Yamaha uses 1 plastic heavy duty helical gear at low torque step (at the motor) for safety (not to stop failures, I hope you understand the difference between safety and failure. If that gear woul be metal it would not cause failure. That plastic geat is to save other metal expensive gears in case you jammed the crank somehow. Another safety reason is that gear shears in case of electrical issue when motor can't stop and rotates at higher not rated torque and you hit the brake) and that gear has no issues, other gears are metal surface hardened gears do not fail even if you hit the crank. Bosch uses plastic for most gears including the high torgue light duty spur gears close the to the ourput crank and those gears fail.
Don't even make me to start explaining to you again all the issues those Bosch gears have and why they actually installed those. That nylon plastic degrade over time making those high loaded spur gears even more prone to fail.
You probably newer asked yourself why Yamaha claims their motors are sealed and maintenance free for life and do not sell any repair or maintenance parts for them and why Bosch sells repair kits for their motors.
Sorry
@TPEHAK Not sure where you're getting your info from? But I feel I must point out a couple of things. Plastic gears are used in most ebike motors for noise suppression, weight and cost. It is not a safety consideration and will not shear even if you put the brakes full on and pedal in 1st gear until the bike stops.
Bosch plastic gears from Gen 2 onwards will not degrade in the lifetime of the motor. We have seen so many old high mileage (50,000 Miles +) motors now with no visible issues. In fact, the only time I have ever seen a plastic gear fail has been when the main motor bearing has collapsed and the owner has tried to keep going once all the bearings balls had dropped out! For information:
Gen 1 gears occasionally crack due to excess pressure from the boss they are pressed onto.
Gen 2 gears do not fail.
Gen 3 The large plastic drive gear can shear its teeth the same as the Yamaha does, but very rare as only fitted to road and gravel bikes.
Gen 4 do not use any plastic gears.
Yamaha are actually one of the only manufacturers that do produce a service manual for all their motors! These show motor parts torque settings, grease type and where it should be applied etc. Other manuals for Japanese bikes show internal motor part numbers etc. This information is not supplied for the EU though. I don't know why you are allowed to fix your Yamaha motor in some countries and not others? It maybe EU law or more likely commercial suicide to admit your motor required intervention when all others don't.
Parts are available, you can find new internal parts on our website for example and we can order most internal parts if you require them.
Proof of service requirement would be the fact, that without fresh lubrication, the bronze bush bearings that support the crankshaft through the large steel drive gear will wear the crankshaft away without fresh grease every 1500 - 2000 Miles.