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Drive belt falls off when clutch/brake is engaged


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I have a 12HP Simplicity RTH tractor with the 36" mower deck that is about 30 years old but is in good running and operating condition -until now.   The drive belt didn't break, rather a piece of it went missing (see pic) and I went to my Sim dealer and obtained the proper replacment and installed it correctly to the best of my knowledge (I've done so at least once before).   The tractor drove fine - until I engaged the clutch/brake at which time it stopped as the belt had fallen off.    I  re-installed it and double-checked all of the belt stop positions and made sure they were tight.   I noticed the belt is tight but not overly so when in the drive position, and very loose when the clutch is engaged (see pic).   I didn't remember how much slack existed previously so figured this was normal, but time after time the same thing happened:  tractor drive as usual, hydro shifts forward/reverse just fine - but the belt falls off when the clutch/brake pedal is pressed.

I've examined all the idlers to make sure the proper hardware is in place and that they turn freely.  There is a small amount of wear that allows the clutch arm assembly to wobbly slightly but it doesn't seem excessive to me.   I know the belt is properly routed and the stops are set per the manual - and from what I can see the clutch link is moving the arm through the complete range of motion, which ought to be applying the required tension to the belt from the clutch spring.   Since the old belt is intact I've compared the old and new belts and they are the same size.

I'm running out of theories and ideas so would really appreciate any thoughts those who are more experienced with these tractors might have.   The 12RTH suits my needs well and I'd really like to get it back in service.   Thanks!



sim belt slack.jpg

Sim belt.jpg

Edited by w9ran
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It almost sounds like you are missing small L shaped belt guides that keep the belt in place when the brake / clutch is pressed.

Note when I used my RBT I never used the brake/clutch, not even when starting it. If I needed to slow down or stop I just moved the hydraulic level. 

hopefully other more knowledgeable will jump in.


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If the belt is coming off the engine drive pulley then the stops for it are not set properly. When stops are set properly the belt should pucker forward off the drive pulley on most tractors. Usually there are 2 stops at the engine pulley that face the rear of the tractor on either side of the belt/pulley.  When the belt is tensioned the stops should just clear the tensioned running belt on either side off pulley center to the rear.


If the belt is coming off the idlers, then the stops are not right on those. On most idlers the stops will be set in towards the pulley far enough that you cant get the belt on or off without loosening the pulley bolt & stops.


If the belt is routed properly and stops are all set properly, you shouldnt have all the slack you have when the clutch is pushed in. 

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I found the two belt guides (156041 and 156331) on ebay but I could make them from an angle bracket if I knew the length of each section.  Does anybody know the measurements?


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