TECH

PLA gears fail to fail in 3D printed bicycle drivetrain

Anyone who has ever snapped a chain or a crank knows how much torque a bicycle’s power train has to absorb on a daily basis; it’s really more than one might naively expect. For that reason, [Well Done Tips]’s idea of 3D printing a gear chain from PLA  did not seem like the most promising of hacks to us.

Contrary to expectations, though, it actually worked; at the end of the video (at about 13:25), he’s on camera going 20 km/h, which while not speedy, is faster than we thought the fixed gearing would hold up. The gears themselves, as you can see, are simple spurs, and were modeled in Fusion360 using a handy auto-magical gear tool. The idler gears are held in place by a steel bar he welded to the frame, and are rolling on good old-fashioned skateboard bearings–two each. (Steel ones, not 3D printed bearings.) The healthy width of the spur gears probably goes a long way to explaining how this contraption is able to survive the test ride.

The design was modeled entirely in Fusion 360 using an automatic gear tool. These are simple spur gears mounted on a steel shaft. The hobbyist uses standard skateboard ball bearings for the bearings—made of steel, not printed. The entire system is based on a fixed gear ratio, which further increases the load. Despite these limitations, the manufacturer achieved a speed of around 20 km/h during his test.

A decisive factor in durability is likely the relatively large width of the gears. This increases the contact area and distributes forces better. The rear wheel drive sprocket was partially reinforced with a steel part from the original cassette, as the flat grooves of the freewheel hub were not suitable for PLA. During the test ride, a grinding noise occurred—possibly due to a lack of lubricant or the straight profile of the teeth. Switching to helical teeth could solve this problem.

While experience shows that PLA can withstand higher loads in the short term, the material is not yet suitable for functional drive elements. Studies and practical experience show that PLA wears quickly due to friction and heat. Nylon or carbon fiber-reinforced filaments would be significantly more robust for permanent applications.

The drive gear at the wheel is steel-reinforced by part of the donor bike’s cassette, as [Well Done Tips] recognized that the shallow splines on the freewheel hub were not exactly an ideal fit for PLA. He does complain of a squeaking noise during the test ride, and we can’t help but wonder if switching to helical gears might help with that. That or perhaps a bit of lubricant, as he’s currently riding the gears dry. (Given that he, too, expected them to break the moment his foot hit the pedal, we can’t hardly blame him not wanting to bother with grease.)

We’ve seen studies suggesting PLA might not be the best choice of plastic for this application; if this wasn’t just a fun hack for a YouTube video, we’d expect nylon would be his best bet. Even then, it’d still be a hack, not a reliable form of transportation. Good thing this isn’t reliable-transportation-a-day!

by: Tyler August