External bearings for stepper shafts
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Here's a NEMA-17 motor with 30+ mm long shaft:
https://www.ebay.com/itm/NEW-Nema-17-Stepper-motor-76oz-in-w-FLAT-CNC-Robot-Reprap-Makerbot-Arduino-11V/321970627102?hash=item4af6f1e21e:g:~wsAAOSwBahU52GO -
@deckingman said in External bearings for stepper shafts:
@Nxt-1 Could be tricky to get everything aligned properly I'd have thought. A bit of misalignment could end up putting more load on the motor bearings than just the belt tension if you didn't take great care.
This is what I am afraid of with this idea as well. If you do not want to make to problem worse then it is, there is very little room for alignment errors.
@deckingman said in External bearings for stepper shafts:
@deckingman While we are in "crazy idea mode", how about a centre hole in the end of the motor shaft, then use something akin to a revolving centre? Like a live centre in the tail stock on a lathe which is used for the sole purpose of supporting the ends of long shafts to prevent them bending while being machined.
In this case if you really want support from a revolving center this small, my guess is that you would need to apply quite a bit of force axially, which would have to be accounted for by the steppers bearings. That is sadly what we are trying to eliminate
@mrehorstdmd said in External bearings for stepper shafts:
Here's a NEMA-17 motor with 30+ mm long shaft:
https://www.ebay.com/itm/NEW-Nema-17-Stepper-motor-76oz-in-w-FLAT-CNC-Robot-Reprap-Makerbot-Arduino-11V/321970627102?hash=item4af6f1e21e:g:~wsAAOSwBahU52GOA few remarks, I have kind of set my mind on nema 23 steppers at this point unless there is some really compelling reason to not do that. Second, that stepper is 1.8°, eliminating it for this project. Also the coil inductance is not specced and guessing from its 12V rating it will be quite high.
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@Nxt-1 said in External bearings for stepper shafts:
there is very little room for alignment errors.
that's where those soft couplers come into play
you get the axle with bearings and pulley where they need to be, you link the axle with the motor using flex coupler, they transfer torque (small alu ones go up to 3Nm) will have no backlash and will allow slight misalignment between two shafts. Even the ghetto couplers we made 10 years ago out of the plastic tubing held pretty good for 3d printing purposes, they even work on the mill, flex couplers would totally work here -
@arhi said in External bearings for stepper shafts:
@Nxt-1 said in External bearings for stepper shafts:
there is very little room for alignment errors.
that's where those soft couplers come into play
you get the axle with bearings and pulley where they need to be, you link the axle with the motor using flex coupler, they transfer torque (small alu ones go up to 3Nm) will have no backlash and will allow slight misalignment between two shafts. Even the ghetto couplers we made 10 years ago out of the plastic tubing held pretty good for 3d printing purposes, they even work on the mill, flex couplers would totally work hereWhat I understand you said is: stepper -> flex coupler -> 2nd shaft with bearings and pulley. If I got that correct then how does that differ from what I suggested in the first post of this thread?
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@Nxt-1 said in External bearings for stepper shafts:
In this case if you really want support from a revolving center this small, my guess is that you would need to apply quite a bit of force axially, which would have to be accounted for by the steppers bearings. That is sadly what we are trying to eliminate
No - you misunderstand. I was thinking along the lines of a mini lathe tail stock with a revolving centre fitted. Actually, not as complicated as a tail stock - just a couple of bearing blocks to hold he centre. Essentially, it's like your diagram but instead of the pulley being the coupling between the two shafts you use a centre - doesn't even have to be a "live" revolving centre, it could be a dead (static) centre. It's difficult to explain if you've never used a lathe - take a gander at this https://www.youtube.com/watch?v=P5os5RRaG58
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@Nxt-1 said in External bearings for stepper shafts:
What I understand you said is: stepper -> flex coupler -> 2nd shaft with bearings and pulley. If I got that correct then how does that differ from what I suggested in the first post of this thread?
exactly, exactly, sorry, long thread I forgot we started with that, but I don't see any reason not to go that way, it's commonly used in many professional machines
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@Nxt-1 indeed, the bearings and shaft are all that we need. However, what's the cost of that? Is it the same as the motor that you are driving? I've seen legitimate motion control product development companies use a stepper with the wires cut off as a bearing block/idler shaft. It's sometimes cheaper than good fiddly bits, if those steppers are already purchased in volume.
I personally like better the even crazier idea of a bearing supporting the pulley.
*not actually a robot
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While we are on the topic of crazy ideas... this one is good sit down.
You know how the ends of stepper motor shafts have little holes in the end? (at least, mine do. But they don't have D cuts either, so they aren't exactly typical.)
I think these are so that you can add a tapped hole to the end, to screw something to the end. Like, perhaps, a shaft extension?
Use an M3 stud (just a threaded shaft) to join the stepper motor (which you have tapped the end with M3 threads) and this.
lololol
*not actually a robot
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@bot said in External bearings for stepper shafts:
@Nxt-1 indeed, the bearings and shaft are all that we need. However, what's the cost of that? Is it the same as the motor that you are driving? I've seen legitimate motion control product development companies use a stepper with the wires cut off as a bearing block/idler shaft. It's sometimes cheaper than good fiddly bits, if those steppers are already purchased in volume.
I personally like better the even crazier idea of a bearing supporting the pulley.
The only real issue with that is space, I believe I do not have space enough to add the length of a coupler in there. I would have to rotate my steppers 90°, so they are square to the tower extrusions. This would also require me to redesign the carriages to accommodate for the rotated belts.
I will double check my CAD files to see how much space I have without rotation.
@bot said in External bearings for stepper shafts:
While we are on the topic of crazy ideas... this one is good sit down.
You know how the ends of stepper motor shafts have little holes in the end? (at least, mine do. But they don't have D cuts either, so they aren't exactly typical.)
I think these are so that you can add a tapped hole to the end, to screw something to the end. Like, perhaps, a shaft extension?
Use an M3 stud (just a threaded shaft) to join the stepper motor (which you have tapped the end with M3 threads) and this.
lololol
That is not to bad of an idea, the issue is that I do not have the capabilities to drill and tap a hole in the stepper shaft, which might even be hardened. Lets alone that I can do it with the accuracy that is required in this case.
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I just checked the CAD model, assuming 54 mm long steppers, they stick outside of the frame by 4.8 mm. That might be acceptable actually. I will think about it.
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I got back from Oriental Motor with regards to my question if steppers with custom shaft are something they do. They do with a MOQ of 500/year, so no option.
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@Nxt-1 said in External bearings for stepper shafts:
I think my plan of action is going to be in this order:
- Ask stepper manufactures if they will make me a stepper with a custom shaft (single shaft, extra long). If they will and for a reasonable price (which I strongly doubt) this is the route to go I think.
- Get a nema 23 dual shaft motor and try 9 mm belts with the corresponding pulleys. This leaves no room for an external bearing in front so the bearing idea is dropped. If the motor seems fine at highish tension, then were good.
- Use the motor from 2) and move to 6 mm belts with the corresponding smaller pulleys. This leaves room for an external bearing in front and back.
- Use the motor from 2) use a rigid coupler to a second shaft, where the pulley for 9 mm belts and two bearings sit on. Redesign the belt paths, carriages and rotate the steppers 90° so they are at a right angle to the tower extrusions.
I am not sure about 3) and 4), they might be swapped around as proper belts are not really cheap and redesigning the carriages only costs me time and effort.
- has been more or less crossed out and I currently debating to skip 2) and 3) and go straight to 4). Assuming that I go for single shaft steppers and stick to 54 mm long steppers at max I can fit a coupler and shaft with needing to rotate the whole shebang. I will start looking into steppers again in the motor thread, I expect to find quite few more options if dual shafts are not a requirement.
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@bot said in External bearings for stepper shafts:
You know how the ends of stepper motor shafts have little holes in the end?
Normally ones without D shaft have those. I have bin told by a machinist that's 'cause you would normally heat press the pulley on that shaft (heat the pulley so it expand, put on the shaft, leave to cool of and grab the shaft) and to remove it you need to use the "radapcieger" (no idea what the english word is, but it's a puller). You attach 3 arms to the pulley and press the center in to the hole of the shaft and pull the pulley off. Without that hole it would be very hard to do it.
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You could also turn the pulley 180 degrees on the shaft; the motor mount looks to be slotted, so after turning it around, you move the motor closer. The result is, there's less moment forces on the shaft, since the bearing point that's at the shell face. The greater distance from the bearing point, the greater the eccentric load on that shaft.
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@arhi it's called a gear puller: https://duckduckgo.com/?q=gear+puller&t=peppermint&ia=images&iax=images
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@mrehorstdmd said in External bearings for stepper shafts:
@arhi it's called a gear puller: https://duckduckgo.com/?q=gear+puller&t=peppermint&ia=images&iax=images
yeah, thanks, I always forget the English name for it .. radapciger here and in Germany:D
https://duckduckgo.com/?q=radapciger&t=peppermint&iar=images&iax=images&ia=images -
I was considering something similar to mrehorstdmd. A bearing carrier with an idler bearing on the shaft above the pulley. Then I saw someone who went as crazy as me and did it. The additional tension he was able to apply to the motor was minimal, less than 5% before the tension on the belt system still stalled out the motors. 3 months later the bearings in his steppers and half his gates pulleys were shot. The best guess is that the extra tension caused the bearings in the pulleys to overheat, seize, and that transferred stress to every component in the drive system. I might still eventually do it, but it will be more to limit stepper shaft deflection during acceleration and speed changes. As for increased belt tension, I think you're better off keeping it simple and using bigger steppers AND idlers.
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My solution, I know it is not perfect but works fine over hals a year. And before that I also broke my stepper motor shaft. -
@Nuramori said in External bearings for stepper shafts:
You could also turn the pulley 180 degrees on the shaft; the motor mount looks to be slotted, so after turning it around, you move the motor closer. The result is, there's less moment forces on the shaft, since the bearing point that's at the shell face. The greater distance from the bearing point, the greater the eccentric load on that shaft.
You are right. When I install the upgrades, that is how they will be mounted for sure. This is how the temporary model looks right now, excuse the lack of both bearings in the stackup, I forgot those when I was drawing it
.
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@dragonn said in External bearings for stepper shafts:
My solution, I know it is not perfect but works fine over hals a year. And before that I also broke my stepper motor shaft.Very nice! How wide is the part of the pulley that sits inside the bearing. With the pulleys I have at the moment, the contact surface would only be ~1mm wide.
