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  1. #1

    Important formulas for DIY compressor builds

    Hi to all, this will be my first post.
    I've become a member here in an effort to expand my knowledge of A/C circuitry. I'm very lacking in the principle behind household wiring, but hopefully that will change soon.

    I'm glad to see that there is a sub-section on here dedicated to compressor talk. This is one field that I DO know a little something about. I've built a total of about 7 compressors in the past- gas and electric- and would like to give to ya'll what I view to be VERY important formulas so you can construct a compressor yourself vs. spending hundreds of dollars at a store. You'd be suprised how much money you can save if you build it yourself.

    First off, there needs to be some understanding of basics. Pumps have max rpm's they can spin at. The closer you can get to this number, the shorter the recovery time of the tank. Sometimes, you'll find that the pump you want to run will require more hp that what you had originally anticipated. If this is the problem you have, a way to combat it would be to install a smaller diameter pulley on the motor...but now you're going to have a longer recovery time, due to the pump not being able to perform at its max speed. That being said. There are two formulas that I'd like to share.

    Let's say that you have a tank and a motor with a 3" pulley already, but want to upgrade your pump. So you buy a pump with an 11" diameter flywheel (also known as a sheive), and this new pump has a max rpm of 1,180, and you need to know if this motor pulley will be big enough to spin your pump at max RPM. Let's say that your motor RPM is 3,450. ALL motors will say on the data plate the RPM. Some are 3,450, some are 1,750, etc. Gas engines are usually 3,500- 3,600 at WOT. Remember it's all about quick recovery time. Here's your formula:

    Take the max RPM of the pump: 1,180
    {multiply that by}
    The diameter of the Shieve on the pump: 11"
    [divide that by]
    The RPM of the motor: 3,450

    So: 1,180 (X) 11 (/) 3450= 3.76"

    In this case, your pulley diameter is 3/4" too small, and you're not spinning the pump at max RPM. Your pump will last longer becasue it won't run as hot, but you're not building pressure as fast as what you could.

    On the same token as the formula above, let's see just how fast your 3" diameter pulley will make that pump spin. You know the max speed the pump can spin is 1,180, and you know that you own a 3" diameter pulley, but you're curious just how much slower your pulley will make the pump spin. Here we go:

    Take the RPM of the motor: 3,450
    {multiply that by}
    pulley diameter of the motor: 3"
    [divide that by]
    Sheive diameter of the pump: 11"

    3,450 (X) 3 (/) 11= 941 RPM

    If you have any questions, let me know. If you're on this forum, then I have the feeling that you share the same passion for building/fabricating as I do. These air compressors are so easy to build it's unreal. A lot of times, I can build a perfectly running compressor for about 100.00 using used parts. my last build was a twin stack (8 gallon air tank). I got the tank for free because someone was throwing it away. I went to the scrap yard and bought a 1hp Baldor electric motor for 10$, found a single cylinder pump on Craigslist for 25$, then 15$ for the check valve, and 30$ for the pressure switch. I already had conduit and copper tubing laying around. This pump is quiet, the compressor is light enough for me to pick up by myself, and is perfect for running on jobsite using a low KW generator.

    If you're in search of pulleys, the best place I've found to get them at for a reasonable price is Tractor Supply Company. Grainger is high dollar, as well as Purvis Industries. The folks at TSC can order you any pulley you need. If they tell you otherwise- they're lazy and don't want to help. Find someone else (older) that feels like using the catalog and still understands what customer service is about.

  2. #2
    Contributing Member MXtras's Avatar
    Join Date
    Nov 2005
    Somewhere in Virginia

    Re: Important formulas for DIY compressor builds

    Great first post! Awesome info.

    This site has been a little dead lately, so please don't forget about us!!

    Welcome to the site.

    If it wasn't for the last minute I wouldn't get anything done

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