GD2 Value Vs GR2 Value

Because D (diameter) = 2* R (Radius), GD²=G*(2R)²=4*GR², Clear ?

Image courtesy lyn

That doesn't answer bushan's question. He already knows there's a factor 4 between them. I don't know why GD² is used either. If you calculate from first principles you need GR² e.g. torque = MoI (GR²)*angular acceleration. I can't think of an occasion where the formula is simpler with GD², while using GR² needs a 4 putting in.

Cheers.........Codey

Sincere apologies to both of you. It was a feeble attempt at some levity, shouldn't have happened.

This document may explain a bit ? ctlg_geared_e_9.pdf

No problem, I thought you were being serious .

On your link, I don't see why page 4 distinguishes between J (Inertia calculation) and GD² (Flywheel effect calculation). All the torque etc calculations, including Flywheel application on page 1, use J (= WR²) and most of the J and GD² formulas differ by factor 4 as expected. But I don't think the data for sphere and cube is correct. For the sphere, GD² = 2/5*WD² is correct, but J should be GD²/4 = 1/10*WD² = 2/5*WR² (the form I remember it as).

For a cube, J = 1/12*W*(a² + b²) (not 1/8), making GD² = 1/3*W*(a² + b²) as stated.

Also it's confusing to say under J --- W : Mass (kg), but under GD² ---- W : Weight (kgf). It should be mass (kg) for both.

Cheers.........Codey

Well, you have gone about 200 miles ahead of my knowledge on this aspect

In the electrical engineering curriculum of those long gone days (1964), a casual mention was made to all these silly things - inertia, torque, etc. We electrical engineers were far superior to all these which were supposed to be handled by lesser mechanical mortals. Alas, when i started in my first job, reality brought me down with a thud, so i started learning all these anew. i still am. GD² is one of them !

Thank you for your reply. still my questions is unaswered. why GD2 value is used instead of GR2. KINDLY HELP ME BY GIVING STRAIGHT ANSWER.

You're not going to get any more help than you've had already, not from me anyway. Have you read and thought about the responses? If you can find a calculation that is simpler with GD² than GR², let us know and maybe we'll comment. In any case it's only a matter of getting an elegant formula, it's not hard to * or / by 4 if necessary.

Codey