Force required to move the wheels on a manual wheelchair

[Wilson123]

Hi,

I'm trying to calculate the force required to move the wheel on a standard wheelchair from stand still and then consistently over a flat surface?

I understand that this has to with the amount of weight that is trying to be moved, this being the weight of the user (approx. 68kg) and the wheelchair (approx. 16kg).

I also understand that the equation for torque requires the distance from the force applied to the axis of rotation which is approximately 0.33m.

Also I have found the average pushing force for a seated human to be roughly 130N.

I feel as though I have enough information to find out the answer I need however I'm unsure exactly how to apply it or if I'm miles off with how I think I need to calculate it.

Any help would be appreciated!

 

[berkeman]

Hi,

I'm trying to calculate the force required to move the wheel on a standard wheelchair from stand still and then consistently over a flat surface?

I understand that this has to with the amount of weight that is trying to be moved, this being the weight of the user (approx. 68kg) and the wheelchair (approx. 16kg).

I also understand that the equation for torque requires the distance from the force applied to the axis of rotation which is approximately 0.33m.

Also I have found the average pushing force for a seated human to be roughly 130N.

I feel as though I have enough information to find out the answer I need however I'm unsure exactly how to apply it or if I'm miles off with how I think I need to calculate it.

Any help would be appreciated!

What surface are you assuming? The rolling resistance on carpet is much higher than on concrete.

On concrete, the main rolling resistance components will be the bearing friction in the axles and the deformation of the hard rubber wheel at the contact patch with the concrete.

 

[Wilson123]

Concrete. I'm just looking for an approximate figure as I'm trying to design a drive train to turn a wheelchair wheel using a lever arm on the driver gear, so I need to get an idea of how much torque I need to provide to the wheel in order to turn it.

Hope this makes sense.

 

[berkeman]

Concrete. I'm just looking for an approximate figure as I'm trying to design a drive train to turn a wheelchair wheel using a lever arm on the driver gear, so I need to get an idea of how much torque I need to provide to the wheel in order to turn it.

Hope this makes sense.

To go slowly on concrete, it probably takes a couple of lbf at the top of the wheel to turn it. But for a practical wheelchair drive train, you will need much more torque to be able to reliably move the wheelchair, especially on different surfaces, up ramps, etc. What is this project?

 

[Baluncore]

Also I have found the average pushing force for a seated human to be roughly 130N.

But if the occupant of the chair pushes forward on the top of the wheel it will only require half that "handle" force.
There are two problems. 1. Accelerating to reach travel speed. 2. Maintaining that speed while countering losses such as tyre friction.

 

[Wilson123]

To go slowly on concrete, it probably takes a couple of lbf at the top of the wheel to turn it. But for a practical wheelchair drive train, you will need much more torque to be able to reliably move the wheelchair, especially on different surfaces, up ramps, etc. What is this project?

I'm designing a rough terrain wheelchair, with the idea of using a drive train powered by a handle that will turn the wheels. I'm trying to understand how big my two sprockets/gears in the drive train need to be. My understanding of how to do this was to work out the force required to move the wheel from static and consistently. Then look at how much force I am providing to the driver gear in the drivetrain in relation to what is required to move the wheelchair.

 

[malemdk]

Assume rolling resistance coefficient is equal to 0.015
rolling resistance equal to 840N x 0.015 =12.6N
resisting torque = 12.6 *.33 = 4.15Nm
Pushing force = 130N
Required radius or toque arm or gear radius = 4.15/130 = 0.032m = 32mm
or 64mm dia gear or sprocket
The above value is the minimum required size , you can take 25-30 % more than this value to meet the uncertainty.

 

[JBA]

By googling "lever powered wheelchair" you can see several existing examples of what you are designing.

 

[Wilson123]

Assume rolling resistance coefficient is equal to 0.015
rolling resistance equal to 840N x 0.015 =12.6N
resisting torque = 12.6 *.33 = 4.15Nm
Pushing force = 130N
Required radius or toque arm or gear radius = 4.15/130 = 0.032m = 32mm
or 64mm dia gear or sprocket
The above value is the minimum required size , you can take 25-30 % more than this value to meet the uncertainty.

Thanks a lot, I think this makes sense to me, gives me a good idea of what I need to be looking at it.

EDIT: after going over this a few times I have a question, I am expecting to have 2 different sized gears/sprockets in my drivetrain, the larger being the driver and the smaller being the driven. The answer given by your calculations gives the dimension for which of these gears? Or am I thinking about this wrong?

 

[malemdk]

The 32 mm size is dia of driver wheel make the driven 3-4 times the driver wheel (100-125mm)
I assuming your are designing somewhat like a bicycle drive chain !

 

[Wilson123]

The 32 mm size is dia of driver wheel make the driven 3-4 times the driver wheel (100-125mm)
I assuming your are designing somewhat like a bicycle drive chain !

Yes it is similar drive chain, however I was under the impression that if I want to provide high torque to my driven wheel, then my driver wheel would be larger than the driven?

 

[malemdk]

No , its other way ie if you need high torque at driven wheel make it larger than the drive wheel

 

[Wilson123]

No , its other way ie if you need high torque at driven wheel make it larger than the drive wheel

My driven wheel/sprocket has the actual bicycle wheel attached to it, therefore I want to provide high torque to this... In the same way that a single speed bike works, I want to provide high torque to the rear wheel therefore my driver wheel should be bigger? See pic for below for reference

See how the driver wheel is larger to provide high torque to the actual bike wheel.

 

[malemdk]

The small sprocket is to increase the speed and not torque, what you want is high torque, so you fix a bigger sprocket at driven wheel and smaller sprocket at drive end.

 

[Carol Sanders]

Hello Wilson! Wheelchairs require some careful thinking. Better consult with the local building code and specialists in caring for the handicapped. There's a whole branch of architecture dealing with this Wheelchairs subject. Most people are helpful with advise when it comes to making things easier for the handicapped. Check with your local home centers. They may have a handy DIY book.