Geometry Correct Lantern Gears - In Onshape!

Backstory

In the 2024 season CHAOS 131 experience some challenges related to our tilt mechanism. We machined the gear arcs and 3d-printed the pinions. The mechanism worked well until we failed to set it up correctly or a software development caused to to strip the teeth off the gear. It didn’t take long for us to recognize the pinions as disposable. I was inspired by this thread as a Lantern Pinion can be self-made in a way that can take a bunch more load on a simple CNC router without much Machining time.

I am happy to present:

Lantern Gear Generator

Lantern gear Generator has the ability to generate:

  • Lantern Gears
  • Lantern Ring Gears
  • Lantern Racks
  • Lantern Pin Wheels (Pinions)

The Generators use function driven curves to create the geometry to ensure high fidelity, smooth running geometry. To test this I created a 3D-printed toy (in the example folder) that uses bearings for the Pin Wheel. The motion is quite smooth.

There are 3 part studios in the main level that act as the core of the generator. I recommend deriving the parts in these into your part studio and modifying them based on the needs of your design. There are also 2 Assemblies that show how the Pin Wheels mesh with the geometry. These assemblies can be good to help you get going quickly, but are not really intended to be used in designs directly .

The generators allow you to specify:

  • The number of “Teeth” or pins on the Pin wheel (Pinion Teeth)
  • The number of Teeth on the Gear/Rack
  • The Diametric Pitch of the mesh.
  • Extra thickness on the Pin Wheel outside of the Pins themselves (PinWallThickness)
  • The thickness of the Gear or Rack
  • The thickness of the Pin Wheel (PinionPlateThickness).
  • The Length of the Pins
  • As well as the bore styles of the Gear and Pin Wheel

Some Helpful Tips:

  1. If you are going to CNC route these, the pin diameter needs to be larger than the endmill you plan on using to machine the teeth.
  2. I recommend ensuring the Diametral Pitch is large enough that the gear teeth are of a similar thickness to the pins.
  3. Lantern Gears can support pinions with fewer teeth than Involute spur gears as the pins are supported in shear and not cantilevered from the hub.
  4. Bearings and bushings can be used over shoulder bolts or pressed dowel pins to make a very smooth mesh.
  5. Onshape is not perfect at regenerating some geometry. If you run into a problem spot, you may be able to make a copy of the document (Please rename it or keep your version private) and use slightly different techniques to get the results you need. The modeling techniques used here are selected to be robust through most regeneration events, but there are simper modeling techniques that can handle a single configuration better.

I’m happy to answer any questions the community has!

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This is sick!

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This is amazing. It would have saved me from drawing the curves for our offseason bot’s latern gear manually in Solidworks (quite annoying). :laughing:

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That, right there, is why I wanted the equation driven generator

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This is awesome. Had to draw it manually in Solidworks with ours.

For those of you at RiverRage tomorrow, I will have my Demo Assembly available to play with.

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I love this stuff! Thanks for putting this together. I hadn’t seen the Parametric Curve FeatureScript before.

It’s a little touchy on the unit system but really powerful.

Is there much of a difference between a “lantern gear” profile, and an equivalent roller chain sprocket? I always assumed they were equivalent.

I decided to try and answer my own question. The short answer is it looks like roller chain sprockets would be pretty compatible (maybe a bit more backlash) when used with lantern gears from this generator. The appropriate settings for 35 sprockets are: pin diameter = 0.2 inches or 5mm, DP = 8.378 1/inch = circular pitch of 0.375 inch.

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Incidentally, here’s a pretty fun (niche) use of lantern gears: The very-Californian “Long Now” foundation is building a clock in the desert to last 10000 years. To meet that requirement they’ve chosen (as far as I can tell) stainless steel lantern gears with ceramic ball bearings on the pins:

Unlike Involute gear geometry, Lantern gear tooth geometry changes as a function of the number of teeth on the pinion. This means that means that a 36 tooth Lantern gear designed for a 5-pin pinion is not interchangeable with a 36 tooth lantern gear designed for a 15-pin pinion. In theory, a Lantern gear designed for use with an infinitely large pinion would be the same as a sprocket, but that fails in practice due to chain dynamics. Usually, a sprocket will have shorter and stubbier teeth than most lantern gears. This is because the chain wraps around the sprocket so you don’t need ideal engagement on the first roller coming in, but you do want additional clearance to handle tolerances and other effects of the chain.

In this screen shot, I took a 36 tooth sprocket and super imposed it over a pair of 36 tooth lantern gears with a similar diametrical pitch. The orange Gear tooth is intended to mate with a 5-tooth pin wheel while the blue tooth is designed to mate with a 15-tooth pin wheel.
image

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I would not recommend using a sprocket as a lantern gear. You wont maintain the law of gearing throughout the tooth engagement cycle. Yes, they can mesh without interference, but the relative velocities of the the gears wont be constant. There will also be non-uniform backlash. In a pinch it can be done, but it is quite a hack.

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