Trying to make sure that calculations are correct. Motor calculation.

[diegoterc3]

Hello I want to make sure that the calculations I am doing are correct.

I am trying to move a refrigerator wirelessly (I know, please don´t ask why).
This refrigerator weighs 70Kg and I´m trying to figure out if I use 2 or 4 motors.
Asumming I will use 4 motors, those 70Kg will be distributed for the 4 motors.
The static friction coefficient is 1. I have attached an image

https://drive.google.com/file/d/0BwBiiNcJ8DICQS13TmFKcFVNaEU/edit

With wheels with 4cm radius I have calculated about 7Nm of torque.
Also the RPM is about 500.

Are my calculations ok? Thank you in advance

 

[dvd]

Are the wheels turning at that rate or is it the refrigerator? Which axis is the refrigerator rotating about? Did you include wind resistance in your power calculations?

I cannot open the attachment.

 

[dineshakabari]

The 500 RPM speed to fast to move object!

 

[BrianE22]

Are your wheels locked up? Assuming they're free to spin you wouldn't be using the coefficient of friction in your calculations. You'd need the rolling resistance (quite a bit lower than 1.0)

 

[racookpe1978]

The refrigerator (no, I won't ask, but I AM CURIOUS!!!!!) is on top of the roller-radio-gadget, right?

If so, you will be much, much more stable if the wheel are not UNDER the refrigerator platform, but are outside of the platform so they have a larger moment arm against acceleration and turning over.

Figure walking speed.
1/2 meter/sec horizontal speed = 50 cm/sec.
From a 4 cm radius wheel = 24 cm circumference = only want a 2 rev/sec or 120 rpm

 

[tbuelna]

You did not describe how the motors drive the wheels, so it's hard to say whether 2 motors are better than 4 motors. If you are driving all 4 wheels, then 1 or 4 motors would likely present the least complexity. One motor with some sort of power distributing mechanical drivetrain would ensure equal and reliable drive torque at each of the 4 wheels. But 4 motors with each driving its own wheel would allow precise speed and torque control using electronics.

Another consideration is how you intend to steer and/or brake the vehicle. 7N-m at each of 4 wheels at 500 rpm is a bit less than 2hp. But the 8cm dia wheels turning at 500 rpm is 410 ft/min, which is a pretty good pace.

 

[chicopee]

If the wheels are turning, rolling friction mentioned above is the number to use, then tack on an additional 10% for bearing friction.