Gears and pulleys?

[jwest22]

hiya, i need some help for a project im working on, basicly i have a compressor pump that runs at 500rpm and @ 12 hp.

 

now the problem is (this is a theoretical problem) i have to power it using a 3480rpm ac motor which has to be less than 10 hp (cost reasons).

 

rough calculations show that a 20cm pulley on the compressor pump and a 3cm pulley on the motor will give me the correct speed reduction, but what do i do about power?. will the speed reduction increase torque giving me the power i need? if this is the case what is the minium sized motor i could use?

 

what if i used gears, id be looking at a ratio of 1:0.15?

 

hope thats clear, any websites or calculations that can help me with this would be amazing as im really baffled.

 

thanks alot.

[YT2095]

gear`s and belt drives (what i assume you meant by Pulleys) do essentially the same thing anyway, and yes a 1:7 ratio geared down would give you the correct speed and increased torque far in excess of the 12hp you need

 

correctly geared you can pull a car with the motor out a sony walkman and couple of penlight batteries!

Edited October 23, 2008 by YT2095

[jwest22]

good, thanks, do you know any equations i can use to proove this? and at the gear ratio of 1:7 what size motor would get me to 12hp?

 

thanks alot

[YT2095]

there aren`t really any equations needed here, you want 500rpm but your motor does 7 times that at 3500 rpm, so you need a gear arrangement whereby the output turns once for every 7 turns of input (the motor)

 

so if you put a gear on the motor shaft with say 10 teeth on it, and have the output gear with 70 teeth on it, you`ll have your 1:7 (or 500rpm from your 3500).

and since 7 doesn`t go into 12 very nicely and we also have to allow for losses due to friction etc... it Does go quite well into 14 to make 2, so a 2hp motor will do the job perfectly.

 

although I think I`d use a worm gear setup personally.

Edited October 23, 2008 by YT2095

[big314mp]

Shouldn't the HP be the same on both ends? i.e. If a 10 HP motor is used, then after the gears belts there should also be 10 HP? I don't think power can be added somewhere in that chain, merely transferred.

[YT2095]

it`s a trade off, you get less speed but greater torque, so a 2hp 3500rpm motor when geared down will be the equiv of a 500rpm motor at about 14hp.

 

a bit like having batteries in series or parallel.

[big314mp]

So does hp = torque or power?

[YT2095]

hp = power

 

but you have to take Rate into account (rpm in this case), and in this case with gearing the equiv to drive his compressor (requiring 500rpm @ 12hp) would be gotten using a 3500rpm @2hp with a 1:7 gear ratio.

[jwest22]

there aren`t really any equations needed here, you want 500rpm but your motor does 7 times that at 3500 rpm, so you need a gear arrangement whereby the output turns once for every 7 turns of input (the motor)

 

so if you put a gear on the motor shaft with say 10 teeth on it, and have the output gear with 70 teeth on it, you`ll have your 1:7 (or 500rpm from your 3500).

and since 7 doesn`t go into 12 very nicely and we also have to allow for losses due to friction etc... it Does go quite well into 14 to make 2, so a 2hp motor will do the job perfectly.

 

although I think I`d use a worm gear setup personally.

 

 

hey, thanks alot for your help thats really useful, how come you'd use a worm gear? which would you put the worm gear on the compressor or the motor? would that be better than belts or chains?

 

thanks

[CaptainPanic]

Power is expressed in watt, or kilowatt, or if you really have to, in horsepowers.

 

A 12 HP engine (about 9 kilowatt) will generate 12 HP on the other end of whatever gearbox you put in between. It's torque that changes...

 

Just have a look at the wikipedia websites of torque and power to see the units. You can do the conversion from one end to the other pretty easily.

 

If the power really changed, then I'd be the first to invest in a Sony Walkman, a bunch of pulleys (the mother of all gearboxes, or MOAG) and power my country with it (the current market needs about 104 GW, or 141 million horsepowers in electricity). But it's not gonna happen.

[gcol]

Worm gear goes on the fast shaft. It gives the greatest speed reduction in the minimum physical space, 40:1 is not uncommon, but friction is vey high.

[YT2095]

CP you`re not taking into account the Rate and you should.

 

for instance, which uses more power to lift 100g to a height of 1 meter, a device that does it in a second or one that takes an hour?

 

in the case of gearing when you half the speed you double the torque, as I said it`s a bit like having a bunch of batteries, you can arrange them to give high current at low voltage or have high voltage at low current, but the power Total will always be the same.

[CaptainPanic]

it`s a trade off, you get less speed but greater torque, so a 2hp 3500rpm motor when geared down will be the equiv of a 500rpm motor at about 14hp.

 

a bit like having batteries in series or parallel.

 

I have a problem with this statement. This is the one I replied to, and I apologize for not quoting it the last time.

This post seems to suggest that the power output changes, which is not true. The horsepowers do not change if you add a gearbox to it. The torque does.

 

CP you`re not taking into account the Rate and you should.

 

for instance, which uses more power to lift 100g to a height of 1 meter, a device that does it in a second or one that takes an hour?

 

in the case of gearing when you half the speed you double the torque, as I said it`s a bit like having a bunch of batteries, you can arrange them to give high current at low voltage or have high voltage at low current, but the power Total will always be the same.

 

This I agree with... so we all agree, which is beautiful, and we can move on into the weekend

p.s. I don't see a mistake in my previous post... I think the above post by YT totally agrees with mine... so linguistically I don't understand YT's opening sentence (it seems to suggest disagreement)?

[npts2020]

CaptainPanic: You could power your entire country with a walkman............for maybe an attosecond.

[CaptainPanic]

I know... that's why I started that paragraph with "If" and ended it with "But it's not gonna happen."... I assume you mean that the energy stored in the batteries could power the country for an attosecond. The power output as used in the walkman (a few Watts at most) cannot even power a toothbrush.

 

If the power really changed, then I'd be the first to invest in a Sony Walkman, a bunch of pulleys (the mother of all gearboxes, or MOAG) and power my country with it (the current market needs about 104 GW, or 141 million horsepowers in electricity). But it's not gonna happen.

[YT2095]

ok, let me explain it a little differently (better maybe), his workload requires a 12hp motor at 500rpm, and I`m saying that a 2hp motor at 3500rpm when geared down 1:7 will do the same work.

[insane_alien]

Yt, the force applied(torque) will change, but the power will not. if you only put say 100 watts into the system, the most you can get out of the system is 100watts- losses.

 

energy conservation, the speed of the machine makes no difference.

[gcol]

ok, let me explain it a little differently (better maybe), his workload requires a 12hp motor at 500rpm, and I`m saying that a 2hp motor at 3500rpm when geared down 1:7 will do the same work.

 

Yes, but it will take 7x longer, surely?

[YT2095]

it will take 7x longer yeah.

[gcol]

That's why I dont see how you can say it does the same work, if work done is force applied against time.

[insane_alien]

work done is force and distance...

[CaptainPanic]

Work has units of "Joule". Power has units of "Joule/second" (="Watt").

 

Work = Force * distance

Power = energy/time

Power = Work * velocity

 

Since these are probably all vectors, it's all simplified a bit. But if you do a dimensional analysis it will hold.

[YT2095]

the distance is taken by a 2hp motor having to turn 7x to get 1 rpm from the output shaft.

the time would come in IF the 2hp motor was only doing 500rpm, but it`s doing 7x that so it doesn`t matter.

[gcol]

I was rather surprised to find that the mathematical definition of work did not include a term for time (living and learning!), neither distance, but more specifically displacement.

Now I am wondering if there is a tricky difference between distance and displacement.

 

If work does not include time, is it o.k. if I take a whole year to shift that pile of bricks and get paid the same as if I had done it in a week?

[YT2095]

hold the press, I think I`v made a mistake!

gimme a few mins to find out exactly where.

==================================

 

ok I think I know where I`ve gone wrong (unless I`m over thinking things?), for a start I`v mixed hp with torque, so I`v had to put horsepower into usable units, I don`t use HP myself, and so this is what I`v done, I`v worked on hp being 750 watts each (I can think better this way).

 

what I have so far is that if the op needs 12hp @ 500 rpm, that translates to 9000W @500 rpm, giving a torque value of 18.

so somehow that figure (18) needs to be plugged into a 3500rpm motor to give a value in watts, which then gets divided by 750 to leave HP.

 

eitherway, my Sincerest apologies to the OP and anyone else I may have confused.

 

am I over analising this? or was I right 1`st time????

Edited October 24, 2008 by YT2095