1. ## Hollow shaft stepper?

Hello.
I discovered this forum a few days ago, and I thought it could be the place to get some help.

(First of all, please forgive my english and my poor scketching skills)

For a little project, we will build a stacking of 3 wooden box (10x10x6 inches). one at the base that will be ground or wall mounted, and two other on the top that will have to rotate independantly.
(Their axis of rotation could not be concentric)

We have a couple of steppers that could do the job.
The thing is that we need to keep the wires hidden in the "sculpture" that is why I 'm trying to create some holow shaft rotations.

Controling the steppers is not a problem but I'm really new to mechanics.
Even if this sound to be basic things, I would need an external advice to know how to achieve this.

Here is a simple sketch. where I'm showing you my starting point.
I have heard about harmonic drive that could offer an hollow shaft but it is far to expensive for our use.

Do think I could achieve this with hardware such as pulleys, belts and strong bearings that could handle a radial weight (if this is wall mounted)

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2. I would suggest installing both steppers in the base and using simple concentric shafting and toothed belt drives to transfer the motion to the second and third boxes thereby enable infinite rotation of the sculpture without the problem of dealing with wires. The intermediate bearings could be a simple ball race such as used for office chairs or lazy susans.

3. Hi Thomas,
I know you need answers but I have to ask you some questions

What is the rotation speed of the wooden box? And the weight of the woden box?
Are you sure that you don't need to build a frame (for example made in steel or aluminium) inside the lower box?
I suppose the lower box is not strong enough to support the weight of the two others and the forces of the transmission system.
Furthermore you need to fix the stepper somewhere otherwise it starts to rotate due to reaction torque couple.

@bluerover.
My rotation axis won't be concentric, so I can't use this solution unfortunately.
@PierArg.
We do not need infinite rotation. when rorated at 360° , the box will go in reverse and so on.
Weight is a reall issue. our prototypes are in wood, but we can make it out aluminium or plastic to get something lighter.
steppers will be fixed inside each box of course ( forgive my sketch which was really basic)
I thought the main problem was to find strong bearings that could handle radial forces and support all the boxes weight. Once these bearings found, I think strong stepper could do the job.
I found something called slewing rings that could handle large radial forces:
http://www.ritbearing.com/images/pro...g-bearings.jpg
have your ever heard of it?

5. Hi Thomas...I believe I begin to understand what you are going to realize.
Can you tell me what is the weight of each box?
And what about the round speed? How long it takes to complete 360 degrees?

Pay attention about the bearings!!! They don't absorb the reaction forces but they just transmit them to the seats in which they are installed. So the woden box, in the bearings seats, are subject to forces.

I think you can create a frame hidden in the first (lower) box wich support the other boxes and the reaction forces of the transmission mechanism.

How do you think to supply the steppers? By an internal battery?
Do you want to use only one stepper for all the boxes?

6. HI PierArg.
Happy to see you trying to help me.

If we make the box a bit smaller and out of plastic, they should weight around 1/2 kg I guess (without stepper in it)
The speed does not need to be higher than 90°/second.

I,now, understand the use of the frame needed in the basis. the wood was very strongly assembled, but plastic will be weeker.
the controller board of the stepper wil be fed with wall power, maybe hidden in the fixed base box .

I think I cannot use one stepper for the 2 rotating boxes on the tob of the base because their speed and rotation direction will be different. So we will use 2 steppers

do you want me to create something more detailed with my beginner solidworks skills?

7. You're welcome.

Ok, I understand more things!

So you want to create a sort of robot-arm made by 3 pieces (box). What is its application? I suppose it is a sort of marketing "totem" for an advertisement application!

If you can write down a mock-up of your device I'd be very happy to help you.
It is not important you do a solidworks model...but is sufficient just a simple hand drawing.

If I was not very busy here at the office, I would have done it for you!

8. Originally Posted by PierArg
You're welcome.

Ok, I understand more things!

So you want to create a sort of robot-arm made by 3 pieces (box). What is its application? I suppose it is a sort of marketing "totem" for an advertisement application!

If you can write down a mock-up of your device I'd be very happy to help you.
It is not important you do a solidworks model...but is sufficient just a simple hand drawing.

If I was not very busy here at the office, I would have done it for you!
It is not something commercial as we are still student but it is kind of a moving plate on which little mockups will be shown. we want to have something more attractive than simple mockups on a table. Ours will be in motion!
As axis of rotation are not concentric, actually it could be compared to a robot arm but with 2 articulations only.
I will make a new sketch.

Are you involved in mechanical design?

9. I'm a mechanical engineer (I'm 28 years old...so I am only 5 years experienced ) and I work for a company that manufactures high pressure pumps.

10. hello again, here is a new quick sketch to show you how boxes and rotation axis will be arranged

11. Ok, I see!
What do you want to put on the planes?
The upper box, during its rotation, could go into collision with the object that you'll put on the second floor (second box).
The same problem is even for the second box that collides with the objects in the first floor.

Have you studied the trajectories of the boxes during their rotational movement?
Have you considered a sort of envelop space?
Once you have this space you have to be careful not to put objects in it.
I hope you understand me...i'm so sorry for my english.

12. Yes there is no issues regarding the boxes trajectories. Little 3d printed object will showcased at the end of each box. No possible collisions between them and the boxes (I admit it is not easy to see that in the 3d view)
I am more concerned about the movement transmission from the steppers and the choices of the bearing.
do you think I could use slewing ring as shown in my previous message?

13. Yes there is no issues regarding the boxes trajectories. Little 3d printed object will showcased at the end of each box. No possible collisions between them and the boxes (I admit it is not easy to see that in the 3d view)
I am more concerned about the movement transmission from the steppers and the choices of the bearing.
do you think I could use slewing ring as shown in my previous message?
what do you mean by "envelop space"?

14. For "envelop space" I mean the space interested by the trajectories of the boxes during their rotational movement.

About the slewing bearings, first of all you need to define your transmission system.
Once you have the forces, you can choose the right bearing.
I think your application require just a simple ball bearing....the slewing ring is too expensive.
Usually the slewing ring are used for heavy loads.

15. thanks PierArg.
I do not really know which transmission to choose... do you have an idea? gear? or pulleys?
I found a stunning piece of art, which is kind of a huge robotic arm. one base is supporting all the moving arms.
How do you think it is made? what kind of bearing are used?
http://vimeo.com/22001314

16. Hi thomas,
I cannot open the link. Appears the error "page not available"
Could you find something similar on youtube?

To choose your most (or best?...what is the correct word?) suitable transmission system you need to do some consideration:
- price (is the most important....)
- what is the reduction ratio you need to realize?
- how much space is available in your machine
- rigid (for example gears) of flexible (pulley-belt)?

I think, but don't give importance on what i'm going to say (first you need to verify the feasibility), you could use a pair of gears.

17. If I understand what you're trying to do, each of the boxes can be supported by the preceding box. The bottom box supported and attached to the floor. The second box attached to the first but eccentric to its rotational axis. The third box would then be supported by the second box and rotate about an axis that is eccentric to both of the preceding boxes. If this is correct then the boxes provide support and the steppers on provide rotation up to 360 degrees of each box from its starting position.
The outer race of bearings such as Kydon Bearings (http://www.kaydonbearings.com/) can be secured to the preceding box with the inner race secured to the moving box. Done correctly, the bearings will allow a good amount of offset load.
If I understand what you're trying to do, each of the boxes can be supported by the preceding box. The bottom box supported and attached to the floor. The second box attached to the first but eccentric to its rotational axis. The third box would then be supported by the second box and rotate about an axis that is eccentric to both of the preceding boxes. If this is correct then the boxes provide support and the steppers on provide rotation up to 360 degrees of each box from its starting position.
The outer race of bearings such as Kydon Bearings (http://www.kaydonbearings.com/) can be secured to the preceding box with the inner race secured to the moving box. Done correctly, the bearings will allow a good amount of offset load.

18. Hi Thomas

Your title, "Hollow shaft stepper," caught my attention, as did your note that you're on a student budget. Those two situations don't go together.

As a student, and even as a stuck engineer, I often try to plagiarize. (As my best mentor said 30 years ago, "Imitation is the sincerest form of flattery.") I like that people are suggesting offsetting the motor from the axis, and appreciate what Randall is doing, pointing you to available hardware.

I would suggest copying a bicycle headset (which lets the handlebar turn). Find drawings (online) or disassemble a junkyard bike. The shaft is hollow, as you need, and the bearings are substantial. The inner and outer tubes can be cut and threaded from water pipe using a lathe, and can be shortened or lengthened to fit your needs. You should be able to beg a bicycle repair shop for a useable cast-off headset (the bearing parts). That would give you a strong hollow-shaft bearing set for about \$10.

Buy a pair of appropriate gears... I think you're dealing with relatively low torque, so you have options. You could try for overstock parts stores (Silicon Valley and perhaps elsewhere), or you could try WM Berg Co online. Center the shaft-side gear onto a lathe and open the hole to fit your shaft. This would be a very good opportunity to try a press fit between gear and shaft, but I'd back it up with a tough epoxy.

Mount the mating gear to your motor, and locate the motor to mesh the gears. It's a stepper... you want minimum play at the gears. Adjustability would be a good thing.

I hope this gives you a few ideas. Cheap need not be ugly. Stretch your imagination.

19. I believe that the steppers in the lower box and the eccentric axes are not self exclusive. The initial shafts can be eccentric to the lowest box and the upper shaft (eccentric to both the lower and central boxes is driven by gears or a belt from the inner of the lower shafts. I hope this makes sense and I have already started to fabricate a "copy" of that which you have explained using two 1 rpm clock motors as it seems like a rather nice kinetic art project.

20. I fully agree with John.
You can even recycle some components of old or broken devices.
For example, if you have an old printer you can extract the steppers.
Or if you have an old washing machine you can use its elecric motor...ok, it is not a stepper but with the right transmission sistem you can obtain the round speed you need.

But I think first you need to write down a simple mock-up with one or more solutions.
So you can start to define better the single parts of your project.

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