I Friction while rotating

  • Thread starter danielp3
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20
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Summary
Designing a motorized chair for indoor purposes, how to calculate the expected friction while rotating?
Say I have a motor connected to a wheel, the wheel is driven forward.
The motor produces a certain torque rotate/turn the wheel right or left, how do I calculate the torque needed to rotate? keep in mind the friction between the surfaces, the mass of the wheel and the tire sag/subsidence due to it's weight.
(no slip situation)
 
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tnich

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Summary: Designing a motorized chair for indoor purposes, how to calculate the expected friction while rotating?

Say I have a motor connected to a wheel, the wheel is driven forward.
The motor produces a certain torque rotate/turn the wheel right or left, how do I calculate the torque needed to rotate? keep in mind the friction between the surfaces, the mass of the wheel and the tire sag/subsidence due to it's weight.
(no slip situation)
I take it that you want to calculate how much torque would be required to overcome the initial static friction and set the wheelchair in motion. (Once rolling, the friction should be reduced.) To do that you would need the coefficients of static friction of the components you mention. You would also need to know the weight that the wheel chair is going to bear, plus the weight of the chair and of a wheel. Then you could calculate the torque.
If you don't have the coefficients of friction available, or even it if you do, you might be better off just building a prototype and measuring for yourself the force required to get it moving. You could then calculate the torque directly.
I suspect that friction is going to be the least of your worries. Moving up a slight slope (like a driveway) or moving on a carpet (which you could consider a high coefficient of friction surface) will probably require much more torque than motion on a hard flat surface.
 

sophiecentaur

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moving on a carpet (which you could consider a high coefficient of friction surface)
I realise that this is the way it tends to be described but the force driving the wheel forward is a friction force, in the same way that the resistive forces on the wheel could be called friction. I feel that calling them both friction introduces a fair bit of cognitive dissonance (I think that's what they call things like that). Better to call those resistive forces Rolling Resistance, perhaps? At least that gives you a chance to deal with the situation when the carpet is slippery or when trying to drive through mud - where the forward force (due to friction) is less than the resistive forces.

how do I calculate the torque needed to rotate?
That would depend on the mass of the load of the chair and the required acceleration. (or, as mentioned above, the slope). Those two are approximately calculable but the rolling resistance can only be found experimentally, I think.
 
20
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I take it that you want to calculate how much torque would be required to overcome the initial static friction and set the wheelchair in motion. (Once rolling, the friction should be reduced.) To do that you would need the coefficients of static friction of the components you mention. You would also need to know the weight that the wheel chair is going to bear, plus the weight of the chair and of a wheel. Then you could calculate the torque.
If you don't have the coefficients of friction available, or even it if you do, you might be better off just building a prototype and measuring for yourself the force required to get it moving. You could then calculate the torque directly.
I suspect that friction is going to be the least of your worries. Moving up a slight slope (like a driveway) or moving on a carpet (which you could consider a high coefficient of friction surface) will probably require much more torque than motion on a hard flat surface.
I aim to calculate the torque needed to steer the chair left or right in a certain radius (let's say R), at a certain velocity (might as well take it to be constant velocity to ease the calculations).
velocity is also known.
All weight is known, so as the coefficient of friction with both the floor and the carpet.

unfortunately, building a prototype is not an option at the moment.

I suspect that friction is going to be the least of your worries. Moving up a slight slope (like a driveway) or moving on a carpet (which you could consider a high coefficient of friction surface) will probably require much more torque than motion on a hard flat surface.
For now, I'm focusing on calculating the torque needed to steer left/right in a certain radius.
 
20
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I realise that this is the way it tends to be described but the force driving the wheel forward is a friction force, in the same way that the resistive forces on the wheel could be called friction. I feel that calling them both friction introduces a fair bit of cognitive dissonance (I think that's what they call things like that). Better to call those resistive forces Rolling Resistance, perhaps? At least that gives you a chance to deal with the situation when the carpet is slippery or when trying to drive through mud - where the forward force (due to friction) is less than the resistive forces.


That would depend on the mass of the load of the chair and the required acceleration. (or, as mentioned above, the slope). Those two are approximately calculable but the rolling resistance can only be found experimentally, I think.a
Actually, I am familiar with rolling resistance. Yet I couldn't find some data about equations and steering.
Also, rolling resistance is different when the wheel is driven and when it is passive.
Do you have any information about that?
 

sophiecentaur

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Actually, I am familiar with rolling resistance. Yet I couldn't find some data about equations and steering.
That's not surprising because it will be very hard to characterise floor surfaces. Also, it's a small and under-funded industry, compared with the car tyre business.

Even if you cannot make a prototype, it would not be hard to experiment with one wheel on an axle (broomstick?) with various loads, surfaces and angles etc.. You really have to do some preparatory practical work if you want to avoid the possibility of your system just not working adequately. The motor and battery sizes would be critical.

Otoh, why not just look at existing systems and use their specifications? Look at reviews and find out the most successful ones. That's as near as I expect you can get to 'data'.
 
20
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That's not surprising because it will be very hard to characterise floor surfaces. Also, it's a small and under-funded industry, compared with the car tyre business.

Even if you cannot make a prototype, it would not be hard to experiment with one wheel on an axle (broomstick?) with various loads, surfaces and angles etc.. You really have to do some preparatory practical work if you want to avoid the possibility of your system just not working adequately. The motor and battery sizes would be critical.

Otoh, why not just look at existing systems and use their specifications? Look at reviews and find out the most successful ones. That's as near as I expect you can get to 'data'.
Experiments would have to include all weights expected and that a bit issue, I'm taking thing in a matter of variables for now. do some calculations before experiments and prototype.

why not just look at existing systems and use their specifications?
Do you have any examples of such existing systems calculations? that would be great.
 

sophiecentaur

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Experiments would have to include all weights expected and that a bit issue
If you are planning to build something then getting a working system that would be a far greater "issue". I'm not suggesting starting with a prototype; my suggestion was about a single wheel with a limited range of loads. The driving force could be manual with a force meter.

Do you have any examples of such existing systems calculations?
I wouldn't expect to find any. There would be privately held data about experiments carried out by manufacturers but why would they publish? My suggestion was to look at specifications which will be available, by looking on the plates of the motors and batteries or even the spare parts catalogues, for instance.

It seems you want a totally theoretical approach to give you meaningful design parameters. It would be a rare piece of Engineering that could work straight from text book calculations. I can only suggest that you look into motor car design publications. There would be far more information about that and there are thousands of amateur enthusiasts who are likely to want to share their know-how. (Perhaps off-road vehicles would be nearest to what you want). It would not be straightforward to extend that information to your different context but that's certainly where I would start.

Personally, I wouldn't attempt what you want to do unless I was prepared to do some experiments. Otoh, if yours is a totally theoretical exercise and no one will want to build a system, you could go straight to automotive publications and do some 'intelligent' scaling. No one could argue with your actual results if your methodology is half decent.

Is this, in fact, a college project that you are discussing?
 
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If you are planning to build something then getting a working system that would be a far greater "issue". I'm not suggesting starting with a prototype; my suggestion was about a single wheel with a limited range of loads. The driving force could be manual with a force meter.
I could check about a force meter, but I still need to know about the torque from the motor.

I wouldn't expect to find any. There would be privately held data about experiments carried out by manufacturers but why would they publish? My suggestion was to look at specifications which will be available, by looking on the plates of the motors and batteries or even the spare parts catalogues, for instance.
Did that, couldn't find any.

It seems you want a totally theoretical approach to give you meaningful design parameters. It would be a rare piece of Engineering that could work straight from text book calculations. I can only suggest that you look into motor car design publications. There would be far more information about that and there are thousands of amateur enthusiasts who are likely to want to share their know-how. (Perhaps off-road vehicles would be nearest to what you want). It would not be straightforward to extend that information to your different context but that's certainly where I would start.
Theoretical approach at first. then some building and experiments.
And yes, It's part of a grad project, at the end there supposed to be a whole system built.
I searched for data about cars and wheelchairs, all I've found was irrelevant since they just give the result and no equations or other detailing.
 

sophiecentaur

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I could check about a force meter, but I still need to know about the torque from the motor.
If you are trying to do a grad project and, if it's practically based then why should a simple thing like a Force meter be in question?
If you have a motor then it will have a type number and a specification. If not, as I said previously, suitable motors are always specified with a given voltage, speed and torque. If you're just making it all up in your head then anything would do. Assume a 10A, 12V motor and look up some DC motors on the Web. The torque and power are related by the speed and, of course, that will depend on the gearing from motor to wheels.
all I've found was irrelevant since they just give the result and no equations or other detailing.
I have to assume that you know the basic equations, needed for a basic mechanical Engineering course so what actual equations do you need? (I very much doubt if there is a specific set of equations published, relating to wheel chair motors and wheels.) Do you have a supervisor on whom you should be able to call on for help?

It's quite likely that electric bike enthusiasts will have posted lots of information of the sort that you want. But, again, you are unlikely to find specific equations; you will have to use some effort and ingenuity to convert what you can find into what you need. I would guess that is part of what 'they' were looking for when you were assigned the task.
 

sophiecentaur

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@danielp3 You don't seem to have had much success with your searching for information on the Web. It is a skill that can be developed and the last thing you must do is to look at the first few hits from just one initial set of search terms and then give up.

Sometimes you need to be very inventive to get the best out of search engines AND to be prepared to spend an hour or more before giving up. Your generation are extremely lucky with the vast amount of data that's available to you. When many of the 'older' PF members were your age, all we had was a local or college library with shelves full of ancient text books and a list of Abstracts of papers.
 
20
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If you have a motor then it will have a type number and a specification. If not, as I said previously, suitable motors are always specified with a given voltage, speed and torque. If you're just making it all up in your head then anything would do. Assume a 10A, 12V motor and look up some DC motors on the Web. The torque and power are related by the speed and, of course, that will depend on the gearing from motor to wheels.
Currently I don't have a motor. I am supposed to choose a motor that will satisfy the required torque.
 
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@danielp3 You don't seem to have had much success with your searching for information on the Web. It is a skill that can be developed and the last thing you must do is to look at the first few hits from just one initial set of search terms and then give up.

Sometimes you need to be very inventive to get the best out of search engines AND to be prepared to spend an hour or more before giving up. Your generation are extremely lucky with the vast amount of data that's available to you. When many of the 'older' PF members were your age, all we had was a local or college library with shelves full of ancient text books and a list of Abstracts of papers.
I am a "bit" familiar with the internet, it's not the first time I look for data and technical data in particular.
I spent lots of hours in search for relevant technical data and specifications and couldn't find any.
"your generation"? how old do you think I am?
all I ask is help from someone who may have encountered calculations of steering torque.
 

sophiecentaur

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"your generation"? how old do you think I am?
Haha. Could have shot myself in the foot there. From what you were saying, you have to be a few / couple of decades younger than I. In my days as a pukka Engineer there was no significant WWW but, since the nineties information has been much more available. Are your searches for data usually successful? PF gets a lot of requests for readily available information that members can easily access themselves. Tell me the search criteria you have used and I could perhaps take it up from there.

Perhaps your needs would be clearer if you posted a diagram to show the scenario and what forces are relevant in this case.. Afaik, powered wheelchairs steer like a tank - different torques to the two main wheels. I have played on a manual chair and found that steering is only an effort in circumstances where going forward in a straight line is also hard work (mud and sand etc.)
 
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Haha. Could have shot myself in the foot there. From what you were saying, you have to be a few / couple of decades younger than I
I am sure you are older than me with much more experience. still, I'm just starting to gain experience as an engineer.

Are your searches for data usually successful? PF gets a lot of requests for readily available information that members can easily access themselves. Tell me the search criteria you have used and I could perhaps take it up from there.
Yes, I usually get the result I seek on the internet.
I am searching for "steering torque calculation" , "rolling resistance" , "rolling resistance in driven wheels" and others.

erhaps your needs would be clearer if you posted a diagram to show the scenario and what forces are relevant in this case.. Afaik, powered wheelchairs steer like a tank - different torques to the two main wheels. I have played on a manual chair and found that steering is only an effort in circumstances where going forward in a straight line is also hard work (mud and sand etc.)
I managed to calculate the driving forward torque on my own.
 

sophiecentaur

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Yes, I usually get the result I seek on the internet.
I am searching for "steering torque calculation" , "rolling resistance" , "rolling resistance in driven wheels" and others.
I would be very surprised if you get anything with "Steering torque calculation"; it's almost as if you expect to find a paper about your chosen subject with all the equations and data already selected and available. You need to decide for yourself, imo, how the existing equations for regular wheel use can apply to what you want.

Also, I don't see how it would apply to two wheels that have differential forces to turn the chair (as I said, 'a tank') There will be low centripetal forces with a wheel chair, the forces on each wheel will be much nearer the rolling resistance. High speeds would only be attained on smooth surfaces so the available torque from a drive that gets the chair through heavy carpet or mud will easily get the chair round typical (safe) turning radii.


I don't need pneumatic wheels or any specific wheels.
There are big differences between different wheels. (See this old PF thread) Also, have you seen this? Compare the rolling resistances of 'typical' wheels under various conditions and see how those forces compare with the centripetal forces for typical wheelchair use. I think you can ignore situations like 'Wheelchair Rugby' which are played on smooth surfaces.

I found this link, which may be of use. Searched for rolling resistance non-pneumatic tyres.

You have to search outside the box! Check your final results against the spec of existing powered wheelchairs. Maintenance manuals do exist for these as I have seen one.
 

WWGD

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Haha. Could have shot myself in the foot there. From what you were saying, you have to be a few / couple of decades younger than I. In my days as a pukka Engineer there was no significant WWW but, since the nineties information has been much more available. Are your searches for data usually successful? PF gets a lot of requests for readily available information that members can easily access themselves. Tell me the search criteria you have used and I could perhaps take it up from there.

Perhaps your needs would be clearer if you posted a diagram to show the scenario and what forces are relevant in this case.. Afaik, powered wheelchairs steer like a tank - different torques to the two main wheels. I have played on a manual chair and found that steering is only an effort in circumstances where going forward in a straight line is also hard work (mud and sand etc.)
It's a mixed bag. The ecstasy of having all the information at your fingertips and the agony of searching through loads of nonsense to find the signal, if not the cliched needle in a haystack.
 

sophiecentaur

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It's a mixed bag. The ecstasy of having all the information at your fingertips and the agony of searching through loads of nonsense to find the signal, if not the cliched needle in a haystack.
But it isn't a random process. If you consider the points I made then you can limit your queries to ones which are actually applicable. Forget the idea that it can all be done with 'special equations'; well known equations can be used in the context of the data in some of those Google hits I found.
 

WWGD

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But it isn't a random process. If you consider the points I made then you can limit your queries to ones which are actually applicable. Forget the idea that it can all be done with 'special equations'; well known equations can be used in the context of the data in some of those Google hits I found.
I agree for this case and the general one where you have a good idea, understanding of what you want/need. You can then easily filter the results. But if you are in this position you are already 90% of the way there. Issue is when you're not so clear, at an advanced stage of knowledge/understanding. Then it is an overwhelming task of deciding even the right term for your search, let alone to choose from within the search results.
 

sophiecentaur

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@WWGD The OP went into this exercise with 'assumptions'. He assumed that he could google a specific answer to his specific problem. He tried that way and got nowhere significant. He seems to have rejected the idea of a different approach. That's his right but I don't think it can work his way.
We all get frustrated with Google and their friends but the service is (nominally) free. Only he who pays the piper can call the tune. A paid consultant would deliver some sort of answer, I am sure.
 

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