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906-1 FADAL 4020 - Un-engaged low gear belt "slapping sound" as machine is running in high gear.


Jun 17, 2021
Heres the run down of sequence of events i have gone thorugh the last couple weeks:

A week ago the machine started making a horrid sound like slapping/crunching whenever i turn on the spindle, but only after the machine has been running for 4-6 hours.

The belts themselves werent in bad condition, but, i seemed to have fixed the problem by switching to new belts and adding oil to hydraulic lines that engage the high/low belts.
One of the lines was leaking at the connection of the cylinder. (this is how i came to the conclusion to add oil as the hydraulic didnt seem to be engaging at the strength it needed to be so i assumed it was slipping.)
which seemed to work for the last week.

Yesterday, the noise returned after 5 hours of run time so I shut it down for the day. Came in, turned it on today to run, it worked great untill about 4 hours run time.

The sound is so bad theres no way i can run the machine through the order i have at the moment.

when i take the cover off and inspect, it seems like the low gear belt is catching the pulley as the high gear is running, but the hydraulic isnt engaged at all.
considering i just fixed it a week ago with new belts i dont think what i did helps in the long term. The belts still seem to be in great condition, owner says the belts could have oil on them but upon inspection
im not convinced thats whats causing it.

last year i changed motor mounts, one solid one rubber sandwich.
so i figure i can rule out motor sag.

If someone with a similar problem, or insight on the issue could que in some ideas it would be greatly appreciated.
This is normally caused by one belt not being fully released. The loss of oil is normally due to the air/oil cylinder being worn out. Replace the two tensioning cylinders and I think you'll be in good shape.
A common problem on the Fadal multi rib belt systems is debris build up in the pulley grooves. This lets the belts ride high on the pulleys and the off-tension belt does not have enough clearance move away from the pulley. When changing belts use a groove cleaning tool (or wire brush) to clean the pulleys, you will be surprised how much junk is in the bottom of the V's.
Also check the idler pulleys for free spool, sometimes rubber builds up in the small gap and drags the pulley.
What motor do you have?

I haven't had a low speed belt in the machine for years, same problem. Too lazy to go back in and clean the pulleys. The machine has the VHT 20/30hp motor, with so much torque runs fine without the low speed belt