Another belt question
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  1. #1
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    887

    Another belt question

    Seeing all the threads about belts made me think of a job I had a few years back.

    Unit had a habit of chewing belts up. Appear to be getting overheated due to damage, cogs being torn off and rubber dried out and hard. Leads me to believe they were slipping.

    1. 60 ton RTU, forward curve supply fan, controlled by VFD, no return fan. 2"WC at outlet, 1.5 duct static during normal operation, VAVs within normal CFM range. Can see as low as 1"WC duct pressure and VFD at 60HZ during unocc recovery.
    2. Belts, pulleys, motor all original sizes and specs. Grooves not worn. Tension checked and aligned. Don't remember sizes.
    3. Installed new belts, checked tension, everything normal till about 57 HZ and then the belts would start floating, then squealing. Belts getting warm.
    4. Rechecked tension and alignment.
    5. Belts fine up to 55HZ.
    6. During this time fan outlet and duct pressure lower due to VAVs being open.
    7. Checked the duct work, no restrictions, fire dampers open.
    7. Calculated fan RPM from pulley diameters and motor RPM and well below max RPM and within units manufacturer specs.

    Seems to me that RPM was too high. I had contacted a local motor shop that does laser alignment and vibration analysis and they has no idea either.

  2. #2
    At 57HZ you are at near full rpm of the motor. The VAVs are full open. Could the fan be loading up too much and spinning/skipping the belt? Did you take an amp draw of the motor to see how loaded it was?

  3. #3
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    are the sheaves worn?
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  4. #4
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    887
    Quote Originally Posted by PPettigrew View Post
    At 57HZ you are at near full rpm of the motor. The VAVs are full open. Could the fan be loading up too much and spinning/skipping the belt? Did you take an amp draw of the motor to see how loaded it was?
    Amperage was close to FLA at 60HZ. My thought was that the fan was at the end of its curve but during this time static pressure at unit was less then when VAVs and duct static reached normal operating conditions. In a CW rotation, motor to the left, the upper length of belts would start lifting a good 3 inches or more and slapping and the lower length would remain taunt.

    Quote Originally Posted by dandyme View Post
    are the sheaves worn?
    No they are not see #2

  5. #5
    Quote Originally Posted by PPettigrew View Post
    At 57HZ you are at near full rpm of the motor. The VAVs are full open. Could the fan be loading up too much and spinning/skipping the belt? Did you take an amp draw of the motor to see how loaded it was?
    THIS!

  6. #6
    Join Date
    Sep 2009
    Location
    Stumptown,USA
    Posts
    1,250
    If this unit has an adjustable motor pulley and it has been used with cogged belts and is more than 3 years old it is time for a new motor pulley. And you might want to measure it and go with a fixed pulley this time. Since it has a vfd you don't need an adjustable pulley. You may need to adjust those belts a little tighter than your tension tool tells you to. Last week the senior service tech and I replaced a 35 horsepower motor for a supermarket main store A/C and I tensioned the belts with my Gates tension tool and they were not tight enough to stop the screeching sound. Go ahead and tighten them a little at a time until the squeeling stops. (after you replace that motor pulley!)
    Challenge yourself, take the CM test --- Certificate Member since 2004 ---Join RSES ---the HVAC/R training authority ---www.rses.org

  7. #7
    Join Date
    Oct 2010
    Location
    AZ
    Posts
    628
    What kind of belts are you using?

    What is the diameter of the smaller sheave?

    What is the maximum RPM of the motor?

    How many horsepower is the motor?

    Adjustable or fixed sheave?

    Check the sheaves very carefully. There may not be a groove, but the angle can be worn to a different angle or worn concave slightly. This may not be really easy to see. There are plastic gauges you can get that may help you see if they are truly the right specs.

  8. #8
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    887
    Its been over a year since I have been to that job. I think 40 HP, 1750 RPM, had cog belts, fixed sheaves, motor side was replaced, first thought of what problem was, had very slight wear. I would have to do scrounging through records to see if I could find the pulley sizes and more info. I ended up increasing the belt tension, setting VFD max HZ to 55, leaving a note for next guy and haven't been back.

  9. #9
    Join Date
    Nov 2011
    Location
    Georgia
    Posts
    24
    Make sure both pulleys are compatible with the type belts you’re using.

  10. #10
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    887
    Quote Originally Posted by acdave17 View Post
    Make sure both pulleys are compatible with the type belts you’re using.
    Yeah been through all that, everything was exactly as shipped from factory and verified to match.

  11. #11
    Join Date
    Feb 2011
    Location
    OH
    Posts
    149
    with out seeing belt and location . i had a machine shop with extream oil in the air and the oil effected the belts so bad went to a belt that lasted a few months longer .

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