# Weight on wheels to keep from spinning wildly

1. Sep 17, 2013

### TimNJ

Hello,

I'm working on my first year engineering project. I'm an electrical engineering major but this is a general engineering course.

I'd like to know how I could figure out how much weight I would need to put on the wheels of a cart that is going to push a 2kg object (for about 3 feet).

The motor will apply 2.6N of force in the x direction with wheels that are r=0.05m. The motor is capable of 5000RPM at full speed, but they can operate a low RPM for a short amount of time with higher torque, which is how I got the 2.6N figure. However, if the cart is too light, the wheels will just spin wildly and not move the 2kg object anywhere. How much weight would the cart need to be to get the book to move?

Thanks!

2. Sep 17, 2013

### SteamKing

Staff Emeritus
What! Were they all out of 10,000 RPM motors?

You haven't provided a lot of information about your cart. Please say you aren't driving the wheels 1:1 with the motor speed.

If you are driving 1:1, you could probably stand on your cart and the wheels would still spin. If you can't develop enough friction between the wheels and the driving surface, the wheel will spin uselessly. Even if the wheels grabbed, it's possible your cart would shoot out across the room out from under the book. Inertia, you know.

If you can't find a slower motor, at least try to put some speed reduction between the motor and the wheels. If you had gears with, say, a 10:1 reduction, not only would the speed at the wheels drop but the torque would be multiplied by 10.

3. Sep 18, 2013

### Baluncore

It would be wise to make the cart somehow wedge itself under the object. That could transfer load to the driving wheels and reduce the object's friction with the surface.

4. Sep 18, 2013

### skeptic2

Correct.

Not correct.

If the coefficient of friction of the wheels is less than that of the book, the wheels will always spin regardless of the reduction. First determine the two coefficients of friction. Then calculate how much weight you need to add to the car to increase its coefficient of friction to more than that of the book. Note: The running coefficient of friction for the car must be greater than the breakaway coefficient for the book.

5. Sep 18, 2013

### SteamKing

Staff Emeritus
Torque multiplication using reduction gears is a well-established principle.

http://en.wikipedia.org/wiki/Gear_ratio

Without it, our cars could never get going once the engine started.

6. Sep 18, 2013

### skeptic2

I agree but all that torque is wasted unless the car can get enough traction to overcome the friction of the book.

7. Sep 18, 2013

### SteamKing

Staff Emeritus
I believe the cart is supposed to carry the book, so I don't know what your statement means.

Wheeled vehicles function at all because there exists a high coefficient of friction between the wheels and the road surface (especially railroads). On icy or wet surfaces, the coefficient of friction is greatly reduced and travel is impeded because the wheels cannot get traction.

8. Sep 18, 2013

### skeptic2

Bolding mine.

9. Sep 19, 2013

### TimNJ

Hello everyone,

I have read all of your responses and they are very helpful. I am absolutely loaded with schoolwork for the next 48 hours so as much as I want to make a meaningful response, I reallly need to focus on my work due in the near future.

To answer a question or two, I was initially thinking of using 1:1, but that seems rather foolish now that I think of it. Even if the rest of it is worked out correctly on paper, if the wheels don't stay on the ground it's useless.

I'm going to look up how gearing works. I'm a freshman electrical engineering student after all. Haha.

And yes for this project, I *think* pushing would be the most effective but I'm not sure.

Thanks.