# Is my power calculation correct?

1. Mar 14, 2012

### FAlonso

I am currently working on designing a conveyor for feeding 1t material to a furnace. Not going into detail of how it works, I would just like to confirm my approach.

Goal: To move 1000 kilograms of mass using steel rope/pulley system. Determining power requirement

It would be a bucket mounted on two wheels running at a very low speed (0.3 m/s) over a mild steel I-beam. From literature around the internet, friciton values are:

mild steel on mild steel (0.74 static, 0.6 dynamic)
rail wheel on steel rail rolling resistance 0.0020

I am calculating power as follows:

Speed desired: 0.3 m/s
Time to achieve the speed: Not important but taking it to be 1s
Mass to be moved: 950 kg + bucket assembly (~50 kg) = 1000 kg

Friction to be overcome while driving: 1000 x 9.8 x 0.002 = 19.6N (2 wheels so 39.2N)
Force to move the mass from rest: F=ma > F=1000 x (0.3-0/1s) = 300N

Total force= 340N

Power required: F x v = 340 x 0.3 = 102 W or 0.13 hp

Am I missing something ? I am a bit skeptical about finding force required to move the mass from rest as it uses the static friction value of 0.72 which makes the required power go up by a large amount.

Regards

2. Mar 15, 2012

### OldEngr63

Stiction is a bear! That is the problem that you are facing here. If you want a realistic solution, you dare not use anything but the static coefficient of friction, and then give yourself a wide margin of extra power to overcome difficulties. Friction is extremely hard to prediction, and when it works against you, it can kill you!

3. Mar 15, 2012

### FAlonso

I went to market and found a winch with 5hp that according to the shopkeeper, people purchase for holding 1000 kg vertically. In my case, the load is moving horizontally. So the 5hp is surely more than my requirement I guess.

4. Mar 16, 2012

### OldEngr63

It really depends. Can you tell yourself with certainty that μstatic ≤ 1? If not, then the frictional resistance could exceed the weight, and this winch may not be enough. A great deal depends upon the surfaces that are in contact - the material being moved and the stationary support surface - and you have not given much information about them. You are the design engineer. Just do what you have to do, and move ahead.

5. Mar 16, 2012

### FAlonso

Thanks for the advice. I will surely do some experimentation to determine the band of values in which the u(static) and u(rolling) falls. Rest I hope my approach is ok ? Just for counter-check