Torque due to moment of inertia (?)

[Mech_LS24]

Atached is FreeBasic code = MS VB.

 

[Baluncore]

The code box for general code was broken, so I used a .pdf file.
I think you should focus on page 3 of Con_Rod_2.pdf
Are you happy with vectors as complex numbers ?
How are you getting on with understanding the modeling of the linkage ?

 

[sophiecentaur]

Result was it worked fine with non-parallel links!

I can assure you that the rods will be under tension or compression for angular displacements of the short motor bar.

It strikes me that the main requirement is for the connecting rods be normal to the 'servo' bar at rest. Then any change in angle will be equal and opposite when there is displacement. It's a matter of the dreaded Perpendicular Distance that counts, I think. A possibly better way would be to use a disc on the servo and connect it to the platter with a belt. That would keep the same ratio of torque under all deflection angles. But what error is tolerable? No need to gild the lilly.

 

[Baluncore]

But what error is tolerable? No need to gild the lilly.

If the error in linkage closure is more than the slack in the linkage, then internal closure forces will greatly increase bearing friction.

It strikes me that the main requirement is for the connecting rods be normal to the 'servo' bar at rest.

Which suggests that, at least then, they will be parallel.

With only one connecting rod, you can get away with almost anything. But more than one requires that the rod lengths be the same as the distance between the centres of rotation. The radius vector length and direction must be the same at both ends of anyone rod. So the rods will be parallel.

A belt would need to be stepped to maintain the required range of movement. To eliminate hysteresis there must be some belt tension, which adds more unnecessary bearing friction. Stepped belts have significantly higher friction than parallel connecting rods with thin pins.

 

[sophiecentaur]

Which suggests that, at least then, they will be parallel.

Not necessarily. I meant that the bar is replaced by a smaller Vee of two radial bars, so that the force from the rods would be normal then the result of displacement would be to balance the length errors. But it all depends on the permissible slop / error. But what you say about is trapezium, of course, correct
If the whole arrangement were to be made coaxial then there would be no problem at all - excepting the construction, which would possibly need to be better than the OP is suggesting.
Also, I wonder why the drive has to be with two rods. A single rod could be driven by the servo and sit between two pegs on the disc. The rod length would change with angle (Cosine) and friction could be an issue but would it be worse than any of the other proposals?

 

[Mech_LS24]

The code box for general code was broken, so I used a .pdf file.
I think you should focus on page 3 of Con_Rod_2.pdf
Are you happy with vectors as complex numbers ?
How are you getting on with understanding the modeling of the linkage ?

Thanks for coming back to me @Baluncore, I appreciate it. I have to admit, I can't follow that code, unfortunately. The focus for me is at the friction and the other suggestions for the mechanical design. It is getting a bit confused in my head. I think we are almost there, but still missing something.

I doesn't need two solid rods, but I though this will help decreasing the amount of force on the rods (as shown below). What are piano wire links/strings? Google searches brings me to real piano's

I can't find the piano wire links meaning, as mentioned in post#29, do you have documentation or so about these?

Thanks for the explanation @Baluncore. Unfortunately it is difficult for me to get a feeling for friction. If I am working on concepts, I try to argument what consideration should be taken into account. With a concept like this, I could say as a disadvantage the amount of friction in the rods. Therefore maybe a system with belt/pulley would be the better choice. But always the question remains, what is that amount of friction? A general, simple calculation would help here. But as I understand, friction is a very difficult subject?

Last, I am wondering how that friction should be calculated. I think we should first focus on a mechanism with the servo, disk and one rod. That should mean the system has two rod ends and the one 7200 bearing. The amount of weight for the mechanism is been determined in post#15. With my current knowledge, this should be the 'input' for calculating the amount of friction right? When the friction is known, I can add the torque for rotating the disk and decide what amount of torque that servo needs to supply.

Thus, I can take the 81.4 N as force and the radius to the path of the ball centre as arm? So:
M = F*arm length
M = 81.4(N)*10(mm) = 814 Nmm = 0.81 Nm

Now I can filter motor with an amount of torque of at least 0.81(Nm) + 1.13 (Nm) = 1.95 Nm. If I add a safety factor of 1.5 makes it, 2.92 Nm.

Correct?

This is the calculation I did in post#19, it is supported with a sketch as well in that post. Could you please verify it? The outcome of those numbers feels 'good' to me, but I doesn't get the feeling of friction in my fingers yet, haha..

Hopefully, a 'push' in the right direction would bring me to solve this :)

 

[Baluncore]

I can't find the piano wire links meaning, as mentioned in post#29, do you have documentation or so about these?

The connecting rods do not need to be heavy since the one pulling does the work, they will not buckle under compression. That is why they can be made from spring steel wire.
https://en.wikipedia.org/wiki/Piano_wire
eBay 'piano wire'
Google 'high carbon steel piano wire'
https://www.fushunspecialsteel.com/the-difference-between-carbon-steel-wire-and-piano-wire/

That should mean the system has two rod ends and the one 7200 bearing.

To build a low friction rod-end you should use a thin pin, NOT a ball and socket.
If the connecting rod is made from piano wire, you could heat the end red-hot and bend the end to a right angle or loop. Avoid heavy rods.
Until the rods and rod ends are defined, the mass and the radii of rod-end bearings cannot be known, so the numbers are meaningless.

A bearing or a pin has a loading force, a radius, and a coefficient of friction. The force or torque due to friction is the product of all three.

 

[Baluncore]

Now I come to think of it, bicycle spokes would make good connecting rod material. Spokes used to be made from piano wire.

 

[sophiecentaur]

Wouldn't polyester line do the job? You could add some light pre-tension into the arrangement and avoid backlash.

 

[Mech_LS24]

Sorry for the late response. I am working on the given information and try to come back with a decent reply to both of you tomorrow.

 

[Mech_LS24]

I have updated the mechanical (concept) design, which has the following features:

• One piano wire acting as rod (instead of two rods)
• Diameter of the disk = 200 mm
• Towerpro Servo with Torque= 1.6kg*cm
• Friction coefficient for steel-steel (dry) = 0.5
• Piano wire has connections with pin with diameter ∅6 mm

Due to the Torque of the servo-motor I could calculate the Force on the rod, which is 1.6 N. This force is inserted into the calculation for the amount of friction and Torque. Concluded I can say:

• The friction force coming from the servo motor is 0.8 N
• The amount of torque due to the friction is 2.4 Nmm

Okay, I have those numbers, but I want to calculate it the other-way around. Because I want to determine the motorsize instead of what torque arises from that motor.

Besides, I calculate the influence of the diameter with friction as @Baluncore mentioned earlier. I found, the smaller the diameter of the pin, the smaller the torque will be.

Wouldn't polyester line do the job? You could add some light pre-tension into the arrangement and avoid backlash.

Polyester lines aren't able to handle compression forces, so during 'pushing' the system the lines doesn't transfer the forces because the lines are 'weak'. Or do I misunderstand you?

 

[Baluncore]

Polyester lines aren't able to handle compression forces, so during 'pushing' the system the lines doesn't transfer the forces because the lines are 'weak'. Or do I misunderstand you?

Where a string is used it is used in tension. The advantage of two strings is that when one is unable to push, the other will pull. Polyester lines also have the advantage of low mass, but there is one problem, they are elastic. There will be hysteresis in the movement and there will be tension in the lines and on the bearings. The permanent tension forces carried by the bearings add an unnecessary friction multiplier to the system.

I might make the rod bearing and pin from the wheel and axle of a model car. The wheel would be grooved, then held in end-loops of the piano wire connecting rod.
The axle in the hot wheel models was made from thin metal guitar string.
https://www.motorbiscuit.com/hot-wheels-turned-a-failed-guitar-design-into-a-cultural-icon/

 

[sophiecentaur]

Or do I misunderstand you?

Yes, you do. I was suggesting replacing the two rods with polyester line. Some pre-tension would be easy to arrange (probably easier than with rods).
Franky, I would say there's been too much theory and not enough actual building and measurement. Any experiment like this has to start with something to work with and then it can be developed. There's no way that the initial setup will work as desired as something is bound to crop up that's not been thought of.
There is mention of simulation software. Simulation can work quite well for a lot of electrical problems but I would question whether this can be simulated in a valid way.

 

[Mech_LS24]

Where a string is used it is used in tension. The advantage of two strings is that when one is unable to push, the other will pull.

Okay I understand the principle. But when one string is in tension the other is limp and just hangs there?

Some pre-tension would be easy to arrange

Could you explain me pre-tension in the lines as you suggest here? Is it like pushing against the lines?

Any experiment like this has to start with something to work with and then it can be developed.

I agreed with this, but if I want to build something, I must roughly know design considerations and one of them is the motor size.

What should both of you advise to determine the torque needed to move a mechanism like this? I have done some calculations now during this topic, but none of them are fully judged...

 

[sophiecentaur]

Could you explain me pre-tension in the lines as you suggest here? Is it like pushing against the lines?

To get rid of any backlash, you can adjust the lengths of the strings so that there is some net tension. A lightweight 'bottle screw' is one way - pr even just a screw hook. That will eliminate any slack around the mid point. Whatever you do, you will need to eliminate backlash in some way and the method depends on the actual setup..

one of them is the motor size.

Newton's second law (adapted for rotary motion) will tell you the minimum motor torque, given the MI and the angular acceleration you want. You can measure the (static) friction torque with a small force meter and a string wrapped round the circumference of the disc.

I have done some calculations now during this topic, but none of them are fully judged...

I don't know what you mean by this but it suggests to me that you are completely new to this. That makes it hard for you. (Me to, aamof, as I have never done a Mech Eng course). Am I right in concluding that you just want to simulate this problem and not do any measurements? I am old school and I am not totally convinced about the use of simulations until you actually know the practical factors. Every variable has to be specified and if you are not careful, an easy-to-use simulator can put in values for you which can result in unrealistic answers.

You seem to be asking practical questions about string etc. but then you quote code. Where do the two come together in your mind? The maths of this doesn't have to be particularly difficult if you want to start yourself off with some ballpark figures.

If you want to commit to a particular motor then you have to look carefully at all the different motor specs you can find. Those things are readily available but sometimes it can be a bit hard to decide what the specs actually mean.

 

[Mech_LS24]

I don't know what you mean by this but it suggests to me that you are completely new to this. That makes it hard for you. (Me to, aamof, as I have never done a Mech Eng course).

I have a Bsc in mechanical engineering with actually quite good results. As well, I acquired a lot of practical experience with working in the workshop. Mostly working on precision parts on conventional lathes and milling machines.

I just don't know yet how to solve problems like this. With 'problems' I mean choose a motor for bringing a disk into movement. I thought it shouldn't be difficult. I can imagine there will be friction and backlash in sections of the concept, but how to bring this friction and backlash into numbers is hard. I started with imagine a disk loaded with a weight bringing into movement at the begin of this topic. Later on, with help of others here (which is great) I found that the given information wasn't sufficient. And now we are here, discussing about piano strings, polyester lines, backlash and pre-load which is great fun, but hardly to follow.

Perhaps I need to step back on this, but then I would like to ask what can I do to get the right knowledge before move on into topics like this. And maybe even more important, how to getting involved in it. I can't sit and wait hoping some day I understand mechanical principles.

 

[Mech_LS24]

Newton's second law (adapted for rotary motion) will tell you the minimum motor torque, given the MI and the angular acceleration you want.

For example, I know Newton's second law, I have studied it, but it is now (I think) already 3 years ago. As well the Moment of inertia, but I need your experiences how to apply it.

I appreciate all the help and try to pick up all the suggestions gaining more knowledge.

 

[sophiecentaur]

I know Newton's second law, I have studied it, but it is now (I think) already 3 years ago.

Haha - I learned it nearly 60 years ago but I was using it from the first lesson and ever since. I think there must be some very mediocre University Engineering courses available these days. Many of them don't seem to help a student with practical experience.
Why would the use of words like piano wire and string be introducing problems for you? All that stuff is the same as the beams and pulley work you must surely have done . You just have to look for the link between what you know and what you need. A lever is a lever and a wheel is a wheel. Did you consider Google Images searches for pictures of the sort of thing you want to do. Don't expect the same experiment to be described but just look around.

 

[Mech_LS24]

Thanks for you help @sophiecentaur , I will dive deeper into it.