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Topic: low RPM, high torque, variable speed, reversible motor set up (Read 1 time) previous topic - next topic

badbanterer

Hello,

I apologize if this is not the right forum, please direct me if it is not.

I stumbled onto this forum and I hope that you are the right people to ask this question. I am looking to create something that can rotate/pivot a load of about 120-200 lbs on a pivot of perhaps 3-4" diameter and would need help in finding/fabricating some kind of motorized kit for this. The speed setting need not be elaborate or super precise, a potentiometer will do.

I am a retired EE and am not well schooled on mechanics so I do not know how to translate those requirements into some torque spec.

I tried it with several drills (crude I know but I am just trying a proof of concept)  and it has more than enough torque but speed control via a trigger is too iffy and thus dangerous but I do like the ability to just use the chuck of the drill to connect or disconnect from the shaft.

Can anyone suggest where and what I might start to look at that would be controllable? I think that the max speed I need is maybe 10-25 RPM.

All suggestions are welcome.

Thanks,
Jorge

jremington

This beefy motor/worm gear combo is worth considering. Rotates about 60 RPM at 12V, but will run correspondingly slower on much lower voltage. Suitable motor drivers are available from Pololu.

I would use a ball bearing turntable to support the weight.

badbanterer

That looks great! Again, forgive me in advance for asking these very basic questions. How would I go about turning a threaded, 1/2 shaft? Can I assume that this is reversible simply by changing polarity? I don't really recognize the shaft (in the photo, if it is what I think it is, it looks more like a hole), is that what is 21 cm (diameter?)

MarkT

Hello,

I apologize if this is not the right forum, please direct me if it is not.

I stumbled onto this forum and I hope that you are the right people to ask this question. I am looking to create something that can rotate/pivot a load of about 120-200 lbs on a pivot of perhaps 3-4" diameter and would need help in finding/fabricating some kind of motorized kit for this. The speed setting need not be elaborate or super precise, a potentiometer will do.

What is the friction of the bearing with that weight on it?

How fast do you want to turn the load?

How fast to you want to rotationally accelerate the load? (Equivalently, how quicking from
stationary to full rotation speed?)

What is the moment of inertia of the load (failing that sketch its size and shape so this can be
estimated).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

badbanterer

Unlike most of the projects here, my project would probably be considered crude (but perhaps more of a challenge due to the changeable and variable nature of the load), not that a range of numbers could not be determined (though I have forgotten how to even start to calculate them now).

This is a device that I am trying to construct that will merely turn or pivot a human from above. So, the load while a constant weight, would probably still be perceived by the motor as dynamic since humans cannot stay perfectly still and balanced.

The bearing would be the tip or ball of a shoe on a smooth, probably hardwood floor.

Acceleration is not a factor since the RPMs are so low, they will be up to speed in just a few seconds.

RPMs would vary based on the nature of the dance (the dancer's turn may be powered by the partner or in the case of ballet, she would be powering it herself. I imagine the range would be anywhere from 5-30 RPM. Even if they rotate slower or faster, that range would simulate their real-world experience.

Sorry to be so vague, I've never tackled anything like this before.

Paul_KD7HB

A drawing of your invention would surely help a lot. How are you envisioning connecting a person to your machine? Be sure to consider the thrust bearing necessary to support the weight of the machine and person(s).

Paul

MarkT

Unlike most of the projects here, my project would probably be considered crude (but perhaps more of a challenge due to the changeable and variable nature of the load), not that a range of numbers could not be determined (though I have forgotten how to even start to calculate them now).

This is a device that I am trying to construct that will merely turn or pivot a human from above. So, the load while a constant weight, would probably still be perceived by the motor as dynamic since humans cannot stay perfectly still and balanced.
Its a highly dynamic load capable of 100's of watts mechanical input working against your input, so
the figure are going to be large - torques in the range 100Nm, powers 100's of watts at a first guess,
your first defence is to select worm gear so back-driving isn't such an issue, and make sure the drive
system can handle the dynamic forces and torques while static, as a first step.

Can you explain more?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

badbanterer

I made a pdf drawing which I believe depicts my idea much better than I have described it.

Thanks for the tutelage.

Paul_KD7HB

Yes, that does help immensely! You might include an activation switch so it only operates when someone pulls down a bit.

Paul

MarkT

Hmm, I notice you have a roller bearings that are expected to have large axial movement - that's not
going to work, roller bearings only rotate - try to move axially and you risk siezing the thing and cracking
things as the roller ends dig in or spall and brinell.

Anyway the top shaft collar needs a thrust bearing.

I think its simpler just to use two angular-contact ball races, like a bicycle wheel, bicycle bottom bracket
or magneto bearings.  Forget the spring arrangement and sense weight some other way.

Anyway looking at the drawing I see enormous torques at the attachment point of the frame to the
axle - this is a huge stress concentration and suggests the axle will have to be very thick and sturdy,
as will the cross-member fixed to it. Something like 24mm or thicker studding might be usable for the
axle and make connecting the cross member easier.  Studding is always loose in a bearing inner race,
so that could be padded out with tape to prevent rattling.

Such thick axle means larger bearings, which will be far stronger than is needed for the actual load
they experience, but I reckon the risk of bending the end of the axle is something its important
to prevent, as that would just jam the mechanism.

And lastly safety - what are the risks if the thing breaks?

You'll have to figure out how to prevent the cross member turning on the axle
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Paul_KD7HB

quote: Hmm, I notice you have a roller bearings that are expected to have large axial movement - that's not
going to work, roller bearings only rotate - try to move axially and you risk siezing the thing and cracking
things as the roller ends dig in or spall and brinell. unquote.

That is why tapered roller bearings are used.

Paul


badbanterer

Wow, this is all great info but I'm afraid, over my EE head.

Ok, so I imagine Paul, that you mean a tapered roller bearing for the 1/2" rod.

Mark:
  • Yes, I can add a thrust bearing under the top collar.
  • I have no idea what " two angular-contact ball races" are. I will try to look it up on McMaster.
  • The threaded rod is 1/2", it isn't bending. Though the unit is sort of tall and "narrow" and thus does tend to want to have the bottom move left/right with respect to the top since that is where the load is.


The spring was to push the assembly back up to then get the drill to turn off.
I also do not know what "studding" is. Can you elaborate or refer me to a link?

Thanks,
Jorge

Paul_KD7HB

Yes, tapered roller bearing for the shaft. AND bearings need to be pressed onto shafts so the inside part of the race turns, and not the shaft inside the bearing. Don't need to be a tight fit. You should be using steel tubing and not solid if there is any chance of bending the shaft. Also a LOT lighter weight.

Paul

badbanterer

So is this the type of bearing? https://www.mcmaster.com/23915t84
This is very expensive compared to the mounted bearing I have now (though now I realize I am using the wrong part).

Can you recommend where/who might have a suitable, less expensive part?
I would need a mounted tapered bearing. This project suddenly got much more expensive just by these bearings alone. Please don't get me wrong, I am still very interested in doing this, the project just got a whole lot more complex. Yes, I realize that it did not get more complex, I did not know what it would take to make this project work.

I am looking at instruction video for the installation of a bearing and I have zero experience or knowledge of this. I however LOVE how much more precise this appears to be.

Now, in order to use this, it looks like I am now having to use an actual shaft rather than merely a "threaded rod" (more money).

Can you explain how one goes about driving the shaft? I was using the drill's chuck because I know nothing about gears or transmissions.

Lastly, if I draw the "top view" of this contraption, can you make suggestions with respect to a more stable set up? This is basically a single spine and will be twisting all over the place so I need more width.

Thanks,
Jorge

Paul_KD7HB

I used a pair of tapered roller bearings on the swiveling part of a fairly large whirley-gig because I had them. They came from a rear axel of my previous pickup, a 1982 Chevrolet. The bearings are mounted in aluminum blocks with locking collars on the vertical shaft keeping them in place.

I also happen to have a small hobby machine shop. I think you need to develop one, also, if you are going to imagine this sort of stuff.

The easiest drive for your mechanism will be "V" belt and pulleys. A single "V" belt can transmit about 10 HP. before slipping. This is much more than you will need.

And you can design the frame connected to the wall to have a plate securing the motor of your choice.

Paul


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