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Pulley speed question

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trw

08-12-2006 07:11:24

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I know someone out there will be able to help me with this one. We have a gas engine that we want to turn a fan. We want the fan to turn approximately 4000 rpm's. We want the motor to turn about 2000 rpm's. According to the formula that I was able to come up with, we need a 7 3/4 inch pulley (that was the size that is on the motor) on the motor and a 3 7/8 inch pulley on the fan. Is this pretty close to the right sizes, or do we need some more info. Thanks a lot!

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Jon Hagen

08-12-2006 19:22:28

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

Is the motor pulling any near full hp on that fan load? Most small aircooled engines of that type are set up for max power and cooling fan output at 3600 rpm. Unless the airseeder fan is a light load for the engine,it may run too hot at only 2000 rpm.

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trw

08-13-2006 09:36:57

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Re: Pulley speed question in reply to Jon Hagen, 08-12-2006 19:22:28

The engine will NOT be using its full potential of horsepower. We were trying to get some speed out of it for cooling. We need to find out from the dealer if this will be cooling enough. Actually the engine will run it at an idle, but we were also concerned about cooling. Thanks to everyone for their info, I believe we are on the right track. I also love the term "redneck engineering"!! We use it a lot, just didn't know what it was called!!!

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Pooh Bear

08-12-2006 11:48:17

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

You want the fan to run at 4000 RPM.
You want the motor to run at 2000 RPM.
The pulley on the motor needs to be
twice as big as the pulley on the fan.
It's a simple 2:1 ratio.
You got a 7.75 pulley on the motor.
Then you need a 3.875 pulley on the fan.
That is a 3 and 7/8 pulley on the fan.

Pooh Bear

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Slowpoke

08-12-2006 09:40:14

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

What kind of fan..agricultural, furnace blower, exhaust, etc? What's the intended use? What's the diameter of the fan? What's it made of? Most "fans" run less than 2000 rpm to as low as 300rpm. I think it will tear itself apart at 4000rpm and injure someone. Post more info.

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trw

08-12-2006 10:42:01

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Re: Pulley speed question in reply to Slowpoke, 08-12-2006 09:40:14

Yes, the fan is designed to run this fast. It is for our air-seeder. We do not have enough hydraulics on our tractor to run the fan, so we put a Honda engine on it. After running the engine in the dust for about 10 years, it is begining to use quite a bit of oil. After pricing a overhaul of the engine, we decided to get a new engine and try to run it slower than the last one and we need to have the fan run the same speed. I used the same formula as Bob did and we came up with the same results. I will wait to see if any other idea are thrown my way.

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Slowpoke

08-12-2006 16:53:59

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Re: Pulley speed question in reply to trw, 08-12-2006 10:42:01

How many quarts of oil will it take to buy a new engine? Maybe you could get more time out of the old engine while finding a new one at a bargain price. Can you design a better air filtration system for the current or replacement engine? Actually, ten years doesn't sound too bad for dusty conditions. Engine hour meters are available for small engines for around $20.

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Bob

08-12-2006 11:19:45

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Re: Pulley speed question in reply to trw, 08-12-2006 10:42:01

I did a similar thing some years ago, to run a centrifugal spray pump from a 16HP Briggs, which bascially ran at idle all day long, and whirled the pump at 3600 RPM's because of the step-up belt drive.

What an improvement that was over the SCREAMING direct-drive engine driven pumps most sprayers have!

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Gerald J.

08-12-2006 09:30:41

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

You need to use pitch diameter, not outside diameter if you want to be precise.

You need to consider power to run the fan which make call for larger diameter pulleys (because the belt only handles a certain maximum tension and that pulling tension multiplied by the radius is torque and torque with speeds gives horsepower) or multiple belts to handle the load.

Belt and pulley vendor catalogs have belt/pulley selection pages with all the formulae needed and also often show typical drives with horsepower ratings.

Gerald J.

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Bob

08-12-2006 11:21:59

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Re: Pulley speed question in reply to Gerald J., 08-12-2006 09:30:41

If the 2 pulleys are of similar construction (belt groove cross section), the ratios between the pitch diameters and the actual diameters will be more or less the same.

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Gerald J.

08-12-2006 16:53:56

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Re: Pulley speed question in reply to Bob, 08-12-2006 11:21:59

No they won't. The pitch diameter is a fixed depth smaller than the O.D. Like 1/4" for a 1/2" belt. So the 7.5" OD pulley is 7.25" PD and the 3.75 is 3.5" PD which makes the ratio 2.07, not 2.00.

Gerald J.

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Bob

08-12-2006 21:31:29

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Re: Pulley speed question in reply to Gerald J., 08-12-2006 16:53:56

I said "more or less the same".

In "redneck engineering", a ratio of 2.07 vs. a ratio of 2.0 is "more or less the same", at least for driving an applicator fan!

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HTR

08-13-2006 18:15:41

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Re: Pulley speed question in reply to Bob, 08-12-2006 21:31:29

Sort of like the old story of the engineer and the scientist in the room, with a pretty, and willing young thing on the other side of the room. They are told they can move half the distance to her every five minutes. The scientist is going to turn around and leave, but the engineer knows he is going to get close to do the job.

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Al m

08-13-2006 05:58:16

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Re: Pulley speed question in reply to Bob, 08-12-2006 21:31:29

"redneck engerneering",love that term,also agree.It's like using 4 in place of pie,close enough 99% of the time and you can do math in your head.
Al

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Stan in Oly, WA

08-13-2006 10:33:53

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Re: Pulley speed question in reply to Al m, 08-13-2006 05:58:16

Hi Al,

Why would you use 4 rather than 3 for pi? It's just as easy to do the math using 3 but the variance from the true figure is only 1/6 as much as using 4.

The actual circumferance of a 12" diameter circle is about 37.7". Using 3 for pi gives a circumferance of 36 but 4 gives 48. 48 wouldn't be close enough for practical purposes 99% of the time.

I totally agree with the concept, though.

All the best, Stan

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Al m

08-13-2006 11:48:02

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Re: Pulley speed question in reply to Stan in Oly, WA, 08-13-2006 10:33:53

Sorry Stan,re-read, yes 3 works as well if not better
Al

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Al m

08-13-2006 11:46:11

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Re: Pulley speed question in reply to Stan in Oly, WA, 08-13-2006 10:33:53

I mean that if you for instance want to calculate rpm for turning a shaft on a lathe 4 would be close enough because the in the end you will be choosing a speed the gearbox has the is close to the mathmatical results.Close counts and cutting speed x 4/ dia I can quickly do in my head.
Al

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I Bleed Green

08-12-2006 09:11:39

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

This is will help you.

Pulley and belt info

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Bob

08-12-2006 07:46:00

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Re: Pulley speed question in reply to trw, 08-12-2006 07:11:24

2000 x 7.75" = 15,500.

15,500 / 3.875" = 4000 RPM's...

Sounds right to me.

(I hope the fan is designed for that speed!)

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Ralph, Ohio

08-12-2006 08:40:01

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Re: Pulley speed question in reply to Bob, 08-12-2006 07:46:00

I think you need to consider the circumference of the pulleys rather than the diameter. The belt travels around the circumference.

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Bob

08-12-2006 09:24:18

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Re: Pulley speed question in reply to Ralph, Ohio, 08-12-2006 08:40:01

Actually, Ralph, the using the diameters is ALL that's needed... nothing fancy, like circumferance.

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