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.