Gears and pulleys?

[Mr Skeptic]

Power = Work * velocity

 

Don't you mean, Power = force * velocity?

[John Cuthber]

Making compressed air takes energy. The compressor will have a rating of so many cubic feet per minute at such and such pressure. That is a power rating. Ignoring the efficiency and things like friction losses that requiremment is why it needs 12HP and 10HP quite simply won't do it.

Given that the displacement (CF/M) will be the same (because the RPM is the same) the pressure will not be as high- if you try to run this system at full load you will overload the motor.

 

Stop pretending that you can ignore the power rating.

[jwest22]

hmm, so where does this leave us? sorry i should of said this is a homework problem. the pump spec is 500 rpm @ 12hp minimum, and i need to locate a motor that can power it which runs at 3000-4000rpm not costing more than $700. therefore i need to use a smaller motor and some hoe get the power required out of it, what do we think?

 

thanks

[YT2095]

your 12hp motor is 9000w, taking 750W as 1hp.

 

my initial calculations factoring 2hp (1500W) by 7 = 10500W

 

which is plenty, however there seems to be argument from some here about this, it would be interesting to see THEIR answers to your question

[Mr Skeptic]

Well, you are going to need a motor with at least 12 HP, and more if you buy a motor that is not 500 rpm, as then you will get transmission losses.

 

Also don't be confused by the common usage of power = strength, as in science power = energy / time. You can increase the strength of a motor by using gears, but you cannot increase the energy/time because of the law of conservation of energy (unless you store energy to use later, that is). Eg if you use a smaller motor to compress a tank of air, and then use the tank of air as your "12 horsepower compressed air pump", you could use a smaller motor so long as it doesn't have to be running continuously.

[traveler]

Power=work/time

 

HP=torque*RPM/5252

 

 

A 12 HP motor at 500 RPM has 12*5252/500=126 lb-ft of torque on the shaft.

 

A 2 HP motor at 3500 RPM has 2*5252/3500=3 lb-ft of torque. A 7:1 gear ratio will give you 500 RPM at the final output shaft, but will only have 21 lb-ft of torque.

 

See what I'm getting at?

 

Also don't be confused by the common usage of power = strength, as in science power = energy / time.

 

Power=WORK/time

 

You can increase the strength of a motor by using gears

 

 

No, you can't increase the "strength" of the motor by switching gears. How do you figure a gear will change a motor?

[John Cuthber]

your 12hp motor is 9000w, taking 750W as 1hp.

 

my initial calculations factoring 2hp (1500W) by 7 = 10500W

 

which is plenty, however there seems to be argument from some here about this, it would be interesting to see THEIR answers to your question

 

How do you justify multiplying the power by 7?

Please do this with the world's power stations and solve the energy crisis.

 

The simple fact is that if you try this trick what will happen is the motor will overheat because you are overrunning it.

If you tried running a 12 HP compressor with a 1 HP motor it would initially run fine but, as the pressure in the air receiver rose the force on the pistons (and thus on the motor) would rise. After a while the pressure would be so high that the motor would stall and burn out.

With 10HP you wil probaly get away with it for a while.

[traveler]

hmm, so where does this leave us? sorry i should of said this is a homework problem. the pump spec is 500 rpm @ 12hp minimum, and i need to locate a motor that can power it which runs at 3000-4000rpm not costing more than $700. therefore i need to use a smaller motor and some hoe get the power required out of it, what do we think?

 

thanks

 

 

12 HP@500 RPM is 126 lb-ft of torque. So you need at least 126 lb-ft at the final output at 500 RPM?

 

So divide that torque by the gear ratio, of say 7:1, and a motor operating at 3500 RPM, using a 7:1 ratio must have at least 18 lb-ft of torque at 3500 RPM, which is 3500*18/5252=12 HP.