The deck has a blade that is spun to cut grass. If the mandrel locks up, the blade won't move. The mower deck is vibrating because of a damaged mandrel. Take care of the mandrels and replace damaged ones.
Follow these steps to fix your issue:
Disconnect the spark plug
Set the parking brake when parking the tractor on a level surface. Remove the key from the switch. You should wear gloves to protect your hands. Remove the spark plug wire from the tractor hood. If your tractor has two cylinders, you should remove the spark plug wires.
Remove the mower deck
The deck should be lowered to its lowest position. Release the belt from the belt keepers by rolling the blade belt off of the pulley. Remove the lift link support from the deck. The left suspension arm must be removed from the tractor frame. The retaining clip and washer can be removed from the deck. Pull the deck out far enough to get the blade cable out of the deck. Release the locking tabs and pull the blade cable out of the deck. Don't use the lift lever until you remove the rear lift links, because the lever exerts more tension after you do so.
Remove the mandrel assembly
To keep the cutting blade from moving, put a wood block between the mower deck and the blade.
Install the new mandrel assembly
Attach the mounting bolts to the new mandrel by threading them into the holes and then removing the bolts. The wood block should be between the blade and mower. Attach the blade to the new blade shaft with the correct side facing up and tighten the blade bolt. New mounting bolts can be used. It is easy to install the lower side of the blade.
Reinstall the mower deck
Attach the blade clutch cable to the idler arm and position the deck under the tractor. Refer to your owner's manual for more on how to put the blade belt in place.
Reconnect the spark plug
Lower the tractor hood if you need to connect the spark plug wire. If your engine has 2 cylinders, you should connect both spark plug wires.
You would need the following tools:
pliers, torque wrench (optional), wood block, work gloves, wrench set.
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