Question about power loss in belt transmission

[berdan]

Howdy all!
In some project I do,I need to design belt transmission,now the tricky part is:
On the output of the belt,I have wheel that needs to turn at 700 rpm,and I need to deliver power of 6KW.
Now the weird question:I need to design the belt transmission so I will have 20 percent power loss in the belt tranmission ! How do I approach such task?
I cant' find any formula that tackles this particular task.
Maybe I'll just limit the available transmitted power (given by H=(f1-f2)*V/33000)?But how I set the f1 and f2 forces?
P.S. I also need the centers of pulleys to be at some given distance from each other ,does that help?
Thanks in advance.

 

[Q_Goest]

Hi berday. If you transmit 6 kW of power and need to have 20% of that absorbed or in some way 'removed' by the belt, then the amount of power you're asking to absorb is 1.2 kW. Given conservation of energy, you need to do something with that energy. If you reject it to the belt, then what you're really saying is you want to convert that energy to heat and put it into the belt. So really, you're saying you want to heat up your belt. If you convert 1.2 kW of power to heat and warm up the belt, it isn't likely to have sufficient surface area to remain cool enough to function correctly. It'll get hot and burn up. Even if you have a very long, wide belt with lots of surface area, there's no easy way to move that energy into the belt except by friction which is still going to be problematic.

If you need to remove that much power for some reason, you could use it to pressurize air (such as by using a fan or blower) or you could pump water or whatever. I'd suggest pressurizing air if you can move ambient air in and out of the vicinity easily because it's an open loop. In other words, you don't need to cool down the air before putting it back into the fan or blower. By putting it right back into the atmosphere, you just have to make sure your inlet and discharge are far enough apart that you won't get air that you compressed (and are venting to atmosphere warmer than it went in) into the inlet.

If you want some software to size belts and obtain valid part numbers for sheaves and belts, download Goodyear MaximizerPro. It's free:
http://www.goodyearep.com/ProductsDetail.aspx?id=23280

 

[Jobrag]

Why do you want to lose twenty percent of your power in the transmission? Do you have a value for normal power loss when using a belt?

 

[Jobrag]

To lose some power in your transmission you could add an extra pulley with a parasitic load (possibly a fan) just tweak the angle of the blades to get the loss you need..

 

[berdan]

Heh,thanks for replies guys,it was just badly formed engineering task,I asked once again and was told that the system just does not have to be efficient to high degree.
I've chosen V-belts as they have efficiency of 70-99 percent.

Anyway,thanks for the detailed responces,this is very appreciated!