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All Yamaha mid-drive motors will break after a while

Yamaha PW X2 broken gear repair
Yamaha PW X2 broken gear repair


All Yamaha mid-drive motors have a design flaw: they contain a nylon gear wheel that cannot withstand the maximum force of the motor. The gear can suddenly break if it is severely overloaded. Repairing is expensive, in some countries you have to buy a new motor that costs around $750. This is a serious problem, because after overload, the gear will definitely break again, so you can keep going this way. That's why I made a cheap replacement gear that you can replace yourself. 

To know if your gearhweel is the same: the diameter is 60mm and the thickness is 20mm.

Replacement gear wheel on Tindie

For the Yamaha PW-X and PW-X2, I have let made replacement gear wheels.
These are made of PA6 GF25 (glass fibre reinforced polyamide 6) which is a bit stronger than the original Yamaha gear material and they are precisely milled.

The gears are available on Tindie here.

Note that I cannot give any warranty as I am not responsible for the weak design of the Yamaha gearbox.

See also the article: Yamaha PW-X2 mid-drive motor tear down.

Buy Bergquist thermal gap filler pads

The two pads with 1mm thickness can better be replaced when the printed circuit has been removed too. This is not necessary with the 3mm thick 10x10mm pads.

Together with the gear wheel you can order a thermal gap filler pad from 56mm x 23mm.

This is for the pads of 46mm x 23mm and 23mm x 10mm.

See more about the thermal gap filler pads here: Yamaha PW-X2 mid-drive motor tear down.

How to replace

Remove the broken gear and glue the new one in place. Use common one or two component PU polyurethane adhesive. PU glue has been tested in practice and is very durable. Here polyurethane glue from Bison, note that the text may differ per country, as long as it is polyurethane then it is fine.

PU glue Bison
PU glue Bison

Yamaha PW-X and PW-X2 broken gear repair
Yamaha PW-X and PW-X2 broken gear repair

Yamaha PW-X and PW-X2 broken gear repair
Yamaha PW-X and PW-X2 broken gear repair

Yamaha PW-X and PW-X2 broken gear
Yamaha PW-X and PW-X2 broken gear

Removing the broken gearwheel
Removing the broken gearwheel

Lubrication

To lubricate the gear wheel, you can apply the excess grease that can be found anywhere in the motor onto the gear wheel.

How to avoid gear overload

Nylon gearwheels have an infinite lifespan when used properly: they are used successfully in hub motors where they rarely break. The problem, however, is that the Yamaha mid-drive motors have a design flaw that causes the maximum motor torque to be greater than the gear can withstand. In order to prevent a failure again, you will have to reduce extreme loads: namely driving at full throttle at low speeds, because than the motor torque is maximum.

The best solution would be a software update from Yamaha where the motor torque is automatically kept below a safe value.

Gearwheel breaks when the crank is blocked

The nylon gearwheel often breaks immediately when the crank is suddenly blocked while driving, for instance when you hit the ground with the pedal. The gear teeth break off due to the high kinetic energy of the fast rotating motor that transfers its energy to the gear. Perhaps Yamaha can prevent this by detecting the blocking with software and then immediately stop the motor via the Mosfets.

Save the nylon gear by reducing the maximum motor current

I have an idea how you could prevent the nylon gear from breaking again. The motor current is measured with Hall effect current sensors. Depending on the motor type, this is the 8-pin ACS723LLCTR-??AU (I don’t know the current version) or the 24-pin ACS709TLLF-45AB (this is 45A).
The solution is to reduce the maximum motor current by replacing the current sensors with lower current versions, for example, replace the ACS723LLCTR-40AU-T with the ACS723LLCTR-20AU-T.

Who would like to try this out?

Yamaha PW SE Hall current sensor
Yamaha PW SE Hall current sensor
Yamaha PW X2 Hall current sensor
Yamaha PW X2 Hall current sensor

Do you have any comments about the website? Please let me know.