Educational Gear Examples 2

My Educational Mechanical Examples Series

This model is one of my educational mechanical mechanism examples on 80mm x 80mm base plates.
You can find all models of the series in this collection => [Mechanical Mechanism Examples]

This set

This present set includes:

• Helical Rack&Pinion
• Helical Internal Gear
• Spiral Bevel Gear
• Multi-Threaded Worm with Worm Wheel
• Helical Crown Gear
• Geneva Drive
• Archimedean Spiral Gear
• Intermittent gear

Worm & Worm wheel, Bevel, Rack & pinion, Crown and Internal gears are also included in my first set of educational examples, but this model provides helical or spiral variations of Bevel, Rack & pinion, Crown and Internal gears, and multiple threaded variation (two, three, and four threads) of Worm & Worm wheel.

The gears on the upper row are included in this model. Those on the lower row are included in the other model (the first set of the gear examples).

Geneva, Spiral and Intermittent gears were not included in the first set.

Flat spur and helical gear, and screw gears are only included in the first set.

Introduction Video

Brief Descriptions

Helical Rack, Helical Internal gear, Spiral Bevel gear and Helical Crown gear are variations of their standard straight-tooth versions that are included in my first set of gear examples. Because their teeth engage gradually along the helix or spiral, contact and separation occur progressively during the rotation. This results in smoother meshing and quieter operation compared to gears with straight teeth. These modified gear types are used when smooth motion, low noise, or high-speed rotation is required.

Multi-threaded Worm has a larger helix angle than a single-thread worm that is included in my first set of gear examples. This model includes two-, three-, and four-thread worms, which have two, three and four continuous teeth around the cylinder. Because of the high gear ratio, the motion transmission between the worm and worm wheel is usually one-way — the worm can drive the worm wheel, but the worm wheel cannot drive the worm. However, for multi-threaded worms, the gear ratio can be small enough for the worm wheel to drive the worm. When this happens, the one-way motion transmission property no longer holds. Increasing the number of threads makes the worm rotate faster for each turn of the wheel, but reduces the self-locking effect. Therefore, multi-threaded worms are used when efficiency and speed are more important than self-locking.

Archimedean Spiral gear is formed by sweeping a rack tooth along an Archimedean spiral curve. When the gear pair is sectioned by a plane containing both axes, the cross-sectional profiles exactly match those of a standard rack and pinion. However, since the spiral teeth are not perpendicular to the radius, the pinion is not a simple spur gear in three dimensions; it is similar to a helical gear. More precisely, the teeth of the pinion are not perfectly helical, but slightly curved to follow the spiral teeth of the mating gear. An Archimedean spiral gear can achieve a very high gear ratio, comparable to that of a worm gear. Because of this, motion is often transmitted in only one direction — that is, from the spiral gear to the pinion, but not in the other way around. The Archimedean is chosen when spatial constraints make it more suitable than a worm gear.

Intermittent gear transmits the motion only during a portion of its rotation and locks the other gear during the rest of the cycle. In this model, both of the driving and driven gears consist of two layers: the upper layer transmits the motion, while the lower layer locks the driven gear while the motion is not being transmitted. With this design, the velocity of the driven gear increases and decreases discontinuously, starting and stopping abruptly. Therefore, this type of gear is not suitable for heavy loads or high-speed applications.

Geneva drive, which is also called a Maltese cross mechanism, is another type of intermittent rotation transmission mechanism. In this mechanism, motion is transmitted by a pin that runs inside a radial slot, while the driven gear wheel is locked by cylindrical slipping contact during the rest of the rotation. Compared with the previous partial-involute gear design, the Geneva drive provides smoother acceleration and decelation of the driven wheel, from and to zero velocity. This reduces vibration, noise, and wear on the mechanism.

These intermittent mechanisms are used when stepwise or timed motion is required, such as in film projectors, indexing tables, and mechanical clocks.

Case

These models are compatible with the case included in my first set.

All the eight gear examples included in this model, excluding two and three threaded worm, can fit in a single case.
Note that the crown gear and the bevel gear, intermittent gear and spiral gear are facing to each other to save the space.

Printing

• Use the models named ???-printable.stl for printing.
  The models named ???-assembled.stl are provided just to show how they should be assembled.
   
• Use well-dried PETG to have better dimensional accuracy.
• Use 0.1 mm or 0.08 mm layer height to have smoother surfaces.
• Use slow printing speed for overhangs.
• Select “Random” seam position to have smoother rotation.
• Printing orientations:
  • The long shafts of worm and pinion for crown gear should be printed in a standing position. Add brims to them for safer printing.
  • Place the worm so as the flat cross section along the axis facing down.
    
  • The short shaft for bevel bear should be printed in a standing position, too.
    
  • Arm of the base of the Spiral gear pinion should be printed in a position where the axis is pointing up.
    
     

All the gear parts except for the worms & worm wheels of two and three threads could be arranged in 209mm x 209mm.

 All the base plates except for the worms & worm wheels of two and three threads could be arranged in 244mm x 244mm.

Filing and Sanding

Sometimes, the gears suffer from the elephant foot effect and/or stringing effect, resulting in a too tight fit to the shafts (they are designed with a 0.15 mm radial clearance). 

If you see rough surface on the shafts due to stringing, sand off the roughness with small piece of sand paper.

You may also want to slightly file off the longitudinal edges of the rack.

If you feel the gears do not rotate smoothly due to an elephant effect, widen the hole slightly by using a thin round bar file.

Without those issues, the gears should rotate very smoothly with minimal friction.

Assembly

No glue is needed except for the worm.

• Worm: If you feel the shaft of the worm is too loose, glue them by using some super glue.
• Rack & pinion: Slide in the rack first, before putting the pinion.
• Internal: Insert the internal gear first, before the other small gears.
• Bevel: Insert the shaft for the smaller gear at last.
• Crown: The thinner pinion can move along the axis. Be surprised to see that the pinion perfectly fits to the crown gear regardless of the position along the axis.
• Spiral: The teeth of the pinion for the spiral gear are bent to have better fit to the spiral gear. Be careful to assemble it in the right direction.

Other educational models

You may be also interested in the mechanical mechanism examples I have published.

Find them in this collection:
https://www.printables.com/@osamutake_3341417/collections/2728214

Happy printing!

Acknowledgement

I got into gears thanks to K.$uzuki's amazing articles and YouTube videos. Many of the mechanisms shown in this series came from the introductions on his website. He also makes excellent gear models himself. This series wouldn’t have existed without his inspiration.

I learned a lot about technical detail of designing gear tooth profiles from Haguruma-No-Hanashi website. I’m truly grateful for that.

Updates

• 2025-10-08
  • Fixed orientation of pinion gear in Intermittent v39-printable.stl.
• 2025-10-07
  • Added the printable SLT files for easier printing.
  • Renamed the assembled STL files for clarity.