Then first remove pos 21 if chisel is locked move the tool holder backwards as far as possible you see the rubber cap pos 21 can be released with a plier or little screwdriver then after that snapring pos 143 to remove with a shaft retaining ring pliers as little set it between the small groove between the two ends of the ring thats on the metal ring its on pos 31 a rubber ring beware PARTS pos 25 blocking elements 2 pieces are in most cases 1 or 2 are broken thats why its not release the chisel in most cases. , beware if this tool has had its working hours , its the best thing to sevice it a 1 617 000 430 a consumables kit and 1 615 437 511 a oil reservoir are not expensive and all the neccesary parts are to in this kit andits made repair more meaningful because carbon brushes and al neccesary o rings are in the kit. also the blocking elements and more . its much cheaper then the parts separate.
All the other answers are wrong. You get a wrench with the sander. It's thin so it fits between the pad and sander to hold the nut in place while you unscrew the pad.
There are a number of reasons why a belt sander will fail to track -
Poor quality belts.
Physical damage (dropped).
Uneven wear of rubber drive roller.
When one becomes troublesome it can take a deal of sorting. Best idea if there is no obvious reason is to start by trying an OE belt.
Make sure that you have a good set of brushes in there. If so and it continues to happen then the motor is no longer drawing the amount of current it needs to function properly, because it should not slow down like that.