# How to Increase RPM without decreasing Torque?

1. Feb 2, 2010

### kashyap

Hi,

I am not a mechanical engineer but an aeronautical engineer. Hence I am very poor with Mechanics & request for your help.

I am working on design which produces approximately 17,400N-m of Torque at 25RPM. I want to maintain this Torque but increase the RPM to 1500. Is that possible? I am planning to use a Gear Train to multiply the RPM but will that decrease the Torque as well?

Regards

Kashyap

2. Feb 2, 2010

### brewnog

Urm, is this an engine?

Power = torque x angular velocity, and this is conserved through a gearbox. So increasing speed, and maintaining torque requires more power; which a gearbox clearly cannot provide.

Perhaps a more thorough description of your quandry would help us to help you?

3. Feb 2, 2010

### kashyap

I am working on a design with a desired output of 4000KW. The input is provided by a rotating shaft. I am able to provide a torque of 17400N-m on the shaft & it rotates at 25RPM. But this gives an output of 113KW only. I am stuck here. I have been asked to do either of the 3 -
1. Increase torque (not possible & its a design constraint)
2. Increase RPM without decreasing Torque (this is my thought but not sure how)
3. Quit (I am not gonna quit)

The reason I was thinking about the 2nd point is because of the this link - https://www.physicsforums.com/archive/index.php/t-67032.html
Not sure though if I got it correctly.

Please help. I am open to make changes if you suggest any to resolve this problem.

Regards

Kashyap

4. Feb 2, 2010

### Staff: Mentor

You cannot convert 113 kW into 4000 kW with a gearbox. Period. Your output power must always be lower than your input power: that's the first law of thermodynamics and it is non-negotiable.

Simply put, you need mechanical motive device with a 4000 kW power output. That would be Option 4: Change your design constraints.

The other thread you linked does not in any way suggest you can increase rpm without decreasing torque. It is a standard reduction gear application: decreasing rpm while increasing torque.

5. Feb 3, 2010

### kashyap

Thanks for your reply. I agree with you on this front & considering Option 4. I will work on this & might be back for more help from soon

6. Feb 5, 2010

### FlexGunship

Hey, if you figure out how to turn 45.6kW into 4000kW there's a job waiting for you here. I'm a motion control engineer and write software and design hardware to operate mechatronics and robotics for industrial machinery.

Free power (sic. free energy) has long been the fevered dreams of the "perpetual motion" niche. Their attempts to break the unrelenting and surly bonds of the law of conservation of energy (in a closed system) has yet failed. If someone has given you an impossible job, it is up to you to accurately report the physical restrictions of the universe.

Too often I am given a task that is not simply "difficult" but literally and physically impossible. I carefully research the reason why, and present it as clearly as possible (being sure not to insult anyone). As engineers we are subject to the impossible whims of managers and administrators. Some of them believe that breaking the intractable laws of physics requires nothing more than a "firmware update."

It's part of your job as an engineer to relate this information in a timely manner. If you persist in an impossible task you waste your time, and by proxy, the company's money. I suggest you consider this the final word on the matter.

Last edited: Feb 5, 2010
7. Feb 5, 2010

### Doug Huffman

I get 290 Watt

I would hope that all here are capable of unit analysis.

8. Feb 5, 2010

### brewnog

Where does 113kW come from anyway? And 290W for that matter?

Power (in Watts) = Torque (in Newton-meters) multiplied by angular speed (rads per second).

9. Feb 5, 2010

### FlexGunship

Actually, yeah... I get 45.6kW.

10. Feb 5, 2010

### Doug Huffman

290 Watts is what you get when you ask Wolfram|Alpha the wrong question.

11. Feb 25, 2010

### familymempire

generally if you want to increase torque it will be done on zccount speed but ageneral rule ther isn`t amplify of power if you can make money from no thing then you can convert 45 kw to 4000 kw

12. Feb 26, 2010

### brewnog

What?

13. Feb 28, 2010

### vargasjc

I think the only way to increase the power from 45.6kW to 4000kW is to increase the torque, that's because a gearbox works based on the difference between gear diameters. So for example, the force on gear one times the radius of that gear equals the force on gear two times the radius of gear two. Thus the force remains the same while the torque actually changes. If you can't up the torque it's not physically possible because it means (according to my very quick calculations) using those 17400Nm of torque to produce 2195 RPM out of thin air.