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Correct me if I'm wrong...

I KNEW that but this guy was sooo convincing he made me question it. Believe me, since then I've been working gear ratios and ten speed bike gears through my head trying to reason it out. I kept coming up with the correct answer but STILL questioned it. I'm just posting here to confirm that I'M not the crazy one. Wait, let me rephrase... ;)


  • Belt driven assemblies...

    IF you take a belt driven device, a fan for example, and increase the DRIVE or motor pulley diameter you will increase DRIVEN pulley speed. Yes or no?
  • John StarcherJohn Starcher Member Posts: 794
    I believe.....

    ....that is correct. A larger drive pulley at the same rpm will produce higher rpm's on the "load" pulley.

  • I'm thinking so too. Geez...

    I JUST talked to an old guy from United Refrigeration. Calls himself "The Motor Man" with "30 years" in the motor business and he INSISTED bigger drive meant slower driven speed. I THOUGHT he was confused but he was so adamant he made me question it myself. I almost had a brain blowout trying to wrap my mind around THAT one. LOL...
  • J.C.A.J.C.A. Member Posts: 2,981
    Think back...

    5th grade math taught us all about "ratios"... TRUE!

    The closer to the size of the driven pulley the drive pulley is...the faster it will turn....and visa versa.
  • LeeLee Member Posts: 19

    higher RPM but less torque, same principal as a car transmission. You can go real fast in 5th but you can't use it from a dead stop. You can't go real fast in 1st but you sure can get a big load moving.
  • radioconnectionradioconnection Member Posts: 68

    Given that there are more inches on the circumference of the larger pulley, that means there will be more inches of belt moved per shaft revolution; so the belt will be moving faster increasing the speed.

    Only exception is for an idler pulley that rides between two others. It's size has no effect.

  • Yup,,,

    I figured it that way too! I just did it again a few minutes ago and was shaking my head over how dumb I was to almost believe him. LOL... I'm telling you, he was convincing. ;)

  • MitchMitch Member Posts: 955
    I think he was

    adamnant because I was taught to never change the motor change the blower pulley. If thinks the same way, then he thought you were increasing the blower pulley and that will be less CFM.

    Changing the motor pulley reduces the available torque. Changing the blower pulley means you need more torque, but by keeping the motor pulley the same, you have more available
  • Then,,,

    Why variable pitch motor pulleys? Meanwhile, during the conversation I checked a couple times to make sure we were talking about the same pulleys. I'm pretty darn sure we were talking motor pulley size and not blower pulley size. But what you're saying about torque DOES ring some kind of bell because that's kind of what he was alluding to. All I care about is I didn't screw up and put a bigger pulley on the Chinese exhaust blower to speed it up when I shoulda put a smaller one. Either way it worked and the Chinese people are happy. Which is really all that counts. ;)
  • bobbob Member Posts: 306
    Did you

    check the current draw of the motor after you changed speed? If so was the door on the blower compartment or off?
  • Yes,,,

    well within range, I could have boosted it a LOT more but somehow running that old thing up over 1000 rpm seemed a bit like not a good idea. I changed out the motor and pulley with identical replacements and they said it wasn't pulling as much as it used to so I slapped a bigger pulley on there and now they are happy again. I cannot understand WHY identiv=cal componenets would do less work but it's all water under the bridge now.
  • larrylarry Member Posts: 1
    Drive Pully

    the actual formual is:

    RPM (fan)/RPM Motor = Sheave Diameter/Pulley Daimeter

    So The bigger the Drive Sheave is the faster the pulley blower motor will go, and move more air. Sheave being the the adjustable motor pulley and the Pulley being the non-ajustable blower pulley.

  • MitchMitch Member Posts: 955
    the drive is adjustable

    because it representst the maximum diameter of the motor pulley without sacrificing can slow the unit down using that. If you need to increase above, change the blower pulley to a smaller diameter, and check the amp draw with the blower door ON.
  • kalkal Member Posts: 60
    yes the \"driven\" will move faster... be carefulll though...

    make sure you are not loading up the drive motor as it will not only be moving slower because of the increased load - but will also be drawing more current, and the motor's internal fan may be inadequate to the task - use an amprobe and don’t let it get past 90% FLA – or ya gona cook it – been there done that with an ajustable sheave!!! Ouch!!!
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    Yes, increasing the size of the drive pully will increase the RPM of the driven pully.

    When both pullies are the same size, RPMs are equal. When the drive pully is smaller than the driven the driven pully has fewer RPMs than the drive; when the drive pully is larger than the driven the driven has greater RPMs than the drive.

    You can easily calculate the RPM difference with simple math.

    Say the drive pully is 4" diameter and the driven pully is 12" diameter.

    A circumference of a 4" pully is 2" * 2 * 3.14 = 12.56"

    The circumference of a 12" pully is 6" * 2 * 3.14 = 37.68"

    37.68 / 12.56 = 3

    That means the drive (smaller) pully must turn three times to produce 1 revolution of the driven pully. If a common 1,750 RPM (full load) motor, the driven pully will be rotating at 1,750 / 3 = 588.33 RPM.

    Be very careful when changing pully sizes as they are typically quite well matched for the motor and work it must accomplish. If you increase the drive pully size in an attempt to increase fan RPM, be very aware that it requires more power to move the fan faster and thus more more air. If the motor does not have sufficient power for the increased load, it will not operate at full speed and will fail prematurely due to overheating.

    I once gave an exceptionally high quality motor salvaged from a 1950s Maytag drier to a friend for him to use in a whole-house fan. Motor RPM was identical, but the pully on the Maytag motor was much larger--told him that he MUST swap the pullies. He didn't and a motor I thought virtually indestructible died in less than a week.

  • elmigueloelmiguelo Member Posts: 1

    If its on a motor its not called a pulley its a sheaved.

    If its a rope type its a pulley.

    The sheave on the motor It's called the driver sheave and on the blower is called driven sheave.

    The sheaves don't wear the sheave grooves does.
This discussion has been closed.


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