Gears tend to be shy.
We are around gears every day, but rarely do we get to see them. They are usually hidden under the cloak of a protective shell or housed deep inside a product or mechanism. There is good reason for this as many gear trains, like in the transmission of your car, need constant lubrication to run cool and friction free. It’s also important to keep debris away from them as it can cause undo wear or cause the gear teeth to lock up or break. While there is good reason to keep them hidden away, because we rarely get to see them, it makes it even harder for us to understand and appreciate them.
There are many different types of gears used to transmit power in the devices we use. In Part 1, I talked about some basic principles of gears focusing mostly on the standard spur gear. Now I will reveal some of these different types of gears and how they are used. I will also discuss ways to prototype innovations with gears to give you the tools to deploy them in your own prototypes.
Types of Gears
Straight Cut Spur Gear
Spur gears are the most common type of gear and the one most people picture when discussing gears. These gears have equally spaced triangular shaped teeth around the circumference of a circle. Most spur gears have teeth on the outside of a shaft, but they can also be formed on the inside bore of a shaft. In this instance they are called internal spur gears.
Helical gears are spur gears with teeth that twist along the axis of the gear like a candy cane. This allows the teeth to engage more gradually, which reduces the stress on the gears and makes them run smoother. They are more expensive to produce than straight spur gears and thus usually reserved for high precision devices. They are common in automotive gear trains where efficiency and longevity is a priority.
Bevel gears have teeth with a conical profile and mesh together at an angle. They are used when you need to change the drive direction in a gear train. These types of gears are sometimes found on old hand crank drills and egg beaters.
Worm gears employ a modified spur gear that is driven by a helical worm gear. This allows for high gear ratios with just two gears. A drawback is that they can be inefficient due to the nature of the motion. They can also be used to change the drive direction in a gear train by 90 degrees.
Alternative Gear Trains
There are some types of gear trains that employ gears in a unique way to achieve desired torque or motion requirements. Here are a few interesting gear train arrangements outside of the standard and compound gear trains discussed in Part 1.
Planetary gears are a great way to get a high gear ratio in a small amount of space with spur gears. In this configuration, a sun gear, which is driven by a motor, is surrounded by a number of planet gears. The planet gears are in turn surrounded by an internal spur gear called a ring gear. As the sun gear rotates, the planets walk around the inside of the ring gear reducing the speed and magnifying torque.
Rack and Pinion
Some applications require linear motion at the output instead of rotation. A rack and pinion uses a circular pinion gear meshed with a rack, which uses the same triangular cut gear teeth arranged in a straight line. This gear arrangement is often found in vehicle steering systems.
Some gear trains use a mix of different types of gears. These are called hybrid gear trains and there are infinite configurations that can be designed. Hybrid gear trains are used to exploit the advantages of the different types of gears to generate the desired motion.
How to Prototype a Gear Train
The best way to do early stage prototypes with gears is to harvest them from another product. Gear boxes can be hacked out of servos, cordless drills, printers or from toys. However, my favorite gearboxes are the hobby gearboxes made by Tamiya. They have a number of different styles like compound, worm gear and planetary arrangements. They are easy to build, come with their own motor, and often have many gear ratio configurations in the same housing, all for between $10-15 at online retailers.
Later in the prototyping process, custom gears may be required, and this is where it gets a little bit tricky. Gears are hard to machine, so it is best if they can be sourced from a gear supply company like Stock Drive or McMaster-Carr. If they cannot be sourced and the tooth pattern is big enough, like 32 pitch and bigger, they can be cut on a laser or water jet cutter with decent enough accuracy.
3D printing is an option, but you have to be careful with the type of material being used. SLA printing is accurate enough to make well formed gears, but the material is not tough enough to withstand many cycles. The SLS 3D printing process uses tougher materials, but the resolution can be too coarse for small gears. Metal printing (DMLS) is a great 3D printing option, but is also one of the most expensive and should be a last resort.
The most important thing to keep in mind when assembling your gear train is to properly space the gears. When assembled properly there should be a little bit of play between each set of gears. This allows them to run smoothly when at speed and keeps them from binding. If you assemble the gear train and it is really noisy, you likely have the mesh too tight and need to rebuild it with the correct spacing.
Let’s wrap up the gears talk
Now that you know what gears are and what they do, it is time to start experimenting. Robert Pirsig wrote in his tome Zen and the Art of Motorcycle Maintenance that “The Buddah resides as comfortably in the circuits of a digital computer or the gears of a motorcycle transmission as he does at the top of a mountain.”
Gears are an integral part of our world and may be an important part of your next innovation. too.
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Also published on Medium.