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Gears as a “Design Pattern”

PeteLinforth / Pixabay

He lives with his creativity in high gear.

John Travolta

On the surface, gears seem pretty simple.  Create a couple short cylinder, put some teeth on it with more or less the same spacing and you are done. I’ve seen a lot of makers do just this and get some workable designs.  So why do mechanical engineers spend so much time optimizing them?

If you have done your own gears, you may have noticed that they can wear out quickly or they may wear unevenly. They may not turn as smoothly as you want. Reversing the gears may create a backlash while the teeth “catch up” to the teeth on the opposite gear. They may push to hard on their mounting axis.   The gears may turns faster or slower than you want.  Here is some information to help you raise your game:

Gears are rotating devices generally used to transmit torque. They have teeth or “spurs” that can interact with other spurs for a variety of tasks. For example:

  •  Increase or decrease mechanical advantage
  •  Change the axis of rotation
  •  Convert between rotational torque and linear force
  •  Speed things up, slow things down

For instance, you might use gears to create a clock where the second hand goes around sixty times for each time the minute hand goes around. Vehicles might “switch gears” to allow the wheels to turn faster or slower while the motor continues to run at the same most efficient RPM.

Mechanical Advantage and Velocity of Gears

Mechanical advantage and velocity change between two connected spur gears are easy to calculate by

counting the number of spurs on each gear.

MA= Teeth on driven gear/Teeth on driving gear

If the driving gear has ten teeth and the driven gear has twenty teeth, we get a mechanical advantage of two. That is, the driven gear will have twice the torque as the input gear.

On the flip side, the driven gear will rotate at half the speed. If you want to create a gear train that speeds up rotation, then you want a big driving gear and a small driven gear. This will cause the driven gear to rotate much faster than the driving gear.

Gear Teeth

The design of the gear spurs has been a topic of deep analysis in mechanical engineering for a long time.

While it is possible to just sketch out some gears that might work, it’s very likely to hit a number of problems:

  • Interference -the teeth can’t mesh in all positions.
  • Excessive friction- The teeth drag across each other causing the gear to wear out very quickly.
  • Excessive stress- The design more easily bends or breaks compared to other designs.
  •  Backlash– A large gap in training faces causes a significant jar in the mechanism when reversing the gears.
  • Jerky or non-constant velocity- the trailing gear may slow down and speed up over and over even though the driving gear is turning at a constant speed.

For these reasons, a large number of rules and conventions have been created for creating quality, efficient gearing. There are entire books on this subject.

For the person that occasionally uses gears in their design, you most likely don’t want to design your own gears each and every time. Fortunately, there are a wide variety of gear design software tools available (leave  comment for others on what your favorite is). Still, even with gear design software, you must enter in your own parameters to generate these gears, so we’ll continue with an overview of gears to help with this in then next post.