# Is it possible to change the Torque in a gearbox by altering the inertia?

1. Feb 13, 2010

### Number_19

Hello guys, I am new here =)

After looking everywhere on Google, I have decided to create an account and give this a shot!

I am trying to understand the concept of Torque generated by a gear box (simple gearbox with 2/3 gears, nothing fancy). I understand that changing the gear ratio changes the Torque of a gear box. More specifically, one creates a reduction gearbox, the Torque increases.

Ok then, now I am wondering:

Keeping in mind this formula:

T = I $$\alpha$$

where T= Torque, I = moment of Inertia, and $$\alpha$$ = angular acceleration

Does it mean that if one changes the moment of Inertia of a gear the Torque changes too?!

More specifically, does it mean that one could increase the Torque being produced by a gearbox by simply increasing the Area/Size of the gears itself (with no need of changing the ratio)?

Thank you for your time, and please let me know if I need to make this any clearer!

Regards
N19!

2. Feb 14, 2010

### brewnog

Your formula is helpful for understanding acceleration. For example, if you had a flywheel at rest (whose moment of inertia is known), the formula describes how much torque is required to cause the flywheel to gain speed at a certain weight. If the inertia is decreased (like on a racing car with a small flywheel), for a given torque its acceleration will be higher.

Don't confuse this with the output torque of a gearbox under constant speed conditions. Look back through this forum, there was a recent discussion on gearbox torque.

3. Feb 20, 2010

### Number_19

Hello sorry for the delay, and thank you for the answer..

Ok, I have got another question. Would changing the kind of gear (for example from spur to helical, for instance) have any effect on the Torque?!

Thank You once again!

Regards
N19

4. Feb 20, 2010

### brewnog

Not really. It's the gear ratio which makes the real difference.

5. Feb 20, 2010

### Number_19

Hmm that sux..I was hoping there would be a way to change the torque (even just slightly) by not changing the ratio.

Thank You all!

6. Feb 20, 2010

### brewnog

Why do you want to?

You can reduce the output torque by reducing the input torque. Share what you're trying to do and we should be able to help out.

7. Feb 20, 2010

### Lsos

If you make the gears lighter or smaller (decrease their moment of inertia) then you will have slightly more output torque available when accelerating....because less torque will be used to accelerate the gears themselves.

8. Feb 21, 2010

### Number_19

To Brewnog, all it is:

Well its a small project where I need to increase the torque of a gearbox.

I could do it easily by increasing the gear ratio. But I would like to try keep the gear ratio the same if possible.

Thank You!

9. Feb 21, 2010

### brewnog

Do you mean the output torque?

You could use a system of pulleys and belts to reduce the output torque of the gearbox, or rubber wheels in place of gears, but you can't bend the laws of physics here. The product of output speed and torque is equal to the product of input speed and torque (less inefficiencies). You can't magically increase the torque without reducing the speed. Failing that, increase the input torque.

When I asked for your application, it was to better understand what the limitation you have is; why can't you use the same gear ratio? What's the gearbox for?

10. Feb 21, 2010

### FredGarvin

You are wanting something for nothing here. You need to make compromises. That is what engineers do. You can not go against the physics.

11. Feb 21, 2010

### Number_19

To be honest, I am actually not sure what the gearbox is for! However, I know I have to increase the output torque (not decrease) by 20%.

I wanted to see if there was a way to do so without changing the gear ratio, for example by changing tooth area, or gear arrangement..but I guess not!

12. Feb 21, 2010

### brewnog

You could increase the input torque by 20%.