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Old Speedaire Compressor RPM question

dottedzebra

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Oct 9, 2008
Messages
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WI
Hi everyone. I've been lurking here for a very long time and have found a lot of great information. I was hoping to get some help with a very specific question about a compressor I bought. Any help or input would be very appreciated.

I bought an old Speedaire compressor from an auto mechanic who was downsizing from two shops to one. His guess is that it was from the late 60's. I don't know how long he had it or how long he ran it the way it's now set up but he said he was using it right up to when he closed the shop.

It's a 60 gallon horizontal tank, The pump is a Kellog 325. When I bought it I didn't know how it should be set up, so I didn't think to check mortor rpm. Right now it has a 3450 rpm, 5 hp, 22 amp 7/8" drive motor on it. After looking up some info on it, I find this out:


Model:325
HP:3
PSIG:125
Pump RPM:836
Piston Displ:16
Free Air:11.7
Motor Pulley 60 Cy:6.4
Motor Pulley 50 Cy:7.4
# & size of groove:2-A


The compressor has a 5.5" (edited, not a 5") drive pulley and the pump has a 13 3/4" (edited, not a 9" pulley), giving the pump 1380 RPM (edited, not 1916), over double what the pump should be run at if I'm reading the above info correctly. I wired it and powered it up before I found the specs for the pump. It appeared to work great, obviously it filled up fast.

I'm assuming that it's a very bad idea to run the pump at well over twice the speed recommened.

My questions are:
Am I reading the info correctly?
If so, what could have been damaged by running it that fast?
Can I use it AT ALL while I look for a slower motor or different pulleys?

From what I can gather, it would need a 2.35" pulley
Would the following pulley work? It says it takes type a belts and is the correct shaft size. I have no idea what else to look for.
It's a 2.75 OD 2 groove pulley that looks like it would bring the rpm's down to 1054
http://www.surpluscenter.com/item.asp?item=1-2BK27-D&catname=powerTrans


All of this is very new to me so if I'm wording something wrong or anythings unclear, let me know.
Thanks for your time.
Mike

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Bob C

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Jul 17, 2012
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Clarify the Motor pulley od size and the flywheel od size.


take motor pulley size times motor RPM then divide by flywheel diameter. that is your pump RPM.
 
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dottedzebra

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Joined
Oct 9, 2008
Messages
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Location
WI
I know what the pump RPM is:

"The compressor has a 5" drive pulley and the pump has a 9" pulley, giving the pump 1916 RPM, well over double what the pump should be run at if I'm reading the above info correctly."


My questions are:
Am I reading the info correctly?
If so, what could have been damaged by running it that fast?
Can I use it AT ALL while I look for a slower motor or different pulleys?
 
OP
D

dottedzebra

Member
Joined
Oct 9, 2008
Messages
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Location
WI
Sorry about that, I guess I don't understand the terminology. The motor pulley (what I call a drive pulley?) is 5" and the compressor pump flywheel (what I called the pump pulley?) has a 9" pulley.

Does that make sense or am I still not writing it correctly?

I'm heading home now, I'll remeasure the pulleys, but I'm pretty sure that's what they are.
 
Last edited:

volaredon

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is that a 1725 or a 3450 RPM motor? oops nevermind, missed it the 1st time I read the post
 

DHS

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Can you show a pic with a tape measure next to the pulleys. From the look of the pic, the flywheel on the compressor is only slightly smaller than the diameter of the tank. It could be just the pic making it look larger than it is.
 

mechanic217

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Messages
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With the information you posted ,the rpm you posted is correct, a 1750 rpm motor will drop the pump rpm to 958.
 
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larry_g

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It looks like the motor and the specs given should be for a 1725 rpm motor. Even then the numbers do not play out correctly. http://www.aircompeq.com/ka.htm This site gives some info on that pump head but not everything. Where did you find your specs you posted in the first post? I do agree that your running the pump way to fast and would do what you can to get it under 900 rpm. 9" seems small for a pump pulley.

lg
no neat sig line
 
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dottedzebra

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Yeah, I'm an idiot. The pump pulley is 13 3/4" and the motor pulley is 5.5". I'm not sure where I got the first numbers, somewhere on my sheet of calculations.

So it looks like it's been running at 1380 RPMs.

A 1725 motor would put it at 690.

Is there a problem with going with a 2.75" pulley to bring it down to those RPMs?
 

DHS

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Yeah, I'm an idiot. The pump pulley is 13 3/4" and the motor pulley is 5.5". I'm not sure where I got the first numbers, somewhere on my sheet of calculations.

So it looks like it's been running at 1380 RPMs.

A 1725 motor would put it at 690.

Is there a problem with going with a 2.75" pulley to bring it down to those RPMs?

I have little experience, but I would think it would be no problem.
 

larry_g

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Larry, I copied it into a spreadsheet, I'll look for the site I got it from tommorrow.

If you look at the site I linked to you will see that they have 890rpm listed as the highest. I would think that as long as you get it under 900rpm you will be ok. The speed listed in my link is for a 3hp motor so you should be ok pushing the pressure and speed a bit since you have a 5hp motor. What is the cutout pressure on the system you have? In my opinion you should be very comfortable running under 900rpm and 150psi cutoff.

lg
no neat sig line
 

Bob C

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Jul 17, 2012
Messages
572
3.25 Motor pulley diameter will set pump RPM at 815 using a 3450 RPM motor. Just Perfect for this pump. Buy it, install it and get to work.
 
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