OK,,,for some time,,the motor on my air compressor has been on the going south trip,,so,I went to look at a job in a guys garage and he gots two motors I would like to buy or trade for this work. Mine is a 3 HP 1750 1 phase 220 volt capacitor start and capacitor run that has been on the compressor forever,,,,Pully size on the motor is 5 1/2"

  He has a 3 HP 1750 1 phase that would be almost a direct changeout.

 He also has what I would really like to have and is a 5HP 3600 1 phase 220 volt.

 The compressor has a 14" flywheel, so that is not changeable. What size pulley would I have to run on the motor to slow it down to where the 1750 is running now,,?

 Well Chit I KNEW I shoulda told all the tidbits on the first go round,,,, Ya'll see tmy shop compressor is an AO Smith V4 SINGLE stage that makes a whole lot of air,,,just not at very high pressures like a two stage would but so far the pressure  it does make 90# has been plenty for my air tools as well as my homemade bead blaster. I don't remember exactly the scfm it does make, (like in the neighborhood of 20 scfm) haS been enough for what I need.

 Michael,,,you do got a point in that I prolly should drive the AO Smith compressor a shade faster for faster pressure build up as it now just loafs along at the speed it now runs at with the existing 3 HP.

  BUT,,I do have a two stage Champion compressor that I traded work for and I completely rebuilt it for a spare and it been under the work bench for bout 5 years. It has a 5" low stage cylinder and a 2" high stage cylinder. The PO of the 2 stage had it driven with a 5 HP 3600  motor with a really small pulley on the motor but I did not measure the pulley back then.

 What I would like to do is buy that guy's 5 HP motor and install it on the AO Smith and run it till the compressor crashes ,,then install the rebuilt Champion with the 5 HP already there,,,,sound like a plan,,,???

 

https://www.blocklayer.com/pulley-belteng.aspx" rel="nofollow - https://www.blocklayer.com/pulley-belteng.aspx

My algebra teacher would ask me to find the "X". I would say "How would I know where the X is, you were the one married to her".

  Well,,,so many THANK YOU'S to all of ya'll Gurus for all the help,,,,, Hey Chask,,I took some screen shots of the link you posted,,that will come in handy,,,if I can talk that dude outa the motor(s)

As I-R quality had gone downhill with India made units, Saylor Beall was a standout replacement "pump", tougher than pretty much all the newer units sold by the old name brands. Hanging intercoolers made them last as they dissipated heat well, then the Chinese copies hit the market.

The "Fushang" copy of the SB is tough and very inexpensive for anyone wanting a good reliable recip replacement pump.

Common shaft sizes, look up the sheave od requirement for different motors and go.

Would not run any recip without a low level oil switch as cheap insurance.

All compressor 'heads' have a range of RPMs. Slower ,they last longer. Faster they'll supply more CFM( pumping more air )

a 2.75" pulley will give same  as you have now, a 3" will be slightly faster, and belt may last longer.

 

That's actually the gas tank cap sitting on the console, should be on the tank under the hood.

Multiply drive (motor) pulley size by drive (motor) rpm divide by driven (compressor) pulley size equals driven (compressor) rpm... work back to find new motor pulley size..

         1750 X 5.5 = 9625 / 14 = 687.5 compressor rpm

          687.5 X 14= 9625 / 3600 = 2.6736 new pulley size

Didn't know Weedeater tagged a riding lawnmower.

I was waiting to see a 16hp weed wacker.

 Hey FarmBoy...great idea,,!! it sure might be the pressure switch as it does not do the slow down all the time,,and when it does,,it will usually speed back up to normal RPM after runnin at the slow down rate,,,,Gonna try filing the contacts today,,,,

 Hey Tim,,it good to hear from you,,Guy,,!! I agree with the rotary screw compressors,,I just don't like the higher speeds they run at to make the air,,,,,Like ole Jay says run em slow and they live longer,,,,

 Say Chask,,,that has to be the very first Weed Eater brand rider I ever seen,,and I've worked on a whole bunch of them,,, Good lookin machine,,!! Just you wait til ole Shameless sees them yellow wheels,,,he gonna think that is one of them Green tractors he like so much,,,,,

  thas great info, TMiller,,THANKS.

 

Hey Fixer, I wish I could sling a 16hp hand held weed wacker around, talk about needin some cannons.

DMiller wrote: I can agree Hubert, PiE R Round, but Pi R Squared too!! I LOVE Pie, square pie, round pie, triangle pie, any kind of Pie!!

I'll drink to that!!!

Okay, so here's some notes to add:

MANY compressors don't run well when going TOO SLOW... it's because they're frequently splash-lubricated.  Some compressors (like the rotary screws Tim was talkin' about) use 'differential pressure' (high-vs-low side) to siphon oil from the reservoir up into the workings and bearings... so one needs to know EXACTLY how they work, before arbitrarily changing speed.

Some compressors (like the big Quincy that I'm rebuilding right now) have the option of running in different modes, and as a result, they have different lubrication methods, if you choose to run in a different mode, the lubrication system must be changed accordingly, but typically, compressors of that caliber are very well documented.

running a compressor slower WILL reduce noice, and heat (Boyle's Law).  Joe's single-stage pump was designed to push a large volume, but it takes special consideration to run high pressure.  My Brunner V4 is a two-stage unit, and while it was designed for a 10hp 3hp motor, I run it on a 2hp capacitor start... and it does fine, but I've made sure everyhing is 'right' for the circumstance.  The unloader mechanism's setup can be critical to starting ability with a small motor.  The unloader's function is to relieve head pressure from the pump on shutdown (keeps from blowing out seals in the pump, and blowing compressor oil all over the shop), and allow for a no-pressure load on startup.  It does this with a check valve at the tank, and a known volume between tank and pump, and a valve that 'dumps' pressure in this volume at shutdown.

In the case of my Brunner, the unloader activates on motor shutdown, it vents the volume between pump and tank check valve.   There's other methods of unloading, and there's different unloading 'modes', particularly on high-quality systems, so know your pump's methods before going willy-nilly.

When you go to a smaller pully, there's less contact surface of the drive belt, which means reduced belt capacity.  This being the case, people who do a motor swap from 4 pole (1800rpm) to 2 pole (3600rpm) motor frequently experience belt slip, and they counteract that by increasing belt tension, which puts substantially more load on the compressor's shaft bearings, and on the motor.  There's a variety of ways to combat this... going to matched dual-belts, or a speed reduction jackshaft... and of course going to a toothed-belt drive.

It will probably be fine with a lower-power motor at 3600 with a smaller sheave, but hopefully these notes will help you understand what you're facing if it doesn't work properly at first try.