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Mechanics and Construction => Mechanics and Construction => Topic started by: huntoon on January 20, 2013, 03:40:58 AM

Title: Correct radius for calculating speed and torque of tank
Post by: huntoon on January 20, 2013, 03:40:58 AM

Hello All.

I'm look looking for your opinion on whether to calculate the speed & torque of a tracked vehicle using the pitch circle radius of the sprocket or the radius from the center of the sprocket to the ground.

(https://lh3.googleusercontent.com/-EJAuyKR3Xa0/UPu6y0TkYeI/AAAAAAAASic/DWnLJ16LGfk/w497-h373/r%2Bor%2Brg.png)

Thanks!

Title: Re: Correct radius for calculating speed and torque of tank
Post by: Pogertt on January 20, 2013, 04:11:10 AM

Use r for your calculations.

Title: Re: Correct radius for calculating speed and torque of tank
Post by: huntoon on January 20, 2013, 01:31:58 PM

Thanks for the fast reply pogertt.
I have a follow-up question.

Would it be best to use r or Rt given below:

(https://lh6.googleusercontent.com/-4c2ajOFoRKc/UPxF0M25NEI/AAAAAAAASi4/FEHaBJydKjI/s718/r+or+rt.png)

Title: Re: Correct radius for calculating speed and torque of tank
Post by: Pogertt on January 20, 2013, 07:33:58 PM

Use Rt.

In your first question, r was chosen because it represented the radius of the wheel, but did not include the thickness of the track.

In the second question Rt was chosen because it DID include the thickness of the track.

Title: Re: Correct radius for calculating speed and torque of tank
Post by: huntoon on January 20, 2013, 07:43:36 PM

Thanks! Would one use Rt for torque and speed calcs?

Title: Re: Correct radius for calculating speed and torque of tank
Post by: jwatte on January 20, 2013, 10:10:40 PM

Yes, one would use Rt for both torque and speed calculations.

Think about it: The speed of the contact surface of the tread is the speed of the contact surface around the wheel, which is the wheel's rotational speed times Rt times two pi.

Also: if your treads are thin, it's not certain that the Rt-vs-r will matter much, because you will get some slippage when driving, and that represents an error term that may be bigger than the Rt-vs-r difference.

Title: Re: Correct radius for calculating speed and torque of tank
Post by: huntoon on January 21, 2013, 12:03:18 AM

Thanks jwatte.

This is as I expected, as this is similar to how one would treat tire's 'rolling radius'  but I was not wholly confident. You guys helped a lot.

Cheers.