

Assuming they are ISO9001 and externally audited, otherwise they could just "claim" to be C3 etc.

I agree with you that it's unlikely a stepper will directly break the bearings, as it would be the X-Axis stepper in a position to damage this worst I think the drivebelt would go first.

I'm not sure if the excessive vibrations going through them from steppers would be all that good for bearing life, but there may be more fundimental problems with this design now I'm looking at the basic geometry.

-Andy.

True enough about the auditing.

I'm looking forward to hearing what the fundamental problems might be. The only thing that makes me nervous about it is using three rods instead of a single profile with a larger polar moment. It seems like it should work, but intuition is notoriously unreliable.

All I was trying to say is that geep-groove bearing is designed to carry axial load to some extent. Quick search shows that static load is about 1.5kN, so axial force possibly generated by stepper motor is orders of magitude lower.

From this calculator [www.tribology-abc.com], is seems that 5mm steel rod should be enough to transfer required moment (300mm length, 0.3Nm, ~1deg error)

I'am affraid that your 3 shaft system won't work - individual shafts can rotate freely, so they won't transfer moment by torsion, they will only bend. Gluing the shafts together will help, but then you can use only partial bushings (but still circular, which may be considrable advantage)

Also assumig 0.3Nm and friction surface 5mm from center of shaft (independent of it's profile), the resulting force will be 60N. With TPFE on steel (cof 0.04) it's 2.4N X-axis friction. This force should be fine (0.015Nm on motor? but maybe I'am wrong here ... )

So maybe using reasonably sized rectangular profile and 2 flat TPFE inserts in small gear could work ...

I agree about the flat bit. A piece of flat bar stock seems ideal. Probably 1/8" (3mm) thick, and as wide as will fit inside the profile of your gear. Might want something with more of a polish to it to reduce friction though.

---

--
I'm building it with Baling Wire

The main reason for the three rods is the need for minimal machining.

The main constriant is the fitting inside the 8mm ID bearing.

I was going to use some kind of "moveable shroud" - maybe even just another pinion that could be "pulled along" to hold the three bars together. This would constrian their position relative to each other, but could still permit torsion around the central axis.

Another alternative would have been to epoxy the three bars together in the middle, effectively making a composite shaft, or an Ideller shaft allowing the three bars to be driven from both ends with just one stepper motor.

Thanks for the input guys, great info on bearings and PTFE CofF.

-Andy

I like the idea of putting the stepper to the side, but i think that the gear will jam on the axis.
what maxbe would be better is to use a very looong gear.
this could be built by printig many small ones and then align them on the axis (rectangular or something)

have you thought of a flexibel shaft like this ?
that maybe would work too and is easyer to built, but i dont know how exact the tork can be transmitted by such a thing...

---

my projects: [www.robotik.dyyyh.de] ...

Goaran,

There may be a few issues which may lead to jamming, most notable the angle of twist on the shaft, but they could probably be designed out.

What factor do you think will lead to the gear jamming?

I've not built a reprap yet, so I've not thought this thoroughly through, but do the print heads move or vibrate much when the extruder stepper isn't running?

If the carriage is on the X-Axis runners, and the gear is held in place by two 608rs bearings, so should remain aligned.

One advantage of the three shafts is that if they are correctly tolerenced (possibly higher tolerance band than the Main Axis Runners) then the chances of high spot (larger diameters) occuring at the same place would even out over the length.

As an aside, I'd absolutely love to make one of the X-Axis Runners multi-purpose, effectively using the Existing Runner to transmit the motion to the pinion. The pinion shaft would become the runner or the runner would become the pinion shaft.

TBH, I've not built even one of the proven design of extruder yet, so I'm probably a few weeks off trying this concept.

I like the idea of a very, very long gear, but that will be quite hard to build.

I have one of those rotary shafts - in - a - sleeve (sorry I don't know their proper name) for my Dremel. They are reasonably heavy, and would still have to move with the Print head. It may be an improvement on the Stepper ~Motor weight, but you still have to move quite a complex system back and forth and then there's the force needed to deform the cable and sleeve. I'll have to check shaft speed and torque those cables can carry, to see if we can use it at its current spec, but dremels are considerably faster speeds than the steppers.

I will put "test with dremel cable" on my to-do list as it may be an easy win, though it will add to the Vitamins we need.

Thanks for the post, two great ideas into the mix there, and certainly another couple of things I really need to try!

-Andy

(*Slow Progress here, I'm having to fit this in around standard engineering activities of drinking, music, coffee work and hang overs)



Edited 1 time(s). Last edit at 02/10/2011 05:29AM by andysuth.

From your pic, I see you still haven't addressed the fact that the gears will unmesh when it's moved in one of the two directions (left-back in your pic). You'd have to put something on both sides of the little gear to keep them meshed after movement, or use a herringbone gear.

Cefiar,

I'm not sure a RepRap would produce satisfactory results in its current incarnation for a Helical Gear, yet alone a Double Helical Gear.

If you rely on the "Rim" concept you suggest, that would increase wear/shaft friction on the system.

However, you have assumed that the bearings aren't directly connected to the pinion shaft, which they would be, though its not apparent from the current pictures.

I'll update these when I get chance. Having produced most of it in SolidEdge so far I'm now switching to SolidWorks as SolidEdge ST3 is far too buggy for me to waste time on.

-AS

andysuth Wrote:
-------------------------------------------------------
> Cefiar,
>
> I'm not sure a RepRap would produce satisfactory
> results in its current incarnation for a Helical
> Gear, yet alone a Double Helical Gear.

I have seen pictures of a Wade's extruder variant with double-helical gears, though I can't remember now where. That part can be done. What concerns me about this idea is that the forces keeping the pinion in position on the spline shaft are not in line with the friction forces between the gear and the spline shaft. With the gears loaded (while extruding) and the X axis moving, I think the pinion may tend to bind on the shaft. Straight cut gears will not maintain mesh by themselves, but it should be almost trivial to design something like transmission shift fingers to move the pinion along the spline shaft.

This all reminds me of something I observed as a child playing with LEGO gears. Gears tend to push away from each other when under load. A pinion mounted on a long shaft may push away hard enough to deflect the shaft and allow the pinion to skip, or at least mesh very inefficiently. You might need to design your gears with a lower than usual pressure angle. Something to consider. On the other hand, my memory or LEGO gears is that they appear to have a pretty high pressure angle, maybe 30°.

...

> I'll update these when I get chance. Having
> produced most of it in SolidEdge so far I'm now
> switching to SolidWorks as SolidEdge ST3 is far
> too buggy for me to waste time on.

Speaking as a long-time SolidWorks user, I find this interesting. SolidWorks is certainly not perfect, but it's hard for me to imagine something significantly worse having any success in the CAD market. At least you'll have me to tell you what went wrong when you run into trouble. For whatever that's worth. However, I know a lot of new SolidWorks users encounter what seems like flaky behavior that is really the result of inexperienced or undeveloped methodologies. I don't know what your level of expertise is, I just want to make sure you're aware that this is a significant factor.

Dale/Andy: The Herringbone geared extruder (by Rhys Jones) has the gears I was talking about.

Yes, your extruder needs to be working well to print it, but a number of people have printed it successfully, especially when using small nozzles (eg: 0.35mm) and 1.75mm filament.

St3 SolidEdge is hideously buggy.

Seriously, when we switched from st2 to st3 I experienced negative work flow: the more I worked on a part, the worse it got, I've never seen anything like it, st3 is a complete joke.

They've taken tasks that previously took 2mouse clicks and now require 4 mouse clicks and keyboard ops. Radius functions produce magic donuts. Text never aligns when producing flatwork. Animation is uncontrollable.

Add in the quirks like solidedge has never had reference geometry axis and so many other bugs, I just now think my life is too short for solidedge.

I speak as someone who is familiar with edge, works, ProE and little bit of inventor, i'd prefere to work on ProE, but works will do as I have access at work.

As to the rest of the post, a lot depends on bearing configuration and shaft design. The rotating shaft wouldn't support the opinion directly, but run through the shaft.

To be honest I think I'm going to put this on the back burner until I've at least built a rep strap, as I'm way behind schedule with that!

Good idea! Save development for the 2nd printer.

I haven't found the earlier discussions on this idea yet, so maybe I'm repeating things already discussed..

Why not use a single square shaft?

Could driving with a remote DC motor and a pinion position encoder down on the extruder allow the drive to be more tolerant of torsional energy storage? If so, it could enable all sorts of power transmission schemes.. springs, speedometer shafts, etc.

Is there any possibility to simplify it even further and use a single round rotating shaft and a pickup wheel with some sort of friction material embedded in the edge (eg. an o-ring?)?

Lastly, if we bolt the pinion at the extruder to the end of the transmission shaft, and let all the sliding along the shaft occur at the motor end, we can further simplify and reduce the mass at the extruder.

In all honesty, I think the square shaft is probably the leading idea.

I like the encoder @ print idea, though the main problem with shaft torsion will be binding /angular misalignment with the sliding pinion.

The problems with a square shaft are:
1. Control of tolerence/flatness/perpendicularity/etc around the shaft and along the length. (easier with round shafts).
2. Machining of the square hole

I say machining as I think its better (at least at first) to use a PTFE insert to lower the friction.

The way this looks like its heading is to enlarge the bearings (means a slight increase of the pinion PCD) to allow the shaft to be bigger.

The Main problem is for me time, I'm away from my home during week and under pressure to finish the DIY at home when I'm back. I'll see what I can do during the week in calculations.

Thanks for adding to this discussion.

-Andy

andysuth Wrote:
-------------------------------------------------------
> I like the encoder @ print idea, though the main
> problem with shaft torsion will be binding
> /angular misalignment with the sliding pinion.

Couldn't we trap the pinion on the shaft between two bearings? Sliding that construct up and down the shaft will be more difficult and whoile we can't eliminate that difficulty, but couldn't we at least locate it off the extruder next to the motor, saving the weight and complexity at the extruder?

> The problems with a square shaft are:
> 1. Control of
> tolerence/flatness/perpendicularity/etc around the
> shaft and along the length. (easier with round
> shafts).

If I'm using an encoder at the pinion, don't the requirements on the shaft become less stringent? It only has to be sufficient that we can slide stuff down it without binding. Wouldn't that allow a very sloppy slider design that can compensate for larger irregularities in the shaft? There's not a lot of force on that shaft since we're driving the pinion and gearing down at the extruder.

> 2. Machining of the square hole

Depends on where we need the hole. If we need it in a printed piece, square shouldn't be too difficult. I'd think a PTFE insert for a square hole to a square shaft should be fairly easy to machine, especially since we could probably make it with four separate pieces (sort of like an inside-out picture frame).

For bearings, perhaps we can get lucky with an SAE/metric matchup and just let the shaft corners slide along the insides of the races? Won't last forever, but maybe long enough?

> I say machining as I think its better (at least at
> first) to use a PTFE insert to lower the
> friction.
> The way this looks like its heading is to enlarge
> the bearings (means a slight increase of the
> pinion PCD) to allow the shaft to be bigger.
>
> The Main problem is for me time, I'm away from my
> home during week and under pressure to finish the
> DIY at home when I'm back. I'll see what I can do
> during the week in calculations.

I'm glad someone here knows how to do the calculations . Lacking that sort of background,. I'm mainly stuck with the experimental approach.

> Thanks for adding to this discussion.
>
> -Andy

For the "long gears" idea, you might be interested in products like McMaster-Carr 6847K11. Known as "gear stock" or "gear rod", it's basically a long bar with the cross-section of a spur gear. They're solid steel and fairly large (13 mm dia) so deflection might not be a major problem, assuming that it's mounted well with bearings on either end.

My main concern with this approach would be sliding friction as the x-axis is moved, and in particular the resulting wear on the extruder gear. But mechanically it's probably the simplest option, and with very low backlash.

However, the inertia of a foot-long spinning steel gear is not negligible, so the extruder may prove difficult to stop and reverse quickly.

I think I'll write this up some time soon, to capture the thought process going on here.

I was off learning about ball screws and ran across ball splines, like these from www.thomsonlinear.com. Low friction lengthwise, even under high torque loads. These are most certainly overkill, probably pricey, and doubtful they could be reprapped, but I thought I'd include them as food for thought.